BRAKE SYSTEM WITH A NEW TYPE OF MUX CONTROL (MUX 2.0), HAVING AN OUTLET VALVE PER BRAKE SYSTEM OR AN OUTLET VALVE PER BRAKE CIRCUIT, AND METHOD FOR CONTROLLING PRESSURE

Abstract

A brake system for motor vehicles may include an actuation device (e.g., brake pedal), a travel simulator to generate a feedback force on the actuation device, a first piston-cylinder unit having at least one piston that separates two working chambers that are connected via at least one hydraulic line to at least one wheel brake of a brake circuit, a control device and a pressure supply unit driven by an electric motor. At least one wheel brake may be assigned to each brake circuit, and each wheel brake may be connected to its associated hydraulic connecting line via a controllable switching valve. An outlet valve may be assigned to a single wheel brake or to a single wheel brake of each brake circuit in a hydraulic connection between the wheel brake and a pressure medium storage container, without any further valve disposed as such.

Claims

1. A brake system for motor vehicles, the brake system including: an actuation device, a travel simulator configured to generate a feedback force on the actuation device, a first piston-cylinder unit, having at least one piston that separates two working chambers from each other, wherein each working chamber is connected via at least one hydraulic connecting line to at least one wheel brake of a brake circuit, wherein at least one wheel brake is assigned to each brake circuit and each wheel brake is enabled to be connected to an associated one of the hydraulic connecting lines via an associated controllable switching valve, a control device, at least one pressure supply unit configured to be driven by an electric motor and having at least one working chamber, wherein the at least one pressure supply unit is configured to build up and/or reduce brake pressure in one or more of the wheel brakes simultaneously or successively, and at least one outlet valve, wherein a respective one of the at least one outlet valve is associated only with a single wheel brake or is associated with one wheel brake of a respective brake circuit, wherein the outlet valve is arranged in a hydraulic connection between the wheel brake with which it is associated and a pressure medium storage container and wherein no further valve is arranged between the respective one of the at least one outlet valve and the pressure medium storage container.

2. The brake system according to claim 1, wherein a respective one of the at least one outlet valve is associated with a front wheel brake.

3. The brake system according to claim 1, wherein the pressure supply unit has only one working chamber, wherein the one working chamber is connected by means of two further hydraulic connecting lines to the respective hydraulic connecting lines of the respective brake circuits or to a working chamber of the first piston-cylinder unit, the brake system further including at least one isolating valve arranged in at least a respective one of the two further hydraulic connecting lines.

4. The brake system according to claim 1, wherein the control device has a memory which contains an actual wheel brake pressure set in a respective wheel brake, wherein, by means of the stored actual wheel brake pressure and a nominal wheel brake pressure, the control unit is configured to determine by taking into account at least one pressure-volume curve of the one or more of the wheel brakes, a calculated time for which a respective outlet valve is to be opened in order to reach the nominal wheel brake pressure in the one or more of the wheel brakes, and wherein the control device is configured to close the respective outlet valve after expiry of the calculated time.

5. The brake system according to claim 4, wherein the control device is configured to reduce brake pressure in the one or more wheel brakes with which an outlet valve is/are associated, via the associated outlet valve, which is opened for the precalculated time, into the pressure medium storage container, wherein, for simultaneous pressure reduction or pressure build-up another wheel brake of a given one of the brake circuits and/or at least one further wheel brake of another one of the brake circuits, the control device opens the respective associated switching valve(s) and sets or regulates the nominal brake pressure in the respective associated wheel brake by means of corresponding activation of the pressure supply unit.

6. The brake system according to claim 1, wherein an outlet valve associated with a wheel brake of one of the brake circuits serves for pressure reduction in another one of the wheel brakes of the one of the brake circuits, wherein for common pressure reduction in one of the brake circuits, the control device is configured to open both the switching valves and the respective outlet valve associated with the one of the brake circuits.

7. The brake system according to claim 6, wherein during opening of the switching valves associated with the one of the brake circuits, the control device is configured to separate the one of the brake circuits from the pressure supply unit by closing an associated isolating valve or by locking the piston of the first piston-cylinder unit.

8. The brake system according to claim 1, wherein, for simultaneous or temporally offset pressure reduction and/or pressure build-up using of the pressure supply unit, the control unit is configured to set a pressure in its at least one pressure chamber (4, 4a) and to open and/or close the switching valves simultaneously and/or with temporal offset in order to set the nominal pressure required in the respective wheel brakes, wherein, by means of the at least one outlet valve, pressure is reduced in the wheel brake or brakes associated with the at least one outlet valve, independently of the pressure supply unit.

9. The brake system according to claim 1, wherein for simultaneous and/or temporally offset pressure reduction and/or pressure build-up, the control device is configured to actuate the pressure supply unit, taking into account wheel brake pressures calculated or prevailing in the wheel brakes, wherein the pressure supply unit is configured to generate a nominal pressure to be set for a respective wheel brake with a respective open switching valve, and after reaching the nominal pressure, the control device is configured to close the respective switching valve to maintain the nominal pressure in the respective wheel brake, wherein the at least one outlet valve is configured to enable pressure to be reduced in a wheel brake associated with a respective one of the at least one outlet valve independently of the pressure-generating unit.

10. The brake system according to claim 1, wherein at least one pressure sensor serves to determine the pressure in at least one brake circuit.

11. The brake system according to claim 1, wherein the control device has a memory configured to store actual wheel brake pressures set in each wheel brake and/or probable wheel brake pressures of each wheel brake continuously calculated and updated with measured values in a control model, wherein, for simultaneous pressure reduction in at least two wheel brakes of one brake circuit, to different nominal pressures in the at least two wheel brakes, the control unit is configured to evaluate a pressure-volume curve of each of the at least two wheel brakes of the one brake circuit and, using a lowest nominal pressure to be generated, is configured to calculate a piston travel of a piston of the pressure supply unit required and to move the piston of the pressure supply unit by the piston travel by means of the electric motor of the pressure supply unit, wherein a switching valve of a particular wheel brake with the lowest nominal pressure remains open until the nominal pressure has been set in the particular wheel brake; and wherein, for the switching valves of the wheel brakes not having the lowest nominal pressure, the control device is configured to individually calculate temporal durations for which the switching valves of the wheel brakes not having the lowest nominal pressure are to remain open so that the pressures in the respective wheel brakes are reduced to respective nominal pressures, and to open the switching valves only for the respective calculated temporal durations.

12. The brake system according to claim 11, wherein the switching valve of the particular wheel brake is opened first at a time at which pressure in an associated brake circuit is equal to or higher than an actual pressure in the particular wheel brake.

13. The brake system according to claim 10, wherein at the same time as, or temporally overlapping, the pressure reduction in the wheel brakes over the brake circuit(s), the pressure is reduced in at least one further wheel brake by opening of an associated outlet valve.

