BRAKE SYSTEM AND METHOD FOR CONTROLLING A BRAKE SYSTEM

Abstract

A brake system and a method for controlling a brake system. The brake system may include a first module having a first pressure supply unit with an electromotive drive, an optional second pressure supply unit and a first control device for controlling the first pressure supply unit; a second module having a third pressure supply unit, isolation valves and brake-pressure-adjusting valves, and a second control device for controlling the brake-pressure-adjusting valves; and a detection unit for detecting a first case of error. The brake system is designed such that, in the first case of error, in order to provide an ABS function and/or a yaw torque intervention, it implements a (wheel-specific and/or selective) adjustment of the pressures in the wheel brakes by actuating at least one of the brake-pressure-adjusting valves of the second module and/or the isolation valves of the second module and the first pressure supply unit.

Claims

1. A brake system including: a first module, comprising a first pressure supply unit having an electromotive drive and a first control apparatus configured to control the first pressure supply unit, wherein the first module is adapted to apply pressure to at least one first brake circuit by way of a first connection point and at least one second brake circuit by way of a second connection point, with a pressurizing medium, wherein the brake circuits are assigned wheel brakes, a second module, comprising a second pressure supply unit, isolation valves and brake pressure adjustment valves, comprising outlet valves and inlet valves, adapted to adjust pressures in the wheel brakes, and a second control apparatus configured to control the brake pressure adjustment valves, wherein, for providing an ABS function, the brake system is adapted to implement adjustment of the pressures in the wheel brakes while actuating at least one of the brake pressure adjustment valves of the second module and/or isolation valves of the second module and the first pressure supply unit, wherein for the adjustment of the pressures, a) at least temporarily, a pressure reduction takes place via at least one outlet valve into the first pressure supply unit, and b) at least temporarily, a pressure build-up takes place using the first pressure supply unit via at least one inlet valve.

2. A brake system including: a first module, comprising a first pressure supply unit having an electromotive drive and a first control apparatus configured to control the first pressure supply unit, wherein the first module is adapted to apply pressure at least one first brake circuit by way of a first connection point and at least one second brake circuit by way of a second connection point, with a pressurizing medium, wherein the brake circuits are assigned wheel brakes, a second module, comprising a second pressure supply unit, isolation valves and brake pressure adjustment valves, comprising at least outlet inlet valves, adapted to adjust pressures in the wheel brakes, and a second control apparatus configured to control the brake pressure adjustment valves, wherein the brake system is configured, in order to provide a yaw torque intervention, to implement a wheel-individual and/or selective adjustment of the pressures in the wheel brakes while actuating at least one of the brake pressure adjustment valves of the second module and/or isolation valves of the second module and the first pressure supply unit, wherein at least one of the inlet valves is energized during a pressure build-up to provide the yaw torque intervention using the pressure supply unit such that it is closed, or at least one of the inlet valves is energized in a pulse-width modulation (PWM) process such that it is partially open.

3. The brake system as claimed in claim 2, wherein the first pressure supply unit is temporarily controlled in such a manner that said first pressure supply unit, when dissipating pressure, generates a pressure sink having a lower pressure than the pressures in the wheel brakes.

4. A brake system having a first module, comprising a first pressure supply unit having an electromotive drive and a first control apparatus configured to control the first pressure supply unit, wherein the first module is adapted to apply pressure to at least one first brake circuit by way of a first connection point and at least one second brake circuit by way of a second connection point, with a pressurizing medium, wherein the brake circuits are assigned wheel brakes, a second module, comprising a second pressure supply unit, isolation valves and brake pressure adjustment valves, comprising at least outlet inlet valves, adapted to adjust pressures in the wheel brakes, and a second control apparatus configured to control the brake pressure adjustment valves, wherein the brake system, in order to provide a yaw torque intervention, is configured to implement a wheel-individual or wheel-selective adjustment of the pressures in the wheel brakes while actuating at least one of the brake pressure adjustment valves of the second module or isolation valves of the second module and the first pressure supply unit, wherein the first pressure supply unit is temporarily controlled in such a manner that said first pressure supply unit, when dissipating pressure, generates a pressure sink having a lower pressure than the pressures in the wheel brakes, whereby the pressure is dissipated via at least one of the inlet valves.

5. The brake system as claimed in claim 1, wherein upon detection of a first error event, the brake system, in order to provide an ABS function and/or a yaw torque intervention, is controlled to implement an adjustment of the pressures in the wheel brakes while actuating at least one of the brake pressure adjustment valves of the second module and/or the isolation valves of the second module and the first pressure supply unit.

6. The brake system as claimed claim 1, wherein at least some of the isolation valves of the first module are disposed and configured to establish a hydraulic connection between the brake pressure adjustment valves and the first and second connection points.

7. The brake system as claimed in claim 1, further including a communications link configured between the first control apparatus and the second control apparatus.

8. The brake system as claimed in claim 1, wherein the first control apparatus or the second control apparatus or a third control apparatus configured to provide an anti-lock braking system (ABS) function and/or a yaw torque intervention is/are configured to control the first pressure supply unit and the brake pressure adjustment valves so as to implement a wheel-individual and/or brake circuit-individual pressure feedback control in the wheel brakes or the brake circuits.

9. The brake system as claimed in claim 1, wherein: a first isolation valve of the isolation valves of the first module is disposed in a first hydraulic line between the first pressure supply unit and the first connection point, and a second isolation valve of the isolation valves of the first module is disposed in a second hydraulic line between the first pressure supply unit and the second connection point, wherein the brake system is configured to detect total failure of the second module, and during the total failure of the second module to control the first pressure supply unit and the first and the second isolation valves to implement at least a brake circuit-individual pressure feedback control in the at least two brake circuits.

10. The brake system as claimed in claim 1, wherein the brake system, is configured: to detect a non-homogenous road condition, and during the total failure of the second module and in the non-homogenous road condition to control the first pressure supply unit to adjust in at least one of the brake circuits a target brake pressure determined as a function of a wheel blocking pressure of a respective wheel brake of the at least one of the brake circuits that has a coefficient of friction that is higher in comparison to another wheel brake of the at least one of the brake circuits.

11. The brake system as claimed in claim 1, wherein at least the second module has wheel sensors for detecting wheel speed, said wheel sensors by way of the communications link being configured to transmit wheel rotational speed signals generated from the detected wheel speed, or the detected wheel speed, to the first module.

12. The brake system as claimed in claim 1, wherein the brake system, temporarily, by means of the first pressure supply unit, is configured to control pressure buildup and pressure dissipation, so as to implement a 1-channel ABS while using wheel rotational speed sensors, and/or is temporarily configured to implement an intermittent braking by modulating a pressure between two fixedly adjusted pressure levels in two of the brake circuits.

13. The brake system as claimed in claim 1, further including at least one pressure sensor configured to detect a brake pressure within at least one of the brake circuits.

14. The brake system as claimed in claim 1, wherein the first module comprises: a rotary pump configured to build up pressure and to dissipate pressure; and a solenoid valve hydraulically connected to a reservoir.

15. The brake system as claimed in claim 14, wherein the first module further comprises at least one pressure transducer configured to provide feedback control of the pressure build-up and the pressure dissipation.

16. The brake system as claimed in claim 1, wherein the first pressure supply unit is configured as a gear pump for building up pressure and dissipating pressure.

17. The brake system as claimed in claim 16, wherein the gear pump is controlled using a pressure transducer or as a function of a measurement of a current and of an angle of a rotor of the electromotive drive.

