A Mill Process System Technical Field The present invention relates to the field of a tumbling mill and to the monitoring of a tumbling mill. The present invention also relates to a method for generating information relating to an internal state of a tumbling mill, and to the field of control of a tumbling mill. The present invention also relates to a method of operating a comminution process in a tumbling mill, and to an apparatus for monitoring of an internal state of a tumbling mill. The present invention also relates to an apparatus for controlling an internal state of a tumbling mill. The present invention also relates to a computer program for monitoring of an internal state of a tumbling mill. The present invention also relates to a computer program for controlling an internal state of a tumbling mill. Description of Related Art In some industries, such as in the mining industry, there is a need to grind material that comes in large pieces to reduce the size of individual pieces of the received material. A tumbling mill can achieve grinding of material. A tumbling mill includes a shell that contains a charge of material to be tumbled and ground when the shell rotates. US 2017/0225172 Al discloses that grinding in tumbling mills may be inefficient particularly where energy is wasted by impact that does not break particles, and that autogenous (AG) and semi-autogenous (SAG) mills sometimes operate in an unstable state because of difficulty in balancing the rate of feed of large particles into the tumbling mill with the consumption of the charge. According to US 2017/0225172 Al it is essential that real-time information be provided as to the current state of the charge in the tumbler in order to control this process. US 2017/0225172 Al discloses using rotor dynamics to determine characteristics of the moving charge within the tumbling mill. According to US 2017/0225172 Al a monitoring apparatus is provided for monitoring a tumble mill. The apparatus includes vibration sensors mounted on the two main bearings of the tumble mill and on a thrust bearing of the mill producing vibration signals corresponding to the bearings on which the sensors are mounted. These vibration signals are transmitted to an analyzer that analyzes the signals and displays an operating condition of the tumble mill either numerically or graphically. In FIG.5 of US 2017/0225172 Al there is disclosure of two orbit plots, one orbit plot generated at time 1 and another orbit plot generated at time 2. By observing the change in the orbit plot from time 1 to time 2, the operator of the mill will observe that the magnitude of vibrations has decreased dramatically and that orbit parameters or frequencies or phases or precessions or other characteristic changes in orbit plots has likewise decreased dramatically, according to US 2017/0225172 Al. This information will, according to US 2017/0225172 Al, tell the operator that something very significant has changed regarding overall operation of the mill rotor and the composite material charge being processed. Summary In view of the state of the art, a problem to be addressed is how to generate improved information relating to an internal state of a tumbling mill and/or how to obtain an improved method of operating a comminution process in a tumbling mill. This problem is addressed by examples presented herein. Brief Description of the Drawings For simple understanding of the present invention, it will be described by means of examples and with reference to the accompanying drawings, of which Figure 1A shows a somewhat diagrammatic and schematic side view of a system including a tumbling mill. Figure 1B shows another somewhat diagrammatic view of a system including a tumbling mill. Figure 1C is a block diagram illustrating a tumbling mill as a box receiving a number of inputs and generating a number of outputs. Figure 2, shows another example of a cross-sectional view taken along line A-A of FIG. 1A Figure 3 is a schematic block diagram of an example of the analysis apparatus shown in Fig 1. Figure 4 is a simplified illustration of the program memory and its contents. Figure 5 is a block diagram illustrating an example of the analysis apparatus. Figure 6A is an illustration of a signal pair S(i) and P(i) as delivered by an A/D converter. Figure 6B is an illustration of a sequence of the signal pair S(i) and P(i) as delivered by the A/D converter. Figure 7 is a block diagram that illustrates an example of a part of a status parameter extractor. Figure 8 is a simplified illustration of an example of a memory and its contents. Figure 9 is a flow chart illustrating an example of a method of operating the status parameter extractor of Figure 7. Figure 10 is a flow chart illustrating an example of a method for performing step S#40 of Figure 9. Figure 11 is a flow chart illustrating another example of a method. Figure 12 is a flow chart illustrating another example of a method for performing step S#40 of Figure 9. Figure 13 is a graph illustrating a series of temporally consecutive position signals P1, P2, P3,..., each position signal P being indicative of a full revolution of the monitored shell. Figure 14A shows another example of a cross-sectional view of the mid-portion 98 of a rotating mill shell during operation. Figure 15A is a block diagram illustrating an example of a status parameter extractor. Figure 15B is a block diagram illustrating another example of a status parameter extractor. Figure 16 is an illustration of an example of a visual indication of an analysis result. Figure 17 is an illustrations of another example of a visual indication of an analysis result. Figure 18 is an illustration of another example of a visual indication of an analysis result. Figures 19A and 19B are illustrations of yet another example of a visual indication of an analysis result in terms of internal status of the tumbling mill. Figure 20 is a block diagram of an example of compensatory decimator. Figure 21 is a flow chart illustrating an embodiment of a method of operating the compensatory decimator of Figure 20. Figures 22A, 22B and 22C illustrate a flow chart of an embodiment of a method of operating the compensatory decimator of Figure 20. Figure 23 shows another example of a cross-sectional view of the mid-portion of a rotating mill shell during operation. Figure 24 shows a somewhat diagrammatic and schematic top view of another system including a tumbling mill. Figure 25 shows a somewhat diagrammatic and schematic top view of yet another embodiment of a system including a tumbling mill. Figure 26 shows a somewhat diagrammatic and schematic top view of yet another embodiment of a system including a tumbling mill. Figure 27 shows a somewhat diagrammatic and schematic top view of yet another embodiment of a system including a tumbling mill. Figure 28 shows a somewhat diagrammatic and schematic top view of yet another embodiment of a system including a tumbling mill. Figure 29 shows a somewhat diagrammatic and schematic top view of yet another embodiment of a system including a tumbling mill. Figure 30 shows another example of a cross-sectional view of the mid-portion of a rotating mill shell during operation. Figure 31 is a block diagram that illustrates another example of a status parameter extractor. Figure 40 is a block diagram of a system for monitoring of an internal state X of a mill and for enabling improved control of a comminition process that occurs in a mill. Figure 41 shows a flowchart of a tumbling mill operating method. Figure 42 shows a flowchart of another tumbling mill operating method. Figure 43 shows a flowchart of yet another tumbling mill operating method. Figure 44 shows a flowchart of yet another tumbling mill operating method. Figure 45 shows a flowchart of yet another tumbling mill operating method. Figure 46 shows a flowchart of yet another tumbling mill operating method. Detailed Description In the following text similar features in different examples will be indicated by the same reference numerals. Figure 1A shows a somewhat diagrammatic and schematic side view of a system 5 including a tumbling mill 10. The tumbling mill 10 may be an autogenous (AG) mill, for example. Alternatively, the tumbling mill 10 may be a semi-autogenous (SAG) mill, for example. Another example tumbling mill 10 is a ball mill 10. Figure 1A also shows a sectional view, section A-A. Section view A-A is also is also identified by the reference 15. The tumbling mill 10 includes a shell 20 having an internal shell surface 22 that forms a chamber 25 for grinding material. In the section view identified by the reference 15 in figure 1A, the shell 20 is illustrated as revolving at a rotational speed f _ROT in a counterclockwise direction, as indicated by the curved arrow f _ROT . The tumbling mill chamber 25, in operation, contains a charge 30 of material to be tumbled and ground. The charge material has a material surface 33, i.e. a boundary between air, in the rotatable shell 20, and material 30 (See Fig 2). Grinding in a tumbling mill serves the purpose of reducing the size of particles of solid material. This may be achieved e.g. by causing pieces of solid material to fall onto other pieces of solid material. Hence, a tumbling mill utilizes a force of nature, gravity, for accelerating particles of the charge against other particles of the charge. According to some embodiments, the walls of the shell 20 comprise a sturdy material, such as for example steel, so as to withstand impact of heavy particles, e.g. large pieces of ore that are tumbled in the chamber 25. According to some embodiments, a wall of the shell 20 comprises an elastic material so as to reduce wear of the wall. According to some embodiments, the elastic material comprises rubber. According to some embodiments, the elastic material comprises a polymer such as polyurethane. According to some embodiments, the internal shell surface 22 includes a surface coating of an elastic material, such as e.g rubber or polyurethane. According to some embodiments, the shell 10 is supported on at least two bearings 40 and 50. The shell 20 is rotatable around an axis of rotation 60. In this connection it is noted that an axis is an imaginary line around which an object spins (an axis of rotation). The rotation of the shell is utilized to lift a part of the charge, including particles of solid material, so that some of the solid particles can fall back down, under the influence of gravity, onto another part of the charge. Hence, it is desirable to select the speed of rotation fROT of the shell 20 to a suitable value in order to obtain a balance between the lifting action and the falling action of the charge 30. With reference to the sectional view 15 of figure 1A, an arrow 62 indicates the direction of the force of gravity g in connection with the rotating shell 20 and its charge 30. Hence, an internal state of the tumbling mill 10 depends, in part, on a balance between the force of gravity 62 and a centripetal force 65 acting to press that part of the charge 30 that is immobile with respect to the internal shell surface 22, in a radial direction from the center, i.e. from the axis of rotation 60. In other words, during operation of the tumbling mill 10, the centripetal force acts to press a part of the charge 30 towards the internal shell surface 22, the centripetal force being dependent on the speed of rotation fROT of the shell 20. In this connection it is noted that the centripetal force, acting on a piece of solid material being in contact with the internal shell surface 22, is dependent on the inner radius of the shell 20. When the force of gravity 62, acting on a particular piece 68 of solid material, is larger than a part 69 of the centripetal force, acting on that piece 68 of solid material in the direction opposite to the direction of gravity, then that piece 68 of solid material will fall. A vibration sensor 70 may be provided for producing a measuring signal SEA. The measuring signal S _EA may be dependent on mechanical vibrations or shock pulses generated when the shell 20 rotates. An example of the system 5 is operative when a vibration sensor 70 is firmly mounted on or at a measuring point on the tumbling mill 10. The measuring point can comprise a connection coupling to which the sensor 70 is firmly attached, or removably attachable. In the example illustrated by figure 1A, the sensor 70 is mounted on the bearing 40. Alternatively, the sensor 70 may be mounted elsewhere on the tumbling mill where the sensor 70 is capable of generating the measuring signal SEA dependent on mechanical vibrations or shock pulses generated when the shell 20 rotates. The tumbling mill 10 has an input side 80 for receiving pieces of solid material, and an output side 90 for delivery of output material 95 that has passed through the tumbling mill 10. The shell 20 may have a mid-portion 98 having a substantially cylindrical shape, the chamber 25 at the mid portion having an internal radius RMIC. The internal radius RMIC may, for example, be in excess of 0,5 meter. Alternatively, the internal radius RMIC may, for example, be in excess of 3 meters. The tumbling mill 10 may alternatively have a chamber mid portion internal radius RMIC in excess of 8 meters. The mid-portion of the shell 20 has a length LMIC from the input side 80 to the output side 90. The mid-portion shell length LMIC may, for example, be in excess of 1meter. According to an embodiment, the mid-portion shell length LMIC may be in excess of 8 meters. It is noted that any herein exemplified internal radius RMIC may be combined with any herein exemplified shell length LMIC. Moreover, it is noted that the shell 20 may have a mid-portion 98 having a polygonal shape. An example of such a polygonal shell shape is a shell that exhibits at least three shell wall portions that are joined to form the chamber 25 of the tumbling mill. In this context it is noted that, for the purpose of this disclosure, a tumbling mill shell having a mid-portion 98 having at least six shell wall portions that are joined to form the chamber 25a may be considered to have a substantially cylindrical shape. Thus, for the purpose of this disclosure, a tumbling mill shell having a mid-portion 98 having a hexagonal shape may be considered to have a substantially cylindrical shape. In the example illustrated by figure 1A, the input side 80 includes a first input 100 for pieces of solid material 110. The solid material 110, also referred to as feed material 110, may include pieces 115 of rock and ore, the pieces 115 having a variety of sizes. However, the solid material 110 fed into the first input 100 may have been treated so that there is a maximum solid material particle size. In other words, the feed material 110 may include pieces 115 of rock and ore, the pieces 115 having a feed particle size distribution. For example, the feed particle size distribution may be such that there is a certain maximum input solid particle volume VISPM, and/or a certain maximum input solid particle diameter D _ISPM . Hence, the maximum solid material particle size in the feed material may be a certain maximum input solid particle volume VISPM. The solid material 110 may, for example include pieces 115 of ore having a particle volume of up to ten (10) cubic decimeters, i.e. an individual input solid particle 115 has a maximum input solid particle volume V _ISP of less than, or at most, ten (10) cubic decimeters. Alternatively, the maximum solid material particle size may be a certain maximum input solid particle diameter DISPM. Hence, an individual input solid particle 115 has a maximum input solid particle diameter D _ISP of less than, or at most 250 mm. The infeed particles 115 may include useful minerals as well as minerals considered less useful. The less useful minerals may be referred to as waste minerals. In order to enable separation of the useful minerals from the waste minerals the solid feed material 110 is ground in the tumbling mill 10. The ground output material 95 delivered from the tumbling mill 10 may include particles 96 whose diameter is around 0,1 mm or less than 0,1 mm. The particles 96 delivered from the tumbling mill 10 may may be referred to as product particles 96. According to some embodiments, the tumbling mill 10 operates to perform dry grinding. According to an embodiment the tumbling mill 10 is a ball mill operating to perform dry grinding. The ball mill 10 includes a number of balls 117 for enhancing the grinding of the feed particles of the solid feed material 110 into ground solid product particles 96. According to an embodiment the balls 117 of the ball mill include steel balls. According to an embodiment the tumbling mill 10 is a ball mill which is used for grinding feed particles 115 of a hard substance into a powder 95 referred to as cement. In this connection it is noted that Portland cement, a form of hydraulic cement, is made by heating limestone, i.e. calcium carbonate, with other materials, such as clay, in a process known as calcination that liberates a molecule of carbon dioxide from the calcium carbonate to form calcium oxide, or quicklime, which then chemically combines with the other materials in the mix to form calcium silicates and other cementitious compounds. According to an embodiment the resulting hard substance is then ground, with an amount of gypsum, into a powder, using the above mentioned ball mill 10 for dry grinding, to make cement. According to some embodiments, the tumbling mill 10 operates to perform grinding of solid material 110. An example of a grinding process employing a tumbling mill 10 that operates to perform grinding of solid material 110 is a tumbling mill 10 in the mining industry. According to some embodiments, the mining industry tumbling mill 10 operates to perform grinding of solid material 110 including a mix of useful minerals and minerals considered less useful. According to some embodiments, the mining industry tumbling mill 10 is an autogenous (AG) mill. Alternatively, the mining industry tumbling mill 10 is a semi-autogenous (SAG) mill. According to some embodiments, the mining industry tumbling mill 10 is a ball mill 10. According to some embodiments, the solid material 110 is an ore having a metal content. The average metal content in the solid material 110 may be, for example, higher than 0,1%. According to some embodiments, the solid material 110 has an average metal content of more than 5% of a desired metal. Alternatively, the average metal content in the solid material 110 may be, for example, 50%. According to some embodiments, the solid material 110 has a content of more than 40% of a desired metal. According to some embodiments, the solid material 110 has a content of more than 40% of a desired metal, the desired metal being iron. In this context it is noted that the content of a desired metal, in the solid material 110, may affect the density of the charge in the tumbling mill 10. Thus, according to some embodiments, the density of the charge in the tumbling mill 10 may be indicative of a relation between a desired metal and waste minerals in the charge in the tumbling mill 10. According to some embodiments, the grinding process may be facilitated by providing a liquid 120. An example of a grinding process facilitated by providing a liquid 120 is a tumbling mill for use in the mining industry. According to some embodiments, the liquid 120 enters the tumbling mill 10 at a second input 130 at the input side 80 of the tumbling mill 10. In the rotating shell 20, the input pieces of solid material 110 are mixed with the input liquid 120 to form the charge 30. When the density of the input liquid 120 differs from the density of the input solid material 110, the density of the charge 30 can be controlled by controlling the proportion of input liquid 120 and input solid material 110. Thus, when the input liquid 120 has a lower density than the density of the input solid material 110, the density of the charge 30 can be lowered by increasing the amount of input liquid 120. The input liquid 120 may comprise water. Water has a density of about 997 kg per cubic metre. The pieces of input solid material typically has a higher density than the density of the input liquid. The pieces of input solid material typically has a density in excess of 1500 kg per cubic metre. The input solid material 110 may comprise ore that holds useful minerals mixed with other minerals. An example of a useful mineral is a mineral that contains a metal, such as e.g. aluminium or iron. Aluminium has a density of about 2700 kg per cubic metre. Iron has a density of about 7870 kg per cubic metre. The “other mineral” mentioned above, may include e.g. granite or other pieces of rock. Granite has a density of about 2700 kg per cubic metre. Table 1 provides some examples of solid material and corresponding material properties. Table 1 In the field of mineralogy, the term tenacity describes a mineral's resistance to breaking, beading, cutting, or other forms of deformation. A material is brittle if, when subjected to stress, it breaks with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. A malleable material is capable of being extended or shaped by beating or by pressure. A ductile material is capable of being pulled or stretched by mechanical force without breaking. Compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size. By contrast, tensile strength is the capacity of a material or structure to withstand loads tending to elongate. In other words, compressive strength resists compression (being pushed together), whereas tensile strength resists tension (being pulled apart). It is noted that gold has a density of 19320 kg per cubic metre, which is significantly higher than that of the other solid materials listed in table 1 above. In this connection it is also noted that gold content in ore comprising some gold is usually low in comparison to the content of other solids that are used as feed material 110 to a tumbling mill. The output side 90 of tumbling mill 10 may include a separator for delivery of output material 95 on an output 200 and for retaining pieces of material whose particle size exceeds a limit value. The separator may include a screen configured to sift out pieces of material that have a particle size smaller than a certain limit value for delivery as output material 95 on the output 200. The ground output material 95 delivered from the tumbling mill 10 may include particles whose diameter is smaller than a certain limit output particle diameter. The limit output particle diameter may be 0,1 mm. One measure of a production quality of the tumbling mill 10 may be the proportion of output particles having an output particle diameter less than 45 µm (here µm means micrometre), or the amount per hour of output particles having an output particle diameter less than 45 µm. Moreover, it is desirable to obtain a high degree of efficiency of the grinding process. One aspect of grinding process efficiency is the amount of ground material per time unit. Hence, it is desirable to improve or optimize the number of kg/hour of ground solid material having a particle size smaller than a limit value. However, the magnitude is usually metric tons per hour of solid material being fed into a tumbling mill 10. Another aspect of grinding process efficiency is the amount of ground material per energy unit, in order to minimize grinding process energy consumption. Hence, it is desirable to improve or optimize the throughput in terms of kg/kilowatt-hour of ground solid material, the ground solid material having a particle size smaller than a limit value. In this context it is noted that a tumbling mill typically may have a power consumption of more than 4 Megawatt. Some tumbling mills have a mean power consumption of 10 Megawatt, and some may require 20 Megawatt as peak consumption. In this context it is noted that when a tumbling mill has a mean power consumption of 10 Megawatt, then the energy consumption is 10000 kWh per hour. Thus, when that tumbling mill is in operation 24 hours a day for a year, then even a small improvement of grinding process energy efficiency, such as a one percent (1%) improvement would render energy savings in the range of six million kWh per year. The efficiency of the grinding process in a tumbling mill 10 depends on a number of variables affecting the internal state of the tumbling mill 10. One variable that has an impact on the efficiency of the grinding process in a tumbling mill 10 is the filling degree of the tumbling mill 10. Hence, it is desirable to control the inflow of input solid material 110 so as to achieve an optimal filling degree. In order to maximise the amount of output material 95 from the tumbling mill 10 it is therefore desirable to control the inflow of input material 110 so as to maintain an optimal state of the tumbling mill process. The optimal internal state of the tumbling mill process may include a certain filling degree of the shell 20, i.e. a certain charge volume. Hence, one variable that has an impact on the efficiency of the grinding process in a tumbling mill 10 is the solid feed rate R _S , i.e. the amount of solid material particles per time unit that is fed into the tumbling mill 10. Another variable that has an impact on the efficiency of the grinding process in a tumbling mill 10 is the mineralogical properties of the input solid material particles 110. In this connection, it is noted that mineralogy is a subject of geology specializing in the scientific study of physical properties of minerals as well as the chemistry, and crystal structure of minerals. Moreover, the mineralogical properties of the particles in the charge 30 are not constant over time, since the composition of solid material 110, such as ore from a mine, typically varies over time. The variation of the mineralogical properties of the particles in the charge 30 may affect the efficiency of the grinding process of the tumbling mill 10. Hence, the efficiency of the grinding process may be variable over time due to the variation of the mineralogical properties of the particles in the charge 30. Thus, a decreased efficiency of the grinding process, during a certain time span, may lead to an increase of the charge volume in the mill 10 if the infeed of material is kept constant. Thus, unless an operator of the tumbling mill is adequately informed about the current charge volume in the mill 10 there is also a risk of overload which may, in a worst case, lead to a complete stop of the grinding process. Yet another variable that has an impact on the efficiency of the grinding process is the size distribution of the solid material particles 110 that are fed into the tumbling mill 10. According to some embodiments the infeed of solid material particles 110 is controlled so that a certain proportion of the solid material particles 110 provided on the first input 100 have individual volumes of more than one cubic decimeter so as to increase the efficiency of the grinding process. It has been concluded that control of the infeed of solid material particles 110 so that a certain proportion of the solid material particles 110 provided on the first input 100 have individual volumes of more than one cubic decimeter increases the efficiency of the grinding process, in particular when the tumbling mill is an AG mill or an SAG mill. The shell 20 is typically opaque, i.e. it is not possible to visually inspect the charge in the shell during operation of the tumbling mill 10. Moreover, the movement of heavy ore that is tumbled during operation of the tumbling mill 10 prevents placing cameras or other sensitive detectors on the inside of the shell 20. It is an object of this document to describe methods and systems for an improved monitoring of an internal state in a tumbling mill during operation. It is also an object of this document to describe methods and systems for an improved Human Computer Interface (HCI) relating to internal state in a tumbling mill during operation. It is also an object of this document to describe methods and systems for an improved Graphical User Interface relating to the grinding process in a tumbling mill 10. The inventor realized that there may exist a mechanical vibration V _IMP indicative of an impact between a protrusion, such as a lifter, on an internal surface of the rotating shell 20 and at least one particle in a toe portion 205 of the material charge 30 during operation of the tumbling mill 10. The inventor also contemplated that such a mechanical vibration V _IMP may be indicative of a current internal state of the tumbling mill 10 and/or a current state of the grinding process. A mechanical vibration VIMP may be generated when a protrusion, such as a lifter, interacts with a particle in a toe portion 205 of a material charge 30 in the chamber 25. The impact force FIMP of the interaction between the rotationally moving lifter and the material charge 30 causes an acceleration of the at least one particle in the toe portion 205 of the material charge 30, the impact causing the mechanical impact vibration VIMP. In fact, the impact force FIMP may cause a mechanical impact vibration VIMP which is indicative of a current internal state of the tumbling mill 10 and/or indicative of a current state of the grinding process. The sensor 70 placed outside the chamber 25 may detect vibrations caused by interaction involving particles of the charge 30 in the chamber 25 during operation of the tumbling mill 10. Hence, with reference to figure 1A, the sensor 70 is capable of generating the measuring signal S _EA dependent on mechanical vibrations or shock pulses generated when the shell 20 rotates. Thus, the measuring signal SEA may be dependent on, and indicative of, the impact force FIMP between a protrusion, such as a lifter, and at least one particle in a toe portion 205 of the material charge 30 during operation of the tumbling mill 10. The sensor 70 may, for example, be an accelerometer 70 configured to generate the measuring signal S _EA having an amplitude that depends on the impact force F _IMP . The inventor concluded that there may exist a mechanical vibration V _IMP indicative of a current internal state of the tumbling mill 10 and/or of a current state of the grinding process, but that conventional methods for measuring vibrations and/or for analysing and/or for visualising such vibrations may hitherto have been inadequate. An analysis apparatus 150 is provided for monitoring of the tumbling mill process. The analysis apparatus 150 may also be referred to as monitoring module 150A. The analysis apparatus 150 may generate information indicative of the internal state of the tumbling mill process dependent on the measuring signal SEA. The sensor 70, generating the measuring signal SEA, is coupled to an input 140 of the analysis apparatus 150 so as to deliver the measuring signal S _EA to the analysis apparatus 150. The analysis apparatus 150 also has a second input 160 for receiving a position signal Ep dependent on the rotational position of the shell 20. A position sensor 170 is provided to generate the position signal Ep dependent on the rotational position of the shell 20. As mentioned above, the shell 20 is rotatable around the axis of rotation 60, and thus the position sensor 170 may generate a position signal Ep having a sequence of shell position signal values PS for indicating momentary rotational positions of the shell 20. A position marker 180 may be provided on an outer surface of the shell 20 such that, when the shell 20 rotates around the axis of rotation 60, the position marker 180 passes by the position sensor 170 once per revolution of the shell, thereby causing the position sensor 170 to generate a revolution marker signal PS. Such a revolution marker signal PS may be in the form of an electric pulse having an edge that can be accurately detected and indicative of a certain rotational position of the monitored shell 20. The analysis apparatus 150 may generate information indicative of a rotational speed fROT of the shell 20 dependent on the position signal Ep, e.g. by detecting a temporal duration between revolution marker signals PS. The position marker 180 may be e.g. an optical device 180, such as a reflex 180, when the position sensor 170 is an optical device, such as e.g. a laser transceiver configured to generate a revolution marker signal PS when the intensity of laser reflection changes due to a laser beam impinging the reflex 180. Alternatively, the position marker 180 may be e.g. a magnetic device 180, such as strong magnet 180, when the position sensor 170 is a device 170 configured to detect a changed magnetic field. An example of a device configured to detect a changed magnetic field is a device including an inductive coil which will generate an electric current in response to a changed magnetic field. Thus, the device 170 configured to detect a changed magnetic field is configured to generate a revolution marker signal PS when passing by the magnetic device 180. Alternatively, the position sensor 170 may be embodied by an encoder 170 which is mechanically coupled to the rotating mill shell 20 such that the encoder generates e.g. one marker signal PS per revolution the rotating mill shell 20. The system 5 may include a control room 220 allowing a mill operator 230 to operate the tumbling mill 10. The analysis apparatus 150 may be configured to generate information indicative of an internal state of the tumbling mill 10. The analysis apparatus 150 also includes an apparatus Human Computer Interface (HCI) 210 for enabling user input and user output. The HCI 210 may include a display, or screen, 210S for providing a visual indication of an analysis result. The analysis result displayed may include information indicative of an internal state of the tumbling mill process for enabling the operator 230 to control the tumbling mill. A tumbling mill controller 240 is configured to deliver a solid material feed rate set point RSSP, and it may also, optionally, deliver a liquid feed rate set point RLSP. According to some embodiments, the set point value RSSP is set by the operator 230. According to some embodiments, also the set point value R _LSP is set by the operator 230. Thus, the tumbling mill controller 240 may include a mill user input/output interface 250 enabling to operator to regulate a solid material feed rate RS and/or a liquid feed rate RL. As mentioned above, the input side 80 of the tumbling mill includes a first input 100 for pieces of solid material 110, and optionally the input side 80 may also have a second input 130 for entry of liquid 120, such as e.g. water, into the chamber 25. The solid material 110 may be transported to the first input 100 by a conveyer belt 260. The conveyer belt 260 runs at a conveyer belt speed to transport a solid material 110 to the first input 100 at the solid material feed rate RS. The solid material feed rate R _S may be for example 10000 kg per minute during operation of the tumbling mill 10 at a certain internal state of the tumbling mill 10. Similarly, the liquid feed rate R _L may be for example 1000 kg per minute during operation of the tumbling mill 10 at the certain internal state of the tumbling mill 10. Control of the liquid feed rate RL is diagrammatically shown in Figure 1A and/or figure 1B by the symbol of a controllable valve 270, receiving the liquid feed rate set point R _LSP from the tumbling mill controller 240. Similarly, control of the solid material feed rate RSF is diagrammatically shown in Figure 1A and/or figure 1B by the symbol of a controllable valve 280, receiving the solid material feed rate set point R _SSP from the tumbling mill controller 240. According to some embodiments, the tumbling mill controller 240 may also generate a set point value f _{ROT_SP} for the rotational speed f _ROT of the mill shell. The rotational speed set point value fROT_SP may also be referred to as U1SP. The rotational speed set point value f _{ROT_SP} , also referred to as U1 _SP , may be generated in response to user input, from operator 230, via user input/output interface 250, as illustrated in Figure 1B. As illustrated in Figure 1B, the tumbling mill controller 240 may also generate set point values U2SP, and U3SP, wherein U2SP is the above mentioned RSSP, and U3SP is the above mentioned R _LSP . Moreover, the tumbling mill controller 240 may also generate a ball feed rate set point value U4 _SP , R _BFSP for setting a ball feed rate U4, R _BF . The ball feed rate U4, R _BF is a number of grinding balls per time unit that is being fed into said ball mill for enhancing the comminution process. Thus, this set point value may be relevant when the mill 10 is a ball mill, i.e a tumbling including grinding balls 1168 (See fig 36). Balls for use in a ball mill may comprise chrome steel or stainless steel. Alternatively, a ball mill may use balls made of a ceramic material. In some examples, a ball mill may use balls comprising a rubber material. The mill user input/output interface 250, in the example illustrated in Figure 1A, is coupled to the regulator 240 and the HCI 210 is coupled to the analysis apparatus 150, or monitoring module 150A, configured to generate information indicative of an internal state of the tumbling mill 10. Thus, when coupled only to monitoring module 150A as shown in figure 1A, the HCI 210 is advantageously possible to add, in a control room 220, without any need to modify any previously existing input/output interface 250 and regulator 240 used by a mill operator 230 to operate the tumbling mill 10. An object to be adressed by solutions and examples disclosed in this document is to describe methods and systems for an improved monitoring of an internal state X in a tumbling mill 10 during operation. It is also an object, to be adressed by solutions and examples disclosed in this document, to describe methods and systems for an improved control of an internal state X in a tumbling mill 10 during operation. Moreover, an object to be adressed by solutions and examples disclosed in this document is to describe methods and systems for an improved Human Computer Interface (HCI) relating to conveying useful information about the internal state X in a tumbling mill during operation. Another object to be adressed by this document is to describe methods and systems for an improved Graphical User Interface relating to the grinding process in a tumbling mill 10. Another object to be adressed by solutions and examples disclosed in this document is to describe methods and systems for an improved control of an an output Y from a tumbling mill 10 during operation. Yet another object to be adressed by solutions and examples disclosed in this document is to describe methods and systems for an improved Human Computer Interface (HCI) relating to conveying useful information about an output Y from a tumbling mill 10 during operation and/or also conveying useful information about a corresonding internal state X in a tumbling mill during operation. Figure 1B shows another somewhat diagrammatic view of a system 320 including a tumbling mill 10. Thus, reference numeral 320 relates to a system including a mill 10 having a rotatable shell 20, as discussed in this document. The system 320 of figure 1B may include parts, and be configured, as described above in relation to figures 1A and 2 and/or as described elsewhere in this document. Whereas the mill user input/output interface 250, in the example illustrated in Figure 1A, is coupled to the regulator 240 and the HCI 210 is a separate input/output interface coupled to the analysis apparatus 150, or monitoring module 150A, the system illustrated in Figure 1B may provide an integrated HCI 210, 250, 210S. Thus, the input/output interface 210 of Figure 1B may be configured to enable all the input and/or output described above in conjunction with interfaces 210 and 250. Figure 1C is a block diagram illustrating a tumbling mill as a box 10B receiving a number of inputs U1, ... Uk, and generating a number of outputs Y1, ... Yn. With reference to figure 1C it is noted that, for the purpose of analysis, a tumbling mill 10 may be regarded as a black box 10B having a number of input variables, referred to as input parameters U1, U2, U3, ... Uk, where the index k is a positive integer. During operation of the black box tumbling mill 10B, the black box tumbling mill 10B has an internal state X, and it produces a number of output variables, also referred to as output parameters Y1, Y2, Y3, ... Yn, where the index n is a positive integer. The internal state X of the mill may be described, or indicated, by a number of internal state parameters X1, X2, X3,..., Xm, where the index m is a positive integer. Using the terminology of linear algebra, the input variables U1, U2, U3,... Uk may be collectively referred to as an input vector U; the internal state parameters X1, X2, X3,..., Xm may be collectively referred to as an internal state vector X; and the output parameters Y1, Y2, Y3, ... Yn may be collectively referred to as an output vector Y. The internal state X of the mill 10, at a point in time termed r, can be referred to as X(r). That internal state X(r) can be described, or indicated, by a number of parameter values, the parameter values defining different aspects of the internal state X(r) of the mill 10 at time r. The internal state X(r) of the black box tumbling mill 10B depends on the input vector U(r), and the output vector Y(r) depends on the internal state vector X(r). An aspect of the internal state X is the total amount of material 30 in the shell 20, and that total amount does not change instantly. Thus, during operation of the mill 10, the internal state X(r) can be regarded as a function of an earlier internal state X(r-1) and of the input U(r): X(r) = f ₁ (X(r-1), U(r) ), wherein X(r-1) denotes the internal state X of the mill 10 at a point in time preceding the point in time termed r. Likewise, the output Y of the black box 10B can be regarded as a function of the internal state X: Y(r) = f ₂ (X(r)) Figure 2, being another example of a cross-sectional view taken along line A-A of FIG. 1A, shows a more detailed example of the mid-portion 98 of the shell 20. The shell 20 has an internal shell surface 22 facing the chamber 25, said internal shell surface 22 including a number of protrusions 310. According to some embodiments, there are provided at least two protrusions 310. The example shell 20, shown in Figure 2, includes twelve protrusions 310 that are placed at equal distances from each other on the internal shell surface 22 of the shell 20. The protrusions 310 may be configured to engage and lift material 30 as the shell rotates about the axis 60. Thus, the protrusions 310 may be referred to as lifters 310. The charge material has a material surface 33, i.e. a boundary between air, in the rotatable shell 20, and material 30. In figure 2, the shell 20 is shown during rotation in a clockwise direction at a speed of rotation f _ROT . Lifters 310 comprise structures such as internal formations, veins, bars, projections and the like which project from internal shell surface 22 towards a center of shell 20. A lifter 310, also referred to as protrusion 310, has a leading edge 312 that engages and lifts the material charge 30 as tumbling mill 10 is rotated about axis 60 such that the material falls upon itself within the interior chamber 25. In one example, lifters 310 comprise elongate bars which are mounted to internal shell surface wall 22 so as to at least partially line the interior shell surface 22 of mill 10. In other examples, lifters 310 are integrally formed as part of a single unitary body with internal shell surface wall 22. According to some embodiments, the leading edges 312 of the protrusions 310 are equidistant. Thus, referring to the example shell 20 shown in Figure 2, including twelve protrusions 310 wherein each protrusion 310 has a leading edge 312, the angular distance between any two adjacent leading edges 312 is 30 degrees. In this context it is noted that, when there are L protrusions 310 on an internal shell surface 22, the L protrusions 310 being positioned such that the leading edges 312 of the protrusions 310 are equidistant, then the angular distance between any two adjacent leading edges 312 is 360/L degrees. Thus, when there are L protrusions 310 at angular positions on the internal shell surface 22, the L protrusions 310 being positioned in a mutually equidistant manner, then the angular distance between any two adjacent protrusions 310 is 360/L degrees. In the example shown in figure 2, the position sensor 170 is mounted in a stationary manner so that it generates a position signal Ep having a sequence of position signal values PS for indicating momentary rotational positions of the shell 20. The position marker device 180 may be provided on an outer wall surface of the shell 20 such that, when the shell 20 rotates around the axis of rotation 60, the position marker 180 passes by the position sensor 170 once per revolution of the shell, thereby causing the position sensor 170 to generate a revolution marker signal value PS. The position sensor 170 may comprise a tachometer 170 that delivers e.g. one position signal pulse Ep per revolution. The position marker device 180 may comprise a metal object. The metal object may be a bolt or a metal bracket, for example. With reference to figure 1B, solid material particles 115 enter the shell 20 of the mill via the material feed inlet 100 and are subjected to breakage from collision with other particles 115, 30 and/or the internal shell surface 22 and/or balls during operation of the mill 10. The breakage results in solid material product particles 96, also referred to as solid material products or product particles 96. The solid material products exit the mill shell 20 via the output 200. An important aspect of the grinding process is the breakage rate. The breakage rate depends e.g. on the collision frequency that solid feed material particles 115 are subjected to after entry into the rotating shell 20. Another important aspect of the grinding process is the size distribution of the solid material product particles 96 resulting from collision. The size distribution may also be referred to as the appearance distribution function. Yet another important aspect of the grinding process is the flow rate of solid material product particles 96 out of the mill shell 20. The particle transport out of the mill shell 20 may also be referred to as the product discharge rate. Thus, in summary: Solid feed particles 115 enter the chamber 25 of mill 10 at a solid material feed rate R _S . The feed particles 115 have a first particle size distribution, also referred to as feed size distribution. The solid feed material may be measured as it is being fed into mill 10. A feed material analyzer 325 may be provided for generating a measurement value indicative of at least one feed material property U5, U6. The at least one feed material property may include a solid feed material size distribution U5, and/or data U6 indicative of the weight per time unit of the solid material 110 that is transported to the first input 100 of the mill 10. Thus a feed material size distribution may be estimated, e.g. by measurement. Alternatively, a solid feed material size distribution U5 may be predetermined. In some examples, the solid feed material size distribution U5 is known because of treatment and/or sorting before delivery of the solid feed material 110 to the conveyer belt 260. Once received in the mill chamber 25 the received particles may be collectively referred to as charge material 30. While being in the mill chamber 25 the solid material particles 30 are subjected to breakage resulting in solid product particles 96 that are discharged from the chamber 25 via the output 200. The breakage causes a change of the size distribution of the particles. Solid material product particles 96 flow out of the mill chamber 25 at a product discharge rate RSDis. The product discharge rate RSDis may be measured, and it may be regarded as an output parameter Y1. The discharged solid product particles 96 have a second particle size distribution, also referred to as product size distribution. The product size distribution may be measured, and values indicative of the product size distribution may be provided, e.g. as output parameter values Y2, Y3 etc. Thus, feed particles 115, having a first particle size distribution or feed size distribution U4, are fed into the mill 10 at a solid material feed rate RS, and product particles 96, having a product size distribution Y2 or second particle size distribution Y2, are discharged from the mill 10 at a product discharge rate Y1 = R _SDis . During transport of a feed particle 115 from the inlet 100 to the outlet 200, the feed particle 115 is transformed into a number of smaller product particles 96. This transformation is due to a comminution process that occurs during operation of the tumbling mill 10. It is believed that the product size distribution depends on - the feed size distribution, and - the internal state(s) X of the mill 10 during the transport of received feed particles 115 from the feed input 100 until the discharge of output material 95 from the mill output 200 in the form of solid product particles 96. Thus, the amount of breakage of a solid feed material particle 115 depends on the above mentioned breakage rate and the internal state X of the mill 10. The internal state X of the mill 10 determines i.a. the duration T _C of the transport of received feed particles 115 from the feed input 100 until the discharge of output material 95 from the mill output 200. An average temporal duration of transport from feed input 100 to mill output 200 may be referred to as a duration TC. With reference to figure 1B, there is shown a cartesian co-ordinate system having three mutually perpendicular axes x, y and z. It is to be understood that, during operation of the mill 10, the material 30 travels in the positive direction of the x-axis from the input side 80 of the mill to the output side 90. Thus, whereas the material 30 is tumbled in the mill 10, the material 30 gradually also travels in a direction paralell to the axis of rotation of the mill, from the input side of the mill to the output side of the mill. However, different individual particles may travel that distance at mutually different speed. A mass of a small feed particle may be transported from the input side of the mill to the output side of the mill to thereafter be discharged via the mill output 200 comparatively fast e.g. because it will be small enough to pass through an exit grid after a shorter time, whereas the largest feed particles need more time in the mill to be ground into product particles small enough to to pass through that same exit grid. Thus, a certain proportion of a large feed particle will remain in the mill for longer than a corresponding proportion of a small feed particle. During steady state operating conditions however, the mass flow into, through, and out of the mill 10 will be constant, or substantially constant. Thus, the transport of material 30 through the mill 10 may be discussed in terms of mass per time unit, e.g as measured in kilograms per minute or in metric tons per hour. In this connection it is noted that the average temporal duration of the transport of feed particles 115 from feed input 100 to mill output 200, and the simultaneous transformation of those feed particles 115 into a number of smaller product particles 96, depends on the average speed of flow v _xA , in the direction of the x-axis, in the mill chamber 25. Accordingly, at least under steady state conditions of the mill process, the average temporal duration of the transport of particles from feed input 100 to mill output 200 depends on the length of the the mill chamber 25, in the x-direction, and the average speed of flow vxA. Figure 3 is a schematic block diagram of an example of the analysis apparatus 150 shown in Fig 1. The analysis apparatus 150 has an input 140 for receiving the analogue vibration signal S _EA , from the vibration sensor 70. The input 140 is connected to an analogue-to-digital (A/D) converter 330. The A/D converter 330 samples the received analogue vibration signal SEA with a certain sampling frequency fS so as to deliver a digital measurement data signal SMD having said certain sampling frequency fS and wherein the amplitude of each sample depends on the amplitude of the received analogue signal at the moment of sampling. The digital measurement data signal SMD is delivered on a digital output 340 which is coupled to a data processing device 350. With reference to Figure 3, the data processing device 350 is coupled to a computer readable medium 360 for storing program code. A computer readable medium 360 may also be referred to as a memory 360. The program memory 360 is preferably a non-volatile memory. The memory 360 may be a read/write memory, i.e. enabling both reading data from the memory and writing new data onto the memory 360. According to an example, the program memory 360 is embodied by a FLASH memory. The program memory 360 may comprise a first memory segment 370 for storing a first set of program code 380 which is executable so as to control the analysis apparatus 150 to perform basic operations. The program memory 360 may also comprise a second memory segment 390 for storing a second set of program code 394. The second set of program code in the second memory segment 390 may include program code for causing the analysis apparatus 150 to process a detected signal. The signal processing may include processing for generating information indicative of an internal state of a tumbling mill, as discussed elsewhere in this document. Moreover, the signal processing may include control of the internal state of a tumbling mill, as discussed elsewhere in this document. Thus, the signal processing may include generating data indicative of an internal state of a tumbling mill, as disclosed in connection with embodiments of status parameter extractor 450 of e.g. figure 5, 15 and/or 24. The memory 360 may also include a third memory segment 400 for storing a third set of program code 410. The set of program code 410 in the third