DE3844871C2 - Method for controlling the gear ratio of a conical pulley belt transmission - Google Patents

Abstract

The invention relates to a method for controlling the transmission ratio of a conical pulley belt transmission for a motor vehicle. Depending on the detection of a certain forward driving range, a control deviation between a target speed of the drive cone pulley and an instantaneous speed of the drive cone pulley is determined, and depending on the size of the control deviation, different control curves are used in order to approach the current speed of the drive cone pulley as quickly as possible to the specified target speed. In this way, a particularly rapid downshifting of the transmission is achieved in engine braking operation without an excessive increase in the clamping force of the drive cone pulley pair.

Description

The invention relates to a method for controlling the transmission ratio of a Conical pulley belt transmission for a motor vehicle according to claim 1. With such a A method prevailing in a cylinder chamber of a drive cone pulley can Hydraulic pressure depending on a gear ratio control device of recorded operating parameters of the motor vehicle, such as vehicle speed, Engine load, set driving range can be controlled.

Infinitely variable transmission of the aforementioned type with appropriate control of the Gear ratios are known (JP 61-105353 A, US 4,735,113).

A variable hydraulic pressure applied to a cylinder chamber of the drive belt disc is placed, leads to a change in the radius of the barrel diameter, in which a V-belt is in contact with the drive pulley, so that this a change in the gear ratio is brought about. The hydraulic pressure is set by an electro-hydraulic module that translates an electronic ratio control unit and a hydraulic control device contains.

The gear ratio control unit is with different types of on gear information supplies and determines the content of the commands sent to an operator device depending on these different types of input formations are given. An output signal from the electronic control unit is given to the actuator in the form of a stepper motor signal to to control a shift control valve. The shift control valve forms part of the hydraulic system pressure control system, and regulates the fluid supply to the cylinder chamber of the type drive pulley and the fluid drainage from this to one in the cylinder chamber Set hydraulic pressure to a value corresponding to a position that the step engine occupies. The shift control valve in no way affects the regulation of the Hydraulic pressure applied to a cylinder of the driven pulley from a pipe  pressure regulating valve is placed. Thus, a line pressure created by the line pressure regulating valve is generated continuously on the cylinder of the output belt disc laid. In this way, the gear ratio is infinitely variable appropriate value regulated for the vehicle conditions and driving conditions is most suitable.

However, some of these known transmissions cannot have a desired high Speed during a transition from an engine drive operating condition Shift down to an engine braking mode. This is because the Water control device for the transmission ratio of the hydraulic pressure in the cylinder Chamber of the drive pulley in the engine brake operating state after the same The shift control regime for the gear ratio is the same as for the engine Drive operating state. The engine braking mode is initiated when a hand selector lever is set to a starting range or low gear range, or if the engine starts to be driven by the traction of the wheels after that Accelerator pedal has been released. Because of the fairly slow speed at which If the gear ratio changes, there is a considerable delay until the Deceleration and engine speed decrease take place, leaving the feeling for the engine braking operation is impaired.

From EP 213 948 A1 and EP 217 606 A1 are infinitely variable, hydraulically be actuated conical pulley belt transmission known when the motor works in overrun mode, the gearbox shifts down quickly.

In the case of a continuously variable transmission of the type outlined, the relationship between the clamping force with which the drive pulley grips the V-belt and the Clamping force with which the driven pulley grips the V-belt is a function the gear ratio and the input torque. The relationship is that Art that the drive pulley requires a high clamping force when the one gear torque increases, and the transmission ratio becomes large. In contrast in addition, the drive pulley speed increases during engine braking  at the same time as the input torque increases in the negative direction. Although the drive pulley is less during engine braking Clamping force in relation to the requirements during motor drive operation required, the clamping force with which the drive pulley grips the V-belt is stronger than required. This is because the hydraulic pressure in the cylinder chamber the drive pulley is increased due to the centrifugal force. This affects one particularly rapid downshifting of the transmission in engine braking mode.

