CONTROL DEVICE FOR A REGENERATIVE BRAKING SYSTEM AND METHOD FOR OPERATING A REGENERATIVE BRAKING SYSTEM

Abstract

A control device for a regenerative braking system having control electronics, the control electronics being designed, in consideration of at least one provided first variable with respect to a utilized coefficient of friction occurring in each case at the at least one wheel which may be regeneratively braked, to determine at least one preset variable with respect to at least one hydraulic minimum braking torque to be exerted on the at least one wheel which may be regeneratively braked, and, in consideration of at least the at least one determined preset variable, to determine the at least one setpoint variable.

Claims

1-15. (canceled)

16. A control device for a regenerative braking system, comprising: control electronics, with the aid of which at least one setpoint variable with respect to at least one setpoint generator braking torque, which is to be exerted in each case with the aid of an electric motor of the regenerative braking system on at least one wheel, which may be regeneratively braked, of a vehicle equipped with the regenerative braking system, is determinable, and at least one control signal corresponding to the at least one determined setpoint variable may be output at the electric motor; wherein the control electronics are designed to determine, in consideration of at least one provided first variable with respect to a utilized coefficient of friction occurring in each case at the at least one wheel which may be regeneratively braked, at least one preset variable with respect to at least one hydraulic minimum braking torque to be exerted on the at least one wheel which may be regeneratively braked, and, in consideration of at least the at least one determined preset variable, to determine the at least one setpoint variable.

17. The control device as recited in claim 16, wherein the preset variable is determinable, with the aid of the control electronics for all wheels which may be regeneratively braked, individually by wheel in consideration of a first variable with respect to the utilized coefficient of friction occurring at the at least one wheel which may be regeneratively braked.

18. The control device as recited in claim 16, wherein the preset variable for the hydraulic minimum braking torque to be exerted equally on all wheels which may be regeneratively braked is determinable in consideration of a minimum of the first variables with the aid of the control electronics.

19. The control device as recited in claim 16, wherein the control electronics are designed to determine at least one first highest generator braking torque for the at least one wheel which may be regeneratively braked as a difference of a total braking torque to be exerted on the particular wheel which may be regeneratively braked and the hydraulic minimum braking torque determined as the preset variable for the wheel which may be regeneratively braked, and, in consideration of the at least one determined first generator braking torque, to determine the at least one setpoint variable.

20. The control device as recited in claim 19, wherein the control electronics are designed to determine, in consideration of a predefined characteristic curve and at least one provided second variable with respect to a minimum of the wheel contact forces occurring in each case on the at least one wheel which may be regeneratively braked, at least one second highest generator braking torque for the at least one wheel which may be regeneratively braked, and to determine the at least one setpoint variable with additional consideration of the at least one determined second highest generator braking torque.

21. The control device as recited in claim 20, wherein the control electronics are designed, in consideration of a provided item of information with respect to a generator potential of the electric motor, to determine at least one third highest generator braking torque for the at least one wheel which may be regeneratively braked, and to determine the at least one setpoint variable with additional consideration of the at least one determined third highest generator braking torque.

22. The control device as recited in claim 21, wherein the control electronics are designed to determine, for the at least one wheel which may be regeneratively braked, at least one minimum variable from at least one of the first highest generator braking torque, the second highest generator braking torque, and the third highest generator braking torque, and to determine the at least one setpoint variable in consideration of the at least one minimum variable.

23. The control device as recited in claim 16, wherein the control electronics are designed to compare at least one wheel velocity value of the at least one wheel which may be regeneratively braked to at least one predefined threshold value and, if the at least one wheel velocity value falls below the at least one predefined threshold value, to permit only the at least one setpoint generator braking torque to be kept constant or reduced upon the determination of the at least one setpoint variable.

