RAUTIAINEN, Kimmo (Pakkalantie 77, Pirkkala, FI-33950, FI)
| Claims 1. A method in connection with a measuring device, the method comprising measuring at least one property of a moving web with a measuring device (1), and the measuring device (1) having at least one measuring head (3, 4) with at least one part in contact or nearly in contact with the moving web, characterized by heating this part. 2. A method according to claim 1, characterized by the part being at a distance of 0 to 100 pm from the moving web. 3.3. A method according to claim 1 or 2, characterized by the part being subject to getting dirty and being heated to facilitate its cleaning and/or to prevent it from getting dirty. 4. A method according to any one of the preceding claims, characterized by heating the part with a heating means carried along with it. 5. A method according to any one of the preceding claims, characterized by arranging the part to be in contact with the moving web. 6. A method according to claim 5, characterized by cleaning the part with the moving web. 7. A method according to any one of the preceding claims, characterized by the part being a reference piece (6) of a device measuring the thickness of the web. 8. A method according to any one of the preceding claims, characterized by stabilizing the moving web with the part. 9. A method according to any one of the preceding claims, characterized by heating the part to a temperature of over 100 °C. 10. An arrangement in connection with a measuring device, the measuring device (1) being arranged to measure at least one property of a moving web, and the measuring device (1) having at least one measuring head (3, 4) with at least one part in contact or nearly in contact with the moving web, characterized in that the arrangement comprises means for heating this part. 11. An arrangement according to claim 10, characterized in that the part is arranged at a distance of 0 to 100 pm from the moving web. 12. An arrangement according to claim 10 or 11, characterized in that the part is subject to getting dirty and is arranged to be heated to facilitate its cleaning and/or to prevent it from getting dirty. 13. An arrangement according to any one of claims 10 to 12, characterized in that the means for heating the part are arranged to be carried along with the part. 14. A method according to any one of claims 10 to 13, characterized in that this part is arranged to be in contact with the moving web. 15. An arrangement according to any one of claims 10 to 14, characterized in that the part is a reference piece (6) of a device measuring the thickness of the web. 16. A method according to any one of claims 10 to 15, characterized in that the part is a part stabilizing the moving web. 17. An arrangement according to any one of claims 10 to 16, characterized in that the part is arranged to be heated to a temperature of over 100°C. |
Background of the invention
[0001] The invention relates to a method in connection with a measuring device, the method comprising measuring at least one property of a moving web with a measuring device, and the measuring device having at least one measuring head with at least one part in contact or nearly in contact with the moving web.
[0002] Further, the invention relates to an arrangement in connection with a measuring device, the measuring device being arranged to measure at least one property of a moving web, and the measuring device having at least one measuring head with at least one part in contact or nearly in contact with the moving web.
[0003] In paper manufacture, qualities of paper, such as paper thickness or some other property, are typically continuously measured while the paper web to be manufactured is moving in the paper machine. Usually, measurement of paper qualities utilizes what are called traversing measuring devices in which the actual measuring sensor or sensors is/are arranged at measuring heads at opposite sides of the paper web, the measuring heads being, in turn, arranged in measuring carriages moving over the whole width of the paper web in the cross direction of the paper web. Between the measuring carriages, there is a narrow slot, i.e. an air gap, where the paper web moves at a high velocity. The measuring devices may, however, be fixedly mounted, in which case they continuously measure the same point in the cross direction of the paper web.
[0004] Nowadays, a paper web in a paper machine moves very fast. At the same time, very high requirements are set for the measuring accuracy. A fast-moving paper web vibrates easily, which causes inaccuracy to the measuring results. To eliminate the inaccuracy caused by the vibration of the paper web, measurements utilize techniques in contact with the paper web, where the measuring instrument or reference piece or support surface or another corresponding member is in contact with the paper web either on both sides thereof, or the paper web is supported on one side for the duration of the measurement. Further, non-contact measuring equipment is also used but, in order to stabilize the paper web, the measuring device and/or reference surface or the like is/are arranged in the vicinity of the paper web in such cases as well, for instance in such a way that there is a thin air cushion between the surface of the paper web and the surface of the part supporting it. Publication FI114944 discloses a measuring element arranged in a flexible film surrounding it in such a way that the measuring element is, by means of the flexible film, movable to touch the paper web during the measurement. Publication FI1 14337 discloses a measuring device with at least one measuring head having a reference piece, the reference piece having a reference surface against which the moving web is supported to measure a web property. Publication Fl 1 19260 discloses measuring equipment comprising at least one first sensor, at least one second sensor and at least one reference piece. When the measuring equipment is calibrated, the moving web is supported against the reference piece. Publication WO 2009/032094 discloses a device for measuring the paper web thickness. The publication presents an embodiment with a first and a second measuring head, the second measuring head comprising a contacting plate arranged to touch one surface of the moving paper web. The publication further presents an embodiment where the first and second measuring heads have floating guides in such a way that there is an air cushion between the measuring heads and the paper web.