14. The brake system according to claim 1, wherein the control device has a memory configured to store an actual brake pressure set in each wheel brake and/or probable wheel brake pressures of each wheel brake continuously calculated and updated with measured values in a control model, wherein for simultaneous pressure build-up in at least two wheel brakes of one brake circuit, to different nominal pressures in the wheel brakes, the control unit is configured to evaluate a respective pressure-volume curve of each of the wheel brakes concerned and to use a highest nominal pressure to be generated to calculate a piston travel of a piston of the pressure supply unit required for this and to move the piston of the pressure supply unit by the piston travel, using the electric motor of the pressure supply unit, wherein a switching valve of a particular wheel brake with a highest nominal pressure remains open until the nominal pressure has been set in the particular wheel brake; and wherein, for the switching valves of the wheel brakes not having the highest nominal pressure, the control device is configured to individually calculate temporal durations for which the switching valves of the wheel brakes not having the highest nominal pressure are to remain open so that the pressure in the respective wheel brakes not having the highest nominal pressure is built up to respective nominal pressures, and to open the respective switching valves only for the respective calculated temporal durations.

15. The brake system according to claim 14, wherein the switching valve of the particular wheel brake is opened first at a time at which pressure in an associated brake circuit is equal to or lower than an actual pressure in the particular wheel brake.

16. The brake system according to claim 14, wherein at the same time as, or temporally overlapping, the pressure build-up in the wheel brakes over the brake circuit(s), the pressure is reduced in at least one further wheel brake by opening of an associated outlet valve.

17. The brake system according to claim 1, wherein the pressure supply unit is a piston-cylinder system have a single piston, wherein the single piston is driven by the electric motor and delimits at least one working chamber.

18. The brake system according to claim 12, wherein a piston of the pressure supply unit separates a first working chamber and a second working chamber tightly from each other, wherein both the first and the second working chamber are enabled, by corresponding actuation of isolating valves, to be used for simultaneous and/or temporally offset pressure build-up and pressure reduction in the wheel brakes of the brake circuits.

19. The brake system according to claim 1, wherein, for pressure build-up and/or pressure reduction in at least one wheel brake, the control device is configured to evaluate a pressure-volume curve of each wheel brake and from a respective pressure rise or fall to be generated, to calculate piston travel of a piston of the pressure supply unit required for the respective pressure rise or fall to be generated, and to correspondingly actuate required valves, and to move the piston of the pressure supply unit by an amount of the piston travel, using the electric motor.

20. The brake system according to claim 1, wherein the control device is configured to determine a pressure in one brake circuit by means of a pressure sensor, and to determine a pressure in another brake circuit via a phase current of the electric motor of the power supply unit, and to take into account determined or calculated brake circuit pressure in controlling pressure build-up or pressure reduction in the wheel brakes.

21. The brake system according to claim 1, wherein the switching valves are digital valves that are open in an unpowered state.

22. The brake system according to claim 1, wherein an interior or armature housing of an inlet valve of a respective wheel brake is connected via a hydraulic line to an associated actuator/brake circuit that includes the respective wheel brake, and wherein a valve seat outlet of the inlet valve is connected via a hydraulic line to the respective wheel brake.

23. The brake system according to claim 1, wherein a working chamber of the pressure supply unit is connected via a hydraulic connection to the pressure medium storage container, the brake system further including a switchable valve configured to shut off the hydraulic connection such that a pressure reduction in at least one wheel brake is enabled to take place time-controlled by opening the associated switching valve of the at least one wheel brake and by opening the switchable valve, wherein a predefined opening time determines the wheel brake pressure to be set.

24. The brake system according to claim 23, wherein, via another pressure chamber of the pressure supply unit, by means of the pressure supply unit, a volume-controlled pressure reduction or pressure build-up is enabled to take place in at least one wheel brake when the switching valves of the respective wheel brakes are open simultaneously.

25. The brake system according to claim 1, wherein the pressure supply unit comprises one pressure chamber, wherein the one pressure chamber is connected via a hydraulic connection to a working chamber of the first piston-cylinder unit, wherein the hydraulic connection is enabled to be shut off by movement of the piston of the first piston-cylinder unit into a shut-off position.

26. The brake system according to claim 1, wherein, on pressure build-up and/or on pressure reduction, the control device is configured to take into account an absolute position of a piston of the pressure supply unit and to determine a necessary movement travel of the piston of the pressure supply unit, depending on actual pressures, nominal pressures and pressure-volume curves of the respective wheel brakes involved in the pressure build-up or reduction.

27. The brake system according to claim 26, wherein, in normal operation with normal brake force amplification, recuperation, and anti-lock braking system (ABS), at low ?, pressure is built up and reduced simultaneously or sequentially in all wheel brakes via the movement travel of the piston of the pressure supply unit, taking into account the respective pressure-volume curves, and wherein, in operating situations with ABS at high ?, ? split, electronic stability program (ESP), and anti-slip regulation (ASR), pressure in at least one wheel brake is reduced via at least one associated outlet valve and/or via one or more outlet valves of the pressure supply unit at the same time.

28. The brake system according to claim 1, wherein, by means of the actuation device, upon failure of the pressure supply unit or upon occurrence of another fault, the at least one piston of the first piston-cylinder device is enabled to be set to build up pressure in at least one wheel brake.

29. The brake system according to claim 1, wherein pressure reduction takes place via a travel-controlled stroke of a double-stroke piston of the pressure supply unit or via a connecting line of a pressure chamber of the double-stroke piston with an open valve into the storage container, wherein the control device is configured to use a pressure measured in a respective brake circuit or a pressure calculated to control the pressure reduction.

30. The brake system according to claim 1, wherein a pressure reduction takes place at high pressures of approximately 200 bar down to pressures in a range close to blocking pressure in normal operation, of approximately 80-100 bar, via pressure or time control of a switching valve disposed in a hydraulic line between a working chamber of the pressure supply unit and the storage container, with optionally simultaneously or temporally offset travel control of a double-stroke piston of the pressure supply unit in advance stroke mode and subsequent pressure reduction to atmospheric pressure by pressure-volume control in return stroke mode of the double-stroke piston.

31. The brake system according to claim 18, wherein the isolating valves are designed for high throughflow and low differential pressures.

32. The brake system according to claim 31, wherein, by means of the pressure supply device before or during opening of an isolating valve, the control unit is configured to set a pressure in a corresponding pressure chamber of the pressure supply unit by a movement of the piston of the pressure supply unit that is so great that a sufficiently small differential pressure is created at the isolating valve to be opened.

