Arrangement and method for supplying a print head with a liquid, and associated control unit

Abstract

According to the invention, a feed pump (7) and a return pump (8) are used in an arrangement (1) and a method for supplying a print head (3) with a liquid (10), and a stroke amplitude (hs, hr) of the feed pump and / or the return pump (7, 8) can be controlled. For this purpose, a measured value of a flow parameter of the liquid (10) is preferably detected.

Description

[0001] PC 24 2167 G January 20, 2025 Arrangement and method for supplying a print head with a liquid, and associated control unit The invention relates to an arrangement for supplying a print head with a liquid. Such arrangements are known in practice. The invention further relates to a method for supplying a print head with a liquid by means of a feed pump and a return pump, wherein a measured value of a flow parameter of the liquid is detected. Such methods are known in practice. The invention further relates to a control unit with which an arrangement for supplying a print head with a liquid can be controlled. Such control units are known in practice. The object of the invention is to improve the liquid supply to print heads.In particular, a consistent fluid supply at the appropriate pressure should be enabled for print heads that print on three-dimensional surfaces using so-called direct-to-shape processes and are moved, for example, by robot arms. To achieve this object, the invention proposes the features of claim 1. In particular, in an arrangement of the type described above, the invention proposes that the arrangement comprise a fluid reservoir, a print head with nozzles, a feed line, a feed pump, a return line, a return pump, a sensor, and a control unit.PC 24 2167 G 2 / 42 January 20, 2025The print head can be movable and, for example, attached to a robot arm. The print head can have a mechanical interface by means of which it can be mechanically connected to a robot arm.The print head can also have an electrical interface by means of which it can be electrically connected to a robot arm. For example, the print head can be used to carry out a direct-to-shape (DTS) process, i.e. directly printing on three-dimensional surfaces, for example walls or vehicles. Since the arrangement has a liquid reservoir, it can have exactly one liquid reservoir or more than one liquid reservoir. The liquid is preferably an ink and the liquid reservoir is preferably an ink reservoir. The liquid can also be another liquid suitable for printing. The arrangement can have exactly one sensor or more than one sensor. The liquid reservoir is connected to the print head via the feed line, and the feed pump is arranged in the feed line.This allows the fluid to be pumped from the fluid reservoir to the print head through the feed line. The feed pump can be located anywhere in the feed line, i.e., between two sections of the feed line. Furthermore, the return line is connected to the print head, and the return pump is located in the return line. This allows the fluid to be pumped from the fluid reservoir, first through the feed line, then through the print head, and finally through the return line. PC 24 2167 G 3 / 42 January 20, 2025 The return pump can be located anywhere in the return line, i.e., between two sections of the return line.Preferably, the return line is also connected to the ink reservoir, so that the liquid can be pumped from the print head back into the liquid reservoir, creating a closed circuit for the liquid. Alternatively, the return line can also be connected to another liquid reservoir, different from the first, into which the liquid is pumped after flowing through the print head. The other liquid reservoir can in turn be connected to the first liquid reservoir by another line, so that a closed circuit for the liquid is again created. Furthermore, the sensor is arranged between a pressure side of the feed pump and a suction side of the return pump. This makes it possible to detect a flow parameter in this area, which includes the print head and the print-head-side sections of the feed line and the return line.The control unit is connected to the sensor, whereby the stroke amplitude of the feed pump can be controlled via the control unit. To achieve this, the feed pump preferably has an oscillating stroke element whose stroke amplitude is adjustable. This has the advantage that by changing the stroke amplitude of the feed pump, the flow conditions of the liquid detected by the sensor, for example a pressure or flow rate, in the feed line can be changed. Another advantage is that by changing the stroke amplitude, a change in the stroke frequency of the feed pump to adapt to the flow conditions in the feed line is no longer necessary or can be suitably adjusted. This is advantageous because pressure pulsations and / or volume flow pulsations in the line depend on changes in the stroke frequency.Furthermore, these pulsations are minimal when a high stroke frequency is set and kept essentially constant. By controlling the stroke amplitude to adapt to flow conditions, the stroke frequency can be set constant and / or at a high value, and pressure and flow rate pulsations can be reduced. Alternatively or additionally, the stroke amplitude of the return pump can be controlled via the control unit. To achieve this, the return pump preferably has an oscillating stroke element with an adjustable stroke amplitude. This is associated with comparable advantages to the controllability of the stroke amplitude of the feed pump, i.e., in particular, the avoidance of pressure and flow rate pulsations to achieve a constant fluid supply to the print head.In an advantageous embodiment, a pulsation damper can be arranged in the feed line, i.e., exactly one or more than one pulsation damper. The pulsation damper can be a hydraulic storage element. This can have two chambers separated from each other by a separating element—for example, an elastomer plate or a diaphragm. One chamber can be in contact with the feed line, i.e., the fluid. The other chamber can be at atmospheric pressure or a different pressure. Furthermore, the other chamber can have a spring element that supports the separating element. Such a pulsation damper can compensate for any pressure and volume flow pulsations generated by the feed pump, so that—in the direction of fluid flow—more uniform flow conditions with reduced pulsations prevail behind the pulsation damper.The pulsation damper is preferably arranged between the feed pump and the print head. This allows the flow of fluid at and in the print head to be designed to be particularly consistent, in particular with low pressure and volume flow pulsations. Alternatively or additionally, a pulsation damper can be arranged in the return line. The pulsation damper is preferably arranged between the return pump and the print head. This makes it possible to achieve the above-mentioned advantages in the return line as well. In a further advantageous embodiment, it can be provided that a pulsation damper is arranged on a suction side or on a pressure side of a pump chamber of the feed pump or the return pump. A pulsation damper can also be arranged on both the suction and pressure sides of the pump chamber of the feed pump or the return pump.One or more pulsation dampers can also be present in the two pumps mentioned. In a further advantageous embodiment, it can be provided that a pump housing encloses the pulsation damper(s) and the lifting element of the feed pump or the return pump. The pump housing can completely enclose the feed pump or the return pump. A common housing can lead to a particularly compact design and thus avoids line-related functional impairments of the pulsation dampers. PC 24 2167 G 6 / 42 January 20, 2025 In a further advantageous embodiment, it can be provided that a channel is formed between a chamber of the pulsation damper and a valve chamber of the feed or return pump. Preferably, the channel opens directly and / or without a connection into the chamber. Such a configuration can be provided for one, several, or all pulsation dampers.This can, optionally in combination with a common pump housing, lead to a particularly compact design with minimal line-related influences, since the channels can be made very short. Alternatively, the hydraulic storage element of the diaphragm pump can also be arranged outside a pump housing and connected to the pump chambers via a line section. In a further advantageous embodiment, it can be provided that a pressure, in particular of a fluid flowing from the feed pump to the return pump, can be measured by means of the sensor. The sensor can also be referred to as a pressure sensor. Based on the measured pressure, the stroke amplitude of the feed pump and / or the return pump can thus be controlled. Preferably, the control is carried out such that the pressure at a measuring point of the sensor is adjusted to a target value.Alternatively or additionally, it can be provided that the sensor can be used to measure the flow of a fluid, particularly from the feed pump to the return pump. For example, a flow rate, a flow volume, or a flow velocity can be measured using the sensor. The sensor can also be referred to as a flow sensor.PC 24 2167 G 7 / 42 January 20, 2025 Thus, based on the measured flow, the stroke amplitude of the feed pump and / or the return pump can be controlled. Preferably, the control is carried out such that the flow at a measuring point of the sensor is adjusted to a target value. Preferably, the pressure and / or the flow can be measured at several points between the feed pump and the return pump by several sensors. This allows more precise control of the arrangement to be achieved.Particularly preferably, the pressure and / or flow rate of the fluid flowing into the print head and / or the fluid flowing out of the print head can be measured. This means that the sensor(s) is / are arranged at or near an inlet and / or an outlet of the print head. This allows precise control of the pressure and / or flow rate in the print head. In a further advantageous embodiment, it can be provided that a sensor, for example the