Method for controlling overlapping shifts of a hydraulically operated dual clutch

Abstract

The invention relates to a method for controlling an overlapping shift of a hydraulically actuated double clutch, the double clutch having two friction clutches (2, 3) which are respectively actuated by slave cylinders (17, 18) and opened without pressure, wherein a system pressure (p(s)) is set by an electrically operated pump unit (4) and a first pressure regulating valve (14) connected downstream thereof, and a second pressure regulating valve (19, 20) is respectively arranged between the first pressure regulating valve (14) and the slave cylinders (17, 18) together with a pressure sensor (24, 25) connected downstream, and the overlapping shift of the friction clutches (2, 3) is carried out by overlapping depressurization of the first slave cylinder (17, 18) associated with the closed first friction clutch (2, 3) and pressurization of the second slave cylinder (18, 17) associated with the opened second friction clutch (3, 2). In order to realize an overlap shift without pressure shock at the friction clutches (2, 3), the system pressure (p(s)) is increased in a first phase (A) of the overlap shift and the system pressure (p(s)) is preset within a preset, essentially constant, negative or positive pressure gradient in a second phase (B) of the overlap shift.

Description

Technical Field

[0001] The invention relates to a method according to the preamble of claim 1 . Background Art

[0002] Power trains with dual clutch transmissions and dual clutches with friction clutches that connect two sub-power trains of the dual clutch transmission to a hybrid drive unit or an internal combustion engine have long been known. A dual clutch transmission can shift between two gears provided on the two sub-transmissions by means of so-called overlapping shifting, in which the engaged friction clutch of the active sub-transmission is opened and the friction clutch of the sub-transmission is engaged overlappingly with the new gear.

[0003] From the publications WO 2015 / 086009 A1 and DE 10 2015 210 877 A1, a hydraulic unit and a method for controlling the hydraulic unit are known for operating the friction clutch of a dual clutch, wherein a system pressure is generated by means of a pump unit and a slave cylinder is loaded with pressure by means of a pressure regulating valve by means of the system pressure in dependence on the switching state of the pressure regulating valve and another slave cylinder is relieved of pressure during overlapping shifting in order to overlap the switching of the friction clutches. Summary of the invention

[0004] The object of the present invention is to improve a method for controlling an overlap shift of a friction clutch of a dual clutch controlled by means of a hydraulic unit. In particular, the object of the present invention is to perform an overlap shift without torque peaks occurring at the friction clutch.

[0005] This object is achieved by the subject matter of claim 1. The claims dependent on claim 1 describe advantageous embodiments of the subject matter of claim 1.

[0006] The proposed method is used to control the overlapping shifting of a hydraulically actuated dual clutch of a dual clutch transmission of a power train of a motor vehicle having a purely electric drive, a hydraulic drive unit or an internal combustion engine. For this purpose, corresponding routines are stored in the software in a control unit for controlling the hydraulic unit for actuating the dual clutch and, if necessary, for controlling and / or cooling other components of the power train.

[0007] The hydraulic unit controls and actuates the two friction clutches of the dual clutch, which are designed as forced-open friction clutches or preferably as forced-closed friction clutches. Forced-closed friction clutches are understood to be clutches which are open in the non-actuated state and which are closed under the action of the hydraulic unit of a slave cylinder which is preferably pressurized by a pump unit.

[0008] According to an advantageous embodiment of the hydraulic unit, each of the two positively closed friction clutches is actuated by a slave cylinder, for example, against the action of a return spring, wherein the required pressure, such as the system pressure, is provided by an electrically operated pump unit. The friction clutch can be designed as a dry single-plate or multi-plate clutch or as a wet-operating diaphragm clutch, which has a diaphragm set consisting of alternately arranged friction diaphragms and counter diaphragms. Accordingly, in a dry-operating friction clutch, the slave cylinder can be formed by a slave cylinder housing and a slave cylinder piston that can be displaced in the slave cylinder housing in dependence on the applied pressure, which, when the friction lining of the clutch disk is tensioned, preloads the pressure plate against the counterpressure plate in a pressure-dependent manner, thereby forming the friction engagement of the friction clutch. In a wet-operating friction clutch, a pressure tank designed as a slave cylinder can be integrated into the clutch housing, which can be axially displaced by loading the pressure chamber and thus preloads the diaphragm set of the friction clutch in a pressure-dependent manner, thereby forming the friction engagement of the friction clutch.

