Method for operating an axial piston machine in which a zero case is taken into account

Abstract

A method is for operating an axial piston machine whose displacement volume is continuously adjustable using a double-acting control cylinder having two oppositely acting control chambers. The method includes providing a corresponding axial piston machine. An electrically adjustable pressure control valve is connected to one control chamber. Another control chamber is connected to an electrically adjustable 3/2-way switching valve. The method includes providing a target control pressure difference. A zero case is defined as a case in which the target control pressure difference is essentially zero. A positive case is defined as a case in which the target control pressure difference is positive. A negative case is defined as a case in which the target control pressure difference is negative. The method further includes calculating a first and a second control pressure. A case differentiation includes determining whether the zero case, the positive case, or the negative case is present.

Claims

1. A method for operating an axial piston machine having a displacement volume that is continuously adjustable using a double-acting control cylinder comprising two oppositely acting control chambers, the method comprising: a) providing a corresponding axial piston machine, wherein an electrically adjustable pressure control valve is connected to one control chamber, wherein another control chamber is connected to an electrically adjustable 3/2-way switching valve; b) providing a target control pressure difference, wherein a zero case is defined as a case in which the target control pressure difference is essentially zero, wherein a positive case is defined as a case in which the target control pressure difference is positive, wherein the target control pressure difference does not fall under the zero case, wherein a negative case is defined as a case in which the target control pressure difference is negative, wherein the target control pressure difference does not fall under the zero case; c) calculating a first control pressure and a second control pressure, wherein in a context of a case differentiation, determining whether the zero case, the positive case, or the negative case is present, wherein a result of the case differentiation is used in the calculation of the first control pressure and the second control pressure; and d) energizing the pressure control valve according to the first control pressure and energizing the 3/2-way switching valve according to the second control pressure.

2. The method according to claim 1, wherein parts b), c) and d) are performed continually and in parallel or quasi-parallel with each other during operation of the axial piston machine.

3. The method according to claim 1, wherein: the pressure control valve is energized with a first control current, the 3/2-way switching valve is energized with a second control current, the first control current is calculated from the first control pressure using an inverse model of the pressure control valve, and the second control current is calculated from the second control pressure using an inverse model of the 3/2-way switching valve.

4. The method according to claim 1, wherein: the first control pressure and the second control pressure are calculated for the zero case, the positive case, and the negative case separately, and at least one result of the calculations is selected based on the case differentiation.

5. The method according to claim 1, wherein: in the zero case, a first pair and a second pair are calculated, the first pair and the second pair each comprise the first control pressure and the second control pressure, in the first pair, the second control pressure equals a pressure in a control supply, the first control pressure is calculated according to the second control pressure and the target control pressure difference, in the second pair, the second control pressure is equal to a pressure in a control return, the first control pressure is calculated according to the second control pressure and the target control pressure difference, and according to a case most recently present in a context of the case differentiation and/or, if desired, a case expected to be present in the future, deciding whether the first pair or the second pair is used as a calculation result.

6. The method according to claim 5, wherein: in a context of the zero case, the first pair and the second pair are calculated, and one calculation result of the two calculation results is selected based on the decision regarding the first pair or the second pair.

7. The method according to claim 1, wherein: in the positive case, the second control pressure is equal to a pressure in a control return, and the first control pressure is calculated according to the second control pressure and the target control pressure difference.

8. The method according to claim 1, wherein: in the negative case, the second control pressure is equal to a pressure in a control supply, the first control pressure is calculated according to the second control pressure and the target control pressure difference.

9. The method according to claim 1, wherein: the axial piston machine includes a first working port and a second working port, a first pressure sensor and a second pressure sensor are provided, a first actual pressure is measured by the first pressure sensor at the first working port and a second actual pressure is measured by the second pressure sensor at the second working port, and the target control pressure difference is calculated according to the first actual pressure and second actual pressure, and a specified target conveying pressure difference.