18. The brake system as claimed in claim 9, wherein at least one third isolation valve of the isolation valves of the first module is disposed and configured in such a manner that, in a closed state of the at least one third isolation valve, the at least one first brake circuit is hydraulically decoupled from the first and the second pressure supply units.

19. The brake system as claimed in claim 9, wherein the first hydraulic line or the second hydraulic line is/are connected to a reservoir by way of a respective suction valve.

20. The brake system as claimed in claim 1, wherein an activation element is disposed on the second pressure supply unit, wherein the second pressure supply unit comprises a master brake cylinder having a single piston that is activatable by means of the activation element.

21. A method of controlling the brake system as claimed in claim 1, said method comprising: controlling the first pressure supply unit of the first module and a plurality of brake pressure adjusting valves, comprising inlet valves and outlet valves in the second module, wherein at least the controlling the first pressure supply unit is performed by means of the first control apparatus, controlling the brake system at least temporarily in such a manner that, for providing an ABS braking operation, adjustment of pressures in the wheel brakes takes place while actuating at least one of the brake pressure adjusting valves of the second module and/or the isolation valves of the second module and the first pressure supply unit), wherein when setting the pressures: a) at least temporarily a pressure reduction takes place via at least one of the outlet valves into the first pressure supply unit; and b) at least temporarily a pressure build-up using the first pressure supply unit takes place via at least one of the inlet valves.

22. A method of controlling the brake system as claimed in claim 2, said method comprising: controlling the first pressure supply unit of the first module and a plurality of brake pressure adjusting valves, comprising inlet valves, in the second module, wherein at least the controlling the first pressure supply unit is carried out by means of the first control apparatus, controlling the brake system at least temporarily in such a manner that, for providing the yaw torque intervention, adjustment of pressures in the wheel brakes takes place while actuating at least one of the brake pressure adjustment valves of the second module or the isolation valves of the second module and the first pressure supply unit, wherein the yaw torque intervention comprises a pressure build-up using the pressure supply unit, in which energizing of at least one of the inlet valves takes place such that it is closed or is operated in a PWM method for partial opening.

23. A method of controlling the brake system as claimed in claim 4, said method comprising: controlling the first pressure supply unit of the first module and a plurality of brake pressure adjusting valves, comprising inlet valves, in the second module, wherein at least the controlling the first pressure supply unit is carried out by means of the first control apparatus, controlling the brake system at least temporarily in such a manner that, for providing the yaw torque intervention, adjustment of the pressures in the wheel brakes takes place while actuating at least one of the brake pressure adjustment valves of the second module or the isolation valves of the second module and the first pressure supply unit, wherein the yaw torque intervention comprises a pressure reduction in which: a pressure sink is generated by means of the first pressure supply unit with a lower pressure than the pressures in the wheel brakes; and the pressure reduction takes place via at least one of the inlet valves.

Description

[0069] Exemplary embodiments of the invention will be explained in more detail hereunder by means of the figures. In the figures, in some instances in a highly simplified illustration:

[0070] FIG. 1a shows a circuit diagram of a first exemplary embodiment of the brake system having a first pressure supply unit according to a first embodiment,

[0071] FIG. 1b shows a circuit diagram of a second exemplary embodiment of the brake system having the first pressure supply unit according to the first embodiment,

[0072] FIG. 2 shows a circuit diagram of the first exemplary embodiment of the brake system having a first pressure supply unit according to a second embodiment,

[0073] FIG. 3 shows a circuit diagram of the first exemplary embodiment of the brake system having a first pressure supply unit according to a third embodiment,

[0074] FIG. 4 shows a circuit diagram of a third pressure supply unit (ESP unit),

[0075] FIG. 5 shows a circuit diagram of the third pressure supply unit (ESP unit) while dissipating a pressure in a first error event in a 4-channel ABS control,

[0076] FIG. 6 shows a circuit diagram of the third pressure supply unit (ESP unit) while building up a pressure in the first error event in a 4-channel ABS control,

[0077] FIG. 7a shows a diagrammatic temporal profile of a select high control in a brake circuit having two wheel brakes,

[0078] FIG. 7b shows a diagrammatic temporal profile of a select low control in a brake circuit having two wheel brakes,

[0079] FIG. 8 shows a circuit diagram of a first exemplary embodiment of the brake system according to the invention having two isolation valves and one feed valve,

[0080] FIG. 9 shows a circuit diagram of a second exemplary embodiment of the brake system according to the invention having four isolation valves and one feed valve,

[0081] FIG. 10 shows a circuit diagram of the third pressure supply unit (ESP unit) while building up a pressure in the first error event in a yaw torque control, and

[0082] FIG. 11 shows a circuit diagram of the third pressure supply unit (ESP unit) while dissipating a pressure in the first error event in a yaw torque control.

[0083] Components with equivalent functions are in some instances provided with the same reference signs in the figures.

[0084] The brake system 2 according to a first exemplary embodiment as illustrated in FIG. 1a has a first pressure supply unit 6 in a first embodiment. In this embodiment, the first pressure supply unit 6 has an electromotive drive 8 which acts on a piston of a piston/cylinder unit. Furthermore, the brake system 2 and in particular the first pressure supply unit 6 has a first control apparatus 9 which feeds in particular the electromotive drive 8 with control signals.

[0085] The first pressure supply installation 6 serves here to impinge a first brake circuit BK1 and a second brake circuit BK2 with a pressurizing medium. To this end, the cylinder of the first pressure supply unit 6 by way of a hydraulic line is hydraulically connected to the first brake circuit BK1 (cf. connection point A1) and to the second brake circuit BK2 (cf. connection point A2).

[0086] In the exemplary embodiment according to FIG. 1a, an isolation valve PD1 by way of which the first pressure supply unit 6 is able to be hydraulically reversibly isolated from the first brake circuit BK1 and from the second brake circuit BK2 is additionally disposed in this hydraulic line. The isolation valve PD1 here is configured as a solenoid valve.

[0087] Additionally, the first pressure supply unit 6 and in particular the cylinder of the first pressure supply unit 6 has a hydraulic connection line to a reservoir 40 in which a check valve is disposed. The hydraulic connection to the reservoir 40 serves for suctioning pressurizing medium from the reservoir 40.

[0088] Furthermore, the brake system 2 has a third pressure supply unit 90 which is only schematically illustrated in FIG. 1a. The third pressure supply unit 90 is also referred to as an ESP unit, or the ESP unit comprises the third supply unit 90, respectively. Moreover, a second control apparatus 95 which controls the third pressure supply unit 90 is provided.

[0089] A communications link 100, in particular a CAN bus link, is configured between the first control apparatus 9 and the second control apparatus 95. The communications link 100 serves for exchanging data and/or signals between the two control apparatuses 9, 95.

[0090] In particular, no valves are disposed in the hydraulic lines of the first brake circuit BK1 as well as of the second brake circuit BK2 in the exemplary embodiment according to FIG. 1a.

[0091] Moreover, a pressure transducer p/U which is disposed between the isolation valve PD1 and the first or the second brake circuit BK1, BK2, respectively, is provided in the hydraulic line. This pressure transducer p/U, in particular in an error event (cf. embodiments hereunder) serves for providing pressure information pertaining to the brake circuits BK1, BK2 in order for pressure to be adjusted in the brake circuits BK1, BK2.