[0009] The pump unit is controlled by a control unit and can be designed as a tandem pump, which provides the system pressure on a common system pressure line (common rail) on the one hand and one or more volume flows for operating and cooling components of the power train on the other hand. For example, a wet-running friction clutch can be cooled by the volume flow of the pump unit. In addition to controlling the rotational speed of the pump unit, the system pressure is controlled - the pump unit is operated specifically for economical operation applications at low pressure with a high volume flow demand or at high pressure with a low volume flow demand.

[0010] In order to regulate the system pressure in the system pressure line, a first pressure regulating valve is connected downstream of the pump unit, which sets the system pressure. To this end, the first pressure regulating valve can be connected to the pressure medium flow. Preferably, the system pressure is set to a stagnation pressure in the system pressure line, and the first pressure regulating valve is connected to the outlet or discharge section of the system pressure line, so that in order to regulate the stagnation pressure, the pressure medium is discharged into the discharge section, for example into a line for cooling, by means of the connection of the first pressure regulating valve. In this case, the closing of the pressure regulating valve causes an increase in the system pressure.

[0011] A second pressure regulating valve is arranged between the stagnation pressure line and the slave cylinder for actuating the friction clutch. In order to monitor the pressure at the slave cylinder, a pressure sensor is arranged between the slave cylinder and the second pressure regulating valve. A check valve is arranged upstream of the second pressure regulating valve so that the pressure set at the respective slave cylinder can be maintained at least temporarily independently of the system pressure. Any pressure drop at the slave cylinder that may occur, in particular due to leakage occurring at the second pressure regulating valve, is reregulated by means of the second pressure regulating valve at a sufficiently set system pressure.

[0012] In this case, the torque which can be transmitted via the friction clutch is set by means of a system pressure which is set by the first pressure regulating valve and is a function of the desired torque.

[0013] The torque transmitted via the closed friction clutch is set by means of the pressure loading of the associated slave cylinder. The corresponding pressure, such as the clutch pressure, is detected by means of a pressure sensor. The detected pressure value is associated with the corresponding torque value that can be transmitted via the friction clutch in the control unit according to a corresponding characteristic curve or characteristic curve group.

[0014] Overlapping shifts from an activated gear of a sub-transmission with a closed friction clutch to another engaged gear of another sub-transmission with an open friction clutch are respectively carried out by overlapping actuation of the friction clutches, which is achieved by overlapping depressurization of a slave cylinder associated with the closed friction clutch and pressurization of another slave cylinder associated with the open friction clutch.

[0015] In order to be able to control the torque curve during the overlap shifting sufficiently accurately and to avoid pressure peaks that are related to the design of the pressure control valve, the system pressure is increased in the first phase of the overlap shifting and is preset within a preset, essentially constant, negative or positive pressure gradient in the second phase of the overlap shifting. An essentially constant, negative or positive pressure gradient is understood to mean that the pressure development of the system pressure is so small that a sudden increase in the system pressure is avoided in order to avoid a rapid adhesion of the friction clutch to be closed. In this case, a negative pressure gradient can be set in the second phase, especially when a rapid shifting is required, while a constant or small positive pressure gradient can be provided in non-time-critical shifting.

[0016] For example, at the beginning of the first phase, the system pressure is increased significantly, for example by actuating the first pressure regulating valve, for example completely closing the passage from the system pressure line to the outlet, and reducing the pump speed of the pump unit. At the beginning of the first phase, the first friction clutch can be temporarily kept closed by means of the increased system pressure by means of the second pressure regulating valve and the non-return valve associated with the closed first friction clutch, despite possible leakage. The first friction clutch is thereby at least briefly exposed to overpressure. The second pressure regulating valve associated with the open second friction clutch is closed. The non-return valve maintains the pressure applied to the slave cylinder.

[0017] In the first phase, the system pressure is reduced by means of a reduced pump speed. After the system pressure has dropped, the second pressure regulating valve associated with the second friction clutch is opened and the second slave cylinder is filled with pressure medium. Due to the reduced system pressure, the second friction clutch does not yet adhere directly and the transition to the second phase of the overlap shifting occurs.

[0018] At the beginning of the second phase, after filling the second slave cylinder, the second pressure regulating valve associated with the second friction clutch is closed and the system pressure is increased. In the second phase, the second pressure regulating valve associated with the first friction clutch is closed in two stages while forming a preferably negative pressure gradient.