33. A method of testing tightness and movability of a floating piston of a master brake cylinder of a brake system, wherein the master brake cylinder has a radially running channel in its cylinder wall which opens into a first pressure chamber of the master brake cylinder, wherein a mouth opening of the channel is configured to be is closed by the floating piston as soon as the floating piston has been moved out of its normal position by an amount corresponding to a diameter of the mouth opening, wherein the channel is hydraulically connected to a pressure chamber of a pressure supply unit, wherein a mechanical stop is arranged in the master brake cylinder and a spring is configured to force-load the floating piston in a direction of the stop, wherein the floating piston is in its normal position when it lies on the stop, the method including: a. building up a pressure, via the pressure supply unit, a in first pressure chamber and in a second pressure chamber of the master brake cylinder, and switching valves such that the pressure in the second pressure chamber of the master brake cylinder is greater than in the first pressure chamber of the master brake cylinder, such that the floating piston moves and closes the channel, thereby closing off the connection between the channel and the pressure supply unit, with open switching valves associated with respective wheel brakes in a first brake circuit and with closed switching valves associated with respective wheel brakes in a second brake circuit; b. producing, by means of the pressure supply unit, a test travel profile of the floating piston and/or a temporal pressure-volume curve in one of the pressure chambers of the master brake cylinder; c. using a signal from a pressure sensor, comparing a pressure rise determined from said producing, to a nominal value curve with open switching valves associated with respective wheel brakes, and evaluating a comparison result.

34. The method of testing the tightness and movability of the floating piston of the master brake cylinder of a brake system according to claim 33, wherein the method further includes: a) storing pressure in the second brake circuit by closure of the valves of the second brake circuit during a parking stop with a vehicle incorporating a brake system comprising the components found in claim 33 stationary; b) subsequently, reducing pressure in the first brake circuit, by means of the pressure supply unit, to 0 to 1 bar; c) subsequently blocking the pressure supply unit, such that no further piston movement of the piston of the pressure supply unit takes place, wherein a control device closes an isolating valve disposed in a hydraulic connecting line between the pressure supply unit an the second brake circuit; d) then, opening the switching valves of the second brake circuit, whereby because of the pressure on a back side of the floating piston of the master brake cylinder, the floating piston moves and thus builds up pressure in the first brake circuit until a pressure equilibrium exists between the first and second brake circuits; and e) measuring pressure in at least one of the first or second brake circuits by means of a pressure sensor in at least one of the first or second brake circuits, and evaluating a pressure curve.

35. A method of reducing pressure at greater than or equal to 100 bar in a brake system with: an actuation device, a travel simulator configured to generate a feedback force on the actuation device, a first piston-cylinder unit, having at least one piston that separates two working chambers from each other, wherein each working chamber is connected via at least one hydraulic connecting line to at least one wheel brake of a brake circuit, wherein at least one wheel brake is assigned to each brake circuit and each wheel brake is enabled to be connected to an associated one of the hydraulic connecting lines via an associated controllable switching valve, a control device, at least one pressure supply unit configured to be driven by an electric motor and having at least one working chamber, wherein the at least one pressure supply unit is configured to build up and/or reduce brake pressure in one or more of the wheel brakes simultaneously or successively, and at least one outlet valve, wherein a respective one of the at least one outlet valve is associated only with a single wheel brake or is associated with one wheel brake of a respective brake circuit, wherein the outlet valve is arranged in a hydraulic connection between the wheel brake with which it is associated and a pressure medium storage container, and wherein no further valve is arranged between the respective one of the at least one outlet valve and the pressure medium storage container, wherein the method includes: a. reducing pressure via pressure control and pressure measurement via a valve disposed between a working chamber of the pressure supply unit and the pressure medium storage container and opening of the switching valves to the respective wheel brakes, b. moving a double-stroke piston of the pressure supply unit in advance stroke mode with simultaneous opening of one or more valve(s) that connect the two working chambers of the first piston-cylinder unit; c. further reducing pressure by means of the double-stroke piston in return stroke mode via pressure-volume control with pressure measurement via a pressure sensor; and d. positioning the double-stroke piston in a starting position corresponding to an initial position for atmospheric pressure, and subsequently delivering of hydraulic medium to the pressure supply unit via check valves.

Description

DESCRIPTION OF THE DRAWINGS

[0059] FIG. 1a: shows a first possible embodiment of the brake system according to the invention with master brake cylinder, pressure supply with outlet valve(s) in one or two brake circuits;

[0060] FIG. 1b: shows an example of a simplified pressure-volume curve;

[0061] FIG. 1c: shows pressure regulation options in the basic system of FIG. 1a;

[0062] FIG. 1d: shows valve circuit with AV/EV in the regulation systems of the prior art;

[0063] FIG. 1e: shows advantageous valve circuit for new regulation system with switching valves and an outlet valve in one brake circuit;

[0064] FIG. 1f: shows inlet valve according to the invention in the brake circuit;

[0065] FIG. 2a: shows pressure-volume control in a closed brake circuit (AV, ZAV closed);

[0066] FIG. 2b: shows pressure-volume curve for wheel brake 1 and wheel brake 2 with offset shift by opening of brake circuit;

[0067] FIG. 3: shows conventional multiplex regulation in sequential order;

[0068] FIG. 3a: shows cycle shortening of multiplex regulation with AV valve in pressure reduction;

[0069] FIG. 3b: shows time control on pressure reduction via outlet valve;

[0070] FIG. 4: shows temporal development of an exemplary regulation with 4 wheel brakes;

[0071] FIG. 4a shows temporal development of an exemplary regulation with 4 wheel brakes;

[0072] FIG. 4b: shows temporal development of an exemplary regulation with 4 wheel brakes;

[0073] FIG. 5: shows advantageous brake system structure with double-stroke piston (DHK);

[0074] FIG. 5a shows pressure build-up regulation in multiplex mode according to the invention with DHK and outlet valve;

[0075] FIG. 5b shows simultaneous pressure build-up and pressure reduction in multiplex mode in DHK return stroke and outlet valve;

[0076] FIG. 5c shows simultaneous pressure build-up and pressure reduction in multiplex mode in DHK advance stroke and outlet valve;

[0077] FIG. 5d shows regulated or controlled pressure reduction in both brake circuits in closed circuit via pressure-generating unit and PD3 valve;

[0078] FIG. 6: shows double-stroke piston system in advantageous dual circuit design.

DESCRIPTION OF THE FIGURES

[0079] FIG. 1a describes the basic embodiment of the brake system according to the invention with master brake cylinder HZE, pressure supply unit DE with single-stroke piston (3) and outlet valve(s) AV1, AV3 in one or optionally two brake circuits. Brake circuit II is advantageously assigned to the front axle. The outlet valve AV1 is optional, i.e. need not necessarily be provided.