[0005] All the above embodiments, i.e. contact, semi-contact and non-contact measuring devices, have the problem that the surface of the measuring device in contact with the paper web or in the vicinity thereof gets dirty due to the impurities in the paper web or impurities carried over by the paper web. Further, some paper pulp types may cause measuring device surfaces to get dirty. Also, for example, the surface of a measuring window off the paper web gets dirty. Getting dirty results in distortions in the measurement result and, particularly in the case of fast paper machines and thin paper grades, the dirt increases holes and other defects in the web, caused by the measuring device, and leads, in the worst case, even to breaks in the web. Thus, the measuring device surfaces have to be cleaned rather often to guarantee measuring accuracy and operation of the paper machine. Cleaning the surfaces is rather laborious and difficult because the pulp forming the web very typically contains, at present, a significant amount of impurities. Publication EP0974828 discloses a solution where the measuring window of a measuring device is kept clean during the measurement by using air blowing. Publication Fl 110720 discloses a solution where the measuring element is cleaned with a washing liquid during the measurement.
Brief description of the invention
[0006] An object of this invention is to provide a method and an arrangement of a novel type in connection with a measuring device.
[0007] The method according to the invention is characterized by heating the measuring element.
[0008] The arrangement according to the invention is characterized in that the arrangement comprises means for heating the measuring element.
[0009] An idea of the invention is that the measuring device arranged to measure at least one property of a moving paper web has at least one measuring head with at least one part in contact or nearly in contact with the moving paper web. This part may be a reference piece, for example. The part is heated. In this way, not so much dirt adheres to the part from the web or from the air carried over by the web, or at least it is relatively easy to remove the dirt from the part.
[0010] The idea of an embodiment is that the part is heated with a heating means carried along with it. Thus, the temperature of the part can be reliably kept at a desired level. The idea of a second embodiment is that the part is arranged either to be in contact with the moving web or in the vicinity of the web to stabilize the movement of the web. In this way, the moving web can be reliably stabilized. In an embodiment where the part in question is arranged to be in contact with the moving web, it is also possible to get the contacting part cleaned with the moving web.
[0011] In the context of this description, the term 'paper' means, in addition to paper, also board, soft tissue and chemical pulp.
Brief description of the figures
[0012] Some embodiments of the invention will be explained in greater detail in the attached drawings, in which
Figure 1 shows schematically a side view and a cross-section of a measuring device for measuring the thickness of a paper web; and
Figure 2 shows a schematic exploded view illustrating the structure of a reference piece of a measuring device according to Figure 1. [0013] In the figures, embodiments of the invention are shown simplified for the sake of clarity. Similar parts are denoted with the same reference numerals in the figures.
Detailed description of the invention
[0014] Figure 1 shows schematically a cross-section of a measuring device 1 according to the invention, the measuring device being in Figure 1 arranged to measure the thickness of a paper web 2 moving in the direction of arrow A. For the sake of clarity, the paper web 2 is in Figure 1 shown essentially thicker in relation to the measuring device 1 than it is in reality. The measuring device 1 comprises a first measuring head 3 and a second measuring head 4, between which there is an air gap 5 where the paper web 2 moves at a high velocity. Typically, the first measuring head 3 and the second measuring head 4 are arranged in measuring carriages moving in a measuring frame extending over the whole width of the paper web 2 in such a way that the first measuring head 3 and the second measuring head 4 move back and forth, i.e. traverse, over the whole width of the web to be manufactured, whereby the measuring device 1 substantially continuously measures the thickness of the paper web 2. For the sake of clarity, the measuring frame and measuring carriages are not shown in Figure 1. The first measuring head 3 and the second measuring head 4 may also be arranged fixedly in connection with the paper machine, in which case they measure the thickness of the paper web 2 only at one point in the lateral direction of the paper web 2.
