Method for a hydraulic system for a dual-clutch gearbox

Abstract

A hydraulic system for an automatic gearbox for a motor vehicle, includes a high-pressure circuit which includes a pressure accumulator, at least one clutch and actuators, and a low-pressure circuit for cooling the clutch, the high-pressure circuit and the low-pressure circuit each containing a hydraulic cooling pump and a hydraulic charging pump that can be driven by a shared electric motor; and a controller which, when it is detected that the pressure accumulator needs to be charged, controls the electric motor to run at a charging setpoint speed, and/or, when it is detected that cooling is needed or another need exists, controls the electric motor to run at a cooling setpoint speed or another setpoint speed. When the pressure accumulator does not need to be charged and there is no need for cooling and no other need, the controller reduces the setpoint speed to zero for a specified period. At the end of the period the controller controls the electric motor to run at least at the test speed.

Claims

1. A hydraulic system for an automatic transmission, in particular a dual-clutch transmission, of a motor vehicle comprising: a high-pressure circuit comprising a pressure accumulator, at least one clutch, and actuators; a low-pressure circuit for cooling the at least one clutch, a cooling hydraulic pump connected to the low pressure circuit; a charging hydraulic pump connectable to the pressure accumulator and to the low pressure circuit, said cooling hydraulic pump and said charging hydraulic pump being drivable via a shared electric motor; a control device having a pressure model unit, said control device being configured to control the electric motor to run at a charging setpoint speed when recognizing a requirement for charging the pressure accumulator to control the electric motor to run at a cooling setpoint speed or other setpoint speed when recognizing a requirement for cooling or other requirement, to lower the speed of the electric motor to zero for a predetermined period of time when neither a requirement for charging the pressure accumulator nor a requirement for cooling nor another requirement exists, and to control the electric motor to run at least at a test speed after expiration of the predetermined period of time, said pressure model unit during an entire operating time of the hydraulic system detecting actuations of the actuators or other events that influence a pressure of the pressure accumulator, and determining partial pressure differences as a function of the detected actuations of the actuators or other events, said pressure modeling unit modeling a pressure decrease in the pressure accumulator over time during the predetermined period of time as a function of the determined partial pressure differences, and as a function of basic leakage pressure losses of the hydraulic system, wherein when the modeling of the pressure modeling unit indicates a minimal pressure of the pressure accumulator, the control device controls the electric motor to run at the test speed.

2. The hydraulic system of claim 1, wherein in a non-charging mode of the hydraulic system, for example after a cooling operation, the cooling hydraulic pump and the charging hydraulic pump are fluidly connected with the low-pressure circuit and the charging hydraulic pump is decoupled from the high-pressure circuit, and wherein in the non-charging mode the hydraulic pumps operate at a low pump load and a low actual current consumption.

3. The hydraulic system of claim 1, wherein in a charging mode of the hydraulic system, the charging hydraulic pump is fluidly connected with the high-pressure circuit and operates at high pump load with high actual current consumption.

4. The hydraulic system of claim 1, wherein the high-pressure circuit and the low-pressure circuit are connected via a bypass line with integrated control valve, said the control valve being configured to automatically switch in dependence on an accumulator pressure in the high-pressure circuit without external energy input between a charging position in which the hydraulic system operates in a charging mode and a non-charging position in which the hydraulic system operates in a non-charging mode for example a cooling mode.

5. The hydraulic system of claim 4, wherein the control valve automatically assumes a charging position when an accumulator pressure in the high-pressure circuit falls below a lower threshold value and the control valve automatically assumes a non-charging position when the accumulator pressure in the high-pressure circuit exceeds an upper threshold value.

6. The hydraulic system according to one of the claims 5, wherein at a charging mode end time point the accumulator pressure in the high-pressure circuit corresponds to the upper threshold value, wherein in particular when exceeding the upper threshold value the control valve in the bypass line automatically switches from the charging position into the cooling position, wherein the upper threshold value is an initial pressure of the pressure model from which partial pressure differences are subtracted or added.

7. The hydraulic system of claim 1, wherein the temperature dependent accumulator pressure in the high-pressure circuit is continuously reduced with a substantially constant slope due to basic leakage pressure losses, and wherein the accumulator pressure is substantially abruptly reduced due to an actuator actuation.