[0080] The brake system consists of a master brake cylinder according to the prior art, comprising a master brake cylinder unit HZE, floating piston SK with return spring 1, a pressure piston DK or ram or an auxiliary piston HiKo, a hydraulically actuated travel simulator WS and corresponding control valves HZV for the function of the piston-cylinder unit, as described for example in the prior art.

[0081] The following embodiments amongst others are possible: [0082] a) master brake cylinder with two pistons in the form of a pressure piston DK and a floating piston SK with connected travel simulator which can be shut off via a valve, [0083] b) 3-piston system with auxiliary piston HS for travel simulator actuation and infeed valve and/or mechanical intervention in the event of a fault, [0084] c) 2-piston system with floating piston SK and auxiliary piston HiKo with infeed.

[0085] In all embodiments, the master brake cylinder unit HZE can be isolated from the pressure supply unit DE. According to variant Var2, this can be achieved via isolating valves TV1 and TV2, or in the second variant Var1 shown, via blocking of the supply of the floating piston. The valve circuit of the HZE ensures that no undesirable feedback occurs to the pedal BP when the pressure supply unit DE is active, and in fall-back level (system failure) the volume of the master brake cylinder unit HZE is guided to the wheel brakes RB1-4. Also, a switching valve SV1-4 is arranged for each wheel brake in the hydraulic connection to the respective associated working chamber A1 or A2 of the brake master cylinder HZE. The concrete embodiment of the master brake cylinder HZE is not however relevant for the brake system according to the invention.

[0086] The brake system has four switching valves SV1, SV2, SV3 and SV4, via which the pressure supply DE and the master brake cylinder HZE are connected to the wheel brakes RB1-4. The switching valves SV1-4 preferably have a low flow resistance and are suitable for MUX operation. In addition, an outlet valve AV3 is provided in a brake circuit for pressure reduction in the wheel brake in RB3 independently of the MUX, and is arranged in the hydraulic connection between the wheel brake RB3 and the storage container 10. Preferably, the outlet valve AV3 is positioned on the front wheel brake RB3 of a brake circuit since, in extreme cases, the pressure in this wheel brake must be reduced quickly and without great time delay because the significant braking effect originates from the front axle.

[0087] The pressure supply unit DE consists of an electric motor M, which via a spindle 2 drives a piston 3 which compresses or shifts the volume in the pressure chamber 4. The motor M of the pressure supply unit may comprise two or three sensors: a) angle sensor 6, b) current measurement sensor for measuring the phase currents of the electric motor 7, and c) if necessary, a temperature sensor 8 for determining the coil temperature of the electric motor M.

[0088] The pressure-generating unit DE is preferably arranged in the valve block or HZE. The pressure chamber 4 of the pressure-generating unit DE is connected to a storage container 10 via a check valve 5. A pressure sensor 9 is arranged at the outlet of the pressure-generating unit DE. The brake circuit II is connected via the isolating valve TV2, and brake circuit I via the isolating valve TV1, to the pressure supply unit DE. The isolating valve TV1 may be omitted if one chamber is separated by the pressure supply unit DE in fall-back level. This can be achieved by a pressure infeed from the pressure-generating unit DE via the blow hole SL of the floating piston SK.

[0089] For pressure modulation in ABS and recuperation, the control device and its regulator determine the necessary pressure change for pressure build-up (referred to below as P.sub.auf) and pressure reduction (referred to below as P.sub.ab). The pressure is regulated by the pressure-generating unit DE, in that simultaneously or with a temporal offset, the individual wheels/wheel cylinders are supplied with pressure. For this, the electric motor M shifts the corresponding volume for pressure change in both directions via e.g. the piston 3.

[0090] Here, the pressure change according to the prior art can be modified by corresponding time control with PMW of the switching valves and pressure control of the pressure of the DE. This however requires a very precise PWM process with complex pressure model. Preferably, therefore, the volume control is used as already described above. For this, the data of the pressure-volume curve (p-V curvesee FIGS. 1a and 2a) of the respective wheel brake RB1-4 involved in the pressure build-up or reduction are stored in the memory of the regulator unit. If now the regulator requests a pressure change ?p, for pressure regulation at the wheel the differential volume ?V is adjusted accordingly by the piston in both directions +?S. For this, one or more switching valves are opened which are closed again after completion of the volume shift. The position of the piston 3, e.g. at the start, middle or end of the stroke, is irrelevant for ?P volume control for the regulation. Here, during the pressure change, a temporal control may be used in order to implement transition functions towards the end of the pressure change, e.g. to reduce the pressure fluctuations and the associated noise.

[0091] A high dynamic is important if two or more wheels require a pressure change simultaneously. For this, the invention proposes that to relieve the load on the motor dynamics, one or two additional outlet valves AV are used. For volume control, in particular also the pressure level in the pressure-generating unit DE and in the wheels is important. It is favourable here that the pressure level on pressure change corresponds to the starting pressure of the wheel to be regulated. This achieves a rapid and low-noise pressure regulation. The temporal developments are illustrated in FIGS. 3, 3a, 3b and 4, 4a, 4b. Suitable pressure-generating units DE are all pumps with single piston, stepped piston, double-stroke piston and also e.g. gear pumps which allow precise volume control.

[0092] In FIG. 1a, for the above-mentioned functions, the pressure generation takes place in one circuit directly via an isolating valve TV1 in BK1 (Var2) or alternatively via a blow hole SL on the front of the SK piston (Var1). The pressure is supplied to brake circuit BK2 via an isolating valve TV2. For infeed via the blow hole SL on the front of the floating piston SK, optionally the isolating valve TV1 may be omitted since, on system failure, the pressure-generating unit DE is isolated from the master brake cylinder effect because piston SK moves and shuts off the pressure supply DE. Alternatively, as drawn in dotted lines, the pressure-generating unit DE may be connected directly to BK1 via TV1 (Var2). Since the SK piston in Var1 is only moved in fall-back level, a special diagnosis circuit is required in which the floating piston SK is moved and checked for tightness.

[0093] FIG. 1b describes the known pressure control based on a simplified pressure-volume curve which forms the basis of the MUX regulation. Depending on the required pressure difference ?p, from the curve a volume change ?V is read which is implemented as a travel change As of the piston 3 by shifting the plunger of the pressure-generating unit DE. This applies for both pressure build-up and pressure reduction.

[0094] FIG. 1c shows a fundamental possibility for pressure regulation in the basic embodiment of FIG. 1. The system itself has the following degrees of freedom in pressure regulation: [0095] pressure build-up and pressure reduction in all brake circuits BKI and BKII mainly with multiplex regulation (pressure regulation with pressure-volume control) in all wheel brake cylinders simultaneously or sequentially; [0096] multiplex regulation in pressure build-up and pressure reduction in brake circuit I via the opened isolating valve TV1 and simultaneous pressure reduction in brake circuit II via the outlet valve AV3; [0097] multiplex regulation via SV1, SV2, SV4 in pressure build-up and pressure reduction in brake circuit I and II for wheel brakes RB1, RB2 and RB4, and simultaneous pressure reduction in wheel brake RB3 via opened outlet valve AV3 with closed switching valve SV3.

