FIELD OF THE INVENTION The present invention generally pertains to a method and device for generating droplets with constant properties, in particular with a constant droplet size and constant droplet speed.

BACKGROUND ART

Inkjet print heads are well known droplet generating devices, wherein droplets of a liquid are generated and expelled. Multiple different kind of inkjet print heads are known, including but not limited to continuous inkjet printing devices and droplet-on-demand inkjet devices. Droplet-on-demand inkjet print heads are known using thermal actuation or actuation by using an electromechanical transducer. Next to the kind of print head, the droplets that are expelled are, in their properties, also dependent on the fluid properties such as viscosity.

It is further known to use a curable liquid, in particular it is known to generate droplets of a radiation curable ink for forming an image on recording substrate. Such radiation curable inks, e.g. UV-radiation curable inks, are very suitable for such image printing, for example because such inks may be cured almost instantaneously allowing quick further handling of the printed recording substrate. Further, such printed images have great durability.

The viscosity of such known curable inks is however not constant over time. Due to the chemically reactive curable components in the ink composition, the viscosity of the ink composition changes slowly but significantly over time. As a result, properties of the droplets generated by an inkjet print head such as droplet speed and droplet size change with the change in viscosity. The changed droplet size and droplet speed may become visible in a printed result, in particular when ink is replenished and droplets of older ink and droplets of fresh ink are printed next to each other.

EP2765003A1 discloses a method to determine a viscosity of an ink in an inkjet print head by detecting a residual pressure wave and calculating the ink viscosity on the basis of a resonance frequency present in the residual pressure wave.

While the method disclosed in the prior art may be suitable to distinguish between different kinds of ink, the accuracy of the known method is however insufficient to detect a viscosity change of a single kind of ink over time, a target viscosity deviation due to manufacturing tolerances and any other small but functionally significant viscosity change or deviation from a target viscosity.

It is an object of the present invention to diminish the visibility of differences between older and fresh ink.

SUMMARY OF THE INVENTION

In an aspect of the present invention, a method according to claim 1 is provided. In particular, the method is configured for controlling a property of a liquid droplet generated in an inkjet print head. The method includes the steps of determining a target viscosity range; determining a liquid viscosity of a liquid arranged in the inkjet print head; maintaining a temperature of the liquid arranged in the inkjet print head constant, if the liquid viscosity is within the target viscosity range; and adjusting a temperature of the liquid arranged in the inkjet print head to adjust the liquid viscosity in order to bring the liquid viscosity within the target viscosity range, if the liquid viscosity is not within the target viscosity range.

Considering that a temperature of a liquid affects the viscosity of the liquid, the present invention provides for the possibility to control the liquid viscosity of the liquid. Having a predetermined target viscosity range, measuring the actual liquid viscosity and adjusting the liquid temperature if the actual liquid viscosity deviates too much from a desired liquid viscosity allows maintaining a constant droplet size and droplet speed by keeping the actual liquid viscosity constant. Hence, in an inkjet printing assembly, a more consistent image quality, i.e. independent from ink aging, can be provided.

In the method according to the present invention, the inkjet print head is provided with a pressure chamber for holding an amount of the liquid, a nozzle operatively connected to the pressure chamber for expelling a droplet of the liquid through the nozzle and an electromechanical transducer for generating a pressure wave in the liquid in the pressure chamber. The step of determining the liquid viscosity comprises the steps of actuating the electromechanical transducer to generate a pressure wave in the liquid in the pressure chamber; sensing a residual pressure wave in the liquid in the pressure chamber; and deriving from the sensed residual pressure wave the liquid viscosity. The electromechanical transducer, such as a piezo-electric actuator or an electrostatic actuator, generates a pressure wave in the liquid and, after actuation, a residual pressure wave remains. The residual pressure wave has properties that depend on a number of factors, one of which is the actual liquid viscosity.