US Pat. No. 4,536,171 describes a method for controlling a translation ratio of a conical pulley belt transmission known. In this cone disc transmission is the translation by a controllable hydraulic Change the likelihood that is applied to a cylinder chamber of a drive cone pulley different. A control valve operated by a stepper motor controls the hydraulics pressure in this cylinder chamber such that different step positions of the step motors each correspond to a specific gear ratio of the transmission. This stepper motor is controlled by a control unit depending on driving conditions controlled. It is provided that the stepper motor with different speed operated to change different switching speeds accordingly to realize the reduction ratio. In particular, the stepper motor with operated at higher speeds when accelerated switching is necessary. Ge According to the printing step, this is the case if there is a difference between an actual under- or translation ratio and a target under- or translation ratio is greater than a predetermined value, d. H. if there is a difference between one Actual position of the stepper motor and a target position of the stepper motor a certain one Value exceeds.

The document EP 0 217 221 B1 shows a method for controlling a transmission, which has an auxiliary transmission in addition to a conical pulley belt transmission to realize different gear ratios. There is a gear lever in front seen the u. a. can assume an engine brake position in which the auxiliary transmission can switch down to a low range, but no longer back to one  High range can be switched. Is the said shift lever in the engine brake position, so the belt transmission is also on a larger reduction ratio set according to a saved speed pattern.

Document EP 0 117 069 B1 describes a method for controlling a cone disc benumschlingungsgetriebes known, in which the gear ratio or Motor output speed according to a variety of stored functions between the degree of throttle valve opening and the target speed for different vehicles speeds is determined. According to the result of a target-actual value ver at the same time, the fluid pressure in the cylinder pressure chamber of a drive cone pulley to set the appropriate reduction ratio.

The invention has for its object a method for controlling the translation ratio of a conical pulley belt transmission, by which is a particularly rapid shift reaction of the transmission, especially in the sense of a Compensation of the centrifugal force-dependent axial force on the drive cone disc at the same time further reduction in the clamping force of the drive cone pulley pair in Detection of engine overrun is achievable.

This object is achieved according to the invention by a method for controlling a Gear ratio of a conical pulley belt transmission for a force vehicle by means of an electronic control unit and a hydraulic switch direction with a switching control valve for the transmission ratio for setting egg hydraulic pressure in a cylinder chamber of a drive cone pulley and with a Line pressure control valve for regulating a hydraulic pressure in a cylinder mer a wear cone of the belt transmission, the hydraulic Switching device one of the control unit as a function of detected operating pa parameters such as vehicle speed, throttle valve opening degree, drive cone disc speed and selectable forward drive range in different positions controllable stepper motor, with a valve spool of the shift control valve  is connected, with the control unit when special forward driving is selected area for "sporty driving"

• - A target speed of the drive cone pulley for this special forward driving range depending on the vehicle speed and the degree of throttle opening is determined
• - A target gear ratio according to the target speed and Fahrzeugge speed is determined,
• - The presence of a motor drive operating state or an engine brake operation condition is determined depending on the degree of throttle opening,
• - Depending on the engine operating state, one of two different characteristic curves for determining a first target position dependent on the target gear ratio the stepper motor is selected,
• - A second target position depending on the control deviation of the target speed is determined by the actual speed of the drive cone pulley,
• - And the stepper motor is controlled according to a target position, which is by Addi tion of the first and the second target position is obtained,
• - With the same target gear ratio, the first target position resulting from the Characteristic for the engine brake operating state determines a control of the step motors for accelerated adjustment of the hydraulic pressure compared to that from the Characteristic for the motor drive operating state determinable first target position be works.

The invention is described below using an exemplary embodiment and associated Drawings explained in more detail. In these shows:

Fig. 1 is an overall view of an infinitely variable, hydraulically actuated disc-cone pulley transmission with a control device for the gear ratio, relatively to one embodiment,

FIG. 2 shows a flow diagram of the steps of an exemplary embodiment of the method carried out by the control device according to FIG. 1, and

Fig. 3 is a characteristic curve f ₂ (i), relating to a gear ratio i with respect to a forward sequence control target position STEPP of a stepper motor, used during a motor drive operation in comparison with a corresponding characteristic line f ₃ (i), which is during a motor Brake operation is used.

In Fig. 1, reference numeral 10 denotes a motor with an output shaft 10 a egg nes motor vehicle. The output shaft 10 a is drivingly connected to a continuously variable transmission, which generally comprises a fluid coupling 12 , a forward / reverse selection device 15 , a continuously variable conical pulley belt transmission 29 and a differential 56 .

The fluid coupling 12 includes a pump impeller 12 b, connected to the motor-driven shaft 10 a and c a turbine runner 12 connected to a turbine shaft. 13 It also contains a bridging device, which is a function of a abwe ence of hydraulic pressure in a lock-up chamber 12 a engaged to me mechanically the impeller 12 and the turbine runner b 12 c to each other.