sensor mentioned above, is arranged between the feed pump and the print head and that an additional sensor is arranged between the print head and the return pump. This embodiment allows a flow parameter, for example a pressure and / or a flow rate, to be measured at two different points. Firstly, a flow parameter can be measured before the fluid enters the print head.For example, the pressure built up by the feed pump can be measured here. Using this measured value, the stroke amplitude of the feed pump can be adjusted so that the measured value corresponds to a specified target value.PC 24 2167 G 8 / 42 January 20, 2025 Secondly, a flow parameter of the fluid as it exits the print head can be measured. This allows, for example, the pressure present at the sensor's measuring point in the return line to be measured. This measured value can be used to control the stroke amplitude of the return pump. The stroke amplitude can be adjusted so that the measured value corresponds to a specified target value. In a further advantageous embodiment, it can be provided that the arrangement comprises a further sensor, by means of which a kinematic parameter of the print head can be measured, and that the further sensor is connected to the control unit.Alternatively or additionally, the arrangement can have an interface via which a measurement signal of a kinematic parameter of the print head can be transmitted to the control unit. For example, the interface can be connected to a position sensor of a robot arm, such as the one mentioned above, to which the print head is attached. A kinematic parameter can be a position or orientation of the print head in space or a temporal derivative thereof, such as a speed or acceleration. The orientation can be specified as an angular value. This additional sensor therefore allows the control unit to take additional parameters into account when adjusting the stroke amplitude, in particular parameters that are not flow parameters of the fluid but nevertheless have an influence on the flow conditions in the arrangement.For example, a change in the geodetic height difference between the print head and the feed pump and / or the return pump can influence the pressure or flow rate of the flowing fluid. The same applies to movements of the print head, whose speed or acceleration can be taken into account by the additional sensor when controlling the arrangement. In a further advantageous embodiment, it can be provided that the arrangement comprises a measuring device by means of which a fluid ejection rate of the print head can be determined, and that the measuring device is connected to the control unit. The measuring device can, for example, measure the activity of the nozzles, from which the fluid ejection rate can be calculated.The measuring device can also be connected to a print control unit or a print management system, which controls the printing process, so that the fluid ejection rate can be calculated from the transmitted data. The fluid ejection rate describes how much fluid leaves the arrangement through the nozzles of the print head and consequently influences the flow conditions in the feed line, the return line, and the print head. By taking this value into account in the control unit, the arrangement can be controlled more precisely. In a further advantageous embodiment, it can be provided that the feed pump and / or the return pump are designed as positive displacement pumps. This can have the advantage that the stroke amplitude allows the feed rate of the fluid to be adjusted particularly reliably. The feed pump and / or the return pump are preferably designed as diaphragm pumps.Diaphragm pumps have, among other things, the advantage that they can be operated at high frequencies. High frequencies, in turn, have the advantage that pressure and volume flow pulsations can be minimized, allowing the print head to be supplied with fluid evenly. PC 24 2167 G 10 / 42 January 20, 2025 Diaphragm pumps are also advantageous because the flow or pressure provided by diaphragm pumps can be varied quickly. By using diaphragm pumps in the arrangement according to the invention, the fluid supply can thus be flexibly adapted to the temporally changing operating and flow conditions. Particularly preferably, the feed pump and / or the return pump are designed as piezo diaphragm pumps. This allows particularly high stroke frequencies and / or particularly small stroke amplitudes to be achieved. In a further advantageous embodiment, a drive for the diaphragm pump can be designed as a linear drive.This means that the diaphragm pump has no rotary drive and that only translational movements can be performed by the drive of the diaphragm pump. By avoiding rotating parts in the drive, the drive can respond more quickly to control signals from the control unit. The linear drive is preferably a direct drive. By using a direct drive, there is no gearbox in the drive, so the drive can respond even more quickly to control signals from the control unit. Furthermore, the need for maintenance work can be minimized, as wear on gearbox components cannot occur. The control unit can comprise a microprocessor or a microcontroller. Furthermore, the control unit can be a digital control unit. The control unit can comprise a memory element and / or a computing unit. The control unit can also comprise a controller.The controller can be a single-variable controller or a multi-variable controller. It is also possible for the control unit to include a pilot control. The control unit is preferably configured such that it controls the stroke amplitude of the feed pump and / or the return pump during operation. In a further advantageous embodiment, the diaphragm pump can have two pump chambers, each delimited by a diaphragm, with the two diaphragms being connected to one another via the lifting element. This embodiment eliminates the need for additional bearing elements for the lifting element, since the lifting element is adequately supported by the two diaphragms. The described double-headed design of the diaphragm pump can be particularly advantageous in the arrangement or method according to the invention for supplying a print head with a liquid. This makes it particularly easy to dispense with a position sensor.If such a sensor is present, the control of the arrangement, in particular the pumps, can also be carried out via this position sensor. If no such sensor is present, the control must be based on the utilization of other information. Since the arrangement according to the invention has a sensor and since a measured value of a flow parameter of the liquid is recorded in the method, such information can be provided and a position sensor that measures the position of a lifting element in the feed and / or return pump is not absolutely necessary. The fact that the two diaphragms are connected to one another via the lifting element also ensures that the two pump chambers can be operated alternately. By means of a lifting movement of the lifting element, liquid can be sucked into one of the pump chambers, while liquid is expelled from the other pump chamber.This allows the two pump chambers to operate without interfering with each other. This design ensures that the pump chambers are always in different phases: while the first pump chamber is in the suction phase, i.e., the intake process, the second pump chamber is in the pressure phase, i.e., the discharge process, and vice versa. This allows the flow conditions on the suction and pressure sides of the diaphragm pump to be stabilized. The diaphragm pump preferably has a pulsation damper on the suction side and a pulsation damper on the pressure side, each connected to both pump chambers. Thus, a pulsation damper common to both pump chambers is connected to the suction side of the two pump chambers. A pulsation damper common to both pump chambers is also connected to the pressure side of the pump chambers.In this way, it is possible to dispense with the need to provide two separate pulsation dampers for each of the two pumping chambers. This allows the installation space of the diaphragm pump to be reduced. The described double-headed diaphragm pump can thus have one, two, three, four, or more pulsation dampers. In a further advantageous embodiment, the diaphragm pump can have pulsation dampers, with at least one of the pulsation dampers being arranged on a suction side of the pumping chambers and / or at least one of the pulsation dampers being arranged on a pressure side of the pumping chambers. Preferably, the diaphragm pump has at least four pulsation dampers, with two of the pulsation dampers being arranged on each suction side of the pumping chambers and two of the pulsation dampers being arranged on each pressure side of the pumping chambers. Thus, a pulsation damper is arranged on each suction and pressure side of each pumping chamber. This allows pressure and pressure dampers to be reduced.January 2025Volume flow pulsations can be reduced even more effectively. In an advantageous embodiment, it can be provided that the control unit is configured to control the arrangement such that a method as described below is carried out. Alternatively or additionally, the features of the independent claim directed to a method are provided according to the invention to achieve the aforementioned object. In particular, to achieve the aforementioned object, in a method of the type described above, it is proposed according to the invention that a stroke amplitude of a stroke element of the feed pump is controlled as a function of the measured value and / or that a stroke amplitude of a stroke element of the return pump is controlled as a function of the measured value.The advantage here is that the control of the feed pump and / or the return pump does not have to be via the stroke frequency, which can therefore be kept constantly at a high value, so that pressure and volume flow pulsations that depend on the stroke frequency can be reduced. At the same time, recorded measured values ​​can be taken into account in the control by adjusting the stroke amplitude of the feed pump and / or the return pump accordingly, e.g., to