8. The hydraulic system of claim 7, wherein the pressure model unit for modeling the decrease of the accumulator pressure over time estimates or determines the slope due to the basic leakage pressure losses.

9. The hydraulic system of claim 8, wherein the pressure model unit calculates the slope based on a time interval between the charging mode end time point and a subsequent charging mode starting time point and based on a pressure difference between the upper threshold value and the lower threshold value of the control valve by taking the partial pressure differences that have occurred in the time interval into account.

10. The hydraulic system of claim 1, further comprising a current measuring device for detecting an actual current consumption of the electric motor and a speed sensor for detecting an actual speed of the electric motor, wherein the control device has an analysis unit, said analysis unit being configured to recognize the requirement for charging the pressure accumulator as a function of and comparison of the actual current consumption of the electric motor with a reference value and as a function of the actual current consumption of the electric motor.

11. The hydraulic system of claim 10, further comprising a constant-speed unit, said constant-speed unit detecting whether the hydraulic pumps run at a constant speed, wherein when the hydraulic pumps do not run at a constant speed the analysis device is deactivated and when the hydraulic pumps run at a constant speed the analysis unit is activated, wherein in particular in the absence of a reference an emergency speed and a charging speed are used as minimal speed.

12. The hydraulic system of claim 11, wherein the constant-speed unit determines that the hydraulic pumps run at a constant speed when a setpoint speed gradient is zero, a filtered setpoint speed gradient is smaller than a threshold value, and an actual rotational speed gradient is smaller than a threshold value.

13. The hydraulic system of claim 1, wherein a reference value for recognizing the requirement for charging the pressure accumulator is determinable from a reference polynomial stored in the control device as a function of the actual speed.

14. The hydraulic system of claim 13, wherein the reference polynomial is generated in an initialization phase, for example at an initial operation of the vehicle, and wherein in the initialization phase, in particular either continuously in a charging mode time interval or continuously in a non-charging mode interval at different actual speeds and optionally different temperatures the corresponding actual current consumptions are detected as reference values and stored.

15. The hydraulic system of claim 1, wherein the control device has an analysis unit, said analysis unit recognizing a requirement for charging the pressure accumulator based on a comparison between the actual current consumption and a filtered value of the actual current consumption.

16. The hydraulic system of to claim 10, wherein the control device has a constant-speed unit, which detects whether the hydraulic pumps are running at a constant speed, and when the hydraulic pumps are running at a constant speed the analysis unit is activated and when the hydraulic pumps are not running at a constant speed the analysis unit is deactivated.

17. The hydraulic system of claim 13, wherein after generation of the reference polynomial the requirement for charging the pressure accumulator is recognizable from a comparison of the actual current consumption with the corresponding reference value.

18. The hydraulic system of claim 1, wherein in the absence of a requirement for cooling or other requirement and also in the absence of a requirement for charging the pressure accumulator, the control device controls the electric motor to run at a test speed for detecting the actual speed and the actual current consumption, said test speed being lower than the charging setpoint speed and the cooling setpoint speed.

Description

(1) In the following the invention and its advantageous embodiments and refinements as well as their advantages are explained in more detail by way of drawings.

(2) It is shown in:

(3) FIG. 1 a block diagram of a dual-clutch transmission for a motor vehicle with seven forward gears and a reverse gear;

(4) FIG. 2 the hydraulic system of the dual-clutch transmission of FIG. 1;

(5) FIG. 3 program modules of the control device for controlling the electric motor of the hydraulic pumps of the hydraulic system in a strongly simplified schematic diagram according to a comparative example which is not part of the invention;

(6) FIG. 4 a constant-speed unit of the control device;

(7) FIG. 5 an analysis unit of the control device;

(8) FIG. 6 a diagram showing the temporal courses of relevant parameters during operation of the hydraulic system;

(9) FIG. 7 in a view corresponding to FIG. 3 a control device according to the exemplary embodiment of the invention;

(10) FIG. 8 a pressure model unit of the control device shown in FIG. 7; and

(11) FIG. 9 a diagram showing the temporal courses of relevant parameters during operation of the hydraulic system according to the second exemplary embodiment.