24. A method for operating a regenerative braking system, comprising: determining at least one setpoint variable with respect to at least one setpoint generator braking torque, which is to be exerted with the aid of an electric motor of the regenerative braking system on at least one wheel, which may be regeneratively braked, of a vehicle equipped with the regenerative braking system; and controlling the electric motor in consideration of the at least one determined setpoint variable; wherein at least the following steps are executed to determine the at least one setpoint variable: determining at least one preset variable with respect to at least one hydraulic minimum braking torque (M.sub.hyd) to be exerted on the at least one wheel which may be regeneratively braked in consideration of at least one first variable with respect to a utilized coefficient of friction occurring in each case at the at least one wheel which may be regeneratively braked; and determining the at least one setpoint variable in consideration of the at least one established preset variable.

25. The method as recited in claim 24, wherein for all wheels which may be regeneratively braked, the preset variable is determined individually by wheel in consideration of the first variable with respect to the utilized coefficient of friction occurring at the particular wheel which may be regeneratively braked.

26. The method as recited in claim 25, wherein the preset variable for the hydraulic minimum braking torque to be exerted equally on all wheels which may be regeneratively braked is determined in consideration of a minimum of the first variables.

27. The method as recited in claim 26, wherein at least one first highest generator braking torque is determined for the at least one wheel which may be regeneratively braked as a difference of a total braking torque to be exerted on the particular wheel which may be regeneratively braked and the hydraulic minimum braking torque determined for the wheel which may be regeneratively braked as the preset variable, and the at least one setpoint variable is determined in consideration of the at least one determined first highest generator braking torque.

28. The method as recited in claim 27, wherein, in consideration of a predefined characteristic curve and at least one provided second variable with respect to a wheel contact force occurring in each case on the at least one wheel which may be regeneratively braked, at least one second highest generator braking torque) is determined for the at least one wheel which may be regeneratively braked, and the at least one setpoint variable is determined with additional consideration of the at least one determined second highest generator braking torque.

29. The method as recited in claim 28, wherein, in consideration of a provided item of information with respect to a generator potential of the electric motor, at least one third highest generator braking torque is determined for the at least one wheel which may be regeneratively braked, and the at least one setpoint variable is determined with additional consideration of the at least one determined third highest generator braking torque.

30. The method as recited in claim 29, wherein at least one wheel velocity value of the at least one wheel which may be regeneratively braked is compared to at least one predefined threshold value and, if the at least one wheel velocity value falls below the at least one predefined threshold value, the at least one setpoint generator braking torque is only permitted to be kept constant or reduced upon the determination of the at least one setpoint variable.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0015] Further features and advantages of the present invention will be explained hereafter with reference to FIG. 1, which shows a block diagram to explain one specific embodiment of the method for operating a regenerative braking system.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0016] FIG. 1 shows a block diagram to explain one specific embodiment of the method for operating a regenerative braking system.

[0017] The method which is schematically shown with the aid of FIG. 1 is suitable for operating a regenerative braking system of a vehicle/motor vehicle. The vehicle/motor vehicle which is equipped with the regenerative braking system may be, for example, an electric vehicle or hybrid vehicle. It is to be noted that the executability of the method is not limited to a specific vehicle type/motor vehicle type. A variety of regenerative braking systems may also be used with a regeneratively operable electric motor to execute the method.

[0018] The executability of the method is therefore also not restricted to a specific braking system type.

[0019] During the execution of the method, at least one setpoint variable M.sub.final is determined with respect to at least one setpoint generator braking torque M.sub.final, which is to be exerted with the aid of the electric motor of the regenerative braking system on at least one wheel, which may be regeneratively braked, of the vehicle equipped with the regenerative braking system. The at least one setpoint variable M.sub.final may be, for example, at least one setpoint generator braking torque which is determined individually by wheel for the particular wheel which may be regeneratively braked or a setpoint generator braking torque M.sub.final which is determined identically for all wheels which may be regeneratively braked. However, it is to be noted that indirect determination of the at least one setpoint generator braking torque M.sub.final is possible with the aid of the at least one setpoint variable M.sub.final. The at least one setpoint variable M.sub.final may therefore also be, for example, a control variable of the electric motor.