achieve the desired flow conditions of the liquid. An arrangement as described above is preferably used for the method. This allows the advantages of the arrangement to be utilized in the method as well. In an advantageous embodiment, it can be provided that the flow parameter is a flow rate PC 24 2167 G 14 / 42 January 20, 2025.This embodiment is preferred, for example, when the flow rate, such as the volume flow, of the liquid is to reach a specific setpoint or be maintained within a specific operating window. Alternatively or additionally, the flow parameter can be a pressure. This embodiment is preferred, for example, when the pressure of the flowing liquid is to reach a specific setpoint or be maintained within a specific operating window. In a further advantageous embodiment, the stroke amplitude can be controlled such that changes in the meniscus pressure in the print head are compensated. The meniscus pressure is the pressure prevailing in the liquid channels in the nozzles of the print head.The meniscus pressure should preferably be maintained within an operating window determined by the following requirements: Firstly, the meniscus pressure should be a slight negative pressure so that the fluid is not forced out of the nozzles. Secondly, the meniscus pressure should not be too negative, so that no ambient air penetrates the print head through the nozzles. The operating window in which the meniscus pressure is to be maintained is therefore preferably between the pressure at which ink is forced out of the print head and the pressure at which air is sucked into the print head. The advantage of the above-mentioned design is that the print head can be operated optimally and no ink escapes from the print head unintentionally. Compensation preferably takes place in real time. In this way, changes in the meniscus pressure can be detected and compensated directly.Alternatively or additionally, the compensation is carried out predictively. The compensation is carried out, for example, by taking into account a kinematic parameter of the print head, such as a speed and / or an acceleration of the print head, during control. The kinematic parameter is preferably used to anticipate changes in the meniscus pressure. This allows changes in the meniscus pressure to be further reduced, in particular avoided. In a further advantageous embodiment, it can be provided that the measured value of the flow parameter is recorded between a pressure side of the feed pump and a suction side of the return pump. The flow parameter is preferably a pressure. Since the print head is also arranged in the aforementioned area, the measured values ​​in this area are particularly relevant for controlling the stroke amplitudes.In a further advantageous embodiment, it can be provided that a setpoint for the flow parameter is specified or calculated and compared with the measured value of the flow parameter, and that the stroke amplitude of the feed pump is controlled depending on a comparison result. This allows the flow parameter, particularly in the feed line, to be maintained within a specified operating window around the setpoint. Alternatively or additionally, the stroke amplitude of the return pump is controlled depending on the comparison result. This allows the flow parameter, particularly in the return line, to be maintained within a specified operating window around the setpoint. Preferably, this adjusts the measured value to the setpoint. This results in the flow parameter being precisely adjusted to the setpoint.Alternatively or additionally, the setpoint for the flow parameter describing a pressure is calculated from a specified average reference pressure and a specified pressure drop for the print head. The advantage here is that the pressure does not have to be measured directly in the print head; instead, a pressure measurement can be taken upstream of the print head in the supply line and a pressure measurement downstream of the print head in the return line. By specifying an average reference pressure, which is preferably a reference value for the meniscus pressure that should prevail in the print head, and a pressure drop for the print head, i.e. a pressure loss of the fluid as it flows through the print head, the respective setpoints at the measuring points can be calculated, and the stroke amplitude can be controlled accordingly.In a further advantageous embodiment, it can be provided that a kinematic parameter of the print head is determined and that the stroke amplitude of the feed pump and / or the return pump is controlled depending on the kinematic parameter. The kinematic parameter can be, for example, a pose, i.e., a position and / or an orientation, of the print head, or a temporal change in the pose, in particular a speed and / or an acceleration. The kinematic parameter can be measured via a sensor attached to the print head. However, the measured value of an external sensor can also be used, for example, a position sensor of a robot arm, such as the one mentioned above, by which the print head is moved in space.Taking into account the spatial position of the print head and its temporal changes offers the possibility of adapting the control of the stroke amplitudes to parameters that are not flow parameters of the fluid, but have a direct influence on these flow parameters. For example, this configuration allows changes in the flow parameters to be controlled and / or predicted in real time and, if necessary, avoided or compensated for preventively. In a further advantageous embodiment, it can be provided that a fluid ejection rate of the print head is determined and that the stroke amplitude of the feed pump and / or the return pump is controlled depending on the determined fluid ejection rate.The fluid ejection rate describes the amount of fluid leaving the print head through the nozzles per unit of time and thus affects the fluid flow parameters, in particular the pressure, the meniscus pressure, and the flow rate. The fluid ejection rate can be determined, for example, by using a measuring device, such as the one mentioned above, with which a measured value is recorded and processed in a calculation step. It is therefore advantageous to consider the fluid ejection rate when controlling the stroke amplitudes of the pumps in order to compensate for resulting changes in the fluid flow conditions. In a further advantageous embodiment, it can be provided that a stroke frequency of a stroke element of the feed pump and / or the return pump is set such that the PC 24 2167 G 18 / 42 20.January 2025 Stroke frequency is between 0.7 and 1.4 times, preferably between 0.8 and 1.2 times, particularly preferably between 0.9 and 1.1 times, most preferably between 0.98 and 1.02 times, a mechanical resonance frequency of the feed pump and / or the return pump, in particular 1.00 times. The lifting element of the respective pump can be mounted, for example, by means of leaf springs or by means of the pump diaphragm. The mechanical resonance frequency of the respective pump is thus the resonance frequency of the system, which is composed of the oscillating lifting element and the element that supports the lifting element, for example, the pump diaphragm and / or the leaf springs. It can be particularly advantageous from an energetic point of view to set the stroke frequency close to the resonance frequency, since this allows the respective pump to be operated in the case of mechanical resonance.In this way, a maximum proportion of the electrical energy used to drive the lifting element can be transferred to pumping the fluid. This can result in reduced energy consumption of the pump. The stroke frequency is preferably kept constant. This allows the energy consumption of the respective pump to be kept at a consistently low level. In a further advantageous embodiment, it can be provided that a stroke frequency of the feed pump and / or the return pump is set so that the stroke frequency lies outside an impedance maximum of a supply or return line of the fluid. PC 24 2167 G 19 / 42 January 20, 2025 Impedance refers to the flow resistance. Impedance is related to volume flow and pressure pulsations that occur in the fluid pumped through the supply or return line, depending on the stroke frequency of the respective pump.Impedance maxima can lead to high volume flow and pressure pulsations and are reached at certain stroke frequencies, which are avoided according to the embodiment of the invention. The stroke frequencies at which impedance maxima occur can be estimated or calculated and can also be determined by a manual or automatic test run of the feed pump and / or the return pump. During this test run, the available stroke frequencies of the respective pump are run through, with the pressure in the supply or return line being recorded by a sensor. Impedance maxima occur at those stroke frequencies at which local pressure maxima are recorded. Outside of an impedance maximum means that the existing impedance is less than fifty percent of the impedance maximum, preferably less than thirty percent of the impedance maximum.The advantage of this design is that the feed pump and / or the return pump experience less resistance from the supply or return line, thus achieving greater energy efficiency. Furthermore, more uniform flow conditions can be achieved in the lines and in the pressure head, as pressure pulsations are reduced, thus making it easier to control the pressure and / or the flow of the fluid. Preferably, the stroke frequency lies at an impedance minimum in the supply or return line. The stroke frequencies at which impedance minima exist can also be estimated, calculated, or determined by a test run described above. In this case, the advantages described above become even more apparent.At an impedance minimum means that the existing impedance is less than twice the impedance minimum, preferably less than 1.3 times, particularly preferably less than 1.1 times, above the impedance minimum. In a further advantageous embodiment, it can be provided that the feed pump and the return pump are controlled synchronously. In particular, this means that the feed pump and the return