[0092] As an alternative to the pressure transducer p/u, an item of information pertaining to the pressure adjusted by means of the first pressure supply unit 6 in this embodiment takes place by estimating the pressure by way of a motor rotary encoder a/U and/or the motor current i/u.

[0093] FIG. 1b shows a circuit diagram of a second exemplary embodiment of the brake system 2 having the first pressure supply unit 6 according to the first embodiment.

[0094] This exemplary embodiment corresponds substantially to the aforementioned exemplary embodiment of the brake system 2 according to FIG. 1a. A difference here lies in that an isolation valve BP1, TV.sub.BK2 is in each case disposed in the hydraulic line to the brake circuits BK1, BK2. An adjustment of a brake circuit-individual brake pressure by means of these two isolation valves BP1, TV.sub.BK2 is in particular possible in an error event.

[0095] A circuit diagram of the first exemplary embodiment of the brake system 2 having the first pressure supply unit 6 according to a second embodiment is illustrated in FIG. 2. This exemplary embodiment of the brake system 2 likewise corresponds substantially to the design embodiment of the brake system 2 according to FIG. 1a. However, the first pressure supply unit 6 in the exemplary embodiment according to FIG. 2 is configured as a rotary pump and in particular as a single-circuit piston pump, in particular a pump having 1 or a plurality of, in particular 3, pistons. The piston pump is embodied in a manner comparable to ESP pump drives in which the piston/pistons by way of an eccentric are driven by a shaft of an electric motor. In this design embodiment, a valve PD2 is additionally provided for enabling the pressure buildup or the pressure dissipation, respectively. When building up pressure, the PD2 valve can advantageously also be used for compensating pressure pulses of a pump driven by an eccentric. The pressure pulses are very high in particular in the case of a 1-piston pump.

[0096] The brake system 2 according to the first exemplary embodiment having a first pressure supply unit 6 according to a third embodiment, as is illustrated in FIG. 3, with the exception of the embodiment of the first pressure supply unit 6, likewise corresponds to the brake system 2 according to FIG. 1a. In the exemplary embodiment according to FIG. 3, the first pressure supply unit 6 is configured as a gear pump. Providing an item of information pertaining to the pressure provided by the pressure supply unit 6 in this exemplary embodiment, as an alternative to the pressure transducer p/u, takes place by estimating a pressure by way of a motor rotary encoder a/U and/or the motor current i/u. By virtue of the mechanical and functional design embodiment of the gear pump, no valve PD2 is required in this exemplary embodiment because pressure can also be dissipated by way of the gear pump by a reversal of the rotation direction, for example by embodying the drive motor of the gear pump as a brushless electric motor operated by way of a B6 bridge circuit and operating the electric motor in the 4-quadrant operation. Moreover, for reasons related to the operating principle, the pressure pulses are significantly weaker than in the case of an eccentric piston pump.

[0097] FIG. 4 shows a circuit diagram of a third pressure supply unit 90 (also referred to as an ESP unit) having the motor/pump unit 91 for use in the brake system 2 according to the invention. The ESP unit is known and has the main components pump P with motor M, the valves HSV1 and HSV2, USV1 and USV2, the inlet and outlet valves EV1 to EV4 and AV1 to AV4 assigned to the wheel brakes RB1, RB2, RB3, RB4, and one storage chamber (SpK) per brake circuit. This system is described in many publications and patent applications. Said system is already marketed as a 2-box brake system e-booster+ESP and is used above all in electric and hybrid vehicles. In this application, only the outlet valves of the ESP unit are actuated by way of the e-booster by way of a CAN interface in interaction with the brake torque of the generator, i.e. recuperation for the avoidance of a buildup of brake pressure in the wheel brakes, and the storage chamber SpK is used for receiving pressurizing medium.

[0098] One aspect of the invention lies in that the first control apparatus 9 by way of a communications link 100 is communicatively connected to the second control apparatus 95 (ECU-ESP) of the ESP unit and, for achieving safety aspects, at least the inlet valves EV1 to EV4 are able to be controlled by the first control apparatus 9.

[0099] A (further) aspect of the invention lies in the wheel-individual pressure dissipation while using the outlet valves AV1 to AV4 and HSV valves of the ESP unit.

[0100] A circuit diagram of the third pressure supply unit 90 (ESP unit) while dissipating pressure in a first error event is illustrated by way of example in a brake circuit in FIG. 5. The first error event here can be understood to mean that a motor M of the third pressure supply unit 90 has failed. In this case, a pressure dissipation for the purpose of feedback control takes place by way of the first pressure supply unit 6. This takes place specifically in that the piston of the first pressure supply unit 6 is moved back (toward the right in the drawing plane, identified by an arrow) and opening of the outlet valves AV4 and AV3 as well as of the isolation valve HSV2 takes place, said outlet valves AV4 and AV3 in the normal state being closed when not energized. The valves which in this state are open for the volumetric flow, for highlighting the opened state, are in each case provided with an asterisk (*) in FIG. 5 (left half of FIG. 5). The state of the other solenoid valves is not explicitly explained. In this way, at least the inlet valves EV1-EV4 are closed by active energizing when dissipating pressure, for example. The pressure dissipation from the wheel brakes RB3 and RB4 is in particular shown in an exemplary manner in FIG. 5 (the flow direction of the pressurizing medium from the wheel brakes to the first pressure supply unit 6 is identified by arrows). For this purpose, the pressure supply unit 90 can according to the invention be equipped with isolation valves HSV1, HSV2 which, as opposed to the typical use in ESP units, according to the invention are operated bidirectionally. The isolation valves HSV1 and HSV2 are also used for resupplying fluid from the reservoir 40 in a normal ESP operation with an active pump. By virtue of the given configuration, pressure from the wheel brakes RB1 and RB2, or RB3 and RB4, (not illustrated), respectively, can be selectively dissipated by opening and closing the isolation valves HSV1 and HSV2 when the isolation valves USV1 and USV2 are closed. A wheel-individual pressure adjustment can take place by correspondingly switching the outlet valves AV1 to AV4.

[0101] In the first error event, the actuation of the valves, in particular of the isolation valves USV1, USV2, HSV1, HSV2 and of the outlet valves AV1 to AV4 can also take place by the first control apparatus 9 and not, as in the normal case, by the second control apparatus 95. When controlling by the first control apparatus 9, the control signals required for this purpose here are transmitted to the third pressure supply unit 90 by means of the communications link 100. However, in the normal operation, in the absence of an error event, the second control apparatus 95 assumes the actuation of the valves. The normal operation here can be understood to mean a pressure buildup required for example for braking a vehicle in comparison to a pressure buildup for the purpose of feedback control (so as to prevent slipping or blocking of the wheel).

[0102] The inlet valves EV1 to EV4 are closed (by energizing) when dissipating pressure. A hydraulic connection to the first pressure supply unit 6 is configured by opening the isolation valve HSV2; an outflow of the pressurizing medium here is then facilitated by means of the first pressure supply unit 6 and not, as customary, by means of the pump P.

[0103] The pressure dissipation illustrated and explained by way of example for two wheel brakes RB3, RB4 in FIG. 5, in an analogous manner can alternatively also take place brake circuit-individually or wheel brake-individually. The wheel brake circuit-individual feedback control is used for the 4-channel ABS operation as well as for yaw torque interventions (also referred to as yaw torque feedback control(s)).

[0104] A pressure is preferably detected by means of the pressure transducer p/U in the ESP unit during this feedback control, so that information pertaining to the pressure for feedback-controlling the pressure dissipation is present at every point in time.

