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

Abstract

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 (8) are used, and that a stroke amplitude (hs, hr) of the feed 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.

Description

The invention relates to an arrangement for supplying a liquid to a print head. 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 recorded. Such processes 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 invention is based on the object of improving the liquid supply of print heads. In particular, in the case of print heads which print three-dimensional surfaces in so-called direct-shape methods and are moved in the process, for example, by robot arms, a stable supply of liquid is to be made possible during the appropriate printing.To achieve this object, the invention proposes the features of claim 1. In particular, it is thus proposed according to the invention in an arrangement of the type described at the beginning that the arrangement comprises a liquid reservoir, a print head with nozzles, a feed line, a feed pump, a return line, a return pump, a sensor and a control unit.The print head can be configured to be movable and can be fastened, for example, 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. Thus, for example, a direct-to-shape (DTS) method can be carried out with the print head, i.e. a direct printing of three-dimensional surfaces, for example walls or vehicles.Since the arrangement has a liquid reservoir, it can have exactly one liquid reservoir or even more than one liquid reservoir. The liquid is preferably an ink and the liquid reservoir is preferably an ink reservoir. The liquid may 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. As a result, the liquid can be conveyed from the liquid reservoir to the print head through the feed line. The feed pump can be arranged at any desired point in the feed line, i.e. between two sections of the feed line.In addition, the return line is connected to the print head and the return pump is arranged in the return line. It is thus possible for the liquid to be conveyable, starting from the liquid reservoir, first through the feed line, then through the print head and subsequently through the return line. The return pump can be arranged at any desired point 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 conveyed from the print head back into the liquid reservoir and a closed circuit for the liquid can be configured. Alternatively, the return line can also be connected to a further liquid reservoir different from the first, into which the liquid is conveyed after flowing through the print head. The further liquid reservoir can in turn be connected to the first liquid reservoir by a further line, so that a closed circuit for the liquid is again formed.In addition, 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 record a flow parameter in this region, which comprises 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, wherein the stroke amplitude of the feed pump can be controlled via the control unit. In order to achieve this, the feed pump preferably has an oscillating stroke element, the stroke amplitude of which can be adjusted. This has the advantage that the flow conditions of the liquid detected by the sensor, for example a pressure or a flow, in the feed line can be changed by changing the stroke amplitude of the feed pump. It is furthermore advantageous that a change in a stroke frequency of the feed pump for adapting flow conditions in the feed line is no longer necessary or can be adjusted in a suitable manner by changing the stroke amplitude. This is advantageous since pressure pulsations and / or volume flow pulsations in the line are dependent on changes in the stroke frequency. Furthermore, these pulsations are minimal when a high stroke frequency is set, which is kept substantially constant. By controlling the stroke amplitude to adapt flow conditions, the stroke frequency can be constant and / or set at a high value and pressure and volume flow pulsations can be reduced.Alternatively or additionally, the stroke amplitude of the return pump can be controllable via the control unit. In order to achieve this, the return pump preferably has an oscillating stroke element, the stroke amplitude of which can be adjusted. This is associated with advantages comparable to the controllability of the stroke amplitude of the feed pump, i.e. in particular the avoidance of pressure and volume flow pulsations in order to achieve a stable liquid supply of the print head.In an advantageous embodiment, it can be provided that a pulsation damper is 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 which are separated from one another by means of a separating element-for example an elastomer plate or a membrane. A chamber can be in contact with the flow line, i.e. with the liquid. The other chamber may be at atmospheric air pressure or at another pressure. Furthermore, the other chamber can have a spring element which supports the separating element. By means of such a pulsation damper, possible pressure and volume flow pulsations which are generated by the feed pump can be compensated for, so that-in the flow direction of the liquid-behind the pulsation damper, more uniform flow conditions with reduced pulsations prevail.Preferably, the pulsation damper is arranged between the feed pump and the print head. The flow of the liquid at and in the print head can thus be designed to be particularly stable, in particular with low pressure and volume flow pulsations.Alternatively or