10. The method according to claim 9, wherein: the axial piston machine includes a speed sensor, an actual speed of the axial piston machine is measured using the speed sensor, and the target control pressure difference is calculated, in a sense of feed-forward control, according to the actual speed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosure is explained in more detail below with reference to the enclosed drawings. The figures show:

(2) FIG. 1 a hydraulic circuit diagram of an axial piston machine according to the disclosure;

(3) FIG. 2 a part of a control diagram implementing the method according to the disclosure;

(4) FIG. 3 another part of the control diagram from FIG. 2; and

(5) FIG. 4 a diagram illustrating the inverse model of the pressure control valve.

DETAILED DESCRIPTION

(6) FIG. 1 shows a hydraulic circuit diagram of an axial piston machine 10 according to the disclosure. The axial piston machine 10 comprises a main pump 18 and a feed pump 13, which are driven by a common drive shaft 19. The actual speed n of the drive shaft 19 may be measured by a speed sensor 70, which is preferably fixedly attached to the axial piston machine 10.

(7) The main pump 18 is configured in an axial piston design, wherein its displacement volume is continuously adjustable, preferably by means of a pivotable swashplate. The swashplate is coupled, with respect to motion, to the control piston of a double-acting control cylinder 20. The control cylinder 20 has a first and a second control chamber 21; 22, wherein the pressures there, the numerical values of which correspond to p_DRE; p_3/2 in FIG. 3, act on the control piston in opposite directions. The first control chamber 21 is fluidly connected directly to a pressure control valve 30, which is preferably in the form of a pressure reducing valve, said pressure control valve being electrically adjustable by means of an actuation solenoid 31. The second control chamber 22 is fluidly connected directly to a 3/2-way switching valve 32, which is electrically adjustable by means of an actuation solenoid 33, said 3/2-way switching valve being biased by means of a restoring spring 34 into a position in which the control supply 15 is connected through (so-called open position). The pressure control valve 30 and/or the 3/2-way switching valve 32 are preferably each controlled with a voltage, the duty cycle of which is selected such that, on average over time, a current results, the numerical value of which corresponds to i_DRE or i_3/2, respectively. For this purpose, preferably in each case a digitally implemented current control loop is used, which is most preferably calculated by the control device which also carries out the method according to the disclosure.

(8) In the present case, the pressure control valve 30 has a positive characteristic curve, such that, in its currentless state, the low pressure in the control return 16 is present in the first control chamber 21. The 3/2-way switching valve is normally open, so that in its currentless state the high pressure in the control supply 15 is present in the second control chamber 22. The present main pump 18 is adjustable across the displacement volume of zero. Accordingly, in the currentless state, a displacement volume of maximum magnitude is set, so long as the drive shaft 19 rotates. The corresponding direction of rotation is configured such that the fan wheel 41 rotates according to the desired cooling air conveying direction. This behavior is desired in the vast majority of cases of fan drives, so that the internal combustion engine to be cooled is sufficiently cooled even in the event of a failure of the electric control.

(9) The feed pump 13 may be in the form of a rotary vane pump, for example. It draws pressurized fluid from a tank 17 and conveys it into the control supply 15 and, via the feed valves 23, also into the closed hydraulic circuit. In FIG. 1, all tank symbols 17 refer to the same tank. The pressurized fluid is preferably a liquid and most preferably hydraulic oil. The output of the feed pump 13 is connected to a feed pressure limiting valve 14. During most of the operating time of the axial piston machine 10, the delivery flow of the feed pump 13 is high enough that the feed pressure limiting valve 14 is triggered, so that the pressure in the control supply 15 is defined by the setting of the feed pressure limiting valve 14. In the present case the feed pressure limiting valve 14 is fixed, although an electrically adjustable feed pressure limiting valve can be used.