(12) FIG. 1 shows a schematic diagram of a dual-clutch transmission for a motor vehicle with all-wheel-drive. The dual-clutch transmission has seven forward gears (see the encircled digits 1 to 7) and a reverse gear RW. In the following the dual-clutch transmission is only described to the degree necessary to understand the invention. The dual-clutch transmission has two input shafts 12, 14, which are arranged coaxial to each other and are alternately connectable with the drive, for example an internal combustion engine, via two hydraulically actuatable multi-disc clutches K1, K2. The input shaft 14 is configured as a hollow shaft in which the input shaft 12, which is configured as solid shaft, is guided. The two input shafts 12, 14 output via gearwheel sets of the forward gears and the reverse gear onto a coaxially arranged output shaft 16 and an intermediate shaft 18, which is configured as hollow shaft. The gearwheel sets of the forward gears 1 to 7 each have fixed gearwheels and idler gearwheels, which can be switched via actuators 22. The actuators 22 can for example be dual-clutch synchronous clutches, which in a neutral position can each switch two neighboring idler gearwheels.

(13) FIG. 2 shows the hydraulic system of the dual-clutch transmission in a very simplified block diagram. By way of the hydraulic system the hydraulic cylinders 23 of the clutches K1, K2 and the actuators 22 are actuated. According to FIG. 2 the hydraulic system has a high-pressure circuit H and a low-pressure circuit N. In the high-pressure circuit H, the clutches' K1, K2 hydraulic cylinders 23 contained in the high pressure circuit and the actuators 22 can be impinged with an accumulator pressure p.sub.s via a pressure accumulator 25, which pressure can be in the range of for example 30 bar. For this a main line 27, which is connected to the pressure accumulator 25, is guided to the hydraulic cylinders 23 via not further described partial lines 31. In the partial lines 31 respective control valves 35 are arranged. The control valves 35 can be controlled via a central control device 39 in a not further shown manner.

(14) The hydraulic system also has a charging hydraulic pump 53, which on the input side is connected with an oil sump 55. The charging hydraulic pump 53 can be controlled by the control unit 39 for charging the pressure accumulator 25 via the electric motor 57. In addition the charging hydraulic pump 53 together with the cooling hydraulic pump 59 is arranged on a common drive shaft 60, which is driven by the electric motor 57. On the output side the cooling hydraulic pump 59 is connected with a low-pressure line 61, which leads to a distributor valve 63. Depending on the position of the distributor valve 63, the hydraulic fluid can be returned to the first and/or second clutch K1, K2 and then into the oil sump when a requirement for cooling exists.

(15) According to FIG. 2, the main line 27 of the high-pressure circuit H branches off at a branching point 65 into a bypass line 67, which is connected with the low-pressure line 61 of the low-pressure circuit N. Downstream of the branching point 65, a later described check valve 69 is arranged. In addition a control valve 71 is integrated in the bypass line 67. The control valve 71 can be switched between a charging position L shown in FIG. 2 and a cooling position (in which the upwards pointing arrow drawn into the control valve 71 connects lines 67 and 67.1) in dependence on the accumulator pressure p.sub.s in the high-pressure circuit. The accumulator pressure p.sub.s in the high-pressure circuit H acts as control pressure with which the control valve 71 can be switched without external energy input, i.e., automatically. The control valve 71 is hereby configured so as to assume the charging position L when the accumulator pressure p.sub.s in the high-pressure circuit H falls below a lower threshold value P.sub.s, low (FIGS. 8 and 9), for example 25 bar. In addition the control valve 71 automatically assumes its cooling position when the accumulator pressure p.sub.s exceeds an upper threshold value P.sub.s, up (FIGS. 8 and 9) for example 28 bar.