memory segment 400 may include program code for causing the analysis apparatus to perform a selected analysis function. When an analysis function is executed, it may cause the analysis apparatus to present a corresponding analysis result on user interface 210, 210S or to deliver the analysis result on a port 420. The data processing device 350 is also coupled to a read/write memory 430 for data storage. Hence, the analysis apparatus 150 comprises the data processor 350 and program code for causing the data processor 350 to perform certain functions, including digital signal processing functions. When it is stated, in this document, that the apparatus 150 performs a certain function or a certain method, that statement may mean that the computer program runs in the data processing device 350 to cause the apparatus 150 to carry out a method or function of the kind described in this document. The processor 350 may be a Digital Signal Processor. The Digital Signal Processor 350 may also be referred to as a DSP. Alternatively the processor 350 may be a Field Programmable Gate Array circuit (FPGA). Hence, the computer program may be executed by a Field Programmable Gate Array circuit (FPGA). Alternatively, the processor 350 may comprise a combination of a processor and an FPGA. Thus, the processor may be configured to control the operation of the FPGA. Figure 4 is a simplified illustration of the program memory 360 and its contents. The simplified illustration is intended to convey understanding of the general idea of storing different program functions in memory 360, and it is not necessarily a correct technical teaching of the way in which a program would be stored in a real memory circuit. The first memory segment 370 stores program code for controlling the analysis apparatus 150 to perform basic operations. Although the simplified illustration of Figure 4 shows pseudo code, it is to be understood that the program code may be constituted by machine code, or any level program code that can be executed or interpreted by the data processing device 350 (Fig.3). The second memory segment 390, illustrated in Figure 4, stores a second set of program code 394. The program code 394 in segment 390, when run on the data processing device 350, will cause the analysis apparatus 150 to perform a function, such as a digital signal processing function. The function may comprise an advanced mathematical processing of the digital measurement data signal SMD. A computer program for controlling the function of the analysis apparatus 150 may be downloaded from a server computer. This means that the program-to-be-downloaded is transmitted to over a communications network. This can be done by modulating a carrier wave to carry the program over the communications network. Accordingly the downloaded program may be loaded into a digital memory, such as memory 360 (See figures 3 and 4). Hence, a program 380 and/or a signal processing program 394 and/or an analysis function program 410 may be received via a communications port, such as port 420 (Figure 1A and/or figure 1B & figure 3), so as to load it into program memory 360. Accordingly, this document also relates to a computer program product, such as program code 380 and/or program code 394 and/or program code 410 loadable into a digital memory of an apparatus. The computer program product comprises software code portions for performing signal processing methods and/or analysis functions when said product is run on a data processing unit 350 of an apparatus 150. The term "run on a data processing unit" means that the computer program plus the data processing device 350 carries out a method of the kind described in this document. The wording "a computer program product, loadable into a digital memory of a analysis apparatus" means that a computer program can be introduced into a digital memory of an analysis apparatus 150 so as achieve an analysis apparatus 150 programmed to be capable of, or adapted to, carrying out a method of a kind described in this document. The term "loaded into a digital memory of an apparatus" means that the apparatus programmed in this way is capable of, or adapted to, carrying out a function described in this document, and/or a method described in this document. The above mentioned computer program product may also be a program 380, 394, 410 loadable onto a computer readable medium, such as a compact disc or DVD. Such a computer readable medium may be used for delivery of the program 380, 394, 410 to a client. As indicated above, the computer program product may, alternatively, comprise a carrier wave which is modulated to carry the computer program 380, 394, 410 over a communications network. Thus, the computer program 380, 394, 410 may be delivered from a supplier server to a client having an analysis apparatus 150 by downloading over the Internet. Figure 5 is a block diagram illustrating an example of the analysis apparatus 150. In the figure 5 example, some of the functional blocks represent hardware and some of the functional blocks either may represent hardware, or may represent functions that are achieved by running program code on the data processing device 350, as discussed in connection with figures 3 and 4. The apparatus 150 in figure 5 shows an example of the analysis apparatus 150 shown in figure 1A and/or figure 1B and/or figure 3. For the purpose of simplifying understanding, figure 5 also shows some peripheral devices coupled to the apparatus 150. The vibration sensor 70 is coupled to the input 140 of the analysis apparatus 150 to deliver an analogue measuring signal S _EA , also referred to as vibration signal S _EA , to the analysis apparatus 150. Moreover, the position sensor 170 is coupled to the second input 160. Thus, the position sensor 170 delivers the position signal Ep, dependent on the rotational position of the shell 20, to the second input 160 of the analysis apparatus 150. The input 140 is connected to an analogue-to-digital (A/D) converter 330. The A/D converter 330 samples the received analogue vibration signal SEA with a certain sampling frequency f _S so as to deliver a digital measurement data signal S _MD having said certain sampling frequency fS and wherein the amplitude of each sample depends on the amplitude of the received analogue signal at the moment of sampling. The digital measurement data signal SMD is delivered on a digital output 340, which is coupled to a data processing unit 440. The data processing unit 440 comprises functional blocks illustrating functions that are performed. In terms of hardware, the data processing unit 440 may comprise the data processing unit 350, the program memory 360, and the read/write memory 430 as described in connection with figures 3 and 4 above. Hence, the analysis apparatus 150 of figure 5 may comprise the data processing unit 440 and program code for causing the analysis apparatus 150 to perform certain functions. The digital measurement data signal S _MD is processed in parallel with the position signal Ep. Hence, the A/D converter 330 may be configured to sample the position signal Ep simultaneously with the sampling of the analogue vibration signal S _EA . The sampling of the position signal Ep may be performed using that same sampling frequency f _S so as to generate a digital position signal EPD wherein the amplitude of each sample P(i) depends on the amplitude of the received analogue position signal Ep at the moment of sampling. As mentioned above, the analogue position signal Ep may have a marker signal value P _S , e.g. in the form of an electric pulse having an amplitude edge that can be accurately detected and indicative of a certain rotational position of the monitored shell 20. Thus, whereas the analogue position marker signal P _S has an amplitude edge that can be accurately detected, the digital position signal EPD will switch from a first value, e.g. “0” (zero), to a second value, e.g. “1” (one), at a distinct time. Hence, the A/D converter 330 may be configured to deliver a sequence of pairs of measurement values S(i) associated with corresponding position signal values P(i). The letter “i” in S(i) and P(i) denotes a point in time, i.e. a sample number. Hence, the time of occurrence of a rotational reference position of said rotating shell can be detected by analysing a time sequence of the position signal values P(i) and identifying the sample P(i) indicating that the digital position signal EPD has switched from the first value, e.g. “0” (zero), to the second value, e.g. “1” (one). Figure 6A is an illustration of a signal pair S(i) and P(i) as delivered by the A/D converter 330. Figure 6B is an illustration of a sequence of the signal pair S(i) and P(i) as delivered by the A/D converter 330. A first signal pair comprises a first vibration signal amplitude value S(n), associated with the sample moment “n”, being delivered simultaneously with a first position signal value P(n), associated with the sample moment “n”. It is followed by a second signal pair comprising a second vibration signal amplitude value S(n+1), associated with the sample moment “n+1”, which is delivered simultaneously with a second position signal value P(n+1), associated with the sample moment “n+1”, and so on. With reference to figure 5, the signal pair S(i) and P(i) is delivered to a status parameter extractor 450. The status parameter extractor 450 is configured to generate an amplitude peak value S _P (r) based on a time sequence of measurement sample values S(i). The amplitude peak value SP(r) may be dependent on an impact force FIMP generated when a projection 310 on an internal shell surface of the rotating shell interacts with a toe portion 205 of the charge material 30 (See Fig.2). In this connection, it is noted that the surface 33 of material 30 will deviate from a horizontal orientation due to a combination of centrifugal force and gravitational force when the shell 20 rotates. The toe portion 205 of the charge material 30 is at the lower edge of the surface 33, as illustrated in figure 2. The status parameter extractor 450 is also configured to generate a temporal relation value R _T (j), also referred to as R _T (r), based on a temporal duration (T _D ) between time of occurrence of the amplitude peak value SP(r) and time of occurrence of a rotational reference position of said rotating shell. As mentioned above, the time of occurrence of a rotational reference position of said rotating shell can be detected by analysing a time sequence of the position signal values P(i) and identifying a sample P(i) indicating that the digital position signal EPD has switched from the first value, e.g. “0” (zero), to the second value, e.g. “1” (one). Figure 7 is a block diagram that illustrates an example of a part of a status parameter extractor 450. According to an example the status parameter extractor 450 comprises a memory 460. The status parameter extractor 450 is adapted to receive a sequence of measurement values S(i) and a sequence of positional signals P(i), together with temporal relations there-between, and the status parameter extractor 450 is adapted to provide a sequence of temporally coupled values S(i), f _ROT (i), and P(i). Thus, an individual measurement value S(i) is associated with a corresponding speed value fROT(i), the speed value fROT(i) being indicative of the rotational speed of the shell 20 at the time of detection of the associated individual measurement value S(i). This is described in detail below with reference to figures 8-13. Figure 8 is a simplified illustration of an example of the memory 460 and its contents, and columns #01, #02, #03, #04 and #05, on the left hand side of the memory 460 illustration, provide an explanatory image intended to illustrate the temporal relation between the time of detection of the encoder pulse signals P(i) (See column #02) and the corresponding vibration measurement values S(i) (See column #03). As mentioned above, the analogue-to-digital converter 330 samples the analogue electric measurement signal S _EA at an initial sampling frequency f _S so as to generate a digital measurement data signal S _MD . The encoder signal P may also be detected with substantially the same initial temporal resolution fS, as illustrated in the column #02 of Figure 8. Column #01 illustrates the progression of time as a series of time slots, each time slot having a duration dt = 1/fSample; wherein fSample is a sample frequency having an integer relation to the initial sample frequency fS with which the analogue electric measurement signal S _EA is sampled. According to a preferred example, the sample frequency f _Sample is the initial sample frequency fS. According to another example the sample frequency fSample is a first reduced sampling frequency fSR1, which is reduced by an integer factor M as compared to the initial sampling frequency fS. In column #02 of figure 8 each positive edge of the encoder signal P is indicated by a “1”. In this example a positive edge of the encoder signal P is detected in the 3:rd, the 45:th, the 78:th time slot and in the 98:th time slot, as indicated in column #02. According to another example, the negative edges of the positional signal are detected, which provides an equivalent result to detecting the positive edges. According to yet another example both the positive and the negative edges of the positional signal are detected, so as to obtain redundancy by enabling the later selection of whether to use the positive or the negative edge. Column #03 illustrates a sequence of vibration sample values S(i). Column #05 illustrates the corresponding sequence of vibration sample values S(j), when an integer decimation is performed. Hence, when integer decimation is performed by this stage, it may e.g. be set up to provide an integer decimation factor M=10, and as illustrated in figure 8, there will be provided one vibration sample value S(j) (See column #05 in figure 8) for every ten samples S(i) (See column #03 in figure 8). According to an example, a very accurate position and time information PT, relating to the decimated vibration sample value S(j), is maintained by setting the PositionTime signal in column #04 to value PT = 3, so as to indicate that the positive edge (see col#02) was detected in time slot #03. Hence, the value of the PositionTime signal, after the integer decimation is indicative of the time of detection of the position signal edge P in relation to sample value S(1). In the example of figure 8, the amplitude value of the PositionTime signal at sample i=3 is PT=3, and since decimation factor M=10 so that the sample S(1) is delivered in time slot 10, this means that the edge was detected M-PT=10-3= 7 slots before the slot of sample S(1). Accordingly, the apparatus 150 may operate to process the information about the positive edges of encoder signal P(i) in parallel with the vibration samples S(i) in a manner so as to maintain the time relation between positive edges of the encoder signal P(i) and corresponding vibration sample values S(i), and/or integer decimated vibration sample values S(j), through the above mentioned signal processing from detection of the analogue signals to the establishing of the speed values fROT. Figure 9 is a flow chart illustrating an example of a method of operating the status parameter extractor 450 of Figure 7. According to an example, the status parameter extractor 450 analyses (Step S#10) the temporal relation between three successively received position signals, in order to establish whether the monitored rotational shell 20 is in a constant speed phase or in an acceleration phase. This analysis may be performed on the basis of information in memory 460, as described above (See Fig 8). If the analysis reveals that there is an identical number of time slots between the position signals, status parameter extractor 450 concludes (in step #20) that the speed is constant, in which case step S#30 is performed. In step S#30, the status parameter extractor 450 may calculate the duration between two successive position signals, by multiplication of the duration of a time slot dt= 1/fs with the number of time slots between the two successive position signals. When the position signal is provided once per full revolution of the monitored shell 20, the speed of revolution may be calculated as V= 1 /(ndiff *dt), wherein n _diff = the number of time slots between the two successive position signals. During constant speed phase, all of the sample values S(j) (see column #05 in Fig 8) associated with the three analyzed position signals may be assigned the same speed value f _ROT =V= 1 /( n _diff *dt), as defined above. Thereafter, step S#10 may be performed again on the next three successively received position signals. Alternatively, when step S#10 is repeated, the previously third position signal P3 will be used as the first position signal P1 (i.e. P1:= P3), so that it is ascertained whether any change of speed is at hand. If the analysis (Step S#10) reveals that the number of time slots between the 1:st and the 2:nd position signals differs from the number of time slots between the 2:nd and 3:rd position signals, the status parameter extractor 450 concludes, in step S#20) that the monitored rotational shell 20 is in an acceleration phase. The acceleration may be positive, i.e. an increase in rotational speed, or the acceleration may be negative, i.e. a decrease in rotational speed also referred to as retardation. In a next step S#40, the status parameter extractor 450 operates to establish momentary speed values during acceleration phase, and to associate each one of the measurement data values S(j) with a momentary speed value Vp which is indicative of the speed of rotation of the monitored mill shell at the time of detection of the sensor signal (SEA) value corresponding to that data value S(j). According to an example the status parameter extractor 450 operates to establish momentary speed values by linear interpolation. According to another example the status parameter extractor 450 operates to establish momentary speed values by non-linear interpolation. Figure 10 is a flow chart illustrating an example of a method for performing step S#40 of Figure 9. According to an example, the acceleration is assumed to have a constant value for the duration between two mutually adjacent position indicators P (See column #02 in Figure 8). Hence, when ^ the position indicator P is delivered once per revolution, and ^ the gear ratio is 1/1: then - the angular distance travelled by the rotating shell 20 between two mutually adjacent position indicators P is one (1) revolution, which may also be expressed as 360 degrees, and - the duration is T = n _diff *dt, ^ where ndiff is the number of slots of duration dt between the two mutually adjacent position indicators P. With reference to Figure 8, a first position indicator P was detected in slot i1= #03 and the next position indicator P was detected in slot i2=#45. Hence, the duration was ndiff1 = i2-i1= 45-3= 42 time slots. Hence, in step S#60 (See Figure 10 in conjunction with figure 8), the status parameter extractor 450 operates to establish a first number of slots n _diff1 between the first two successive position signals P1 and P2, i.e. between position signal P(i=3) and position signal P(i=45). In step S#70, the status parameter extractor 450 operates to calculate a first speed of revolution value VT1. The first speed of revolution value VT1may be calculated as VT1= 1 /(ndiff1 *dt), wherein VT1 is the speed expressed as revolutions per second, ndiff1 = the number of time slots between the two successive position signals; and dt is the duration of a time slot, expressed in seconds. Since the acceleration is assumed to have a constant value for the duration between two mutually adjacent position indicators P , the calculated first speed value VT1 is assigned to the time slot in the middle between the two successive position signals (step S#80). Hence, in this example wherein first position indicator P1 was detected in slot iP1= #03 and the next position indicator P2 was detected in slot i _P2 =#45; the first mid time slot is slot i _P1-2 = i _P1 + (i _P2 - i _P1 )/2= 3+ (45-3)/2= 3+21)=24. Hence, in step S#80 the first speed of revolution value VT1 may be assigned to a time slot (e.g. time slot i= 24) representing a time point which is earlier than the time point of detection of the second position signal edge P(i=45), see Figure 8. The retro-active assigning of a speed value to a time slot representing a point in time between two successive position signals advantageously enables a significant reduction of the inaccuracy of the speed value. Whereas state of the art methods of attaining a momentary rotational speed value of a tumbling mill shell 20 may have been satisfactory for establishing constant speed values at several mutually different speeds of rotation, the state of the art solutions appear to be unsatisfactory when used for establishing speed values for a rotational tumbling mill shell 20 during an acceleration phase. By contrast, the methods according to examples disclosed in this document enable the establishment of speed values with an advantageously small level of inaccuracy even during an acceleration phase. In a subsequent step S#90, the status parameter extractor 450 operates to establish a second number of slots ndiff2 between the next two successive position signals. In the example of Figure 8, that is the number of slots n _diff2 between slot 45 and slot 78, i.e. n _diff2 = 78-45=33. In step S#100, the status parameter extractor 450 operates to calculate a second speed of revolution value VT2. The second speed of revolution value VT2 may be calculated as VT2= Vp61= 1 /(n _diff2 *dt), wherein ndiff2 = the number of time slots between the next two successive position signals P2 and P3. Hence, in the example of Figure 8, ndiff2 = 33 i.e. the number of time slots between slot 45 and slot 78. Since the acceleration may be assumed to have a constant value for the duration between two mutually adjacent position indicators P , the calculated second speed value VT2 is assigned (Step S#110) to the time slot in the middle between the two successive position signals. Hence, in the example of Figure 8, the calculated second speed value VT2 is assigned to slot 61, since 45+(78-45)/2 = 61,5. Hence the speed at slot 61 is set to V(61) := VT2. Hence, in this example wherein one position indicator P was detected in slot i2= #45 and the next position indicator P was detected in slot i3=#78; the second mid time slot is the integer part of: iP2-3= iP2 + (iP3 - iP2)/2= 45+ (78-45)/2= 45+33/2=61,5 Hence, slot 61 is the second mid time slot i _P2-3 . Hence, in step S#110 the second speed value VT2 may advantageously be assigned to a time slot (e.g. time slot i= 61) representing a time point which is earlier than the time point of detection of the third position signal edge P(i=78), see Figure 8. This feature enables a somewhat delayed real-time monitoring of the rotational speed while achieving an improved accuracy of the detected speed. In the next step S#120, a first acceleration value is calculated for the relevant time period. The first acceleration value may be calculated as: a12 = (VT2-VT1)/((iVT2 - iVT1)*dt) In the example of figure 8, the second speed value VT2 was assigned to slot 61, so i _VT2 = 61 and first speed value VT1 was assigned to slot 24, so iVT1 = 24. Hence, since dt=1/fs, the acceleration value may be set to a12 = fs* (VT2-VT1)/(i _VT2 - i _VT1 ) for the time period between slot 24 and slot 60, in the example of Figure 8. In the next step S#130, the status parameter extractor 450 operates to associate the established first acceleration value a12 with the time slots for which the established acceleration value a12 is valid. This may be all the time slots between the slot of the first speed value VT1 and the slot of the second speed value VT2. Hence, the established first acceleration value a12 may be associated with each time slot of the duration between the slot of the first speed value VT1 and the slot of the second speed value VT2. In the example of Figure 8 it is slots 25 to 60. This is illustrated in column #07 of Figure 8. In the next step S#140, the status parameter extractor 450 operates to establish speed values for measurement values s(j) associated with the duration for which the established acceleration value is valid. Hence speed values are established for each time slot which is associated with a measurement value s(j), and associated with the established first acceleration value a12. During linear acceleration, i.e. when the acceleration a is constant, the speed at any given point in time is given by the equation: V(i) = V(i-1) + a * dt, wherein V(i) is the momentary speed at the point of time of slot i V(i-1) is the momentary speed at the point of time of the slot immediately preceding slot i a is the acceleration dt is the duration of a time slot According to an example, the speed for each slot from slot 25 to slot 60 may be calculated successively in this manner, as illustrated in column #08 in Figure 8. Hence, momentary speed values Vp to be associated with the detected measurement values Se(25), Se(26), Se(27)...Se(59), and Se(60) associated with the acceleration value a12 may be established in this manner (See time slots 25 to 60 in column #08 in conjunction with column #03 and in conjunction with column #07 in Figure 8). Hence, momentary speed values S(j) [See column #05] to be associated with the detected measurement values S(3), S(4), S(5), and S(6) associated with the acceleration value a12 may be established in this manner. According to another example, the momentary speed for the slot 30 relating to the first measurement value s(j)= S(3) may be calculated as: V(i=30) = Vp30 = VT1+ a* (30-24)*dt = Vp24 + a * 6*dt The momentary speed for the slot 40 relating to the first measurement value s(j)= S(4) may be calculated as: V(i=40) = Vp40 = VT1+ a* (40-24)*dt = Vp40 + a* 16*dt or as: V(i=40) = Vp40 = V(30) + (40-30)*dt = Vp30 + a* 10*dt The momentary speed for the slot 50 relating to the first measurement value s(j)= S(5) may then subsequently be calculated as: V(i=50) = Vp50 = V(40) + (50-40)*dt = Vp40 + a* 10*dt and the momentary speed for the slot 60 relating to the first measurement value s(j)= S(6) may then subsequently be calculated as: V(i=60) = Vp50 + a* 10*dt When measurement sample values S(i) [See column #03 in Figure 8] associated with the established acceleration value have been associated with a momentary speed value, as described above, an array of data including a time sequence of measurement sample values S(i), each value being associated with a speed value V(i), fROT(i), may be delivered on an output of said status parameter extractor 450. Alternatively, if a decimation of sample rate is desired, it is possible to do as follows: When measurement sample values S(j) [See column #05 in Figure 8] associated with the established acceleration value have been associated with a momentary speed value, as described above, an array of data including a time sequence of measurement sample values S(j), each value being associated with a speed value V(j), fROT(j), may be delivered on an output of said status parameter extractor 450. With reference to figure 11, another example of a method is described. According to this example, the status parameter extractor 450 operates to record (see step S#160 in Fig 11) a time sequence of position signal values P(i) of said position signal (Ep) such that there is a first temporal relation n _diff1 between at least some of the recorded position signal values (P(i)), such as e.g. between a first position signal value P1(i) and a second position signal value P2(i). According to an example, the second position signal value P2(i) is received and recorded in a time slot (i) which arrives n _diff1 slots after the reception of the first position signal value P1(i) (see step S#160 in Fig 11). Then the third position signal value P3(i) is received and recorded (see step S#170 in Fig 11) in a time slot (i) which arrives ndiff2 slots after the reception of the second position signal value P2(i). As illustrated by step S#180 in Fig 11, the status parameter extractor 450 may operate to calculate a relation value a12= ndiff1 / ndiff2 If the relation value a12 equals unity, or substantially unity, then the status parameter extractor 450 operates to establish that the speed is constant, and it may proceed with calculation of speed according to a constant speed phase method. If the relation value a12 is higher than unity, the relation value is indicative of a percentual speed increase. If the relation value a12 is lower than unity, the relation value is indicative of a percentual speed decrease. The relation value a12 may be used for calculating a speed V2 at the end of the time sequence based on a speed V1 at the start of the time sequence, e.g. as V2 = a12 * V1 Figure 12 is a flow chart illustrating an example of a method for performing step S#40 of Figure 9. According to an example, the acceleration is assumed to have a constant value for the duration between two mutually adjacent position indicators P (See column #02 in Figure 8). Hence, when ^ the position indicator P is delivered once per revolution, and ^ the gear ratio is 1/1: then - the angular distance travelled between two mutually adjacent position indicators P is 1 revolution, which may also be expressed as 360 degrees, and - the duration is T = n*dt, ^ where n is the number of slots of duration dt between the first two mutually adjacent position indicators P1 and P2. In a step S#200, the first speed of revolution value VT1 may be calculated as VT1= 1 /( ndiff1*dt), wherein VT1 is the speed expressed as revolutions per second, ndiff1 = the number of time slots between the two successive position signals; and dt is the duration of a time slot, expressed in seconds. The value of dt may e.g be the inverse of the initial sample frequency fs. Since the acceleration is assumed to have a constant value for the duration between two mutually adjacent position indicators P, the calculated first speed value VT1 is assigned to the first mid time slot in the middle between the two successive position signals P(i) and P(i+ndiff1). In a step S#210, a second speed value VT2 may be calculated as VT2= 1 /(ndiff2 *dt), wherein VT2 is the speed expressed as revolutions per second, ndiff2 = the number of time slots between the two successive position signals; and dt is the duration of a time slot, expressed in seconds. The value of dt may e.g. be the inverse of the initial sample frequency fs. Since the acceleration is assumed to have a constant value for the duration between two mutually adjacent position indicators P , the calculated second speed value VT2 is assigned to the second mid time slot in the middle between the two successive position signals P(i+ndiff1) and P(i+ndiff1+ ndiff2). Thereafter, the speed difference VDelta may calculated as VDelta = VT2 – VT1 This differential speed VDelta value may be divided by the number of time slots between the second mid time slot and the first mid time slot. The resulting value is indicative of a speed difference dV between adjacent slots. This, of course, assumes a constant acceleration, as mentioned above. The momentary speed value to be associated with selected time slots may then be calculated in dependence on said first speed of revolution value VT1, and the value indicative of the speed difference between adjacent slots. When the measurement sample values S(i), associated with time slots between the first mid time slot and the second mid time slot, have been associated with a momentary speed value, as described above, an array of data including a time sequence of measurement sample values S(i), each value being associated with a speed value V(i) is delivered on an output of said status parameter extractor 450. The momentary speed value V(i) may also be referred to as fROT(i). In summary, according to some examples, a first momentary speed value VT1 may be established in dependence of the angular distance delta-FI _p1-p2 between a first positional signal P1 and a second positional signal P2, and in dependence of the corresponding duration delta-Tp1-p2 = tP2 – tP1. Thereafter, a second momentary speed value VT2 may be established in dependence of the angular distance delta-FI _p2-p3 between the second positional signal P2 and a third positional signal P3, and in dependence of the corresponding duration delta-Tp2-p3= tP2 – tP1. Thereafter, momentary speed values for the rotational shell 20 may be established by interpolation between the first momentary speed value VT1 and the second momentary speed value VT2. In other words, according to examples, two momentary speed values VT1 and VT2 may be established based on the angular distances delta-FIp1-p2, delta-FIp2-p3 and the corresponding durations between three consecutive position signals, and thereafter momentary speed values for the rotational shell 20 may be established by interpolation between the first momentary speed value VT1 and the second momentary speed value VT2. Figure 13 is a graph illustrating a series of temporally consecutive position signals P1, P2, P3,..., each position signal P being indicative of a full revolution of the monitored shell 20. Hence, the time value, counted in seconds, increases along the horizontal axis towards the right. The vertical axis is indicative of speed of rotation, graded in revolutions per minute (RPM). With reference to Figure 13, effects of the method according to an example are illustrated. A first momentary speed value V(t1) = VT1 may be established in dependence of the angular distance delta-FIp1-p2between the first positional signal P1 and the second positional signal P2, and in dependence of the corresponding duration delta-T1-2 = tP2 – tP1. The speed value attained by dividing the angular distance delta-FIp1-p2 by the corresponding duration (tP2 – tP1) represents the speed V(t1) of the rotational shell 20 at the first mid time point t1, also referred to as mtp (mid time point) , as illustrated in figure 13. Thereafter, a second momentary speed value V(t ₂ ) = VT2 may be established in dependence of the angular distance delta-FI between the second positional signal P2 and a third positional signal P3, and in dependence of the corresponding duration delta-T2-3= tP3 – tP2. The speed value attained by dividing the angular distance delta-FI by the corresponding duration (t _P3 – t _P2 ) represents the speed V(t ₂ ) of the rotational shell 20 at the 2:nd mid time point t ₂ (2:nd mtp), as illustrated in figure 13. Thereafter, momentary speed values for time values between the first first mid time point and the 2:nd mid time point may be established by interpolation between the first momentary speed value VT1 and the second momentary speed value VT2, as illustrated by the curve fROTint. Mathematically, this may be expressed by the following equation: V(t12) = V(t1) + a * (t12 – t1) Hence, if the speed of the shell 20 can be detected at two points of time (t1 and t2), and the acceleration a is constant, then the momentary speed at any point of time can be calculated. In particular, the speed V(t12) of the shell at time t12, being a point in time after t1 and before t ₂ , can be calculated by V(t12) = V(t1) + a * (t12 – t1) wherein a is the acceleration, and t1 is the first mid time point t1 (See Figure 13). The establishing of a speed value as described above, as well as the compensatory decimation as described with reference to Figures 20, 21, and 22, may be attained by performing the corresponding method steps, and this may be achieved by means of a computer program 94 stored in memory 60, as described above. The computer program may be executed by a DSP 50. Alternatively the computer program may be executed by a Field Programmable Gate Array circuit (FPGA). The establishing of a speed value f _ROT (i) as described above may be performed by the analysis apparatus 150 when a processor 350 executes the corresponding program code 380, 394, 410 as discussed in conjunction with Figure 4 above. The data processor 350 may include a central processing unit 350 for controlling the operation of the analysis apparatus 14. Alternatively, the processor 50 may include a Digital Signal Processor (DSP) 350. According to another example the processor 350 includes a Field programmable Gate Array circuit (FPGA). The operation of the Field programmable Gate Array circuit (FPGA),may be controlled by a central processing unit 350 which may include a Digital Signal Processor (DSP) 350. Identification of data relating to the toe of the charge in a tumbling mill As mentioned above, the tumbling mill shell 20 has an internal shell surface 22 facing the chamber 25, the internal shell surface 22 including a number of protrusions 310, also referred to as lifters, that may be configured to engage and lift material 30 as the shell rotates about the axis 60 (See e.g. figure 2). The number of protrusions 310 provided on the internal shell surface 22 facing the chamber 25 is herein termed with the variable L. Whereas figure 2 illustrates a case when there are twelve protrusions 310, i.e. L=12, the number L of protrusions 310 may be higher or lower. According to some embodiments the number L of protrusions 310 may be at least one, i.e. the number L of protrusions 310 may be L=1. According to some embodiments the number L of protrusions 310 may be any number higher than L=1. According to some embodiments the number L of protrusions 310 may be anywhere in the range from L=2 to L=60. According to some embodiments the number L of protrusions 310 may be anywhere in the range from L=2 to L=35. The number L of protrusions 310 is an important factor in relation to analysis of the vibrations resulting from rotation of the mill shell 20. The inventor realized that the interaction of a protrusion 310 with the toe of the charge, forcing the material of the charge to accelerate in the direction of movement of the protrusion 310 causes a mechanical vibration VIMP. The inventor also realized that this mechanical vibration VIMP, caused by the interaction of protrusions 310 with the toe of the charge, will be repetitive, i.e. there will be a repetition frequency fR. With reference to figure 2, it is noted that it illustrates the rotating mill shell 20 at the moment in time when a protrusion 310C impacts the toe 205 of the charge material 30. The impact of the protrusion 310C against the mass of material in the toe portion 205 causes a mass of toe portion material to accelerate in the direction A _ACC of movement of the protrusion 310C, this acceleration causing a force FIMP against the leading edge surface of the protrusion 310C. Incidentally, this impact force F _IMP may be estimated to be of a magnitude: FIMP = m205 * a205 wherein m ₂₀₅ is the mass of the portion of the toe that is accelerated, a ₂₀₅ is the amount of acceleration of that portion of the toe Hence, the measurement signal S _MD (See e.g. Fig 5) may include at least one vibration signal signature S _FIMP dependent on a vibration movement of the rotationally moving tumbling mill shell 20; wherein said vibration signal signature SFIMP has a repetition frequency fR which depends on the speed of rotation fROT of the rotationally moving tumbling mill shell 20. Moreover, the magnitude of the peak amplitude of the vibration signal signature SFIMP appears to depend on the magnitude of the impact force FIMP. Accordingly, the inventor concluded that a measure of the energy, or of the amplitude, of the vibration signal signature SFIMP appears to be indicative of the magnitude of the impact force FIMP. The existence of a vibration signal signature SFIMP which is dependent on the vibration movement of the rotationally moving mill shell 20 may therefore provide an indication of a toe portion 205 of the charge in of the monitored tumbling mill shell 20. In fact, the vibration signal signature S _FIMP which is dependent on the vibration movement of the rotationally moving mill shell 20 may provide an indication of a position of a toe portion 205 of the charge in of the monitored tumbling mill shell 20, the position being indicated in relation to a reference position value. The inventor concluded that the repetition frequency fR of the mechanical vibration VIMP, caused by the interaction of protrusions 310 with the toe of the charge, depends on the number L of protrusions 310 provided on the internal shell surface 22 and on the speed of rotation fROT of the shell 20. When the monitored tumbling mill shell 20 rotates at a constant rotational speed such a repetition frequency f _R may be discussed either in terms of repetition per time unit or in terms of repetition per revolution of the shell being monitored, without distinguishing between the two. However, if the tumbling mill shell 20 rotates at a variable rotational speed the matter is further complicated, as discussed elsewhere in this disclosure, e.g. in connection with Figures 20, 21, 22A, 22B, and 22C. In fact, it appears as though even very small variations in rotational speed of the mill shell may have a large adverse effect on detected signal quality in terms of smearing of detected vibration signals. Hence, a very accurate detection of the rotational speed f _ROT of the mill shell 20 appears to be of essence. Moreover, the inventor realized that, not only the amplitude of the mechanical vibration V _IMP but also the time of occurrence of the mechanical vibration V _IMP may be indicative of data relating a toe portion 205 of the charge in a tumbling mill. Thus, the measurement signal SMD (See e.g. Fig 5) may include at least one vibration signal amplitude component S _FIMP dependent on a vibration movement of the rotationally moving tumbling mill shell 20; wherein said vibration signal amplitude component SFIMP has a repetition frequency f _R which depends on the speed of rotation fROT of the rotationally moving tumbling mill shell 20 and that also depends on the number L of protrusions 310 provided on the internal shell surface 22 of the mill shell 20; and wherein there is a temporal relation between the occurrence of the repetitive vibration signal amplitude component S _FIMP and the occurrence of a position signal P(i) which has a second repetition frequency fP dependent on the speed of rotation fROT of the rotationally moving tumbling mill shell 20. As regards constant rotational speed, the inventor concluded that if the speed of rotation f _ROT is constant, the digital measurement signal S _MD , comprising a temporal sequence of vibration sample values S(i), has a repetition frequency fR, that depends on the number L of protrusions 310 provided on the internal shell surface 22. The status parameter extractor 450 may optionally include a Fast Fourier Transformer (FFT) coupled to receive the digital measurement signal SMD, or a signal dependent on the digital measurement signal S _MD (See Figure 15A and/or figure 15B). In connection with the analysis of a tumbling mill, having a rotating shell 20, it may be interesting to analyse signal frequencies that are higher than the rotation frequency fROT of the rotating shell 20. In this context, the rotation frequency fROT of the shell 20 may be referred to as ”order 1”. If a signal of interest occurs at, say ten times per revolution of the shell, that frequency may be referred to as Order 10, i.e. a repetition frequency f _R (measured in Hz) divided by rotational speed fROT ( measured in revolutions per second, rps) equals 10 Hz/rps, i.e. order Oi = f _R /f _ROT = 10 Referring to a maximum order as OMAX, and the total number of frequency bins in the FFT to be used as Bn, the inventor concluded that the following applies according to an example: Oi* B _n =N _R * O _MAX . Conversely, NR = Oi * Bn / OMAX, wherein OMAX is a maximum order; and B _n is the number of bins in the frequency spectrum produced by the FFT, and Oi is the number L of protrusions 310 in the monitored tumbling mill shell 20. The above variables OMAX, Bn, and Oi, should be set so as to render the variable NR a positive integer. In connection with the above example it is noted that the FFT analyzer is configured to receive a reference signal, i.e. a position marker signal value PS, or EP, once per revolution of the rotating shell 20. As mentioned in connection with Figure 2, a position marker device 180 may be provided such that, when the shell 20 rotates around the axis of rotation 60, the position marker 180 passes by the position sensor 170 once per revolution of the shell 20, thereby causing the position sensor 170 to generate a revolution marker signal value PS, E _P . Incidentally, referring to the above example of FFT analyzer settings, the resulting integer number NR may indicate the number of revolutions of the monitored shell 20 during which the digital signal S _MD is analysed. According to an example, the above variables O _MAX , B _n , and Oi, may be set by means of the Human Computer Interface, HCI, 210, 210S (See e.g. Fig 1 and/or fig.5 and/or fig.15A and/or fig.15B). Consider a case when the digital measurement signal S _MD is delivered to an FFT analyzer: In such a case, when the FFT analyzer is set for ten protrusions, i.e. L=10, and Bn = 160 frequency bins, and the user is interested in analysing frequencies up to order O _MAX = 100, then the value for N _R becomes N _R = Oi * B _n /O _MAX = 10*160/100 = 16. Hence, it is necessary to measure during sixteen shell revolutions (NR = 16) when Bn = 160 frequency bins is desired, the number of protrusions is L=10; and the user is interested in analysing frequencies up to order O _MAX = 100. In connection with settings for an FFT analyzer, the order value O _MAX may indicate a highest frequency to be analyzed in the digital measurement signal SMD. According to some embodiments, the setting of the FFT analyzer should fulfill the following criteria when the FFT analyzer is configured to receive a reference signal, i.e. a position marker signal value PS, once per revolution of the rotating shell 20: The integer value Oi is set to equal L, i.e. the number of protrusions 310 in the shell 20, and the settable variables O _MAX , and B _n are selected such that the mathematical expression Oi * Bn /OMAX becomes a positive integer. Differently expressed: When integer value Oi is set to equal L, then settable variables OMAX and Bn should be set to integer values so as to render the variable N _R a positive integer, wherein NR = Oi* Bn /OMAX According to an example, the number of bins Bn is settable by selecting one value Bn from a group of values. The group of selectable values for the frequency resolution B _n may include B _n =200 B _n = 400 Bn = 800 Bn = 1600 B _n = 3200 An example of Constant speed phase As mentioned in connection with step S#30 in figure 9, the status parameter extractor 450 may identify a constant speed phase, i.e. a status of constant rotational speed fROT of the shell 20. Figures 14A and 14B show another example of a cross-sectional view of the mid-portion 98 of a rotating mill shell 20 during operation. This view could be taken e.g. along line A-A of FIG.1A. According to the example of Fig.14A, the tumbling mill shell 20 has six protrusions 310 configured to engage the charge material 30 as the shell rotates about the axis 60, i.e. the number L=6. The inner diameter of the shell 20 may be e.g.600 cm, and the speed of rotation may be constant, at e.g.13,6 revolutions per minute. For the purpose of this example, the sample frequency is such that there are n= 7680 samples per revolution at that. rotational speed f _ROT of the shell 20. As mentioned above, the shell 20 is rotatable around the axis of rotation 60, and thus the position sensor 170 may generate a position signal Ep for indicating momentary rotational positions of the shell 20. A position marker 180 may be provided on an outer surface of the shell 20 such that, when the shell 20 rotates around the axis of rotation 60, the position marker 180 passes by the position sensor 170 once per revolution of the shell, thereby causing the position signal Ep to exhibit a position marker signal value PS. Each such position marker signal value PS is indicative of a stationary position, i.e. a position of the immobile stator. Figure 14A illustrates a rotational position of the rotating shell 20 wherein the position marker 180 is located at the same rotational position as the static position sensor 170, and a protrusion 310A has passed through the toe portion 205. The protrusion 310A is followed by an adjacent protrusion 310B. Figure 14B illustrates another rotational position of the rotating shell 20, a short time later than the position shown in Figure 14A. In figure 14B the adjacent protrusion 310B is located at the position of impact with the toe 205. The time of impact causes a vibration VIMP which leads to a signal signature event in the vibration signal. Thus, the rotational position illustrated in figure 14B is an event position 205E of the rotating shell 20. The event position 205E is a rotational position of the rotating shell 20 at the time of protrusion impact with the toe 205. Thus, the event position 205E is indicative of the toe position 205. Accordingly, the position 205E of the toe 205, can be expressed as a percentage of the distance between two adjacent static positions P3 and P4, as illustrated in figure 14B. When there is one position position marker signal value PS per revolution and the rotational speed f _ROT is constant, or substantially constant, there will be a constant, or substantially constant, number of vibration sample values S(i) for every revolution of the mill shell 20. For the purpose of this example, the position signal P(0) is indicative of the vibration sample i=0, as shown in table 2 (See below). For the purpose of an example, the position of the position signal P(0) in relation to the shell 20 may not be important, as long as the repetition frequency f _P is dependent on the speed of rotation f _ROT of the rotationally moving tumbling mill shell 20. Hence, if the position signal Ep has one pulse Ps per revolution of the shell 20, the digital position signal will also have one Position signal value P(i) = 1 per revolution, the remaining Position signal values being zero. Table 2 Thus, at a certain constant speed fROT there may be n time slots per revolution, as indicated by table 2, and n may be a positive integer. In the example of table 2, n = 7680. Having one position signal Ps per revolution, we know that the position signal will be repetitive every n slots since the rotational speed fROT is constant. Thus, a number of virtual position signals PC may be generated by calculation. In an example, consider that virtual position signals P _C are generated. The provision of one virtual position signal P _C per protrusion 310 may be used for establishing a temporal relation between the occurrence of the repetitive vibration signal amplitude component S _FIMP and the occurrence of a position signal P(i) which has a second repetition frequency fP dependent on the speed of rotation fROT of the rotationally moving tumbling mill shell 20. Having L equidistant protrusions 310 in the mill shell and one position signal Ps per revolution and a constant speed of rotation fROT it is possible to generate one virtual position signal P _C per protrusion, so that the total number of position signals Ps, P _C are evenly distributed. Each such position marker signal value P _S and Pc is indicative of a stationary position, i.e. a position of the immobile stator, as illustrated by “Ps” and “Pc” in figures 14A and 14B. Thus, a position signal Ps or P _C will occur at every n/L sample value position, as indicated in Table 3, when there are provided n time slots per revolution. In table 3, n=7680, and L=6, and thus there is provided a position signal PC at every 1280 sample, the calculated position signals being indicated as 1C. As illustrated in the Figure 14 example, the position marker signal value PS and Pc are indicative of L stationary positions P1, P2, P3, P4, P5 and PL, where L =6, since there are 6 protrusions 310 in the illustrated shell 20. It may be assumed that the position of the toe portion 205 of the mill is substantially constant during a single revolution of the mill shell 20. In other words, the position of the toe portion 205 is substantially immobile. Since the vibration signal amplitude component S _FIMP , S _P is generated by interaction of a protrusion and the toe of the charge (See figure 14B), it will be repetitive with the frequency of one vibration signal amplitude component S _FIMP , S _P per protrusion 310. Thus, it can be assumed that the temporal relation between the occurrence of the repetitive vibration signal amplitude component SFIMP, SP and the occurrence of a position signal P, PC will be substantially constant for each of the L data blocks, L being L=6 in this example. Table 3 illustrates the principle of a temporal progression of position signal values P(i) with calculated Positions signal values P(i) being indicated as “1C”. Table 3 Table 4 Table 5 As mentioned above, the shell 20 is rotatable around the axis of rotation 60, and thus the position sensor 170, being mounted in an immobile manner, may generate a position signal Ep having a sequence of shell position signal values PS for indicating momentary rotational positions of the shell 20. As shown in Figure 23 a position marker 180 may be provided on an outer surface of the shell 20 such that, when the shell 20 rotates around the axis of rotation 60, the position marker 180 passes by the position sensor 170 during one revolution of the shell 20, thereby causing the position sensor 170 to generate a revolution marker signal value P _S . As mentioned above, the position sensor 170 may generate a position signal Ep having a sequence of shell position signal values PS for indicating momentary rotational positions of the shell 20 when the shell 20 rotates. With reference to tables 2-4 in this document, such a marker signal value PS is illustrated as “1” in column #2 in tables 2-4. When the rotating shell is provided with one position marker device 180, the marker signal value PS will be provided once per revolution. The marker signal value