(10) In the present case, the first and second working ports 11; 12 of the main pump 18 and of the axial piston machine 10 are fluidly connected to a hydraulic motor 40 in the sense of a closed hydraulic circuit. The hydraulic motor 40 drives a fan wheel 41 directly, which, for example, cools the internal combustion engine of a relatively large vehicle, in particular a mobile work machine. The disclosure is intended primarily for this application, although of course it can also be used for other applications. The target conveying pressure difference explained below can be used as a control variable, for example in a superposed control loop by means of which the speed of the fan wheel 41 is controlled. The method according to the disclosure is intended to require only the first and second pressure sensors 71; 72 and the speed sensor 70; no other sensors are absolutely necessary. Of course, for example a pivot angle sensor may be optionally used on the main pump 18 or another speed sensor may be optionally used on the hydraulic motor 40 to improve the operating behavior of the overall system.

(11) With the first pressure sensor 71, a first actual pressure p_A can be measured at the first working port 11. With the second pressure sensor 72, a second actual pressure p_B can be measured at the second work port 12.

(12) Reference should also be made to the two feed valves 23, via which pressurized fluid can be conveyed from the feed pump 13 into the closed hydraulic circuit to compensate for leakages. It is conceivable to equip the axial piston machine 10 with a purge valve by which, in addition to said leakages, pressurized fluid can be drawn from the closed hydraulic circuit in a controlled way so that the pressurized fluid there does not overheat. However, this hazard is low in the case of the present fan drive.

(13) FIG. 2 shows a part of a control diagram implementing the method according to the disclosure. The part of the control diagram shown in FIG. 2 may be subject to significant variations without departing from the scope of the disclosure. For example, the feed-forward control with the trajectory planning filter 50 and with the inverse model 51 of the axial piston machine may be used alone. It is also conceivable that the controller 52 is used alone. In any case, a functionally capable overall system is provided; FIG. 2 shows the most complex variant with the best operating properties.

(14) A target conveying pressure difference dp_target is specified to the axial piston machine from the outside, wherein the difference between the first and second actual pressures p_A; p_B measured by means of the first pressure sensor and the second pressure sensor, respectively, is to be set to the target conveying pressure difference dp_target.

(15) To make this setting happen as quickly as possible, feed-forward control based on an inverse model 51 of the axial piston machine is first provided. The description in DE 10 2019 210 003 A1 specifies mathematical formulas by means of which the behavior of the axial piston machine can be modeled. DE 10 2021 200 693 A1 specifies a method by which a mathematical model of the axial piston machine can be determined in an experimental manner. Common to all of these models is that, in the case of an inversion, they provide useful results only if the inputted temporal profile of the conveying pressure difference can in fact be set in a real axial piston machine.

(16) However, the target conveying pressure difference dp_target may have abrupt changes that cannot occur in the actual conveying pressure difference because the actual conveying pressure difference can change only continuously. Therefore, the target conveying pressure difference dp_target is first fed to a trajectory planning filter 50 before being fed to the inverse model 51 of the axial piston machine. In the simplest case, the trajectory planning filter 50 is a low-pass filter with a fixed cut-off frequency. In this way, the discussed abrupt changes can be easily eliminated. However, preferably the trajectory planning filter 50 disclosed in DE 10 2019 210 003 A1 is used, by means of which the performance capability of the axial piston machine can be optimally utilized so that, in the context of the feed-forward control, the target conveying pressure difference dp_target is approached particularly quickly.

(17) Besides the modified target conveying pressure difference, the measured values from the sensors, namely the actual speed n of the drive shaft and the first and second actual pressures p_A; p_B, are also fed to the inverse model 51 of the axial piston machine, for the calculation of the model 51. As a result, the inverse model 51 of the axial piston machine provides a target control pressure difference dpX_target which theoretically must be set in order to achieve the fastest possible approach of the actual conveying pressure difference toward the target conveying pressure difference dp_target. Of course, this theoretical value does not perfectly match the real circumstances. Therefore, closed-loop control is superposed on the feed-forward control.