(16) During driving operation, pressure losses occur due to actuation of the clutches K1, K2 and the actuators 22. In addition further pressure losses occur due to basic leakage, i.e., due to valve gaps or the like in the high-pressure circuit H. As a result the accumulator pressure p.sub.s is reduced during driving operation. In the case the accumulator pressure p.sub.s falls below the lower threshold value p.sub.s, low (when a requirement for charging the pressure accumulator exists) the control valve 71 automatically assumes its charging position L (FIG. 2). When recognizing the requirement for charging the pressure accumulator, the control device 39 controls the electric motor 57 according to FIG. 6 to run at a charging setpoint speed n.sub.1 , setpoint, charging. As a result the charging hydraulic pump 53 can charge the pressure accumulator 53. In such a charging mode the charging hydraulic pump 53 operates with high pump load and therefore with correspondingly high actual current consumption I.sub.actual. When the accumulator pressure p.sub.s exceeds the upper threshold value p.sub.s, up (i.e., a requirement for charging the pressure accumulator no longer exists) the control valve 71 automatically assumes its cooling position K. In the cooling position K the charging hydraulic pump 53 pumps hydraulic oil into the low-pressure circuit N via the now open bypass line 67. At the same time the high-pressure circuit H is closed pressure tight via the check valve 69. Correspondingly the charging hydraulic pump 53 no longer operates with a high pump load but with a reduced pump load and correspondingly with a reduced actual current consumption I.sub.actual

(17) As mentioned above when recognizing a requirement for charging the pressure accumulator charging, the control device 39 controls the electric motor 57 to run at a charging setpoint speed n.sub.setpoint charging. For recognizing such a requirement for charging the pressure accumulator, the invention does not require a pressure sensor in the high-pressure circuit H or a charging sensor in the control valve 71. Instead the control device 39 has an analysis device 73 (FIGS. 3 and 5). According to FIG. 2, the analysis device 73 is in signal communication with a current-measuring device 75 integrated in the motor control unit, which current measuring device detects an actual current consumption I.sub.actual of the electric motor 57 and with a speed sensor 77 which detects an actual speed n.sub.actual of the electric motor 57. When a requirement for charging the pressure accumulator is determined, the analysis unit 73 generates a charging setpoint speed.sub.setpoint, charging. When no requirement for charging the pressure accumulator exists, the analysis unit 73 does not generate a setpoint speed, i.e., the charging speed n.sub.setpoint, charging is set to zero.

(18) According to FIG. 3, the control device 39 also has a further analysis unit 79 with which a requirement for cooling the clutches K1, K2 or another requirement is recognized. Overall the analysis unit 79 can recognize multiple requirements depending on different input parameters. Depending on the recognized requirement, the analysis unit 79 for example generates a cooling setpoint speed n.sub.setpoint cooling shown in FIG. 3, an emergency speed n.sub.emergency or another setpoint speed. In the analysis unit 79 shown in FIG. 3 a clutch temperature T.sub.K1, K2 is exemplary compared with a predetermined threshold temperature T.sub.thresh. When the clutch temperature T.sub.K1, K2 exceeds the threshold temperature T.sub.thresh, a requirement for cooling is recognized and correspondingly a cooling setpoint speed n.sub.setpoint cooling is generated. When no requirement for cooling exists the test speed n.sub.test is generated. The speeds generated in the analysis units 73 and 79 are transmitted to a comparer module 81. The comparer module 81 prioritizes the individual speed requests according to relevance and according to speed. This means a speed request of a lower speed that was categorized as relevant can be prioritized over a speed request of a greater speed, which was categorized as less relevant. The speed request prioritized by the comparer module 81 is transmitted to the electric motor 57 as setpoint speed n.sub.setpoint.

(19) As further shown in FIG. 3 a constant-speed unit 83 is assigned to the analysis unit 73 for recognizing a requirement for charging the pressure accumulator. By means of this unit 83 it is detected whether the hydraulic pumps 53, 59 are running at constant speed. In the absence of a constant actual speed (i.e. when the speed changes) the analysis unit 73 is deactivated via a stop signal S.sub.block. On the other hand in case of a constant actual speed, the analysis unit 73 is activated with an activation signal S.sub.activation. The constant-speed unit 83 is shown in more detail in FIG. 4. Accordingly a constant speed is concluded when the setpoint speed gradient {dot over (n)}.sub.setpoint equals zero and the filtered setpoint speed gradient {dot over (n)}.sub.setpoint filter is lower than a threshold value x.sub.1. In addition the actual speed gradient {dot over (n)}.sub.actual has to be smaller than a further threshold value x.sub.2 as shown in the block diagram of FIG. 4.