[0015] To measure the thickness of the paper web 2, in connection with the first measuring head 3, measuring means based on both electromagnetism and optics are arranged, and in connection with the second measuring head 4, a reference piece 6 is arranged against which the moving paper web 2 is supported and which forms, at the same time, the reference surface for the measurement. Preferably, the reference piece 6 is plate-like in the manner shown in Figure 1 but the appearance of the reference piece 6 may, however, vary. The measuring means based on electromagnetism may be a coil 7, for example, which is shown very schematically in Figure 1. The reference piece 6 may, in turn, be manufactured of a material conducting electricity well, for example of steel, aluminium or copper, whereby the distance between the coil 7 and the reference piece 6 may be determined in the manner known as such by a person skilled in the art. The reference piece 6 may also be merely coated with a material conducting electricity well, in which case the reference piece 6 may be manufactured of even a great variety of materials. The measuring means based on optics may be, for example, a laser 8 comprising a transmitter part 9 for transmitting a measuring beam 11 towards the paper web 2, and a receiver part 10 for receiving a measuring beam 11' reflected from the surface of the paper web 2. The measuring means based on electromagnetism determine the distance between the first measuring head 3 and the second measuring head 4 while the measuring means based on optics determine the distance between the first measuring head 3 and the paper web 2, whereby the thickness of the paper web 2 may be determined by subtracting the distance between the first measuring head 3 and the paper web 2 from the distance between the measuring heads 3 and 4. Both the coils 7 and 8 are arranged in a support element 12, which is fixedly arranged in connection with the first measuring head 3. Further, the measuring device 1 comprises control means for controlling the operation of the coils 7 and the laser 8 as well as analyzing equipment for processing the measuring data of the coils 7 and laser 8, the control means and the analyzing equipment not being shown in Figure 1 for the sake of clarity.
[0016] Generally, in measuring devices utilizing optics and measuring the thickness of a moving web, it is extremely important to keep the moving web as straight and uniform as possible at the measuring time, the intention being to ensure this by supporting the moving web against the reference surface in the measuring device.
[0017] The reference piece 6 may consist of a suction plate 23, a heating foil 24, an insulator layer 25 and a support plate 26, as illustrated in Figure 2, whereby the different parts of the reference piece 6 are attached to form one entity by means of screws 28. Below the suction plate 23, at its edges, protrusions 16 are formed in such a way that an open air space 17 remains below the suction plate 23.
[0018] The upper surface of the reference piece 6 forms a reference surface 13 for measuring the thickness of the paper web 2. The suction plate 23 is provided with a plurality of holes 14 extending through it.
[0019] The second measuring head 4 comprises a feed channel 18, a flow channel 19 and a nozzle slot 20 for feeding gaseous medium into the space between the second measuring head 4 and the paper web 2, as well as means, for example a blower 21 and a blower control unit 22 used for controlling it, for feeding gas into the feed channel 18. For the sake of clarity, Figure 1 does not show the valves used in feeding gas. The gas to be fed is preferably air but it may also be another gas. The gas to be fed into the feed channel 18 flows from the feed channel 18 through the flow channel 19 into the nozzle slot 20. In the embodiment according to Figure 1 , the flow channel 19 is annular, as is the nozzle slot 20. The nozzle slot 20 is surrounded by a curved control surface, whereby the nozzle slot 20 and the curved control surface form a kind of a Coanda nozzle. From the nozzle slot 20, the gas is discharged towards the paper web 2, and the curved control surface in the immediate vicinity of the nozzle slot 20 turns the gas flow to the space between the paper web 2 and the second measuring head 4. The pressure of the gas and thus its flow rate are arranged to be so high that the gas flow between the paper web 2 and the second measuring head 4 generates negative pressure, which draws the paper web 2 towards the second measuring head 4. Due to the effect of the Coanda nozzle, the negative pressure generated influences the air space 17 below the suction plate 23, and further the space between the reference surface 13 and the paper web 2 at the point of the reference piece 6 via the holes 14 formed through the suction plate 23 in such a way that the paper web 2 is supported, due to the effect of this negative pressure, against the upper surface of the reference piece 6, i.e. against the reference surface 13.
[0020] The feed pressure of the gas to be fed into the space between the moving paper web 2 and the second measuring head 4 and thus its flow rate may vary, depending on, for example, the velocity, tightness, grade and basis weight of the paper web 2. The flow rate of the gas can be changed for instance by means of the blower 21 , which is controlled with the blower control unit 22. The air space 17 may also be provided with a negative- pressure sensor 27 measuring the negative pressure in the air space 17, and the blower control unit 22 can control the operation of the blower 21 with the aid of the measuring data on the negative pressure, transmitted by this sensor. The pressure difference that must be generated between the reference surface 13 and the air space 17 to support the paper web 2 against the reference surface 13 is very small. Even a pressure in the air space 17 which is about 1 % lower than the pressure between the reference surface 13 and the paper web 2 is sufficient to support the web against the reference surface 13 without vibration or waviness. Depending on the velocity, basis weight, grade and tightness of the paper web 2, the pressure difference typically varies between 1 and 20 % but it may also be smaller or greater. The smaller the pressure difference is, the less dust carried along with the web gets through the holes 14.
[0021] The heating foil 24 consists of a foil having a resistance wire connected to it. When electric current is switched on to the heating foil 24, the suction plate 23 heats up. The insulator layer 25 protects the support plate 26 against heating and thus makes the heating of the suction plate 23 more efficient. The heating foil 24 touches the edges of the suction plate 23, i.e. protrusions 16, whereby the edges of the suction plate 23 heat up by conduction. The rest of the suction plate 23 heats up by the effect of thermal radiation.