The method according to the present invention comprises deriving a damping factor from the sensed residual pressure wave and deriving the liquid viscosity from the damping factor. The damping factor is one of the easily derivable properties of the residual pressure wave suitable to derive the actual liquid viscosity accurately.

Another factor affecting the natural damping of the residual pressure wave is inertance of a volume of the liquid in the nozzle. This nozzle inertance changes due to a fill rate of the nozzle. For a reliable derivation of the liquid viscosity, the changes in nozzle inertance should be kept to a minimum. The nozzle inertance is substantially constant if little or no liquid flows into or out of the nozzle. So, therefore, in the present invention, the damping factor is determined when an oscillation of liquid in the nozzle is small.

In an embodiment, the damping factor may be determined based on a part of the residual pressure wave, which part of the pressure wave corresponds to a period in which the oscillation of the ink in the nozzle is small. With natural damping of the residual pressure wave, the amount of liquid actually flowing is reduced and hence oscillation of liquid in the nozzle is also reduced. A later part of the residual pressure wave thus corresponds to a period in which the oscillation in the nozzle is small.

In another embodiment, the electromechanical transducer is actuated with such small amplitude that the oscillation of the liquid in the nozzle is small. With small amplitude of the actuation, only a minor pressure wave is initiated and at least at the time of the residual pressure wave being sensed the oscillation in the nozzle is small. In a particular example, said small amplitude is equal to or smaller than a third of a drive amplitude, wherein the drive amplitude corresponds to an actuation of the electromechanical transducer for expelling a droplet. Large amplitude is needed to expel a droplet, which requires a significant oscillation in the nozzle, of course. The inventors have found that at a third of the amplitude for expelling a droplet, or smaller amplitude, the oscillation in the nozzle is sufficiently small for reliably deriving the liquid viscosity from the natural damping of the residual pressure wave.

It is noted that for preventing oscillation of the liquid in the nozzle it is not sufficient to prevent a droplet ejection. With certain amplitude, no droplet will be ejected, while still a flow of ink to and from the nozzle is generated, i.e. a fill rate of the nozzle oscillates. In accordance with the present invention, a residual pressure wave during such oscillation of the fill rate of the nozzle is excluded for viscosity detection. So, if no droplet is ejected, a drive amplitude may be reduced to a level where said oscillation becomes insignificant to the viscosity detection or a part of the residual pressure wave signal is not taken into account during the viscosity calculation.

As used herein, the liquid viscosity of the liquid in the inkjet print head may be derived as an averaged liquid viscosity by performing the method for at least two of a multiple of pressure chambers in such inkjet print head. Averaging the determined liquid viscosities provides for the averaged liquid viscosity as a more accurate liquid viscosity

representative of the actual liquid viscosity by averaging out any inaccuracies in the performance of the method, such as but not limited to statistical errors and noise contributions.

In an embodiment, the liquid is a curable ink, in particular a radiation curable ink, more in particular a UV-radiation curable ink. These inks are chemically reactive and have an increasing viscosity over time. Hence, the method according to the present invention is particularly suitable for reliably and consistently generating droplets of such inks.

As herein described, the viscosity of an ink may change over time. It is noted that the viscosity of an ink may likewise vary between manufacturing batches of the ink. As apparent to those skilled in the art, the present invention is as well suitable for adapting to such viscosity variations or any viscosity variations due to any other cause.

The present invention further provides for an inkjet device comprising an inkjet print head and a control unit, wherein the control unit is configured to perform the method according to the present invention, and is in particular configured to determine a liquid viscosity of the liquid arranged in the inkjet print head; store a predetermined target viscosity range; determine whether the liquid viscosity is within the target viscosity range; maintain a temperature of the liquid arranged in the inkjet print head constant, if the liquid viscosity is within the target viscosity range; and adjust a temperature of a liquid arranged in the inkjet print head to adjust the liquid viscosity in order to bring the liquid viscosity within the target viscosity range, if the liquid viscosity determined in step b is not within the target viscosity range.