pump are operated at the same stroke frequency. This simplifies the control of the method and improves the control result. In a further advantageous embodiment, it can be provided that the feed pump and the return pump are controlled with a phase offset from one another. Preferably, the feed pump and the return pump are controlled with a phase offset from one another by a value of 160° to 200°, particularly preferably by 180°. This can reduce pressure and volume flow pulsations at the print head.Alternatively or additionally, the stroke frequencies of the feed pump and the return pump are the same. This allows pressure pulsations at the pressure head to be further reduced. In a further advantageous embodiment, it can be provided that a manipulated variable of the stroke amplitude is updated in time steps, whereby the time steps are selected depending on the stroke frequency of the feed pump and / or the return pump. This allows control signals to be taken into account by the pumps particularly quickly.PC 24 2167 G 21 / 42 January 20, 2025The time steps are preferably proportional to the current inverse of the stroke frequency. The inverse of the stroke frequency is the stroke period. Thus, a new manipulated variable of the stroke amplitude can be specified for each stroke period or for any number of stroke periods, whereby the pressure or flow of the fluid can be adapted particularly quickly and precisely to current measured values.Alternatively or additionally, the features of the independent claim directed to a control unit are provided to achieve the object stated at the outset. In particular, it is therefore proposed according to the invention that a control unit is set up to control an arrangement as described above in such a way that a method as described above is carried out. The advantages described above can be implemented by means of this control unit. The invention will now be described in more detail using an exemplary embodiment, but is not limited to this exemplary embodiment. Further variants of the invention and exemplary embodiments arise from combining the features of individual or multiple claims with one another and / or with individual or multiple features of the exemplary embodiments and / or the previously described variants of devices and methods according to the invention. Fig. 1 shows an arrangement for supplying a print head with a liquid, Fig.2 shows a diaphragm pump according to the invention, which can be used as a feed and / or return pump. PC 24 2167 G 22 / 42 January 20, 2025. Fig. 3 shows a view of a diaphragm pump according to the invention with two pump chambers, which can be used as a feed and / or return pump. Fig. 4 shows a view of another diaphragm pump according to the invention with two pump chambers. Fig. 5 shows a view of another diaphragm pump according to the invention with two pump chambers, which has only two pulsation dampers. In the arrangement 1 shown in Fig. 1 for supplying a print head 3 with a liquid 10, the print head 3 is connected to a liquid reservoir 2 via a feed line 5 and a return line 6, so that a closed circuit is formed for the liquid 10. In the embodiment shown, the liquid 10 is ink. The print head 3 is a print head 3 of an inkjet printer and has a plurality of nozzles 4 for this purpose.In the feed line 5 there is arranged a feed pump 7 with drive M, which is designed as a diaphragm pump 17 and has a lifting element 9. Furthermore, in the return line 6 there is arranged a return pump 8 with drive M, which is also designed as a diaphragm pump and has a lifting element 9'. The pressure head 3 is arranged between a pressure side 13 of the feed pump 7 and a suction side 14 of the return pump 8. In the feed line 5, between the feed pump 7 and the pressure head 3, a sensor 11 is arranged, with which a pressure p. s can be measured in the supply line. Furthermore, a sensor 11 is arranged in the return line 6 between the pressure head 3 and the return pump 8, with which a pressure p rPC 24 2167 G 23 / 42 January 20, 2025 in the return line. The print head 3 also has a further sensor 16 with which a kinematic parameter of the print head 3 can be measured. This is, in particular, a position of the print head 3, which is specified with the coordinates x, y, z, and an orientation of the print head 3, which is specified with the angles ψ, θ, ϕ. A measuring device 18 is placed on the print head 3, with which the fluid ejection rate Q jet from the nozzles 4 of the print head 3. This can be done, for example, as described above. The recorded measured values ​​ps, pr, Qjet, x, y, z, ψ, θ, ϕ are sent to a control unit 12. Based on these values, the control unit 12 outputs a stroke amplitude h to the feed pump 7 and the return pump 8, respectively. s , h r and a stroke frequency f s , f r The control unit provides the control variables for the stroke amplitude h s , h rand the stroke frequency f s , f r in time steps Δt. Furthermore, in the control unit 12, a mean reference pressure p m,ref , which in one embodiment corresponds to a target meniscus pressure for the print head 3, and a pressure drop Δp ref via print head 3. These are predefined target values. From these values, the target values ​​for the pressure p s , which is measured by the sensor 11 on the pressure side 13 of the feed pump 7, and for the pressure p r , which is detected by the sensor 11 on the intake side 14 of the return pump 8, can be calculated. The arrangement 1 has a total of four pulsation dampers 15. Two pulsation dampers 15 are arranged upstream and downstream of the feed pump 7. Two further pulsation dampers 15 are arranged upstream and downstream of the return pump 8. The pulsating delivery rates Q s , Q rthe feed pump 7 and the return pump 8 are hydraulically balanced so that the flows Q hs , Q hr in the feed line 5 and the return line 6 are no longer pulsating, but continuous. With the arrangement 1, in particular, a method for supplying a print head 3 with a liquid 10 can be carried out. In such a method, in the embodiment described here, the pressure p in the feed line 5 between the feed pump 7 and the print head 3 can be measured by means of a sensor 11. s measured. This pressure p s is then sent to the control unit 12. The same applies to the pressure p r , which is measured by a sensor 11 in the return line 6. In the control unit 12, a mean reference pressure p m,ref and a pressure drop Δp ref The pressure drop Δp refrefers to the pressure loss that the liquid 10 experiences when flowing from the sensor 11 in the supply line 5, through the pressure head 3, to the sensor 11 in the return line 6. From the two specified reference values ​​for the pressure, a setpoint for the pressure p s and a setpoint for the pressure p r The setpoint for the pressure p s the sum of the mean reference pressure p m,ref and half the pressure drop Δp ref . As setpoint for the pressure p r the difference between the mean reference pressure p m,ref and half the pressure drop Δp ref used. The mean reference pressure p m,ref corresponds to the meniscus pressure that should be present in the nozzles 4 of the print head 3. The control unit 12 compares the measured pressure values ​​p s , p rwith the calculated setpoints. Based on the comparison result, the control unit 12 adjusts the control variables for the stroke amplitudes hs, hr of the supply and return pumps 7, 8. The stroke amplitude h s the feed pump 7 is controlled so that the pressure p s in the flow line 5 is adjusted to its setpoint. The pressure p r in the return line 6 is controlled accordingly by the stroke amplitude h r of the return pump 8 is adjusted to its setpoint. The control unit 12 updates the control variables for the stroke amplitudes hs, hr at each time step Δt – or alternatively at any multiple of the time step Δt. The time step Δt corresponds to the current reciprocal of the stroke frequency f s the feed pump 7 or the current reciprocal of the stroke frequency f rof the return pump 8. Preferably, the feed and return pumps 7, 8 are operated at the same stroke frequency fs, fr, so that the time step Δt is adapted to both pumps 7, 8. The time step Δt thus corresponds to the pumping period of the pumps 7, 8. In this way, a new stroke amplitude h s , h r which allows for a fast response control of the pressure p s , p r Other process parameters that are taken into account in the control unit 12 are the liquid discharge rate Q jet as well as the kinematic parameters x, y, z, ψ, θ, des Print head 3. The liquid ejection rate Q jetDescribes the amount of fluid that leaves the print head 3 through the nozzles 4 per unit of time. This is the fluid 10 used for printing, for example, for printing on a vehicle or other three-dimensional object. The fluid ejection rate Q jet has a direct influence on the pressure p s , p r in lines 5, 6 and also on the pressure in the print head. Since the fluid ejection rate Q jet when controlling the stroke amplitudes h s , h r taken into account, changes in the pressures p s , p r be compensated preventively, even before they are detected by the sensors 11 in the lines 5, 6.The kinematic parameters x, y, z, ψ, θ, of the print head 3 include its position x, y, z in space and also its orientation ψ, θ, Temporal changes in these parameters – such as speed or acceleration – are also taken into account in the control unit 12. The position and orientation influence, among other things, changes in the geodetic height difference between print head 3 and pumps 7, 8. In addition, temporal changes – particularly due to rapid or jerky movements of the print head 3 – cause changes in the pressures p s , p r in lines 5, 6 and in print head 3. When controlling the stroke amplitudes h s , h r can be determined by recording kinematic parameters x, y, z, ψ, θ, somit Pressure changes are compensated preventively. For particularly high energy efficiency, the stroke frequencies f s , f rof the pumps 7, 8 are controlled so that they are close to the mechanical resonance frequencies of the pumps 7, 8. In addition, the impedance of the supply line 5 and / or the return line 6 is taken into account by adjusting the stroke frequencies f s , f r outside the respective impedance