additionally, it can be provided that a pulsation damper is arranged in the return line. Preferably, the pulsation damper is arranged between the return pump and the print head. This also makes it possible to achieve the above-mentioned advantages in the return line.In a further advantageous embodiment, it can be provided that a pressure, in particular a liquid 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. Thus, based on the measured pressure, the stroke amplitude of the feed pump and / or of the return pump can be controlled. The control is preferably carried out in such a way that the pressure at a measurement location of the sensor is adjusted to a setpoint value.Alternatively or additionally, it can be provided that a flow rate of a liquid, in particular flowing from the feed pump to the return pump, can be measured by means of the sensor. By means of the sensor, for example, a flow rate, a flow volume or a flow speed can be measured. The sensor can also be referred to as a flow sensor.Thus, based on the measured flow rate, the stroke amplitude of the feed pump and / or of the return pump can be controlled. The control is preferably carried out in such a way that the flow rate at a measurement location of the sensor is adjusted to a setpoint value.Preferably, the pressure and / or the flow rate can be measurable by a plurality of sensors at a plurality of points between the feed pump and the return pump. This allows more accurate control of the arrangement to be achieved.Particularly preferably, the pressure and / or the flow rate of the liquid flowing into the print head and / or of the liquid flowing out of the print head can be measured. This means that the sensor or sensors is / are arranged at or in a vicinity of an inlet and / or an outlet of the print head. As a result, the pressure and / or the flow in the print head can be controlled exactly.In a further advantageous embodiment, it can be provided that a sensor, for example the aforementioned sensor, 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. By means of this configuration, a flow parameter, for example a pressure and / or a flow, can 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. With this measured value, the stroke amplitude of the feed pump can be adjusted such that the measured value corresponds to a predefined setpoint value.On the other hand, a flow parameter of the fluid can be measured upon exiting the print head. As a result, for example, the pressure which is present at the measuring point of the sensor in the return line can be measured. This measurement value can be used for controlling the stroke amplitude of the return pump. The stroke amplitude can be set in such a way that the measured value corresponds to a predefined setpoint 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 may be connected to a position sensor of a robot arm, for example the one mentioned above, to which the print head is attached. A kinematic parameter can be a position or an orientation of the print head in space or a time derivative thereof such as a speed or an acceleration. The alignment can be given as an angle value.By means of this further sensor, therefore, further parameters can be taken into account in the adaptation of the stroke amplitude by the control unit, in particular parameters which are not flow parameters of the liquid 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 the flow rate of the flowing liquid. The same applies to movements of the print head, the speed or acceleration of which can be taken into account by the further sensor in the control of the arrangement.In a further advantageous embodiment, it can be provided that the arrangement comprises a measuring device, by means of which a liquid ejection rate of the print head can be determined, and that the measuring device is connected to the control unit. By way of the measuring device, for example, the activity of the nozzles can be measurable, from which the liquid ejection rate can be calculated. The measuring device can also be connected to a print control device or a print management system, by means of which the printing process is controlled, so that the liquid ejection rate can be calculated from the transmitted data.The liquid ejection rate describes how much liquid exits the array through the nozzles of the printhead and thus affects the flow conditions in the feed line, the return line and the printhead. By taking this value into account in the control unit, the arrangement can be regulated more precisely.In a further advantageous embodiment, it can be provided that the feed pump and / or the return pump are designed as a positive displacement pump. This can have the advantage that the delivery rate of the liquid can be adjusted particularly reliably by the stroke amplitude.The feed pump and / or the return pump are preferably designed as a diaphragm pump. Diaphragm pumps have the advantage, among other things, that they can be operated at high frequencies. High frequencies in turn have the advantage that pressure and volume flow pulsations can be minimized and the print head can thus be supplied with liquid uniformly. Diaphragm pumps are also advantageous because the flow rate or pressure provided by diaphragm pumps can be varied rapidly. By using diaphragm pumps in the arrangement according to the invention, the liquid supply can thus be adapted flexibly to the operating and flow conditions that change over time.Particularly preferably, the feed pump and / or the return pump are designed as a piezo membrane pump. As a result, particularly high stroke