PS is illustrated as “1” in column #2 in tables 2-4. Having L equidistant protrusions 310 in the mill shell and one position signal P per revolution and a constant speed of rotation f _ROT it is possible to generate one virtual position signal PC per protrusion, so that the total number of position signals P, PC are evenly distributed, as discussed above. Thus, a position signal P or PC will occur at every n/L sample value position, as indicated in Table 3, when there are provided n time slots per revolution. In table 3, n=7680, and L=6, and thus there is provided a position signal PC at every 1280 sample, the calculated position signals being indicated as 1C. It is believed that the mutually equidistant positions of the protrusions 310 is of importance when the marker signal value PS, illustrated as “1” in column #2 in tables 2-4, is provided once per revolution and virtual position signal values PC are generated in an evenly distributed manner such that a position signal P or P _C will occur at every n/L sample value position, as indicated in Table 3, when there are provided n time slots per revolution in a sequence of shell position signal values for indicating momentary rotational positions of the shell 20. In table 3 an actually detected revolution marker signal value P _S is reflected as “1” (see column #2, time slot “0” and time slot “7680” in table 3), and virtual position signal values PC are reflected as “1C” (see column #2, time slot “0” and time slot “7680” in table 3). This is believed to be of importance for some embodiments of this disclosure since the position markers 180 cause the generation of position reference signal values, and the protrusions 310, when engaging material in the charge of a rotating mill, cause the generation of a signal event, such as e.g. an amplitude peak value, in the vibration signal (See references SEA, SMD, Se(i), S(j), S(q) e.g. in figures 1 and15). Moreover, the temporal duration between the occurrence of a position reference signal value and the occurrence of a signal event in the vibration signal, caused by a protrusion 310 engaging material in the charge of the rotating mill shell may be indicative of an internal state of the operating mill, as discussed elsewhere in this disclosure Table 4 is an illustration of the first block, i.e. Block I, having n/L = 7680/6=1280 consecutive time slots. It is to be understood that if there is a constant speed phase (See Fig 9) for the duration of a complete revolution of the shell 20, then each of the blocks I to VI (See table 3) will have the same appearance as Block I being illustrated in table 4. According to embodiments of this disclosure, with reference to column #03 in table 4, the vibration sample values S(i) are analyzed for detection of a vibration signal signature S _FIMP . The vibration signal signature SFIMP may be manifested as a peak amplitude sample value Sp. According to an example, with reference to column #03 in table 4, the vibration sample values S(i) are analyzed by a peak value detector for detection of a peak sample value Sp. With reference to table 5, the peak value analysis leads to the detection of a highest vibration sample amplitude value S(i). In the illustrated example, the vibration sample amplitude value S(i=760) is detected to hold a highest peak value Sp. Having detected the peak value Sp to be located in time slot 760, a temporal relation between the occurrence of the repetitive vibration signal amplitude component Sp and the occurrence of a position signal P(i) can be established. In table 5 the time slots carrying position signals P(i) are indicated as 0% and 100%, respectively, and all the slots in between may be labelled with their respective locations, as illustrated in column #02 in table 5. As illustrated in the example in col. #02 of table 5, the temporal location of slot number i = 760 is at a position 59% of the temporal distance between slot i=0 and slot i=1280. Differently expressed, 760/1280= 0,59 = 59% Consequently, the inventor concluded that the temporal relation between the occurrence of the repetitive vibration signal amplitude component S _FIMP and the occurrence of a position signal P(i) may be used as an indication of the relative physical position of the toe portion 205 of the charge between two consecutive protrusions 310 in the rotating shell 20. Accordingly, a position of the toe 205, expressed as a percentage of the distance between two adjacent leading edges (see 312C and 312D in Fig 2 in conjunction with table 5), can be obtained by: Counting a total number of samples (NB – N0 = NB – 0 = NB =1280) from the first reference signal occurrence in sample number N0 = 0 to the second reference signal occurrence in sample number NB=1280, and Counting another number of samples (N _P – N ₀ = N _P –0 = N _P ) from the first reference signal occurrence at N0 = 0 to the occurrence of the peak amplitude value Sp at sample number N _P , and generating said first temporal relation (R _T (r); T _D ; FI(r)) based on said another number NP and said total number NB. This can be summarized as: R _T (r) = R _T (760)= (N _P – N ₀ ) / (N _B – N ₀ ) = (760 - 0) / (1280-0) = 0,59 = 59% Thus, a relative toe position may be generated by: Counting a total number of samples (N _B ) from the first reference signal occurrence to the second reference signal occurrence, and Counting another number of samples (NP) from the first reference signal occurrence to the occurrence of the peak amplitude value Sp at sample number NP, and generating said first temporal relation (R _T (r); T _D ; FI(r)) based on a relation between said sample number NP and said total number of samples i.e. NB. With reference to figure 14, it is noted that, at the point in time illustrated, the position marker 180 is depicted just as it passes by the position by the position sensor 170. Thus, the point in time illustrated may be the point in time indicated by time slot 1280, i.e. when the position signal P(i=1280) is generated. Since the shell rotates in a clockwise direction, the most recent peak sample value Sp was generated by the impact of protrusion 310A with the toe portion 205 (See Figures 14A and 14B in conjunction with table 5). Thus, the vibration sample amplitude value S(i=760), detected to hold a highest peak value Sp, occurred at a time T _SP = dt*(1280-760) before the occurrence of the position signal P(i=1280). Since S= v*t, wherein S= distance, v= a constant speed, and t is time, the temporal relation can be directly translated into a distance. Consequently, col. #02 of table 5, can be regarded as indicating the physical location of the toe portion 205 at a position 59% of the distance between protrusion 310A and protrusion 310B (see figure 14 in conjunction with col. #02 of table 5). According to another example, with reference to table 6, the temporal relation between the occurrence of the repetitive vibration signal amplitude component Sp and the occurrence of a position signal P(i) can be regarded as a phase deviation, expressed in degrees. Table 6 In fact, by using the position signal as a reference signal for the digital measurement signal S _MD , S(i), S(j), and adjusting the settings of a Fast Fourier Transformer in a certain manner, the Fast Fourier Transformer may be used for extracting the amplitude top value as well as the phase value, as discussed below. Consequently, col. #02 of table 6, can be regarded as indicating the physical location of the toe portion 205 at a position 213,75 degrees of the distance between protrusion 310A and protrusion 310B when the total distance between protrusion 310A and protrusion 310B is regarded as 360 degrees (see figure 14 in conjunction with col. #02 of table 6). The physical location of the toe portion 205, when expressed as a part of the distance between two adjacent protrusions 310, may be referred to as a relative position of the toe 205. In other words, this disclosure provides a manner of identifying a relative toe position, for the toe 205 of a charge in a tumbling mill. Hence, this disclosure provides a manner of generating information indicative of the location of the toe portion 205, when expressed as a part of the distance between two adjacent protrusions 310 in a rotating shell 20. With reference to figure 15A and/or figure 15B and figure 16 the relative toe position may be presented as a phase angle FI(r), as discussed in connection with figures 15 and 16 below. According to embodiments of this disclosure, the relative toe position can be presented as a percentage (see col. #02 of table 5 above). Moreover, according to embodiments of this disclosure, the relative toe position can be presented as a temporal duration, or as a part of a temporal duration. As discussed above, in connection with table 5, since S= v*t, wherein S= distance, v= the speed of a protrusion, and t is time, the temporal relation can be directly translated into a distance. In this context it is noted that the speed v of a protrusion depends on the angular velocity fROT of the shell 20 and of the radius RMIC of the shell 20 (See fig 14). Figure 15A is a block diagram illustrating an example of a status parameter extractor 450. The status parameter extractor 450 of fig.15A includes a shell speed detector 500 that receives the digital vibration signal S _MD , S(i) and the digital position signal (Pi). The shell speed detector 500 may also be referred to as a shell speed value generator 500. The shell speed detector 500 may generate the three signals S(j), P(j) and fROT(j) on the basis of the received digital vibration signal S _MD , S(i) and the digital position signal (Pi). This may be achieved e.g. in the manner described above in relation to figures 7 to 13. In this connection it is noted that the three signals S(j), P(j) and fROT(j) may be delivered simultaneously, i.e. they all relate to the same time slot j. In other words, the the three signals S(j), P(j) and f _ROT (j) may be provided in a synchronized manner. The provision of signals, such as S(j), P(j) and fROT(j), in a synchronized manner advantageously provides accurate information about about temporal relations between signal values of the individual signals. Thus, for example, a speed value f _ROT (j) delivered by the shell speed value generator 500 is indicative of a momentary rotational speed of the shell 20 at the time of detection of the amplitude value S(j). It is noted that the signals S(j) and P(j), delivered by the shell speed value generator 500, are delayed in relation to the signals S(i) and (Pi) received by the shell speed value generator 500. It is also noted that the signals S(j) and P(j) are equally delayed in relation to the signals S(i) and (Pi), thus the temporal relation between the two has been maintained. In other words, the signals S(j) and P(j) are synchronously delayed. The shell speed detector 500 may deliver a signal indicative of whether the speed of rotation has been constant for a sufficiently long time, in which case the signals S(j) and P(j) may be delivered to a Fast Fourier Transformer 510. The variables Y, Z, and L, should be set so as to render the variable X a positive integer, as discussed above. According to an example, the above variables Y, Z, and L, may be set by means of the Human Computer Interface, HCI, 210, 210S (See e.g. Fig 1 and/or fig.5 and/or fig.15A and/or fig.15B). As mentioned above the resulting integer number X may indicate the number of revolutions of the monitored tumbling mill shell 20 during which the digital signals S(j) and P(j) are analysed by the FFT 510. Thus, based on the settings of the variables Y, Z, and L, the FFT 510 may generate the value X, indicative of the duration of the analysis of a measurement session, and after a measurement session, the FFT 510 delivers a set of status value Sp(r) and FI(r). The notion “r”, in status values Sp(r) and FI(r), indicates a point in time. It is to be noted that there may be a delay in time from the reception of a first pair of input signals S(j), P(j) at the inputs of the FFT 510 until the delivery of a pair of status values Sp(r) and FI(r) from the FFT 510. A pair of status values Sp(r) and FI(r) may be based on a temporal sequence of pairs of input signals S(j), P(j). The duration of the temporal sequence of pairs of input signals S(j), P(j) should include at least two successive position signal values P(j) = 1 and the corresponding input signal pairs. The status values Sp(r) and FI(r) may also be referred to as CL and ФL, respectively, as explained below. As noted above in relation to figure 2, the vibration signal SEA, SMD, S(j), S(r) will exhibit a signal signature S _FIMP indicative of the impact of a protrusion with the toe portion 205, and when there are L protrusions 310 in the shell 20 (See fig 1 in conjunction with figure 14 and figure 15A and/or figure 15B) then that signal signature SFIMP will be repeated L times per revolution of the shell 20. For the purpose of conveying an intuitive understanding of this signal processing it may be helpful to consider the superposition principle and repetitive signals such as sinus signals. A sinus signal may exhibit an amplitude value and a phase value. In very brief summary, the superposition principle, also known as superposition property, states that, for all linear systems, the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually. Acoustic waves are a species of such stimuli. Also a vibration signal, such as the vibration signal SEA, SMD, S(j), S(r) including the signal signature SFIMP indicative of the impact of a protrusion with the toe portion 205 is a species of such stimuli. In fact, the vibration signal SEA, SMD, S(j), S(r) including the signal signature SFIMP may be regarded as a sum of sinus signals, each sinus signal exhibiting an amplitude value and a phase value. In this connection, reference is made to the Fourier series (See Equation 1 below): n=∞ F(t) = ∑ Cn sin(nωt + Фn ) (Eq.1) n=0 wherein n=0 the average value of the signal during a period of time (it may be zero, but need not be zero), n=1 corresponds to the fundamental frequency of the signal F(t), n=2 corresponds to the first harmonic partial of the signal F(t) ω = the angular frequency i.e. (2*π*fROT), f _ROT = the shell speed of rotation expressed as periods per second, t= time, Фn= phase angle for the n:th partial, and C _n = Amplitude for the n:th partial It follows from the above Fourier series that a time signal may be regarded as composed of a superposition of a number of sinus signals. An overtone is any frequency greater than the fundamental frequency of a signal. In the above example, it is noted that the fundamental frequency will be fROT, i.e. the shell speed of rotation, since the FFT 510 receives a marker signal value P(j)=1 only one time per revolution of the shell 20 (See e.g. figure 14). Using the model of Fourier analysis, the fundamental and the overtones together are called partials. Harmonics, or more precisely, harmonic partials, are partials whose frequencies are numerical integer multiples of the fundamental (including the fundamental, which is 1 times itself). With reference to Figure 15A and/or figure 15B and equation 1 above, the FFT 510 may deliver the amplitude value Cn(r) for n=L, i.e. CL (r) = Sp(r). The FFT 510 may also deliver phase angle for the partial (n=L), i.e. ФL(r) = FI(r). Now consider an example when a mill shell rotates at a speed of 10 revolutions per minute (rpm), the shell having ten (10) protrusions 310. A speed of 10 rpm renders one revolution every 6 seconds, i.e. f _ROT = 0,1667 rev/sec. The shell having ten protrusions (i.e. L=10) and running at a speed of fROT = 0,1667 rev/sec renders a repetition frequency fR of 1,667 Hz for the signal relating to the protrusions 310, since the repetition frequency fR is the frequency of order 10. The position signal P(j), P(q) (see Figure 15A and/or figure 15B) may be used as a reference signal for the digital measurement signal S(j),S(r). According to some embodiments, when the FFT analyzer is configured to receive a reference signal, i.e. the position signal P(j), P(q), once per revolution of the rotating shell 20, then the setting of the FFT analyzer should fulfill the following criteria: The integer value Oi is set to equal L, i.e. the number of protrusions 310 in the shell 20, and the settable variables O _MAX , and B _n are selected such that the mathematical expression Oi * Bn /OMAX becomes a positive integer. Differently expressed: When integer value Oi is set to equal L, then settable variables O _MAX and B _n should be set to integer values so as to render the variable N _R a positive integer, wherein NR = Oi* Bn / OMAX OMAX is a maximum order; and B _n is the number of bins in the frequency spectrum produced by the FFT, and Oi is a frequency of interest, expressed as an integer in orders, and wherein f _ROT is the frequency of order 1, i.e. the fundamental frequency. In other words, the speed of rotation fROT of the shell 20 is the fundamental frequency and L is the number of protrusions 310 in the impeller 20. Using the above setting , i.e. integer value Oi is set to equal L, and with reference to Figure 15A and/or figure 15B and equation 1 above, the FFT 510 may deliver the amplitude value C _n for n=L, i.e. C _L = Sp(r). The FFT 510 may also deliver phase angle for the partial (n=L), i.e. Ф _L = FI(r). Thus, according to embodiments of this disclosure, when the FFT 510 receives a position reference signal P(j), P(q) once per revolution of the rotating shell 20, then the FFT analyzer can be configured to generate a peak amplitude value C _L for a signal whose repetition frequency fR is the frequency of order L, wherein L is the number of equidistantly positioned protrusions 310 in the rotating shell 20. With reference to the discussion about equation 1 above in this disclosure, the amplitude of the signal whose repetition frequency fR is the frequency of order L may be termed Cn for n=L, i.e. CL. Referring to equation 1 and figure 15A and/or figure 15B, the amplitude value C _L may be delivered as a peak amplitude value indicated as Sp(r) in figure 15A and/or figure 15B. Again with reference to equation 1, above in this disclosure, the phase angle value Ф _L for the signal whose repetition frequency f _R is the frequency of order L may be delivered as a temporal indicator value, the temporal indicator value being indicative of a temporal duration TD1 between occurrence of an impact force FIMP and occurrence of a rotational reference position of said rotating shell. Hence, according to embodiments of this disclosure, when the FFT 510 receives a position reference signal P(j), P(q) once per revolution of the rotating shell 20, then the FFT analyzer can be configured to generate a phase angle value Ф _L for a signal whose repetition frequency f _R is the frequency of order L, wherein L is the number of equidistantly positioned protrusions 310 in the rotating shell 20. Hence, using the above setting, i.e. integer value Oi being set to equal L, and with reference to Figure 15A and/or figure 15B and equation 1 above, the FFT 510 may generate the phase angle value ФL. With reference to Figure 15A and/or figure 15B in conjunction with figure 1A, the status values Sp(r) = CL and FI(r) = ФL may be delivered to the Human Computer Interface (HCI) 210 for providing a visual indication of the analysis result. As mentioned above, the analysis result displayed may include information indicative of an internal state of the tumbling mill process for enabling the operator 230 to control the tumbling mill. Figure 15B is a block diagram illustrating an example of a status parameter extractor 450. The example status parameter extractor 450 in figure 15B comprises a shell speed detector 500, a speed variation compensatory decimator 470, a Time Synchronous Averager 471, and a Fast Fourier Transformer 510, FFT. In this disclosure the abbreviation TSA may be used for Time Synchronous Averager. The example status parameter extractor 450 may be the status parameter extractor 450 described in figure 15A with the addition of the time synchronous averager, TSA, 471. The TSA 471 is configured to receive vibration signal values S(q) and corresponding position signal values P(q) as delivered by the speed variation compensatory decimator 470. The TSA 471 is configured to receive vibration signal values S(q) and corresponding position signal values P(q) corresponding to a plurality of revolutions or cycles, and to generate average vibration signal values S(t), wherein an average vibration signal value S(t) is based on a number NTSA of vibration values detected at the same rotational position of the shell 20. As discussed elsewhere in this disclosure, the compensatory decimator 470 is configured to generate a decimated digital vibration signal SMDR such that the number NV of vibration sample values per revolution of said rotating shell is kept at a constant value, or at a substantially constant value, even when said rotational speed varies. Accordingly, when the compensatory decimator 470 delivers NV vibration sample values per revolution of said rotating shell, then every N _V :th vibration sample value relates to the same rotational position. Thus, when NV = 100, then the speed variation compensatory decimator 470 outputs one hundred (100) vibration signal values S(q) per revolution. Thus, when NV = 100 and the TSA 471 is configured to generate an indidual average vibration signal value S _TSA (t) as an average of NTSA vibration signal values S(q), then the output average value STSA(t) may be generated as: S _TSA (t) = ( S(q) + S(q+100) + S(q+200) )/ N _TSA For example, if NTSA= 3, then the TSA 471 delivers STSA(t) wherein each value is based on three vibration signal values S(q). Thus, it is to be understood that the signal values S(q), S(q+100), and S(q+200) all represent the q:th position. Thus, the TSA 471 delivers average signal values S _TSA (t) with the same number of elements as the number N _V of outputs per revolution provided by the speed variation compensatory decimator 470. For example, if a speed variation compensatory decimator 470 outputs NV =100 (one hundred) vibration signal values S(q) per revolution, then the TSA 471 delivers NV =100 (one hundred) average vibration signal values S(t) per revolution. The combination of shell speed detector 500, speed variation compensatory decimator 470, and time synchronous averager 471 allows for an output from the TSA 471 with vibration values averaged over several revolutions which advantageously reduces noise. It is to be noted that the TSA is configured to generate the averaged vibration values such that an average vibration value represents an average of a number of vibration values detected at the same rotational position of the shell 20. As illustrated in figure 15B, the output signals P _TSA and S _TSA of the TSA 471 may be provided to the FFT 510. In some examples, the output PTSA STSA of the TSA 471 is provided to the HCI 210. In some examples, the HCI 210 is arranged to set the number of revolutions or cycles the TSA 471 is configured to average. Information about the current internal state X of the mill 10 may be conveyed by one or more internal state values such that it intuitively makes sense to an operator 230 of the mill 10. Figure 16 is an illustration of an example of a visual indication of an analysis result. According to an example, the visual indication of the analysis result may include the provision of a polar coordinate system 520. A polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point 530 and an angle from a reference direction 540. The reference point 530 (analogous to the origin of a Cartesian coordinate system) is called the pole 530, and the ray from the pole in the reference direction is the polar axis. The distance from the pole is called the radial coordinate, radial distance or simply radius, and the angle is called the angular coordinate, polar angle, or azimuth. According to an example, the amplitude value Sp(r) is used as the radius, and the temporal relation value FI(r), Ф(r), T _D is used as the angular coordinate. In this manner an internal status of the monitored tumbling mill may be illustrated by providing an internal status indicator object 550, on the display 210S (Figure 16 in conjunction with figure 1A and/or figure 1B). Figure 16 in conjunction with figure 1A and/or figure 1B and figure 14 may be useful for understanding the following example. Hence, an example relates to an electronic tumbling mill monitoring system 150, 210S for generating and displaying information relating to a grinding process in a tumbling mill 10 having a shell 20 that rotates around an axis 60 at a speed of rotation fROT for grinding charge material 30 by tumbling the charge material in the rotating shell. The example monitoring system 150 includes: a computer implemented method of representing an internal state of said grinding process in said tumbling mill on a screen display 210S, the method comprising: displaying on said screen display 210S a polar coordinate system 520, said polar coordinate system 520 having a reference point (O, 530), and a reference direction (0°, 360°, 540); and a first internal status indicator object (550, S _P1 , T _D1 ), indicative of said internal state of said grinding process, at a first radius (Sp(r), S _P1 ) from said reference point (O) and at a first polar angle (FI(r), Ф(r), TD, TD1) in relation to said reference direction (0°,360°, 540), said first radius (Sp(r), S _P1 ) being indicative of an impact force (F _IMP ) generated when a projection (310) on an internal shell surface of the rotating shell interacts with a toe portion 205 of the charge material (30), and said first polar angle (FI(r), Ф(r), T _D , T _D1 ) being indicative of a position of the toe portion 205 between two protrusions 310 in the rotating shell 20. As mentioned above, the status parameter extractor 450 may be configured to generate successive pairs of the status values Sp(r) and FI(r). The status parameter extractor 450 may also generate time derivative values of the status values Sp(r) and FI(r), respectively. This may be done e.g. by subtracting a most recent previous status value Sp(r-1) from the most recent status value Sp(r) divided by the temporal duration between the two values. Similarly a numerical derivative of the internal status value FI may be achieved. Thus, derivative values dSp(r) and dFI(r) may be generated. The derivative values dSp(r) and dFI(r) may be used for indicating movement of the first internal status indicator object (550, SP1, TD1). Figures 17 and 18 are illustrations of another example of a visual indication of an analysis result. With reference to figures 17 and 18 the above mentioned derivative values may be used for displaying, on said screen display 210S, an arrow 560 originating at the position of the first internal status indicator object (550, SP1, TD1) and having an extension that depends on the magnitude of the derivative values. In other words, the absence of an arrow 560 means that the internal status is stable, not having changed for the temporal duration. The arrow 560 in figure 18 is longer than the arrow 560 in figure 17, thereby indicating a faster ongoing change of the internal status of the mill represented in figure 18 than that of the mill represented in figure 17. Figures 19A and 19B are illustrations of yet another example of a visual indication of an analysis result in terms of internal status of the tumbling mill 10. A most recent internal status indicator object 550(r) indicates a current internal status of the mill 10. Another internal status indicator object 550(r-1) indicates a most recent previous internal status of the mill 10. An internal status indicator object 550(1), shown as a small empty circle, indicates an internal status of the mill 10 at a nearly empty degree of filling. It is noted that when starting up a tumbling mill from an empty state, the initial internal status indicator object appears at an initial polar angle Ф(1) that represents a very first detected toe position of the mill. In figures 19A and 19B the first thirty-one (31) detected toe positions are indicated as empty circles, starting with the small empty circle 550(1). Based on experimental measurements, it appears as though the initial polar angle Ф(1) may be used as a reference toe position value. Hence, the initial polar angle Ф(1) may thus be termed reference toe position value ФTR. For the particular tumbling mill whose internal state is represented by the display 210S illustrated by figures 19A and 19B, the reference toe position corresponds to an angular value Ф _TR of about 47 degrees, as can be seen in figures 19A and 19B. Whereas the first thirty-one (31) detected toe positions are indicated as empty circles, a thereafter following sequence of toe positions are indicated as shaded circles, one of the shaded circles being indicated as 550(p) in figure 19A. The shaded circles in figure 19A represent higher filling degrees of the mill shell 20 than the filling degrees indicated by empty circles. The full, black, circles in figure 19A represent higher filling degrees of the mill shell 20 than the filling degrees indicated by shaded circles. Hence, it is noted that an initial, lowest detected, filling degree appears to be represented by a comparatively small radius, i.e. a low peak amplitude value Sp, at an initial polar angle Ф(1). With reference to figure 19A, a gradually increasing detected toe position FI(r), and in a corresponding manner a gradually increasing filling degree of the mill shell 20, renders an image of a spiral arm that whirls outwards in a counter-clockwise direction, as indicated by a curved arrow 560A in figure 19A, starting from the first internal status indicator object 550(1). In this manner, the current internal status of the tumbling mill 20 may be represented and visualized such that it intuitively makes sense to an operator 230 of the mill system 5. It is to be noted that, whereas the display of a single internal status indicator object 550, as shown in figure 17, represents a current internal status, or a latest detected internal status of the mill 10, the display of a temporal progression of internal status indicator objects ranging from an initial status 550(1) via intermediate states, such as 550(p) and 550(r-1) to 550(r), as shown in figure 19A, represents a current internal status 550(r) as well as a history of several earlier internal states 550(p), 550(p+1), 550(r-1), of the mill 10. In other words, a gradually increasing polar angle FI(r) in combination with a gradually increasing radius value SP(r) renders an image of a spiral arm that whirls outwards, as indicated by a curved arrow 560A in figure 19A, starting from the first internal status indicator object 550(1). An “angular length” X6(r) of the spiral arm, from the initial polar angle Ф(1) of the first internal status indicator object 550(1) up to the current, or latest detected, toe position FI(r), appears to be indicative of an absolute position X6(r) of the toe portion 205 (see e.g. figure 2 and figure 14). In this connection, it is noted that 360 degrees in the polar coordinate system 520 of figure 19A corresponds to 100% of the distance between the leading edges of two adjacent protrusions, such as 312C and 312D in figure 2. An example of variable speed phase status parameter extractor As mentioned above, the analysis of the measurements data is further complicated if the tumbling mill shell 20 rotates at a variable rotational speed f _ROT . In fact, it appears as though even very small variations in rotational speed of the mill shell may have a large adverse effect on detected signal quality in terms of smearing. Hence, a very accurate detection of the rotational speed f _ROT of the mill shell 20 appears to be of essence, and an accurate compensation for any speed variations appears to also be of essence. With reference to figure 15A and/or figure 15B, the shell speed detector 500 may deliver a signal indicating when the speed of rotation varies, as discussed in connection with figure 9. Referring again to figure 15A and/or figure 15B, the signals S(j) and P(j) as well as the speed value fROT(j) may be delivered to a speed variation compensatory decimator 470. The speed variation compensatory decimator 470 may also be referred to as a fractional decimator. The decimator 470 is configured to decimate the digital measurement signal S _MD based on the received speed value fROT(j). According to an example, the decimator 470 is configured to decimate the digital measurement signal S _MD by a variable decimation factor D, the variable decimation factor D being adjusted during a measuring session based on the variable speed value fROT(j). Hence, the compensatory decimator 470 is configured to generate a decimated digital vibration signal SMDR such that the number NV of vibration sample values per revolution of said rotating shell is kept at a constant value, or at a substantially constant value, when said rotational speed varies. According to some embodiments, the number NV of sample values per revolution of said rotating shell is considered to be a substantially constant value when the number of sample values per revolution varies less than 5 %. According to a preferred embodiment, the number of sample values per revolution of said rotating shell is considered to be a substantially constant value when the number of sample values NV per revolution varies less than 1 %. According to a most preferred embodiment, the number of sample values per revolution of said rotating shell is considered to be a substantially constant value when the number of sample values per revolution varies by less than 0,2 %. Thus, the Figure 15A embodiment, as well as the Figure 15B embodiment, includes the fractional decimator 470 for decimating the sampling rate by a decimation factor D = N/UD, wherein both UD and N are positive integers. Hence, the fractional decimator 470 advantageously enables the decimation of the sampling rate by a fractional number. Hence, the speed variation compensatory decimator 470 may operate to decimate the signals S(j) and P(j) and f _ROT (j) by a fractional number D = N/ U _D . According to an embodiment the values for U _D and N may be selected to be in the range from 2 to 2000. According to an embodiment the values for UD and N may be selected to be in the range from 500 to 1500. According to yet another embodiment the values for UD and N may be selected to be in the range from 900 to 1100. In this context it is noted that the background of the term “fraction” is as follows: A fraction (from Latin fractus, "broken") represents a part of a whole or, more generally, any number of equal parts. In positive common fractions, the numerator and denominator are natural numbers. The numerator represents a number of equal parts, and the denominator indicates how many of those parts make up a unit or a whole. A common fraction is a numeral which represents a rational number. That same number can also be represented as a decimal, a percent, or with a negative exponent. For example, 0.01, 1%, and 10−2 are all equal to the fraction 1/100. Hence, the fractional number D = N/ UD may be regarded as an inverted fraction. Thus, the resulting signal S _MDR , which is delivered by fractional decimator 470, has a sample rate of fSR = fS /D = fS * UD/N where fS is the sample rate of the signal SRED received by fractional decimator 470. The fractional value U _D /N is dependent on a rate control signal received on an input port 490. The rate control signal may be a signal indicative of the speed of rotation fROT of the rotating shell. The variable decimator value D for the decimator may be set to D= fS/ fSR, wherein fS is the initial sample rate of the A/D converter, and fSR is a set point value indicating a number of samples per revolution in the decimated digital vibration signal S _MDR . For example, when there are twelve (12) protrusions in the mill shell to be monitored, the set point value fSR may be set to 768 samples per revolution, i.e. the number of samples per revolution is set to fsr in the decimated digital vibration signal S _MDR . The compensatory decimator 470 is configured to generate a position signal P(q) at a regular interval of the decimated digital vibration signal SMDR, the regular interval being dependent on the set point value fSR. For example, when fSR is set to 768 samples per revolution, a position signal P(q) may be delivered once with every 768 sample of the decimated vibration signal S(q). Hence, the sampling frequency f _SR , also referred to as f _SR2 , for the output data values R(q) is lower than input sampling frequency f _S by a factor D. The factor D can be set to an arbitrary number larger than 1, and it may be a fractional number, as discussed elsewhere in this disclosure. According to preferred embodiments the factor D is settable to values between 1,0 to 20,0. In a preferred embodiment the factor D is a fractional number settable to a value between about 1,3 and about 3,0. The factor D may be obtained by setting the integers UD and N to suitable values. The factor D equals N divided by UD: D = N/ U _D According to an embodiment, the integers UD and N are settable to large integers in order to enable the factor D=N/UD to follow speed variations with a minimum of inaccuracy. Selection of variables UD and N to be integers larger than 1000 renders an advantageously high accuracy in adapting the output sample frequency to tracking changes in the rotational speed of the shell 20. So, for example, setting N to 500 and UD to 1001 renders D=2,002. The variable D is set to a suitable value at the beginning of a measurement and that value is associated with a certain speed of rotation of a rotating part to be monitored. Thereafter, during measuring session, the fractional value D is automatically adjusted in response to the speed of rotation of the rotating part to be monitored so that the output signal S _MDR provides a substantially constant number of sample values per revolution of the rotating shell. Figure 20 is a block diagram of an example of compensatory decimator 470. This compensatory decimator example is denoted 470B. Compensatory decimator 470B may include a memory 604 adapted to receive and store the data values S(j) as well as information indicative of the corresponding speed of rotation f _ROT of the monitored rotating mill shell . Hence the memory 604 may store each data value S(j) so that it is associated with a value indicative of the speed of rotation fROT(j) of the monitored mill shell at time of detection of the sensor signal SEAvalue corresponding to the data value S(j). The provision of data values S(j) associated with corresponding speed of rotation values fROT(j) is described with reference to Figures 7 - 13 above. Compensatory decimator 470B receives the signal SMD, having a sampling frequency fSR1, as a sequence of data values S(j), and it delivers an output signal S _MDR , having a reduced sampling frequency fSR, as another sequence of data values R(q) on its output 590. Compensatory decimator 470B may include a memory 604 adapted to receive and store the data values S(j) as well as information indicative of the corresponding speed of rotation fROT of the monitored rotating mill shell. Memory 604 may store data values S(j) in blocks so that each block is associated with a value indicative of a relevant speed of rotation of the monitored mill shell, as described below in connection with Figure 21. Compensatory decimator 470B may also include a compensatory decimation variable generator 606, which is adapted to generate a compensatory value D. The compensatory value D may be a floating number. Hence, the compensatory number can be controlled to a floating number value in response to a received speed value fROT so that the floating number value is indicative of the speed value fROT with a certain inaccuracy. When implemented by a suitably programmed DSP, as mentioned above, the inaccuracy of floating number value may depend on the ability of the DSP to generate floating number values. Moreover, compensatory decimator 470B may also include a FIR filter 608. In this connection, the acronym FIR stands for Finite Impulse Response. The FIR filter 608 is a low pass FIR filter having a certain low pass cut off frequency adapted for decimation by a factor D _MAX . The factor D _MAX may be set to a suitable value, e.g.20,000. Moreover, compensatory decimator 470B may also include a filter parameter generator 610. Operation of compensatory decimator 470B is described with reference to Figures 21 and 22 below. Figure 21 is a flow chart illustrating an embodiment of a method of operating the compensatory decimator 470B of Figure 20. In a first step S2000, the speed of rotation fROT of the mill shell to be monitored is recorded in memory 604 (Fig 20 & 21), and this may be done at substantially the same time as measurement of vibrations begin. According to another example the speed of rotation of the mill shell to be monitored is surveyed for a period of time. The highest detected speed fROTmax and the lowest detected speed fROTmin may be recorded, e.g. in memory 604 (Fig 20 & 21). In step S2010, the recorded speed values are analysed, for the purpose of establishing whether the speed of rotation varies. In step S2020, the user interface 210, 210S displays the recorded speed value fROT or speed values f _ROTmin , f _ROTmax , and requests a user to enter a desired order value Oi. As mentioned above, the mill shell rotation frequency fROT is often referred to as ”order 1”. The interesting signals may occur about ten times per mill shell revolution (Order 10). Moreover, it may be interesting to analyse overtones of some signals, so it may be interesting to measure up to order 100, or order 500, or even higher. Hence, a user may enter an order number Oi using user interface 210, 210S. In step S2030, a suitable output sample rate f _SR is determined. The output sample rate f _SR may also be referred to as fSR2 in this disclosure. According to an embodiment output sample rate f _SR is set to f _SR = C * Oi * f _ROTmin wherein C is a constant having a value higher than 2,0 Oi is a number indicative of the relation between the speed of rotation of the monitored mill shell and the repetition frequency of the signal to be analysed. f _ROTmin is a lowest speed of rotation of the monitored mill shell to expected during a forthcoming measurement session. According to an embodiment the value fROTmin is a lowest speed of rotation detected in step S2020,as described above. The constant C may be selected to a value of 2,00 (two) or higher in view of the sampling theorem. According to embodiments of the present disclosure the Constant C may be preset to a value between 2,40 and 2,70. According to an embodiment the factor C is advantageously selected such that 100*C/ 2 renders an integer. According to an embodiment the factor C may be set to 2,56. Selecting C to 2,56 renders 100* C = 256 = 2 raised to 8. In step S2050, a compensatory decimation variable value D is determined. When the speed of rotation of the mill shell to be monitored varies, the compensatory decimation variable value D will vary in dependence on momentary detected speed value. According to an embodiment, a maximum compensatory decimation variable value D _MAX is set to a value of D _MAX = f _ROTmax / f _ROTmin , and a minimum compensatory decimation variable value DMIN is set to 1,0. Thereafter a momentary real time measurement of the actual speed value fROT is made and a momentary compensatory value D is set accordingly. f _ROT is value indicative of a measured speed of rotation of the rotating mill shell to be monitored In step S2060, the actual measurement is started, and a desired total duration of the measurement may be determined. The total duration of the measurement may be determined in dependence on a desired number of revolutions X of the monitored mill shell . When measurement is started, a digital signal SMD is delivered to input 480 of the compensatory decimator. In the following the signal S _MD is discussed in terms of a signal having sample values S(j), where j is an integer. In step S2070, record data values S(j) in memory 604, and associate each vibration data value S(j) with a speed of rotation value fROT(j). In a subsequent step S2080, analyze the recorded speed of rotation values, and divide the recorded data values S(j) into blocks of data dependent on the speed of rotation values. In this manner a number of blocks of block of data values S(j) may be generated, each block of data values S(j) being associated with a speed of rotation value . The speed of rotation value indicates the speed of rotation of the monitored mill shell , when this particular block data values S(j) was recorded. The individual blocks of data may be of mutually different size, i.e. individual blocks may hold mutually different numbers of data values S(j). If, for example, the monitored rotating mill shell first rotated at a first speed f _ROT1 during a first time period, and it thereafter changed speed to rotate at a second speed fROT2 during a second, shorter, time period, the recorded data values S(j) may be divided into two blocks of data, the first block of data values being associated with the first speed value f _ROT1 , and the second block of data values being associated with the second speed value fROT2. In this case the second block of data would contain fewer data values than the first block of data since the second time period was shorter. According to an embodiment, when all the recorded data values S(j) have been divided into blocks, and all blocks have been associated with a speed of rotation value, then the method proceeds to execute step S2090. In step S2090, select a first block of data values S(j), and determine a compensatory decimation value D corresponding to the associated speed of rotation value f _ROT . Associate this compensatory decimation value D with the first block of data values S(j). According to an embodiment, when all blocks have been associated with a corresponding compensatory decimation value D, then the method proceeds to execute step S2100. Hence, the value of the compensatory decimation value D is adapted in dependence on the speed fROT. In step S2100, select a block of data values S(j) and the associated compensatory decimation value D, as described in step S2090 above. In step S2110, generate a block of output values R in response to the selected block of input values S and the associated compensatory decimation value D. This may be done as described with reference to Figure 22. In step S2120, Check if there is any remaining input data values to be processed. If there is another block of input data values to be processed, then repeat step S2100. If there is no remaining block of input data values to be processed then the measurement session is completed. Figures 22A, 22B and 22C illustrate a flow chart of an embodiment of a method of operating the compensatory decimator 470B of Figure 20. In a step S2200, receive a block of input data values S(j) and an associated specific compensatory decimation value D. According to an embodiment, the received data is as described in step S2100 for Figure 21 above. The input data values S(j) in the received block of input data values S are all associated with the specific compensatory decimation value D. In steps S2210 to S2390 the FIR-filter 608 (See Fig.20) is adapted for the specific compensatory decimation value D as received in step S2200, and a set of corresponding output signal values R(q) are generated. This is described more specifically below. In a step S2210, filter settings suitable for the specific compensatory decimation value D are selected. As mentioned in connection with Figure 20 above, the FIR filter 608 is a low pass FIR filter having a certain low pass cut off frequency adapted for decimation by a factor DMAX. The factor DMAX may be set to a suitable value, e.g.20. A filter ratio value FR is set to a value dependent on factor DMAX and the specific compensatory decimation value D as received in step S2200. Step S2210 may be performed by filter parameter generator 610 (Fig.20). In a step S2220, select a starting position value x in the received input data block s(j). It is to be noted that the starting position value x does not need to be an integer. The FIR filter 608 has a length F _LENGTH and the starting position value x will then be selected in dependence of the filter length FLENGTH and the filter ratio value FR. The filter ratio value FR is as set in step S2210 above. According to an embodiment, the starting position value x may be set to x:= F _LENGTH / F _R . In a step S2230 a filter sum value SUM is prepared, and set to an initial value, such as e.g. SUM := 0,0 In a step S2240 a position j in the received input data adjacent and preceding position x is selected. The position j may be selected as the integer portion of x. In a step S2250 select a position Fpos in the FIR filter that corresponds to the selected position j in the received input data. The position Fpos may be a compensatory number. The filter position Fpos, in relation to the middle position of the filter, may be determined to be Fpos = [(x-j) * FR] wherein FR is the filter ratio value. In step S2260, check if the determined filter position value Fpos is outside of allowable limit values, i.e. points at a position outside of the filter. If that happens, then proceed with step S2300 below. Otherwise proceed with step S2270. In a step S2270, a filter value is calculated by means of interpolation. It is noted that adjacent filter coefficient values in a FIR low pass filter generally have similar numerical values. Hence, an interpolation value will be advantageously accurate. First an integer position value IFpos is calculated: IFpos := Integer portion of Fpos The filter value Fval for the position Fpos will be: Fval = A(IFpos) + [A(IFpos+1) – A(IFpos)] * [Fpos – IFpos] wherein A(IFpos) and A(IFpos+1) are values in a reference filter, and the filter position Fpos is a position between these values. In a step S2280, calculate an update of the filter sum value SUM in response to signal position j: SUM := SUM + Fval * S(j) In a step S2290 move to another signal position: Set j := j-1 Thereafter, go to step S2250. In a step 2300, a position j in the received input data adjacent and subsequent to position x is selected. This position j may be selected as the integer portion of x. plus 1 (one), i.e j:= 1 + Integer portion of x In a step S2310 select a position in the FIR filter that corresponds to the selected position j in the received input data. The position Fpos may may be a compensatory number. The filter position Fpos, in relation to the middle position of the filter, may be determined to be Fpos = [(j-x) * F _R ] wherein FR is the filter ratio value. In step S2320, check if the determined filter position value Fpos is outside of allowable limit values, i.e. points at a position outside of the filter. If that happens, then proceed with step S2360 below. Otherwise proceed with step S2330. In a step S2330, a filter value is calculated by means of interpolation. It is noted that adjacent filter coefficient values in a FIR low pass filter generally have similar numerical values. Hence, an interpolation value will be advantageously accurate. First an integer position value IFpos is calculated: IFpos := Integer portion of Fpos The filter value for the position Fpos will be: Fval (Fpos) = A(IFpos) + [A(IFpos+1) – A(IFpos)] * [Fpos – IFpos] wherein A(IFpos) and A(IFpos+1) are values in a reference filter, and the filter position Fpos is a position between these values. In a step S2340, calculate an update of the filter sum value SUM in response to signal position j: SUM := SUM + Fval * S(j) In a step S2350 move to another signal position: Set j := j+1 Thereafter, go to step S2310. In a step S2360, deliver an output data value R(j). The output data value R(j) may be delivered to a memory so that consecutive output data values are stored in consecutive memory positions. The numerical value of output data value R(j) is: R(j) := SUM In a step S2370, update position value x: x := x + D In a step S2380, update position value j j := j+1 In a step S2390, check if desired number of output data values have been generated. If the desired number of output data values have not been generated, then go to step S2230. If the desired number of output data values have been generated, then go to step S2120 in the method described in relation to Figure 21. In effect, step S2390 is designed to ensure that a block of output signal values R(q), corresponding to the block of input data values S received in step S2200, is generated, and that when output signal values R corresponding to the input data values S have been generated, then step S2120 in Fig.21 should be executed. The method described with reference to Figure 22 may be implemented as a computer program subroutine, and the steps S2100 and S2110 may be implemented as a main program. Figure 23 shows another example of a cross-sectional view of the mid-portion 98 of a rotating mill shell 20 during operation. This view could be taken e.g. along line A-A of FIG.1A. According to the example of Fig.23, the tumbling mill shell 20 has six protrusions 310 configured to engage the charge material 30 as the shell rotates about the axis 60, i.e. the number L=6. For the purpose of clarity, the protrusions in the example of Fig.23 have been invidually referenced as 310 ₁ , 310 ₂ , 310 ₃ , 310 ₄ , 310 ₅ , and 310 ₆ . A position sensor 170 is provided to generate the position signal Ep dependent on the rotational position of the shell 20. As mentioned above, the shell 20 is rotatable around the axis of rotation 60, and thus the position sensor 170, being mounted in an immobile manner, may generate a position signal Ep having a sequence of shell position signal values PS for indicating momentary rotational positions of the shell 20. As shown in Figure 23 a number of position markers 180 may be provided on an outer surface of the shell 20 such that, when the shell 20 rotates around the axis of rotation 60, several position markers 180 pass by the position sensor 170 during one revolution of