(18) In the present case, the control variable of the controller 52 is a further target control pressure difference, which is additively superposed 54 with that of the feed-forward control in order to obtain dpX_target. It is conceivable that the control variable of the controller 52 starts at a suitable point of the inverse model 51 of the axial piston machine in order to improve the control behavior.

(19) First, on the input side of the controller 52 the actual conveying pressure difference is calculated 55 as the difference 53 between the first and second actual pressures p_A; p_B. The control deviation results from the difference 53 between the target conveying pressure difference dp_target and the actual conveying pressure difference. The control deviation is fed to the controller 52, which is preferably in the form of a continuous linear controller, in particular a PID regulator. The controller 52 is preferably calculated discretely in time in the context of the round-based calculation method explained above.

(20) FIG. 3 shows another part of the control diagram from FIG. 2. The interface of FIGS. 2 and 3 is formed by the target control pressure difference dpX_target, which is also the primary input variable of the method according to the disclosure.

(21) In the context of the disclosure, particular control valves are used, namely a pressure control valve and a 3/2-way switching valve. This selection has considerable advantages in the case of a fan drive, because different fan variants common on the market can be realized with the same components. In particular, it can be freely selected on which side of the control cylinder the pressure control valve is to be arranged, with the 3/2-way switching valve being arranged on the other side. This advantage comes at the expense of a significant disadvantage, which is rectified with the disclosure. In particular, the control behavior of the control valves at a target control pressure difference dpX_target equal to zero has a significant discontinuity which relates to both control valves. Thus, with a conventional, purely hydraulic control device, the present control valves are not usable, even if they were hydraulically actuatable. Therefore, in the context of the disclosure a method is used which is preferably implemented by means of a programmable digital computer, which in particular comprises a microprocessor.

(22) The mentioned discontinuity is taken into account by distinguishing three cases, namely a zero case 60, a positive case 61, and a negative case 62. In the zero case 60, the target control pressure difference dpX_target is essentially zero, and the cases in which, because of control inaccuracies, it is not certain whether the actual control pressure difference actually set is positive or negative are also to be included.

(23) In the positive case 61, the target control pressure difference dpX_target is positive, and the zero case 60 is not present. In the positive case 61, the second control chamber is connected to the control return via the 3/2-way switching valve, so that in the second control chamber essentially the pressure of zero is present. In the case of the pressure control valve with rising characteristic curve shown in FIG. 1, the current i_DRE is thus approximately proportionally increased to the desired target control pressure difference dpX_target. The corresponding relationship is shown in FIG. 4.

(24) In the negative case 62, the target control pressure difference dpX_target is negative, and the zero case is not present. In this case, the second control chamber is connected to the control supply via the 3/2-way switching valve, so that a high pressure is present in the second control chamber, this pressure simultaneously defining the maximum pressure that can be set with the pressure control valve. In the case of pressure control valve with positive characteristic curve shown in FIG. 1, the current i_DRE is decreased approximately inversely proportionally to the magnitude of the desired target control pressure difference dpX_target.

(25) In the zero case 60, two zero settings of the control valves (numbers 30; 32 in FIG. 1) are possible. Either both of the control chambers may be connected to the control supply or both of the control chambers (numbers 21; 22 in FIG. 1) may be connected to the control return. In both cases of the zero setting, an effective pressure of zero acts on the control piston. Theoretically, the pressure control valve must not be set to an intermediate value. However, this is unavoidable in practice, in particular when a transition from the positive case 61 to the negative case 62 or vice versa occurs (zero crossing). This problem is addressed by the fact that the corresponding switchover is carried out only when it is actually necessary, and it is then carried out particularly quickly.

(26) For this purpose, preferably all four conceivable cases of the first and second control pressures p_DRE; p_3/2 are calculated continually and quasi-simultaneously or, in the case of the zero settings, provided as constants, wherein only in the context of a case differentiation 63 is it determined which of the four cases is used to control the control valves (numbers 30; 32 in FIG. 1).