(20) FIG. 5 shows the analysis unit 73 for recognizing a requirement for charging the pressure accumulator in a more detailed block diagram. According to this it is first determined in a program module 85 whether reference values I.sub.ref for a comparison between an actual current consumption I.sub.actual of the electric motor 57 are already present. If this is not the case an initiation phase is started. In the initiation phase the electric motor 57 is controlled to run at a high speed. In addition according to FIG. 5 a requirement for charging the pressure accumulator is determined in a comparer module 86, i.e. from a difference I of the actual current consumption I.sub.actual with a filtered value I.sub.actual, filter of the actual current consumption I.sub.actual. When the difference I between the actual current consumption I.sub.actual and the filtered value I.sub.actual filter is at zero no sudden current consumption change has resulted from the switching of the control valve 71 (due to different pump loads).

(21) On the other hand, in such an automatic switching of the control valve 71 a significant difference exists between the actual current consumption I.sub.actual and the filtered value I.sub.actual filter. When such a significant difference is detected it can be recognized (in dependence on the algebraic sign of the difference) when the control valve 71 is in its charging position L or in its cooling position K (i.e., whether a requirement for charging the pressure accumulator exists or not). When recognizing such a requirement for charging the pressure accumulator, the control device 39 controls the electric motor 57 to run at the charging setpoint speed n.sub.setpoint, charging in order to ensure a proper charging process of the pressure accumulator 25. When no requirement for charging the pressure accumulator exists, no setpoint speed is generated in the comparer module, i.e., the setpoint speed n.sub.setpoint charging is set to zero.

(22) In the above-described initiation phase additionally a reference polynomial P, indicated in FIG. 5, which can be stored in the control unit 39, is determined, i.e. on the basis of value pairs consisting of different actual speeds and correlating actual current consumptions of the electric motor 57. The reference polynomial P, however, has to be generated either continuously during a cooling mode time interval or continuously during a charging mode time interval in order to obtain a meaningful reference polynomial P with the detected value pairs.

(23) After generating the reference polynomial P (for example continuously during the cooling mode time interval), the requirement for charging the pressure accumulator is no longer recognized by monitoring the difference I between the actual current consumption I.sub.actual and the filtered value I.sub.actual filter (i.e. in the comparer module 86) but rather in a comparer module 88, i.e., from a comparison of the actual current consumption I.sub.actual and the correlating reference value I.sub.ref (is determined from the reference polynomial P). For this, according to FIG. 5 the actual current consumption I.sub.actual is transmitted via the program module 85 to the comparer module 88. When the actual current consumption I.sub.actual approximately corresponds to the corresponding reference value I.sub.ref the charging hydraulic pump 53 does not work against the high accumulator pressure p.sub.s but with low actual current consumption against the relatively low pressure in the low-pressure circuit N. Correspondingly the control valve 71 is in its cooling position K and the accumulator pressure p.sub.s is still sufficient so that no pressure requirement for charging the accumulator exists. In this case no speed request is generated by the comparer module 88, i.e., the setpoint speed n.sub.setpoint charging is set to zero.

(24) In the case that the actual current consumption I.sub.actual is significantly higher than the corresponding reference value I.sub.ref, the charging hydraulic pump 53 works against the high accumulator pressure p.sub.s in the high-pressure circuit H (with correspondingly high current consumption). In this case a requirement for charging the pressure accumulator is recognized in the comparer module 88 and a charging setpoint speed n.sub.setpoint, charging is generated and transmitted to the comparer module 81.

(25) FIG. 6 shows a diagram which illustrates the temporal courses of the relevant parameters in the hydraulic system, i.e., on top the temporal course of automatically occurring switching processes of the switching valve 71, in the center of the image the temporal course of the actual speed n.sub.actual and on the bottom the temporal course of the actual current consumption I.sub.actual. According to this the control valve 71 switches at a time point t.sub.A from the cooling position K (i.e. the non-charging position) into the charging position L and at a time point t.sub.E again into its cooling position K. In the present operating situation neither a requirement for cooling nor a requirement for charging the pressure accumulator charging nor any other requirement exists up to the time point t.sub.A. The control device 39 therefore controls the electric motor 57 to run at a setpoint test speed n.sub.test (for example 600 rpm), which results in a corresponding actual speed n.sub.actual.