the shell 20, each marker 180 thereby causing the position sensor 170 to generate a revolution marker signal value P _S . According to an embodiment, there are L position markers 180 provided on the shell 20 such that, when the shell 20 rotates around the axis of rotation 60, the position markers 1801...180L consequtively pass by the position sensor 170, thereby causing the position sensor 170 to generate L revolution marker signal values P _S during one revolution of the shell 20. According to the embodiment shown in figure 23, there are six protrusions 310, i.e. L=6, and there are six position markers 180 ₁ , 180 ₂ , 180 ₃ , 180 ₄ , 180 ₅ , and 180 ₆ . It is believed to be important that the placement, in terms of angular positions, of the position markers 180 mirrors the the placement, in terms of angular positions, of the protrusions 310 on the internal surface 22 of shell 20. In the embodiment of figure 23 the L position markers 180 are positioned in a mutually equidistant manner on the perimeter of the shell 20, thereby causing the position sensor 170 to generate a marker signal PS every 360/L degrees during a revolution of the shell 20. In this context it is noted that, in the embodiment of figure 23, the L protrusions 310 ₁ , 310 ₂ , 3103, 3104, 3105, and 310L are positioned in a mutually equidistant manner on the internal surface 22 of shell 20. It is believed that the mutually equidistant positions of the protrusions 310 and the mutually equidistant positions of the position markers 180 is of importance for some embodiments of this disclosure. This is believed to be of importance for some embodiments of this disclosure since the position markers 180 cause the generation of position reference signal values, and the protrusions 310, when engaging material in the charge of a rotating mill, cause the generation of a signal event, such as e.g. an amplitude peak value, in the vibration signal (See references SEA, SMD, Se(i), S(j), S(q) e.g. in figures 1 and15). Moreover, the temporal duration between the occurrence of a position reference signal value and the occurrence of a signal event in the vibration signal, caused by a protrusion 310 engaging material in the charge of the rotating mill shell may be indicative of an internal state of the operating mill, as discussed elsewhere in this disclosure. For example, the temporal duration between the occurrence of a position reference signal value and the occurrence of a signal event in the vibration signal, caused by a protrusion 310 engaging material in the charge of the rotating mill shell may be indicative of an internal state, such as e.g. the position of the toe 205. However, the actual placing of the position markers 180 in relation to the positions of the protrusions 310 is believed to be of less importance. Thus, whereas figure 23 illustrate the position markers 180 being placed at the same angular positions as the protrusions 310, it is to be noted that the position markers 180 may well be displaced in terms of angular positions. However, if the position markers 180 are displaced in terms of angular positions, it is believed to be of importance that all of the position markers 180 are equally displaced so as to maintain the mutually equidistant position of the position markers 180. More particularly, it is believed to be important that the placement, in terms of angular positions, of the position markers 180 mirrors the placement, in terms of angular positions, of the protrusions 310 on the internal surface 22 of shell 20. As noted above, in connection with figures 19A and 19B, it has been observed that when starting up a tumbling mill from an empty state, the initial internal status indicator object appears at an initial polar angle Ф(1) that represents a very first detected toe position 205 of the mill. Based on experimental measurements, it appears as though the initial polar angle Ф(1) may be used as a reference toe position value. Hence, the initial polar angle Ф(1) may thus be termed reference toe position value ФTR. For the particular tumbling mill whose internal state is represented by the display 210S illustrated by figures 19A and 19B, the reference toe position corresponds to an angular value ФTR of about 47 degrees, as can be seen in figures 19A and 19B. With reference to figures 2 and 14, it is believed that the angular value of the reference toe position value ФTR would be changed to a numerically different angular value if the position marker 180 is physically moved to a different placement, in terms of angular position. The set-up of the rotating mill shell 20, as illustrated in Figure 23 may be used in combination with the status parameter extractors 450 as exemplified in this disclusure. With reference to Figure 15A and/or figure 15B, the set-up of the rotating mill shell 20, as illustrated in Figure 23 may be used for generating the marker signal P(i) which is delivered to shell speed value generator 500. Thus, the shell speed value generator 500 will receive a marker signal P(i) having a position indicator signal value every 360/L degrees during a revolution of the shell 20. Thus, the Fast Fourier Transformer 510 will receive a marker signal value P(j)=1, from the speed value generator 500, every 360/L degrees during a revolution of the shell 20 when the rotational speed fROT is constant. Alernatively, the Fast Fourier Transformer 510 will receive a marker signal value P(q)=1, from the decimator 470, 470B, every 360/L degrees during a revolution of the shell 20 when the rotational speed fROT varies. Moreover, the speed value generator 500 will be able to generate even more accurate speed values fROT(j) when it receives a marker signal P(i) having a position indicator signal value, e.g. P(i)=1, every 360/L degrees during a revolution of the shell 20. As for appropriate settings of the FFT 510 when it receives a marker signal value P(j)=1 every 360/L degrees during a revolution of the shell 20, this means that the fundamental frequency will be the repetition frequency f _R . As noted above in relation to figure 2, the vibration signal SEA, SMD, S(j), S(q) will exhibit a signal signature SFIMP indicative of the impact of a protrusion with the toe portion 205, and when there are L protrusions 310 in the shell 20 (See Fig 23 in conjunction with eq.2 below) then that signal signature SFIMP will be repeated L times per revolution of the shell 20. Again, reference is made to the Fourier series (See Equation 2 below): n=∞ F(t) = ∑ Cn sin(nωt + Фn ) (Eq.2) n=0 wherein n=0 the average value of the signal during a period of time (it may be zero, but need not be zero) n=1 corresponds to the fundamental frequency of the signal F(t). n=2 corresponds to the first harmonic partial of the signal F(t). ω = the angular frequency of interest i.e. (2*π*f _R ) f _R = a frequency of interest, expressed as periods per second t= time Ф _n = phase angle for the n:th partial C _n = Amplitude for the n:th partial In this embodiment it is noted that the fundamental frequency will be one per protrusion 310 when the FFT 510 receives a marker signal value P(j)=1 every 360/L degrees during a revolution of the shell 20. As noted above, the settings of the FFT 510 should be done with a consideration of the reference signal. As noted above, the position signal P(j), P(q) (see Figure 15A and/or figure 15B) may be used as a reference signal for the digital measurement signal S(j),S(q). According to some embodiments, when the FFT analyzer is configured to receive a reference signal, i.e. the position signal P(j), P(q), once every 360/L degrees during a revolution of the shell 20 and L is the number of protrusions 310 in the shell 20, then the setting of the FFT analyzer should fulfill the following criteria: The integer value Oi is set to unity, i.e. to equal 1, and the settable variables Y, and Z are selected such that the mathematical expession Oi * Z/Y becomes a positive integer. Differently expressed: When integer value Oi is set to equal 1, then settable variables Y and Z should be set to integer values so as to render the variable X a positive integer, wherein X= Oi * Z/Y Using the above setting , i.e. integer value Oi is set to equal unity, and with reference to Figure 15A and/or figure 15B and equation 2 above, the FFT 510 may deliver the amplitude value Cn for n=1, i.e. C1 = Sp(r). The FFT 510 may also deliver phase angle for the fundamental frequency (n=1), i.e. Ф1 = FI(r). With reference to Figure 15A and/or figure 15B in conjunction with figure 1A and/or figure 1B and equation 2 above, the status values Sp(r) = C1 and FI(r) = Ф1 may be delivered to the Human Computer Interface (HCI) 210 for providing a visual indication of the analysis result. As mentioned above, the analysis result displayed may include information indicative of an internal state of the tumbling mill process for enabling the operator 230 to control the tumbling mill. With reference to figures 16, 17, 18, 19A, and 19B, the example illustrations of visual indications of analysis results are valid for the set-up of the rotating mill shell 20, as illustrated in Figure 23, whereby the FFT 510 will receive a marker signal P(i), P(j), P(q) having a position indicator signal value every 360/L degrees, wherein L is the number of protrusions 310 in the shell 20. Whereas the above discussion in relation to settings of the FFT 510 refers to the Fourier series and equations 1 and 2 for the purpose of conveying an intuitive understanding of the background for the settings of an FFT transformer 510, it is noted that the use of digital signal processing may involve the discrete Fourier transform (See Equation 3 below): Equation 3: Thus, according to embodiments of this disclosure the above discrete Fourier transform (DFT) may be comprised in signal processing for generating data indicative of the internal state of a tumbling mill, such as that discussed in connection with embodiments of the status parameter extractor 450. In this connection, reference is made to e.g. figures 3, 4, 5, 15 and/or 24. In view of the above discussion on the subject of FFT and the Fourier series, the discrete Fourier transform will not be discussed in further detail, as the skilled reader of this disclosure is well acquainted with it. Whereas figure 23 illustrates that a number of position markers 180 may be provided on an outer surface of the shell 20, each marker 180 thereby causing the position sensor 170 to generate a revolution marker signal value PS, it is noted that such a position signal may alternatively be generated by an encoder 170 which is mechanically coupled to the rotating mill shell 20. Thus, the position sensor 170 may be embodied by an encoder 170 which is mechanically coupled to the rotating mill shell 20 such that the encoder generates e.g. one marker signal P _S per protrusion 310 in the rotating shell 20 during rotation of the mill shell 20. In summary, as regards appropriate settings of the FFT 510 and the above equations 1 and 2, it is noted that the phase angle for the n:th partial, i.e. Ф _n , may be indicative of the relative position of the toe 205. In particular, the phase angle for the n:th partial, i.e. Фn, may be indicative of the position of the toe 205, expressed as a part of the distance between two adjacent protrusions 310 in a rotating shell 20. With reference to table 6 above and figure 14, the total distance between two adjacent protrusions may be regarded as 360 degrees, and value of the phase angle for the n:th partial, i.e. Фn, divided by 360 may be indicative of a percentage of the total distance between the two adjacent protrusions. This can be seen e.g. by comparing col. #2 in table 5 and table 6 above. As mentioned above, Фn= phase angle for the n:th partial, and Cn =Amplitude for the n:th partial. As discussed above, considering the number L of protrusions in the rotating shell 20 and the number of reference signals being generated and, as a consequence thereof, the order Oi of a signal of interest, the FFT 510 may be set so as to deliver a phase angle for the n:th partial, Фn, and an amplitude for the n:th partial, Cn, so that the phase angle for the n:th partial, i.e. Ф _n , may be indicative of the relative position of the toe 205. Moreover, as noted above, the FFT 510 may be set so as to render the variable X a positive integer, wherein X= Oi* Z/Y and wherein Oi is set to a integer value, Y is set to a integer value, Z is set to a integer value. Figure 24 shows a somewhat diagrammatic and schematic top view of another system 700 including a tumbling mill 10. The tumbling mill 10 may be an autogenous (AG) mill, for example. Alternatively, the tumbling mill 10 may be a semi-autogenous (SAG) mill. Another example tumbling mill 10 is a ball mill 10. The tumbling mill 10 includes a shell 20 having an internal shell surface 22 that forms a chamber 25 for grinding material. The tumbling mill system 700 of figure 24 may be configured as described in any of the other embodiments described in this disclosure, e.g. in relation to figures 1-31. However, whereas the tumbling mill system of figure 1A and/or figure 1B was described to have a vibration sensor 70 by the input side of the mill, it is noted that the tumbling mill system 700 of figure 24 may be configured to have a first vibration sensor 70IN for producing a first measuring signal SEAIN as well as a second vibration sensor 70 _OUT for producing a second measuring signal S _EAOUT . The signal treatment of the first measuring signal SEAIN generated by the first vibration sensor 70 _IN may be as described in relation to the signal S _EA in any of the other embodiments described in this disclosure, e.g. in relation to figures 1-31. Likewise, the signal treatment of the second measuring signal SEAOUT generated by the second vibration sensor 70OUT may be as described in relation to the signal SEA in any of the other embodiments described in this disclosure, e.g. in relation to figures 1-31. Thus, a difference as compared the above described embodiments is that in the system 700 there will be provided data indicative of the internal state of the input side of the tumbling mill, based on the first measuring signal S _EAIN as well as data indicative of the internal state of the output side of the tumbling mill, based on the second measuring signal SEAOUT. Accordingly, the tumbling mill system 700 of figure 24 may be configured as described in any of the described embodiments of this disclosure as regards the provision of a position signal or reference signal. The analysis apparatus 150 shown in figure 24 may comprise a first status parameter extractor 450 ₁ as well as a second status parameter extractor 450 ₂ . The status parameter extractors 4501 and 4502 may operate as described in any of the other described embodiments, e.g. with reference to figure 5 and or 15 and/or as described in relation to figure 30 -31. Thus first status parameter extractor 450 ₁ may be configured to generate parameters SP1(r), RT1(r), fROT(r), dSP1(r), and dRT1(r). Similarly the second status parameter extractor 4502 may be configured to generate parameters S _P2 (r), R _T2 (r), f _ROT (r), dS _P2 (r), and dR _T2 (r). However, the rotational speed f _ROT (r) of the shell will be the same, of course, and thus it may suffice if one of the status parameter extractors delivers the rotational speed value fROT(r). With reference to figure 24, there is shown a cartesian co-ordinate system having three mutually perpendicular axes x, y and z. It is to be understood that, during operation of the mill 10, the material 30 travels in the positive direction of the x-axis from the input side 80 of the mill to the output side 90. The tumbling mill system 700 of figure 24 advantageously provides parameters indicative of the internal state of the input side of the tumbling mill: S _P1 (r), R _T1 (r), dS _P1 (r), and dR _T1 (r), as well as parameters indicative of the internal state of the output side of the tumbling mill: SP2(r), RT2(r), dSP2(r), and dRT2(r). A comparison of an input side parameter with the corresponding output side parameter can advantageously add yet another dimension to the understanding of internal state of the mill 10. For example, the relation between RT2(r) and RT1(r) indicates whether - the toe position is the same at input and output side, or - the toe position is higher at the input side, indicated when R _T1 (r) > R _T2 (r); or - the toe position is higher at the output side, indicated when RT2(r) > RT1(r). The toe position being higher at the output side may be indicative of an incipient disorder. For example, when the outflow of output material 95 is decreased, perhaps due to a blockage, while the inflow of solid material 110 continues at an unreduced pace, there will be an increased risk of overload that may lead to a decreased efficiency of the grinding process in the tumbling mill. Accordingly, tumbling mill system 700 of figure 24 may advantageously enable an early indication of an incipient disorder. Thus, based on a comparison of an input side parameter with the corresponding output side parameter the tumbling mill system 700 may enable adjustments of control parameters so as to avoid e.g. disorders such as overload of the mill. With reference to figure 24 it is noted that the vibration sensor 70 _OUT is attached to a non- rotating portion of the body of mill structure 10, and the vibration sensor 70OUT is positioned so as to primarily detect vibrations in the horizontal direction Y (see the cartesian co-ordinate system having three mutually perpendicular axes x, y and z, wherein Y is a horisontal direction). Likewise, the vibration sensor 70IN is attached to a non-rotating portion of the body of mill structure 10, and the vibration sensor 70IN is positioned so as to primarily detect vibrations in the horizontal direction Y. Experimental measurements appear to indicate that an improved vibration signal quality is obtained when a vibration sensor is configured to primarily detect vibrations in a horizontal direction Y, as compared to a vibration signal quality obtained when a vibration sensor is configured to primarily detect vibrations in a vertical direction Z. As noted above, e.g. in connection with Figure 2, the interaction of a protrusion 310 with the toe 205 of the charge, forcing material in the toe of the charge to accelerate in the direction of movement of the protrusion 310, as illustrated in figure 2, causes a mechanical vibration V _IMP . The impact of the protrusion 310C against the mass of material in the toe portion 205 causes a mass of toe portion material to accelerate in the direction AACC of movement of the protrusion 310C, this acceleration causing a force F _IMP against the leading edge surface of the protrusion 310C. Since the mass of solid material in charge 30 of the mill is in the magnitude of metric tons, this impact force FIMP is of of a significant magnitude. However, since the mill structure will typically rest on a very hard floor surface that tends to mitigate vibrations in the vertical direction, it appears as though the detection of vibrations in a horizontal direction Y renders an improved vibration signal quality. Figure 25 shows a somewhat diagrammatic and schematic top view of yet another embodiment of a system 720 including a tumbling mill 10. The tumbling mill system 720 of figure 25 may be configured as described in connection with figure 24. However, whereas the tumbling mill system 700 of figure 24 was described to have a vibration sensor 70 _OUT attached to a non-rotating portion of the body of mill structure 10, and a vibration sensor 70IN attached to another non-rotating portion of the body of mill structure 10, the tumbling mill system 720 of figure 25 differs in that it provides a vibration sensor 70 ₂₀ that is attached to the rotating shell 20 of the mill structure 10. The provision of a vibration sensor 7020 directly on the rotating shell 20 will render high vibration amplitudes, in particular when the vibration sensor 7020 is positioned on the outside of the shell directly on the opposite side of the shell wall from a projection 310, as illustrated in figure 25. The tumbling mill system 720 of figure 25 may optionally comprise a first vibration sensor 7020IN for producing a first measuring signal SEAIN as well as a second vibration sensor 7020OUT for producing a second measuring signal S _EAOUT . As shown in figure 25, the first vibration sensor 70 _20IN may be firmly attached to the outer surface of the shell 20 at a measuring point position 310IN which is closer to the input side 80 than it is to the output side 90. Moreover, the second vibration sensor 70 _20OUT may be firmly attached to the outer surface of the shell 20 at a measuring point position 310OUT which is closer to the output side 90 than it is to the input side 80. The first vibration sensor 7020IN and the second vibration sensor 7020OUT may be equipped to communicate in a wireless manner with the apparatus 150, e.g. via transciever units 740 and 750, respectively. A sensor 70 ₂₀ , 70 _20IN , 70 _20OUT on the the outer surface of the shell 20 may be supplied with power via a battery, or alternatively by means of an inductive device (not shown) attached to outer surface of the rotating shell 20 that operates as a generator by interaction with one or several permanent magnets that are immobile. In this manner, when the shell 20 rotates, it will repeatedly cause the inductive device to to pass through the magnetic fields of the immobile one or several permanent magnets, thereby inducing electric current that may be used as power for the sensor 70 ₂₀ , 70 _20IN , 70 _20OUT . The tumbling mill system 720 of figure 25 may also advantageously provide parameters indicative of the internal state of the input side of the tumbling mill S _P1 (r), R _T1 (r), dS _P1 (r), and dRT1(r), as well as parameters indicative of the internal state of the output side of the tumbling mill: SP2(r), RT2(r), dSP2(r), and dRT2(r). Hence, the skilled reader of this disclosure directly and unambiguosly derives that the tumbling mill system 720 of figure 25 may advantageously enable an early indication of an incipient disorder in a manner substantially similar to that of the tumbling mill system 700 of figure 24. In particular, the tumbling mill system 720 of figure 25 may advantageously enable a comparison of an input side parameter with the corresponding output side parameter in the manner described above in relation to the tumbling mill system 700. Thus also the the tumbling mill system 720 of figure 25 may advantageously enable adjustments of control parameters so as to avoid e.g. disorders such as an overload of the mill. Figure 26 shows a somewhat diagrammatic and schematic top view of yet another embodiment of a system 730 including a tumbling mill 10. The tumbling mill 10 may be an autogenous (AG) mill, for example. Alternatively, the tumbling mill 10 may be a semi- autogenous (SAG) mill. Another example tumbling mill 10 is a ball mill 10. The tumbling mill 10 includes a shell 20 having an internal shell surface 22 that forms a chamber 25 for grinding material. The tumbling mill system 730 of figure 26 may include parts, and be configured, as described in any of the other embodiments described in this disclosure, e.g. in relation to figures 1-25 and/or as described in relation to figure 30 -31. In particular, the apparatus 150, shown in figure 26 may be configured as described in any of the other embodiments described in this disclosure, e.g. in relation to figures 1-25 and/or as described in relation to figure 30 -31. However, in the embodiment of the system 730 illustrated in figure 26, the apparatus 150 includes a monitoring module 150A as well as a control module 150B. Although the drawing illustrates the apparatus 150 as two boxes, it is to be understood that the apparatus 150 may well be provided as a single entity 150 including a monitoring module 150A as well as a control module 150B, as indicated by the unifying reference 150. The system 730 is configured to control an internal state of in a tumbling mill 10 having a shell 20 that rotates around an axis 60 at a speed of rotation f _ROT for grinding a charge of material 30 by tumbling the material in the rotating shell. The shell 20 has an internal shell surface 22 including a first number L of protrusions 310 configured to engage material as the shell 20 rotates about the axis 60. The system 730 may comprise a device 170, 180 for generating a position signal. The device 170, 180 may incude the position sensor 170 and the marker 180 as described elsewhere in this disclosure. The position signal is E _P , P(i), P(j), P(q) indicative of a rotational position of said rotating shell 20, said position signal including a time sequence of position signal sample values P(i), P(j), P(q). A sensor 70, 70IN, 70OUT, 330 is povided and it is configured to generate a vibration signal SEA, SMD, Se(i), S(j), S(q) dependent on mechanical vibrations VIMP emanating from rotation of said shell. The vibration signal S _EA , Se(i), S(j), S(q) may include a time sequence of vibration sample values Se(i), S(j), S(q). The apparatus 150 of the system 730 may comprise a monitoring module 150A and a control module 150B. The monitoring module 150A comprises a status parameter extractor 450, 4501, 4502450C configured to detect a first occurrence of a first reference position signal value in said time sequence of position signal sample values P(i), P(j), P(q) (See tables 2, 3 and 4 above, wherein column #2 illustrates the position signal having values 1; 1C). The status parameter extractor 450 may be configured to detect a second occurrence of a second reference position signal value 1; 1C; 100% in said time sequence of position signal sample values P(i), P(j), P(q)). The status parameter extractor 450 may also be configured to detect an occurrence of an event signature S _P (r); Sp in said time sequence of vibration sample values Se(i), S(j), S(q). The event may be caused by the impact of a protrusion 310 into the toe portion 205 of the charge 30, causing an impact vibration that may cause a vibration signal signature, as discussed elsewhere in this disclosure. The status parameter extractor 450 may be configured to generate data indicative of a first temporal relation RT(r); TD; FI(r), X1(r) between the event signature occurrence, and the first and second occurences. As mentioned above, the system 730 includes a control module 150B configured to receive data indicative of an internal state of the mill 10 from the mill monitoring module 150, 150A. The data indicative of an internal state can include any of the information generated or delivered by the status parameter extractor 450, as described in relation to any of the figures 1-31 in this disclosure. With reference to figure 26, the control module 150B includes a regulator 755 for controlling an angular toe position FI(r), A _TOE (See figure 26 in conjunction with figure 2) based on a toe position reference value FI _REF (r) (See fig 26), said first temporal relation R _T (r); T _D ; FI(r); X1(r) (See figs 1 -31), and a toe position error value FIERR(r) (see fig 26). The toe position error value (FIERR(r) ) depends on said toe position reference value FI _REF (r), and said first temporal relation R _T (r); T _D ; FI(r) (See figs 3 -26). The toe position reference value FIREF(r) may be generated by manual input (not shown in Fig 26, but it may be done as discussed e.g. in connection with figure 1A and/or figure 1B above. As shown in figure 26, the said toe position error value (FIERR(r) ) may depend on a difference between said toe position reference value FIREF(r) ), and the first temporal relation RT(r); TD; FI(r); X1(r). The regulator 755 may be configured to control the solid material feed rate set point R _SSP in dependence on the toe position reference value FIREF(r). The solid material feed rate RS, discussed in connection with figure 1A, depends on the solid material feed rate set point R _SSP (See fig 26). As mentioned in connection with figure 1A, the solid material feed rate RS is an amount of solid material per time unit that is fed into the input 100 of said tumbling mill 10. The regulator may also be configured to control a liquid feed rate set point R _LSP in dependence on said toe position reference value FIREF(r). The liquid feed rate RL may depend on said liquid feed rate set point R _LSP . As mentioned in connection with figure 1A, the liquid feed rate R _L may be an amount of liquid per time unit that is being fed into an input 130 of the tumbling mill 10. The event signature may be indicative of an impact force F _IMP generated when a projection 310 on an internal shell surface 22 of the rotating shell 20 interacts with a toe portion 205 of the charge material 30. The status parameter extractor 450 may be configured to generate said first temporal relation RT(r); TD; FI(r); X1(r) as a phase angle (FI(r). The first temporal relation R _T (r); T _D ; FI(r); X1(r) is indicative of a toe position 205, A _TOE (r) (See fig 2 in conjunction with fig 26). The first temporal relation RT(r); TD; FI(r); X1(r) may be indicative of a proportion of a distance between two adjacent of said protrusions 310 in the mill shell. Alternatively, the relation value X1(r) may be indicative of a relative position of the toe portion 205, i.e. the position of the toe portion 205 in relation to two predermined stator positions separated from each other in a manner corresponding to the positions of two adjacent protrusions 310. Moreover, the relation value X1(r) may be indicative of an absolute toe position. An absolute toe position may be generated based on a combination of the relation value X1(r) and the second internal status parameter X2(r), also referred to as Sp(r). In particular, an absolute toe position may be generated based on a temporal progression of the combination of the first internal status parameter X1(r) and the second internal status parameter X2(r), as discussed e.g. in connection with figures 16, 17, 18, 19A and 19B. As illustrated in figure 19A, an inital data set 550(r) = 550(1), corresponding to X1(r=1) and X2(r=1), represents an empty, or near empty state of the mill. As the filling degree of the mill increases, in the direction of arrow 560A, the absolute toe position increases. In figures 19A and 19B, the “angular length”, from the initial angle FITR, to the current indicator 550(r) is indicative of the absolute toe position. With reference to figure 19B in conjunction with figure 2, the toe angle 0 degrees at the vertical line 960 in figure 2 may correspond to the initial angle FITR in figure 19B. An increase in the value X2(r), as r increases is shown as an increased distance from origo i fig 19B, and it is indicative of an increased mass of the toe portion 205, as discussed elsewhere in this document. When the value of the parameter X1(r) increases to a value equal to 360 degrees, it is indicative of the toe position 205 being at an angular distance 360/L from the the initial angle FITR. When the value of the parameter X1(r) increases to a value exceeding 360 degrees, it is indicative of the absolute toe position 205 being at an angular distance 360/L + X1(r) from the the initial angle FITR. The status parameter extractor 450 may be configured to generate said event signature as an amplitude value SP(r); Sp; CL(r); C1(r); X2(r). The status parameter extractor 450 may comprise a Fourier Transformer 510 (see fig.15A and/or fig.15B) configured to generate said first temporal relation RT(r); TD; FI(r) X1(r). As discussed in connection with table 5, the status parameter extractor 450 may be configured to count a total number of samples NB from the first occurence to the second occurrence. Moreover, the status parameter extractor 450 may be configured to count another number of samples N _P from the first occurence to the occurrence of the event, and said status parameter extractor 450 may be configured to generate said first temporal relation RT(r); TD; FI(r) X1(r) based on said another number and said total number. The status parameter extractor 450 may be configured to count a total number of samples NB from the first occurence to the second occurrence, and the status parameter extractor 450 may be configured to count another number of samples NP from the first occurence to the occurrence of the event. Moreover, the status parameter extractor 450 may be configured to generate said first temporal relation RT(r); TD; FI(r) based on a relation between said another number and said total number, wherein said relation between said another number and said total number may be indicative of a toe position 205. According to one embodiment, the regulator 755 operates to control the toe position on the input side (FI _IN (r), A _{TOE_IN} ) based on a toe position reference value (FI _REFIN (r) ), said first temporal relation (FIIN(r)), and a toe position error value (FI _ERRI (r) ), wherein said toe position error value (FI _ERRI (r) ) depends on said toe position reference value (FIREFIN(r) ), and said first temporal relation (RT(r); TD; FIIN(r)). According to one embodiment, the regulator 755 operates to control the solid material feed rate set point RSSP in dependence on the toe position reference value FIREFIN(r) According to one embodiment, the regulator 755 operates to control the solid material feed rate set point RSSP based on a toe position reference value (FIREFIN(r) ), said first temporal relation (FI _IN (r)), and a toe position error value (FIERRI(r) ), wherein said toe position error value (FI _ERRI (r) ) depends on said toe position reference value (FI _REFIN (r) ), and said first temporal relation (FIIN(r)). Although figures 26, 28 and 29 illustrate two feedback signals, i.e. first temporal relation FIIN(r) relating to an input side of the mill 10, and first temporal relation FIOUT(r) relating to an output side of the mill 10, it is to be understood that the system may operate with a single feedback signal. Thus, for example, the regulator may be provided with a single input, e.g. for receiving the first temporal relation FIIN(r) relating to the status at the input side of the mill. According to one embodiment, the regulator 755 operates to control the solid material feed rate set point RSSP based on a toe position reference value (FI _REFOUT (r) ), said first temporal relation (FI _OUT (r)), and a toe position error value (FIERRO(r) ), wherein said toe position error value (FIERRO(r) ) depends on said toe position reference value (FI _REFOUT (r) ), and said first temporal relation (FI _OUT (r)). The regulator 755 may be configured to include a proportional–integral–derivative controller (PID controller). Alternatively, the regulator 755 may be configured to include a proportional–integral controller (PI controller). Alternatively, the regulator 755 may be configured to include a proportional controller (P controller). Alternatively, the regulator 755 may be configured to include Kalman filtering, also known as linear quadratic estimation (LQE). Kalman filtering is an algorithm that uses a series of measurements observed over time, including statistical noise and other inaccuracies, and produces estimates of unknown variables that tend to be more accurate than those based on a single measurement alone, by estimating a joint probability distribution over the variables for each timeframe. Figure 27 shows a schematic block diagram of a distributed process monitoring system 770. Reference numeral 780 relates to a client location with a mill 10 having a rotatable shell 20, as discussed above in relation to preceding drawings in this document. The client location 780, which may also be referred to as client part or mill location 780, may for example be the premises of a mining company, or the premises of an ore mill plant, or e.g. a manufacturing plant for the manufacture of cement. The distributed process monitoring system 770 is operative when one sensor 70 is, or several sensors 70, 70 _IN , 70 _OUT are, attached on or at measuring points related to the shell 20. As mentioned above such measuring points may be e.g. at a bearing 40, 50 (See Fig.26 & 27) or at a measuring point position 310IN, 310OUT (See Fig.25). The measuring signals S _EA, S _EAIN , S _EAOUT , and E _P (See e.g. Figs.1, 27, 26, 25) may be coupled to input ports of a mill location communications device 790. The mill location communications device 790 may include an Analogue-to-Digital converter 795 for A/D- conversion of the measuring signals S _EA, S _EAIN , S _EAOUT , and E _P . The A/D converter 975 may operate as disclosed in relation to A/D converter 330 elsewhere in this document, e.g. in connection with figure 3 and 5. The mill location communications device 790 has a communication port 800 for bi-directional data exchange. The communication port 800 is connectable to a communications network 810, e.g. via a data interface 820, for enabling delivery of digital data corresponding to the measuring signals SEA, SEAIN, SEAOUT, and EP. The communications network 810 may be the world wide internet, also known as the Internet. The communications network 810 may also comprise a public switched telephone network. A server computer 830 is connected to the communications network 810. The server 830 may comprise a database 840, user input/output interfaces 850 and data processing hardware 852, and a communications port 855. The server computer 830 is located on a server location 860, which is geographically separate from the mill location 780. The server location 860 may be in a first city, such as the Swedish capital Stockholm, and the mill location 780 may be on the countryside near a mill, and/or in another country such as for example in Norway, Australia or in the USA. Alternatively, the server location 860 may be in a first part of a county and the mill location 780 may be in another part of the same county. The server location 860 may also be referred to as supplier part 860, or supplier location 860. According to an example a central control location 870 comprises a monitoring computer 880 having data processing hardware and software for monitoring and/or controlling an internal state of a mill 10 at a remote mill location 780. The monitoring computer 880 may also be referred to as a control computer 880. The control computer 880 may comprise a database 890, user input/output interfaces 900 and data processing hardware 910, and a communications port 920, 920A, or several communications ports 920, 920A, 920B. The central control location 870 may be separated from the mill location 780 by a geographic distance. The central control location 870 may be in a first city, such as the Swedish capital Stockholm, and the mill location 780 may be on the countryside near a mill, and/or in another country such as for example in Norway, Australia or in the USA. Alternatively, the central control location 870 may be in a first part of a county and the mill location 780 may be in another part of the same county. By means of communications port 920, 920A the control computer 880 can be coupled to communicate with the mill location communications device 790. Hence, the control computer 880 can receive the measuring signals SEA, SEAIN, SEAOUT, and EP (See e.g. Figs.1, 27, 26, 25) from the mill location communications device 790 via the communications network 810. The system 770 may be configured to enable the reception of measuring signals SEA, SEAIN, S _EAOUT , and E _P in real time, or substantially in real time or enabling real time monitoring and/or real time control of the mill 10 from the location 870. Moreover, the control computer 880 may include a monitoring module 150, 150A as disclosed in any of the examples in this document, e.g. as disclosed in connection with any of the drawings 1-26 above. A supplier company may occupy the server location 860. The supplier company may sell and deliver apparatuses 150 and/or monitoring modules 150A and/or software for use in an such apparatuses 150 and/or monitoring modules 150A. Hence, supplier company may sell and deliver software for use in the control computer 880 at the central control location 870. Such software 370, 390, 400 is discussed e.g. in connection with Figure 4. Such software 370, 390, 400 may be delivered by transmission over said communications network 810. Alternatively such software 370, 390, 400 may be delivered as a a computer readable medium 360 for storing program code. Thus the computer program 370, 390, 400 may be provided as an article of manufacture comprising a computer storage medium having a computer program encoded therein. According to an example embodiment of the system 770 the monitoring computer 880 may substantially continuously receive measurement signals measuring signals SEA, SEAIN, S _EAOUT , and E _P (See e.g. Figs.1, 27, 26, 25) from the mill location communications device 790, e.g via the communications network 810, so as to enable continuous or substantially continuous monitoring of the internal state of the mill 10. The user input/output interfaces 900 at the central control location 870 may comprise a screen 900S for displaying images and data as discussed in connection with HCI 210 elsewhere in this document. Thus, user input/output interfaces 900 may include a display, or screen, 900S, 210S for providing a visual indication of an analysis result. The analysis result displayed may include information indicative of an internal state of the tumbling mill process for enabling an operator 930 at the central control location 870 to control the tumbling mill 10. Moreover, the monitoring computer 880 at the central control location 870 may be configured to deliver information indicative of an internal state of the tumbling mill process to the HCI 210, via the communications port 920, 920B and via the communications network 810. In this manner, the monitoring computer 880 at the central control location 870 may be configured to enable an operator 230 at the client location 780 to control the tumbling mill. The local operator 230 at the client location 780 may be placed in the control room 220 (See figure 1A and/or figure 1B and/or Figure 27). Thus, the client location 780, 220 may include a second mill location communications device 790B. The second mill location communications device 790B has a communication port 800B for bi- directional data exchange, and the communication port 800B is connectable to the communications network 810, e.g. via a data interface 820B. Although it has, for the purpose of clarity, been described as two location communications devices 790, 790B, there may, alternatively, be provided a single mill location communications device 790, 790B, and/or a single communications port 800, 800B for bi- directional data exchange. Thus, the items 790 and 790B may be integrated as one unit at the mill location 780, and likewise, the items 820 and 820B may be integrated as one unit at the mill location 780. Figure 28 shows a schematic block diagram of yet another embodiment of a distributed process monitoring system 940. Reference numeral 780 relates to a mill location with a mill 10 having a rotatable shell 20, as discussed above in relation to preceding drawings in this document. The distributed process monitoring system 940 of figure 28 may include parts, and be configured, as described in any of the other embodiments described in this disclosure, e.g. in relation to figures 1-31. In particular, the monitoring apparatus 150, also referred to as monitoring module 150A, shown in figure 28 may be configured as described in any of the other embodiments described in this disclosure, e.g. in relation to figures 1-31. In particular, the process monitoring system 940 illustrated in figure 28, may be configured to include a monitoring module 150A, as disclosed in connection with figure 27, but located at the central control location 870. Moreover, in the process monitoring system 940 illustrated in figure 28, the mill location 780 includes a control module 150B, as described above e.g. in connection with figure 26. Thus, the internal state of the mill 10 may be automatically controlled by control module 150B located at or near the mill location 780, whereas the monitoring computer 880 at the central control location 870 may be configured to deliver information indicative of an internal state of the tumbling mill process to the HCI 900, 900S for enabling an operator 930 at the central control location 870 to monitor the internal state of the tumbling mill 10. The measuring signals SEA, SEAIN, SEAOUT, and EP (See e.g. Figs.1, 27, 26, 25) may be coupled to input ports of the mill location communications device 790. The mill location communications device 790 may include an Analogue-to-Digital converter 795 for A/D- conversion of the measuring signals SEA, SEAIN, SEAOUT, and EP. The A/D converter 975 may operate as disclosed in relation to A/D converter 330 elsewhere in this document, e.g. in connection with figure 3 and 5. The mill location communications device 790 has a communication port 800 for bi-directional data exchange. The communication port 800 is connectable to the communications network 810, e.g. via a data interface 820. The communication port 800 is connectable to a communications network 810, e.g. via a data interface 820, for enabling delivery of digital data corresponding to the measuring signals S _EA, S _EAIN , S _EAOUT , and E _P . Moreover, the client location 780 may include a second mill location communications device 790B. The second mill location communications device 790B has a communication port 800B for bi-directional data exchange, and the communication port 800B is connectable to the communications network 810, e.g. via a data interface 820B so as to enable reception, by the control module 150B, of data indicative of an internal state of the mill 10. As illustrated in Figure 28, data indicative of an internal state of the mill 10 may be generated by the monitoring module 150A at the central location 870. Although figure 28, for the purpose of clarity, describes two location communications devices 790, 790B, there may, alternatively, be provided a single mill location communications device 790, 790B, and/or a single communications port 800, 800B for bi- directional data exchange. Thus, the items 790 and 790B may be integrated as one unit at the mill location 780, and likewise, the items 820 and 820B may be integrated as one unit at the mill location 780. Figure 29 shows a schematic block diagram of yet another embodiment of a distributed process control system 950. Again, reference numeral 780 relates to a mill location with a mill 10 having a rotatable shell 20, as discussed above in relation to preceding drawings in this document.The distributed process monitoring system 950 of figure 29 may include parts, and be configured, as described in any of the other embodiments described in this disclosure, e.g. in relation to figures 1-31. In particular, the monitoring apparatus 150, also referred to as monitoring module 150A, shown in figures 28 and 29 may be configured as described in any of the other embodiments described in this disclosure, e.g. as discussed in relation to figures 1-31. Moreover, the process monitoring system 950 illustrated in figure 29, may be configured to include a control module 150B, as described above e.g. in connection with figure 26 as well as a monitoring module 150A, as disclosed in connection with figure 27. In the example of figure 29, the monitoring module 150A and the control module 150B are provided at the control location 870. The control location 870 may be remote from the mill location 780. Communication of data between the control location 870 and the mill location 780 may be provided via data ports 820 and 920 and ther communications network 810, as discussed above in connection with preceding figures. Figure 30 shows another example of a cross-sectional view of the mid-portion 98 of a rotating mill shell 20 during operation. This view could be taken e.g. along line A-A of FIG.1A. According to the example of Fig.30, the tumbling mill shell 20 has four protrusions 310 configured to engage the charge material 30 as the shell rotates about the axis 60, i.e. the number L=4. For the purpose of clarity, the protrusions in the example of Fig.23 have been invidually referenced as 3101, 3102, 3103, and 3104. A position sensor 170 is provided to generate the position signal Ep dependent on the rotational position of the shell 20. As illustrated in figure 30, the position sensor 170 is positioned along a vertical line 960 from the axis of rotation 60, around which the shell 20 is rotatable. Moreover, the position marker 180 is provided on an outer surface of the shell 20 such that, when a protrusion 310 passes by the vertical line 960, then the position sensor 170 generates a marker signal PS. When the tumbling mill shell 20 has four protrusions 310, as ilustrated in figure 30, ande there is provided a single static position sensor 170, the method according to an example may receive a first static position signal Ps1, P1 in response to a first passage of the position marker 180, and the method may receive a second static position signal Ps2 in response to a second passage of the position marker 180, and the method may then generate virtual static position signals PC in a recorded time sequence of position signal sample values. The generated virtual static position signals PC are inserted so as to be evenly distributed in the recorded time sequence of position signal sample values when the protrusions are evenly distrbuted on the inner circumference of shell 20. This advantageously leads to, a recorded time sequence of position signal sample values indicative of L static positions that are evenly distributed, as shown in figure 30. In the figure 30 example, there are four protrusions, i.e. L=4, so the recorded time sequence of position signal sample values are indicative of four static positions P1, P2, P3, and P4 = PL. The L static position signals Ps, Pc can thereafter be used as reference position signals. The event signal signatures will also occur L times per revolution in a time sequence of vibration signal sample values, and the occurrence of an event signal signature can be analysed in relation to one static position signal Ps, Pc or two static position signal Ps, Pc. As noted above, in connection with figures 19A and 19B, it has been observed that when starting up a tumbling mill from an empty state, the initial internal status indicator object appears at an initial polar angle Ф(1) that represents a very first detected toe position 205 of the mill. Based on experimental measurements, it appears as though the initial polar angle Ф(1) may be used as a reference toe position value. Hence, the initial polar angle Ф(1) may thus be termed reference toe position value ФTR. For the particular tumbling mill whose internal state is represented by the display 210S illustrated by figures 19A and 19B, the reference toe position corresponds to an angular value Ф _TR of about 47 degrees, as can be seen in figures 19A and 19B. With reference to figures 2 and 14, it is believed that the angular value of the reference toe position value ФTR would be changed to a numerically different angular value if the position marker 180 is physically moved to a different placement, in terms of angular position. With reference to Figure 30, it is believed that the position marker 180 being provided on an outer surface of the shell 20 such that, when a protrusion 310 passes by the vertical line 960, then the position sensor 170 generates a revolution marker signal value P _S that will cause the reference toe position value ФTR to have a very small value, or a zero value, since the initial polar angle Ф(1) = ФTR represents a very first detected toe position of the mill when starting up a tumbling mill from an empty state. In this connection, a zero degree absolute toe position X6 can be represented when the toe is at a lowest part of the shell 20, at the position indicated by the vertical line 960 from the axis of rotation 60 (See figure 30 in conjunction with figure 2). With reference to figures 30, 2 and 14A and 14B, it is believed that the angular value of the reference toe position value ФTR would be changed to a numerically different angular value if the position marker 180 is physically moved to a different placement, in terms of angular position. Figure 31 is a block diagram that illustrates another example of a status parameter extractor 450, referred to as status parameter extractor 450C. The status parameter extractor 450C may include i.a. a vibration event signature detector and position signal value detector and a relation generator, as discussed below. The vibration event signature detector may be embodied by a peak detector, as discussed below. According to aspects of the solution disclosed in this document, reference position signal values Ep, 1,1C are generated at L predetermined rotational positions of the rotatable shell 20, the L predetermined rotational positions following a pattern that reflects the angular positions of the L protrusions 310 in the shell 20. The provision of such reference position signal values Ep, 1,1C together with the provision of vibration event signature detection in a manner as herein disclosed, makes it possible to generate data indicative of a position of a toe portion 205 in an advantageously accurate manner. Although it has been examplified with protrusions 310 that are positioned in an equidistant pattern, i.e. evenly distributed in the shell 20, this solution is also operable with other patterns of angular positions of the L protrusions 310 in the shell 20. When other patterns of angular positions of the L protrusions 310 in the shell is used, it is of importance that the reference position signal values Ep, 1,1C are generated at L predetermined rotational positions of the rotatable shell 20, the L predetermined rotational positions following a pattern that reflects the angular positions of the L protrusions 310 in the shell 20. With reference to figure 5, the A/D converter 330 may be configured to deliver a sequence of pairs of vibration measurement values S(i) associated with corresponding position signal values P(i) to the status parameter extractor 450. The status parameter