frequencies and / or particularly small stroke amplitudes can be achieved.In a further advantageous embodiment, it can be provided that a drive of the diaphragm pump is designed as a linear drive. This means that the diaphragm pump does not have a rotational drive and that only translatory movements can be carried out by the drive of the diaphragm pump. By avoiding rotatable parts in the drive, a more rapid responsiveness of the drive to actuating signals of the control unit can be achieved.The linear drive is preferably a direct drive. By using a direct drive, no gear is present in the drive, so that the responsiveness of the drive to actuating signals of the control unit can be even faster. In addition, the need for maintenance work can be minimized in this way, since wear of gear parts cannot occur.The control unit may comprise a microprocessor or a microcontroller. Furthermore, the control unit can be a digital control unit. The control unit can comprise a storage element and / or a computing unit. The control unit can also comprise a controller. The controller may be a single-variable controller or a multi-variable controller. It is also possible for the control unit to comprise a pilot control.The control unit is preferably configured such that during operation the stroke amplitude of the feed pump and / or of the return pump is controlled.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, in order to achieve the object mentioned at the beginning, the features of the subordinate claim directed to a method are provided according to the invention. In particular, in order to achieve the object mentioned, it is therefore proposed according to the invention in a method of the type described at the beginning 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.It is advantageous here that control of the feed pump and / or the return pump does not have to take place via the stroke frequency and the latter can therefore be kept constant at a high value, so that pressure and volume flow pulsations which are dependent on the stroke frequency can be reduced. At the same time, recorded measured values can be taken into account in the control by the stroke amplitude of the feed pump and / or of the return pump being adapted accordingly, for example in order to achieve the desired flow conditions of the liquid.Preferably, an arrangement as described above is used for the method. As a result, the advantages which exist as a result of the arrangement can also be used in the method.In an advantageous embodiment, it can be provided that the flow parameter is a throughflow. This embodiment is preferred, for example, when the flow rate, such as the volume flow rate, of the liquid is to reach a specific setpoint value or is to be kept in a specific operating window.Alternatively or additionally, it can be provided that the flow parameter is a pressure. This embodiment is preferred, for example, when the pressure of the flowing liquid reaches a specific setpoint value or is to be kept within a specific operating window.In a further advantageous embodiment, it can be provided that the stroke amplitude is controlled in such a way that changes in a meniscus pressure in the print head are compensated for.Meniscus pressure is the pressure prevailing in the liquid channels in the nozzles of the print head. The meniscus pressure is preferably to be kept in an operating window which is determined by the following requirements: On the one hand, the meniscus pressure should be a slight negative pressure so that the liquid is not forced out of the nozzles. Secondly, the meniscus pressure should not be too high a negative pressure, so that no air from the environment penetrates into the print head through the nozzles. The operating window in which the meniscus pressure is to be maintained is thus 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.An advantage of the above-mentioned embodiment is that the print head can be operated optimally and no ink unintentionally escapes from the print head.Preferably, the compensation takes place in real time. In this way, changes in meniscus pressure can be detected and compensated directly.Alternatively or additionally, the compensation is predictive.The compensation is effected, 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 the control.Preferably, the kinematic parameter is used to predict changes in meniscus pressure. As a result, changes in the meniscus pressure can 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 said region, the measured values in this region are particularly relevant for the control of the stroke amplitudes.In a further advantageous embodiment, it can be provided 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 of the feed pump is controlled as a function of a comparison result. Thus, in particular in the feed line, the flow parameter can be kept around the setpoint value in a defined operating window.Alternatively or additionally, the stroke amplitude of the return pump is controlled as a function of the comparison result. Thus, in particular in the return line, the flow parameter can be kept around the setpoint value in a defined operating window.Preferably, the measured value is hereby adjusted to the desired value. This results in an exact setting of the flow parameter to the desired value.Alternatively or additionally, the setpoint value for the flow parameter describing a pressure is calculated from a predefined mean reference pressure and a predefined pressure drop for the print head. It is advantageous here that the pressure does not have to be