In the inkjet device according to the present invention, the inkjet print head comprises a pressure chamber for holding an amount of the liquid; a nozzle operatively connected to the pressure chamber for expelling a droplet of the liquid through the nozzle; and an electromechanical transducer for generating a pressure wave in the liquid in the pressure chamber. Further, a residual pressure wave, which is a pressure wave remaining in the liquid after actuation, comprises a single oscillation frequency and the control unit is configured to sense the residual pressure wave and derive the liquid viscosity from the sensed residual pressure wave based on a damping factor of the single oscillation frequency. Using a print head with a pressure chamber that functions as a Helmholtz resonator and thus results in a residual pressure wave having a single dominant resonance frequency, deriving the damping factor can be performed with high accuracy resulting in a highly accurate derivation of the liquid viscosity, allowing highly accurate control of droplet size and droplet speed, inter alia.

In an embodiment, the inkjet print head comprises multiple pressure chambers with respective nozzles and respective electromechanical transducers, wherein the control unit is configured to determine the liquid viscosity for at least two of the multiple pressure chambers and to average the liquid viscosity of the at least two of the multiple pressure chambers to determine an averaged liquid viscosity.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying schematical drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

Fig. 1 A shows a perspective view of an exemplary inkjet printing assembly;

Fig. 1 B schematically illustrates a scanning inkjet printing assembly;

Fig. 1 C shows a perspective view of another exemplary inkjet printing assembly;

Fig. 2 shows a graph of an exemplary residual pressure wave;

Fig. 3 shows a graph for illustrating the method according to the present invention; Fig. 4A schematically illustrates an inkjet device according to the present invention; and Fig. 4B shows a flow diagram for elucidating an embodiment of the method according to the present invention. DETAILED DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawings, wherein the same reference numerals have been used to identify the same or similar elements throughout the several views.

Fig. 1A shows an inkjet printing assembly 36, wherein printing is achieved using a wide format inkjet printer. The wide-format inkjet printing assembly 36 comprises a housing 26, wherein the printing assembly, for example the ink jet printing assembly shown in Fig. 1 B is arranged. The inkjet printing assembly 36 also comprises a storage means for storing image receiving member 28, 30, a delivery station to collect the image receiving member 28, 30 after printing and storage means for marking material 20. In Fig. 1A, the delivery station is embodied as a delivery tray 32. Optionally, the delivery station may comprise processing means for processing the image receiving member 28, 30 after printing, e.g. a folder or a puncher. The wide-format inkjet printing assembly 36 furthermore comprises means for receiving print jobs and optionally means for manipulating print jobs. These means may include a user interface unit 24 and/or a control unit 34, for example a computer.

Images are printed on an image receiving member, for example paper, supplied by a roll 28, 30. The roll 28 is supported on the roll support R1 , while the roll 30 is supported on the roll support R2. Alternatively, cut sheet image receiving members may be used instead of rolls 28, 30 of image receiving member. Printed sheets of the image receiving member, cut off from the roll 28, 30, are deposited in the delivery tray 32.

Each one of the marking materials for use in the printing assembly are stored in four containers 20 arranged in fluid connection with the respective print heads for supplying marking material to said print heads.

The local user interface unit 24 is integrated to the print engine and may comprise a display unit and a control panel. Alternatively, the control panel may be integrated in the display unit, for example in the form of a touch-screen control panel. The local user interface unit 24 is connected to a control unit 34 placed inside the printing apparatus 36. The control unit 34, for example a computer, comprises a processor adapted to issue commands to the print engine, for example for controlling the print process. The inkjet printing assembly 36 may optionally be connected to a network N. The

connection to the network N is diagrammatically shown in the form of a cable 22, but nevertheless, the connection could be wireless. The inkjet printing assembly 36 may receive printing jobs via the network. Further, optionally, the controller of the printer may be provided with a USB port, so printing jobs may be sent to the printer via this USB port.