The forward / reverse selector 15 includes a planetary gear set 17 , a forward clutch 40, and a reverse clutch 50 . The planetary gear set 17 contains a sun gear 19 , a pinion carrier 25 which rotatably carries double pinions 21 and 23 , and a ring gear 27 . The sun gear 19 is connected to the turbine shaft 13 . The pinion carrier 25 is ver bindable via the forward clutch 40 with the turbine shaft 13 and is connected to a drive cone shaft 14 . The ring gear 27 is provided to be held stationary on a stationary part via the reverse clutch or brake 50.

The conical disc belt transmission 29 includes a driving bevel wheel 16 which is rotatable with the drive cone pulley shaft 14, a driven bevel wheel 26 which is rotatable with an output shaft 28, and a V-belt 24, the conical disk between the drive 16 and the output bevel wheel is tensioned 26th The drive cone disk 16 includes a conical drive cone disk part 18 which is integral with the drive cone disk shaft 14 for matching rotation therewith, and an axially movable, conical conical disk part 22 . The movable, conical conical disk part 22 acts together with the other conical conical disk part 18 to form a wedge-shaped conical disk groove between these parts. The movable, conical conical disk part 22 is axially displaceable along the drive conical disk shaft 14 as a function of a hydraulic fluid pressure which is effective in a cylinder chamber 20 of the drive conical disk 16 . The cylinder chamber 20 of the on drive pulley disk 16 includes two chambers 20 a and 20 b and or limited forms an effective pressure acting area of a size that is of those two times, which is formed in the cylinder chamber 32 of the driven pulley disk 26th A selection device for selecting an effective diameter of the cylinder chamber 20 will be explained in detail below with reference to FIG. 2.

The driven conical disk contains a conical conical disk part 30 , which is formed integrally with the driven shaft 28 for rotation therewith, and an axially movable, conical conical disk part 34 . The movable, conical cone disc part 34 cooperates with the conical part 30 to form a wedge-shaped conical disc groove between these elements. The movable, conical conical disk part 34 is axially displaceable along the driven conical disk shaft 34 as a function of the hydraulic pressure in the cylinder chamber 32 of the driven conical disk 26 .

On the output shaft 28 , a drive gear 46 is fixed, which is in engagement with an intermediate gear 48 which is fixed to the intermediate shaft 52 . On the intermediate shaft 52 , a pinion 54 is fixed, which is in engagement with a final gear 44 of the differential 56 .

The differential 56 includes a pair of gears 58 , 60 rotatable integrally with the final gear 44 and a pair of side gears 62 and 64 . The Seitenzahnrä the (side gears) 62 and 64 are fixedly coupled to a pair of output shaft 66 and 68 ge.

The type of change in the gear ratio in the continuously variable Ke Gel disc belt transmission is briefly explained.

A change in the gear ratio of the continuously variable cone pulley belt transmission can be done by axially moving the movable, conical Ke gelscheibenteile 22 and 34 to change their respective barrel diameter with the wedge belt 24 . The hydraulic pressure within the cylinder chamber 20 of the drive cone pulley 16 is changed in relation to the hydraulic pressure within the cylinder chamber 32 of the driven cone pulley 26 in accordance with a control command. For example, increasing the width of the wedge-shaped cone grooved in the drive cone pulley 16 causes the V-belt 24 to be made to narrow the width of the wedge-shaped conical grooved groove in the driven cone pulley 26 , thereby changing the gear ratio toward a downshift, with the result of a decrease in Circumferential speed or speed of the driven conical shaft 28 . In contrast, a reduction in the width of the wedge-shaped conical disk groove of the drive conical disk 16 causes the V-belt 24 to enlarge the width of the wedge-shaped conical disk groove of the driven conical disk 26 .

The control device for the gear ratio is explained below.

As shown in Fig. 1, the transmission ratio control means includes a hydraulic transmission ratio control section (hydraulic switch device) 70 and an electronic transmission ratio control section (electronic control unit) 90 which drives a stepping motor 71 of the hydraulic transmission ratio control section 70 . The hydraulic transmission ratio control section 70 regulates the fluid supply to the cylinder chamber 20 of the drive cone disc 16 and the fluid discharge thereof, while it allows the application of line pressure to the cylinder chamber 32 of the output cone disc 26 .