measured directly in the print head, but that a pressure measurement can take place upstream of the print head in the feed line and a pressure measurement can take place downstream of the print head in the return line. By presetting an average reference pressure, which is preferably a reference value for the meniscus pressure, which is to prevail in the print head, and a pressure drop for the print head, i.e. a pressure loss of the liquid when flowing through the print head, the respective target values can be calculated at the measurement points 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 of the return pump is controlled as a function of the kinematic parameter.The kinematic parameter can be, for example, a pose, i.e. a position and / or an alignment, of the print head or else a change over time in the pose, in particular a speed and / or an acceleration. The kinematic parameter can be measured via a sensor which is fastened to the print head. However, the measurement value of an external sensor can also be used, for example a position sensor of a robot arm, for example the one mentioned above, by which the print head is moved in space.Taking into account the spatial position of the print head and the change over time of the latter makes it possible to adapt the control of the stroke amplitudes to parameters which are not flow parameters of the liquid but have a direct influence on these flow parameters. For example, this configuration allows changes in the flow parameters to be controlled and / or anticipated in real time and, if appropriate, avoided or preventively compensated.In a further advantageous embodiment, it can be provided that a liquid ejection rate of the print head is determined and that the stroke amplitude of the feed pump and / or of the return pump is controlled as a function of the determined liquid ejection rate. The liquid ejection rate describes the amount of liquid that exits the printhead per unit time through the nozzles and thus affects the liquid flow parameters, particularly pressure, meniscus pressure and flow. The liquid ejection rate can be determined, for example, by using a measuring device, for example the aforementioned measuring device, with which a measured value is recorded and processed in a computing step. It is therefore advantageous to take into account the liquid discharge rate when controlling the stroke amplitudes of the pumps in order to compensate for changes in the flow conditions of the liquid which arise therefrom.In a further advantageous embodiment, it can be provided that a stroke frequency of a stroke element of the feed pump and / or of the return pump is set such that the stroke frequency lies between 0.7 and 1.4 times, preferably between 0.8 and 1.2 times, particularly preferably between 0.9 and 1.1 times, very particularly preferably between 0.98 and 1.02 times, a mechanical resonance frequency of the feed pump and / or of the return pump, in particular corresponds to 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 resonant frequency of the respective pump is thus the resonant frequency of the system which is composed of the oscillating stroke element and of the element which supports the stroke element, that is to say for example the pump diaphragm and / or the leaf springs.It can be particularly advantageous in terms of energy to set the stroke frequency close to the resonant frequency, since the respective pump can thereby be operated in the mechanical resonant case. In this way, a maximum proportion can be transferred into the delivery of the liquid by the electrical energy with which the lifting element is driven. This may result in reduced power consumption of the pump.Preferably, the stroke frequency is kept constant. Thus, the energy consumption of the respective pump can be kept at a constantly low level.In a further advantageous embodiment, it can be provided that a stroke frequency of the feed pump and / or of the return pump is set such that the stroke frequency lies outside an impedance maximum of an inlet or return line of the liquid.The impedance denotes the flow resistance. The impedance is connected with volume flow pulsations and pressure pulsations which occur in the liquid which is conveyed through the inlet 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 achieved at specific stroke frequencies, which are avoided according to the development 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 traversed, wherein the pressure in the inlet or return line is detected by means of a sensor. At those stroke frequencies at which local maxima of the pressure are detected, impedance maxima are present.Outside an impedance maximum means that the present impedance is less than fifty percent of the impedance maximum, preferably less than thirty percent of the impedance maximum.It is advantageous in this embodiment that the feed pump and / or the return pump experiences less resistance through the inlet or return line, so that a higher energy efficiency can be achieved. Furthermore, more uniform flow conditions can be achieved in the lines and in the print head, since pressure pulsations are reduced, and thus also a simpler controllability of the pressure and / or of the flow of the liquid.Preferably, the stroke frequency lies in a minimum impedance of the inlet or return line. The stroke frequencies at which impedance minima are present can also be estimated, calculated or determined by a test run described above. In this case, the above-described advantages are more prominent. In an impedance minimum, it means that the present impedance is less than twice the impedance minimum, preferably less than 1.3 times, particularly preferably less than 1.1 times, over the impedance minimum.In a further advantageous embodiment, it can be provided that the feed pump and the return pump are controlled in a synchronized manner. 