(27) This case differentiation 63 is simple when clearly the positive or negative case 61; 62 is present. It becomes more difficult when a zero crossing occurs. Here, it is conceivable that the target control pressure difference dpX_target is abruptly adjusted across zero. In this sub-case, the simple case differentiation of the positive and negative cases 61; 62 already leads to the desired result, wherein the discontinuity caused by the control system is not troublesome due to the discontinuous adjustment of the target control pressure difference dpX_target.

(28) If the zero crossing takes place in the context of a continuous adjustment of the target control pressure difference dpX_target, it can be predicted when theoretically the zero crossing will take place by observing the derivative of the target control pressure difference with respect to time. Then, the zero setting that does not require an abrupt adjustment of the control valves is set a short period of time before the theoretical zero crossing. During the zero case 60, the target control pressure difference dpX_target is further observed. If the zero case 60 is clearly exited in the sense of a zero crossing, the other zero setting is abruptly set.

(29) With the inverse model 64 of the pressure control valve, the first control current i_DRE required for setting the desired pressure is determined. This may be accomplished with the static model shown in FIG. 4, which is simply the valve characteristic curve. However, it is also conceivable that a dynamic model is used which takes into account how quickly the pressure control valve reacts to changes in the first control current i_DRE. Due to the particularly fast adjustment desired in the zero case 60, a dynamic inverse model 64 is advantageous.

(30) The inverse model 65 of the 3/2-way switching valve is used to determine the second control current i_3/2 required to set the desired pressure. Due to the underlying switching characteristic, basically only two cases are to be differentiated here. In one case, i_3/2 equals zero. In the other case, i_3/2 is as large as possible without allowing overheating of the corresponding actuation magnet (number 33 in FIG. 1). It is conceivable that a greater second control current i_3/2 is set during the switchover time period than in a later time period in which the 3/2-way switching valve is intended only to maintain its position. Firstly, a fast switchover can be achieved in this way, and secondly, the current consumption and the risk of overheating are minimized.

(31) FIG. 4 shows a diagram illustrating the inverse model of the pressure control valve. It is a static model in the form of the valve characteristic curve. On the horizontal axis the (average) current in the actuation solenoid is plotted, and on the vertical axis the pressure applied at the outlet of the pressure control valve is plotted. The pressure control valve is preferably designed to have at least a monotonic characteristic curve. This ensures that the model can be clearly inverted. If a first control pressure p_DRE is specified, there is only one single possible first control current i_DRE to be set for said first control pressure.

(32) The present characteristic curve is approximately linear. In the context of the preferred digital control, the present approximation quality is readily sufficient, since the remaining non-linearity is compensated by the inverse model, without fear of technical disadvantages.

(33) Finally, reference is made to a parallel patent application of the present patent applicant which has the same priority date and which relates to further details of the axial piston machine according to the disclosure.

REFERENCE SIGNS

(34) 10 Axial piston machine 11 First working port 12 Second working port 13 Feed pump 14 Feed pressure limiting valve 15 Control supply 16 Control return 17 Tank 18 Main pump 19 Drive shaft 20 Control cylinder 21 First control chamber 22 Second control chamber 23 Feed valve 30 Pressure control valve 31 Actuation solenoid of the pressure control valve 32 3/2-way switching valve 33 Actuation solenoid of the 3/2-way switching valve 40 Hydraulic motor 41 Fan wheel 50 Trajectory planning filter 51 Inverse model of axial piston machine 52 Controller 53 Target/actual value comparison 54 Superposition of feed-forward control and closed-loop control 55 Determination of the actual conveying pressure difference 60 Zero case 61 Positive case 62 Negative case 63 Case differentiation 64 Inverse model of the pressure control valve 65 Inverse model of the 3/2-way switching valve 70 Speed sensor 71 First pressure sensor 72 Second pressure sensor dpX_target Target control pressure difference dp_target Target conveying pressure difference i_DRE First control current i_3/2_ Second control current p_A First actual pressure p_B Second actual pressure n Actual speed p_DRE First control pressure p_3/2 Second control pressure