[0098] The pressure reduction p.sub.ab via switching valves SV1 and SV2 in BK I takes place mainly via pressure-volume control, sequentially or simultaneously. For this, the respective switching valve SVi is always opened. For simultaneous pressure reduction p.sub.ab at different starting pressures, optionally by deviation from the MUX regulation, switching valves SV1 and SV2 may be opened with a time offset, and pressure reduction p.sub.ab controlled via a switching valve SV2. The isolating valve TV1 is always opened on pressure reduction. In this exemplary embodiment, the wheel brake RB1 has a higher pressure, therefore the associated switching valve SV1 is opened before the switching valve SV2. On the basis of knowledge of the pressure differencethe pressures in wheel brakes RB1 and RB2 and the pressure in the pressure-generating unit DE are knownthe time control may be dimensioned precisely. SV2 is opened when the pressure in the pressure-generating unit DE is approximately reached. Further pressure reduction then takes place simultaneously in both wheel brake cylinders RB1 and RB2 by control via piston 3 when switching valves SV1, SV2 and TV1 are open. When the target pressure of a wheel is reached, the corresponding switching valve SV1 or SV2 is closed. If further pressure reduction is desired in one wheel, the further pressure reduction can take place in the respective wheel brake.

[0099] As already described, to simplify the system, preferably the PWM control is omitted, in particular also for noise reduction.

[0100] Exemplary temporal curves of the pressure reduction are described in FIGS. 4a to 4c.

[0101] FIG. 1d shows a conventional valve circuit for ABS with four inlet valves EV and four outlet valves AV. If this is also used with fewer AV for MUX, e.g. with differential pressure in MUX, as well as this pressure the fault case must be taken into account in which, on an asymmetric road surface, the pressure-generating unit and also the valve actuation fail suddenly e.g. due to the ECU, and at the same time the pressure-generating unit has a low pressure level. In this case for example, EV1 has 130 bar and EV2 0 bar. On failure of the pressure-generating unit, this means that for EV1 the return spring of the valve armature must open around 130 bar. To enable this, the magnet circuit of the valve must be sufficiently large, whereby the valve becomes costly. Alternatively, a pressure-relieved valve may be used, but its costs are also high.

[0102] In dimensioning of the valve seat, it must also be taken into account that this should be as large as possible in order to generate a small backup pressure if the brake pressure is to be built up rapidly by the pressure-generating unit. The backup pressure is introduced directly into the motor torque or power.

[0103] FIG. 1e shows a modified throughflow of the switching valves SV. The hydraulic medium flows from the brake circuit or pressure-generating unit through the armature chamber to the valve seat and onto the wheel cylinder. If the above fault case occurs, the wheel pressure opens the switching valve. The magnetic force must however also close around 130 bar, which takes place however with a small armature air gap in the valve end position. The return spring of the switching valve SV need therefore only be slightly strengthened so that the switching valve does not snatch at correspondingly high volume flow. Since conventional inlet valves must close at around 220 barin FIG. 1e, 130 bar, with the same magnet dimensioning the valve seat area may be increased which means a smaller backup pressure or flow resistance and is advantageous for MUX mode. The valve circuit depicted in FIG. 1e is therefore advantageous for the brake system according to the invention.

[0104] FIG. 1f shows a possible embodiment of the inlet valve EV according to the invention and the connection to the brake circuit BK and the pressure supply DV and wheel brakes RBi.

[0105] The inlet valve EV has a magnet armature MA, a magnetic base body MGK and an exciter coil ES. When the magnetic valve EV is powered, the magnet force MK shifts the armature MA from position S.sub.A0 to position S.sub.A2 by the differential travel S.sub.A. The magnet armature MA moves a ram MSt? by the same travel, so that the ram MSt? comes to rest on the valve seat VS and closes the outlet Ea of the magnetic valve. The armature MA at this point still has a residual air gap S.sub.0 from the magnetic base body MKG, which is provided so that the armature MA does not stick to the magnetic housing MGK when the power to the exciter coil ES of the valve EV is switched off, due to re-magnetisation losses of the iron circuit. When the valve current is switched off, the return spring RF moves the armature MA back to the starting position. The magnet force F.sub.M rises nonlinearly with a smaller air gap, e.g. with increasing travel. The return spring F.sub.RF is dimensioned such that the magnetic force F.sub.M in the starting position S.sub.A0 is greater than the spring force, so that a secure closure of the valve is guaranteed. The spring force increases with the increasing travel S.sub.A and in the end position S.sub.A2 is however smaller than the magnet force F.sub.M. Preferably, a linear spring is used, so that the magnet force F.sub.M in the end position for a given current is significantly higher than the return force, so that the valve can be retained with low current, or secure closure is guaranteed even at high differential pressures between the wheel brake and the pressure supply. This retention is also ensured at high differential pressures since the magnet force increases strongly nonlinearly at the closed valve position. The return spring must however also be dimensioned such that the function as an unpowered open valve can be ensured and the valve always opens safely.

[0106] The outlet E.sub.a of the valve is connected to the wheel brakes RBi (RB1-RB4), the inlet E, to one brake circuit BKi or to the pressure supply unit DV (20). With such connections, the inlet valve EV can be opened both by the return spring RF and by the pressure in the wheel brake, which is very important in particular in the event of a fault or malfunction in the brake system (e.g. loss of voltage to the valve). Also, even at high pressures in the brake circuit and small pressures in the wheel brake, only the pressure difference between inlet Ei and outlet Ea acts on the ram MSt?. This differential pressure at the valve is relatively low in pressure build-up, but must however be taken into account in the spring design RF so that the pressure difference does not lead to the valve being pushed back on pressure build-up when the volume of the pressure supply DV is delivered to the wheel brake. Valves with large opening cross-section ?Q or low flow losses reduce this effect.

[0107] In particular on pressure build-up with pressure-volume control or time control with low differential pressure between the pre-pressure and actual pressure in the wheel brake, the valves described above with large opening cross-section may be used since the regulation accuracy is very high. This in turn has advantages in that only low flow losses occur, in particular with rapid pressure build-up (TTL), and the drive motor requires only a low power for rapid pressure build-up in a very short time (TTL=150 ms).