maxima. The stroke frequencies fs, fr, at which impedance maxima are present,PC 24 2167 G 27 / 42 20 January 2025are determined empirically either during commissioning of arrangement 1. For this purpose, the available stroke frequencies f s , f r of the pumps 7, 8 and at each stroke frequency f s , f r Measured values ​​for the pressure p s , p rThe impedance maxima occur at those stroke frequencies at which the measured pressure pulsations are highest. Alternatively, the stroke frequencies fs, fr at which impedance maxima occur are calculated. The aforementioned stroke frequencies fs, fr are stored in the control unit 12 after measurement or calculation so that they can be avoided during operation. In addition, the pumps 7, 8 are operated with a phase shift of 180°. This means that when the feed pump 7 discharges liquid 10, the return pump 8 draws in liquid 10, and vice versa. In this way, the pumps 7, 8 do not interfere with each other, and energy consumption can be reduced. In addition, the pressure and volume flow pulsations in the pressure head 3 are minimized. Figure 2 shows a diaphragm pump 17 according to the invention, which can be integrated into an arrangement 1 according to the invention, for example the arrangement 1 from Figure 1, as a feed pump 7 and / or as a return pump 8.The diaphragm pump 17 has a pumping chamber 20. The pumping chamber 20 is defined by a diaphragm 19. The diaphragm 19 is connected to a lifting element 9. Figure 2 also shows the coil windings 27 of the linear drive of the diaphragm pump 17. In this embodiment, the linear drive consists of a rigid coil armature and a mobile rotor, the lifting element 9, which has permanent magnets. If electrical current flows through the coil windings, the lifting element 9 is set in motion, causing a suction or discharge process in the pumping chamber 20, depending on the direction of impact of the lifting element 9. The diaphragm pump 17 comprises two pulsation dampers 15. These are connected via pump lines 23 to the suction valve 28 and the discharge valve 29 of the diaphragm pump 17.The pump lines 23 are short channels 34 that connect a valve chamber 33 of the diaphragm pump 17 and a chamber 32 of the pulsation damper 15. The channels 34 open directly and seamlessly into the respective chamber 32. The diaphragm pump 17 has a pump housing 26, which, among other things, comprises the pulsation damper 15, the lifting element 9, the diaphragm 19, and the drive M. The suction valve 28 and the discharge valve 29 are in turn connected to the pump chamber 20 via pump lines 23. The pulsation dampers 15 each have a separating layer 25, which in this example is an elastomer plate. In this example, the pulsation dampers 15 have only one chamber 32, which is in contact with the liquid 10 to be pumped. The other chamber of the respective pulsation damper 15 is omitted since the pulsation dampers 15 operate with atmospheric air pressure.Thus, the respective separating layer 25 only separates the chamber 32 of the pulsation damper 15, in which the liquid 10 is located, from the environment. On the suction side 21 of the pump chamber 20 of the diaphragm pump 17, an orifice plate 24 is arranged at an inlet 30 of the diaphragm pump 17 in front of the pulsation damper 15. This improves the damping behavior of the suction-side pulsation damper 15. PC 24 2167 G 29 / 42 January 20, 2025 On the pressure side 22 of the pump chamber 20, an orifice plate 24 is also arranged at an outlet 31 of the diaphragm pump 17 behind the pulsation damper 15. This also improves the damping behavior of the pressure-side pulsation damper 15. Figure 3 shows a diaphragm pump 17 according to the invention, which can be used in an arrangement 1 according to the invention, for example the arrangement 1 of Figure 1, as a feed pump and / or as a return pump and which has two pumping chambers 20, 20'. These pumping chambers 20, 20' are each delimited by a diaphragm 19, 19'.The two diaphragms 19, 19' are connected to each other via a lifting element 9. This couples the pump chambers 20, 20' to each other. This means that when one of the pump chambers 20, 20' is in a suction process or suction phase, the other of the pump chambers 20, 20' is in an expulsion process or pressure phase. In the view shown in Figure 3, the lifting element 9 of the diaphragm pump 17, which is a positive displacement pump, is in a rest position. For this reason, the pump chambers 20, 20' are of equal volume in the pumping phase shown. Figure 3 also shows the coil windings 27 of the linear drive of the diaphragm pump 17, which is designed as a direct drive. In this embodiment, the linear drive consists of a rigid coil armature and a mobile rotor, the lifting element 9, which has permanent magnets.If electrical current flows through the coil windings 27, the lifting element 9 is set in motion, causing suction or discharge processes to take place in the pump chambers 20, 20', respectively, PC 24 2167 G 30 / 42 January 20, 2025 in phase shifts. In this embodiment, the diaphragm pump 17 comprises four pulsation dampers 15. These are connected via pump lines 23, i.e., fluid lines within the diaphragm pump 17, to the suction valves 28 and the discharge valves 29 of the diaphragm pump 17, which in turn are connected via pump lines 23 to the pump chambers 20, 20'. Furthermore, the pulsation dampers 15 are connected via pump lines 23 to the inlet 30 and outlet 31 of the diaphragm pump 17. These pump lines 23 are short channels 34 that connect a valve chamber 33 of the flow and / or return pump 7, 8 and a chamber 32 of the pulsation damper 15. The channels 34 open directly and seamlessly into the respective chamber 32.The diaphragm pump 17 has a pump housing 26, which, among other things, comprises the pulsation damper 15, the lifting element 9, the diaphragm 19, and the drive M. The pulsation dampers 15 each have a separating layer 25, which in this example is an elastomer plate. In this example, the pulsation dampers 15 have only one chamber, which is in contact with the liquid 10 to be pumped. The other chamber of the respective pulsation damper 15 is omitted, since the pulsation dampers 15 shown operate with atmospheric pressure. Thus, the respective separating layer 25 only separates the chamber of the pulsation damper 15, in which the liquid 10 is located, from the environment. On the suction side 21 of the diaphragm pump 17, an orifice plate 24 is arranged in the pump lines 23 in front of an inlet opening of the pulsation damper 15. This improves the damping behavior of the pulsation damper 15.PC 24 2167 G 31 / 42 20.January 2025 On the pressure side 22, an orifice plate 24 is also arranged in the pump line 23 at an outlet opening of the pulsation damper 15. This also improves the damping behavior of the pressure-side pulsation damper 15. Figure 4 shows a diaphragm pump 17 according to the invention, which can be used in an arrangement 1 according to the invention, for example the arrangement 1 from Figure 1, as a feed pump 7 and / or as a return pump 8. The structure of the diaphragm pump 17 is similar to that of the diaphragm pump 17 from Figure 3. The only difference is that the orifices 24 are not arranged in front of the inlet and outlet openings of the pulsation damper 15, as is the case in Figure 3. Instead, an orifice plate 24 is arranged at the inlet 30 of the diaphragm pump 17 and at the outlet 31 of the diaphragm pump 17.As a result, the two pulsation dampers 15 on the suction side 21 of the pump chambers 20 can be connected to one another via a pump line 23 to form a large pulsation damper. On the pressure side 22, the two pulsation dampers 15 can also be connected via a pump line 23. This can have an advantageous effect on the damping behavior of the pulsation dampers 15. The diaphragm pump 17 according to the invention shown in Figure 5 can be used in an arrangement 1 according to the invention, for example the arrangement 1 from Figure 1, as a feed pump 7 and / or as a return pump 8. The structure of the diaphragm pump 17 is similar to that of the diaphragm pump 17 in Figure 3. The only difference is that the diaphragm pump 17 has only two pulsation dampers 15 instead of four. One of the pulsation dampers 15 is arranged on the suction side 21 of the pump chambers 20 of the diaphragm pump 17. Unlike in the embodiment according to Fig.4, in which the separating layer 25 of the pulsation damper 15 is arranged on a transverse side of the diaphragm pump 17, in the embodiment shown in Fig. 5, the separating layer 25 is arranged on a longitudinal side of the diaphragm pump 17. A "transverse side" is a side of the diaphragm pump 17 that forms a closure of the diaphragm pump 17 in the stroke direction. A "longitudinal side" is a side located to the side of the stroke element 9. The separating layer 25 is divided into two sections, wherein the sections are preferably spatially separated from one another centrally by the inlet 30 of the diaphragm pump 17. The liquid 10 thus enters the suction-side pulsation damper 15 via the inlet 30 and an orifice plate 24. The other pulsation damper 15 is arranged on the pressure side 22 of the pump chambers 20 of the diaphragm pump 17. Here too, the outlet 31 of the diaphragm pump 17 is preferably located centrally between two sections of the separating layer 25 of the pressure-side pulsation damper 15.The separating layer 25 is arranged on a transverse side of the diaphragm pump 17, which is opposite the transverse side on which the separating layer 25 of the suction-side pulsation damper 15 is arranged. A baffle 24 is also arranged at the outlet 31. The pulsation dampers 15 are each connected to both pump chambers 20, 20' via pump lines 23. By providing only two pulsation dampers 15, a particularly compact design of the diaphragm pump 17 can be achieved. It is proposed that, in an arrangement 1 and a method for supplying a print head 3 with a liquid 10, a feed pump 7 and a return pump 8PC 24 2167 G 33 / 42 January 20, 2025 be used, and that a stroke amplitude h. s , h r the flow and / or return pump 7, 8 is controllable. For this purpose, a measured value of a flow parameter of the liquid 10 is preferably recorded.