[0020] To determine the at least one setpoint variable M.sub.final, at least one preset variable M.sub.hyd is determined with respect to at least one hydraulic minimum braking torque M.sub.hyd to be exerted on the at least one wheel which may be regeneratively braked. The particular determined hydraulic minimum braking torque M.sub.hyd is preferably to be exerted with the aid of at least one associated wheel brake cylinder (and/or at least one associated wheel brake caliper) on the at least one wheel which may be regeneratively braked. Preferably, after the determination of the at least one preset variable M.sub.hyd, at least one hydraulic component of the regenerative braking system, for example, at least one valve and/or at least one pump, is operated/controlled in such a way that in each case a brake pressure corresponding to the at least one preset variable M.sub.hyd is set in the at least one associated wheel brake cylinder. The at least one preset variable M.sub.hyd may therefore be, for example, hydraulic minimum braking torque M.sub.hyd to be exerted on the at least one wheel which may be regeneratively braked and/or a corresponding control variable of the at least one hydraulic component of the regenerative braking system. Other variables may also be determined as the at least one preset variable M.sub.hyd.

[0021] The at least one preset variable M.sub.hyd is determined in consideration of at least one first variable fr1 and fr2 with a respect to a (utilized) coefficient of friction fr1 and fr2 occurring in each case at the at least one wheel which may be regeneratively braked. Therefore, the at least one preset variable M.sub.hyd may be determined with respect to hydraulic minimum braking torque M.sub.hyd to be exerted on the at least one wheel which may be regeneratively braked in a simple way so that in the ABS case, a standard (hydraulic) ABS regulation is executable to avoid/remedy locking of the particular affected wheel. By way of the (direct or indirect) consideration of the at least one utilized coefficient of friction fr1 and fr2 in the determination of the at least one setpoint variable M.sub.final, optimization thereof is possible on the wheel level.

[0022] The at least one preset variable M.sub.hyd is preferably determined so that particular hydraulic minimum braking torque M.sub.hyd linked thereto is higher the more critical the at least one first variable fr1 and/or fr2 is with respect to possible locking of the at least one wheel which may be regeneratively braked. For example, to determine the at least one preset variable M.sub.hyd in consideration of the at least one first variable fr1 and fr2, a corresponding first characteristic curve k1 may be used.

[0023] Subsequently, the at least one setpoint variable M.sub.final is determined in consideration of the at least one preset variable M.sub.hyd (and/or corresponding hydraulic minimum braking torque M.sub.hyd). The electric motor is controlled thereafter in consideration of the at least one determined setpoint variable M.sub.final. The control of the electric motor may be carried out in particular so that, with the aid of the electric motor, at least one actual generator braking torque corresponding to the at least one setpoint generator braking torque M.sub.final, which is determined using the at least one setpoint variable M.sub.final, is exerted on the particular wheel which may be regeneratively braked. (The electric motor used for this purpose may be in particular the electric drive motor of the vehicle/motor vehicle).

[0024] The above-described determination of the at least one preset variable M.sub.hyd in consideration of the at least one first variable fr1 and fr2 ensures that at all times during the execution of the method, locking of the at least one wheel which may be regeneratively braked may be reacted to with a reduction of the brake pressure existing in the adjacent wheel brake cylinder. At the same time, with the aid of the method described here (by way of the advantageous determination of the at least one setpoint variable M.sub.final), the generator operation of the electric motor is adapted to the at least one hydraulic minimum braking torque M.sub.hyd. It is therefore ensured that in spite of maintaining the at least one hydraulic minimum braking torque M.sub.hyd, a braking intent requested by the driver and/or an automatic braking mechanism (for example, an ACC and/or an emergency braking system) is not exceeded. In particular, the method provides a master-slave relationship, in which ensuring the at least one hydraulic minimum braking torque M.sub.hyd assumes a master status, with respect to which the generator operation of the electric motor is optimized in the slave status. A standard (hydraulic) ABS regulating method is therefore executable in any needed case without a restriction or a termination of the generator operation of the electric motor.