[0105] A pressure buildup in the first error event is illustrated by way of example by the circuit diagram of the third pressure supply unit 90 according to FIG. 6. In this case, the second control apparatus 95 controls the inlet valves EV1 to EV4 of the third pressure supply unit 90 as in the normal operation of the third pressure supply unit 90. The outlet valves AV1 to AV4 are (non-energized) closed when building up pressure. Additionally, the valve USV2 or USV1 is opened during the pressure buildup, while the valves HSV1, HSV2 remain (non-energized) closed. The pressure buildup in the two wheel brakes RB3, RB4 is shown by way of example in FIG. 6, so reference here is made in each case to the isolation valves HSV2 and USV2 situated within this brake circuit BK1. Alternatively, the pressure buildup illustrated and explained by way of example for two wheel brakes RB3, RB4 in FIG. 6, in an analogous manner can also take place brake circuit-individually or wheel brake-individually, as a result of which a wheel-individual pressure buildup and yaw torque intervention can take place.

[0106] The isolation valve PD1, if provided, which isolates the first pressure supply unit 6 from the brake circuits BK1, BK2 is operated so as to be open during the pressure buildup. The first pressure supply unit 6 by way of the hydraulic line conveys pressurizing medium into the wheel brakes RB3, RB4. Also in this exemplary embodiment, the pressure transducer p/U, which according to FIG. 6 is disposed in the second brake circuit BK2, is preferably utilized for detecting information pertaining to pressure. Alternatively, the controlling of the valves of the third pressure supply unit 90 in this exemplary embodiment can also be assumed by the first control apparatus 9 by way of the communications link 100.

[0107] A temporal profile of vehicle speed V.sub.F, wheel circumferential speed V.sub.R, reference speed V.sub.RFE, brake circuit pressure P.sub.h for high wheel, P.sub.L for low wheel is in each case illustrated in FIGS. 7a and 7b. The slippage coefficient is the wheel speed at which a wheel becomes unstable, and is approximately equal to the reference speed V.sub.RFE. In this way, the typical key indicators such as the -limit or (reference speed V.sub.RFE), subpoint 1, 1, 2, 4 for the pressure dissipation P.sub.ab (slippage coefficient exceeded) and time points 3 and 5 for the pressure buildup P.sub.auf (slippage coefficient undershot) are illustrated in FIGS. 7a and 7b.

[0108] In homogenous conditions (all vehicle wheels are situated on asphalt, for example) switching takes place to a select-low feedback control (FIG. 7b), i.e. a corresponding pressure which is so low that no wheel is blocked is adjusted. In this way, approx. 20% of the full braking action is dispensed with.

[0109] In non-homogenous conditions, for example u-split, i.e. wheels on a vehicle side on ice, the other vehicle side on a wet or dry road, the select-high feedback control (FIG. 7a) sets in, i.e. the wheels that are not blocked are feedback-controlled, while the wheels with the low coefficient of friction remain blocked. Here too, approx. 20% of the optimum braking action is dispensed with.

[0110] As has already been explained, FIG. 7a shows a select-high feedback control. The description of the ABS feedback control assumes the general principles that are known from patent applications, brake manuals and brochures. As a result of the tire slippage characteristic, a slippage between the vehicle speed V.sub.F and the wheel circumferential speed V.sub.R=wheel slippage is thus formed as a brake pressure increases. In the case of a slippage coefficient that is a function of many factors, the maximum of the tire circumferential force is exceeded, the latter in the absence of feedback control leading to the wheel blocking. By way of the feedback-controller which evaluates the wheel acceleration (positive and negative) and the slippage , the pressure feedback control with the pressure dissipation P.sub.ab and the pressure buildup P.sub.auf becomes effective with a view to the desired, optimum brake force and cornering force. A reference speed=-limit, which corresponds to the optimum slippage , is likewise formed by the feedback-controller using complex algorithms.

[0111] In this way, FIG. 7a specifically shows an exemplary temporal profile of a select-high feedback control in a brake circuit having two wheel brakes. At the time point 1, as a result of the pressure buildup P.sub.auf by the first pressure supply unit 6, the blocking limit at the wheel V.sub.R1 (at a low coefficient of friction low-) is reached at the pressure p.sub.1, this wheel as a consequence in the case of a further pressure buildup P.sub.auf reaching a wheel circumferential speed V.sub.R=0 and thus blocking. Consequently, the pressure continues to be built up. A further pressure buildup P.sub.auf has the effect that the wheel V.sub.R2 at the time point 2, shortly upon exceeding the -limit at the pressure level p.sub.2, likewise becomes unstable and the wheel circumferential speed V.sub.R2 decreases sharply. Consequently, the pressure is reduced by the pressure supply, for example by restoring the piston. The pressure differential P of the previously determined pressures p.sub.1 and p.sub.2 is evaluated. If the pressure differential P=P.sub.2P.sub.1 is significant, i.e. pressure p.sub.2 exceeds pressure p.sub.1 by more than 30%, the select-high feedback control (also referred to as selective high- control) is initiated. The pressure is then moderately reduced by p.sub.ab=20% in both brake circuits, i.e. the circuit isolation valves (BP1/BP2, TV.sub.BK2; cf. FIG. 9) are not utilized for a selective pressure dissipation and are in the opened state.

[0112] As a consequence, the wheel V.sub.R2 does not block at the time point 3 and again undershoots the -slippage limit at the time point 3. A pressure buildup in stages follows from the time point 3. In a first stage, the pressure is increased by 70% of the previous p.sub.ab value, for example, and in a second step increased by a further 30%, so that the pressure p.sub.2 is reached again and subsequently exceeded. In this phase, the pressure transducer p/U is preferably used for pressure measurement. The slippage limit is again exceeded at the time point 4. Thereafter, the pressure is reduced again, as at the time point 2, and subsequently increased again in stages so that the wheel undershoots the slippage limit again at the time point 5. This feedback control method is continued during the feedback controlling.

[0113] FIG. 7b specifically shows an exemplary temporal profile of a select-low feedback control in a brake circuit having two wheel brakes. Here, the pressure differential P=P.sub.2P.sub.1 is relatively minor in the range between 10% and 20%. As a consequence, the wheels with minor pressure differentials become unstable. This is an indication for an operation on a homogenous carriageway. As described previously in the context of the select-high feedback control, the pressure is reduced by p.sub.ab and increased again in stages. As opposed to the select-high feedback control, the pressure in the case of the select-low feedback control is however dissipated more intensely, for example p.sub.ab=40%, so that the low wheel is relieved from the blocked state at the time point 6, i.e. as opposed to the select-high feedback control, no wheel is operated in the blocked state. The pressure is kept low until first the wheel V.sub.R2 and then the wheel V.sub.R1 undershoots the -limit at the time point 3; then only is the pressure increased again in stages. The slippage limit of the wheel V.sub.R1 is exceeded again at the time point 4, and the pressure is lowered again and subsequently increased in stages.

[0114] FIGS. 7a and 7b show only the general aspects of the select-low/select-high feedback control. Many enhancements are conceivable, such as another test in a select-high feedback control, when the pressure level is reduced in the high wheel. Alternatively, the select-low wheel can also exit the blocked state again without feedback control and exceed the A limit. This potential wheel speed profile is identified by X in FIG. 7a. Thereafter, another select-low/select-high test can take place, optionally with switching over from a select-high feedback control to a select-low feedback control.