[0002] PC 24 2167 G 34 / 42 20 January 2025List of reference symbols1 Arrangement2 Fluid reservoir3 Print head 4 Düsen 5 Flow line 6 Return line 7 Flow pump 8 Return pump 9 Lifting element 10 Liquid 11 Sensor 12 Control unit 13 Pressure side (of 7) 14 Suction side (of 8) 15 Pulsation damper 16 Additional sensor 17 Diaphragm pump 18 Measuring device 19 Diaphragm (of 17) 20 Pump chamber (of 17) 21 Suction side (of 20) 22 Pressure side (of 20) 23 Pump line 24 Blende 25 Separating layer (of 15)26 Pump housing27 Coil windings (of 17)28 Suction valve29 Pressure valve30 Inlet (of 17)31 Outlet (of 17) 32 Kammer 33 Valve room PC 24 2167 G 35 / 42 20 January 2025 34 Kanal M Antrieb f r Stroke frequency (of 8) f s Stroke frequency (of 7) h r Stroke amplitude (of 8) h s Stroke amplitude (of 7) p m,ref mean reference pressure p r Pressure (in 6) p s Pressure (in 5) Q hr Flow (in 6) Q hs Flow (in 5) Q jetLiquid discharge rate Q r Delivery rate (of 8) Q s Delivery rate (of 7)x Position (of 3) in x-directiony Position (of 3) in y-directionz Position (of 3) in z-directionΔp ref Pressure drop (for 3)Δt time stepψ Orientation (of 3) around the x-axisθ Orientation (of 3) around the y-axisϕ Orientation (of 3) around the z-axis