[0022] Impurities in the paper web and carried along with it do not adhere to a heated suction plate 23 as easily as to an unheated suction plate 23. Thus, in use, the suction plate 23 remains clean significantly longer than before. On the other hand, cleaning the suction plate 23 is, owing to the heating, relatively easy.
[0023] Cleaning the surface of the suction plate 23, i.e. the reference surface 13, may take place in such a way, for example, that the measuring head is run to the edge of the web, to the outside, and the surface is wiped. Wiping may take place manually, or the wiping action may be automated to take place mechanically. On the other hand, owing to the heating, a separate cleaning stage aside the web is not necessarily needed but the paper web as such may keep the heated suction plate 23 clean.
[0024] The suction plate 23 is preferably heated to a temperature of over 90 °C, particularly preferably to a temperature of over 100 °C. The required temperature depends on, for example, the type and amount of dirt gathered on the surface of the suction plate 23, the properties of the suction plate 23 and the temperature of the web to be measured. On the lower surface of the suction plate 23, for instance in the middle of it, a temperature sensor may be arranged with which the temperature of the suction plate 23 is measured to control the required heating capacity. If the diameter of the suction plate 23 is in the range of 10 cm, a heating capacity of 200 W or 300 W, for example, may be used. Further, heating provides the advantage that the temperature of the suction plate 23 can be stabilized. This contributes to maintaining a good measuring result with regard to accuracy.
[0025] In the embodiment of the attached drawings, there is an air space 17 between the heating foil 24 and the suction plate 23. If desired, the heating foil 24 can be arranged to be throughout in contact with the suction plate 23. Naturally, the heating foil 24 must then be provided with holes at the points corresponding to those where the suction plate 23 has holes 14. Further, heating the suction plate 23 may be implemented by arranging a resistance wire to be directly in contact with the suction plate 23. Further, a heating resistor may be integrated into the structure of the suction plate 23. Still further, the suction plate 23 itself may be formed of appropriately resistive material, whereby it serves as the heating resistor in itself. In addition, the suction plate 23 may be heated according to the induction principle, for instance. Furthermore, heating may take place by blowing hot gas, such as hot air or hot steam. The heating means may be arranged to move along with the measuring device, in which case the temperature of the reference surface can be kept constant owing to the continuous heating.
[0026] The reference surface may also be heated outside the web. In this case, the heating may take place for instance by means of an infrared radiation or induction heater, warm-air heater or burner. The heating may take place at intervals of 10 minutes or an hour, for example. At the same time, cleaning of the surface may be carried out manually or mechanically.
[0027] A reference piece in contact with the web may also be used in measuring devices which measure other properties of the moving web than thickness and in which there is a need to stabilize the movement of the web to achieve an accurate measuring result. Such properties include the colour, glare or roughness of the web, for example. The solution may also be used for porosity measurement. Further, the part in contact or nearly in contact with the web, to which the above heating solution can be applied, may be, for example, a support plate stabilizing the web, which may be utilized in topography measurement, for example. Still further, the part may be a measuring sensor as such. Furthermore, the reference piece 6 does not necessarily have holes 14. Thus, the part does not necessarily have to be in contact with the web but heating it in ways described above is advantageous in keeping it clean, also in cases where the part is separated from the web only by a thin air cushion while stabilizing the web as well as in cases where the part is a measuring window of an optical or radiometric measuring device, for example, positioned at a distance from the web. Stabilizing the web may thus take place with the part to be heated, or alternatively other manners to stabilize the web may be used instead of or in addition to it, for example air seals, rolls or another suitable manner. The part to be heated may thus be in contact with the web or at a distance from it. Further, if desired, the part to be heated may be arranged to be in contact with the web only for some time, in which case the part to be heated may be in contact with the web for instance during the measurement and/or the cleaning of the part. In such a case, the part is arranged to move upwards and downwards in the elevational direction of the measuring head, and the surface of the part is caused to touch the moving web by raising and lowering the measuring element. Preferably, the heatable part in contact or nearly in contact with the web is at a distance of 0 to 100 μιτι from the moving web. Thus, it is typically subject to getting dirty but, as described, this problem can be alleviated with the invention. The closeness of the part to the web, in turn, brings about advantages in that the measuring accuracy is at a good level and/or the part stabilizes the web efficiently.
[0028] In some cases, features disclosed in this application may be used as such, irrespective of other features. On the other hand, features disclosed in this application may, if required, be combined to form various combinations.
[0029] The drawings and the related description are only intended to illustrate the idea of the invention. Details of the invention may vary within the claims. Instead of a paper web, the moving web may as well be another web, such as a plastic film or a textile web.