[0019] In this case, the first pressure regulating valve and the second pressure regulating valve associated with the second friction clutch can be gradually opened by means of the second stage of the second pressure regulating valve that closes the first friction clutch. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention is based on Figures 1 to 3 The embodiment shown in is elaborated in detail. It shows:

[0021] Figure 1 A schematically illustrated hydraulic wiring diagram of a hydraulic unit for actuating a dual clutch is shown;

[0022] Figure 2 Show Figure 1 A schematic diagram of a pressure regulating valve of a hydraulic unit; and

[0023] Figure 3 A graph showing overlap shifts versus time is shown. DETAILED DESCRIPTION

[0024] Figure 1 The schematic diagram shows a hydraulic unit 1 for actuating two friction clutches 2, 3 of a dual clutch of a dual clutch transmission. The hydraulic unit 1 can also actuate and / or cool components (not shown) of a drive train with a dual clutch transmission. This function of the hydraulic unit 1 is not shown.

[0025] The hydraulic unit 1 has an electrically operated and regulated pump unit 4, which is designed here as a tandem pump 5. The pump unit 4 delivers pressure medium from an oil sump 6 via a filter unit 7. A larger pump part 8 delivers a large volume of pressure medium at low pressure into a cooling line 9 and via a heat exchanger 10 to components 11 to be cooled, such as wet-operating friction clutches 2, 3, brakes, transmission elements and / or similar components.

[0026] The smaller pump part 12 builds up a system pressure p(s) in a system pressure line 13. The applied system pressure p(s) is set on the one hand by the rotational speed of the pump unit 4 and on the other hand by a pressure regulating valve 14. The pressure regulating valve 14 connects the system pressure line 13 to the cooling line 9 for setting the system pressure p(s).

[0027] Two pressure lines 15, 16 can be connected to the system pressure line 13, which pressure lines act on the slave cylinders 17, 18 with clutch pressures p(K1) and p(K2) for actuating the friction clutches 2, 3 by means of pressure regulating valves 19, 20 and non-return valves 21, 22. Another component 23 connected to the system pressure line 13, such as a parking lock or the like, is not essential for the function of actuating the friction clutches 2, 3. The clutch pressures p(K1) and p(K2) acting on the outputs 17, 18 are detected by means of pressure sensors 24, 25 and evaluated by a control unit, which controls the contact force of the friction clutches 2, 3 and thus the torque that can be transmitted. The control unit also controls the pump unit 4 and the pressure regulating valves 14, 19, 20.

[0028] In normal driving operation outside of gearshifting in the dual clutch transmission, one of the friction clutches 2, 3 is closed and the other is open. For this purpose, the respective pressure regulating valve 19, 20 of the closed friction clutch 2, 3 is connected to the system pressure line 13, while the other pressure regulating valve 20, 19 is connected to the oil pan 6. The non-return valves 21, 22 keep the clutch pressure p(K1), p(K2) of the closed friction clutch 2, 3 below the respective clutch pressure p(K1), p(K2) independently of the decrease in the system pressure p(s) by opening the pressure regulating valve 14 and / or correspondingly controlling the pump unit 4. If the clutch pressure p(K1), p(K2) of the closed friction clutch 2, 3 drops to a value corresponding to a preset desired torque below the clutch pressure p(K1), p(K2), the system pressure p(s) is increased, for example by closing the pressure regulating valve 14, until the required clutch pressure p(K1), p(K2) and, if necessary, an overpressure are reached.

[0029] In the case of a shift between a currently activated gear of a sub-transmission with closed friction clutches 2, 3 and a newly activated gear of a sub-transmission with open friction clutches 3, 2, in an overlapping shift, the closed friction clutches 2, 3 are opened and the open friction clutches 3, 2 are closed with temporal overlap. In order to carry out an overlapping shift, it is advantageous to change the installation of the pressure regulating valves 19, 20 on the friction clutches 2, 3 and the installation of the pressure regulating valve 14, 19, 20, taking into account and avoiding pressure peaks acting on the friction clutches 2, 3 during the shifting process of the pressure regulating valves 14, 19, 20. For this purpose, the overlapping shift is carried out as in Figure 3 The process is performed in two stages as described in the diagram 100 of FIG.