[0115] As has already been explained, in one exemplary embodiment, in the second error event switching from a select-low feedback control to a select-high feedback control takes place by way of the first control apparatus 9 when the first control apparatus 9 detects that the vehicle is situated on a non-homogenous hard ground, for example a partially icy road. For this purpose, it is necessary that the brake system 2 according to the invention by means of the first pressure supply unit 6 can adjust different pressures in the individual brake circuits BK1, BK2. The design embodiments already shown schematically by means of FIGS. 1b, 8 and 9 are particularly suitable for this purpose. In order for this control strategy to be implemented, the control apparatus 9 monitors the pressures in the individual wheel brakes RB1, RB2, RB3, RB4 that lead to a blocking of the wheels. If these pressures between two wheels, in particular within one brake circuit BK1, BK2, deviates by more than 30% from one another, switching by the first control apparatus 9 takes place from a select-low feedback control to a select-high feedback control, so as to still achieve a very positive braking result even within said error event.

[0116] FIG. 8 shows a circuit diagram of a brake system 2 which comprises a first module (referred to as X-boost) and a second module. The first modulethe X-boosthas a first pressure supply unit 6 having an electromotive drive 8, as well as a second pressure supply unit 14 having a master brake cylinder 22 and an activation element 26 having a brake pedal. Furthermore provided is a valve installation having various solenoid and check valves.

[0117] The second module and in particular the third pressure supply unit 90 comprises an electrically driven motor/pump unit 91 having a pump with an electromotive drive. The third pressure supply unit 90 can be any arbitrary ESP unit. A suitable ESP unit is described in detail in DE 10 2014 205 645 A1. Alternatively, a standard ABS unit without ESP function can be used as the second module.

[0118] The two modules (X-boost and ESP unit) are specified for impinging two brake circuits BK1 and BK2 with pressurizing medium, wherein the modules are preferably hydraulically connected in series. In one exemplary embodiment, the X-boost is fastened to the scuttle of a vehicle, the second module (ESP unit) at two hydraulic interfaces or connection points A1, A2 (cf. solid black points in FIG. 8 relating to BK1, BK2), respectively, being connected thereto by way of hydraulic lines.

[0119] The first pressure supply unit 6 by way of a first hydraulic line HL1 is connected to the first brake circuit BK1, or to the corresponding interface, respectively. Furthermore provided is a second hydraulic line HL2 for connecting the first pressure supply unit to the second brake circuit, or to the corresponding interface, respectively.

[0120] According to the invention, the second pressure supply unit 14 of the X-boost only has one master brake cylinder 22 having a piston 24 and a piston chamber. In the exemplary embodiment, the second pressure supply unit 14 is embodied with a single circuit and by way of a third hydraulic line HL3 and a feed valve 69 is connected to the brake circuit BK1, or to the corresponding hydraulic interface, respectively. A fluidic connection to the second hydraulic line HL2 runs by way of an optional first isolation valve BP1 (highlighted by a border with dashed lines). The second pressure supply unit 14 by closing the feed valve 69 is able to be isolated from the brake circuits BK1, BK2 in such a manner that the activation element 26 in the normal brake-by-wire operation without errors (for example without a brake circuit failure) acts only on a travel simulator 28.

[0121] In the exemplary embodiment as per FIG. 8, the brake circuits BK1 and BK2 are able to be isolated by way of the optional first isolation valve BP1 (preferably open when not energized), if present. According to the invention, in the event of a failure of the first pressure supply unit 6, the master brake cylinder 22 of the second pressure supply unit 14 by opening the first isolation valve BP1 can in this way be connected either only to the first brake circuit BK1 or to the first and the second brake circuit BK1, BK2. For this emergency operation, the feed valve 69 is configured as a valve that is open when not energized. To the extent that a current is still applied, said feed valve 69 is open so that the second pressure supply unit 14 is no longer hydraulically decoupled from the brake circuits BK1, BK2.

[0122] The first pressure supply unit 6 likewise selectively acts on the second brake circuit BK2 (first isolation valve BP1 closed) or both brake circuits BK1, BK2 (first isolation valve BP1 opened or open when not energized). The first isolation valve BP1 is open in the normal operation so that the first pressure supply unit 6 supplies both brake circuits BK1, BK2 with pressure, and the second pressure supply unit 14 by the closed feed valve 69 is decoupled from the first brake circuit BK1. If it is established that volume is lost from the brake circuits BK1, BK2, the brake circuit BK1 by means of the first isolation valve BP1 can be decoupled from the first pressure supply unit 6 so that, in the event of a leakage in the first brake circuit BK1, the second brake circuit BK2 can continue to be operated without hydraulic fluid losses.

[0123] In the exemplary embodiment, the isolation valve BP1 is embodied as a solenoid valve, wherein the ball seat of the isolation valve BP1 by way of a connector (valve seat connector) is connected to the portion of the hydraulic line that leads to the first pressure supply unit 6. In this way, the isolation valve BP1 can also be reliably closed by energizing in the event of a failure of the first brake circuit BK1, and is not forced open by higher pressures in the operation of the first pressure supply unit 6.

[0124] The second pressure supply unit 14 upon activation of the activation element 26 feeds the travel simulator 28 by way of a breather bore in a wall of the master brake cylinder 22, such that a progressive haptic resistance in the form of a restoring force as a function of a variable of the activation of the activation element 26 can be felt. The variable of the activation here can be understood to mean how firmly and/or how far a driver activates the activation element 26 configured as a brake pedal, and thus pushes the piston 24 into the master brake cylinder 22. The progressive haptic resistance is also referred to as a pedal characteristic.

[0125] A travel simulator valve 29 can be provided for blocking the connection to the travel simulator 28.

[0126] The second pressure supply unit 14 has at least one breather bore 38 which by way of hydraulic lines is connected to a reservoir 40. The reservoir 40 is likewise part of the brake system 2.

[0127] In the exemplary embodiment, a check valve RVHZ as well as a throttle DR can be disposed in the hydraulic line between the breather bore 38 and the reservoir 40. By means of this check valve RVHZ and the first pressure supply unit 6 it is possible to carry out a diagnosis pertaining to a state of preservation of sealing elements disposed within the first pressure supply unit 6 as well as within the travel simulator 28. The travel simulator valve 29, if present, can be closed when checking the seal of the master brake cylinder 22.

[0128] As illustrated, the master brake cylinder 22 has two sealing elements 42a, 42b, which are configured as annular seals. The breather bore 38 is disposed between the two sealing elements 42a, 42b. A throttle DR is disposed in the connection between the breather bore 38, which is disposed between the two sealing elements 42a, 42b, and the reservoir 40.

[0129] The throttle DR in terms of the flow rate thereof is sized such that the pedal characteristic is not substantially changed (3 mm pedal travel in 10 s) in the event of a failure of the sealing element 42a. Moreover, a temperature-related volumetric compensation of the pressurizing medium can take place by way of the throttle DR.

[0130] High pressure peaks in the brake circuits BK1 and BK2, which can significantly stress the first pressure supply unit 6, can be created in an ABS operation of the third pressure supply unit 90. In the variant of design embodiment according to FIG. 8, a pressure limitation valve V is connected to the piston chamber of the first pressure supply unit 6 by way of a bore, so that the high pressure peaks are dissipated and damage to the system is avoided.

[0131] A suction valve NV is likewise fluidically connected to the piston chamber of the first pressure supply unit 6 and enables pressurizing medium to be resupplied from the reservoir 40. In this way, the first pressure supply unit 6 can independently introduce additional pressurizing media into the brake circuits BK1, BK2. An additional breather bore provided in the cylinder of the first pressure supply unit 6 enables a volumetric compensation in the initial position of the piston of the first pressure supply unit 6.