Fig. 1 B shows an ink jet printing assembly 3. The ink jet printing assembly 3 comprises supporting means for supporting an image receiving member 2. The supporting means are shown in Fig. 1 B as a medium support surface 1 , but alternatively, the supporting means may be a flat surface. The medium support surface 1 , as depicted in Fig. 1 B, is a rotatable drum, which is rotatable about its axis as indicated by arrow A. The supporting means may be optionally provided with suction holes for holding the image receiving member in a fixed position with respect to the supporting means. The ink jet printing assembly 3 comprises print heads 4a - 4d, mounted on a scanning print head carriage 5. The scanning print head carriage 5 is guided by suitable guiding means 6, 7 to move in reciprocation in the main scanning direction B. Each print head 4a - 4d comprises an orifice surface 9, which orifice surface 9 is provided with at least one orifice 8. The print heads 4a - 4d are configured to eject droplets of marking material onto the image receiving member 2. The medium support surface 1 , the carriage 5 and the print heads 4a - 4d are controlled by suitable controlling means 10a, 10b and 10c, respectively. The image receiving member 2 may be a medium in web or in sheet form and may be composed of e.g. paper, cardboard, label stock, coated paper, plastic or textile.

Alternatively, the image receiving member 2 may also be an intermediate member, endless or not. Examples of endless members, which may be moved cyclically, are a belt or a drum. The image receiving member 2 is moved in the sub-scanning direction A by the medium support surface 1 along four print heads 4a - 4d provided with a fluid marking material.

The scanning print head carriage 5 carries the four print heads 4a - 4d and may be moved in reciprocation in the main scanning direction B parallel to the medium support surface 1 , such as to enable scanning of the image receiving member 2 in the main scanning direction B. Only four print heads 4a - 4d are depicted for demonstrating the invention. In practice an arbitrary number of print heads may be employed. In any case, at least one print head 4a - 4d per color of marking material is placed on the scanning print head carriage 5. For example, for a black-and-white printer, at least one print head 4a - 4d, usually containing black marking material is present. Alternatively, a black-and- white printer may comprise a white marking material, which is to be applied on a black image-receiving member 2. For a full-color printer, containing multiple colors, at least one print head 4a - 4d for each of the colors, usually black, cyan, magenta and yellow is present. Often, in a full-color printer, black marking material is used more frequently in comparison to differently colored marking material. Therefore, more print heads 4a - 4d containing black marking material may be provided on the scanning print head carriage 5 compared to print heads 4a - 4d containing marking material in any of the other colors. Alternatively, the print head 4a - 4d containing black marking material may be larger than any of the print heads 4a - 4d, containing a differently colored marking material. The print head carriage 5 is guided by guiding means 6, 7. These guiding means 6, 7 may be rods as depicted in Fig. 1 B. The rods may be driven by suitable driving means (not shown). Alternatively, the print head carriage 5 may be guided by other guiding means, such as an arm being able to move the print head carriage 5. Another alternative is to move the image receiving material 2 in the main scanning direction B. Each print head 4a - 4d comprises an orifice surface 9 having at least one orifice 8, in fluid communication with a pressure chamber containing fluid marking material provided in the print head 4a - 4d. On the orifice surface 9, a number of orifices 8 is arranged in a single linear array parallel to the sub-scanning direction A. Eight orifices 8 per print head 4a - 4d are depicted in Fig. 1 B, however obviously in a practical embodiment several hundreds of orifices 8 may be provided per print head 4a - 4d, optionally arranged in multiple arrays. As depicted in Fig. 1 B, the respective print heads 4a - 4d are placed parallel to each other such that corresponding orifices 8 of the respective print heads 4a - 4d are positioned in-line in the main scanning direction B. This means that a line of image dots in the main scanning direction B may be formed by selectively activating up to four orifices 8, each of them being part of a different print head 4a - 4d. This parallel positioning of the print heads 4a - 4d with corresponding in-line placement of the orifices 8 is advantageous to increase productivity and/or improve print quality. Alternatively multiple print heads 4a - 4d may be placed on the print carriage adjacent to each other such that the orifices 8 of the respective print heads 4a - 4d are positioned in a staggered configuration instead of in-line. For instance, this may be done to increase the print resolution or to enlarge the effective print area, which may be addressed in a single scan in the main scanning direction. The image dots are formed by ejecting droplets of marking material from the orifices 8.