[0030] Figure 2 Reference Figure 1 A schematic diagram of one of the pressure regulating valves 14, 19, 20 is shown, which has a pressure inlet along the arrow 26 and a pressure outlet along the arrow 27. The pressure regulating valves 14, 19, 20 are operated by means of a control piston 29 which is axially displaceable along the double arrow 28 and which controls the flow of pressure medium through the pressure inlet and outlet by means of control edges 30, 31. The control piston 29 is operated by means of a magnetic force F(M) of an electronically controlled magnetic valve along the double arrow 28 in opposition to the action of a spring element (not shown) and the action of a restoring force F(C). In particular, at the beginning of connecting the control edges 30, 31 to the respective pressure inlet or pressure outlet, pressure peaks occur at the pressure regulating valves 14, 19, 20, which can have a negative effect on the system pressure p(s) of the system pressure line 13 or on the torque which can be transmitted via the friction clutches 2, 3 in the pressure lines 15, 16 and thus via the slave cylinders 17, 18. For example, a friction clutch that is to be engaged can suddenly transmit torque and an open friction clutch can engage again during the coasting phase. Both processes result in uncomfortable overlapping shifts.

[0031] In order to avoid pressure peaks at the friction clutches 2 and 3, Figure 3 Reference Figure 1 A diagram 100 shows a typical overlapping shift by disengaging the friction clutch 2 and engaging the friction clutch 3. The diagram 100 shows seven sub-diagrams I, II, III, IV, V, VI, VII over the time t.

[0032] In the subgraph with the speed n over time t, the speed curve 101 shows the speed n(GE1) of the transmission input shaft of the sub-transmission with the friction clutch 2 closed at the beginning of the overlap shift, and the speed curve 102 shows the speed n(GE2) of the transmission input shaft of the sub-transmission with the friction clutch 3 open at the beginning of the overlap shift over time t and thus the output speeds of the friction clutches 2, 3. The speed curve 103 shows the speed n(M) of the crankshaft of the internal combustion engine and thus the input speeds of the friction clutches 2, 3 over time t.

[0033] Subchart II shows the torque M of the friction clutches 2 and 3 with respect to time t. Torque variation curve 104 shows the desired torque M (K1, soll) to be transmitted via friction clutch 2, as expected torque, torque variation curve 105 shows the desired torque M (K2, soll) of friction clutch 3, torque variation curve 106 shows the actual torque M (K1, ist) transmitted via friction clutch 2, and torque variation curve 107 shows the actual torque M (K2, ist) transmitted via friction clutch 3.

[0034] The subgraphs III and IV show the unfiltered and filtered valve flows i, i(F) over time t for actuating the pressure regulating valves 14, 19, 20. The flow curve 108 shows the unfiltered valve flow i(14) of the pressure regulating valve 14, the flow curve 109 shows the unfiltered valve flow i(19) for actuating the friction clutch 2, and the flow curve 110 shows the unfiltered valve flow i(20) of the pressure regulating valve 20 for actuating the friction clutch 3.

[0035] Flow curve 111 shows the filtered valve flow i ( 14 , F) of the pressure regulating valve 14 , flow curve 112 shows the filtered valve flow i ( 19 , F) for actuating the friction clutch 2 , and flow curve 113 shows the filtered valve flow i ( 20 , F) of the pressure regulating valve 20 for actuating the friction clutch 3 .

[0036] Partial diagram V shows the pump speed n(P) over time t in a speed curve 114 .

[0037] In the subgraph VI of pressure p over time t, the system pressure p(s) is shown in the pressure curve 115, the clutch pressure p(K1) of the pressure line 15 is shown in the pressure curve 116, and the clutch pressure p(K2) of the pressure line 16 is shown over time t in the pressure curve 116. The pressure curve 117 shows the system pressure p(s) over time t.

[0038] Finally, in the partial diagram VII with the travel s over time t, the travel curve 118 shows the position s( 19 ) of the control piston of the pressure control valve 19 and the travel curve 119 shows the position s( 20 ) of the control piston of the pressure control valve 20 over time t.

[0039] Until time t, in the sub-drive train with closed friction clutch 2, the torque is transmitted from the internal combustion engine to the drive wheels, the speed n(M) of the internal combustion engine and the speed n(GE1) are identical except for the slight slip at the friction clutch 2. Flow flows through the pressure control valve 19 and the pressure control valve 14 is open and only briefly closes in the event of a clutch pressure p(K1) that drops due to leakage, otherwise the non-return valve 21 maintains the clutch pressure p(K1).