extractor 450C, of Figure 31, is adapted to receive a sequence of measurement values S(i) and a sequence of positional signals P(i), together with temporal relations there-between. Thus, an individual measurement value S(i) is associated with a corresponding position value P(i). Such a signal pair S(i) and P(i) are delivered to a memory 970. With reference to figure 31, the status parameter extractor 450C comprises a memory 970. The memory 970 may operate to receive data, in the form of a signal pair S(i) and P(i), so as to enable analysis of temporal relations between occurrences of events in the received signals. Columns #2 and #3 in Table 3 provide an illustration of an example of the data collected in the memory 970 during one full revolution of a shell, when a position signal 1, 1C is provided six times per revolution, since there are L=6 protrusions 310 in the shell 20. Table 4 and table 5 provide more detailed information about example signal values in the first 1280 time slots of table 3. The position signal 1, 1C may be generated by physical marker devices 180 and/or some position signals 1C may be virtual position signals. The time sequence of position signal sample values P(i), P(j), P(q)) should be provided at an occurrence pattern that reflects the angular positions of the protrusions 310 in the shell 20. For example, when there are six (L= 6) equidistant protrusions 310 in the shell 20, the angular distance between any two adjacent protrusions 310 is 60 degrees. This is since 360 degrees is one full revolution and, when L=6, the angular distance between any two adjacent protrusions is 360/L = 360/6 = 60. Accordingly, the corresponding time sequence of position signal sample values P(i), representing a full revolution of the shell 20, should include six (L= 6) position signal values 1, 1C with a corresponding occurrence pattern, as illustrated in table 3. The status parameter extractor 450C further comprises a position signal value detector 980 and vibration event signature detector 990. The vibration event signature detector 990 may be configured to detect a vibration signal event such as an amplitude peak value in the received sequence of measurement values S(i). The output of the position signal value detector 980 is coupled to a START/STOP input 995 of a reference signal time counter 1010, and to a START input 1015 of an event signature time counter 1020. The output of the position signal value detector 980 may also coupled to a START/STOP input 1023 of vibration event signature detector 990 for indicating the start and the stop of the duration to be analyzed. Detector 990 transmits on its output when a position signal value 1, 1C is detected. The vibration event signature detector 990 is configured to analyse all the sample values S(i) between two consecutive position signal values 1, 1C for detecting a highest peak amplitude value Sp therein. The vibration event signature detector 990 has a first output 1021 which is coupled to a STOP input 1025 of the event signature time counter 1020. The reference signal time counter 1010 is configured to count the duration between two consecutive position signal values 1, 1C, thereby generating a first reference duration value TREF1 on an output 1030. This may be achieved, e.g. by reference signal time counter 1010 being a clock timer that counts the temporal duration between two consecutive position signal values 1, 1C. With reference to figure 14B, the first reference duration value TREF1 may in this manner be indicative of the temporal duration between static position signal P4 and static position signal P5. Alternatively, the reference signal time counter 1010 may count the number of time slots (See column #01 in table 3) between two consecutive position signal values 1, 1C. The event signature time counter 1020 is configured to count the duration from the occurrence of a position signal value 1, 1C to the occurrence of a vibration signal event such as an amplitude peak value. This may be attained in the following manner: - The event signature time counter 1020 starts counting when receiving, on START input 1015, information that position signal value detector 980 detected an occurrence of a position signal value 1, 1C. - The event signature time counter 1020 stops counting when receiving, on STOP input 1025, information that vibration event signature detector 990 detected a vibration signal event such as an amplitude peak value in the received sequence of measurement values S(i). In this manner, the event signature time counter 1020 may be configured to count the temporal duration from the occurrence of a position signal value 1, 1C to the occurrence of a an amplitude peak value. The temporal duration from the occurrence of a position signal value 1, 1C to the occurrence of a an amplitude peak value is here referred to as a second reference duration value TREF2. The second reference duration value TREF2 may be delivered on an output 1040. With reference to figure 14B, the second reference duration value T _REF2 may in this manner be indicative of the temporal duration between the occurrence of static position signal P4 and the occurrence of an amplitude peak value. With reference to figure 31, the output 1040 is coupled to an input of a relation generator 1050 so as to provide the second reference duration value TREF2 to the relation generator 1050. The relation generator 1050 also has an input coupled to receive the first reference duration value TREF1 from the output 1030 of reference signal time counter 1010. The relation generator 1050 is configured to generate a relation value X1 based on the received second reference duration value TREF2 and the received first reference duration value TREF1. The relation value X1 may also be referred to as R _T (r); T _D ; FI(r). The relation value X1 may be generated L times per revolution of the shell 20. Moreover, the L times generated relation values X1 from a single revolution of the shell may be averaged to generate one value X1(r) per revolution of the shell 20. In this manner, the status parameter extractor 450C may be configured to deliver an updated value X1(r) once per revolution. For the purpose of clarity, an example of a relation value X1 is generated in the following manner: Please refer to column #03 in table 4 in conjunction with figure 31: The vibration sample values S(i) are analyzed, by vibration event signature detector 990 , for the detection of a vibration signal signature S _FIMP . The vibration signal signature SFIMP may be manifested as a peak amplitude sample value Sp. With reference to table 5, the peak value analysis leads to the detection of a highest vibration sample amplitude value S(i). In the illustrated example, the vibration sample amplitude value S(i=760) is detected to hold a highest peak value Sp. Having detected the peak value Sp to be located in time slot 760, a temporal relation value X1 can be established. In table 5 the time slots, in a time sequence of position signal sample values P(i), carrying position signal values 1, 1C are indicated as 0% and 100%, respectively. As illustrated in the example in col. #02 of table 5, the temporal location of slot number i = 760 is at a position 59% of the temporal distance between slot i=0 and slot i=1280. Differently expressed, 760/1280= 0,59 = 59% Accordingly, a position of the toe 205, expressed as a percentage of the distance between two adjacent static positions PC, (see static positions P4 and P5 in Fig 14B in conjunction with table 5), can be obtained by: Counting a total number of samples (N _B – N ₀ = N _B – 0 = N _B =1280) from the first reference signal occurrence in sample number N0 = 0 to the second reference signal occurrence in sample number NB=1280, and Counting another number of samples (N _P – N ₀ = N _P –0 = N _P ) from the first reference signal occurrence at N0 = 0 to the occurrence of the peak amplitude value Sp at sample number NP, and generating said first temporal relation (X1, RT(r); TD; FI(r)) based on said another number N _P and said total number N _B . This can be summarized as: X1(r) = R _T (r) = R _T (760)= (N _P – N ₀ ) / (N _B – N ₀ ) = (760 - 0) / (1280-0) = 0,59 = 59% Thus, a relative toe position X1, RT may be generated by: Counting a total number of samples (NB) from the first reference signal occurrence to the second reference signal occurrence, and Counting another number of samples (NP) from the first reference signal occurrence to the occurrence of the peak amplitude value Sp at sample number NP, and generating said first temporal relation (X1; RT(r); TD; FI(r)) based on a relation between said sample number NP and said total number of samples i.e. NB. The relation generator 1050 may generate an update of relation value X1 with a delivery frequency that depends on the rotational speed of the shell 20. As noted above, the status parameter extractor 450C may be configured to deliver an updated value X1(r) once per revolution. In this manner a delivered updated value X1(r) may be based on L values generated during one revolution. The latest update, number r, of the first internal status parameter X1(r) may be delivered on a first status parameter extractor output 1060. With reference to figure 31, the vibration event signature detector 990 may be configured to detect a peak amplitude sample value Sp. The vibration event signature detector 990 has an output 1070 for delivering a detected vibration signal amplitude peak value Sp. The detected vibration signal amplitude peak value Sp may be delivered from the output 1070 of vibration signal peak amplitude detector 990 to an output 1080 of status parameter extractor 450C. The output 1080 constitutes a second status parameter extractor output for delivery of a second internal status parameter X2(r), also referred to as Sp(r). The second internal status parameter X2(r) is delivered at the same delivery frequency as the first internal status parameter X1(r). Moreover, the first internal status parameter X1(r) and the second internal status parameter X2(r) are preferably delivered simultaneously, as a set of internal status parameter data (X1(r); X2(r)). In the notation X1(r), the “r” is a sample number indicating a time slot, i.e. increasing number value of “r” indicates temporal progression, in the same manner as the number “i” in column #01 in table 3. As mentioned elsewhere in this document, the magnitude of the peak amplitude sample value Sp of the vibration signal signature SFIMP appears to depend on the magnitude of the impact force FIMP. The impact force FIMP of the interaction between a rotationally moving protrusion 310 and the material charge 30 causes an acceleration of the at least one particle in the toe portion 205 of the material charge 30, the impact causing the mechanical impact vibration VIMP. With reference to figure 2, it is noted that the impact of the protrusion 310C against the mass of material in the toe portion 205 causes a mass of toe portion material to accelerate in the direction AACC. The direction AACC is the direction in which the protrusion 310C moves. The acceleration causes a force FIMP against the leading edge surface 312C of the protrusion 310C. This impact force FIMP may be estimated to be of a magnitude: F _IMP = m ₂₀₅ * a ₂₀₅ wherein m ₂₀₅ is the mass of the portion of the toe that is accelerated, a ₂₀₅ is the amount of acceleration of that portion of the toe In view of the above, the inventor concluded that the magnitude of a detected peak amplitude sample value Sp may advantageously be indicative of the density of the charge in the tumbling mill 10. In this context it is noted that the content of a desired metal, in the solid material 110 of the charge material 30, affects the density of the charge in the tumbling mill 10, as discussed in connection with table 1 in this document. Thus, the density of the charge in the tumbling mill 10 may be indicative of a relation between a desired metal and waste minerals in the charge in the tumbling mill 10. Accordingly, the inventor concluded that the magnitude of a detected peak amplitude sample value Sp may advantageously be indicative of a relation between a desired metal and waste minerals in the charge in the tumbling mill 10. Moreover, the inventor concluded that the magnitude Sp of a detected peak amplitude sample value, in combination with data indicative of the toe position i.e. the above discussed relation value X1 may advantageously be indicative of the filling degree of the tumbling mill 10. In this connection, it is noted that the filling degree of the tumbling mill 10 has an impact on the efficiency of the grinding process. In order to maximise the amount of output material 95 from the tumbling mill 10 it is therefore desirable to control the inflow of input material 110 so as to maintain an optimal state of the tumbling mill process, including an optimized filling degree. The optimal internal state of the tumbling mill process may include a certain filling degree of the shell 20, i.e. a certain charge volume. Accordingly, the solid material feed rate set point RSSP may be controlled in dependence on the combination of the relation value X1(r) and the magnitude of a detected peak amplitude sample value X2(r). Moreover, the inventor concluded that the magnitude Sp(r) = X2(r) of a detected peak amplitude sample value, in combination with data indicative of the toe position i.e. the above discussed relation value X1(r) may advantageously be indicative of the absolute toe position value X6(r) of the tumbling mill 10. It can be seen, in figures 19A and 19B, that a gradually increasing polar angle X1(r) = FI(r) in combination with a gradually increasing radius value X2(r)=S _P (r) renders an image of a spiral arm that whirls outwards, as indicated by a curved arrow 560A in figure 19A, starting from the first internal status indicator object 550(1). An “angular length” X6(r) of the spiral arm, from the initial polar angle Ф(1) of the first internal status indicator object 550(1) up to the current, or latest detected, toe position FI(r), appears to be indicative of an absolute position X6(r) of the toe portion 205 (see e.g. figure 2 and figure 14). In this connection, it is noted that 360 degrees in the polar coordinate system 520 of figure 19A corresponds to the relative toe position value X1 indicating 100% of the distance between two adjacent static reference positions, as discussed in relation to figure 14B. In particular it is noted, with reference to figures 19A and 19B there is a certain slowness in the change of the absolute toe position X6. Figure 32 is a block diagram of the system 5, 320, 770 including a tumbling mill illustrated as a box 10 receiving a number of inputs U1, ... Uk, and generating a number of outputs Y1, ... Yn. With reference to figure 32 and fig.1C it is noted that, for the purpose of analysis, a tumbling mill 10 may be regarded as a black box 10B having a number of input variables, referred to as input parameters U1, U2, U3, ... Uk, where the index k is a positive integer. During operation of the tumbling mill 10, 10B, the tumbling mill has an internal state X, and for the purpose of analysis, the tumbling mill 10 may be regarded as the black box 10B having a number of output variables, also referred to as output parameters Y1, Y2, Y3, ... Yn, where the index n is a positive integer. The internal state X of the mill may be described, or indicated, by a number of internal state parameters X1, X2, X3,..., Xm, where the index m is a positive integer. Using the terminology of linear algebra, the input variables U1, U2, U3,... Uk may be collectively referred to as an input vector U. Thus, the dimension of input vector U is k: Input vector U: Dim (U) = k Likewise, the internal state parameters X1, X2, X3,..., Xm may be collectively referred to as an internal state vector X. The dimension of internal state vector X is m: Internal state vector X: Dim (X) = m The output parameters Y1, Y2, Y3, ... Yn may be collectively referred to as an output vector Y. The dimension of output vector Y is n: Output vector Y: Dim (Y) = n The internal state X of the mill 10, at a point in time termed r, can be referred to as X(r). That internal state X(r) can be described, or indicated, by a number of internal state parameters X1, X2, X3,..., Xm, as discussed above. These internal state parameters define different aspects of the internal state X(r) of the mill 10 at time r. The internal state X(r) of the the tumbling mill 10 depends on the input vector U(r). An aspect of the internal state X is the total amount of material 30 in the shell 20, and that total amount does not change instantly. Thus, during operation of the mill 10, the internal state X(r) can be regarded as a function of an earlier internal state X(r-1) and of the input U(r): X(r) = f1(X(r-1), U(r) ), (eq.4) wherein X(r-1) denotes the internal state X of the mill 10 at a point in time preceding the point in time termed r. The output Y of the tumbling mill 10 can be regarded as a function of the internal state X. Thus, using the terminology of linear algebra, the output vector Y(r) depends on the internal state vector X(r): Y(r) = f ₂ (X(r)) (eq.5) It is an object of an aspect of this document to address the problem of how to maintain the internal comminution process of the mill 10 at a suitable operating point. Thus, during operation of the the mill 10 it may be desirable to counteract deviations from such a suitable operating point. This problem may be addressed by providing a linearized model of the comminution process at an operating point. When regarding the above functions f ₁ and f ₂ , respectively, at operating points near a suitable operating point, the funcions may be linear. Accordingly, at a selected operating point, the internal state X(r) can be regarded as a function of an earlier internal state X(r-1) and of the input U(r) in accordance with a linear model which may be written as follows: X(r) = A * X(r-1) + B * U(r), (eq.6) wherein A and B are coefficient matrices. In this connection it is noted that in linear algebra, a coefficient matrix is a matrix consisting of the coefficients of the variables in a set of linear equations. As the skilled reader of this document knows, the coefficient matrix is used in solving systems of linear equations. In this connection it is noted that the coefficients in matrices A and B, respectively, may be constants. Similarly, at a selected operating point, the output vector Y(r) depends on the internal state vector X(r) in accordance with a linear model which may be written as follows: Y(r) = C * X(r) (eq.7) wherein C is a coefficient matrix. However, equation 7 does not mean that a change in the state X must be immediately conveyed into a change of the state Y, since there may, perhaps sometimes, be a delay from the occurrence of a changed internal state X to the occurrence of a corresponding change of the state Y(r) of the product material 95,96. When operating at a steady state, however, there appears to be a causal link between the internal state X in the comminution process occuring in the mill 10 at time r and the state Y(r) of the product material 95,96 at the same time r. Thus Equation 7 is valid, at least when operating the tumbling mill 10 at steady state. Referring to equation 7, the coefficients in matrix C may be constants. The constant values for the coefficients in matrix C may be set to the derivatives C = dY/dX at a selected operating point XOP. With reference to figure 32, the system comprises a monitoring module 150A for generating an internal state vector X of dimension m, wherein m is a positive integer. In an example Dim(X) is at least 2. The values in the internal state vector X may be generated in a manner as disclosed in relation to any of figures 1A to 31 above. The Monitoring Module 150A may be adapted to convey 1122 information describing the internal state X of the mill during operation of the mill 10, e.g via a user interface 210, as indicated by arrow 1122. Thus, one or several values in the internal state vector X may be conveyed to an operator 230 via user interface 210. This advantageously simplifyes for the operator 230 of the mill 10 to make suitable adjustments 1124 to set point values (indexed SP) for influencing the input vector U. Thus, by adjusting e.g. the speed set point value U1SP (See fig 32 in conjunction with figure 1B) the operator 230 can adjust the speed fROT, U1. In this manner the operator, by adjusting the relevant set point value(s) USP can adjust the corresponding input variable(s) U1, U2, U3,... Uk. The set point values U1SP, U2SP, U3SP,... Uk may be collectively referred to as a set point vector U _SP . Thus, the dimension of set point vector U _SP is k: set point vector U _SP : Dim (U _SP ) = k The system 5,320,770 of figure 32 may include a Monitoring Module 150A as described in any of the other embodiments described in this disclosure, e.g. in relation to any of figures 1-31. Figure 33 is a block diagram of another system 730, 940, 950 including a tumbling mill illustrated as a box 10 receiving a number of inputs U1, ... Uk, and generating a number of outputs Y1, ... Yn. The system 940 of figure 33 may include a Monitoring Module 150A as described in any of the other embodiments described in this disclosure, e.g. in relation to any of figures 1-31. Moreover, the system 940 of figure 33 may include a control module 150B as described in any of the other embodiments described in this disclosure, e.g. in relation to figure 28. The Monitoring Module 150A of figure 33 may be adapted to convey information describing the internal state X of the mill during operation of the mill 10, e.g via a user interface 210. Thus, one or several values in the internal state vector X may be conveyed 1122 to an operator 230 via user interface 210, as indicated by arrow 1122. This advantageously simplifyes for the operator 230 of the mill 10 to make suitable adjustments 1126 to mill set point values U and/or internal state reference values X _REF (indexed REF) for influencing the internal state X of the mill during operation of the mill 10. Arrow 1126 indicates user input relating e.g. to a desired internal state XREF. The internal state reference parameters X1 _REF , X2 _REF , X3 _REF ,..., Xm _REF may be collectively referred to as an internal state reference vector X REF. The dimension of internal state reference vector X _REF is m: Internal state reference vector XREF: Dim (XREF) = m In this manner the operator 230, by adjusting mill set point values U and/or relevant internal state reference parameter value(s) X1REF, X2REF, X3REF,..., XmREF can influence the internal state X of the mill during operation of the mill 10. Thus, the user interface 210, in response to user input, may be configured to generate values for the internal state reference vector X _REF . The internal state reference vector X REF is delivered to a reference input of a Control Module 150B, as illustrated in figure 33. Referring to figure 33 in conjunction with figure 26, the Control Module 150B is a multivariable Control Module that also receives, from the Monitoring Module 150A, the above described internal state vector X. In this connection, the internal state vector X may be indicative of a current state of a communition process in the mill 10, and the internal state reference vector X REF is indicative of a desired state of the communition process. The multivariable Control Module 150B may be adapted to generate, based on the received internal state reference vector X _REF and the received internal state vector X, an internal state error vector X _ERR . The internal state error vector X ERR includes internal state error values X1ERR, X2 ERR, X3ERR,..., Xm ERR The dimension of internal state error vector X _ERR is m: Internal state error vector X _ERR : Dim (X _ERR ) = m The error vector is delivered to regulator 755, 755C. The regulator 755, 755C of figure 33 is a multivariable regulator adapted to generate a set point vector U _SP . Accordingly, the set point vector USP includes the above described set point value(s) for controlling or adjusting corresponding input variable(s) U1, U2, U3,... Uk (See fig 33 in conjunction with figure 34). Thus, the system desribed in relation to Figure 33 advantageously simplifyes for the operator 230 of the mill 10 by conveying 1122 information indicative of the internal state X of the mill during operation, while also allowing the operator to provide 1126 information describing a desired internal state, e.g. in the form of reference values for the above described internal state reference vector X REF. The regulator 755, 755C may be a multi-variable regulator configured to include a multi- variable proportional–integral–derivative controller (PID controller). Alternatively, the regulator 755, 755C may be configured to include a multi-variable proportional–integral controller (PI controller). Alternatively, the regulator 755, 755C may be configured to include a multi-variable proportional controller (P controller). Alternatively, the regulator 755, 755C may be configured to include Kalman filtering, also known as linear quadratic estimation (LQE). Kalman filtering is an algorithm that uses a series of measurements observed over time, including statistical noise and other inaccuracies, and produces estimates of unknown variables that tend to be more accurate than those based on a single measurement alone, by estimating a joint probability distribution over the variables for each timeframe. Figure 34 shows another somewhat diagrammatic view of a system 1130 including a tumbling mill 10. Thus, reference numeral 1130 relates to a system including a mill 10 having a rotatable shell 20, as discussed in this document. The system 1130 of figure 34 may include parts, and be configured, as described above in relation to figure 1A and/or figure 1B and/or as described in any of the other examples described in this disclosure, e.g. in relation to figures 1-33. The Monitoring Module 150A may include status parameter extractor functionality as described elsewhere in this document for generating internal state parameter values X1, X2, X3,..., Xm. It is to be noted that the internal state X of the mill 10, at a point in time termed r, can be referred to as X(r). That internal state X(r) can be described, or indicated, by a number of parameter values, the parameter values defining different aspects of the internal state X(r) of the mill 10 at time r. Thus, values of the internal state parameters X1, X2, X3,..., Xm at the time r may be collectively referred to as an internal state vector X(r). The system illustrated in Figure 34 may provide an integrated HCI 210, 250, 210S. Thus, the input/output interface 210 of Figure 34 may be configured to enable all the input and/or output described above. Additionaly, the input/output interface 210 of figure 34 may be configured to provide 1132 information relating to a state of the output material. The state of the output material may be described by the output parameters Y1, Y2, Y3, ... Yn, collectively referred to as output vector Y. As mantioned above, the dimension of output vector Y is n: Output vector Y: Dim (Y) = n The vector Y may also be referred to as discharge material state vector Y. System 1130 of figure 34 includes a regulator 1190. The regulator 1190 may be configured to enable all functions described with reference to regulator 240, which is described elsewhere in this document. Alternatively, regulator 1190 may be configured to enable all functions described with reference to regulator 755, which is described elsewhere in this document. In addition to functions described in regulator 240 and/or regulator 755 the regulator 1190 may be configured to perform additional functions, such as e.g. to convey and/or receive information relating to the output material 95, 96, e.g. in the form of output parameters Y1, Y2, Y3, ... Yn. Thus regulator 1190 may also be referred to by reference number 240C and/or 755C. Thus, regulator 1190 may be configured to convey information relating to the output material 95, 96 to a mill operator 230, as indicated by arrow 1132. Moreover, regulator 1190 may be configured to receive, from a mill operator 230, information relating to the output material 95, 96, as indicated by arrow 1196. Figure 35 is a schematic general overview of information that may be conveyed by input/output interface 210 of Figure 34. With reference to figures 34 and 35 it is noted that the regulator 1190, 755C of fig.34 is coupled, via coupling 1100, for data exchange with input/output interface 210. Information to be transferred via coupling 1100 includes reference values for the above described internal state reference vector X REF. Referring to figure 34, the system 1130 comprises a product analyzer 1140 configured to analyze at least a portion of said product particles 96. The analyzer 1140 is configured to generate at least one product measurement value Y1, Y2, Y3, ... Yn based on said product particle analysis. The product analyzer 1140 may include, for example, an analyzer of the type Outotec PSI 300i. Alternatively, the product analyzer 1140 may comprise a PSI 500i Particle Size Analyzer, which is commercially available from the company Metso Outotec. Moreover, the product analyzer 1140 may comprise a device, such as a scale, configured to measure, or estimate, the mass per time unit of the material 95 that is discharged from the mill. However, in some examples the product analyzer 1140 may be configured to analyze or measure a portion of the product particles 96 instead of analyzing or measuring all of the product particles 96. In effect, the at least one output material measurement value Y1, Y2, Y3, ... Yn may be indicative of a discharge material state Y, the discharge material state Y being a momentary state of the output material 95. When analyzer 1140 provides two or more output material measurement values, these values may be provided in the form of the above mentioned output vector Y. The at least one product measurement value may, for example, include a value indicative of a product discharge rate RSDis. The product discharge rate RSDis constitutes data indicative of the weight per time unit of the solid material that is discharged from the mill after grinding. Thus, the product discharge rate R _SDis is a measure of the mass per time unit of the material 95 that is discharged from the mill. The product discharge rate RSDis may also be referred to as output parameter Y1. However, the inventor concluded that a measure of the product discharge rate RSDis, Y1, does not necessarily require analysis of the material 95 that is discharged from the mill. In fact, data indicative of the the product discharge rate R _SDis , Y1, may be obtained by analysis of the solid feed material 110. The ability to control the toe position advantageously renders an improved ability of maintaining the mill at steady state operating conditions even when the characteristics of the solid feed material changes. During steady state the outflow from the mill equals the inflow to the mill and thus, the mass per time unit of solid discharge material 95 equals the mass per time unit of solid feed material 110 during steady state. If there is not a steady state, the toe position will change, and as discussed elsewhere in this disclosure, action can then be taken to control the toe position, e.g. so as to so as to return to a steady state operation. For the purpose of analysis of the mass per time unit of the ground material, i.e. of product particles, it is therefore possible to measure the mass of the feed material 110 (See e.g. figure 1A). As mentioned elsewhere in this disclosure, the solid feed material 110, also referred to as feed material 110, may include pieces 115 of rock and ore, the pieces 115 having a variety of sizes. The solid material 110 may be transported to the first input 100 of the mill 10 by a conveyer belt 260. The conveyer belt 260 runs at a controllable conveyer belt speed to transport a solid feed material 110 to the first input 100 at the solid material feed rate R _S . According to an example, the conveyer belt 260 includes a scale configured to measure the weight of the solid material 110 on the conveyer belt 260 having a controllable conveyer belt speed. Hence, the conveyer belt and scale combination 325 therefore provides information about weight per time unit of the solid material 110 that is transported to the first input 100 of the mill 10. In other words, data U6 indicative of the weight per time unit of the solid material 110 that is transported to the first input 100 of the mill 10 can be obtained based on weight information from the scale and information about conveyer belt speed. With reference to figure 34, the conveyer belt and scale combination is schematically illustrated as a feed material analyzer 325. The feed material analyzer 325 may be provided for generating a measurement value indicative of at least one feed material property U5, U6. The at least one feed material property may include a solid feed material size distribution U5, and/or data U6 indicative of the weight per time unit of the solid material 110 that is transported to the first input 100 of the mill 10. As an example: When, during steady state, the conveyer belt and scale combination is configured to weigh solid material 110 on a stretch of 20 meters of the conveyer belt 260, and the conveyer belt 260 moves at a speed of 10 meters per second, and the scale indicates 100 kg, that means that the flow is about 100 kg in a duration of 2 seconds. Thus, there may be a weight per time unit of solid feed material 110 of about 180 metric tons per hour. The product discharge rate R _SDis , also referred to as output parameter Y1, may be estimated on the basis of data U6 indicative of the weight per time unit of the solid material 110 that is transported to the first input 100 of the mill 10. According to an example, the product discharge rate R _SDis , Y1, is estimated to be equal to the data U6 indicative of the weight per time unit of the solid material 110 that is transported to the first input 100 of the mill 10. As mentioned above, the product discharge rate RSDis, Y1 from the mill equals the solid material feed rate U6 to the mill during steady state, and thus the product discharge rate R _SDis , Y1 can be estimated with the solid material feed rate U6 during steady state operating conditions. The momentary state of the output material 95, i.e. the discharge material state Y, may be identified by measurement of at least one output material measurement value Y1, Y2, Y3, ... Yn. In practice it may be desirable to generate more than one output material measurement value in order to obtain information indicative of the discharge material state (Y). The at least one output material measurement value may be one or many selected from the group: - a value Y1 indicative of a mass per time unit of said output material 95; - a value Y1 indicative of a mass per time unit of said product particles 96; - a value Y2 indicative of a median particle size; - a value Y3 indicative of a mass per time unit of said product particles 96 having a size that falls below a predetermined product particle size limit; - a value Y4 indicative of a proportion, or a percentage share, of said product particles that have a product particle size in a range between a lower product particle size limit Y8 and an upper product particle size limit Y9; -a value Y5 indicative of a count, i.e. a number of product particles, of said product particles with product particle size in a range between a lower product particle size limit and an upper product particle size limit; - a value Y6 indicative of a product particle size distribution, such as a standard deviation; and - a value Y7 indicative of a product particle size. Said product particle size value Y7 may be at least one selected from the group: - a product particle median size value; - a product particle mean size value; - a product particle median diameter value; and - a product particle mean diameter value. Said product particle size limit values may be at least one selected from the group: - a product particle diameter value; and - a product particle maximum width value. Said value Y6 indicative of a product particle size distribution may be at least one selected from the group: - a standard deviation value; - a variance value; - range between the highest and lowest size; - interquartile range. Said range between a smallest particle size value and a largest particle size value may be between 30 micrometres and 20 millimetres; 150 micrometres and 300 micrometres; 200 micrometres and 220 micrometres; and/or 0 millimetres and 40 millimetres. The product analyzer 1140 may thus be configured to analyze at least a portion of said product particles 96 so as to generate at least one product measurement value Y1, Y2, Y3, ... Yn based on said product particle analysis. The at least one product measurement value Y1, Y2, Y3, ... Yn may be provided with information indicative of a point in time when the at least one product measurement value Y1, Y2, Y3, ... Yn was generated. Moreover, the discharge material state Y, at a point in time termed w, can be referred to as Y(w). That discharge material state Y(w) can be described, or indicated, by a number of parameter values Y1(w), Y2(w), Y3(w), ... Yn(w), the parameter values defining different aspects of the material 95,96 discharged from of the mill 10 at time w. Thus, values of the discharge material parameter values Y1, Y2, Y3, ... Yn at time w may be collectively referred to as discharge material state vector Y(w), also referred to as output vector Y(w). As noted above, there is a causal relationship between between a certain internal state X(r) and a certain output Y(r), and thus the output Y of the tumbling mill 10 can be regarded as a function of the internal state X. Referring to figure 34, the output vector Y may be delivered to a first input of a correlator 150C1. Moreover, the internal state vector X may be delievered by the module 150A to a second input of the correlator 150C1. The correlator 150C1 is configured to identify a correspondence between the internal state X and the corresponding output Y. However, in order to perform a correlation it is desirable to ensure that a measured value of the output Y(w) refers to, at least approximately, the same point in time as the internal state X(r). In other words, the values in the internal state vector X(r) may need to be synchronized with the values in the corresponding output vector Y(w). Referring to figure 34, the output vector Y(w) may be delivered to a first input of an optional synchronizer 1150. The synchronizer 1150 is optional because it may not be needed, e.g. when the internal state vector X(r) and the corresponding output vector Y(w) are generated in a synchronized manner such that - the point in time w is the same point in time as the point in time r, or - such that the point in time w is at least approximately the same point in time as the point in time r. Temporally Synchronized vectors X(t) and Y(t) are received by a correlation data generator 1160, as illustrated in figure 34. The correlation data generator 1160 generates a correlation data set 1170. According to an example, the correlation data generator 1160 generates a correlation data set by performing correlation of a received at least one status parameter value, such as e.g. X1(t) and a received at least one corresponding product measurement value, such as e.g. Y2(t). The correlation data generator 1160 may receive a number of time stamped internal state vectors X(r) and a number of time stamped corresponding output vector Y(w). The received information vectors may be received in a temporally interleaved fashion such as X(10), Y(12), X(14), Y(16), X(18), Y(20), X(22), Y(24), wherein the synchronizer 1150 receives a vector X in a time period between the reception of two consecutive vectors Y. That is the case e.g. when vector X(18) is time stamped in the time period between t=20 and t=16, and the Y-vectors Y(16) and Y(20), respectively, are time stamped at the points in time t=16 and t=20. When operating the mill 10 at a steady state condition, i.e. when all the values in vectors X and Y are stable over time, the synchronizer 1150 may generate pairs of vectors X and Y by adjusting the time stamps so that a generated pair of vectors X and Y have the same time stamp. That same time stamp may e.g. be an intermediate time stamp. For example, the synchronizer 1150 when receiving the above mentioned vectors X(18) and Y(20) may arrange them as a vector pair stamped with an intermediate time t= 19. Thus, the synchronizer 1150 may, in response to reception of vectors X(t) and Y(t+2) generate a vector pair X(t+1) and Y(t+1) for delivery to correlation data generator 1160. Morever, the delivery frequency of the X-vectors and the Y-vectors may be different. This problem may be adressed, for example, by configuring the synchronizer 1150 to deliver, to correlation data generator 1160: pairs of received vextors X and Y such that each time stamped vector Y is associated with that vector X having the closest earlier time stamp. As a consequence, the synchronizer 1150 may have to discard or reject some vectors. Thus, for example, when the delivery frequency of the X-vector lower than the delivery frequency of the Y-vector, the synchronizer 1150 may receive vectors as follows: vector X(34), vector Y(36), vector X(37), vector Y(38), vector X(40), vector Y(40), vector Y(42) vector X(43), vector Y(44), then the synchronizer 1150 may deliver, to correlation data generator 1160, pairs 1165 of vectors X and Y such that each time stamped vector Y is associated with that vector X having the closest earlier time stamp. In the above example, the following pairs could be delivered by synchronizer 1150: vector X(34) vector Y(36), vector X(37), vector Y(38), vector X(40), vector Y(40), vector X(43), vector Y(44), and as a cosequence vector Y(42) may be discarded. Table 7 below is an example of successive pairs 1165 of vectors X and Y arranged in temporal order. Table 7: Successive pairs 1165 of vectors X and Y arranged in temporal order. The example of successive pairs 1165 of vectors X and Y, illustrated by table 7, includes information indicative of a toe position X1, and information indicative of a corresponding output parameter Y2. The output parameter Y2 is indicative of a median size of particles produced by a tumbling mill. The correlation data generator 1160, may be configured to perform a correlation based on received pairs 1165 of vectors X and Y. According to an example the correlation data generator 1160 may be configured to perform a regression analysis based on a large number of received pairs 1165 of vectors X and Y. The regression analysis may use one or several statistical processes for estimating the relationships between the dependent variables, i.e the values in the vector Y and one or more independent variables, i.e. the values in the vector X. Figure 36 is of a cross-sectional view of the shell 20 of a ball mill 10 during operation at a sped of rotation rotation fROT. Thus, figure 36 is comparable to the cross-sectional views of figures 1A, 2, 14A, 14B, 23, and 30 above. The tumbling mill 10 of figure 36 operates as a ball mill, and thus the charge material 30 comprises a number of grinding balls 1168 as well as solid feed material 110, 115. The charge material 30 may, in some examples, also include liquid feed material, such as water. Figure 37A is an example of a graph of an output from a Particle Size Analyzer comprised in the product analyzer 1140. As mentioned above, in connection with figure 34, the system 1130 comprises a product analyzer 1140 configured to analyze at least a portion of said product particles 96. The analyzer 1140 is configured to generate at least one product measurement value Y1, Y2, Y3, ... Yn based on said product particle analysis. As illustrated by the graph of Figure 37A, the Particle Size Analyzer may generate data values indicative of a particle size distribution. Based of such data values indicative of a particle size distribution a data value Y2 indicative of indicative of a median particle size may be obtained. In the example graph of figure 37A, it can be seen that a highest count of particles is represented by a peak value of the graph in the range between 230 – 280 mikrometers. Also indicated in figure 37A is a desired particle size range, i.e. a Target range of 200 to 220 mikrometers. As mentioned above, the product analyzer 1140 may include, for example, an analyzer of the type Outotec PSI 300i. Alternatively, the product analyzer 1140 may comprise a PSI 500i Particle Size Analyzer, which is commercially available from the company Metso Outotec. Figure 37B is a plot of a large number of successive pairs 1165 of vectors X1 and Y2, for a ball mill, like the one illustrated in figure 36, running at a constant, or substantially constant speed of rotation U1= fROT =18 rpm. In figure 37B, the density of black dots, each dot representing a pair of values X1-Y1, that density is very high at values arund X1= 60 degrees. The high density of black dots at values arund X1= 60 degrees indicates that the mill operated very often at Operating Points of X1 being on or near 60 degrees. It can be seen that X1 = 60 corresponds to a median particle size Y1 of about 260 mikrometers. The density of black dots is lower at around X1=40 to 50 degrees, indicating that the mill operated less often at operating points of X1 being between 40 and 50 degrees. With reference to figure 1A in conjunction with figure 36, the ball mill had a radius of R _MIC = 1930 millimetres and it had 28 protrusions 310 (not shown in fig.36). During operation of the ball mill 10, there may be a grinding ball feed rate U4 set or selected so that the amount of grinding balls remains constant, or substantially constant. Referring to figure 34, the correlation data generator 1160 may be configured to perform a regression analysis, also referred to as correlation analysis, based on received pairs 1165 of values X1 and Y2 as illustrated in figure 37B. The regression analysis may, for example, employ linear regression. The linear regresssion analysis, when applied to a single dependent variable Y2 and a single independent variable, such as X1 or X6, will operate to identify a linear relation, i.e a line, 1180 that most closely fits the data according to a specific mathematical criterion. For example, the method of ordinary least squares computes the unique line 1180 that minimizes the sum of squared differences between the true data and that line. Thus, the line 1180 in figure 37B is an illustration of a result of linear regression based on received pairs of values X1-Y1, as discussed above. Accordingly, the correlation data set 1170, generated by correlator 150C1, may include a table of data or alternatively a linear equation. The linear equation for a line in a two- dimensional space, such as the two-dimensional space X1-Y2, can be established from points that the line passes. For example, when a line passes through two points (X1A,Y2A) and (X1 _B ,Y2 _B ), then the symmetric equation for that line is given by equation 8: (X1 - X1A) / (X1B - X1A) = (Y2 – Y2a) / (Y2A – Y2B) (eq.8) wherein X1A and Y2A are the numerical values defining one point in the two-dimensional space X1-Y2, and X1B and Y2B are the numerical values defining another point in the two-dimensional space X1-Y2. Thus, equation 8 defines the linear correlation between the internal state parameter X1 and the output parameter Y2 at an operating point of the tumbling mill (See figure 37C). Figure 37C is a plot of a generated linear regression result. Thus, the plot of figure shows the line identifyed line 1180 that illustrates the relation between X1 and Y2, at least when that ball mill operates at Operating Points of 0 < X1 < 80 degrees, at a constant, or substantially constant speed of rotation fROT =18 rpm. Thus, the correlation data generator 1160 may deliver correlation data 1170 indicative of the linear relation 1180 when the linear regresssion analysis was applied to a single dependent variable Y2 and a single independent variable. With reference to figure 34, the correlation data set 1170, generated by correlator 150C1 may be delivered to an internal state reference value generator 150c2. The internal state reference value generator 150c2 may be configured to use the received correlation data 1170 for transforming a desired value Y _REF into a corresponding internal state reference value XREF. Table 8 is an illustration of an example of a data transformation table for transforming a desired value Y2REF into a corresponding internal state reference value X1 _REF . In fact, table 8 is an example data set corresponding to the information in table 7 above. Y2 _REF X1 _REF 195 => 62 198 => 63 201 => 64 204 _=> 65 207 _=> 66 210 => 67 213 => 68 216 => 69 219 => 70 222 => 71 225 => 72 228 => 73 231 => 74 234 _=> 75 Table 8: A correlation data set 1170 in the form of a correlation table for transforming a desired product property Y2REF into an internal state parameter reference value X1REF The example correlation data table 1170, an example of which is illustrated by table 8, indicates a correlation between internal status parameter value X1, indicative of a toe position, and output parameter Y2, indicative of a median size of particles produced by a tumbling mill. Based on equation 8 above and any two points (X1, Y2) or (X1 _REF , Y2 _REF ) from the correlation data set 1170, an linear equation defining the correlation may be generated. Expressing the dependent variable Y2 as a function of the independent variable X1, the line 1180 in figure 37C may be described by rewriting equation 8 into equation 9: Y2 = (Y2B - Y2A) / (X1B - X1A) * X1 - (Y2B - Y2A) / (X1B - X1A) * X1A + Y2A (Eq.9) With reference to figure 37C, the illustrated linear relation example 1180 comprises the two points: (X1 _A , Y2 _A ) = (40, 200), and (X1 _B , Y2 _B ) = (47, 220) Thus, by insertion of these example values in equation 9 and expressing the dependent variable Y2 as a function of the independent variable X1, the line 1180 in figure 37C may be described by the equation: Y2 = k1 * X1 + m1 = 2,86 X1 + 85,71 As mentioned above in connection with figure 34, the correlation data generator 1160 generates a correlation data set 1170. The correlation data set 1170 may be delivered in the form of a linear equation 1170 for transforming a desired value YREF into a corresponding internal state reference value X _REF . Again with reference to figure 37C and the illustrated linear relation example 1180 comprising the two points: (X1 _A , Y2 _A ) = (40, 200), and (X1 _B , Y2 _B ) = (47, 220) it is to be understood that equation 8 above may be re-written into equation 10 (below) so as to express a desired internal state value, i.e. an internal state reference value X1REF, as a function of the desired value Y2 _REF : X1REF= k2 * Y1REF + m2 (Eq.10) wherein k2= (X1 _B - X1 _A ) / (Y2 _B - Y2 _A ) m2= X1A - (X1B - X1A) / (Y2B - Y2A) * Y2A Accordingly, with reference to figure 34, the correlation data generator 1160 may generate the correlation data set 1170 in the form of a linear equation 1170 for transforming a desired value Y _REF into a corresponding internal state reference value X _REF , and the linear equation 1170 may, for example be as shown by equation 10 above. Figure 37D part I is an example of a graph that illustrates a density of toe position values X6 during a time period of running the mill. From the graph in Figure 37D part I it can be deduced that during this time period, the mill most often operated with a toe position value X6 in the range from 550 degrees 600 degrees. In this connection, a zero degree toe position value X6 may indicate an empty mill, and 720 degrees (= 2*360) may indicate the toe position ATOE shown in figure 2. As discussed above, the distance between two adjacent protrusions 310 may be indicated as a phase value of 360 degrees, and thus a toe position A _TOE as shown in figure 2 would correspond to about 720 degrees. The actual values for the parameter X6 will depend on the number of protrusions 310, also referred to as lifters 310. Figure 37D part II is an example of a graph that illustrates product the discharge rate RSDis, Y1 as dependent on the toe position values X6, ATOE (See figure 2). The grey dots 1183 indicate detected maximum values for the product discharge rate R _SDis , Y1 as dependent on toe position value X6. The curved black dotted line 1185 indicates an average of the values 1183 for the product discharge rate RSDis, Y1 as dependent on toe position value X6. From the graph 1185 in figure 37D part II it can be deduced that during measurements on this particular mill the product discharge rate RSDis, Y1 exhibited a maximim value Y1max for toe position values X6 in the range from about 550 degrees to about 600 degrees. The line 1188 indicates a toe position value X6 at or near the toe position value X6 that caused the highest product discharge rate value RSDis, Y1 of about 850 tons per hour. The grey dots 1187 indicate mean values for the all of the detected product discharge rate values RSDis, Y1 as dependent on toe position value X6. The grey dots 1187 are dense enough to form a line that indicates mean values for product discharge rate values RSDis, Y1 as dependent on toe position value X6. From the grey dots 1187 in figure 37D part II it can be deduced that the product discharge rate RSDis, Y1 quickly decreases for toe position values X6 in the range lower than about 500 degrees. Accordingly, a product discharge rate limit value Y1limit could be identified, and a corresponding toe position limit value X6limit can also be identified, based on the correlation. In summary, figure 37D part II is an illustration of a deteced correlation between toe position value X6 and product discharge rate RSDis, Y1. Accordingly, it is concluded that correlation as disclosed in this disclosure advantageously makes it possible to control the mill so as to operate with a toe position values X6 that optimizes the product discharge rate values R _SDis , Y1. Data used for figure 37D was based on product discharge