[0025] In the specific embodiment of FIG. 1, the one preset variable M.sub.hyd for the one hydraulic minimum braking torque M.sub.hyd to be exerted equally on all wheels which may be regeneratively braked is determined in consideration of a minimum Min (fr1, fr2) of (utilized) coefficients of friction fr1 and fr2. By way of example, particular occurring (utilized) coefficients of friction fr1 and fr2 are output for this purpose at a computer block 2 for minimum calculation, and subsequently the one preset variable M.sub.hyd is determined as hydraulic minimum braking torque M.sub.hyd in consideration of minimum Min(fr1, fr2) and predefined first characteristic curve k1. However, particular preset variable M.sub.hyd may also be determined individually by wheel for all wheels which may be regeneratively braked in consideration of particular first variable fr1 or fr2 with respect to coefficient of friction fr1 or fr2 occurring at the particular wheel which may be regeneratively braked.

[0026] In the method of FIG. 1, the at least one first variable fr1 or fr2 is calculated as particular (utilized) coefficient of friction fr1 or fr2 from a wheel radius r1 or r2, respectively, of the particular wheel which may be regeneratively braked, a wheel contact force F1 or F2, respectively, of the particular wheel which may be regeneratively braked (or at least one second variable corresponding thereto), and a wheel braking torque M1 or M2 to be exerted hydraulically and regeneratively on the particular wheel which may be regeneratively braked. The at least one first variable fr1 or fr2 may be, for example, at least one variable derived from at least one measured value. At computer blocks 3 and 4, a product P1 and P2 is calculated from particular radius r1 or r2 and particular wheel contact force F1 or F2 (or the at least one corresponding second variable) for each wheel which may be regeneratively braked. (Particular (dynamic) wheel contact force F1 and F2 may be derived/calculated from measured or estimated values for the vehicle longitudinal acceleration and the vehicle lateral acceleration in a conventional way.) In addition, in computer blocks 5 and 6, for each wheel which may be regeneratively braked, hydraulic brake pressure P.sub.hyd1 or P.sub.hyd2 thereof, actual generator braking torque M.sub.gen1 or M.sub.gen2 thereof, and principle of linear momentum (principle of angular momentum) D1 or D2 thereof are analyzed to determine its wheel braking torque M1 and M2 which is to be exerted hydraulically and regeneratively. With the aid of a minimum calculation of signals S1 and S2 of computer blocks 5 and 6 in each case with a value 0.0, it is ensured at computer blocks 7 and 8 for minimum calculation that wheel braking torques M1 and M2 are less than or equal to zero. Subsequently, (utilized) coefficients of friction fr1 or fr2 are determined at computer blocks 9 and 10 for each wheel which may be regeneratively braked in each case as a quotient of wheel braking torque M1 and M2 to be exerted hydraulically and regeneratively and product P1 or P2. Utilized coefficients of friction fr1 or fr2 are therefore determinable reliably, which ensures an advantageous determination of the at least one hydraulic minimum braking torque M.sub.hyd for each wheel which may be regeneratively braked. This ensures, if needed, the desired regulating quality of a standard (hydraulic) ABS regulation at each wheel which may be regeneratively braked.

[0027] In addition, in the method of FIG. 1, at least one first highest generator braking torque M.sub.max1 for the at least one wheel which may be regeneratively braked is determined as the difference of a total braking torque M.sub.total to be exerted on the particular wheel which may be regeneratively braked and hydraulic minimum braking torque M.sub.hyd determined for the particular wheel which may be regeneratively braked. By way of example, total braking torque M.sub.total is determined in that wheel braking torques M1 and M2 are output at a computer block 12 for minimum calculation. Thereafter, hydraulic minimum braking torque M.sub.hyd determined for all wheels which may be regeneratively braked is subtracted from total braking torque M.sub.total at a computer block 14. With the aid of a maximum calculation of a signal S3 of computer block 14 and a value 0.0,negative signals S3 are filtered out at a computer block 15 (for maximum calculation). A signal S4 output by computer block 15 for maximum calculation is multiplied at a computer block 16 by the number of wheels which may be regeneratively braked in each case (the value 2.0 here). In this way, the at least one first highest generator braking torque M.sub.max1 is obtained. (In the procedure described here, only the one first highest generator braking torque M.sub.max1 is determined. Alternatively, a determination individually by wheel of multiple highest generator braking torques M.sub.max1 may also be carried out.)