[0132] The third pressure supply unit 90 is only schematically illustrated in FIG. 8. Said pressure supply unit 90 ultimately supplies four wheel brakes RB1, RB2, RB3 and RB4. In the schematic illustration, the wheel brakes RB1, RB2 operate a front axle VA of the vehicle, and the wheel brakes RB3 and RB4 operate a rear axle HA of the vehicle. An electric drive motor for driving the vehicle is situated on the rear axle HA of the vehicle. The vehicle can be a purely electric vehicle or a hybrid vehicle.

[0133] The first brake circuit BK1 is connected to the wheel brakes RB1 and RB2, and the second brake circuit BK2 is connected to the wheel brakes RB3 and RB4. A corresponding allocation is advantageous for the hydraulic assembly illustrated in FIG. 8.

[0134] The third pressure supply unit 90 furthermore possesses a control apparatus 95 (ECU-ESP).

[0135] The second pressure supply unit 14 likewise possesses a printed circuit board which has a level sensor NST which detects the position of a magnetic float gauge NS within the reservoir 40. The PCB furthermore has sensors 30a, 30b for detecting the pedal travel as well as a difference in the distance of travel between the piston 24 and the pedal travel.

[0136] A suction valve 70b, which connects the pump of the third pressure supply unit 90 to the reservoir 40, is provided in the first brake circuit BK1 for providing additional pressurizing medium for the third pressure supply unit 90.

[0137] When the pump of the third pressure supply unit 90 requires pressurizing medium for the second brake circuit BK2, the latter can thus be provided from the reservoir 40 by way of the suction valve 70c.

[0138] In this way, for suctioning pressurizing medium, the two brake circuits BK1, BK2 by the respective hydraulic lines HL1, HL2 are in each case connected to the reservoir 40 by way of one suction valve 70b or 70c, respectively. In order to achieve optimum suctioning of the pressurizing medium, the suction valve 70c preferably has a diameter in the range from 30 mm to 50 mm and in particular a diameter of 40 mm.

[0139] The exemplary embodiment optionally possesses a control of the clearance between the brake pads and the disk brake. The wheel brakes RB1, RB2, RB3, RB4 (cf. FIG. 8) can be configured as frictionless wheel brakes RB1, RB2, RB3, RB4. In a brake-by-wire system, disk brakes having brake pads which are spaced apart by way of a clearance in the absence of pressure in the brake system enable the frictional resistance to be reduced. This can be achieved by the use of rollback seals, restoring springs of the brake pads, or by actively retracting the brake pads by generating a vacuum by means of the pressure supply 6, as is explained in EP 2 225 133 by the applicant.

[0140] The clearance in the wheel brake RB1, RB2, RB3, RB4, which is variable during operation, can be measured in a wheel-individual or brake circuit-individual manner by evaluating the pressure profile by means of the first pressure supply unit 6. According to the invention, corresponding measuring can take place when servicing, or else during the operation of the vehicle. The measurement is preferably performed in a stationary vehicle or after braking.

[0141] Using the known clearance values of the wheel brakes RB1, RB2, RB3, RB4, the clearance when activating the wheel brake RB1, RB2, RB3, RB4 is first rapidly overcome by means of a piston travel control of the first pressure supply unit 6. In this respect, the use of a brushless motor as an electromotive drive 8 of the first pressure supply unit 6 with a small time constant is to be preferred, because the action of overcoming the clearance can be implemented without the driver perceiving the latter when activating the brake.

[0142] Moreover, the brake system 2 can be controlled so that the vehicle electric motor is active in the phase of the clearance. In this way, a braking action is generated immediately when activating the brake.

[0143] In one exemplary embodiment of the invention, differences in the clearances of the wheel brakes RB1, RB2, RB3, RB4 are compensated for in that the inlet valves EV1 to EV4 of the second module (ESP unit) are actuated, and/or the electric motor of one or a plurality of vehicle axles is utilized for generating a braking action at the beginning of braking. By way of the clearance, stick-slip effects of new brake systems at low speeds can generally be reduced or avoided.

[0144] In one exemplary embodiment, the brake system 2 according to the invention in the event of a failure (error event 4) of the ESP unit implements a very simple variant of an intermittent brake. Locking of the wheels is avoided and the steerability is maintained by moving the piston of the first pressure supply unit 6 in a reciprocating manner between an upper and lower pressure range. As opposed to a 1-channel ABS operation, no measurement values, for example pressure and wheel speeds, are required in this form of deceleration.

[0145] The automated intermittent brake leads to sufficient stopping distances (approx. 200% of the stopping distance in the ABS mode in comparison to a full-fledged wheel-individual ABS) and to acceptable stability by maintaining the steerability.

[0146] The brake system according to the invention can provide the decisive advantage that the brake pedal acts only on the piston 24 and by way of the feed valve 69 is isolated from the brake circuits BK1, BK2. In this way, the function of the automated intermittent brake with the X-boost or X-booster, respectively, cannot be interfered with by the driver as opposed to the prior art (WO2011/098178).

[0147] Alternatively to the intermittent brake, a 1-channel ABS operation with select-low feedback control (error event 3) can be implemented. This leads to a further deterioration of the stopping distance (approx. 400% stopping distance in comparison to the stopping distance with a full-fledged wheel-individual ABS) but to an unrestricted vehicle stability and in terms of this characteristic is superior to the intermittent brake. In this form of the 1-channel ABS operation, measurement values such as, for example, pressure and wheel speeds are required, which can be imported from the ESP unit by way of a communications link/interface, for example a CAN interface.

[0148] In order to further increase the availability of the brake system 2 according to the invention according to FIG. 8, the electromotive drive 8 of the first pressure supply unit 6 is connected to the control unit 9 (ECU-DV) of the X-boost by way of two redundant three-phase strands, and the electronic system is embodied so as to be (partially) redundant. For example, two B6 bridges can be provided for each strand. Moreover, in at least one exemplary embodiment, the electronic system is connected to two redundant voltage supplies. In this way, the failure probability of the electromotive drive 8 can be reduced by the factor of 4-10, and the error event (failure of the first pressure supply unit 6) can be further significantly reduced.

[0149] The control apparatus 95 of the ESP unit 90, as well as the control unit 9 (ECU-DV) of the X-boost, are connected to one another by way of the communications link 100, for example a CAN bus. To this extent, it is possible for control commands to be released to the third pressure supply unit 90, said control commands causing an activation of the drive 91 and/or of the provided valves (cf. also FIG. 8).