The hydraulic gear ratio control section 70 includes the stepping motor 71 , a link lever device 72 , a gear ratio control valve 73 , an oil pump 74 , a line pressure control valve 75 , a vacuum diaphragm 76 , a throttle valve 77 and a manual valve 78 .

The stepper motor 71 actuates via the connecting lever device 72 , valve spool 73 a and 75 a, the transmission ratio control valve 73 and the line pressure control valve 75 .

The control valve 73 for the gear ratio contains a valve spool 73 a, which is connected to the connecting lever device 72 .

The arrangement is such that when the valve spool 73 a shifts in Fig. 1 to the left, the supply of hydraulic fluid to the cylinder chamber 20 of the on drive cone pulley 16 is increased, while the discharge of hydraulic fluid from the Zylin the chamber 20 of the drive cone pulley 16 is reduced, thereby causing an increase in hydraulic fluid pressure inside the cylinder chamber 20 of the drive cone disk 16 .

The line pressure control valve 75 effects pressure regulation for the hydraulic fluid from the oil pump 74 and generates the line pressure, which is variable depending on the throttle valve opening degree and transmission ratio.

The throttle valve 77 effects a pressure regulation under the influence of the vacuum membarn 76 and generates a throttle pressure depending on an engine intake vacuum in the intake pipe.

The manual valve 78 is a selector valve which is actuated by a selector lever 79 . When selecting one of the forward driving ranges including a D range (drive range) and an S range (sporty driving range), the manual valve 78 allows the throttle pressure to reach the forward clutch 40 . When an R range (reverse travel range) is selected, the manual valve 78 allows the throttle pressure to be applied to the reverse brake 50 .

The S range differs from the D range in that the same vehicle speed and the same degree of throttle valve opening in the S range a smaller Gear ratio is provided as in the D range.

The electronic translation control device 90 includes a sensor 91 with respect to a selected position of the selector lever 79 , a drive or input cone disk speed sensor 92 , a vehicle speed sensor 93 , a throttle valve opening degree sensor 94 , a control unit 95 and a drive circuit 96 for the stepper motor 71 . The sensor 91 for the selected position detects which loading area is selected by the selector lever 79 . Thus, an output signal of the sensor 91 which detects the selected position is representative of the position S _p which is selected by the selection lever 79 .

The speed sensor or peripheral speed sensor 92 for the drive cone disc 16 detects the circumferential speed of the drive pulley disk 16 and generates an output signal for the detected rotational speed N _IN of the disk-drive bevel representative.

The vehicle speed sensor 93 detects the peripheral speed or rotational speed of the driven cone 26 or that of the axle shaft 66 or 68 and generates an output signal which represents the vehicle speed V _SP .

The throttle valve opening degree sensor 94 detects the opening degree of the engine throttle valve and generates an output signal representing the detected throttle opening degree T _VO .

The control unit 95 is a microcomputer-based control device with a central processor unit CPU, a random access memory ROM and an input / output interface. The output signals of the sensors 91 , 92 , 93 and 94 are applied to the control unit 95 . An output signal from the control unit 95 is applied to the motor drive circuit 96 for the stepping motor 71 .

The hydraulic gear ratio control device 70 and the electronic gear ratio control device 90 are e.g. B. explained in U.S. Patent No. 4,735,113.

The mode of operation of the exemplary embodiment is explained below.

The control unit 95 stores a control program in its read-only memory ROM, as shown by a flow chart in FIG. 2. The execution of this control program is repeated after a certain period of time.

In step 100 , the selected position S _P , the peripheral speed or speed N _{IN of} the drive cone disk 16 , the vehicle speed V _SP and the degree of opening T _{VO of} the throttle valve are obtained by reading in the output signals from the sensors 91 , 92 , 93 and 94 .

In step 101 , a decision is made as to whether the selected position S _P indicates the N range or P range or not. If the decision is that the N range or the P range is set, control goes to step 102 . In step 102 , an N or P range control is carried out, the hydraulic fluid of the cylinder chamber 20 of the drive cone disk 16 being relieved or drained off.

If the answer to the question in step 101 is negative, the control proceeds to step 103 by deciding whether the selected position S _P relates to the R range or not. If it is determined that the R range is selected, control transfers to step 104 . In step 104 , an R-range control is carried out, in which the hydraulic pressure within the cylinder chamber 20 of the drive cone disk 60 is kept at a certain value.