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 phase-shifted with respect to one another. The feed pump and the return pump are preferably controlled phase-shifted with respect to one another by a value of 160° to 200°, particularly preferably of 180°. As a result, pressure and volume flow pulsations at the print head can be reduced.Alternatively or additionally, the stroke frequencies of the feed pump and of the return pump are the same. As a result, pressure pulsations at the print head can be reduced even further.In a further advantageous embodiment, it can be provided that a control variable of the stroke amplitude is updated in time steps, wherein the time steps are selected depending on the stroke frequency of the feed pump and / or the return pump. Actuating signals can thus be taken into account particularly quickly by the pumps.The time steps are preferably proportional to the respectively present reciprocal value of the stroke frequency. The reciprocal of the stroke frequency is the stroke period. Thus, a new control variable of the stroke amplitude can be preset for each stroke period or for any desired number of stroke periods, whereby the pressure or the flow rate of the liquid can be adapted particularly quickly and at the same time precisely to current measured values.Alternatively or additionally, to achieve the object mentioned at the beginning, the features of the subordinate claim directed to a control unit are provided. In particular, it is thus proposed according to the invention that a control unit is configured to control an arrangement as described above in such a way that a method as described above is carried out. This control unit can realize the advantages described above.The invention will now be described in more detail with reference to an exemplary embodiment, but is not limited to this exemplary embodiment. Further variants of the invention and exemplary embodiments result from the combination of the features of individual claims or of a plurality of claims with one another and / or with individual features or of a plurality of features of the exemplary embodiments and / or of the variants of devices and methods according to the invention described above.It shows FIG. 1 shows an arrangement for supplying a print head with a liquid.In the arrangement 1 shown in FIG. 1 for supplying a print head 3 with a liquid 10, the print head 3 is connected via a feed line 5 and via a return line 6 to a liquid reservoir 2, so that a closed circuit is formed for the liquid 10. In the exemplary 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, a feed pump 7 with drive M is arranged, which is designed as a diaphragm pump 17 and has a lifting element 9. Furthermore, a return pump 8 with drive M is arranged in the return line 6, which pump is likewise designed as a diaphragm pump and has a lifting element 9'. The print head 3 is arranged between a print side 13 of the feed pump 7 and a suction side 14 of the return pump 8.In the feed line 5, a sensor 11 is arranged between the feed pump 7 and the print head 3, with which a pressure p s in the feed line can be measured. Furthermore, a sensor 11 is arranged in the return line 6 between the print head 3 and the return pump 8, with which a pressure p r in the return line can be measured.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 indicated with the coordinates x, y, z and an orientation of the print head 3 which is indicated with the angles ψ, θ, φ.A measuring device 18 is placed on the print head 3, with which the liquid ejection rate Q jet from the nozzles 4 of the print head 3 is determined. This can be effected, for example, as described above.The acquired measured values p s, p r, Q jet, x, y, z, ψ, θ, φ are given to a control unit 12. On the basis of these values of the feed pump 7 and the return pump 8, the control unit 12 specifies a stroke amplitude h s, h r and a stroke frequency f s, f r respectively. The control unit specifies the manipulated variables for the stroke amplitude h s, h r and the stroke frequency f s, f r in time steps Δt.Furthermore, in the control unit 12, an average reference pressure p m,ref, which in one exemplary embodiment corresponds to a setpoint meniscus pressure for the print head 3, and a pressure drop Δp ref across the print head 3 are taken into account. These are predetermined setpoint values. From these values, the set values for the pressure p s, which is detected 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 suction 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 disposed in front of and behind the feed pump 7. Two further pulsation dampers 15 are arranged in front of and behind the return pump 8. The pulsating delivery rates Q s, Q r of the feed pump 7 and the return pump 8 are hydraulically compensated by the pulsation dampers 15, so that the flow rates Q hs, Q hr in the feed line 5 and the return line 6 are no longer pulsating but are 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 exemplary embodiment described here, the pressure p s can be measured in the feed line 5 between the feed pump 7 and the print head 3 by means of a sensor 11. This pressure p s is then applied 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, as the set values for the pressure, an average reference