rate R _SDis , Y1 that was estimated on the basis of data U6 indicative of the weight per time unit of the solid material 110 that is transported to the first input 100 of the mill 10, as discussed mor in detail above in connection with figure 34. A more complex case of a multi-variable monitoring system Figures 37B and 37C serve as illustration of the function of the correlation data generator 1160 in the relatively simple case of regression analysis applied to a single dependent variable Y2 and a single independent variable X1. In fact, the ball mill inherently appears to exhibit smaller variations in toe position than do non-ball mill tumbling mills, in particular when the ball mill is also operated a constant, or substantially constant speed of rotation U1, as discussed above. However, is also an object to be adressed by solutions and examples disclosed in this document, to describe methods and systems for improved monitoring and/or control of an internal state X in a tumbling mill 10 during operation. When the tumbling mill 10 runs at a varible speed of rotation X5 = U1 and it also exhibits variations in toe position X1, the above described regression analysis as applied to a single dependent variable Y2 and a single independent variable X1 may not suffice. In order to address this problem, however, the correlation data generator 1160 may apply regression analysis to a number of data pairs 1165 comprising a received internal state vector X(t) of dimension m and a received corresponding output vector Y(t) of dimension n, wherein m and n are positive integers. Thus, when m status parameter values X1, X2, X3,..., Xm are to be correlated with n product measurement values Y1, Y2, Y3, ... Yn, the correlation data generator 1160 may be configured to generate a correlation data 1170 set by performing correlation of a received internal state vector X(t) and a received corresponding output vector Y(t) wherein X(t) is a m*1 vector and m is a positive integer, and Y(t) is a n*1 vector and n is a positive integer. Accordingly, in this case the correlation data generator 1160 may be configured to perform a regression analysis so as to identify a more complex linear combination (i.e more complex than a line in a two-dimensional space) that most closely fits the data according to a specific mathematical criterion. For example, the correlation data generator 1160 may perform a method of ordinary least squares, applied to a number of received vectors X(t) of dimension m and a number of received corresponding output vectors Y(t) of dimension n, so as to compute a unique hyperplane that minimizes the sum of squared differences between the received data and that hyperplane. Figure 38 is an example illustration of a plane 1180B in a three-dimensional space X5, X1, Y2. Thus, figure 38 is an example illustration of a plane that was computed based on a method of ordinary least squares, applied to two received vectors X5(t) and X1(t) and one received corresponding output vectors Y2(t). The plane 1180B appears to illustrate particle size y2 being dependent both on the speed of rotation X5, f _ROT of the shell and on the position X1, X6 of the toe portion 205. In this connection it is noted that if the particle size depends on relative toe position X1, then it also depends on absolute toe position X6. Accordingly, the correlation data generator 1160, when receiving vectors X(t) of dimension m and a number of received corresponding output vectors Y(t) of dimension n, is configured to generate a multi-dimensional correlation data set 1170. According to an example, the multi-dimensional correlation data set 1170 may be delivered as data 1170 indicative of the above mentioned hyperplane. Alternatively, the multi-dimensional correlation data set 1170 may be delivered as data 1170 indicative of the coefficient matrix C, as discussed in relation to equation 7 above. According to an example, correlation data generator 1160 may be configured to include Kalman filtering, also known as linear quadratic estimation (LQE), when generating a correlation data set 1170. This solution advantageously enables identification and/or determination of a cause and effect relationship between the internal state X of the comminution process and the at least one output material measurement value Y. Moreover, this solution advantageously enables identification and/or determination of a cause and effect relationship between the internal state X of the comminution process and the discharge material state Y. The discharge material state Y may also be referred to as the product material state Y. This solution is versatile in that it allows for the defining of a desired discharge material state YREF, and for testing of alternative internal states, also referred to as operating points X _OP , of the comminution process in order to search and identify an internal state X _BEP of the comminution process that causes, or produces, the desired discharge material state YREF or that causes or produces a discharge material state Y as near as possible to the desired discharge material state YREF. Such an internal state may be referred to as a Best Operating Point, BEP. The values of the parameters at BEP may collectively be referred to as internal state BEP vector XBEP. Moreover, the recording of a detected momentary comminution process internal state X(r) in association with a corresponding momentary discharge material state Y(r), produces correlation data indicative of a correlation between a momentary comminution process internal state X(r) and a corresponding momentary discharge material state Y(r). By performing repeated recording of a number of mutually different detected momentary comminution process internal states X(r) in association with momentary discharge material states Y(r) that were caused by the respective momentary comminution process internal states X(r), wherein r is a number variable indicative of a number of different points in time, a correlation data set may be produced. Such a correlation data set is indicative of a correlation between a number of momentary comminution process internal states X(r) and a number of corresponding momentary discharge material states Y(r). The ball mill operating characteristic curve, or BMOC curve, of a ball mill is a graphical plot that illustrates the median size (Y2) of product particles generated by a ball mill when the toe position value is varied, such as figures 37 and 38. The BMOC curve is created by plotting the toe position value against the median size (Y2) of product particles at various toe positions. The tumbling mill operating point, or X _OP or TOP, is a specific point within the operation characteristic of a tumbling mill. It has been found that when the toe position value is varied within as certain range of toe position values around a particular tumbling mill operating point (XOP, TOP) there is a linear relation between the toe position value and the product particle size distribution (Y). In the context of this document, the term mill operation area (MOA) may be used to describe such a certain range of toe position values. The mill operating characteristic curve, or MOC curve, of a tumbling mill is a graphical plot that illustrates the product particle size distribution (Y) of product particles generated by a tumbling mill when at least one of the status parameter values (X1, X2, X3, X4, X5, X6,) is varied. Thus, for example, a MOC curve is created by plotting a measure of the product particle size distribution (Y) against the toe position value when e.g. the rotational speed (fROT) of the shell is kept constant. It has been found that the there is a linear relation between the median size (Y2) of product particles generated by a ball mill and the toe position value, at least when the toe position value X1 is varied within as certain range of toe position values. Referring again to figure 34, the internal state reference value generator 150c2 may be configured to use the received correlation data 1170 for transforming a desired value YREF into a corresponding internal state reference value XREF. In this connection it is noted that there may also be a linear relation between a single dependent variable such as the median size Y2 of product particles generated by a ball mill on the one hand, and on the other hand two independent variables, such as the toe position value, at least when the toe position value X1 is varied within as certain range of toe position values, and the rotational speed X5, f _ROT of the shell 20. In such a case the linear relation 1170 may be expressed by the equation of a plane in a three-dimensional space. Accordingly, with reference to figure 34, the correlation data generator 1160 may generate the correlation data set 1170 in the form of the equation of a plane in a three-dimensional space. The equation of a plane in a three-dimensional space, such as the space (X5, X2, Y2) can be expressed as equation 11: a * X5 + b* X2 + c * Y2 = 0 (Eq.11) The equation of a plane written in the form of equation 11 is called a linear equation in three-dimensional space. Use of the correlation data for operating a mill With reference to figure 34, an operator 230 in the control room 220 is tasked with operation of the the tumbling mill 10. The operator may use regulator 1190 for operating the mill 10. The regulator 1190 is coupled to the user interface 210, 210B also referred to as Human Computer Interface (HCI) 210B, as shown in figure 34. The example control room 220, shown in figure 34, includes an internal state control system 1200 comprising the internal state reference value generator 150c2 and the user interface 210, 210B and regulator 755C or regulator 240C. The internal state control system 1200 may be configured to perform the following steps: (Step S3000:) cause the user interface 210 to convey information requesting the operator to provide user input indicative of a desired discharge material state YREF. The user input indicative of a desired discharge material state YREF may be indicative of at least one desired output material measurement value, such as Y1 and/or Y2, as discused above. For example, the user input may be indicative of a desired product particle median size Y2 _REF , and/or desired product particle size distribution Y3 _REF , Y4 _REF , or a desired amount of output material per time unit Y1 _REF . This request, S3000, may be generated by software included in the regulator 755C, or by software included in the regulator 240C, or by software included in the internal state reference value generator 150c2. The internal state control system 1200 may also be configured to: (Step S3005:) receive, e.g. via user interface 210, data indicative of a desired discharge material state Y _REF and/or desired product particle median size Y2 _REF and/or desired product particle size distribution Y2, Y3, Y4. Moreover, the internal state control system 1200 may be configured to perform a method comprising the following steps: S3010: generate a toe position reference value (X1REF; FIREF) based on said data indicative of said desired discharge material state Y _REF and/or said desired product particle median size (Y2REF) and/or desired product particle size distribution Y2REF, Y3REF, Y4REF, and a correlation data set (1170); said correlation data set (1170) being indicative of a causal relationship between a certain toe position value (X1(r), FI(r), TD, RT(r); X6, ATOE(r) ) and a corresponding certain product particle median size (Y2), at said speed of shell rotation (U1, f _ROT ); and/or indicative of a causal relationship between a certain internal state XREF and a corresponding certain discharge material state Y _REF . The corresponding certain discharge material state YREF may include a certain product particle size distribution (Y2, Y3, Y4). The step S3010 may involve the delivery of the received data, from the user interface 210 to the internal state reference value generator 150c2 (See figure 34 and/or figure 35 and/or figure 39). The internal state reference value generator 150c2 is configured to transform data relating to desired discharge material state Y _REF into data indicative of a corresponding desired internal state X _REF and/or data indicative of a corresponding desired toe position reference value X1REF (r), FIREF (r), as discussed above. With reference to figure 34 in conjunction with figure 35, the internal state control system 1200 may also be configured to: S3020: cause the user interface (210, 210S, 240, 250) to convey information indicative of the corresponding desired internal state X _REF and/or data indicative of the corresponding desired toe position reference value (X1 _REF (r), FI _REF (r),and S3020: causing a user interface (210, 210S, 240, 250) to convey information indicative of an actual toe position value (X1(r), FI(r), TD, RT(r); X6, ATOE(r) ), e.g. received from the monitoring module 150A, S3020: receiving, via a user interface (210, 210S, 240, 250), first user input relating to said solid material feed rate (U2, RS ); S3020: generating a solid material feed rate set point value (U2 _SP , R _SSP ) thereby influencing said internal state (X) for controlling or affecting said desired discharge material state YREF product particle median size (Y2); wherein said generated solid material feed rate set point value (U2 _SP , R _SSP ) is based on said received first user input. According to an example, the generated solid material feed rate set point value U2 _SP , being based on said received first user input, causes the material in the rotating shell (20) to be tumbled at said influenced internal state (X) for causing the product particles to be generated with a product particle median size (Y2) corresponding to said influenced internal state (X) of said comminution process. System for monitoring and providing improved comminution process information content to an operator Figure 39 is a block diagram of the system 1130 for monitoring of an internal state X of a mill 10 and for providing improved information content to an operator 230 of the mill 10. The system 1130 includes a tumbling mill 10, as discussed in connection with figures 34 to 38 above. In Figure 39 the system 1130 is shown as a block diagram including a tumbling mill illustrated as a box 10 receiving a number of inputs U1, ... Uk, and generating a number of outputs Y1, ... Yn. Thus, in terms of signal processing and analysis, the mill 10 receives an input vector U, and it generates an output vector Y, in the manner discussed elsewhere in this document. The system 1130 of figure 39 may include parts, and be configured, as described above in relation to figure 1A and/or figure 1B and/or as described in any of the other examples described in this disclosure, e.g. in relation to figures 1-38. The system 1130 includes a Monitoring Module 150A and/or a Correlation Module 150C, as shown in fig.39. The Correlation Module 150C may operate to generate the correlation data set 1170 during operation of the mill 10, as described above, and/or Correlation Module 150C may operate to transform data relating to desired discharge material state YREF into data indicative of a corresponding desired internal state XREF, the transformation step being based on a correlation data set 1170 that is relevant for the mill 10 being operated. The system 1130 shown in figure 39, includes an internal state control system 1200 comprising the internal state reference value generator 150c2 and the user interface 210, 210B and regulator 240C. The internal state control system 1200 may be configured to perform the following steps: (Step S3000:) cause the user interface 210 to convey information requesting the operator to provide user input indicative of a desired discharge material state YREF. The user input indicative of a desired discharge material state YREF may be indicative of at least one desired output material measurement value, such as Y1 and/or Y2, as discused above. For example, the user input may be indicative of a desired product particle median size Y2REF, and/or desired product particle size distribution Y3REF, Y4 REF, or a desired amount of output material per time unit Y1 _REF . This request, S3000, may be generated by software included in the regulator 240C. The internal state control system 1200 may also be configured to: (Step S3005:) receive, e.g. via user interface 210, data indicative of a desired discharge material state YREF and/or desired product particle median size Y2REF and/or desired product particle size distribution Y2, Y3, Y4. Moreover, the internal state control system 1200 may be configured to perform a method comprising the following steps: S3010: generate a corresponding desired internal state X _REF (also referred to as internal state reference vector X _REF ) which may include a toe position reference value (X1 _REF ; FIREF). The internal state reference vector XREF may be based on said data indicative of said desired discharge material state Y _REF and/or said desired product particle median size (Y2 _REF ) and/or desired product particle size distribution Y2REF, Y3REF, Y4REF, and a correlation data set (1170); said correlation data set (1170) being indicative of a causal relationship between a certain internal state XREF and a corresponding certain discharge material state YREF. The corresponding certain discharge material state YREF may include a certain product particle size distribution (Y2, Y3, Y4), and/or a certain product discharge rate Y1REF. The step S3010 may involve the delivery of the received data (i.e. indicative of a desired discharge material state YREF), from the user interface 210 to the Correlation Module 150C (See figure 39). The Correlation Module 150C may include an internal state reference value generator 150c2 configured to transform data relating to desired discharge material state YREF into data indicative of a corresponding desired internal state XREF and/or data indicative of a corresponding desired toe position reference value X1 _REF (r), FI _REF (r), as discussed above. With reference to figure 39 in conjunction with figure 35, the internal state control system 1200 may also be configured to: S3020: cause the user interface (210, 210S, 240, 250) to convey information indicative of the corresponding desired internal state X _REF and/or data indicative of the corresponding desired toe position reference value (X1REF (r), FIREF (r),and S3020: causing a user interface (210, 210S, 240, 250) to convey information indicative of an actual toe position value (X1(r), FI(r), T _D , R _T (r); X6, A _TOE (r) ), e.g. received from the monitoring module 150A, S3020: receiving, via a user interface (210, 210S, 240, 250), first user input relating to said solid material feed rate (U2, R _S ); S3020: generating a solid material feed rate set point value (U2 _SP , R _SSP ) thereby influencing said internal state (X) for controlling or affecting said desired discharge material state YREF product particle median size (Y2); wherein said generated solid material feed rate set point value (U2SP, RSSP) is based on said received first user input. According to an example, the generated solid material feed rate set point value U2SP, being based on said received first user input, causes the material in the rotating shell (20) to be tumbled at said influenced internal state (X) for causing the product particles to be generated with a product particle median size (Y2) corresponding to said influenced internal state (X) of said comminution process. System for Monitoring Mill Product and providing improved Process Control Figure 40 is a block diagram of a system 1130B for monitoring of an internal state X of a mill 10 and for enabling improved control of a comminition process that occurs in a mill 10. The system 1130B may include some, or all, of the features discussed in connection with figure 39. Thus, the system 1130B may include some, or all, of the features of system 1130 of figure 39. The system 1130B includes a Correlation Module 150C, as shown in fig.39, and system 1130B may also include a Monitoring Module 150A. The Correlation Module 150C may operate to generate the correlation data set 1170 during operation of the mill 10, as described above, and/or Correlation Module 150C may operate to transform data relating to desired discharge material state YREF into data indicative of a corresponding desired internal state X _REF , the transformation step being based on a correlation data set 1170 that is relevant for the mill 10 being operated. The system 1130 shown in figure 39, includes an internal state control system 1200 comprising the internal state reference value generator 150c2 and the user interface 210, 210B and regulator 240C. The system 1130B may be configured to perform the following steps: (Step S3000:) cause the user interface 210 to convey information requesting the operator to provide user input indicative of a desired discharge material state YREF. The user input indicative of a desired discharge material state Y _REF may be indicative of at least one desired output material measurement value, such as Y1 and/or Y2, as discussed above. For example, the user input may be indicative of a desired product particle median size Y2REF, and/or desired product particle size distribution Y3REF, Y4 REF, or a desired amount of output material per time unit Y1 _REF . This request, S3000, may be generated by software included in the regulator 150B, or by software included in the Correlation Module 150C, or by internal state control system 1200. The system 1130B may also be configured to: (Step S3005:) receive, e.g. via user interface 210, data indicative of a desired discharge material state Y _REF and/or desired product particle median size Y2 _REF and/or desired product particle size distribution Y2, Y3, Y4. Moreover, the system 1130B may be configured to perform a method comprising the following steps: S3010: generate a corresponding desired internal state XREF, also referred to as internal state reference vector X _REF ) which may include a toe position reference value (X1 _REF ; FIREF). The internal state reference vector XREF may be based on said data indicative of said desired discharge material state YREF and/or said desired product particle median size (Y2REF) and/or desired product particle size distribution Y2REF, Y3REF, Y4REF, and a correlation data set (1170); said correlation data set (1170) being indicative of a causal relationship between a certain internal state X _REF and a corresponding certain discharge material state Y _REF . The corresponding certain discharge material state YREF may include a certain product particle size distribution (Y2, Y3, Y4), and/or a certain product discharge rate Y1 _REF . The step S3005 may involve the delivery of the received data (i.e. indicative of a desired discharge material state Y _REF ), from the user interface 210 to the Correlation Module 150C (See figure 40). The Correlation Module 150C may include an internal state reference value generator 150c2 configured to transform data relating to desired discharge material state Y _REF into data indicative of a corresponding desired internal state XREF and/or data indicative of a corresponding desired toe position reference value X1REF (r), FIREF (r), as discussed above. Moreover, the system 1130B may be configured to perform a method comprising the following steps: controlling via a regulator 755C, 755 said discharge material state (Y) based on said at least one status parameter reference value (X1REF; FIREF) included in an internal state reference vector X _REF , at least one status parameter value (X1, X2, X3, X4, X5, X6, X7 ) or an internal state vector (X) including said at least one status parameter value indicative of a current internal state (X) of the comminution process, and at least one status parameter error value (X1 _ERR , X2 _ERR , X3 _ERR , X4 _ERR , X5ERR, X6ERR, X7ERR) or an internal state error vector X ERR including said at least one status parameter error value, wherein said at least one status parameter error value (X1ERR, X2ERR, X3ERR, X4ERR, X5ERR, X6ERR, X7ERR) depends on said at least one status parameter reference value (X1REF; FIREF), and said at least one status parameter value (X1, X2, X3, X4, X5, X6, X7 ). Moreover, the system 1130B may be configured to perform a method comprising the following steps: controlling via a regulator 755C, 755 said discharge material state (Y) based on an internal state reference vector XREF indicative of a current internal state (X) of the comminution process, and an internal state vector (X) indicative of a current internal state (X) of the comminution process, and an internal state error vector X _ERR including at least one status parameter error value, wherein said internal state error vector X ERR depends on said internal state reference vector X _REF , and said internal state vector (X). Moreover, the system 1130B may be configured to perform a method comprising the following steps: receiving, via a user interface (210, 210S, 240, 250), a first user input relating to said solid material feed rate (U2, RS ); and generating said solid material feed rate set point value (U2 _SP , R _SSP ); wherein said generated data indicative of solid material feed rate set point value (U2SP, RSSP) is based on said received first user input. Any individual step of the methods disclosed below could be carried out or reiterated simultaneously to another individual step or steps of the method, unless it would involve a logical contradiction. In continuous operation of the tumbling mill, the steps are typically reiterated continuously, such as sequentially in a loop. Although actions defined in this method are typically performed as such steps, they could be carried out according to this disclosure as actions in a more general sense than steps. Figure 41 illustrates in a flowchart a method of operating a tumbling mill, such as the tumbling mill 10 (with or without the system 1120, 5, 720, 730 that includes the tumbling mill 10, as a particular context may imply) described herein. Thus, this method, including all tumbling mill features or aspects thereof, can be understood with reference to the tumbling mill 10, and in particular according to explanations and elaborations made herein with references to figures 34 – 40. However, it is foreseen that this method could be applicable to a tumbling mill in a more general sense, as long as at least the tumbling mill features or aspects, to which the steps of this method relate, are present in the tumbling mill in question. This method comprises the following steps. Monitoring S4101 a toe position of a first portion of material inside the mill, wherein the toe position is determined responsive to signals measured from a vibration sensor. Determining S4102 a measure of a discharge property of a second portion of material discharged from the mill. Correlating S4103 the toe position with the measure of discharge property. Storing S4104 the correlation, i.e. a result of the correlating S4103, in a non-transitory electronic memory for future use in the control of an operation parameter of the mill based on the stored correlation. The method according to figure 41 may also comprise one or more of the following. The monitored toe position is at steady state. The measure of output particle size is determined when the toe position is at steady state or substantially constant. Steady state is achieved when an amount or a rate of input material entering the mill is equal to, or substantially equal to, an amount or a rate of material leaving the mill. The toe position of the material inside the rotating container is determined responsive to the vibration signal and a position signal. The position signal is indicative of a rotational position of a rotating container. The discharge property comprises an output particle size corresponding to said second portion of material discharged from the mill. Figure 42 illustrates in a flowchart another method of operating a tumbling mill, such as the tumbling mill 10 (with or without the system 1120, 5, 720, 730 that includes the tumbling mill 10, as a particular context may imply) described herein. Thus, this method, including all tumbling mill features or aspects thereof, can be understood with reference to the tumbling mill 10, and in particular according to explanations and elaborations made herein with references to figures 34 – 40. However, it is foreseen that this method could be applicable to a tumbling mill in a more general sense, as long as at least the tumbling mill features or aspects, to which the steps of this method relate, are present in the tumbling mill in question. This method comprises the following steps. Monitoring S4201 a toe position of a first portion of material inside the mill, wherein the toe position is determined responsive to signals measured from a vibration sensor. Determining S4202 a measure of a discharge property of a second portion of material discharged from the mill. Correlating S4203 the toe position with the measure of discharge property. Storing S4204 the correlation, i.e. a result of the correlating S4203, in a non-transitory electronic memory for future use in the control of an operation parameter of the mill based on the stored correlation. Displaying S4205, on a user interface, the determined operation parameter as a suggestion to a user, which step of displaying S4205 is related to the above description of figure 39. Operating S4206 the tumbling mill based on manual input, may in itself be seen as a further method step, as shown in figure 42, or just as a categorization of this particular method. The steps of this method may be repeated in a loop as indicated in figure 42. Figure 43 illustrates in a flowchart another method of operating a tumbling mill, such as the tumbling mill 10 (with or without the system 1120, 5, 720, 730 that includes the tumbling mill 10, as a particular context may imply) described herein. Thus, this method, including all tumbling mill features or aspects thereof, can be understood with reference to the tumbling mill 10, and in particular according to explanations and elaborations made herein with references to figures 34 – 40. However, it is foreseen that this method could be applicable to a tumbling mill in a more general sense, as long as at least the tumbling mill features or aspects, to which the steps of this method relate, are present in the tumbling mill in question. This method comprises the following steps. Monitoring S4301 a toe position of a first portion of material inside the mill, wherein the toe position is determined responsive to signals measured from a vibration sensor. Determining S4302 a measure of a discharge property of a second portion of material discharged from the mill. Correlating S4303 the toe position with the measure of discharge property. Storing S4304 the correlation, i.e. a result of the correlating, in a non-transitory electronic memory for future use in the control of an operation parameter of the mill based on the stored correlation. Delivering S4305 a signal to a regulator to change the operation of the mill to correspond to the determined operation parameter, which step of delivering S4305 is related to the above description of figure 40. Operating S4306 the tumbling mill by automatic regulation, may in itself be seen as a further method step, as shown in figure 43, or just as a categorization of this particular method. The steps of this method may be repeated in a loop as indicated in figure 43. Figure 44 illustrates in a flowchart a method of operating a tumbling mill, such as the tumbling mill 10 (with or without the system 1120, 5, 720, 730 that includes the tumbling mill 10, as a particular context may imply) described herein. Thus, this method, including all tumbling mill features or aspects thereof, can be understood with reference to the tumbling mill 10, and in particular according to explanations and elaborations made herein with references to figures 34 – 40. However, it is foreseen that this method could be applicable to a tumbling mill in a more general sense, as long as at least the tumbling mill features or aspects, to which the steps of this method relate, are present in the tumbling mill in question. This method comprises the following steps. Monitoring S4401 a toe position of a first portion of material inside the mill, wherein the toe position is determined responsive to signals measured from a vibration sensor. Determining S4402 a measure of a discharge property of a second portion of material discharged from the mill. Receiving S4403 an input corresponding to a desired discharge property of discharge material. Determining S4404 an operation parameter of the mill based on the determined toe position, determined in the step of monitoring S4401, and the received desired discharge property, received in the step of receiving S4403, and a predetermined correlation between toe position and discharge property. Further, the operation parameter may comprise a rotation speed of the tumbling mill and/or the operation parameter may comprise one or more input material parameters. The method according to figure 44 may also comprise one or more of the following. The one or more input material parameters comprise an amount or a rate of solid material entering the mill. The one or more input material parameters comprise an amount or a rate of liquid material entering the mill. The monitored toe position is at steady state. The measure of output particle size is determined when the toe position is at steady state or substantially constant. Steady state is achieved when an amount or a rate of input material entering the mill is equal to, or substantially equal to, an amount or a rate of material leaving the mill. The toe position of the material inside the rotating container is determined responsive to the vibration signal and a position signal. The position signal is indicative of a rotational position of a rotating container. The discharge property comprises an output particle size corresponding to said second portion of material discharged from the mill. The predetermined correlation is a stored correlation that is provided according to the method of figure 41 above. Figure 45 illustrates in a flowchart another method of operating a tumbling mill, such as the tumbling mill 10 (with or without the system 1120, 5, 720, 730 that includes the tumbling mill 10, as a particular context may imply) described herein. Thus, this method, including all tumbling mill features or aspects thereof, can be understood with reference to the tumbling mill 10, and in particular according to explanations and elaborations made herein with references to figures 34 – 40. However, it is foreseen that this method could be applicable to a tumbling mill in a more general sense, as long as at least the tumbling mill features or aspects, to which the steps of this method relate, are present in the tumbling mill in question. This method comprises the following steps. Monitoring S4501 a toe position of a first portion of material inside the mill, wherein the toe position is determined responsive to signals measured from a vibration sensor. Determining S4502 a measure of a discharge property corresponding to a second portion of material discharged from the mill. Receiving S4503 an input corresponding to a desired discharge property of discharge material. Using S4504 the desired discharge property YREF to retrieve a correlated internal state reference value XREF from non-transitory electronic memory storage, wherein the step of using S4504 may be seen merely as a subroutine to the step of receiving S4503 or, as shown in figure 45, an individual step of the method. Determining S4505 an operation parameter of the mill based on the determined toe position, determined in the step of monitoring S4501, and the received desired discharge property, received in the step of receiving S4503, and a predetermined correlation between toe position and discharge property. Displaying S4506, on a user interface, the determined operation parameter as a suggestion to a user, which step of displaying S4506 is related to the above description of figure 39. Operating the tumbling mill based on manual input S4507, may in itself be seen as a further method step, as shown in figure 45, or just as a categorization of this particular method. The steps of this method may be repeated in a loop as indicated in figure 45. Further, the operation parameter may comprise a rotation speed of the tumbling mill and/or the operation parameter may comprise one or more input material parameters. The method according to figure 45 may also comprise one or more of the following. The one or more input material parameters comprise an amount or a rate of solid material entering the mill. The one or more input material parameters comprise an amount or a rate of liquid material entering the mill. The monitored toe position is at steady state. The measure of output particle size is determined when the toe position is at steady state or substantially constant. Steady state is achieved when an amount or a rate of input material entering the mill is equal to, or substantially equal to, an amount or a rate of material leaving the mill. The toe position of the material inside the rotating container is determined responsive to the vibration signal and a position signal. The position signal is indicative of a rotational position of a rotating container. The discharge property comprises an output particle size corresponding to said second portion of material discharged from the mill. The predetermined correlation is a stored correlation that can be provided according to the method of figure 41 above. Figure 46 illustrates in a flowchart another method of operating a tumbling mill, such as the tumbling mill 10 (with or without the system 1120, 5, 720, 730 that includes the tumbling mill 10, as a particular context may imply) described herein. Thus, this method, including all tumbling mill features or aspects thereof, can be understood with reference to the tumbling mill 10, and in particular according to explanations and elaborations made herein with references to figures 34 – 40. However, it is foreseen that this method could be applicable to a tumbling mill in a more general sense, as long as at least the tumbling mill features or aspects, to which the steps of this method relate, are present in the tumbling mill in question. This method comprises the following steps. Monitoring S4601 a toe position of a first portion of material inside the mill, wherein the toe position is determined responsive to signals measured from a vibration sensor. Determining S4602 a measure of a discharge property corresponding to a second portion of material discharged from the mill. Receiving S4603 an input corresponding to a desired discharge property of discharge material. Using S4604 the desired discharge property YREF to retrieve a correlated internal state reference value XREF from non-transitory electronic memory storage, wherein the step of using S4604 may be seen merely as a subroutine to the step of receiving S4603 or, as shown in figure 46, an individual step of the method. Determining S4605 an operation parameter of the mill based on the determined toe position, determined in the step of monitoring S4601, and the received desired discharge property, received in the step of receiving S4603, and a predetermined correlation between toe position and discharge property. Delivering S4606 a signal to a regulator to change the operation of the mill to correspond to the determined operation parameter, which step of delivering S4606 is related to the above description of figure 40. Operating S4607 the tumbling mill by automatic regulation, may in itself be seen as a further method step, as shown in figure 46, or just as a categorization of this particular method. The steps of this method may be repeated in a loop as indicated in figure 46. Further, the operation parameter may comprise a rotation speed of the tumbling mill and/or the operation parameter may comprise one or more input material parameters. The method according to figure 46 may also comprise one or more of the following. The one or more input material parameters comprise an amount or a rate of solid material entering the mill. The one or more input material parameters comprise an amount or a rate of liquid material entering the mill. The monitored toe position is at steady state. The measure of output particle size is determined when the toe position is at steady state or substantially constant. Steady state is achieved when an amount or a rate of input material entering the mill is equal to, or substantially equal to, an amount or a rate of material leaving the mill. The toe position of the material inside the rotating container is determined responsive to the vibration signal and a position signal. The position signal is indicative of a rotational position of a rotating container. The discharge property comprises an output particle size corresponding to said second portion of material discharged from the mill. The predetermined correlation is a stored correlation that is provided according to the method of figure 41 above. The methods disclosed above with reference to the figures 41 – 46 may be performed at least partly by a computer including a processor 350 and an associated memory 360. These may be coupled to and controlled via a graphical user interface 210, 210S. Various examples are disclosed below, starting with example 1. An example 1 relates to a system 5 for grinding material, the system comprising: a tumbling mill having a shell that rotates around an axis ( ) at a speed of rotation (fROT) for grinding material by tumbling a charge of the material in the rotating shell; wherein said shell has an internal shell surface, said internal shell surface including at least one protrusion configured to engage material within the shell; a vibration sensor configured to generate an analogue measurement signal (SEA) dependent on mechanical vibrations (V _IMP ) emanating from rotation of said shell; a position sensor configured to generate a position signal indicative of a rotational position of said rotating shell; a signal recorder adapted to record - a time sequence of measurement sample values (Se(i), S(j)) of said digital measurement data signal (SMD, SENV, SMD), and - a time sequence of said position signal values (P(i)), and - time information (i, dt; j) such that an individual measurement data value (S(j)) is associated with data indicative of time of occurrence of the individual measurement data value (S(j)), and such that an individual position signal value (P(i)) is associated with data indicative of time of occurrence of the individual position signal value (P(i)); a signal processor adapted to detect the occurrence of an amplitude peak value in said recorded time sequence of measurement sample values (Se(i), S(j)); said signal processor being adapted to generate data indicative of a temporal duration between said position signal value occurrence and said amplitude peak value occurrence. 2. The system of example 1, wherein said signal processor is configured to generate a shell charge data set, said shell charge data set being indicative of an internal charge state in said shell; said shell charge data set comprising said amplitude peak value and said temporal duration. 3. The system according any preceding example, wherein said shell charge data set being indicative of a speed of rotation of said rotating mill shell. 4. The system according any preceding example, wherein the rotating shell is configured to hold a charge of material exceeding 500 kg during operation of the tumbling mill. An example 5 relates to an electronic tumbling mill monitoring system for generating and displaying information relating to an internal state of a grinding process in a tumbling mill (10) having a shell that rotates around an axis (60) at a speed of rotation (f _ROT ) for grinding charge material (30) by tumbling the charge material in the rotating shell, the tumbling mill monitoring system comprising: a status parameter extractor (450) for generating a a first internal status indicator data structure (550, S _P1 , T _D1 ), indicative of said internal state of said grinding process, said first internal status indicator data structure (550, S _P1 , T _D1 ) including a first impact force indicator value (S _P1 ) and a first temporal indicator value (T _D1 ) ; said first impact force indicator value (SP1) being indicative of an impact force (FIMP) generated when a projection on an internal shell surface of the rotating shell interacts with a toe portion of the charge material, and said first temporal indicator value (TD1) being indicative of a temporal duration (TD1) between occurrence of said impact force (FIMP) and occurrence of a rotational reference position of said rotating shell. 6. The tumbling mill monitoring system according to example 5, wherein said status parameter extractor (450) further generates a second internal status indicator data structure (S _P2 , T _D2 ) , indicative of said internal state of said grinding process, said second internal status indicator data structure (550, S _P1 , T _D1 ) including a second impact force indicator value (S _P2 ) and a second temporal indicator value (T _D2 ) said second impact force indicator value (SP2 ) being indicative of an impact force (FIMP) generated when a projection on an internal shell surface of the rotating shell interacts with a toe portion of the charge material, and said second temporal indicator value (TD2) being indicative of a temporal duration (TD1) between occurrence of said impact force (FIMP) and occurrence of a rotational reference position of said rotating shell; wherein said first internal status indicator data structure (SP1, TD1) is indicative of said internal state of said grinding process at a first point in time, and said second internal status indicator data structure (SP2, TD2) is indicative of said internal state of said grinding process at a second point in time. 7. The tumbling mill monitoring system according to example 6, wherein said first internal status indicator data structure (S _P1 , T _D1 ) in conjunction with said internal status indicator data structure (SP2, TD2) is indicative of a temporal progression of said internal state of said grinding process. 8. The tumbling mill monitoring system according to any preceding example, wherein said status parameter extractor (450) includes a shell speed detector (500) configured to generate a value indicative of a tumbling mill shell speed of rotation (f _ROT (j)) based on a digital position signal (P(i)), said shell speed detector (500) being configured to associate said value indicative of a tumbling mill shell speed of rotation (fROT(i)) with a point of time (i). 9. The tumbling mill monitoring system according to any preceding example, wherein said shell speed detector (500) is configured to associate said first impact force indicator value (S _P1 ; (S(i)) with said value indicative of a tumbling mill shell speed of rotation (fROT(j)). 10. The tumbling mill monitoring system according to any preceding example, wherein said status parameter extractor (450) is configured to maintain a synchronized temporal relation between said first impact force indicator value (S _P1 ; (S(i); S(j)) and said value indicative of a tumbling mill shell speed of rotation (f _ROT (i); f _ROT (j)). Example 11: In an electronic tumbling mill monitoring system for generating and displaying information relating to a grinding process in a tumbling mill having a shell that rotates around an axis (60) at a speed of rotation (fROT) for grinding charge material (30) by tumbling the charge material in the rotating shell; wherein said shell has an internal shell surface, said internal shell surface including at least one projection configured to engage material as the shell rotates about the axis (60), a computer implemented method of representing an internal state of said grinding process in said tumbling mill on a screen display, the method comprising: displaying on said screen display a polar coordinate system, said polar coordinate system having a reference point (O), and a reference direction (0,360); and a first internal status indicator object (S _P1 , T _D1 ), indicative of said internal state of said grinding process, at a first radius (S _P1 ) from said reference point (O) and at a first polar angle (TD1) in relation to said reference direction (0,360), said first radius (S _P1 ) being indicative of an impact force (F _IMP ) generated when a projection on an internal shell surface of the rotating shell interacts with the charge material, and said first polar angle (TD1) being indicative of a temporal duration (TD1) between occurrence of said impact force (F _IMP ) and occurrence of a rotational reference position of said rotating shell. 12. The method according to example 11, wherein the method further comprises displaying on said screen display a second internal indicator object (SP2, TD2) at a second radius (SP2) from said reference point (O) and at a second polar angle (TD1) in relation to said reference direction (0,360), said second radius (SP2) being indicative of an impact force (SP; FIMP) generated when a projection on an internal shell surface of the rotating shell interacts with the charge material, and said second polar angle (TD1) being indicative of a temporal duration (TD1) between occurrence of said impact force (FIMP) and occurrence of a rotational reference position of said rotating shell; wherein said first internal indicator object (SP1, TD1) is indicative of said internal state of said grinding process at a first point in time, and said second internal indicator object (SP1, TD1) is indicative of said internal state of said grinding process at a second point in time. 13. The method according to example 12, wherein a simultaneous displaying on said screen display of said first internal state point (SP1, TD1) and said second internal state point (SP1, T _D1 ) is indicative of a temporal progression of said internal state of said grinding process. An example 14 relates to an electronic tumbling mill monitoring system for generating and displaying information relating to an internal state of a grinding process in a tumbling mill (10) having a shell that rotates around an axis (60) at a speed of rotation (f _ROT ) for grinding charge material (30) by tumbling the charge material in the rotating shell, the tumbling mill monitoring system comprising: a status parameter extractor (450) for generating a first internal status indicator data structure (550, S _P1 , T _D1 ), indicative of said internal state of said grinding process, said first internal status indicator data structure (550, SP1, TD1) including a first impact force indicator value (SP1) and a first temporal indicator value (P; T _D1 ) ; said first impact force indicator value (SP1) being indicative of an impact force (FIMP) generated when a projection on an internal shell surface of the rotating shell interacts with a toe portion of the charge material, and said first temporal indicator value (TD1) being indicative of a temporal duration (TD1) between occurrence of said impact force (FIMP) and occurrence of a rotational reference position of said rotating shell; wherein said status parameter extractor (450) includes a shell speed detector (500) configured to generate a value indicative of a tumbling mill shell speed of rotation (f _ROT (j)) based on a digital position signal (P(i)), said shell speed detector (500) being configured to associate said value indicative of a tumbling mill shell speed of rotation (fROT(i)) with a point of time (i). 15. The tumbling mill monitoring system according to any preceding example, wherein said shell speed detector (500) is configured to associate said first impact force indicator value (SP1; S(j)) with said value indicative of the tumbling mill shell speed of rotation (f _ROT (j)) so that said speed of rotation (f _ROT (j)) value indicates said tumbling mill shell speed of rotation (fROT(j)) at the point of time (j) of occurrence of said impact force (FIMP). 16. The tumbling mill monitoring system according to any preceding example, wherein said status parameter extractor (450) is configured to generate a temporal progression of vibration signal values (S(i)) and a temporal progression of rotational reference position signals; said status parameter extractor (450) further comprising a speed variation compensatory decimator (470); the decimator (470) being configured to decimate the temporal progression of vibration signal values (S(i); S _MD ) based on the speed value (f _ROT (j) so as to generate a decimated vibration signal (S _MDR ) comprising a decimated temporal progression of vibration signal values (R(q); SP(r)). 17. The tumbling mill monitoring system according to any preceding example, wherein said status parameter extractor (450) further comprises a fast Fourier transformer (510) configured to generate said first impact force indicator value (S _P1 ) and said first temporal indicator value (T _D1 ) based on said decimated vibration signal (SMDR). 18. The system according to any preceding example, wherein said material includes pieces of said material, said material pieces comprising a mineral. 19. The system according to any preceding example, wherein said tumbling mill (10) operates to perform dry grinding. 