Claims

PC 24 2167 G 36 / 42 20 January 2025 Claims 1. Arrangement (1) for supplying a print head (3) with a liquid (10) comprising - a liquid reservoir (2), - a print head (3) with nozzles (4), - a feed line (5), - a feed pump (7), - a return line (6), - a return pump (8), - a sensor (11) and - a control unit (12), wherein a stroke amplitude (h s ) of an oscillating lifting element (9) of the feed pump (7) and / or a stroke amplitude (h r) of an oscillating lifting element (9) of the return pump (8) is adjustable, wherein the liquid reservoir (2) is connected to the pressure head (3) via the feed line (5), the feed pump (7) is arranged in the feed line (5), the return line (6) is connected to the pressure head (3), the return pump (8) is arranged in the return line (6), the sensor (11) is arranged between a pressure side (13) of the feed pump (7) and a suction side (14) of the return pump (8), wherein the control unit (12) is connected to the sensor (11) and wherein the stroke amplitude (h s , h r) of the feed pump (7) and / or the return pump (8) is controllable.

2. Arrangement (1) according to the preceding claim, characterized in that a pulsation damper (15) is arranged in the feed line (5), in particular between the feed pump (7) and the pressure head (3), and / or in the return line (6), in particular between the return pump (8) and the pressure head (3), and / or that a PC 24 2167 G 37 / 42 20 January 2025 pulsation damper (15) is arranged on a suction side (21) or on a pressure side (22) of a pump chamber (20, 20') of the feed pump (7) or the return pump (8).

3. Arrangement (1) according to one of the preceding claims, characterized in that by means of the sensor (11) a pressure (p s , p r ) and / or a flow (Q hs , Q hr) of a liquid (10) flowing from the feed pump (7) to the return pump (8) can be measured.