[0028] As an advantageous refinement, in the method of FIG. 1, in consideration of a predefined second characteristic curve k2 and the at least one wheel contact force F1 and F2 occurring in each case at the at least one wheel which may be regeneratively braked (or the at least one corresponding second variable), at least one second highest generator braking torque M.sub.max2 is determined for the at least one wheel which may be regeneratively braked. For this purpose, a minimum is ascertained at a computer block 18 from wheel contact forces F1 and F2 of the wheels which may be regeneratively braked. Signal S5 of computer block 18 for minimum calculation is subsequently doubled and multiplied by the wheel radius in computer block 20. In addition, a signal S6 is obtained from vehicle deceleration a and second characteristic curve k2, which is multiplied at a computer block 22 by a signal S7 of computer block 20. A signal S8 of computer block 22 is compared to a value of 0.0 at a computer block 24 for maximum calculation, to ensure that the at least one second highest generator braking torque M.sub.max2 which is output by computer block 22 is greater than zero.

[0029] With the aid of the advantageous utilization of second characteristic curve k2, variables which characterize the vehicle dynamics, for example, understeering and oversteering tendencies, may also be taken into consideration in the determination of the at least one second highest generator braking torque M.sub.max2. This ensures at least one second highest generator braking torque M.sub.max2 based on characteristic variables of the vehicle level, while the above-described at least one first highest generator braking torque M.sub.max1 is based on characteristic variables of the wheel level.

[0030] In the method of FIG. 1, in consideration of a provided item of information with respect to a generator potential of the electrical motor, at least one third highest generator braking torque M.sub.max3 is also determined for the at least one wheel which may be regeneratively braked. The provided information may include, for example, an item of information about an instantaneous vehicle velocity of the particular vehicle and/or about an instantaneous charge level of a battery chargeable with the aid of the electric motor. The instantaneous driver braking command may also be taken into consideration in the determination of the at least one third highest generator braking torque M.sub.max3. It may therefore also be taken into consideration that the generator potential of the electric motor is restricted in the event of a present vehicle velocity less than a predefined minimum velocity suitable for regenerative braking or in the event of a complete charge of the particular battery. Because processes for determining the generator potential and/or the at least one third highest generator braking torque M.sub.max3 are known, they are not illustrated in FIG. 1. The items of information for the generator potential may be externally transmitted, and are therefore easily modifiable.

[0031] In the method of FIG. 1, the at least one setpoint variable M.sub.final is determined in consideration of the at least one first highest generator braking torque M.sub.max1, the at least one second highest generator braking torque M.sub.max2, and the at least one third highest generator braking torque M.sub.max3. For this purpose, for the at least one wheel which may be regeneratively braked, at least one minimum variable Min is determined from particular first highest generator braking torque M.sub.max1, particular second highest generator braking torque M.sub.max2, and particular third highest braking torque M.sub.max3. By way of example, a signal S9 is first ascertained as a minimum of second highest generator braking torque M.sub.max2 and third highest generator braking torque M.sub.max3 at a computer block 26 for minimum calculation. Thereafter, the at least one minimum variable Min is determined as the minimum of first highest generator braking torque M.sub.max1 and signal S9 of computer block 26 with the aid of a computer block 28 for minimum calculation.

[0032] In the specific embodiment of FIG. 1, the driver and/or the automatic braking mechanism is able to specify whether the electric motor is to be used for braking the vehicle or not. A corresponding switching signal 30, which may be triggered by the driver and/or the automatic braking mechanism, is provided at a switch 32 together with the at least one determined minimum variable Min and a value 0.0. Switch 32 is switchable with the aid of switching signal 30 in such a way that, if generator operation of the electric motor is desired by the driver and/or the automatic braking mechanism, the at least one minimum variable Min is output by switch 32 as a signal S10. Otherwise, instead of the at least one minimum variable Min, value 0.0 may be output by switch 32 as signal S10.