[0150] The following safety-relevant redundancies can be implemented using the brake system 2 as per FIG. 8: [0151] ensuring a sufficient braking action for meeting the statutory requirements in the event of a brake circuit failure, failure a) of the second pressure supply unit 14, b) of the first pressure supply unit 6, or c) of the first pressure supply unit 6 and the third pressure supply unit 90 (simultaneously), i.e. also meeting statutory requirements in the case of double errors: [0152] error event 1failure of the third pressure supply unit 90: deceleration by boosting the brake force by way of the first pressure supply unit 6 in both brake circuits BK1, BK2; [0153] error event 2failure of the third pressure supply unit 90 and of the brake circuit BK1: deceleration by boosting the brake force by way of the first pressure supply unit 6, for example on the rear axle; [0154] error event 3failure of the third pressure supply unit 90 and of the second brake circuit BK2: deceleration by the second pressure supply unit 14, for example on the front axle (first isolation valve BP1 closed); [0155] error event 4failure of the first pressure supply unit 6: deceleration by boosting the brake force by way of the third pressure supply unit 90; [0156] error event 5failure of the first pressure supply unit 6 and of the first brake circuit BK1 or of the second brake circuit BK2: deceleration by boosting the brake force by way of the third pressure supply unit 90 in one of the brake circuits BK1, BK2, optionally facilitated by the vehicle electric motor on one axle; [0157] error event 6failure of the first pressure supply unit 6 and of the third pressure supply unit 90: braking by the master brake cylinder on the front axle VA and optionally by the electric drive motor on the rear axle HA; [0158] error event 7failure of the on-board network: braking by the second pressure supply unit 14 optionally on the front axle VA and the rear axle HA; [0159] electronic brake force distribution (EBV) in the event of failure of the ESP unit by generating pressure in the first brake circuit BK1 by way of the third pressure supply unit 90, and generating pressure in the second brake circuit BK2 by way of the first pressure supply unit 6, with the first isolation valve BP1 closed, and controlling the first pressure supply unit 6 by way of a sensor assembly of the second pressure supply unit 14. Required to this end is a S/W brake circuit split, i.e. the wheels of the front axle VA are connected to the first brake circuit BK1, and the wheels of the rear axle HA are connected to the second brake circuit BK2; [0160] controlling the clearance between the brake pads and the disk brake; [0161] 4-channel ABS operation and/or yaw torque feedback-control when actuating the valves of the ESP unit; [0162] 1-channel ABS operation or implementation of an automated intermittent brake.

[0163] FIG. 9 shows an alternative design embodiment of the X-boost according to FIG. 8. As opposed to the exemplary embodiment according to FIG. 8, a second isolation valve TVBK2 is disposed in the second hydraulic line HL2 in FIG. 9. This second isolation valve TVBK2 enables the second brake circuit BK2 to be hydraulically decoupled from the first pressure supply unit 6. In this way, the first pressure supply unit 6 can selectively provide pressurizing medium in the first brake circuit BK1 or in the second brake circuit BK2 or in both brake circuits. When volumetric loss is detected in the second brake circuit BK2, the latter can be decoupled.

[0164] Furthermore, the exemplary embodiment according to FIG. 9 differs in that a third isolation valve BP2 is provided in the first hydraulic line HL1 between the first isolation valve BP1 and the first connection point A1 for the first brake circuit BK1. This third isolation valve BP2 is preferably disposed such that the third hydraulic line, in a hydraulic connection between the first isolation valve BP1 and the third isolation valve BP2, opens into the first hydraulic line HL1. The third isolation valve BP2 enables the first brake circuit BK1 to be hydraulically decoupled from the first pressure supply unit 6 as well as from the second pressure supply unit 14. In the case of a failed first pressure supply unit 6, it is thus possible for pressurizing medium, proceeding from the second pressure supply unit 14, to be introduced into the second brake circuit BK2 by way of the feed valve 69, the first isolation valve BP1 and the second isolation valve TV.sub.BK2. No pressurizing medium is dispensed into the first brake circuit BK1 when the third isolation valve BP2 is closed.

[0165] The following safety-relevant redundancies can be implemented using the brake system 2 as per FIG. 9: [0166] ensuring a sufficient braking action in the event of a failure of the one or the plurality of pressure supply units, [0167] error events 1-7: see embodiment 1; [0168] error event 8failure of the feed valve 69 (for example leaky), or failure of the electric actuation: closure of the third hydraulic line HL3 by the isolation valves BP1 and BP2 so that the travel simulator 28 is fully effective; the first pressure supply unit 6 adjusts wheel pressures in the brake circuit BK2 and/or the ESP unit adjusts wheel pressures in both brake circuits BK1 and BK2, [0169] further degree of freedom: selectively feeding the pressure of the master brake cylinder into the brake circuit BK1 or BK2 in the event of a failure of a brake circuit; [0170] ensuring a 4-channel ABS feedback control and/or a yaw torque feedback control when actuating the valves of the ESP unit, [0171] 2-channel ABS operation according to the select-low and select-high method, or 1-channel ABS according to the select-low method with wheel rotational speed sensors; [0172] Electronic brake force distribution (EBV) in the event of a failure of the ESP unit by generating pressure in the brake circuit BK1 by way of the second pressure supply unit 14, and generating pressure in the brake circuit BK2 by way of the first pressure supply unit 6, with a closed first isolation valve BP1, and controlling the pressure supply by way of the sensor assembly of the second pressure supply unit 14. Required to this end is the S/W brake circuit distribution, and the brake force distribution into the brake circuits is controlled by way of the isolation valves BP1, BP2 and TVBK2. According to the invention, the piston of the first pressure supply unit 6 for applying a suitable pressure can be controlled in a reciprocating stroke movement. Optionally, an adjustment of pressure can take place by way of a PWM control of the valves, in particular of the isolation valves; [0173] The clearance control is already embodied in the exemplary embodiment as per FIG. 8. The exemplary embodiment as per FIG. 9 offers the additional potential of compensating for the unequal clearance in the wheel brakes RB1, RB2, RB3, RB4 of the brake circuits BK1, BK2 by a corresponding pilot control prior to the brake force boosting operation by sequentially opening the isolating valves BP1, TV.sub.BK2. Alternatively, the PWM operation may also be used so that different flow cross sections to the brake circuits BK1, BK2 are established and the unequal clearance can simultaneously be compensated for. A S/W brake circuit split is suitable here. This method is readily possible because the brake circuit isolation valves are a component part of the X-boost module and can be implemented without any temporal delay and susceptibility to faults (utilizing an interface between the X-boost and the ESP unit, for example). In this way, the brake system can be designed in such a manner, for example, that no clearance is provided on the brake pads on the front axle, and a clearance is provided on the rear axle. In this way, a failure of the first pressure supply unit 6 does not lead either to a temporal delay in braking when pressure is generated by the activation unit and acts according to the invention on the wheel brakes RB1, RB2, RB3, RB4 of the front axle VA. Moreover, a greater braking action can be generated by the front axle VA.

[0174] A circuit diagram of the third pressure supply unit (ESP unit) while dissipating pressure (cf. FIG. 11) or building up pressure (cf. FIG. 11) in the first error event during yaw torque feedback control is in each case shown in FIGS. 10 and 11. In principle, the feedback control here is performed in a manner similar to the 4-channel ABS likewise possible in the first error event. In the case of the yaw torque feedback control, however, the pressure dissipation as well as the pressure buildupas opposed to the 4-channel ABS feedback controltakes place by way of the inlet valves EV1-EV4 as well as the USV valves. Opened valves relevant for the flow are in each case provided with an asterisk (*) in FIG. 10 as well as FIG. 11. The state of the other solenoid valves is not explicitly explained. For example, at least the inlet valves EV2, EV3, EV4 are thus closed by active energizing during pressure dissipation. To the extent that the valves are valves actuatable by means of a PWM signal, open in the context of this application can also be understood to mean that these valves are actuated by actuation with a PWM signal, such that a predefined opening cross section is established thereby. In this way, a flow rate through the respective valve is able to be controlled by actuating the valves by means of a PWM signal. Specifically, the inlet valves EV1-EV4 and the valves USV1, USV2 in FIGS. 10 and 11 are actuatable by means of a PWM signal. In this way, a flow rate through these valves in the situations described hereunder is able to be feedback-controlled or controlled, respectively.