If the answer to the question that we decided in step 103 is negative, control proceeds to step 105 by deciding whether or not the selected position S _P relates to the D range. If it is determined that the D range is selected, control transfers to step 106 . In step 106 , a D-range control is processed, in which the hydraulic fluid pressure, which is applied to the cylinder chamber 20 of the drive cone disk 16 , is set in order to bring about a transmission ratio that is suitable for the vehicle conditions and the corresponding driving conditions.

If the answer to the question in step 105 is negative, ie if the S range is selected, then an S range control is carried out, which is explained below.

In this case, control proceeds from step 105 to step 107 . In step 107 , a target peripheral speed N _IN * of the drive pulley 16 is obtained using the vehicle speed V _SP and the throttle valve opening degree T _{VO obtained} in step 100 using the following general equation, which is expressed as follows:

N _IN * = f ₁ (V _SP , T _VO )

This function is represented by the curves indicated in full lines in block 107 of the flow chart according to FIG. 2. In this block, the chain line represents the values of the target speed of the drive pulley as a function of various values of the vehicle speed when T _VO = 0 in the D range. From the full line, which is denoted by T _VO = 0, it is clear that if T _VO is 0, a higher target speed for the drive belt pulley in the S range than the target speed for the same vehicle speed is given is the drive pulley, which is obtained in the D range.

Control then passes to step 108 by using the target speed N _IN * for the drive cone pulley 16 obtained in step 107 and the speed N _{IN of} the drive pulley 16 obtained in step 100 following equation results:

e = N _IN * - N _IN

In the following step 109 , a target gear ratio i is obtained from the following equation:

i = N _IN * / V _SP CONST (CONST: constant).

In the next step 110 , the throttle valve opening _degree T _{VO is} compared with a predetermined value of the throttle valve opening _degree T _VOO , which represents a throttle valve opening _degree close to 0, and it is determined whether T _{VO is} less than or equal to T _VOO or not. In this step 110 , it is determined whether the vehicle is running in the engine drive mode or the engine brake mode.

Following step 110 , motor drive control for the stepper motor 71 is performed in two different ways, depending on whether the motor drive operating state or the motor braking operating state is present. This motor-drive control combines a feedforward control with a feedback control.

Control continues to step 111 during the motor drive mode when the answer to the question in step 110 is NO. In step 111 , a target forward follow-up step (first target position) STEP _{FF is} obtained from the gear ratio obtained in step 109 using a function shown in a diagram shown in FIG. 3. Control then passes to step 112 , in which the two values K _P1 and K _{i1 are set} as two forward follow-up control constants K _P , K _i, respectively. In the next step 113 , the absolute values of | e | received and it is determined whether | e | is greater than or equal to a predetermined deviation value e ₀ . If | e | is greater than or equal to e ₀ , control transfers to step 114 , where zero is set as a constant K ₃ . If | e | is less than e ₀ , control passes to step 115 , in which a constant K ₃ is set to one.

During the engine braking mode, when the answer to the question in step 110 is YES, control proceeds to step 116 by the target forward sequence control position step STEP _FF obtained through the gear ratio i obtained in step 109 using the function f ₃ shown in Fig. 3 is obtained. Control then passes to step 117 , where the two values K _P2 and K _{i2 are set} as forward sequence control constants K _P and K _i . K _P1 is less than K _P2 while K _{i1 is} less than K _i2 . In the next step 118 , the constant K _{3 is} set to one.

After step 114 or 115 or 118 , control transfers to step 119 , which a target feedback control step STEP _{FB is} obtained by the constants K _P , K _i and K ₃ and the deviation e using the following equation:

STEP _FB = K ₃ (K _P. E + K _i . ∫e)

The control then goes to step 120 , a target position or target step STEP * for the stepper motor is obtained by adding the target position STEP _FF for the forward sequence control to the target position STEP _FB for the feedback control. Finally, in step 121 a drive signal for stepper motor 71 is emitted or the latter itself is moved in such a direction that a difference between the actual position of stepper motor 71 and the desired drive position STEP * of stepper motor 71 is reduced towards zero. The stepper motor drive signal is applied to the motor drive circuit 96 . After the repeated processing of this flowchart 4 , the actual position of the stepper motor 71 is set to the target position STEP * for the stepper motor.