[0033] As an advantageous refinement, in the method of FIG. 1, in addition at least one wheel velocity value v1 or v2 of the at least one wheel which may be regeneratively braked is compared to at least one predefined threshold value v0. For this purpose, firstly a signal S11 is ascertained as a minimum of the at least one wheel velocity value v1 and v2 with the aid of a computer block 34 for minimum calculation and subsequently signal S11 of computer block 34 is compared in a computer block 36 to the at least one predefined threshold value v0. If the at least one wheel velocity value v1 and v2 falls below the at least one predefined threshold value v0, i.e., signal S11 is less than the at least one predefined threshold value v0, the at least one setpoint generator braking torque is only permitted to be kept constant or reduced upon the determination of the at least one setpoint variable M.sub.final. This may be understood to mean that, if the at least one wheel velocity value v1 and v2 falls below the at least one predefined threshold value v0, a redetermination of the at least one setpoint variable M.sub.final is only possible if setpoint generator braking torque M.sub.final corresponding to redetermined setpoint variable M.sub.final is less than a setpoint generator braking torque M.sub.final* determined (directly or indirectly), immediately beforehand. In contrast, if the at least one wheel velocity value v1 and v2 falling below the at least one predefined threshold value v0 may not be determined, a redetermination of the at least one setpoint variable M.sub.final is also possible if setpoint generator braking torque M.sub.final corresponding to redetermined setpoint variable M.sub.final is greater than a setpoint generator braking torque M.sub.final* determined (directly or indirectly) immediately beforehand.

[0034] Therefore, in the method of FIG. 1, the wheel behavior of the at least one wheel which may be regeneratively braked may be observed. If a wheel slip threshold or velocity threshold which is predefined with the aid of the at least one threshold value v0 is fallen below, an instantaneous increase of the at least one setpoint generator braking torque M.sub.final may be suppressed, in that setpoint generator braking torque M.sub.final*, which was determined immediately beforehand and is instantaneously active, is maintained. Therefore, in the event of falling below the predefined wheel slip threshold or velocity threshold, the at least one setpoint generator braking torque M.sub.final is no longer increased further, but rather only kept constant or reduced. An increase of the at least one setpoint generator braking torque M.sub.final may only be permitted again with the aid of a corresponding redetermination of the at least one setpoint variable M.sub.final when all wheels which may be regeneratively braked again display a stable wheel behavior.

[0035] An increase of the at least one setpoint generator braking torque M.sub.final in relation to setpoint generator braking torque M.sub.final*, which was determined immediately beforehand and is instantaneously active, is preventable in that firstly signal S10 of switch 32 is output together with presently active setpoint generator braking torque M.sub.final* at a computer block 38 for minimum calculation. Computer block 38 outputs the ascertained minimum as signal S12. If the at least one wheel velocity value v1 and v2 falls below the at least one predefined threshold value v0, or if signal S11 is less than the at least one predefined threshold value v0, computer block 36 outputs a corresponding signal S13 at a further switch 40, at which signals S10 and S12 are also provided. Switch 40 is switchable with the aid of signal S13 in such a way that, if signal S11 is less than the at least one predefined threshold value v0, switch 40 outputs signal S10 as a signal 514. Otherwise, switch 40 outputs signal S12 as signal S14.