Upon ejection of the marking material, some marking material may be spilled and stay on the orifice surface 9 of the print head 4a - 4d. The ink present on the orifice surface 9 may negatively influence the ejection of droplets and the placement of these droplets on the image receiving member 2. Therefore, it may be advantageous to remove excess of ink from the orifice surface 9. The excess of ink may be removed for example by wiping with a wiper and/or by application of a suitable anti-wetting property of the surface, e.g. provided by a coating.

Fig. 1 C shows another embodiment of an inkjet printing assembly 14 (herein also referred to as a printing apparatus), in which the medium support surface 1 is a flat surface. On the flat surface a flexible medium or a non-flexible flat medium may be arranged and may be printed on. The medium support surface 1 is supported on a suitable support structure 12 and a guide beam 16 is arranged over the medium support surface 1. Such guide beam 16 is also known in the art as a gantry. The guide beam 16 supports the print head carriage 5 such that the print head carriage 5 is enabled to scan in an X-direction. The guide beam 16 is arranged and configured to be enabled to reciprocate in a Y-direction, wherein the Y-direction is usually substantially

perpendicular to the X-direction. In a known printing apparatus 14, the guide beam 16 is also arranged and configured to be enabled to move in a Z-direction, which is substantially perpendicular to the X-direction and the Y-direction such to enable to adapt the printing apparatus 14 to a thickness of the recording medium being arranged on the medium support surface 1 and/or to be enabled to print multiple layers on top of each other such to generate height differences in a printed image.

In the printing assemblies as shown in Figs. 1A - 1 C, the marking material may be a liquid curable ink that comprises a chemically reactive compound. The chemically reactive compound may change the liquid viscosity of the ink over time. In order to obtain consistent print results by application of consistent droplet size, droplet speed and other droplet properties, the method according to the present invention may be applied.

Fig. 2 shows a graph of a residual pressure wave that may be used for determining an actual liquid viscosity in accordance with a generically known method. Using an inkjet print head having pressure chamber with an associated electromechanical transducer, a pressure wave may be generated and a residual pressure wave, i.e. a pressure wave remaining in the liquid after the actuation pulse is finished, may be sensed by the electromechanical transducer in a sensing mode or by a separate sensor. An exemplary residual pressure wave is illustrated in Fig. 2, wherein the horizontal axis represents time in microseconds and the vertical axis represents amplitude of the residual pressure wave in arbitrary unit (a.u.). The solid line shows the actual residual pressure wave, while the dotted lines show a decaying envelope of the residual pressure wave. The residual pressure wave has a dominant resonance frequency with a period of about 5.5 microseconds. Such a single resonance frequency may be expected when the pressure chamber functions as a Helmholtz resonator. With such Helmholtz resonator, the residual pressure wave may be described by A(t) = a ^■ e ^{"& i■} cos(2 ^■ π ^■ / ^■ t + φ) (eq. 1 ) wherein t represents time, a represents an amplitude, b represents a damping factor, f represents a frequency and φ represents a phase. In particular, the exponential factor describes the envelope function as shown in dotted lines in Fig. 2. The damping factor b is dependent on many factors, most of which are constants. Two factors are however relevant to the present invention. First of all, the damping factor b is dependent on the viscosity of the liquid and, hence, the damping factor may be used for deriving the liquid viscosity. Second, the damping factor b is dependent on an ink oscillation in the nozzle. In the time period t = 0 to about 6 microseconds, a large amplitude is apparent, which seems to deviate from the envelope. This can be presumed to be a result of a significant variation of a volume of the ink in the nozzle. From t = about 10 microseconds, the envelope seems to describe the exponential factor of Equation 1 (eq. 1 ) quite well. So, for determining the damping factor from the residual pressure wave, the first period of about two cycles may be ignored. Then, based on the residual pressure wave in the period wherein the variation of the volume of the ink in the nozzle has become insignificant, i.e. after about 10 microseconds, any suitable mathematical method may be applied for deriving the damping factor b and deriving from the damping factor b the liquid viscosity. This is well known from the prior art and is not further elucidated herein. It is however noted that best results are obtained by mathematically matching the residual pressure wave and Equation 1 , while in the prior art only simplified methods have been described.