[0108] Also, because of the low flow losses of the advantageously configured inlet valves, a pressure reduction can take place quickly via the inlet valves. Precise pressure reduction via the inlet valves EV can take place with corresponding control of the piston movement of the pressure supply unit 20. Optionally, it is also possible to implement the known MUX process with the valve circuit described above, or with pressure reduction control via outlet valves AV in one brake circuit, in particular for consumers with low volume balance, e.g. the wheel brakes on the rear axle. In other words, a combination is also possible which uses the MUX process in connection with the new valve circuit only in two wheel brakes (e.g. front axle), and the pressure reduction takes place conventionally on two further wheel brakes. This would mean that two wheel brakes/actuators are provided with inlet and outlet valves (EV+AV) and two wheel brakes/actuators only with inlet or switching valves EV. In this case, only the wheel brakes of the front axle are equipped with the new valve circuit according to the invention as shown in FIGS. 1a and 1b, and a standard circuit/standard valves are used on the rear axle.

[0109] FIG. 2a shows the pressure-volume curve of the wheel/wheel cylinder with connecting lines as far as the switching valve SV and pressure sensor. Two curves are shown. Curve P.sub.aufa corresponds to a so-called stiff curve, the other curve P.sub.auf requires substantially more volume. This may in extreme cases cause vapour bubbles due to e.g. play or poor purging.

[0110] This means that the values for V.sub.a e.g. for ?P=P.sub.1?P.sub.2 are equal to V.sub.1?V.sub.2=?V.sub.a=?S.sub.a and at V.sub.auf=A.sub.p equal to V.sub.1a?V.sub.2a=?V=?S. This curve for p.sub.auf and p.sub.ab is stored e.g. for the first time on line-end tests in the memory of the control device both for the individual wheel brakes and for the brake circuits for both p.sub.auf and p.sub.ab. On each braking, the curve is measured by comparison of pressure P with the volume V.sub.(?S). If a great deviation occurs, with a stationary vehicle, the curves can be recorded or adapted as in the above-mentioned test. It is also significant that the values can fluctuate between P.sub.auf and P.sub.ab. It is normal that due to play, V.sub.0 is greater on pressure build-up P.sub.auf but not on pressure reduction P.sub.ab. When the play has been eliminated, the curves are almost equal.

[0111] With poor purging or vapour bubbles, the curves behave similarly but with greater volume for the corresponding pressure value.

[0112] For regulation, the p-V curves are used for pressure build-up P.sub.auf and pressure reduction p.sub.ab.

[0113] FIG. 2b describes the simplified relevant pressure-volume curves for pressure regulation without hysteresis in a closed brake circuit or shifting after pressure reduction with opened outlet valve AV. Starting from a pressure p1, by defining a nominal differential pressure ?p, the necessary volume shift ?V or travel change ?s of the piston can be read from the curve. These differ and are dependent on whether the pressure is changed in one or more brake circuits. The piston is then moved accordingly. If the pressure is reduced via one or more outlet valves, there is a volume loss in the pressure-generating unit. For further pressure reduction or pressure build-up in the closed brake circuit, the travel allocation of the pressure-volume curve is determined by detecting the pressure. This is required in regulation for monitoring the volume balance, since the working chamber of the pressure-generating unit only has a limited volume and thus towards the end of the piston stroke, the piston would travel to the stop. If the piston of the pressure-generating unit travels close to the stop after a pressure change and a further pressure rise is impending, the piston is retracted briefly with closed switching valves SV in order to draw in volume from the storage container. In the design with double-stroke piston (FIG. 5-6), this is retracted or switched to return stroke mode.

[0114] FIG. 3 shows a time development of the MUX regulation as known from WO 2006/111393 A1 or WO 2010/091883 A1. This system is known as the 4-channel MUX in which, except in the critical case of simultaneous pressure reduction (simultaneous p.sub.ab), the pressure reduction p.sub.ab is processed serially per wheel channel (cylinder). In the worst case scenario, this leads to a great delay time which causes large speed differences or even slip because of the individual response times of the valve, motor and the time for the respective pressure reduction p.sub.ab. This both reduces the stability of the braking and disadvantageously extends the braking distance. Optimisation is performed on the response time of the switching valves, motor and pressure reduction gradients. Costs however limit optimisation. The case of simultaneous pressure reduction p.sub.ab for all channels however occurs rarely in practice.

[0115] A further restriction exists in the regulation concept known from WO 2006/111393 A1 or WO 2010/091883 A1 in the necessary priority for the pressure reduction p.sub.ab. If a pressure reduction is required, no pressure build-up p.sub.auf can take place. Since usually the time for pressure build-up p.sub.auf in the regulation cycle is around 200 ms, and two or three small p.sub.auf take place per control cycle each with approximately 10 ms delay time, this was not considered critical but is noted as a minor defect of the 4-channel MUX.

[0116] The brake system according to the invention with its regulation concept offers the following improvements: [0117] introduction of an additional outlet valve on the front axle; [0118] various control methods and strategies for control and regulation of the wheel brakes, e.g. VA, corner braking; [0119] possibility of pressure build-up p.sub.auf with simultaneous pressure reduction p.sub.ab (described in FIG. 5b and FIG. 5c).

[0120] FIG. 3 shows the pressure curve in the individual wheel brakes over time for conventional MUX mode in which the pressure in the wheel brakes is reduced temporally successively. V1 and V2 are the front wheel brakes, H1 and H2 the rear wheel brakes. At X, the signal is given for simultaneous pressure reduction p.sub.ab. The response time t.sub.vSV of the switching valve SV is around 5 ms. The response time t.sub.vM of the motor is around 10 ms. It is taken into account here that the pistons of the pressure-generating unit must first be positioned at (1) for the different pressure levels of the individual wheels before the pressure change. Then the pressure reduction p.sub.ab takes place, assuming t.sub.ab of around 10 ms, wherein during this time the pressure is reduced by ?p equal to around 20 bar.

[0121] The response times for the switching valves and motor assumed in FIG. 3 are also assumed, for an objective comparison, for the depiction of the temporal pressure development in FIG. 3a which corresponds to the pressure development according to the invention.

[0122] With conventional ABS systems with 4 inlet and 4 outlet valves, the ABS regulator always determines a ?p and then determines the time for which the outlet valve must be opened in order for the required pressure reduction to take place in the wheel brake. This time control is known to be subject to tolerances, which limits the precision of the pressure regulation. Also, on closure of the outlet valve AV, pressure fluctuations always occur which cause disadvantageous noise.

[0123] The ABS regulator here determines the necessary pressure difference ?p substantially from the wheel angular acceleration and partially from the wheel slip, with correction factors for a) wheel inertia moment, b) gear stage and c) fading detection.

[0124] In contrast to time control, in conventional MUX as shown in FIGS. 1 and 1a, a volume control of the pressure supply is used in which ?v=?p, wherein this takes place with evaluation of the pressure-volume curve of the wheel. Thus the precision of the pressure regulation is much greater, and the temporal pressure development can be influenced towards the end of the pressure reduction so that only slight pressure fluctuations occur.