pressure p m,ref and a pressure drop Δp ref are set. The pressure drop Δp ref denotes the pressure drop which the liquid 10 experiences when flowing from the sensor 11 in the feed line 5, through the print head 3, to the sensor 11 in the return line 6. From the two predetermined reference variables for the pressure, a setpoint value for the pressure p s and a setpoint value for the pressure p r are therefore calculated. The setpoint value for the pressure p s is the sum of the mean reference pressure p m,ref and half the pressure drop Δp ref. The desired value for the pressure p r is 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 here to the meniscus pressure which is to be present in the nozzles 4 of the print head 3.The control unit 12 compares the measured pressure values p s, p r with the calculated target values. On the basis of the comparison result, the control unit 12 adjusts the manipulated variables for the stroke amplitudes h s, h r of the feed pump and the return pump 7, 8. In this case, the stroke amplitude h s of the feed pump 7 is controlled in such a way that the pressure p s in the feed line 5 is adjusted to its setpoint value. The pressure p r in the return line 6 is adjusted accordingly to its desired value by controlling the stroke amplitude h r of the return pump 8.The control unit 12 updates the manipulated variables for the stroke amplitudes h s, h r. at each time step Δt-or alternatively at any desired multiple of the time step Δt. The time step Δt corresponds to the current reciprocal of the stroke frequency f s of the feed pump 7 or the current reciprocal of the stroke frequency f r of the return pump 8.The time step Δt thus corresponds to the pump period of the pumps 7, 8. In this way, a new stroke amplitude h s, h r can be predefined for each pump period, as a result of which a quick control of the pressure p s, p r is made possible.Further process parameters which are taken into account in the control unit 12 are the liquid ejection rate Q jet and the kinematic parameters x, y, z, ψ, θ, φ of the print head 3.The liquid discharge rate Q jet describes the amount of liquid that leaves the print head 3 through the nozzles 4 per unit time. This is the liquid 10 which is used for printing, for example for printing a vehicle or another three-dimensional object. The liquid ejection rate Q jet has a direct influence on the pressure p s, p r in the lines 5, 6 and also on the pressure in the print head. Since the liquid discharge rate Q jet is taken into account in the control of the stroke amplitudes h s, h r changes in the pressures p s, p r can be preventively compensated even before they are detected by the sensors 11 in the lines 5, 6.The kinematic parameters x, y, z, ψ, θ, φ of the print head 3 comprise its position x, y, z in space and also its orientation ψ, θ, φ. Temporal changes of these parameters-i.e. a speed or also an acceleration-are also taken into account in the control unit 12. The position and the alignment influence, among other things, changes in the geodetic height difference between the print head 3 and the pumps 7, 8. In addition, their temporal changes cause, in particular due to rapid or jerking movements of the print head 3, changes in the pressures p s, p r in the lines 5, 6 and in the print head 3.For a particularly high energy efficiency, the stroke frequencies f s, f r of the pumps 7, 8 are controlled such that they are close to the mechanical resonance frequencies of the pumps 7, 8. In addition, the impedance of the feed line 5 and / or of the return line 6 is taken into account in that the stroke frequencies f s, f r are kept outside the respective impedance maxima.The stroke frequencies f s, f r, at which impedance maxima are present, are determined empirically either during startup of the arrangement 1. For this purpose, the available stroke frequencies f s, f r of the pumps 7, 8 are traversed and measured values for the pressure p s, p r are recorded at each stroke frequency f s, f r. The impedance maxima are present at those stroke frequencies at which the measured pressure pulsations are the highest.Alternatively, the stroke frequencies f s, f r, at which impedance maxima are present, are calculated.The stroke frequencies f s, f r mentioned are stored in the control unit 12 after the measurement or calculation, so that they can be avoided during operation.In addition, the pumps 7, 8 are operated with a phase offset of 180°. That is, when the feed pump 7 discharges liquid 10, the return pump 8 sucks liquid 10, and vice versa. In this way, the pumps 7, 8 do not interfere with each other and the power consumption can be reduced. In addition, the pressure and volume flow pulsations in the print head 3 are minimized.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 8 are used and that a stroke amplitude h s, h r 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 recorded.List of reference characters1 Arrangement 2 Liquid reservoir 3 Print head 4 Nozzles 5 Feed line 6 Return line 7 Feed pump 8 Return pump 9 Stroke element 10 Liquid 11 Sensor 12 Control unit 13 Print side (from 7) 14 Suction side (from 8) 15 Pulsation damper 16 Further sensor 17 Diaphragm pump 18 Measuring device M Drive f r Stroke frequency (from 8) f s Stroke frequency (from 7) h r Stroke amplitude (from 8) h s Stroke amplitude (from 7) p m,ref Mean reference pressure p r Pressure (in 6) p s Pressure (in 5) Q hr Throughflow (in 6) Q hs Throughflow (in 5) q jet liquid ejection rate Q r delivery rate (of 8) Q s delivery rate (of 7) x position (of 3) in x-direction y position (of 3) in y-direction z position (of 3) in z-direction Δp ref pressure drop (for 3) Δt time step ψ orientation (of 3) about the x-axis θ orientation (of 3) about the y-axis φ orientation (of 3) about the z-axis