It is further noted that the residual pressure wave illustrated in Fig. 2 may be considered to be the result of a droplet generation. The deviating part of the residual pressure wave in the period of the first 10 microseconds may be avoided by application of a drive pulse with a lowered amplitude, in particular smaller than a third compared to a droplet generation pulse. So, without expelling a droplet, the liquid viscosity may be determined.

Fig. 3 illustrates the method according to the present invention. The graph shows a dependency of a liquid viscosity (vertical axis) on a temperature (horizontal axis) for a first liquid L1 and a second liquid L2. For both liquids holds that with an increasing temperature, the liquid viscosity decreases. As a result and as known from the prior art, with a known viscosity behavior of a liquid, determining a viscosity may be used to determine a liquid temperature. For example, for the first liquid L1 , determining that the first liquid L1 has a first viscosity μ1 results in deriving that the temperature of the first liquid L1 has a first temperature T1 .

Having a chemically reactive liquid, wherein the chemical reactive components make that due to aging the viscosity changes, means that the viscosity behavior is not well known. In other words, due to aging, the viscosity behavior may change from the behavior of the second liquid L2 to the behavior of the first liquid L1. More in particular, at the first temperature T1 , a freshly prepared chemically reactive ink may be expected to have a second viscosity μ2. Due to aging and once introduced in a printing assembly, the viscosity may have increased - although still being kept at the first temperature T1 - to the first viscosity μ1 . The printing assembly is however configured to generate droplets based on an ink having a viscosity corresponding to the second viscosity μ2 or, more in general, having a viscosity in a target viscosity range TVR between a lower limit viscosity μΙ_ and an upper limit viscosity μΗ.

Considering that the generic temperature dependence of the viscosity is predictable (curves for the first liquid L1 and for the second liquid L2 are similar in shape), the present invention applies a temperature increase from the first temperature T1 to a second temperature T2 to lower the liquid viscosity from the first viscosity μ1 to the second viscosity μ2, which lies in the target viscosity range TVR. Thus, with a simple temperature increase, a desired viscosity in the target viscosity range TVR is obtained.

It is noted that for control purposes, the inkjet device according to the present invention may comprise a temperature sensor in the inkjet print head for determining a

temperature of the liquid in the inkjet print head and the temperature sensor may be operatively coupled to the control unit. The control unit may be configured to determine a temperature change for adapting the viscosity and control the inkjet print head to increase or decrease the temperature of the liquid. The temperature change may be determined mathematically using a priori known information of the viscosity behavior or the temperature change may be based on a feedback control loop, for example.

Moreover, a feed forward control loop is as well contemplated considering that droplet ejection by actuation also results in energy dissipation and hence heat generation. So, with printing of an image, if many droplets need to be ejected in a short time, a lot of heat may be generated and if fewer droplets need to be ejected, less heat is generated. In order to maintain a constant viscosity, the temperature of the print head may be controlled in accordance with the expected future heat generation by droplet ejection actuation.