20. The system according to any preceding example, wherein said tumbling mill (10) operates to perform dry grinding of particles of a hard substance into a powder including cement. An example 21 relates to a method for generating information relating to an internal state of a tumbling mill (10) having a shell (20) that rotates around an axis (60) at a speed of rotation (f _ROT ) for grinding a charge of material (30) by tumbling the material in the rotating shell; said shell (20) having an internal shell surface (22) including a first number (L) of protrusions (310) configured to engage material as the shell (20) rotates about the axis (60), the method comprising generating a position signal (E, P, P(i), P(j), P(q)) indicative of a rotational position of said rotating shell (20), said position signal including a time sequence of position signal sample values (P(i), P(j), P(q)); detecting a first occurrence of a first reference position signal value (1; 1C, 0%) in said time sequence of position signal sample values (P(i), P(j), P(q)); detecting a second occurrence of a second reference position signal value (1; 1C; 100%) in said time sequence of position signal sample values (P(i), P(j), P(q)); generating a vibration signal (S _EA , Se(i), S(j), S(q)) dependent on mechanical vibrations (VIMP) emanating from rotation of said shell, said vibration signal (SEA, Se(i), S(j), S(q)) including a time sequence of vibration sample values (Se(i), S(j), S(q)); detecting a third occurrence of an event signature (S _P (r); S _P ) in said time sequence of vibration sample values (Se(i), S(j), S(q)); generating data indicative of a first temporal relation (RT(r); TD; FI(r)) between said third occurrence i.e. said event signature occurrence, and said first and second occurences. 22. The method according to any preceding example, wherein: said first temporal relation (RT(r); TD; FI(r)) is indicative of a proportion of a distance between two adjacent of said protrusions (310). 23. The method according to any preceding example, wherein: Said first temporal relation (RT(r); TD; FI(r)) is indicative of a relative toe position (205). 24. The method according to any preceding example, wherein: said event signature is indicative of an impact force (FIMP) generated when a projection (310) on an internal shell surface (22) of the rotating shell (20) interacts with a toe portion (205) of the charge material (30). 25. The method according to any preceding example, further comprising: generating said first temporal relation (RT(r); TD; FI(r)) as a phase angle (FI(r)). 26. The method according to any preceding example, further comprising: generating said event signature as an amplitude value (S _P (r); S _p ; C _L (r); C ₁ (r)). 27. The method according to any preceding example, wherein: Said first temporal relation (R _T (r); T _D ; FI(r)) is generated by a Fourier Transformation. 28. The method according to any preceding example, further comprising: Counting a total number of samples (N _B ) from the first occurrence to the second occurrence, and Counting another number of samples (NP) from the first occurrence to the third occurrence, and generating said first temporal relation (R _T (r); T _D ; FI(r)) based on said another number and said total number. 29. The method according to any preceding example, further comprising: Counting a total number of samples (NB) from the first occurrence to the second occurrence, and Counting another number of samples (N _P ) from the first occurrence to the third occurrence, and generating said first temporal relation (RT(r); TD; FI(r)) based on a relation between said another number and said total number. 30. The method according to example 29, wherein: Said relation between said another number and said total number is indicative of a relative toe position (205). 31. The method according to example 29 or 30, wherein: Said relation between said another number and said total number is indicative of a position of a toe (205) of said charge (30) expressed as a portion of a distance between two of said protrusions (310), said two protrusions (310A, 310B) being mutually adjacent on said internal shell surface (22). 32. The method according to any preceding example, further comprising: generating said reference position signal value (1; 1C, 0%) at least one time per revolution of said rotating shell (20). 33. The method according to example 32, further comprising: generating said reference position signal value (1; 1C, 0%) a second number of times per revolution of said rotating shell (20); said second number being equal to said first number (L). 34. The method according to example 32, further comprising: generating said reference position signal value (1; 1C, 0%) a second number of times per revolution of said rotating shell (20); said second number being lower than said first number (L). 35. The method according to any preceding example, further comprising: generating said reference position signal value (PS; 1; 1C, 0%) based on detection of a rotating position marker (180), wherein the rotation of said rotating position marker (180) is indicative of the rotation of said rotating shell (20). 36. The method according to example 32, wherein said reference position signal value (1; 1C, 0%) being generated at least one time per revolution of said rotating shell (20) is based on detection of a rotating position marker (180), wherein the rotation of said rotating position marker (180) is indicative of the rotation of said rotating shell (20). 37. The method according to example 36, wherein at least one of said first reference position signal value (1; 1C, 0%) and said second reference position signal value (1; 1C; 100%) is generated by calculation based on said first number (L). 38. The method according to example 36, wherein at least one of said first reference position signal value (1; 1C, 0%) and said second reference position signal value (1; 1C; 100%) is generated at an angular position; wherein a full revolution of said shell is virtually or mathematically divided into a third number of mutually equal parts. 39. The method according to example 38, wherein Said third number is equal to said first number; and wherein said mutually equal parts correspond to a first number of equal distances between said protrusions (310). 40. The method according to any preceding example, wherein: Said protrusions are mutually substantially equidistant. 41. The method according to any preceding example, further comprising: recording said time sequence of vibration sample values (Se(i), S(j), S(q)); detecting the occurrence of said event signature in said recorded time sequence of vibration sample values (Se(i), S(j), S(q)). 42. The method according to any preceding example, wherein: Said event signature is an amplitude peak value. 43. The method according to any preceding example, further comprising: associating an individual vibration sample value (Se(i), S(j), S(q)) with an individual position signal sample value (P(i), P(j), P(q)). 44. The method according to any preceding example, further comprising: generating data indicative of a momentary rotational speed value based on a second temporal relation (R _T (r); T _D ; FI(r)) between said first occurrence of said first reference position signal value (1; 1C, 0%) and said second occurrence of said second reference position signal value (1; 1C; 100%); said momentary rotational speed value (f _ROT ) being indicative of said speed of rotation (fROT). 45. The method according to any preceding example, further comprising: recording, in a memory, said time sequence of position signal sample values (P(i), P(j), P(q)); and recording, in said memory, said time sequence of vibration sample values (Se(i), S(j), S(q)); wherein said step of detecting the occurrence of a reference position signal value (1; 1C) involves detecting the occurrence of said reference position signal value (1; 1C) in said recorded time sequence of position signal sample values (P(i), P(j), P(q)). 46. The method according to any preceding example, wherein: Said first temporal relation (R _T (r); T _D ; FI(r)) is indicative of a first internal state of said tumbling mill. 47. The method according to any preceding example, wherein: Said first temporal relation (RT(r); TD; FI(r)) is indicative of a first internal state of said tumbling mill. 48. The method according to any preceding example, further comprising: Generating data indicative of an absolute toe position value based on said relative toe position value. 49. The method according to any preceding example, wherein: said event signature is a peak amplitude value. 50. The method according to any preceding example, wherein: Said speed of rotation (f _ROT ) is a variable speed of rotation (f _ROT ). An example 51 relates to a system for grinding material, the system comprising: a tumbling mill having a shell that rotates around an axis (60) at a speed of rotation (f _ROT ) for grinding material by tumbling a charge of the material in the rotating shell; wherein said shell has an internal shell surface, said internal shell surface including a first number of protrusions configured to engage material within the shell, said protrusions being arranged at equal mutual distances on said internal shell surface; said first number being at least two; a vibration sensor configured to generate an analogue measurement signal (SEA) dependent on mechanical vibrations (V _IMP ) emanating from rotation of said shell; a position sensor configured to generate a position signal indicative of a rotational position of said rotating shell; a signal recorder adapted to record - a time sequence of measurement sample values (Se(i), S(j)) of said digital measurement data signal (SMD, SENV, SMD), and - a time sequence of said position signal values (P(i)), and - time information (i, dt; j) such that an individual measurement data value (S(j)) is associated with data indicative of time of occurrence of the individual measurement data value (S(j)), and such that an individual position signal value (P(i)) is associated with data indicative of time of occurrence of the individual position signal value (P(i)); a signal processor adapted to detect the occurrence of an amplitude peak value in said recorded time sequence of measurement sample values (Se(i), S(j)); said signal processor being adapted to generate a second number of reference position signals per revolution of said shell, said second number of reference position signals being generated at equal angular distances based on said position signal; said second number being equal to said first number; and data indicative of a temporal duration between said reference position signal value occurrence and said amplitude peak value occurrence. An example 52 relates to a system for controlling an internal state of in a tumbling mill (10) having a shell (20) that rotates around an axis (60) at a speed of rotation (f _ROT ) for grinding a charge of material (30) by tumbling the material in the rotating shell; said shell (20) having an internal shell surface (22) including a first number (L) of protrusions (310) configured to engage material as the shell (20) rotates about the axis (60), the system comprising a device (170, 180 ) for generating a position signal (EP, P(i), P(j), P(q)) indicative of a rotational position of said rotating shell (20), said position signal including a time sequence of position signal sample values (P(i), P(j), P(q)); a sensor (70, 70 _IN , 70 _OUT , 330) configured to generate a vibration signal (S _EA , S _MD , Se(i), S(j), S(q)) dependent on mechanical vibrations (V _IMP ) emanating from rotation of said shell, said vibration signal (SEA, Se(i), S(j), S(q)) including a time sequence of vibration sample values (Se(i), S(j), S(q)); a status parameter extractor (450) configured to detect a first occurrence of a first reference position signal value (1; 1C, 0%) in said time sequence of position signal sample values (P(i), P(j), P(q)); said status parameter extractor (450) being configured to detect a second occurrence of a second reference position signal value (1; 1C; 100%) in said time sequence of position signal sample values (P(i), P(j), P(q)); said status parameter extractor (450) being configured to detect a third occurrence of an event signature (S _P (r); S _P ) in said time sequence of vibration sample values (Se(i), S(j), S(q)); said status parameter extractor (450) being configured to generate data indicative of a first temporal relation (R _T (r); T _D ; FI(r)) between said third occurrence i.e. said event signature occurrence, and said first and second occurrences; and a regulator for controlling an angular toe position (FI(r), ATOE ) based on a toe position reference value (FI _REF (r) ), said first temporal relation (RT(r); TD; FI(r)), and a toe position error value (FI _ERR (r) ), wherein said toe position error value (FI _ERR (r) ) depends on said toe position reference value (FIREF(r) ), and said first temporal relation (RT(r); TD; FI(r)). 53. The system according to example 52, wherein said toe position error value (FIERR(r) ) depends on a difference between said toe position reference value (FI _REF (r) ), and said first temporal relation (RT(r); TD; FI(r)). 54. The system according to example 52 or 53, wherein Said regulator is configured to control a solid material feed rate set point (R _S S _P ) in dependence on said toe position reference value (FIREF(r) ), and wherein a solid material feed rate (R _S ) depends on said solid material feed rate set point (R _S S _P ), said solid material feed rate (R _S ) being an amount of solid material per time unit that is being fed into an input (100) of said tumbling mill (10). 55. The system according to example 52, 53, or 54, wherein Said regulator is configured to control a liquid feed rate set point (R _L S _P ) in dependence on said toe position reference value (FIREF(r) ), and wherein a liquid feed rate (R _L ) depends on said liquid feed rate set point (R _L S _P ), said liquid feed rate (R _L ) being an amount of liquid per time unit that is being fed into an input (130) of said tumbling mill (10). 56. The system according to according to any preceding example, wherein said first temporal relation (RT(r); TD; FI(r)) is indicative of a proportion of a distance between two adjacent of said protrusions (310). 57. The system according to according to any preceding example, wherein Said first temporal relation (RT(r); TD; FI(r)) is indicative of a toe position (205, ATOE(r)). 58. The system according to according to any preceding example, wherein said event signature is indicative of an impact force (F _IMP ) generated when a projection (310) on an internal shell surface (22) of the rotating shell (20) interacts with a toe portion (205) of the charge material (30). 59. The system according to according to any preceding example, wherein said status parameter extractor (450) is configured to generate said first temporal relation (RT(r); TD; FI(r)) as a phase angle (FI(r)). 60. The system according to according to any preceding example, wherein said status parameter extractor (450) is configured to generate said event signature as an amplitude value (SP(r); Sp; CL(r); C1(r)). 61. The system according to according to any preceding example, wherein said status parameter extractor (450) comprises a Fourier Transformer configured to generate said first temporal relation (R _T (r); T _D ; FI(r)). 62. The system according to according to any preceding example, wherein said status parameter extractor (450) is configured to count a total number of samples (N _B ) from the first occurrence to the second occurrence, and said status parameter extractor (450) is configured to count another number of samples (N _P ) from the first occurrence to the third occurrence, and said status parameter extractor (450) is configured to generate said first temporal relation (RT(r); TD; FI(r)) based on said another number and said total number. 63. The system according to according to any preceding example, wherein said status parameter extractor (450) is configured to count a total number of samples (NB) from the first occurrence to the second occurrence, and said status parameter extractor (450) is configured to count another number of samples (NP) from the first occurrence to the third occurrence, and said status parameter extractor (450) is configured to generate said first temporal relation (RT(r); TD; FI(r)) based on a relation between said another number and said total number, wherein: said relation between said another number and said total number is indicative of a toe position (205). An example 64 relates to a method for determining and visualizing an internal state of in a tumbling mill (10) having a rotatable shell (20) for rotating around an axis (60) at a speed of rotation (f _ROT ) for grinding charge material (30) by tumbling the charge material in the rotatable shell (20); wherein the rotatable shell (20) has an internal shell surface (22), the internal shell surface including a certain number (L) of internal projections (310) for engaging material (30) when the shell rotates, thereby causing a mechanical vibration (VIMP) having a repetition frequency (fR) dependent on the rotational speed (fROT) of the rotatable shell (20), the method comprises - receiving a measurement signal (E _P , P(i), P(j), P(q)) indicative of a rotational position of said rotating shell; and - receiving a signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)) indicative of said vibration (V _IMP ); - determining a value (R _T (r); T _D ; FI(r)) indicative of a position of a toe (205) of a charge of said material (30) based on said vibration signal and said position signal, 65. The method according to example 64, wherein receiving a signal (E _P , P(i), P(j), P(q)) indicative of a rotational position of said rotating shell comprises measuring rotation at said rotatable shell (20) utilizing at least one sensor 170. 66. The method according to example 64 or 65, wherein receiving a signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)) indicative of said vibration (VIMP) comprises measuring vibrations at said rotatable shell (20) utilizing at least one sensor 70. 67. The method according to any preceding example, further comprising - controlling said tumbling mill (10) based on said value (RT(r); TD; FI(r)) indicative of a position of a toe (205) of a charge of said material (30). 68. The method according to any preceding example, further comprising - providing a visual representation of said value (RT(r); TD; FI(r)) indicative of a position of a toe (205) of a charge of said material (30). 69. The method according to example 68, wherein providing a visual representation comprises providing a polar diagram representing a time-series of values (R _T (r); T _D ; FI(r)) indicative of a position of a toe (205) of a charge of said material (30). 70. An example computer program for performing the method according to any preceding example, the computer program comprising computer program code means adapted to perform the steps of the method according to any preceding example when said computer program is run on a computer. 71. The computer program according to any preceding example, the computer program being embodied on a computer readable medium. An example 72 relates to a system for monitoring an internal state of a tumbling mill (10) having a rotatable shell (20) including an internal shell surface (22) having a number (L) of protrusions (310) for engaging material when the shell rotates, thereby causing a vibration (V _IMP ) having a repetition frequency (f _R ) dependent on a speed of rotation (f _ROT ) of said shell (20); said system (150) comprising: a monitoring unit (150A) for receiving a signal (E _P , P(i), P(j), P(q)) indicative of a rotational position of said rotating shell, and a signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)) indicative of said vibration (VIMP), said monitoring unit being configured to extract, from said vibration signal and said position signal, a value (R _T (r); T _D ; FI(r)) indicative of a position of a toe (205) of a charge of said material (30). 73. The system according to example 72, wherein said monitoring unit is arranged to receive a signal (EP, P(i), P(j), P(q)) comprising a time sequence of vibration sample values (Se(i), S(j), S(q)) indicative of vibration indicative of a rotational position of said rotating shell; and a signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)) comprising a time sequence of vibration sample values (Se(i), S(j), S(q)) indicative of vibration; and wherein said monitoring unit is arranged to detect a first occurrence of a first reference position signal value in said time sequence of position signal sample values (P(i), P(j), P(q)), a second occurrence of a second reference position signal value in said time sequence of position signal sample values (P(i), P(j), P(q)), and an occurrence of an event signature (SP(r); SP) in said time sequence of vibration sample values (Se(i), S(j), S(q)). 74. The system according to example 73, wherein said monitoring unit is arranged to determine said value (R _T (r); T _D ; FI(r)) indicative of the position of the toe (205) of the charge of said material (30) based on a temporal relationship between said detected first and second occurrence of said first reference position signal value, and said occurrence of an event signature. 75. The system according to example 73 or 74, wherein said monitoring unit is arranged to determine a first duration between said first and second occurrence of said first reference position signal value, a second between occurrence of said event signature and said first and/or second occurrence of said first reference position signal value, and wherein said monitoring unit is arranged to generate data indicative of a first temporal relation (R _T (r); T _D ; FI(r)) between said first duration and second duration. 76. The system according to example 75, wherein said monitoring unit is arranged to determine an internal state of said tumbling mill (10) based on an operating point reference value (FIREF(r)), said first temporal relation (RT(r); TD; FI(r)), and a operating point error value (FI _ERR (r)), wherein said operating point error value (FIERR(r)) depends on said operating point reference value (FIREF(r)), and said first temporal relation (R _T (r); T _D ; FI(r)). 77. The system according to any of examples 72 to 76, comprising a measuring unit comprising at least one sensor (70,170) arranged at the tumbling mill (10), and arranged to provide said signal (EP, P(i), P(j), P(q)) indicative of a rotational position of said rotating shell, and provide said signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)) indicative of said vibration (V _IMP ). 78. The system according to example 77, wherein said measuring unit comprises at least one vibration sensor, wherein said vibration sensor is arranged at said rotatable shell (20) generating a vibration signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)); said vibration sensor being configured to generate said vibration signal based on vibration exhibited by said rotatable shell (20). 79. The system according to example 77 or 78, wherein said measuring unit comprises at least one position sensor is configured to generate a position signal indicative of a predetermined rotational position of said rotatable shell (20). 80. The system according to example 79, wherein at least one position marker (180) is provided at said rotatable shell (20), wherein said at least one position sensor is arranged to detect the at least one position marker (180), and wherein said position signal comprises a time sequence of position signal values (P(i), P(j), P(q)). 81. The system according to example 77 or 78, comprising a control unit (150B) comprising a regulator arranged to control an angular toe position (FI(r), A _TOE ) based on said extracted value (RT(r); TD; FI(r)) indicative of a position of the toe (205) of the charge of said material (30). 82. The system according to any of example 77 or 78, wherein the said measuring unit, said monitoring unit and/or said control unit are arranged at different locations and arranged to communicate via a communications network. 83. The system according to example 82, wherein said monitoring unit and/or said control unit are arranged at a location geographically distant from said tumbling mill (10). 84. The system according to any preceding example, wherein said monitoring unit and said measuring unit are arranged at the tumbling mill (10). 85. The system according to any preceding example, wherein said measuring unit comprises a first sensor for generating a first vibration signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)); said first sensor being configured to generate said first vibration signal based on vibration exhibited at a first part of said rotatable shell (20); and a second sensor for generating a second vibration signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)); said second sensor being configured to generate said second vibration signal based on vibration exhibited at a second part of said rotatable shell (20); wherein said monitoring unit is arranged to detect a fourth occurrence of an event signature (SP(r); SP) in a time sequence of first vibration signal sample values (Se(i), S(j), S(q)); said monitoring unit being configured to detect a fifth occurrence of said event signature (S _P (r); S _P ) in a time sequence of second vibration signal sample values (Se(i), S(j), S(q)); said monitoring unit being configured to generate data indicative of an order of occurrence between said fourth occurrence and said fifth occurrence; and, determining said value (R _T (r); T _D ; FI(r)) indicative of a position of the toe (205) of the charge of said material (30) based on said order of occurrence. An example 86 relates to computer implemented method of representing an internal state of a grinding process in a tumbling mill (10) on a screen display (210S) of a digital tumbling mill monitoring system for generating and displaying information relating to a grinding process in a tumbling mill (10) having a rotatable shell (20) for rotating around an axis (60 ) at a speed of rotation (fROT) for grinding charge material (30) by tumbling the charge material in the rotatable shell (20); wherein the rotatable shell (20) has an internal shell surface (22), the internal shell surface including a certain number (L) of internal projections (310) for engaging material (30) when the shell rotates, thereby causing a mechanical vibration (VIMP) having a repetition frequency (fR) dependent on the rotational speed (fROT) of the rotatable shell (20), the method comprising: - receiving a signal (EP, P(i), P(j), P(q)) indicative of a rotational position of the rotating shell (20), - generate a position reference value (1; 1C, 0%; 100%) based on said position signal (E _P , P(i), P(j), P(q)) such that said position reference value is provided a first number of times per revolution of the rotatable shell (20), said first number of position reference values being indicative of a first number of predetermined rotational positions of the rotatable shell (20), and - receiving a vibration signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)) based on the mechanical vibrations (V _IMP ) emanating from rotation of said shell (20); detecting an occurrence of an event signature (SP(r); SP) in said vibration signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)); - displaying on said screen display (210S) a polar coordinate system, said polar coordinate system having a reference point (O), and a reference direction (0,360); and a first internal status indicator object (S _P1 , T _D1 ), indicative of said internal state of said grinding process at a first polar angle (TD1) in relation to said reference direction (0,360), said first polar angle (T _D1 ) being indicative of a temporal duration (T _D1 ) between occurrence of said event signature (SP(r); SP) and occurrence of a rotational reference position of said rotating shell; and/or said first polar angle (T _D1 ) being indicative of an angular position of the rotatable shell (20) at the occurrence of said event signature (S _P (r); S _P ); and/or said first polar angle (TD1) being indicative of an angular position of a charge material toe (205) at the occurrence of said event signature (S _P (r); S _P ). 87. The method according to any preceding example, wherein said first number is at least two and/or said first number is equal to said certain number. 88. The method according to any preceding example, wherein said vibration signal includes a time sequence of vibration sample values (Se(i), S(j), S(q)); and wherein said detection includes detecting an occurrence of an event signature (SP(r); Sp) in said time sequence of vibration sample values (Se(i), S(j), S(q)). An example 89 relates to computer implemented method of representing an internal state of a grinding process in a tumbling mill (10) on a screen display (210S) of a digital tumbling mill monitoring system for generating and displaying information relating to a grinding process in a tumbling mill (10) having a rotatable shell (20) for rotating around an axis (60 ) at a speed of rotation (f _ROT ) for grinding charge material (30) by tumbling the charge material in the rotatable shell (20); wherein the rotatable shell (20) has an internal shell surface (22), the internal shell surface including a certain number (L) of internal projections (310) for engaging material (30) when the shell rotates, thereby causing a mechanical vibration (VIMP) having a repetition frequency (fR) dependent on the rotational speed (fROT) of the rotatable shell (20), the method comprising: receiving a signal (E _P , P(i), P(j), P(q)) indicative of a rotational position of the rotating shell (20), - receiving a vibration signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)) dependent on mechanical vibrations (V _IMP ) emanating from rotation of said shell; - detecting an occurrence of an event signature (SP(r); Sp) in said vibration signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)); - displaying on said screen display a polar coordinate system, said polar coordinate system having a reference point (O), and a reference direction (0,360); and a first internal status indicator object (S _P1 , T _D1 ), indicative of said internal state of said grinding process at a first polar angle (TD1) in relation to said reference direction (0,360), said first polar angle (T _D1 ) being indicative of a temporal duration (T _D1 ) between occurrence of said event signature (SP(r); Sp) and occurrence of a rotational reference position of said rotating shell. 90. The method according to any preceding example, wherein said first internal status indicator object (SP1, TD1) is displayed, on said screen display, at a first radius (SP1) from said reference point (O). 91. The method according to any preceding example, wherein said first internal status indicator object (S _P1 , T _D1 ) is displayed, on said screen display, at a first radius (S _P1 ) from said reference point (O), and wherein said first radius (SP1) is indicative of an impact force (FIMP) generated when a projection on an internal shell surface of the rotating shell interacts with the charge material. 92. The method according to any preceding example, wherein said vibration signal includes a time sequence of vibration sample values (Se(i), S(j), S(q)); An example 93 relates to a system for monitoring an internal state of a tumbling mill including a rotatable shell configured with an internal shell surface having a certain number (L) of internal projections for engaging material when the shell rotates, thereby causing a vibration having a repetition frequency dependent on a speed of rotation of said shell, said system comprising: - a monitoring unit for receiving a position signal indicative of a predetermined rotational position of said rotating shell, said position signal including a time sequence of position signal values (P(i), P(j), P(q)); and a signal (S _EA , Se(i), S(j), S(q)) indicative of said vibration, said vibration signal (S _EA , Se(i), S(j), S(q)) including a time sequence of vibration sample values (Se(i), S(j), S(q));wherein said monitoring unit is configured to generate a position reference value based on said position signal such that said position reference value is provided a first number of times per revolution of said shell, said first number of position reference values being indicative of a first number of predetermined rotational positions of said rotatable shell, said first number of predetermined rotational positions corresponding to positions of said internal projections on the internal shell surface of said rotatable shell; said first number being at least two and/or said first number being at most equal to said certain number; and wherein said monitoring unit is configured to extract, from said vibration signal, a signal signature that occurs when said internal projection engages with a toe portion of said material; said signal signature being extracted from said vibration signal said certain number of times (L) per revolution of said shell; said monitoring unit being configured to measure a first duration from occurrence of a first position reference value to occurrence of a second position reference value; measure a second duration between occurrence of said signal signature and said occurrence of said first position reference value, or between occurrence of said signal signature and said occurrence of said second position reference value; and generate a relation value based on said second duration and said first duration; said relation value being indicative of a momentary position of said toe portion (205) between two said predetermined rotational positions of said rotatable shell (20) during rotation of said shell. 94. The system according to any preceding example, wherein said monitoring unit is arranged to extract said signal signature from said vibration signal said certain number of times per revolution of said shell. 95. The system according to any preceding example, wherein said monitoring unit being configured to generate said temporal relation value at least once during one revolution of said shell, and/or generate said temporal relation value said certain number of times during one revolution of said shell, and/or generate said temporal relation value said certain number of times per revolution of said shell. An example 96 relates to a system for monitoring an internal state of a tumbling mill (10) including a rotatable shell (20) configured with an internal shell surface having a certain number (L) of internal projections for engaging material when the shell rotates, thereby causing a vibration having a repetition frequency dependent on a speed of rotation of said shell, said system comprising: - a monitoring unit for receiving a position signal indicative of a predetermined rotational position of said rotating shell, said position signal including a time sequence of position signal values (P(i), P(j), P(q)); and a signal (S _EA , Se(i), S(j), S(q)) indicative of said vibration, said vibration signal (SEA, Se(i), S(j), S(q)) including a time sequence of vibration sample values (Se(i), S(j), S(q));wherein said monitoring unit is configured to generate a position reference value based on said position signal such that said position reference value is provided a first number of times per revolution of said shell, said first number of position reference values being indicative of a first number of predetermined rotational positions of said rotating shell, said first number being at least two; and wherein said monitoring unit is configured to extract, from said vibration signal, a signal signature that occurs when said internal projection engages with a toe portion of said material; said monitoring unit being configured to measure a first duration from occurrence of a first position reference value to occurrence of a second position reference value; measure a second duration between occurrence of said signal signature and said occurrence of said first position reference value, or between occurrence of said signal signature and said occurrence of said second position reference value; and generate a relation value based on said second duration and said first duration; said relation value being indicative of a momentary position of said toe portion between two said predetermined rotational positions of said rotatable shell during rotation of said shell. 90. The system according to any preceding example, wherein said occurrence of said second position reference value being consecutive to said occurrence of said first position reference value. An example 96 relates to a system for monitoring an internal state of a tumbling mill (10) including a rotatable shell (20) configured with an internal shell surface having a certain number (L) of internal projections for engaging material when the shell rotates, thereby causing a vibration having a repetition frequency dependent on a speed of rotation of said shell (20), said system comprising: - a monitoring unit for receiving a position signal indicative of a predetermined rotational position of said rotating shell, and a signal indicative of said vibration, wherein said monitoring unit is configured to provide a rotational position indicator signal based on said position signal such that said rotational position indicator signal is provided a first number of times per revolution of said shell; and wherein said monitoring unit is configured to extract, from said vibration signal, a signal signature that occurs when said internal projection engages with a toe portion of said material; said monitoring unit being configured to measure a first duration from the provision of a first rotational position indicator signal to the provision of a second rotational position indicator signal; measure a second duration between the occurrence of said signal signature and the occurrence of said first rotational position indicator signal, or between the occurrence of said signal signature and the occurrence of said second rotational position indicator signal; and generate a temporal relation value based on said second duration and said first duration; said temporal relation value being indicative of a momentary position of said toe portion between (in relation to) two consecutive predetermined rotational positions of said rotating shell; said first number being at least two. An example 97 relates to a system for monitoring an internal state of a tumbling mill (10) including a rotatable shell (20) configured with an internal shell surface having a certain number of internal projections for engaging material when the shell rotates, thereby causing a vibration having a repetition frequency dependent on a speed of rotation of said shell, said system comprising: a monitoring unit for receiving a position signal indicative of a predetermined rotational position of said rotating shell, and a signal indicative of said vibration, wherein said monitoring unit is configured to detect a first occurrence of a first reference position signal value (1; 1C, 0%) in a time sequence of position signal sample values (P(i), P(j), P(q)); said monitoring unit is configured to provide a rotational position indicator signal based on said position signal such that said rotational position indicator signal is provided a first number of times per revolution of said shell; and wherein said monitoring unit is configured to extract, from said vibration signal, a signal signature that occurs when said internal projection engages with a toe portion of said material; said monitoring unit being configured to measure a first duration from the provision of a first rotational position indicator signal to the provision of a second rotational position indicator signal; measure a second duration from the provision of said first rotational position indicator signal to the occurrence of said signal signature; and generate a temporal relation value based on said first duration and said second duration; said temporal relation value being indicative of a position of said toe portion between two consecutive predetermined rotational positions of said rotating shell; said certain number being at least two. 98. The system of example 97, wherein said monitoring unit is configured to generate said temporal relation value at least twice per revolution of said rotating shell; Said certain number being at least two. 99. The system of example 97 or 98, wherein said monitoring unit being configured to generate a relation value based on said signal signature and two position signals, said relation value being generated at least twice per revolution of said rotating shell; Said certain number being at least two. An example 100 relates to a tumbling mill arrangement (730; 780; 720) having a rotatable shell (20) including an internal shell surface (22) having a number (L) of protrusions (310) for engaging material (30) when the shell rotates, thereby causing a vibration (VIMP) having a repetition frequency (fR) dependent on a speed of rotation (fROT) of said shell (20); the tumbling mill arrangement comprising - a vibration sensor for generating a signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)) indicative of said vibration (V _IMP ); - a position sensor for generating a signal (E _P , P(i), P(j), P(q)) indicative of a rotational position of said rotating shell, and - a first tumbling mill arrangement data port (800, 820), connectable to a communications network; - a first tumbling mill arrangement communications device (790) being configured to deliver, via said first tumbling mill arrangement data port (820): data indicative of said vibration signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)), and data indicative of said position signal (EP, P(i), P(j), P(q)). 101. The tumbling mill arrangement of example 100, wherein said communications network comprises the world wide internet, also known as the Internet. 102. The tumbling mill arrangement according to example 100 or 101, further comprising: - a second tumbling mill arrangement data port (800B; 820B), connectable to a communications network; - a second tumbling mill arrangement communications device (790B) being configured to receive, via said second tumbling mill arrangement data port (800B; 820B): data (TD; FI(r); RT(r); X1(r); X2, Sp(r); X5, fROT, dRT(r), X4; dSp(r), X3) indicative of an internal state of said mill process. 103. The tumbling mill arrangement according to any preceding example, further comprising: - a second tumbling mill arrangement data port (800B; 820B), connectable to a communications network; - a second tumbling mill arrangement communications device (790B) being configured to receive, via said second tumbling mill arrangement data port (800B; 820B): data (RT(r); TD; FI(r); X1(r); X2, Sp(r), fROT, dRT(r); d Sp(r)) indicative of a position of a toe (205) of a charge of said material (30). 104. The tumbling mill arrangement according to any preceding example, further comprising: a Human Computer Interface (HCI; 210) for enabling user input/output; and a screen display (210S); and wherein said Human Computer Interface (HCI; 210) is configured to display, on said screen display (210S), data (TD; FI(r); RT(r); X1(r); X2, Sp(r); X5, fROT, dRT(r), X4; dSp(r), X3) indicative of said internal state (X) of said mill process. 105. The tumbling mill arrangement according to any preceding example, further comprising: a Human Computer Interface (HCI; 210) for enabling user input/output; and a screen display (210S); and wherein said Human Computer Interface (HCI; 210) is configured to display, on said screen display (210S), data (T _D ; FI(r); R _T (r); X1(r); X2, Sp(r); X5, f _ROT , dR _T (r), X4; dSp(r), X3) indicative of indicative of a position of a toe (205) of a charge of said material (30). 106. The tumbling mill arrangement according to any preceding example, wherein: second tumbling mill arrangement communications device (790B) is said first tumbling mill arrangement communications device (790) and said second tumbling mill arrangement data port (800B; 820B) is said first tumbling mill arrangement data port (820). 107. The tumbling mill arrangement according to any preceding example, further comprising: a control module (150, 150B) configured to receive said data (T _D ; FI(r); R _T (r); X1(r); X2, Sp(r); X5, fROT, dRT(r), X4; dSp(r), X3) indicative of an internal state of said mill process. 108. The tumbling mill arrangement according to any preceding example, wherein: said control module (150, 150B) includes - a regulator (755) configured to control a solid material feed rate into said tumbling mill based on said data (TD; FI(r); RT(r); X1(r); X2, Sp(r); X5, fROT, dRT(r), X4; dSp(r), X3) indicative of an internal state of said mill process; and/or - a regulator configured to control the rotational speed (fROT) of the rotatable shell (20) based on said data (T _D ; FI(r); R _T (r); X1(r); X2, Sp(r); X5, f _ROT , dR _T (r), X4; dSp(r), X3) indicative of an internal state of said mill process; and/or a regulator configured to control a liquid feed rate into said tumbling mill based on said data (T _D ; FI(r); R _T (r); X1(r); X2, Sp(r); X5, f _ROT , dR _T (r), X4; dSp(r), X3) indicative of an internal state of said mill process. 109. The tumbling mill arrangement according to any preceding example, wherein: said control module (150, 150B) includes a regulator (755) configured to control a solid material feed rate into said tumbling mill based on said value (RT(r); TD; FI(r)) indicative of a position of a toe (205) of a charge of said material (30), and/or a regulator configured to control the rotational speed (fROT) of the rotatable shell (20) based on said value (RT(r); TD; FI(r)) indicative of a position of a toe (205) of a charge of said material (30), and/or a regulator configured to control a liquid feed rate into said tumbling mill based on said value (RT(r); TD; FI(r)) indicative of a position of a toe (205) of a charge of said material (30). An example 105 relates to a monitoring apparatus (870; 880; 150; 150A) for cooperation with a tumbling mill arrangement according to any preceding example, or according to any of examples 100 to 109, the monitoring apparatus comprising: - a monitoring apparatus data port (920, 920A), connectable to a communications network (810), for data exchange with a tumbling mill arrangement; wherein - said monitoring apparatus (870; 880; 150; 150A) is configured to receive, via said monitoring apparatus data port (920, 920A): data indicative of a vibration signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)), and data indicative of a position signal (E _P , P(i), P(j), P(q)); the monitoring apparatus (870; 880; 150; 150A) further comprising: a status parameter extractor (450) being configured to generate data (T _D ; FI(r); R _T (r); X1(r); X2, Sp(r); X5, f _ROT , dR _T (r), X4; dSp(r), X3) indicative of an internal state of said mill process based on said vibration signal and said position signal. 110. The monitoring apparatus according to any preceding example, wherein: said monitoring apparatus (870; 880; 150; 150A) is configured to transmit, via said monitoring apparatus data port (920, 920A): generated data (T _D ; FI(r); R _T (r); X1(r); X2, Sp(r); X5, f _ROT , dR _T (r), X4; dSp(r), X3) indicative of said internal state of said mill process to said tumbling mill arrangement. 111. The monitoring apparatus according to any preceding example, wherein said monitoring apparatus (870; 880; 150; 150A) is configured to generate and transmit a value (RT(r); TD; FI(r)) indicative of a position of a toe (205) of a charge of said material (30) to said tumbling mill arrangement. 112. The monitoring apparatus according to any preceding example, wherein said monitoring apparatus (870; 880; 150; 150A) is configured to utilize a server (830) at a remote server location (860) to generate and/or transmit a value (R _T (r); T _D ; FI(r)) indicative of a position of a toe (205) of a charge of said material (30) to said tumbling mill arrangement, and/or store and/or retrieve data indicative of a vibration signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)), and/or data indicative of a position signal (EP, P(i), P(j), P(q)). 113. The monitoring apparatus according to any preceding example, wherein said monitoring apparatus (870; 880; 150; 150A) comprises a memory storage ( 890) and said monitoring apparatus is configured to store on and/or retrieve from said memory storage ( 890), data indicative of a vibration signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)), and/or data indicative of a position signal (E _P , P(i), P(j), P(q)). An example 114 relates to an assembly for cooperation with a tumbling mill arrangement according to any preceding example, or according to any of examples 100 to 113, the assembly comprises a monitoring module (150; 150A), a control module (150; 150B), and at least one assembly data port (920, 920A, 920B), connectable to a communications network (810), for data exchange with a tumbling mill arrangement; wherein said monitoring module (150; 150A) is configured to receive, via said assembly data port port (920, 920A): data indicative of a vibration signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)), and data indicative of a position signal (EP, P(i), P(j), P(q)); the monitoring module (150; 150A) being configured to generate data (TD; FI(r); RT(r); X1(r); X2, Sp(r); X5, fROT, dRT(r), X4; dSp(r), X3) indicative of an internal state of said mill process based on said vibration signal and said position signal, said control module (150; 150B) is arranged to communicate with said tumbling mill arrangement via an assembly data port (920, 920B), and said control module (150, 150B) includes - a regulator (755) configured to control a solid material feed rate into said tumbling mill based on said data (TD; FI(r); RT(r); X1(r); X2, Sp(r); X5, fROT, dR _T (r), X4; dSp(r), X3) indicative of an internal state of said mill process; and/or - a regulator configured to control the rotational speed (f _ROT ) of the rotatable shell (20) based on said data (TD; FI(r); RT(r); X1(r); X2, Sp(r); X5, fROT, dRT(r), X4; dSp(r), X3) indicative of an internal state of said mill process; and/or a regulator configured to control a liquid feed rate into said tumbling mill based on said data (T _D ; FI(r); R _T (r); X1(r); X2, Sp(r); X5, f _ROT , dR _T (r), X4; dSp(r), X3) indicative of an internal state of said mill process. 115. The assembly according to any preceding example, wherein the assembly is arranged at a location geographically distant from said tumbling mill (10). 116. A method for generating information relating to an internal state (X) of a tumbling mill (10) having a shell (20) that rotates at a speed of rotation (fROT) for grinding a charge of material (30) by tumbling the material in the rotating shell; said shell (20) having an internal shell surface (22) including a first number (L) of protrusions (310) configured to engage material as the shell (20) rotates about the axis (60), thereby causing a vibration (VIMP) having a first repetition frequency (fR) dependent on the speed of rotation (fROT); the method comprising receiving a position signal (E, P, P(i), P(j), P(q)) indicative of a rotational position of said rotating shell (20) such that said position signal (EP, P(i), P(j), P(q)) has a second repetition frequency (f _RP ) dependent on said speed of rotation (f _ROT ); receiving a vibration signal (S _EA , Se(i), S(j), S(q)) dependent on mechanical vibrations (VIMP) emanating from rotation of said shell, said vibration signal (SEA, Se(i), S(j), S(q)) including a time sequence of vibration sample values (Se(i), S(j), S(q)); detecting, in said time sequence of vibration sample values (Se(i), S(j), S(q)), an event signature (SP(r); SP) having an event signature occurrence frequency (fR), said event signature occurrence frequency being equal to said first repetition frequency (fR); generating, based on said event signature occurrence frequency, a periodic event signal exhibiting said first number (L) of periods per revolution of said shell during operation of the tumbling mill (10); generating, based on said position signal (E, P, P(i), P(j), P(q)), a periodic reference signal exhibiting said first number (L) of periods per revolution of said shell during operation of said tumbling mill (10); generating data indicative of a first temporal relation (X1(r), R _T (r); T _D ; FI(r)) between said periodic event signal, and said periodic reference signal; said temporal relation being indicative of said internal state (X) of the tumbling mill (10). 117. The method according to any preceding example, wherein: said periodic event signal is a sinusoidal event signal; and said periodic reference signal is a sinusoidal reference signal; and said data indicative of a first temporal relation (RT(r); TD; FI(r)) is indicative of a first temporal relation (RT(r); TD; FI(r)) between said sinusoidal event signal, and said sinusoidal reference signal. 118. The method according to any preceding example, wherein: said periodic reference signal is generated based on said first number (L) and said position signal (E, P, P(i), P(j), P(q)) such that said periodic reference signal is configured to exhibit said first number (L) of periods per revolution of said shell during operation of said tumbling mill (10). 119. The method according to any preceding example, wherein: said periodic reference signal is generated based on said first number (L) and said position signal (E, P, P(i), P(j), P(q)) such that said periodic reference signal is configured to exhibit said first number (L) of periods per revolution of said shell during operation of said tumbling mill (10), and a reference amplitude value, such as a peak value, based on a certain position signal value (E, P, P(i), P(j), P(q)). 120. The method according to any preceding example, wherein: said periodic reference signal is configured to exhibit least two periods per revolution of said shell during operation of said tumbling mill (10). 121. The method according to any preceding example, wherein: said position signal includes a time sequence of position signal sample values (P(i), P(j), P(q)); and said second repetition frequency (f _RP ) is a frequency lower than, or equal to, said first repetition frequency (f _R ). 