[0175] A wheel-selective yaw torque feedback control during a pressure buildup in the wheel brake RB4 is shown in an exemplary manner in FIG. 10. To this end, the inlet valve EV1 assigned to the respective wheel brake, here the wheel brake RB4, and the isolation valve USV2 assigned to the respective brake circuit, here the first brake circuit BK1, are passed through by a flow of pressurizing medium. The valves in this embodiment do not have to be actively actuated because said valves are passively open in the non-energized open state and bidirectionally permit a volumetric flow of the pressurizing medium. For the selective pressure generation in a wheel brake RB4, the other inlet valves EV1-EV3, by way of which pressure is not to be built up (RB1-RB3) are actuated in such a manner that the solenoid valves are moved from the open state to the energized closed state. In this context, actuating in the case of a valve that is open when not energized can be understood to mean that the inlet valves EV1-EV3 are closed, i.e. switched so as not to conduct pressurizing medium. Likewise, the HSV valves for the selective pressure generation in the wheel brake RB4 are closed, i.e. switched so as not to conduct pressurizing medium.

[0176] In this way, an impingement of pressure from the first pressure supply unit 6 by way of the isolation valve USV2 and the inlet valve EV4 takes place exclusively to the wheel brake RB4 (schematically indicated by an arrow). In addition to a wheel brake RB1, RB2, RB3, RB4, a yaw torque can be generated in a plurality of wheel brakes RB1, RB2, RB3, RB4. To this end, those inlet valves EV1-EV4 in the wheel brakes RB1, RB2, RB3, RB4 are closed in each case by way of which a pressure is not to be built up. By way of this enhancement, a yaw torque can be simultaneously generated in, for example, 2 wheel brakes RB1, RB2, RB3, RB4 of one vehicle side. Since brake circuits are typically embodied so as to be black and white, or diagonal, in this instance consequently one wheel brake RB1, RB2, RB3, RB4 of one brake circuit is in each case impinged with pressure. A further potential enhancement of the yaw torque feedback control is possible as a result of a sequential or simultaneous multiplex operation of the circuit isolation valves BP1/BP2 and TV.sub.BK2 of the first module (embodiment according to FIG. 9). In this way, a pressure in a wheel brake RB1, RB2, RB3, RB4 (for example RB4 of the right rear wheel) can be brought to a pressure level, wherein the circuit isolation valve TV.sub.BK2 upon reaching the pressure valve is closed in order to maintain the pressure.

[0177] Moreover, another pressure level can be adjusted in a wheel brake RB1, RB2, RB3, RB4 of the other brake circuit (for example RB2 of the right front wheel), wherein the second brake circuit isolation valve BP1 or alternatively BP2 is closed in order to maintain the pressure. The brake circuit isolation valves BP1/BP2 and TV.sub.BK2 are required for maintaining pressure because the inlet valves of the wheel brakes RB1, RB2, RB3, RB4 have check valves connected in parallel. In this way, maintaining pressure in the second module (ESP unit) is impossible when the pressure has dissipated, or when a lower pressure level is adjusted in the second brake circuit.

[0178] The following states thus result for the relevant valves of the third pressure supply unit 90 for the pressure buildup according to FIG. 10: [0179] HSV1: closed (not energized) [0180] HSV2: closed (not energized) [0181] EV4: open (open when not energized, or energized by the PWM method, i.e. partially opened) [0182] EV1-EV3: closed (energized)

[0183] All other valves in the hydraulic, in particular non-energized, initial state.

[0184] In the case of pressure dissipation, as shown in FIG. 11, for example, the return of pressurizing medium takes place in an analogous but reversed manner from the wheel brake RB4 by way of the inlet valve EV4 and the isolation valve USV2 to the first pressure supply unit 6. In an analogous manner, the pressure dissipation then also takes place in the event of yaw torque interventions in a plurality of wheel brakes. Here too, the multiplex method is preferably used.

[0185] In one embodiment, a plurality of, in particular all four, wheel brakes RB1, RB2, RB3, RB4 can additionally be actuated individually and wheel-selectively in an analogous manner, and a wheel-selective yaw torque feedback control can thus be implemented. Alternatively or additionally, the yaw torque feedback control in one embodiment can take place in a brake circuit-selective manner, so that two wheel brakes of one brake circuit are in each case conjointly actuated.

[0186] Specifically, the following states are thus derived for the relevant valves of the third pressure supply unit 90 for the pressure dissipation according to FIG. 11: [0187] HSV1: closed (non-energized) [0188] HSV2: closed (non-energized) [0189] EV4: open (open when non-energized or energized by the PWM method, i.e. partially opened) [0190] USV1: closed (closed when energized) [0191] EV1-EV3: closed (energized)

[0192] All other valves in the hydraulic, in particular non-energized, initial state.

[0193] At this point it is to be pointed out that all parts described above are in each case to be considered individually-even without features which have been additionally described in the respective context, even when said features have not been explicitly identified individually as optional features in the respective context, for example by using: in particular, preferably, for example, e.g., optionally, parentheses, etc.and in combination or any arbitrary sub-combination as independent design embodiments or refinements of the invention, respectively, as defined in particular in the introduction to the specification and the claims. Deviations therefrom are possible. Specifically, it is to be pointed out that the word in particular or parentheses do not identify features which are mandatory in the respective context.

List of Reference Signs

[0194] 2 Brake system [0195] 6 First pressure supply unit [0196] 8 Electromotive drive [0197] 9 Control apparatus (ECU-DV) [0198] 14 Second pressure supply unit [0199] 22 Master brake cylinder [0200] 24 Piston [0201] 26 Activation element [0202] 28, WS Travel simulator [0203] 28a, 28b Sealing element of the travel simulator [0204] 29 Travel simulator valve [0205] 30a, 30b Pedal travel sensor [0206] 38 Breather bore of the second pressure supply unit [0207] 40 Reservoir [0208] 42a, 42b Sealing element of the auxiliary piston [0209] 69 Feed valve [0210] 70b, 70c, 80d, Suction valve (check valve) [0211] RV1, RV2, NV Suction valve (check valve) [0212] RVHZ Check valve (master cylinder) [0213] 74, PD1, PD2 Isolation valve [0214] 80, V Pressure limitation valve [0215] 90 Third pressure supply unit [0216] 91 Motor/pump unit [0217] 95 Control apparatus of the ESP unit [0218] 100 Communications link (CAN bus) [0219] A1, A2 Connection point [0220] B1, B2 Electrical connections (three-phase) [0221] P Pump [0222] M Motor [0223] BP1, TV1 First isolation valve [0224] TVBK2, TV2 Second isolation valve [0225] BP2 Third isolation valve [0226] RB1, RB2, RB3, RB4 Wheel brake [0227] DR Throttle [0228] BK1 First brake circuit [0229] BK2 Second brake circuit [0230] HL1 First hydraulic line [0231] HL2 Second hydraulic line [0232] HL3 Third hydraulic line [0233] HL4 Fourth hydraulic line [0234] VA Front axle [0235] HA Rear axle [0236] NS Float gauge [0237] NST Level sensor [0238] HSV1, HSV2, Isolation valves of the ESP unit [0239] USV1, USV2 Isolation valves of the ESP unit [0240] AV1, AV2, AV3, AV4 Outlet valve of the ESP unit [0241] EV1, EV2, EV3, EV4 Inlet valve of the ESP unit