4. Arrangement (1) according to one of the preceding claims, characterized in that one or the sensor (11) is arranged between the feed pump (7) and the print head (3) and that an additional sensor (11) is arranged between the print head (3) and the return pump (8).

5. Arrangement (1) according to one of the preceding claims, characterized in that the arrangement (1) comprises a further sensor (16) by means of which a kinematic parameter (x, y, z, ψ, θ, ϕ) of the print head (3) can be measured, and in that the further sensor (16) is connected to the control unit (12) and / or in that the arrangement (1) has an interface via which a measurement signal of a kinematic parameter (x, y, z, ψ, θ, ϕ) of the print head (3) can be transmitted to the control unit (12).6.Arrangement (1) according to one of the preceding claims, characterized in that the arrangement (1) comprises a measuring device (18) by means of which a liquid ejection rate (Qjet) of the print head (3) can be determined, and in that the measuring device (18) is connected to the control unit (12).PC 24 2167 G 38 / 42 January 20, 20257. Arrangement (1) according to one of the preceding claims, characterized in that the feed pump (7) and / or the return pump (8) are designed as a positive displacement pump, in particular as a diaphragm pump (17).

8. Arrangement (1) according to the preceding claim, characterized in that a drive (M) of the diaphragm pump (17) is designed as a linear drive, in particular wherein the linear drive is a direct drive.9.Arrangement (1) according to one of the two preceding claims, characterized in that the diaphragm pump (17) has two pump chambers (20, 20') each delimited by a diaphragm (19, 19'), the two diaphragms (19, 19') being connected to one another via the lifting element (9).

10. Arrangement (1) according to one of the two preceding claims, characterized in that the diaphragm pump (17) has pulsation dampers (15), wherein at least one of the pulsation dampers (15) is arranged on a suction side (21) of the pump chambers (20, 20') and / or at least one of the pulsation dampers (15) is arranged on a pressure side (22) of the pump chambers (20, 20'), in particular wherein the diaphragm pump (17) has at least four pulsation dampers (15), wherein two of the pulsation dampers (15) are each arranged on a suction side (21) of the pump chambers (20, 20') and two of the pulsation dampers (15) are each arranged on a pressure side (22) of the pump chambers (20, 20').11.Arrangement (1) according to one of the preceding claims, characterized in that the control unit (12) is set up to control the arrangement (1) such that a method according to one of the subsequent method claims is carried out.PC 24 2167 G 39 / 42 January 20, 202512. Method for supplying a print head (3) with a liquid (10) by means of a feed pump (7) and a return pump (8), in particular using an arrangement (1) according to one of the preceding claims, wherein a measured value of a flow parameter of the liquid (10) is recorded, characterized in that a stroke amplitude (h. s , h r) of a lifting element (9, 9') of the feed pump (7) and / or the return pump (8) is controlled as a function of the measured value.

13. Method according to the preceding claim, characterized in that the flow parameter is a flow (Qhs, Qhr) and / or a pressure (ps, pr).

14. Method according to one of the preceding method claims, characterized in that the stroke amplitude (h s , h r ) is controlled so that changes in a meniscus pressure in the print head (3) are compensated, in particular wherein the compensation takes place in real time and / or predictively.

15. Method according to one of the preceding method claims, characterized in that the measured value of the flow parameter, in particular the pressure (p s , p r), between a pressure side (13) of the feed pump (7) and a suction side (14) of the return pump (8).

16. Method according to one of the preceding method claims, characterized in that a setpoint value for the flow parameter is specified or calculated and is compared with the measured value of the flow parameter and that the stroke amplitude (h s , h r ) of the feed pump (7) and / or the return pump (8) is controlled depending on a comparison result, in particular whereby the measured value is adjusted to the setpoint valuePC 24 2167 G 40 / 42 20 January 2025and / or whereby the setpoint value for the one pressure (p s , p r ) describing flow parameters from a given mean reference pressure (p m,ref ) and a given pressure drop (Δp ref) for the print head (3) is calculated.

17. Method according to one of the preceding method claims, characterized in that a kinematic parameter (x, y, z, ψ, θ, ϕ) of the print head (3) is determined and that the stroke amplitude (h s , h r ) of the feed pump (7) and / or the return pump (8) depending on the kinematic parameter (x, y, z, ψ, θ, is controlled.

18. Method according to one of the preceding method claims, characterized in that a liquid ejection rate (Qjet) of the print head (3) is determined and that the stroke amplitude (h s , h r ) of the feed pump (7) and / or the return pump (8) depending on the determined liquid discharge rate (Q jet ) is controlled.

19. Method according to one of the preceding method claims, characterized in that a stroke frequency (f s , f r) of a lifting element (9, 9') of the feed pump (7) and / or the return pump (8) is adjusted so that the stroke frequency (fs, fr) is between 0.7 and 1.4 times a mechanical resonance frequency of the feed pump (7) and / or the return pump (8), in particular wherein the stroke frequency (f s , f r ) is kept constant.

20. Method according to one of the preceding method claims, characterized in that a stroke frequency (f s , f r ) of the feed pump (7) and / or the return pump (8) is adjusted so that the stroke frequency (f s , f r) lies outside an impedance maximum of a supply or return line of the liquid, in particular wherein the stroke frequency lies in an impedance minimum.PC 24 2167 G 41 / 42 20 January 202521. Method according to one of the preceding method claims, characterized in that the feed pump (7) and the return pump (8) are controlled in a synchronized manner.

22. Method according to one of the preceding method claims, characterized in that the feed pump (7) and the return pump (8) are controlled out of phase with respect to one another, in particular wherein the feed pump (7) and the return pump (8) are controlled out of phase with respect to one another by a value of 160° to 200°, preferably by 180°, and / or wherein the stroke frequency (f s , f r) of the feed pump (7) and the return pump (8) is the same.

23. Method according to one of the preceding method claims, characterized in that a control variable of the stroke amplitude (hs, hr) is updated in time steps (Δt), wherein the time steps (Δt) are selected as a function of the stroke frequency (fs, fr) of the feed pump (7) and / or the return pump (8), in particular wherein the time steps (Δt) are proportional to the respective current reciprocal of the stroke frequency (f s , f r ).

24. Control unit (12) which is arranged to control an arrangement (1) according to one of the preceding device claims in such a way that a method according to one of the preceding method claims is carried out.

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