[0125] At V1 in FIG. 3, after (X), the response time of t.sub.vM and t.sub.v of SV1 acts. After opening of the switching valve SV1, the motor M is able to reduce the pressure over the time t.sub.ab. Then SV1 is closed again at (2). First however, the motor has already reached the required pressure level via the described volume control of the pressure supply.

[0126] Then the motor already begins the pressure reduction p.sub.ab of the front wheel V2, which takes place after opening of SV2 of V2. Thus the sequence V1-H2 has a total delay time of 60 ms with the above assumptions. This corresponds approximately to a regulation deviation ?v of around 15 km/h.

[0127] FIG. 3a shows the temporal pressure curve for the regulation concept according to the invention. The pressure reduction p.sub.ab at the front wheel brake V1 at time (1) corresponds to (1) in FIG. 3. At the front wheel brake V2, the additional outlet valve AV is used for pressure reduction p.sub.ab. The pressure reduction p.sub.ab takes place almost without delay at (11) via a time control ?t described above, at which at (12) after closure of SV2 the pressure fluctuation occurs. Thus regulation of the front wheels V1 and V2which at high ? make a considerably greater contribution to the brake force effect than the rear wheelsis almost not delayed. During pressure reduction p.sub.ab of V1, the motor at (13) is already prepared for pressure change at the rear wheel H1. This take place at (14) to (15) by volume control by means of the pressure-generating unit DE. Since the rear wheels H1, H2 often have the same higher pressure level than V1 or V2, a simultaneous pressure reduction p.sub.ab can take place by volume control. Alternatively, at H2 at (16) a time-controlled pressure reduction p.sub.ab into the brake circuit can take place. One condition for this is that the MUX pressure level in the wheel brake of the rear wheel H2 is lower than at the rear wheel H1, at which the pressure reduction takes place by means of volume control. It is also possible that the pressure reduction in the rear wheel brake H2 takes place by time-controlled opening of the associated outlet valve.

[0128] Variants of the front axle and rear axle regulation are shown in detail and described in FIGS. 4, 4a, 4b. A comparison of the delay times tv in relation to the conventional MUX methods in FIG. 3 show a significant improvement at (10) with 60 ms and (17) with 25 ms. This is possible by the use of the time-controlled pressure reduction p.sub.ab at V2 and H2 by means of outlet valve or into the brake circuit, partially simultaneous p.sub.ab at H1 and H2, and priority control at H1 and (13).

[0129] FIGS. 4-4b show the pressure development with different actual pressure levels and variants of the front and rear wheels H1, H2, V1 and V2 with the regulation concept according to the invention.

[0130] FIG. 4 shows the temporal pressure development V1 to H2 in different phases. Phase 0-X shows a pressure development in which, because of the different pressure levels in the wheel brakes, no simultaneous pressure build-up p.sub.auf and pressure reduction p.sub.ab can take place, which is most often the case. Consequently, here also full multiplex mode is active i.e. precise ?p regulation via volume control both for pressure build-up p.sub.auf and for pressure reduction p.sub.ab. At pressure build-up p.sub.auf, sometimes e.g. at 20 with simultaneous pressure build-up requirement, a temporally offset pressure build-up p.sub.auf takes place at H1 and H2. However also a partially simultaneous pressure build-up P.sub.auf may take place. Also, a partially simultaneous pressure reduction is possible as shown in FIG. 4b.

[0131] At X in FIG. 4, the signal p.sub.ab for simultaneous pressure reduction in all wheel brakes is given, which is implemented without time delay. These two variants A and B are shown in more detail in FIG. 4a and FIG. 4b.

[0132] FIG. 4a shows the variant A, again starting at point X, with p.sub.ab at V1 and V2 as described in FIG. 3a. At the front wheel V1, the pressure reduction p.sub.ab takes place by means of volume control by the pressure-generating unit DE. For the rear wheel H2, at 21 a controlled p.sub.ab takes place over time dt=f(dp). This pressure reduction takes place when a sufficient differential pressure ?p between H2-V1 is present. The pressure levels of all wheels are known in the regulation concept according to the invention or MUX method, so that a relatively precise pressure reduction p.sub.ab in the rear wheel H2 is achieved by the time control or opening of the switching valve SVH2. The necessary opening time ?t may, because of the change in pressure level, be adapted flexibly by M1 (MUX). The pressure reduction p.sub.ab of the rear wheel H1 also takes place via volume control, starting from the preparation at 13, and then by opening of the associated switching valve at 22.

[0133] As a result, there is a relatively small tvmax as described in FIG. 3a. At 11, the time-controlled pressure reduction p.sub.ab for the front wheel V2 takes place. Here again, the pressure difference from the storage container is known and hence a precise pressure control is possible by the time-controlled opening of the outlet valve.

[0134] FIG. 4b shows variant B for partially simultaneous pressure reduction at the rear wheels H1 and H2, starting from a relatively small pressure difference between the rear wheels H1 and H2. Here, after preparation at 13, at 23 the pressure reduction p.sub.ab takes place with MUX, i.e. volume control by means of the pressure-generating unit. At 24, the rear wheel H2 is switched to pressure reduction p.sub.ab by opening of the switching valve SVH2 assigned to the rear wheel H2. At 25, the controlled pressure reduction p.sub.ab for H2 is achieved, so the switching valve SVH2 is closed. At 26, via the volume control, the ?p for the rear wheel H1 is reached, so at 26 the switching valve SVH1 is closed.

[0135] Both methods allow a short delay time. In some cases, the controlled pressure reduction p.sub.ab causes the pressure fluctuations, which however only occur in extreme cases with simultaneous pressure reduction p.sub.ab.

[0136] To summarise and in addition, the following features apply: [0137] the pressure of each wheel at the start and end of the pressure reduction p.sub.ab (FIG. 4, 4a) is stored in the memory; the two values are used as reference for the subsequent pressure changes of the wheel or following wheels; [0138] the pressure of the last pressure build-up p.sub.auf (FIG. 4) is stored in the memory and thus forms the basis for setting the pressure of the pressure-generating unit DE in preparation for the following pressure reduction p.sub.ab; [0139] the outlet valves AV are time-controlled, wherein for this the pressure difference from the storage container is taken into account; one wheel of the front axle is connected to the brake circuit only via switching valve SV, wherein a switching valve SV towards the brake circuit and an outlet valve AV towards the storage container are assigned to the second wheel of the front axle, so that switching valves SV of the rear axle are time-controlled to p.sub.ab, while MUX controls the wheels of the front axle with low pressure level, in which the time control t.sub.ab of the switching valves SV of the rear axle HA evaluates the differential pressure ?p; [0140] as well as the differential pressure, for the time control (valves are opened for a predefined time), the ?p of the corresponding outlet valve AV is evaluated from the pressure-volume curve. Priority control of the MUX with orientation towards driving stability with braking distance, e.g. wheel of the front axle VA has priority and also, at positive ? jump with highest negative p.sub.ab or positive acceleration p.sub.auf, since here the ?p to be regulated is greatest; [0141] the pressure change by volume control by means of the pressure-generating unit, and the time-controlled opening of outlet valve or switching valve used in parallel, forms the combined MUX regulator; [0142] in time control, the corresponding volume must be taken into account in the volume delivery, corresponding to the pressure change ?p determined by the regulator.