[0040] At time t1, the overlapping shifting begins and begins with the first phase A within the time interval Δt(A). At time t1, the system pressure p(s) is increased significantly by closing the pressure control valve 14. This results in a pressure increase in the clutch pressure p(K1), which is maintained at the non-return valve 21. The pressure peaks caused by switching the pressure control valve 14 are unimportant, since the friction clutch 2 is closed anyway and the pressure control valve 20 of the friction clutch 3 to be closed is still open. After the pressure peaks have decreased, in phase A, the system pressure p(s) decreases and the pressure line 16 and the slave cylinder 18 are filled quickly at a low system pressure p(s) and a high volume flow. After the filling is completed, the system pressure p(s) increases again at the end of phase A due to the increased stiffness of the filled pressure line 16.

[0041] Phase B, which takes place within the time interval Δt(B) between the times t2 and t3, begins at the time t2 with the system pressure p(s) being reduced again by opening the pressure regulating valve 14, and the clutch pressure p(K2) being reduced again, wherein the pressure line 16 and the slave cylinder 18 remain filled. The friction clutch 2 is slowly lowered by opening the pressure regulating valve 14 and, due to leakage and by opening the pressure regulating valve 19 in a time-staggered manner, the control piston of the pressure regulating valve 19 is set in such a way that no pressure medium can enter the pressure line 15, but a pressure medium outflow into the oil sump 6 is possible. The system pressure p(s) increases again near the end of phase B, so that the torque transmission at the friction clutch 3 gradually builds up, while the torque transmission at the friction clutch 2 is reduced.

[0042] The main purpose of the two-stage control of the overlapping shift in phases A and B is to avoid undesirable torque peaks at the friction clutches 2, 3 due to pressure peaks occurring at the pressure regulating valves 14, 19, 20 during their stroke change. The first pressure peak of the pressure regulating valve 14 in phase A is avoided by the friction clutch 2 being held in contact with an increased pressure, while the pressure line 16 and the slave cylinder 18 of the still open friction clutch 3 are filled. A second pressure surge on the open friction clutch 3 can be avoided by avoiding the entry of pressure medium into the pressure line 15, and by setting a corresponding position s(19, ü) of the control piston of the pressure regulating valve 19, an outflow of pressure medium can be achieved, thereby enabling a slow and controlled opening of the friction clutch 2. A decreasing pressure gradient of the clutch pressure p(K1) is thereby achieved. Alternatively, in phase B, the system pressure p(S) can be increased slowly in order to avoid pressure pulses in the system pressure line 13, which propagate into the pressure line 15. The described procedure can be provided in particular in the case of non-time-critical overlapping shifts.