122. A method for generating information relating to an internal state (X) of a tumbling mill (10) including a rotatable shell (20) configured with an internal shell surface having a first number (L) of internal protrusions (310) for engaging material (30) when the shell rotates, thereby causing a vibration having a repetition frequency dependent on a speed of rotation of said shell, the method comprising the steps: receive a position signal relating to rotational position of said rotating shell, and detect, in a time sequence of position signal values (P(i), P(j), P(q)), a first occurrence of a first reference position signal value (1; PS) indicative of a predetermined rotational position of said rotating shell; provide a reference signal (1, 1C, PS, PC, 0%) based on said position signal such that said reference signal is provided a certain number (L) of times per revolution of said shell; said certain number being at least two; and receive a signal indicative of said vibration, detect, in said vibration signal, a signal event signature that occurs when a said internal protrusion (310) engages with a toe portion of said material; measure a first duration (100%) from the provision of a first reference signal (1, 1C, PS, PC, 0%) to the provision of a subsequent reference signal (1, 1C, PS, PC, 100%); and measure a second duration between the provision of a reference signal to the occurrence of a subsequent said signal event signature, or measure the second duration between the occurrence of said signal event signature to the provision of a subsequent reference signal; and generate a temporal relation value based on said second duration and said first duration (100%); said temporal relation value being indicative of said internal state (X) of the tumbling mill (10). 123. The method according to any preceding example, wherein: said temporal relation value is indicative of a position of said toe portion (205) between two consecutive predetermined rotational positions (Ps, Pc) of said rotating shell. 124. The method according to any preceding example, wherein: a said protrusion (310) is positioned, on said internal shell surface (22), in a mutually equidistant manner in relation to another said protrusion (310). 125. The method according to any preceding example, wherein: a said protrusion (310) is an elongated protrusion (310) having a direction of elongation, the direction of elongation extending in a direction substantially perpendicular to a direction of movement of said protrusion (310) during operation of said tumbling mill (10), said elongated protrusion (310) moving at said speed of rotation (fROT) of said shell (20) during operation of said mill (10). 126. The method according to any preceding example or according to any example dependent on example 116, further comprising: generating said first temporal relation (RT(r); TD; FI(r)) as a phase angle (FI(r)) between said periodic event signal, and said periodic reference signal. 127. The method according to any preceding example, wherein: said first temporal relation (RT(r); TD; FI(r)) is indicative of a proportion of a certain distance, said certain distance being the distance between two adjacent of said protrusions (310). 128. The method according to any preceding example, wherein: Said first temporal relation (RT(r); TD; FI(r)) is indicative of a relative toe position (205). 129. The method according to any preceding example, wherein: said event signature is indicative of an impact force (F _IMP ) generated when a protrusion (310) on an internal shell surface (22) of the rotating shell (20) interacts with a toe portion (205) of the charge material (30). 130. The method according to any preceding example, further comprising controlling an angular toe position (FI(r), X1(r), A _TOE ) based on a toe position reference value (FIREF(r), X1REF(r) ), said first temporal relation (R _T (r); T _D ; FI(r)), and a toe position error value (FI _ERR (r), X1 _ERR (r) ), wherein said toe position error value (FIERR(r), X1ERR(r) ) depends on said toe position reference value (FIREF(r), X1REF(r) ), and said first temporal relation (R _T (r); T _D ; FI(r)). 131. The method according to any preceding example, wherein: said toe position error value (FI _ERR (r) ) depends on a difference between said toe position reference value (FIREF(r) ), and said first temporal relation (RT(r); TD; FI(r)). 132. The method according to any preceding example or according to example 130 or 131, further comprising controlling a solid material feed rate set point (R _SSP ) in dependence on said toe position reference value (FI _REF (r) ), wherein a solid material feed rate (RS ) depends on said solid material feed rate set point (RSSP), said solid material feed rate (RS ) being an amount of solid material per time unit that is being fed into an input (100) of said tumbling mill (10). 133. The method according to any preceding example or according to any of examples 130 - 132, further comprising controlling speed of rotation set point (fROTSP) in dependence on said toe position reference value (FIREF(r) ), and wherein said speed of rotation (fROT) depends on said speed of rotation set point (fROTSP); and/or controlling a liquid feed rate set point (RLSP) in dependence on said toe position reference value (FI _REF (r) ), and wherein a liquid feed rate (R _L ) depends on said liquid feed rate set point (R _LSP ), said liquid feed rate (RL ) being an amount of liquid per time unit that is being fed into an input (130) of said tumbling mill (10). 134. The method according to any preceding example or according to any of claims 130 to 133, wherein: said tumbling mill (10) is located at a mill location (780), and wherein at least a part of the method is performed at a location (870) remote from said mill location (780), and/or wherein at least a part of the method is performed at a remote location (870), said remote location (870) being geographically separated from the mill location (780) by a geographic distance; wherein the method further comprises the step: transfer at least some of said signals between said mill location (780) and said remote location (870). 135. The method according to any preceding example, wherein said geographic distance exceeds one kilometre; and/or wherein said mill location (780) is in a first country constituting a first jurisdiction, and said remote location (870) is in a second country constituting a second jurisdiction such that at least a part of the method is performed in said first country and at least a part of said method is performed in said second country. 136. The method according to any preceding example, wherein at least a part of said signal transfer is performed by a communications network (810), such as e.g. the Internet. 137. The method according to any preceding example or according to any of examples 122 - 136, wherein said event signature is indicative of an impact force (FIMP) generated when a projection (310) on an internal shell surface (22) of the rotating shell (20) interacts with a toe portion (205) of the charge material (30). 138. The method according to any preceding example or according to any of examples 122 - 137, wherein said event signature is an amplitude value (SP(r); Sp; CL(r); C1(r)), such as e.g. a peak amplitude value. 139. The method according to any preceding example or according to any of examples 122 - 138, wherein said first temporal relation (R _T (r); T _D ; FI(r)) is generated by a Fourier Transformer configured to generate said first temporal relation (RT(r); TD; FI(r)). 140. The method according to any preceding example or according to any of examples 126 - 139, wherein said first duration, between two consecutive reference signals, is measured by counting a total number of samples (N _B ) from the occurrence of a first reference signal to the occurrence of the consecutive reference signal; and said second duration is measured by counting another number of samples (N _P ) between the provision of a reference signal to the occurrence of a subsequent said signal event signature, or by counting another number of samples (NP) between the occurrence of said signal event signature to the provision of a subsequent reference signal; the method further comprising: generating said first temporal relation (RT(r); TD; FI(r)) based on said another number (NP) and said total number (NB). 141. The method according to any preceding example or according to any of examples 122 - 140, wherein said first temporal relation (RT(r); TD; FI(r)) is based on a relation between said another number and said total number. 142. The method according to any preceding example or according to any of examples 122 - 141, wherein said first temporal relation (R _T (r); T _D ; FI(r)) is generated by a status parameter extractor (450) configured to generate said first temporal relation (R _T (r); T _D ; FI(r)). 143. A method of operating a comminution process in a tumbling mill (10) including a rotatable shell (20) having an internal shell surface (22) with a first number (L) of protrusions (310) configured to engage material when the shell (20) rotates for grinding a charge of material (30) by tumbling the material in the rotating shell so as to generate product particles (95; 96) at a mill output (200), thereby causing a vibration (V _IMP ) having a first repetition frequency (f _R ) dependent on a speed of rotation (U1, f _ROT ) when a protrusion (310) engages with a toe portion (205) of said material (30); the method comprising receiving a vibration signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)) indicative of said vibration (VIMP); receiving a position signal (EP, P(i), P(j), P(q)) indicative of a rotational position of said rotating shell; generating at least one status parameter value (X1(r), FI(r), TD, RT(r); X2, Sp(r); X3, dSp(r); X4, dRT(r); X5, fROT; X6, ATOE(r); X7 ) indicative of an internal state (X) of said comminution process based on said vibration signal and said position signal; said at least one status parameter value including a toe position value (X1(r), FI(r), TD, RT(r); X6, ATOE(r) ) indicative of a position of the toe portion (205). 144. The method according to example 140, wherein said at least one status parameter value (X1(r), FI(r), TD, RT(r); X2, Sp(r); X3, dSp(r); X4, dR _T (r); X5, f _ROT ; X6, A _TOE (r); X7 ) is said first temporal relation as defined in any of examples 112-138, or wherein said at least one status parameter value (X1(r), FI(r), TD, RT(r); X2, Sp(r); X3, dSp(r); X4, dRT(r); X5, fROT; X6, ATOE(r); X7 ) is said temporal relation value as defined in any of examples 118 to 138 or wherein said at least one status parameter value (X1(r), FI(r), T _D , R _T (r); X2, Sp(r); X3, dSp(r); X4, dRT(r); X5, fROT; X6, ATOE(r); X7 ) is generated as defined in any of examples 1 to 111. 145. The method according to any of examples 143 to 144, further comprising providing a solid material feed rate set point value (U2SP, RSSP) for setting a solid material feed rate (U2, R _S ); said solid material feed rate (U2, R _S ) being an amount of solid material feed particles (115) per time unit that is being fed into an input (100) of a tumbling mill (10) thereby influencing said internal state (X); said feed particles (115) having a feed particle size distribution. 146. The method according to any of examples 143 to 145, further comprising analysing at least a portion of said product particles (96); generating at least one product measurement value (Y1; Y2) based on said product particle analysis; said at least one product measurement value (Y1; Y2) being indicative of a discharge material state (Y(r)). 147. The method according to any preceding example or according to any of examples 143 to 146, further comprising performing correlation of said at least one product measurement value (Y1; Y2) and said at least one status parameter value (X1(r), FI(r), TD, RT(r); X2, Sp(r); X3, dSp(r); X4, dR _T (r); X5, f _ROT ; X6, A _TOE (r); X7 ); and generating, by said correlation, a correlation data set indicative of a causal relationship between said at least one status parameter value (X1(r), TD, FI(r), RT(r); X2, Sp(r); X3, dSp(r); X4, dR _T (r); X5, f _ROT ; X6, A _TOE (r); X7 ) and said at least one product measurement value (Y1; Y2) and/or a correlation data set indicative of a causal relationship between said internal state (X) and said discharge material state (Y(r)). 148. The method according to any preceding example or according to any of examples 143 to 147, further comprising receiving data indicative of a desired discharge material state (YREF(r)); generating at least one status parameter reference value (X1 _REF ; FI _REF ) based on said data indicative of said desired discharge material state (YREF(r)) and said correlation data set; the generated at least one status parameter reference, value (X1REF; FIREF) including a toe position reference value (X1REF(r), FI REF (r), TDREF; X6 _REF , A _TOEREF (r)); 149. The method according to any preceding example or according to any of examples 143 to 148, further comprising receiving data indicative of a desired discharge material state (YREF(r)); generating at least one status parameter reference value (X1REF; FIREF) based on said data indicative of said desired discharge material state (YREF(r)) and a correlation data set; wherein the generated at least one status parameter reference value (X1REF; FIREF) includes a toe position reference value (X1 _REF (r), FI _REF (r), T _DREF ; X6 _REF , A _TOEREF (r)); and wherein said correlation data set is indicative of a causal relationship between said at least one status parameter value (X1(r), T _D , FI(r), R _T (r); X2, Sp(r); X3, dSp(r); X4, dRT(r); X5, fROT; X6, ATOE(r); X7 ) and said at least one product measurement value (Y1; Y2) and/or wherein said correlation data set is indicative of a causal relationship between said internal state (X) and said discharge material state (Y(r)). 150. The method according to any preceding example or according to any of examples 143 to 149, further comprising displaying, by a user interface (210, 210S, 240, 250), said at least one status parameter reference value (X1REF; FIREF) displaying, by said user interface (210, 210S, 240, 250), said at least one status parameter value (X1(r) T _D , FI(r), R _T (r); X2, Sp(r); X3, dSp(r); X4, dR _T (r); X5, f _ROT ; X6, ATOE(r); X7 ) including said toe position value (X6, ATOE(r); X1(r), FI(r), TD, RT(r) ) for enabling an operator (230) to adjust a mill set point value (U; U1; U2; U3); receiving, by said user interface (210, 210S, 240, 250), a mill set point value (U; U1; U2; U3); said received mill set point value including a received solid material feed rate set point value (U2SP, RSSP); providing said received solid material feed rate set point value (U2SP, R _SSP ) so that it sets said solid material feed rate (U2, R _S ) thereby influencing said internal state (X) to control or influence said discharge material state (Y(r)). 151. The method according to any preceding example or according to any of examples 143 to 150, further comprising controlling said discharge material state (Y(r)) based on said at least one status parameter reference value (X1 _REF ; FI _REF ), said at least one status parameter value (X1(r), FI(r); X6(r), ATOE(r); X7 ), and a status parameter error value (X1 _ERR (r), FI _ERR (r); X6 _ERR , A _{TOE_ERR} (r); X7 _ERR ), wherein said status parameter error value (X1 _ERR (r), FI _ERR (r); X6 _ERR , A _{TOE_ERR} (r); X7 _ERR ) depends on said at least one status parameter reference value (X1REF; FIREF), and said at least one status parameter value (X1(r), FI(r); X6(r), ATOE(r); X7 ). 152. The method according to any preceding example or according to any of examples 143 to 151, further comprising controlling a mill set point (U; U1; U2; U3) including said solid material feed rate set point value (U2SP, RSSP), thereby influencing said internal state (X) to control or affect said at least one product measurement value (Y1; Y2) and/or said discharge material state (Y(r)), based on said at least one status parameter reference value (X1REF; FIREF), said at least one status parameter value (X1(r), FI(r); X6(r), ATOE(r); X7 ), and a status parameter error value (X1 _ERR (r), FI _ERR (r); X6 _ERR , A _{TOE_ERR} (r); X7ERR), wherein said status parameter error value (X1 _ERR (r), FI _ERR (r); X6 _ERR , A _{TOE_ERR} (r); X7 _ERR ) depends on said at least one status parameter reference value (X1REF; FIREF), and said at least one status parameter value (X1(r), FI(r); X6(r), A _TOE (r); X7 ). 153. The method according to any preceding example or according to any of examples 143 to 152, wherein said tumbling mill (10) is a ball mill, said method further comprising providing a ball feed rate set point value (U4SP, RBFSP,) for setting a ball feed rate (U4, RBF); said ball feed rate (U4, RBF) being a number of grinding balls per time unit that is being fed into an input (100) of said ball mill for enhancing said comminution process; said grinding balls having a feed ball size distribution. 154. The method according to any preceding example or according to any of examples 143 to 153, wherein said discharge material state (Y(r)) is a momentary product particle size distribution (Y), said momentary product particle size distribution being indicative a product particle size distribution measured during a measurement moment time period, said measurement moment time period being equal to or shorter than ten minutes. 155. The method according to any preceding example or according to any of examples 122 to 154, further comprising generating, based on said position signal, a second number (L) of static position indications or a second number (L) of static position indication values (P1, P _C , P1, P2, P3, PL), wherein a static position indication value is indicative of an immobile rotational position; generating, based on said vibration signal, a first number (L) of variable position indications or variable position indication values, wherein a variable position indication value is indicative of a variable position between two of said immobile rotational positions. 156. The method according to any preceding example or according to example 155, further comprising generating, based on said variable position indications and said static position indications, a relation value; said relation value being indicative of a position of said toe portion between two of said static positions, 157. The method according to any preceding example or according to any of examples 122 to 156, further comprising generating, based on said event signature, a force value indicative of impact force; generating, based on a combination of said force value and said speed value, a mass value indicative of a mass of at least a part of said charge of material. 158. The method according to any preceding example or according to any of examples 122 to 157, further comprising generating, based on said mass value and said relation value, an absolute toe position value . 159. The method according to any preceding example or according to any of examples 122 to 158, further comprising generating a mass value based on said absolute toe position value and said speed value and historic data relating to said force value as combined with said absolute toe position value and said speed value, said mass value being indicative of a total mass of said charge of material (30). 160. The method according to any of examples 143 to 159, wherein the tumbling of the material (30) in the rotating shell generates output material (95) including said product particles (96) at the mill output (200). 161. The method according to example 160, wherein the output material (95) comprises said product particles (96) and liquid. 162. The method according to any of examples 143 to 161, wherein the tumbling mill (10) includes a rotatable shell (20) having an internal shell surface (22) with a first number (L) of protrusions (310) configured to engage material when the shell (20) rotates for dry grinding a charge of material (30) by tumbling the material in the rotating shell so as to generate output material product particles (95, 96) at the mill output (200). 163. A method of operating a comminution process in a tumbling mill (10) including a rotatable shell (20) having an internal shell surface (22) with a first number (L) of protrusions (310) configured to engage material when the shell (20) rotates for grinding a charge of material (30) by tumbling the material in the rotating shell so as to generate output material (95) including product particles (95; 96) at a mill output (200), thereby causing a vibration (V _IMP ) having a first repetition frequency (f _R ) dependent on a speed of rotation (U1, f _ROT ) when a protrusion (310) engages with a toe portion (205) of said material (30); the method comprising receiving a vibration signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)) indicative of said vibration (VIMP); receiving a position signal (EP, P(i), P(j), P(q)) indicative of a rotational position of said rotating shell; generating, based on said vibration signal and said position signal, at least one status parameter value (X1(r), FI(r), TD, RT(r); X2, Sp(r); X3, dSp(r); X4, dRT(r); X5, fROT; X6, ATOE(r); X7 ) indicative of an internal state (X) of said comminution process; said at least one status parameter value including a first status parameter value (X1(r), FI(r), T _D , R _T (r); X6, ATOE(r) ) indicative of a position of the toe portion (205). 164. The method according to example 163, wherein said first status parameter value (X1(r), FI(r), TD, RT(r); X2, Sp(r); X3, dSp(r); X4, dRT(r); X5, fROT; X6, ATOE(r); X7 ) is said first temporal relation as defined in any of examples 112-138, or wherein said first status parameter value (X1(r), FI(r), T _D , R _T (r); X2, Sp(r); X3, dSp(r); X4, dRT(r); X5, fROT; X6, ATOE(r); X7 ) is said temporal relation value as defined in any of examples 118 to 138 or wherein said first status parameter value (X1(r), FI(r), T _D , R _T (r); X2, Sp(r); X3, dSp(r); X4, dRT(r); X5, fROT; X6, ATOE(r); X7 ) is generated as defined in any of examples 1 to 111. 165. The method according to any of examples 163-164 or according to any of examples 143 to 163, further comprising providing a solid material feed rate set point value (U2SP, RSSP) for setting a solid material feed rate (U2, R _S ); said solid material feed rate (U2, R _S ) being an amount of solid material feed particles (115) per time unit that is being fed into an input (100) of a tumbling mill (10) thereby influencing said internal state (X); said feed particles (115) having a feed particle size distribution. 166. The method according to any of examples 163 to 165 or according to any of examples 143 to 165, further comprising analysing at least a portion of said output material (95); generating at least one output material measurement value (Y1; Y2) based on said output material analysis. It is to be understood that each output material measurement value (Y1; Y2) may be associated with a timestamp or a time period corresponding to said output material analysis. 167. The method according to example 162 or any of examples 143 to 145 or according to any of examples 143 to 166, wherein said at least one output material measurement value (Y1; Y2) is indicative of an output material quality measure. 168. The method according to example 167 or any of examples 143 to 145 or according to any of examples 143 to 167, wherein said at least one output material measurement value (Y1; Y2) is indicative of a discharge material state (Y); the discharge material state (Y) being a momentary state of the output material (95). 169. The method according to any of examples 166 -168 or according to any of examples 143 to 145 or according to any of examples 143 to 168, wherein said at least one output material measurement value (Y1; Y2) is one or several selected from the group: - a value (Y1; Y2) indicative of a mass per time unit of said output material (95); - a value (Y1; Y2) indicative of a mass per time unit of said product particles (96); - a value (Y1; Y2) indicative of a mass per time unit of said product particles (96), wherein said product particles (96) have a product particle size in a range between a smallest product particle size limit value and a largest product particle size limit value; - a value (Y1; Y2) indicative of a percentage of said product particles (96) having a product particle size in a range between a smallest product particle size limit value and a largest product particle size limit value; - a value (Y1; Y2) indicative of a product particle size distribution (Y), such as a standard deviation; - a value (Y1; Y2) indicative of a product particle size (Y1; Y2). 170. The method according to example 169, wherein said product particle size (Y1; Y2) is at least one selected from the group: - a product particle median size value; - a product particle mean size value; - a product particle median diameter value; and - a product particle mean diameter value. 171. The method according to example 169, wherein said product particle size limit values are at least one selected from the group: - a product particle diameter value; and - a product particle maximum width value. It is to be understood that said smallest product particle size limit value may be set to zero. Said range between the smallest product particle size limit value and the largest product particle size limit value may be defined even with the smallest product particle size limit value is omitted, or the largest product particle size limit value is omitted, whereby the range becomes the values below the largest product particle size limit value, or values above the smallest product particle size limit value respectively. This solution advantageously enables identification and/or determination of a cause and effect relationship between the internal state of the comminution process and the at least one output material measurement value. Moreover, this solution advantageously enables identification and/or determination of a cause and effect relationship between the internal state of the comminution process and the discharge material state (Y). The discharge material state (Y) may also be referred to as the product material state (Y). This solution is versatile in that it allows for the defining of a desired discharge material state (YREF), and for testing of alternative internal states of the comminution process in order to search and identify an internal state of the comminution process that causes, or produces, the desired discharge material state (YREF) or that causes or produces a discharge material state (Y) as near as possible to the desired discharge material state (YREF). Moreover, the recording of a detected momentary comminution process internal state (X(r)) in association with a corresponding momentary discharge material state (Y(r)), produces correlation data indicative of a causal relationship between a momentary comminution process internal state (X(r)) and a corresponding momentary discharge material states (Y(r)). By performing repeated recording of a number of mutually different detected momentary comminution process internal states (X(r)) in association with momentary discharge material states (Y(r)) that were caused by the respective momentary comminution process internal states (X(r)), a correlation data set may be produced. Such a correlation data set is indicative of a causal relationship between a number of momentary comminution process internal states (X(r)) and a number of corresponding momentary discharge material states (Y(r)). 172. The method according to any of examples 166 -171 or according to any of examples 143 to 145 or according to any of examples 143 to 171, further comprising performing correlation of said at least one product measurement value (Y1; Y2) and said at least one status parameter value (X1(r), FI(r), TD, RT(r); X2, Sp(r); X3, dSp(r); X4, dRT(r); X5, fROT; X6, ATOE(r); X7 ); and generating, by said correlation, a correlation data set indicative of a causal relationship between said at least one status parameter value (X1(r), TD, FI(r), RT(r); X2, Sp(r); X3, dSp(r); X4, dR _T (r); X5, f _ROT ; X6, A _TOE (r); X7 ) and said at least one product measurement value (Y1; Y2) and/or a correlation data set indicative of a causal relationship between said internal state (X(r)) and said discharge material state (Y(r). In this connection it is noted that the internal state (X(r)) may be indicated by a combination of parameter values, the combination of internal state parameter values being referred to as an internal state vector (X(r)). In the same manner the discharge material state (Y(r) may be indicated by a combination of parameter values, the combination of discharge material state parameter values being referred to as a discharge material state vector (Y(r). 173. The method according to any of examples 166 -172 or according to any of examples 143 to 145 or according to any of examples 143 to 172, further comprising receiving data indicative of a desired discharge material state (YREFD(r)); generating at least one status parameter reference value (X1REF; FIREF) based on said data indicative of said desired discharge material state (YREF _D (r)) and a correlation data set; the generated at least one status parameter reference, value (X1REF; FIREF) including a first status parameter reference value (X1 _REF (r), FI _REF (r), T _DREF ; X6 _REF , A _TOEREF (r)). 174. The method according to any of examples 166 -173 or according to any of examples 143 to 145 or according to any of examples 143 to 173, further comprising receiving data indicative of a desired discharge material state (YREFD(r)); generating at least one status parameter reference value (X1 _REF ; FI _REF ) based on said data indicative of said desired discharge material state (YREFD(r)) and a correlation data set; wherein the generated at least one status parameter reference value (X1REF; FIREF) includes a first status parameter reference value (X1REF(r), FI REF (r), TDREF; X6REF, ATOEREF(r)); and wherein said correlation data set is indicative of a causal relationship between said at least one status parameter value (X1(r), T _D , FI(r), R _T (r); X2, Sp(r); X3, dSp(r); X4, dR _T (r); X5, f _ROT ; X6, A _TOE (r); X7 ) and said at least one product measurement value (Y1; Y2) and/or wherein said correlation data set is indicative of a causal relationship between said desired discharge material state (YREFD(r)) and a corresponding reference internal state (XREF(r) ) 175. The method according to any of examples 166 -174 or according to any of examples 143 to 145 or according to any of examples 143 to 174, further comprising causing a user interface (210, 210S, 240, 250) to convey information indicative of said first status parameter reference value (X1 _REF (r), FI _REF (r), T _DREF ; X6 _REF , A _TOEREF (r)); and causing a user interface (210, 210S, 240, 250) to convey information indicative of said first status parameter value (X1(r), FI (r), T _D ; X6, A _TOE (r)), said first status parameter value being indicative of a position of the toe portion (205); receiving, via a user interface (210, 210S, 240, 250), first user input relating to said solid material feed rate (U2, R _S ); generating said solid material feed rate set point value (U2SP, RSSP) thereby influencing said internal state (X) for controlling or affecting said discharge material state (Y(r)); wherein said generated solid material feed rate set point value (U2SP, RSSP) is based on said received first user input. This solution advantageously generates information about a first status parameter reference value. The generated first status parameter reference value is indicative of a desired toe position that corresponds to a desired discharge material state (YREF(r)). Moreover, this solution advantageously generates information about an actual first status parameter value. The generated actual first status parameter value is indicative of a position of the toe portion (205), and thus it is indicative of the actual internal state (X) of said comminution process. Thus, this solution advantageously conveys, to a user via a user interface, information relating to the actual internal state (X) of said comminution process as well as information relating to a desired internal state (X) of said comminution process. Such conveyed information may be useful to an operator (230) wishing to adjust a solid material feed rate (U2, RS) for controlling or affecting said discharge material state (Y(r)). In this document “desired” values may be referred to as “reference” values. Thus, for example, the above mentioned “first status parameter reference value“ relates to a “desired first status parameter value“. In the context of this document, the term “user” may relate to a person operating a tumbling mill, and such a user may also be referred to as an operator. 176. The method according to any of examples 166 -175 or according to any of examples 143 to 145 or according to any of examples 143 to 175, further comprising generating said solid material feed rate set point value (U2SP, RSSP) thereby influencing said internal state (X) for controlling or affecting said discharge material state (Y(r)); wherein said generated solid material feed rate set point value (U2 _SP , R _SSP ) is based on said first status parameter reference value (X1REF(r), FI REF (r), TDREF; X6REF, ATOEREF(r)); and said first status parameter value (X1(r), FI (r), T _D ; X6, A _TOE (r)), said first status parameter value being indicative of a position of the toe portion (205); This solution advantageously generates information about a first status parameter reference value that is indicative of a desired toe position that corresponds to a desired discharge material state (YREF(r)). Moreover, this solution advantageously generates information about an actual first status parameter value that is indicative of an actual position of the toe portion (205), and thus it is indicative of the current actual internal state (X) of said comminution process. Thus, this solution advantageously automatically, generates a solid material feed rate set point value (U2SP, RSSP) which in turn affects the solid material feed rate (U2, RS) for controlling or affecting said discharge material state (Y(r)). In this document “desired” values may be referred to as “reference” values. Thus, for example, the above mentioned “first status parameter reference value “relates to a “desired first status parameter value“. 177. A system for operating a comminution process in a tumbling mill (10), the system comprising one or more hardware processors configured to perform all, or at least some, of the steps of the method according to any preceding example or according to any of examples 112 to 172. 178. A first system for operating a comminution process in a tumbling mill (10), wherein said tumbling mill (10) is located at a mill location (780), and wherein the system comprises one or more hardware processors, located at said mill location (780), said one or more hardware processors being configured to perform at least some of the steps of the method according to any preceding example or according to any of examples 122 to 177. 179. A second system for co-operation with the first system according to example 178, wherein the second system comprises one or more hardware processors, located at a remote location (870), said remote location (870) being geographically separated from the mill location (780) by a geographic distance; and wherein said one or more hardware processors being configured to perform at least some of the steps of the method according to any preceding example or according to any of examples 122 to 178, wherein at least a part of the method is performed at a remote location (870), said remote location (870) being geographically separated from the mill location (780) by a geographic distance; wherein the method further comprises the step: transfer at least some of said signals between said mill location (780) and said remote location (870). the system comprising one or more hardware processors configured to perform at least some of the steps of the method according to any preceding example or according to any of examples 122 to 178. wherein: said tumbling mill (10) is located at a mill location (780), and wherein at least a part of the method is performed at a location (870) remote from said mill location (780), and/or wherein at least a part of the method is performed at a remote location (870), said remote location (870) being geographically separated from the mill location (780) by a geographic distance; wherein the method further comprises the step: transfer at least some of said signals between said mill location (780) and said remote location (870). 180. The method according to any preceding example, wherein said geographic distance exceeds one kilometre; and/or wherein said mill location (780) is in a first country constituting a first jurisdiction, and said remote location (870) is in a second country constituting a second jurisdiction such that at least a part of the method is performed in said first country and at least a part of said method is performed in said second country. 181. The method according to any preceding example, wherein at least a part of said signal transfer is performed by a communications network (810), such as e.g. the Internet. 182. A method of operating a mill, the method comprising: monitoring a toe position of a first portion of material inside the mill, said toe position determined responsive to signals measured from a vibration sensor; determining a measure of output particle size corresponding to a second portion of material processed in the mill and discharged from the mill; correlating the toe position with the measure of output particle size; storing the correlation in a non-transitory electronic memory; and controlling an operation parameter of the mill based on the stored correlation. 183. The method of example 182, wherein the operation parameter comprises a rotation speed of the mill. 184. The method of claim 182, wherein the operation parameter comprises one or more input material parameters. 185. The method of example 184, wherein the one or more input material parameters comprise an amount or a rate of solid material entering the mill. 186. The method of example 184, wherein the one or more input material parameters comprise an amount or a rate of liquid material entering the mill. 187. The method of example 182, wherein the monitored toe position is at steady state. 188. The method of example 187, wherein the measure of output particle size is determined when the toe position is at steady state or substantially constant. 189. The method of example 187, wherein steady state is achieved when an amount or a rate of input material entering the mill is equal to, or substantially equal to, an amount or a rate of material leaving the mill. 190. A system for operating a mill, said system comprising: a sensor configured to generate a vibration signal; a position sensor configured to generate a position signal corresponding to a rotational position of a rotating container; and one or more hardware processors configured to: receive an input corresponding to a discharge property of discharge material; determine a toe position of the material inside the rotating container responsive to the vibration signal and the position signal; and determine an operation parameter of the mill based on a predetermined correlation between toe position and discharge property, the determined toe position, and the received discharge property. 191. The system of example 190, wherein the discharge property comprises a measure of particle size of the discharge material. 192. The system of example 190, wherein the discharge property comprises a rate of the discharge material leaving the mill. 193. The system of example 190, wherein the operation parameter comprises a rotation speed of the mill. 194. The system of claim 190, wherein the operation parameter comprises one or more input material parameters. 195. The system of example 194, wherein the one or more input material parameters comprise an amount or a rate of solid material entering the mill. 196. The system of example 194, wherein the one or more input material parameters comprise an amount or a rate of liquid material entering the mill. 197. The system of example 190, wherein the predetermined correlation comprises a correlation between toe positions and particle sizes of the discharge material. 198. The system of example 190, wherein the one or more hardware processors are further configured to display the determined operation parameter as a suggestion to a user on a user interface. 199. The system of example 190, wherein the one or more hardware processors are further configured to transmit a signal to a regulator to change the operation of the mill to correspond to the determined operation parameter. 200. A system for operating a mill, said system comprising: a sensor configured to generate a vibration signal; a position sensor configured to generate a position signal corresponding to a rotational position of a rotating container; and one or more hardware processors configured to: determine a toe position of the material inside the rotating container responsive to the vibration signal and the position signal; determine an expected discharge property of a discharge material based on a predetermined correlation between toe position and discharge property and the determined toe position; and display the determined toe position and the determined expected discharge property, wherein the toe position is determined without opening or visually inspecting inside the mill. 201. The system of example 200, wherein the one or more hardware processors are further configured to: receive a first input corresponding to a discharge property of discharge material; compare the expected discharge property with the received input discharge property; and determine an operation parameter of the mill based on the comparison of the expected discharge property and the received input discharge property. 202. A method of operating a tumbling mill, the method comprising: monitoring a toe position of a first portion of material inside the mill, said toe position determined responsive to signals measured from a vibration sensor; determining a measure of a discharge property of a second portion of material discharged from the mill; correlating the toe position with the measure of discharge property; storing the correlation in a non-transitory electronic memory for future use in the control of an operation parameter of the mill based on the stored correlation. 203. A method of operating a tumbling mill, the method comprising: monitoring a toe position of a first portion of material inside the mill, said toe position determined responsive to signals measured from a vibration sensor; determining a measure of a discharge property corresponding to a second portion of material discharged from the mill; receiving an input corresponding to a desired discharge property of discharge material; and determining an operation parameter of the mill based on the determined toe position, and the received desired discharge property, and a predetermined correlation between toe position and discharge property. 204. The method of example 203, wherein the operation parameter comprises a rotation speed of the mill and/or wherein the operation parameter comprises one or more input material parameters. 205. The method of example 204, wherein the one or more input material parameters comprise an amount or a rate of solid material entering the mill. 206. The method of example 204, wherein the one or more input material parameters comprise an amount or a rate of liquid material entering the mill. 207. The method of example 202 or 203, wherein the monitored toe position is at steady state. 208. The method of example 207, wherein the measure of output particle size is determined when the toe position is at steady state or substantially constant. 209. The method of example 207, wherein steady state is achieved when an amount or a rate of input material entering the mill is equal to, or substantially equal to, an amount or a rate of material leaving the mill. 210. The method of example 202 or 203, wherein the toe position of the material inside the rotating container is determined responsive to the vibration signal and a position signal. 211. The method of example 210, wherein the position signal is indicative of a rotational position of a rotating container. 212. The method of 203, wherein said predetermined correlation is the stored correlation according to example 202. 213. The method of example 202 or 203, wherein said discharge property comprises an output particle size corresponding to said second portion of material discharged from the mill. 214. The method of example 202 or 203, further comprising displaying, on a user interface, the determined operation parameter as a suggestion to a user 215. The method of example 202 or 203, further comprising delivering a signal to a regulator to change the operation of the mill to correspond to the determined operation parameter. (PO9) 216. A system (150, 450, 1140, 150A, 150C) for operating a comminution process in a tumbling mill (10) including a rotatable shell (20) having an internal shell surface (22) with a first number (L) of protrusions (310) configured to engage material when the shell (20) rotates for grinding a charge of material (30) by tumbling the material in the rotating shell so as to generate product particles (95; 96) at a mill output (200), thereby causing a vibration (VIMP) having a first repetition frequency (fR) dependent on a speed of rotation (U1, fROT) of the rotatable shell (20) when a protrusion (310) engages with a toe portion (205) of said material (30); the system comprising a sensor (10) for generating a vibration signal (S _FIMP ; S _EA , S _MD , Se(i), S(j), S(q)) indicative of said vibration (VIMP); a position signal generator for generating a position signal (EP, P(i), P(j), P(q)) indicative of a rotational position of said rotatable shell (20); one or more hardware processors configured to: generate at least one status parameter value (X1(r), FI(r); X2(r); X3(r); X4(r); X5, f _ROT ; X6, A _TOE (r); X7(r) ) indicative of an internal state (X) of the comminution process based on said vibration signal and said position signal; deliver a solid material feed rate set point value (U2SP, RSSP) for setting a solid material feed rate (U2, RS) for feeding solid material (115) into a mill solid material feed input (100); said solid material feed rate (U2, R _S ) being an amount of solid material feed particles (115) per time unit; said solid material feed rate set point value (U2SP, RSSP) thereby influencing said internal state (X); a product analyzer (1140) configured to analyze at least a portion of said product particles (96); the product analyzer (1140) being configured to generate at least one product measurement value (Y1, Y2, Y3, ... Yn) based on said product particle analysis; said at least one product measurement value (Y1, Y2, Y3, ... Yn) being indicative of a discharge material state (Y(r), Y1, Y2, Y3, ... Yn); the system further comprising an input for receiving data indicative of a desired discharge material state (YREF(r), Y1REF, Y2REF, Y3REF, ... YnREF); A1) a user interface (210, 210S, 240, 250) configured to convey information indicative of at least one internal status parameter reference value (X1REF(r), FI REF (r); X6REF, ATOEREF(r)), said internal status parameter reference value (X1REF; FIREF) being indicative of a desired internal state (XREF) for causing said desired discharge material state (YREF(r)); and A2) said user interface (210, 210S, 240, 250) further being configured to convey information indicative of said status parameter value (X1(r), FI (r); X6, A _TOE (r)); or B1) a user interface (210, 210S, 240, 250) configured to convey information indicative of at least one internal status parameter error value (X1 _ERR (r), FI _ERR (r); X6ERR, ATOE_ERR (r)), said internal status parameter error value (X1ERR(r), FI ERR (r); X6ERR, ATOE_ERR (r)) being indicative of a diffference between a desired internal state (XREF) for causing said desired discharge material state (Y _REF (r)) and said status parameter value (X1(r), FI (r); X6, A _TOE (r)); said user interface (210, 210S, 240, 250) being configured to receive first user input relating to said solid material feed rate (U2, R _S ) for generating said solid material feed rate set point value (U2 _SP , R _SSP ) thereby influencing said internal state (X(r)) for controlling or affecting said discharge material state (Y(r)). This solution advantageously generates and conveys information indicative of an internal status reference value. The internal status parameter reference value is indicative of a desired a desired internal state (XREF(r) ) of the comminution process that corresponds to a desired discharge material state (YREF(r)). Moreover, this solution advantageously generates information about an actual status parameter value. The generated actual status parameter value is indicative of the actual internal state (X) of said comminution process. Thus, this solution advantageously conveys, to a user via a user interface, information relating to the actual internal state (X) of said comminution process as well as information relating to a desired internal state (X) of said comminution process. Such conveyed information may be useful to an operator (230) wishing to adjust a solid material feed rate (U2, RS) for controlling or affecting said discharge material state (Y(r)). In this document “desired” values may be referred to as “reference” values. Thus, for example, the above mentioned “first status parameter reference value“ relates to a “desired first status In the context of this document, the term “user” may relate to a person operating a tumbling mill, and such a user may also be referred to as an operator. 217. The system according to example 216, wherein said at least one status parameter value includes a value (X1(r), FI(r); X6, ATOE(r) ) indicative of a position of the toe portion (205). 218. The system according to example 216 or 217, wherein said one or more hardware processors are further configured to: generate said solid material feed rate set point value (U2 _SP , R _SSP ) based on said received first user input. 219. The system according to example 216, 217, or 218, wherein said at least one internal status parameter reference value (X1 _REF ; FI _REF ) comprises at least one reference value (X1REF(r), FIREF(r); X6REF, ATOEREF(r) ) indicative of a desired position of the toe portion (205), and/or at least one reference value (X8 _REF (r)) indicative of a desired filling degree. 220. The system according to any preceding claim, wherein, said data indicative of a desired discharge material state (Y _REF (r), Y1 _REF , Y2 _REF , Y3REF, ... YnREF) includes a desired discharge material state vector (YREF(r)); said desired discharge material state vector (YREF(r)) including data (Y1 _REF ) indicative of desired mass per time unit of said product particles (95; 96), and/or data (Y2REF) indicative of a desired a median particle size (Y2REF). (PO8) 221.1. A system (150, 450, 1140, 150A, 150C) for operating a comminution process in a tumbling mill (10) including a rotatable shell (20) having an internal shell surface (22) with a first number (L) of protrusions (310) configured to engage material when the shell (20) rotates for grinding a charge of material (30) by tumbling the material in the rotating shell so as to generate product particles (95; 96) at a mill output (200), thereby causing a vibration (VIMP) having a first repetition frequency (fR) dependent on a speed of rotation (U1, fROT) of the rotatable shell (20) when a protrusion (310) engages with a toe portion the system comprising a sensor (10) for generating a vibration signal (SFIMP; SEA, SMD, Se(i), S(j), S(q)) indicative of said vibration (VIMP); a position signal generator for generating a position signal (E _P , P(i), P(j), P(q)) indicative of a rotational position of said rotatable shell (20); one or more hardware processors configured to: generate at least one status parameter value (X1(r), FI(r); X2(r); X3(r); X4(r); X5, fROT; X6, ATOE(r); X7(r) ) indicative of an internal state (X) of the comminution process based on said vibration signal and said position signal; deliver a solid material feed rate set point value (U2 _SP , R _SSP ) for setting a solid material feed rate (U2, R _S ) for feeding solid material (115) into a mill solid material feed input (100); said solid material feed rate (U2, RS ) being an amount of solid material feed particles (115) per time unit; said solid material feed rate set point value (U2 _SP , R _SSP ) thereby influencing said internal state (X); a product analyzer (1140) configured to analyze at least a portion of said product particles (96); the product analyzer (1140) being configured to generate at least one product measurement value (Y1, Y2, Y3, ... Yn) based on said product particle analysis; said at least one product measurement value (Y1, Y2, Y3, ... Yn) being indicative of a discharge material state (Y(r), Y1, Y2, Y3, ... Yn); the system further comprising A) an input for receiving data indicative of a desired discharge material state (YREF(r), Y1REF, Y2REF, Y3REF, ... YnREF); and a controller (150B) for generating said solid material feed rate set point value (U2 _SP , RSSP) based on said desired discharge material state (YREF(r), Y1REF, Y2REF, Y3REF, ... Yn _REF ); and said at least one status parameter value (X1(r), FI(r); X2(r); X3(r); X4(r); X5, fROT; X6, ATOE(r); X7(r) ). 222. The system according to example 221, wherein said at least one status parameter value includes a value (X1(r), FI(r); X6, ATOE(r) ) indicative of a position of the toe portion (205). 223. The system according to example 221 or 222, further comprising a reference generator (150C) configured to generate at least one internal status parameter reference value (X1REF(r), FI REF (r); X6REF, ATOEREF(r)) based on said data indicative of said desired discharge material state (Y _REF (r), Y1 _REF , Y2 _REF , Y3 _REF , ... Yn _REF ); said internal status parameter reference value (X1REF; FIREF) being indicative of a desired internal state (X _REF ) for causing said desired discharge material state (Y _REF (r)); and wherein said controller (150B) is configured to generate said solid material feed rate set point value (U2SP, RSSP) based on said at least one internal status parameter reference value (X1REF(r), FI REF (r); X6 _REF , A _TOEREF (r)); and said at least one status parameter value (X1(r), FI(r); X2(r); X3(r); X4(r); X5, fROT; X6, ATOE(r); X7(r) ).