In an embodiment, the temperature may be constantly controlled to keep the liquid viscosity at the second viscosity μ2, although the viscosity may already be in the target viscosity range TVR. In such embodiment, the printing device may not eject droplets when the viscosity is not within the target viscosity range TVR and the printing device may eject droplets when the viscosity is within the target viscosity range TVR. Figs. 4A and 4B illustrate an embodiment of the method and the inkjet printing device according the present invention. Fig. 4A schematically shows an embodiment of the inkjet printing device having a pressure chamber 50 filled with a liquid, an

electromechanical transducer 51 operatively coupled to the pressure chamber 50 for generating a pressure wave in the liquid and for sensing a residual pressure wave in the liquid. The inkjet printing device further comprises a nozzle operatively coupled to the pressure chamber 50 for expelling a droplet of the liquid and a control unit 54 operatively coupled to the electromechanical transducer for actuating the transducer and for receiving a signal corresponding to the sensed residual pressure wave.

Fig. 4B shows a flow diagram of an embodiment of the method, applied in the inkjet printing device according to Fig. 4A.

Referring to Fig. 4B, the method is initiated in a first step S1. In a second step S2, the electromechanical transducer 51 is actuated by the control unit 54 such to generate a pressure wave in a liquid, such as an UV-curable inkjet ink, which is provided in the pressure chamber 50 of the inkjet print head.

After actuation, in a third step S3, the electromechanical transducer 51 is used as a sensor and a residual pressure wave in the liquid in the pressure chamber 50 is detected. A corresponding signal is supplied to the control unit 54.

The control unit 54 is configured to calculate a damping factor of a relevant part of the residual pressure wave, which is performed in a fourth step S4. In particular, as above explained, a first part of the residual pressure wave may have a damping factor that is not only affected by the liquid viscosity, but also by a relatively large variation of a volume of the ink in the nozzle 52, which significantly affects a value of the damping factor (b in Equation 1 (eq. 1 )). Such a part is preferably omitted in the calculations in order to obtain a high accuracy of the calculated viscosity. Further, in a particular embodiment, in the fourth step S4, the residual pressure wave may be examined for disturbances in the pressure chamber 50. For example, as well known in the art, disturbances such as air bubbles or dirt result in a disturbed residual pressure wave. From such disturbed residual pressure wave, the disturbance may be detected. On the other hand, in view of the method of the present invention, it is preferred to detect such a disturbed residual pressure wave as such disturbed residual pressure wave will not be represented by equation 1. Hence, a damping factor calculated based on a disturbed residual pressure wave will not represent the damping factor b of equation 1 , effectively resulting in an incorrect viscosity measurement.

Having determined the damping factor in the fourth step S4, it is enabled to calculate a liquid viscosity in a fifth step S5 and use the determined liquid viscosity in a sixth step S6 where the liquid viscosity is compared to a predetermined target viscosity range. It is however noted that, in another embodiment, the fifth step S5 may be omitted and the sixth step S6 may include comparing the calculated damping factor with a target range for the damping factor, wherein such target range for the damping factor corresponds to the target viscosity range.

If the liquid viscosity is within the target viscosity range, the method continues to a seventh step S7 and thus the method is concluded. If the liquid viscosity is however not within the target viscosity range, the method continues with an eighth step S8, where it is determined whether the liquid viscosity is higher than an upper limit of the target viscosity range. If so, the temperature of the liquid in the pressure chamber 50 is controlled to be increased (ninth step S9). If the viscosity is lower than a lower limit of the target viscosity range, the temperature of the liquid is decreased (tenth step S10). After adapting the temperature, the method may proceed at step S2 again in order to verify the liquid viscosity resulting from the temperature adaptation.

It is noted that in an embodiment, such resulting liquid viscosity may be employed in an iterative learning control method in the control unit for further improving an accuracy and/or speed of the viscosity control. Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. In particular, features presented and described in separate dependent claims may be applied in combination and any advantageous combination of such claims is herewith disclosed.

Further, it is contemplated that structural elements may be generated by application of three-dimensional (3D) printing techniques. Therefore, any reference to a structural element is intended to encompass any computer executable instructions that instruct a computer to generate such a structural element by three-dimensional printing techniques or similar computer controlled manufacturing techniques. Furthermore, such a reference to a structural element encompasses a computer readable medium carrying such computer executable instructions.

Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.