[0043] Description of Reference Numerals

[0044] 1 Hydraulic unit

[0045] 2 Friction clutch

[0046] 3 Friction clutch

[0047] 4 Pump unit

[0048] 5 Series Pumps

[0049] 6 Oil sump

[0050] 7 Filter unit

[0051] 8 Pump section

[0052] 9 Cooling lines

[0053] 10 Heat exchanger

[0054] 11 Components

[0055] 12 Pump section

[0056] 13 System pressure line

[0057] 14 Pressure regulating valve

[0058] 15 Pressure line

[0059] 16 Pressure line

[0060] 17 Slave Cylinder

[0061] 18 Slave Cylinder

[0062] 19 Pressure regulating valve

[0063] 20 Pressure regulating valve

[0064] 21 Check valve

[0065] 22 Check valve

[0066] 23 Components

[0067] 24 Pressure Sensor

[0068] 25. Pressure Sensor

[0069] 26 Arrow

[0070] 27 Arrow

[0071] 28 Double Arrow

[0072] 29 Control piston

[0073] 30 Control Edge

[0074] 31 Control Edge

[0075] 100 Charts

[0076] 101 Speed ​​change curve

[0077] 102 Speed ​​change curve

[0078] 103 Speed ​​change curve

[0079] 104 Torque variation curve

[0080] 105 Torque variation curve

[0081] 106 Torque variation curve

[0082] 107 Torque variation curve

[0083] 108 Current change curve

[0084] 109 Current change curve

[0085] 110 Current change curve

[0086] 111 Current change curve

[0087] 112 Current change curve

[0088] 113 Current change curve

[0089] 114 Speed ​​change curve

[0090] 115 Pressure change curve

[0091] 116 Pressure change curve

[0092] 117 Pressure change curve

[0093] 118 Stroke change curve

[0094] 119 Stroke change curve

[0095] Phase A

[0096] Phase B

[0097] F(C) Gravity

[0098] F(M)Magnetic force

[0099] i Valve Flow

[0100] i(F) Valve flow

[0101] i(14) Valve flow

[0102] i(14, F) valve flow

[0103] i(19) Valve flow

[0104] i(19, F) valve flow

[0105] i(20) Valve flow

[0106] i(20, F) valve flow

[0107] M torque

[0108] M(K1,ist) actual torque

[0109] M(K1, soll) expected torque

[0110] M(K2,ist) actual torque

[0111] M(K2, soll) expected torque

[0112] Speed

[0113] n(GE1) speed

[0114] n(GE2) speed

[0115] n(M) speed

[0116] n(p) pump speed

[0117] pPressure

[0118] p(K1) Clutch pressure

[0119] p(K2) Clutch pressure

[0120] p(s) system pressure

[0121] s Stroke

[0122] s(19) position

[0123] s(19,ü) position

[0124] s(20) position

[0125] t time

[0126] t1 time point

[0127] t2 time point

[0128] t3 time point

[0129] Δt(A) time interval

[0130] Δt(B) time interval

Claims

1. A method for controlling an overlapping shift of a hydraulically actuated double clutch, the double clutch having two friction clutches which are each actuated by a slave cylinder and which are opened without pressure, wherein the system pressure is set by an electrically operated pump unit and a first pressure regulating valve connected downstream of the pump unit, and a second pressure regulating valve is respectively arranged between the first pressure regulating valve and the slave cylinders together with a pressure sensor connected downstream, and the overlapping shift of the friction clutches is carried out by overlapping depressurization of a first slave cylinder associated with the closed first friction clutch and pressurization of a second slave cylinder associated with the opened second friction clutch, It is characterized in that In a first phase of the overlap shift, the system pressure is increased, and in a second phase of the overlap shift, the system pressure is predefined within a predefined, essentially constant, negative or positive pressure gradient.

2. The method according to claim 1, It is characterized in that At the beginning of the first phase, a first pressure regulating valve arranged between the system pressure line and the cooling line is completely closed and the pump speed of the pump unit is reduced.

3. The method according to claim 2, It is characterized in that At the beginning of the first phase, the first friction clutch is kept closed by means of the system pressure increased via the second pressure regulating valve and the non-return valve associated with the first friction clutch.

4. The method according to claim 2 or 3, It is characterized in that In the first phase, after the system pressure has been reduced, the second pressure regulating valve associated with the second friction clutch is opened and the second slave cylinder is filled with pressure medium.

5. The method according to claim 4, It is characterized in that At the beginning of the second phase, after the second slave cylinder has been filled, the second pressure regulating valve associated with the second friction clutch is closed and the system pressure is increased.

6. The method according to claim 2 or 3, It is characterized in that In the second phase, the second pressure regulating valve associated with the first friction clutch is set closed in the direction of the first slave cylinder and open in the direction of the oil sump.

7. The method according to claim 6, It is characterized in that In the corresponding position of the second pressure regulating valve, the system pressure is increased with the aid of the first pressure regulating valve, and the second pressure regulating valve associated with the second friction clutch is gradually opened, and the corresponding position is the corresponding position of the control piston of the second pressure regulating valve that enables the outflow of pressure medium and thus enables slow and controlled opening of the first friction clutch.

8. The method according to any one of claims 1 to 3, It is characterized in that The torque that can be transmitted via the friction clutch is set by means of the system pressure that is set by the first pressure regulating valve and is set as a function of the desired torque.

9. The method according to any one of claims 1 to 3, It is characterized in that The torque transmitted via the closed friction clutch is set by means of pressure loading of the associated slave cylinder, wherein the clutch pressure applied to the slave cylinder is maintained by means of a check valve and leakage in the subsequent pressure line to the slave cylinder is regulated by means of pressure regulation via the second pressure regulating valve.

10. The method according to claim 9, It is characterized in that The clutch pressure is detected by means of the pressure sensor and the pressure value detected by the pressure sensor is correlated in a control unit according to a characteristic curve with a corresponding torque value transmittable via the friction clutch.

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