Claims

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 stroke element (9) of the feed pump (7) and / or a stroke amplitude (h r) of an oscillating stroke element (9) of the return pump (8) can be adjusted, wherein the liquid reservoir (2) is connected to the print 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 print 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 forward 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 forward pump (7) and / or of the return pump (8) can be controlled via the control unit (12).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 print head (3), and / or in the return line (6), in particular between the return pump (8) and the print head (3).Arrangement (1) according to one of the preceding claims, characterized in that a pressure (p s, p r) and / or a throughflow (Q hs, Q hr) of a liquid (10) flowing from the feed pump (7) to the return pump (8) can be measured by means of the sensor (11).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 in that an additional sensor (11) is arranged between the print head (3) and the return pump (8).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).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 (Q jet) of the print head (3) can be determined, and in that the measuring device (18) is connected to the control unit (12).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).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.Arrangement (1) according to one of the preceding claims, characterized in that the control unit (12) is configured to control the arrangement (1) such that a method according to one of the following method claims is carried out.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 measurement value of a flow parameter of the liquid (10) is recorded, characterized in that a stroke amplitude (h s, h r) of a stroke element (9, 9') of the feed pump (7) and / or of the return pump (8) is controlled as a function of the measurement value.Method according to the preceding claim, characterized in that the flow parameter is a flow rate (Q hs, Q hr) and / or a pressure (p s, p r).Method according to one of the preceding method claims, characterized in that the stroke amplitude (h s, h r) is controlled in such a way that changes in a meniscus pressure in the print head (3) are compensated for, in particular wherein the compensation takes place in real time and / or predictively.Method according to one of the preceding method claims, characterized in that the measured value of the flow parameter, in particular of 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), is recorded.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 in that the stroke amplitude (h s, h r) of the feed pump (7) and / or of the return pump (8) is controlled as a function of a comparison result, in particular wherein the measured value is thereby matched to the setpoint value and / or wherein the setpoint value for the flow parameter describing a pressure (p s, p r) is calculated from a specified mean reference pressure (p m,ref) and a specified pressure drop (Δp ref) for the print head (3).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 in that the stroke amplitude (h s, h r) of the feed pump (7) and / or of the return pump (8) is controlled as a function of the kinematic parameter (x, y, z, ψ, θ, φ).Method according to one of the preceding method claims, characterized in that a liquid discharge rate (Q jet) of the print head (3) is determined, and in that the stroke amplitude (h s, h r) of the feed pump (7) and / or of the return pump (8) is controlled as a function of the determined liquid discharge rate (Q jet).Method according to one of the preceding method claims, characterized in that a stroke frequency (f s, f r) of a stroke element (9, 9') of the feed pump (7) and / or of the return pump (8) is set such that the stroke frequency (f s, f r) lies between 0.7 and 1.4 times a mechanical resonance frequency of the feed pump (7) and / or of the return pump (8), in particular wherein the stroke frequency (f s, f r) is kept constant.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 of the return pump (8) is set such that the stroke frequency (f s, f r) lies outside an impedance maximum of an inlet or return line of the liquid, in particular wherein the stroke frequency lies in an impedance minimum.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.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 phase-shifted manner with respect to one another, in particular wherein the feed pump (7) and the return pump (8) are controlled in a phase-shifted manner with respect to one another by a value of 160° to 200°, preferably of 180°, and / or wherein the stroke frequency (f s, f r) of the feed pump (7) and of the return pump (8) is the same.Method according to one of the preceding method claims, characterized in that a control variable of the stroke amplitude (h s, h r) is updated in time steps (Δt), wherein the time steps (Δt) are selected as a function of the stroke frequency (f s, f r) of the feed pump (7) and / or of the return pump (8), in particular wherein the time steps (Δt) are proportional to the respectively current reciprocal value of the stroke frequency (f s, f r).Control unit (12) which is configured 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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