(26) At the time point t.sub.A the control valve 71 automatically assumes its charging position L. Upon reaching the charging position L first only the actual current consumption I.sub.actual increasesat still set test rotational speed n.sub.test. This actual current increase results (at still set test rotational speed n.sub.test) solely from the load increase associated with the change into the charging position L. Only when at a time point t.sub.A (FIG. 6) the comparer module 88 (FIG. 5) recognizes that the actual current consumption I.sub.actual has exceeded the reference value I.sub.ref(n.sub.test) the comparer module 88 (FIG. 5) generates a speed request. Correspondingly the speed is increased to the set point charging speed n.sub.setpoint charging, whereupon a corresponding actual speed n.sub.actual is established (FIG. 6, center of image). The increase of the setpoint speed to the charging speed n.sub.setpoint charging occurs at the time point t.sub.A1 and is associated with a further increase of the actual current consumption I.sub.actual. Thus two actual current increases occur, i.e., a first current increase due to the load increase (up to the time point t.sub.A1) and a second current increase due to the setpoint speed increase.

(27) At the time point t.sub.E the control valve 71 automatically switches back to its cooling position (or non-charging position) K. When the constant-speed unit detects the presence of a constant speed it is again detected based on the actual current consumption I.sub.actual whether a requirement for charging the pressure accumulator exists or not. When reaching the cooling position K firstwhile the charging setpoint speed n.sub.setpoint charging is still generated by the comparer module 88only the actual current consumption I.sub.actual decreases. This actual current decrease results (while the target charge rotational speed n.sub.setpoint charging is still generated by the comparer module 88) solely from the load decrease associated with the change into the cooling position K. Only when the comparer module 88 at a time point T.sub.E1 (FIG. 6) recognizes that the actual current consumption I.sub.actual has fallen below a reference value I.sub.ref(n.sub.charging), the setpoint speed is reduced to the test speed n.sub.test. The speed reduction to the test speed n.sub.test thus occurs at the time point T.sub.E1 and is associated with a further actual current decrease. Thus two actual current decreases result, i.e., a first current decrease due to the load decrease (up to the time point T.sub.E1) and a second current decrease due to the rotational speed reduction.

(28) In the preceding exemplary embodiment of the control device 39, the electric motor 57 is controlled to run at a setpoint test speed n.sub.test when neither a requirement for charging the pressure accumulator nor a requirement for cooling or other requirement is recognized. The target test speed n.sub.test is selected so that the actual current consumption I.sub.actual and the actual speed n.sub.actual can be reliably measured. A disadvantage hereby is that the electric motor 57 has to be permanently operated at least at this setpoint test speed n.sub.test when no requirement for charging the pressure accumulator exists and no requirement for cooling or other requirement exists, in order to ensure the ability to measure the actual current consumption I.sub.actual and the actual rotational speed n.sub.actual of the electric motor 57.

(29) In contrast to this, FIGS. 7, 8 and 9 show a control device 39 according to the exemplary embodiment of the invention. The general construction and the functioning of this control device 39 are essentially identical to that of the preceding comparative example. Insofar reference is made to the description above. In contrast to the preceding exemplary embodiment, the control device 39 according to FIGS. 7 to 9 can lower the setpoint speed n.sub.setpoint for a predetermined standstill time period t.sub.off (FIG. 9) as low as zero when neither a requirement for charging the pressure accumulator nor a requirement for cooling nor other requirement is recognized. After expiration of this standstill time period t.sub.off the control device 39 controls the electric motor 57 at least with the target test rotational speed n.sub.test in order to enable measuring by the control device again.