The type of gear ratio control is then explained is executed while the vehicle is within the selected S range is driven.

(A) Engine drive operating condition

During the motor drive mode, while the S range is selected, control proceeds from step 110 to step 119 via steps 111 , 112 , 113 and 115 (or 114 ). Since the forward sequence control constants K _P and K _{i have been set} equal to the suitable values K _P1 and K _i1 , which are suitable for driving in the S range, this process causes a control of the gear ratio with a low-delay, quick response an external disturbance. If the absolute value of the deviation | e | is greater than the specified deviation value e ₀ , the target position (target position) for the feedback control of the stepper motor STEP _FB is set to zero, since the constant K _{3 is} zero (see step 114 ).

(B) engine braking operation

During engine braking operation in the S-select area, control proceeds from step 110 through steps 116 , 117 and 118 to step 119 . As can be easily understood from a comparison of the characteristic curves f ₃ (i) with the characteristic curve f ₂ (i), with the same transmission ratio that was obtained in step 109 , the target position STEP _{FF of} the forward sequence control, given in step 116 , is lower than the one obtained in step 111 . A smaller transmission ratio is thus established, which creates a larger reduction ratio. Because the larger value K _P2 and K _i2 coupling constant than the return K _P and K _i are set in step 119 is greater Sollpo sition STEP _FB set for the feedback control in step 119th

Thus, the stepper motor 71 is activated at an increased speed to cause a rapid drop in the hydraulic pressure within the cylinder chamber 20 of the drive pulley 16 . As a result, an increase in the hydraulic pressure due to the effect of the centrifugal force is compensated for by the aforementioned rapid pressure drop in the hydraulic pressure and ensures a sufficiently rapid change in the gear ratio during a transition phase to the motor-brake operation.

Therefore, a preferred engine braking feel or behavior is created that the occurrence of longitudinal acceleration at an early, initial stage and stages um and a rapid increase in engine speed results.

Claims (1)

1. A method for controlling a transmission ratio of a cone-belt transmission ( 29 ) for a motor vehicle by means of an electronic control unit ( 90 ) and a hydraulic switching device ( 70 ) with a switching control valve ( 73 ) for the transmission ratio for setting a hydraulic pressure in a cylinder chamber ( 20 ) a drive cone disc ( 16 ) and with a line pressure control valve ( 75 ) for regulating a hydraulic pressure in a cylinder chamber ( 32 ) an output cone disc ( 26 ) of the belt transmission ( 29 ), the hydraulic switching device ( 70 ) being one of the control unit ( 90 ) depending on recorded operating parameters such as vehicle speed (V _SP ), throttle valve opening degree (T _VO ), drive cone pulley speed (N _IN ) and selectable forward driving range (D range, S range) in different positions on controllable stepper motor ( 71 ), the one with a valve spool ( 73 a) of the shift control valve ( 73 ) is connected, with the control unit ( 90 ) when a special forward driving range is selected for "sporty driving" (S range)

• - A target speed (N _IN *) of the drive cone pulley ( 16 ) for this particular forward driving range (S range) is determined depending on the vehicle speed (V _SP ) and the throttle valve opening degree (T _VO ),
• a target gear ratio (i) is determined in accordance with the target speed (N _IN *) and vehicle speed (V _SP ),
• the presence of a motor drive operating state or an engine brake operating state is determined as a function of the throttle valve opening degree (T _VO ),
• - Depending on the motor operating state, one of two different characteristic curves (f ₂ (i), f ₃ (i)) is selected for determining a first set position (STEP _FF ) of the stepping motor ( 71 ) that depends on the set gear ratio (i) ,
• a second set position (STEP _FB ) is determined as a function of the control deviation (e) of the set speed (N _IN *) from the actual speed (N _IN ) of the drive cone pulley ( 16 ),
• - And the stepper motor ( 71 ) is driven according to a target position (STEP *), which is obtained by adding the first and the second target position (STEP _FF + STEP _FB ),
• - With the same target gear ratio (i), the first target position (STEP _FF ), which is determined from the characteristic curve (f ₃ (i)) for the engine brake operating state, a control of the stepper motor ( 71 ) for accelerated adjustment of the hydraulic pressure compared to the first setpoint position (STEP _FF ) which can be determined from the characteristic curve (f ₂ (i)) for the motor drive operating state.