[0036] Signal S14 is processed by an asymmetrical filter. Increases of the at least one setpoint generator braking torque M.sub.final are filtered in such a way that a soft buildup of the at least one setpoint generator braking torque M.sub.final is achievable. In contrast, reductions of the at least one setpoint generator braking torque M.sub.final are implemented immediately. For this purpose, signal S14 is compared to presently active setpoint generator braking torque M.sub.final* at a computer block 42 for minimum calculation. The minimum of signal S14 and presently active setpoint generator braking torque M.sub.final* is output by computer block 42 for minimum calculation as a signal S15 at a switch 44, at which presently active setpoint generator braking torque M.sub.final* is also provided. Switch 44 is switchable like above-described switch 32 with the aid of switching signal 30. A signal S16 of switch 44 is subtracted with the aid of computer block 46 from signal S14. Subsequently, a signal S17 of computer block 46 is multiplied at a computer block 48 by a filter time constant f. A signal S18 of computer block 48 is subsequently added at a computer block 50 to signal S16. This causes the desired asymmetrical filtering of the at least one setpoint generator braking torque M.sub.final. (In addition, filter time constant f may be adapted to fadeout operation (fading out of the torque).) In addition, as a supplement, presently active setpoint generator braking torque M.sub.final* may also be adapted to a present actual braking torque M*, in that presently active setpoint generator braking torque M.sub.final* and present actual braking torque M* are provided at a computer block 52 for minimum calculation. The computer block subsequently (re-) determines signal M.sub.final* output at computer block 38.

[0037] The method of FIG. 1 ensures that even in the case of generator operation of the electric motor, a minimum brake pressure is always maintained in each wheel brake cylinder of the at least one wheel which may be hydraulically braked. The optimum wheel deceleration is therefore settable at any time with the aid of the ABS regulating algorithm. The ABS regulating algorithm does not have to control the electric motor directly for this purpose. This task is omitted as a result of the advantageous determination of the at least one setpoint variable M.sub.final with the aid of the above-described method. (A reduction of the at least one setpoint generator braking torque M.sub.final may be triggered indirectly via a brake pressure reduction of the ABS.)

[0038] The method of FIG. 1 enables a specification of the at least one setpoint generator braking torque M.sub.final (i.e., the regeneration torque) in the sense of a pilot control. FIG. 1 relates to a vehicle having an axle differential on exactly two (drivable) wheels which may be regeneratively braked with the aid of the electric motor. For example, the two wheels which may be regeneratively braked may be understood as the wheels connected to the front axle of the vehicle equipped with the regenerative braking system. However, it is to be noted that the method of FIG. 1 is also applicable in an altered version for use on the rear axle, or on two axles which may be electrically braked, or in the case of individual wheel electric drives.

[0039] The method of FIG. 1 may cooperate with an external ABS algorithm. The at least one setpoint generator braking torque M.sub.final (i.e., the regeneration torque) may therefore also be at least partially maintained during an active ABS regulation. A mean brake pressure level of the at least one wheel brake cylinder of the regenerative braking system is significantly lower during ABS braking using regeneration at corresponding (utilized) coefficient of friction than in the case of solely hydraulic braking with equal vehicle deceleration. A thermal potential of the wheel brakes on at least one axle may therefore often be reduced, which results in a savings in cost and weight on the braking system.

[0040] The input variables required for executing the method illustrated with the aid of FIG. 1 are generally available in a vehicle network. In addition, the required input variables may be measured or derived easily. A control unit architecture optimized with respect to the signal runtimes is easily implementable.

[0041] The method of FIG. 1 may be executable in particular with the aid of a control device for a regenerative braking system. Such a control device has control electronics, with the aid of which the at least one setpoint variable M.sub.final is determinable with respect to the at least one setpoint generator braking torque M.sub.final, which is to be exerted with the aid of the electric motor of the regenerative braking system on the at least one wheel which may be regeneratively braked. In addition, at least one control signal corresponding to the at least one determined setpoint variable M.sub.final may be output at the electric motor with the aid of the control electronics. As already described above, the control electronics are designed, in consideration of the at least one provided first variable fr1 and fr2 with respect to coefficient of friction fr1 and fr2 occurring in each case at the at least one wheel which may be regeneratively braked, to determine the at least one preset variable M.sub.hyd with respect to hydraulic minimum braking torque M.sub.hyd to be exerted on the at least one wheel which may be regeneratively braked, and, in consideration of at least the at least one determined hydraulic minimum braking torque M.sub.hyd, to determine the at least one setpoint variable M.sub.final. Furthermore, the control electronics may be refined to execute the above-described method steps.