(30) For determining the standstill time period t.sub.off the control device 39 additionally has a pressure model unit 89. The pressure model unit 89 is according to FIG. 7 for example in signal communication with the signal input of the comparer module 81. When the standstill conditions (i.e., neither a requirement for charging the pressure accumulator nor a requirement for cooling and other requirement exists) is satisfied the pressure model unit 89 sets the setpoint speed n.sub.setpoint to zero. The standstill time period t.sub.off begins at the end time point t.sub.E of the charging mode (at the time point at which the control valve 71 automatically switches from the charging position L into the cooling position or K).

(31) In the pressure model unit 89 according to FIG. 8, the temporal course of the accumulator pressure p.sub.s(t) is modeled during the entire operating time. For this the pressure model unit 89 detects every event relevant for the pressure course in the high-pressure circuit H that leads to an increase (i.e., in FIG. 8 the partial pressure differences p.sub.y) or to a reduction (i.e., in FIG. 8 the partial pressure differences p.sub.x) of the accumulator pressure p.sub.s. Depending on the event the partial pressure differences have positive or negative algebraic signs. The partial pressure differences p.sub.x are exemplary linked with actuator actuations. Each of these actuator actuations is assigned a partial pressure difference px.sub.1, px.sub.2, . . . (FIG. 8 or 9). The partial pressure differences px.sub.1, px.sub.2 . . . exemplary shown in FIGS. 8 and 9 correlate respectively with the hydraulic fluid withdrawals from the high-pressure circuit H associated with the actuator actuations. In addition also the pressure loss p.sub.xB due to permanent basic leakage is taken into account in the modeled temporal course of the accumulator pressure p.sub.s(t).

(32) According to FIG. 8 or 9 the initial value of the temporal accumulator pressure course corresponds to the already mentioned upper threshold value p.sub.s, up of the control valve 71. When reaching a minimal pressure p.sub.min the standstill time period t.sub.off ends so that the control device 39 controls the electric motor 57 to run at the test speed n.sub.test. Subsequently it is determined with the analysis unit 73 whether a requirement for charging the pressure accumulator exists or not. The requirement for charging the pressure accumulator, however is only determined when the constant-speed conditions are satisfied. For a meaningful measurement of the actual current consumption it is also advantageous when a defined minimal measurement time (measuring window) exists in which the constant conditions are satisfied.

(33) In FIG. 8 the switching timer point t.sub.E is recognized by means of the program module 90 of the pressure model unit 89, at which the control valve 71 is switched from its charge position L into its non charge position/cooling position K. The recognition of the switching time point t.sub.E occurs by means of the testing of the actually present charging setpoint speed n.sub.sepoint, charging and the charging setpoint speed n.sub.sepoint, charging n1 present at a previous measuring time point. When the conditions stored in the program module 90 (i.e., n.sub.setpoint, charging, n10 and n.sub.sepoint, charging=0) are satisfied, an initialization occurs at which the temporal accumulator pressure course p.sub.s(t) modeled in the pressure model is reset to its initial value, i.e., to the upper threshold value p.sub.s up as shown in the diagram of FIG. 9 with the accumulator pressure charge curve between the time points tA and t.sub.E.

(34) As mentioned above the accumulator pressure p.sub.s during operation is reduced due to for example the basic leakage pressure losses .sub.pB and pressure losses px.sub.1, px.sub.2 . . . due to actuator actuations. Especially the basic leakage losses lead to a continuous reduction with substantially constant slope (FIG. 8 or 9) of the accumulator pressure course p.sub.s(t). In light of the foregoing the slope due to basic leakage can approximated as follows: first a time interval t.sub.cooling (FIG. 9) between the end time point t.sub.E of a charging operation and the starting time point t.sub.A of a subsequent charging operation (i.e., the time period between two recognized requirements for charging) is detected. Subsequently the pressure difference between the upper threshold value p.sub.s, up and the lower threshold value p.sub.s, low of the control valve 71 is determined by taking the partial pressure differences p.sub.y or p.sub.x of the accumulator pressure p.sub.s occurring in the time interval t.sub.cooling into account. From this the slope can be easily determined, which allows optimizing the modeling of the temporal accumulator pressure course. The accumulator pressure p.sub.s is strongly temperature dependent. For a realistic modeling it is therefore preferred when the temporal accumulator pressure course is modeled by taking this strong temperature dependence into account.