[0143] FIG. 5 describes a further embodiment of the pressure supply unit DE according to the invention with pressure regulation with master brake cylinder, master brake cylinder valves HZV, pressure supply with double-stroke piston, switching and outlet valves.

[0144] The master brake cylinder HZE is connected to brake circuits BKI and BKII. For the separation logic, the same applies as in FIG. 1a. The advance stroke chamber 4 of the double-stroke piston 3 is connected to brake circuit BI via the isolating valve TV1, and to BK II via the isolating valve TV2. The return stroke chamber 4a is connected via the isolating valve TV2b to BK II and via HZE to BK I. The transmission preferably takes place by the floating piston SK. The advance stroke chamber 4 and return stroke chamber 4a can be connected together hydraulically via a switching valve ShV. This switching valve SvH allows a short-circuit between the two chambers, and is used in particular on advance stroke (towards the left) to reduce the hydraulically active area of the piston 3. The return stroke chamber 4a of DHK 3 is connected via the switching valve PD1 to the storage container 10. The two chambers 4 and 4a are also each connected via a check valve to the storage container 10. This system configuration offers the following degrees of freedom: [0145] pressure build-up and pressure reduction in all brake circuits with multiplex regulation (pressure regulation with pressure-volume control) in all wheel brake cylinders simultaneously or sequentially via isolating valves TV1, TV2 and PD1, and the switching valves SV1-SV4 of the wheel brakes RB1-RB4; [0146] multiplex regulation in pressure build-up and pressure reduction in brake circuit I and pressure reduction in brake circuit II via outlet valves AV3, ZAV; [0147] multiplex regulation with simultaneous pressure reduction in brake circuit BK I and pressure build-up in BK II via double-stroke piston control; [0148] multiplex regulation with simultaneous pressure reduction in brake circuit BK II and pressure build-up in BK I via double-stroke piston control; [0149] pressure reduction RB3 at any time via AV3 in multiplex regulation.

[0150] FIG. 5a shows as an example some of the pressure regulation possibilities which may take place temporally in parallel with each other: [0151] controlled pressure reduction in RB1 via SV1, TV1 by pressure-volume control by means of return stroke of the double-stroke piston 3 with open PD3 valve, or alternatively pressure control of pressure reduction via pressure estimation based on phase current measurement in brake circuit I; [0152] controlled pressure reduction in RB2 via SV2, TV1 via pressure-volume control by means of return stroke of double-stroke piston 3 with open PD3 valve, or alternatively pressure control of pressure reduction via pressure estimation based on phase current measurement in brake circuit I; [0153] pressure reduction in RB3 via AV3 with time control of outlet valve AV3; [0154] pressure reduction in RB4 by means of double-stroke piston 3 with time control of one of the switching valves SV4 or PD1, wherein the other valve must also be opened at this time, or alternatively pressure control of pressure reduction via pressure estimation based on pressure measurement in brake circuit II.

[0155] For simultaneous pressure reduction p.sub.ab at different starting pressures, optionally a deviation may occur from the MUX regulation in that the switching valves SV1 and SV2 are opened with time offset. The isolating valve TV1 is here opened continuously on pressure reduction. Since a higher pressure prevails in RB1, the switching valve SV1 is opened before switching valve SV2. On the basis of knowledge of the pressure difference (wheel pressure RB1 and RB2 and pressure in the advance stroke chamber of the pressure supply unit), the time control can be dimensioned precisely. If the pressure in the advance stroke chamber of the pressure supply unit DE is not determined precisely, because at the same time a pressure reduction via ZAV is taking place in wheel brake RB4, and TV2 is closed, the pressure in the advance stroke chamber may be used via pressure estimation p/i from the torque of the electric motor. The switching valve SV2 is open when the pressure of the pressure-generating unit DE is approximately reached. The further pressure reduction then takes place simultaneously in both wheel brake cylinders by control via piston 3 with open SV1, SV2 and TV1. When the target pressure of a respective wheel is reached, the corresponding valve SV1 or SV2 is closed. If further pressure reduction is required in a wheel, further pressure reduction can take place only in one wheel brake.

[0156] In parallel to the pressure reduction control in MUX mode, in BK II the pressure can be reduced by time control of AV3. This can be determined temporally freely because closure of SV3 does not influence the other wheel brake cylinders. Also, the temporal activation of the pressure reduction in wheel brake RB4 can be selected freely on pressure reduction of BK 1 in MUX mode.

[0157] FIG. 5b shows as an example some of the pressure regulation possibilities which may take place temporally in parallel with each other: [0158] controlled pressure reduction in RB1 via switching valves SV1 and TV1 via pressure-volume control by means of return stroke of double-stroke piston 3 with closed PD1 valve; [0159] controlled pressure reduction in RB2 via switching valves SV2 and TV1 via pressure-volume control by means of return stroke of double-stroke piston 3 with closed PD1 valve; [0160] pressure reduction in RB3 via outlet valve AV3 with time control ?t (opening of AV3 valve for period ?t); [0161] pressure build-up in RB4 via isolating valve TV2b (ZAV) with pressure-volume control by means of return stroke of double-stroke piston with closed PD1 valve.

[0162] For simultaneous pressure reduction and pressure build-up in wheel brake RB4, the pressure build-up dynamic is determined by the pressure reduction dynamic and the effective piston area and hydraulic differential pressures. This must be taken into account in the regulation. When the target pressure is reached in wheel brake RB4, the switching valve SV4 is closed. If the pressure in BKI is to be reduced further, PD1 is opened for further pressure reduction in brake circuit I.

[0163] FIG. 5c shows as an example some of the pressure regulation possibilities which may take place temporally in parallel with each other: [0164] controlled pressure reduction in RB1 via switching valves SV1 and TV1 via pressure-volume control by means of advance stroke of double-stroke piston 3 with open PD1 valve; [0165] controlled pressure reduction in RB2 via switching valves SV2 and TV1 via pressure-volume control by means of advance stroke of double-stroke piston 3 with open PD1 valve; [0166] pressure reduction in RB3 via outlet valve AV3 with time control of outlet valve AV3; [0167] pressure reduction in RB4 via double-stroke piston 3 with time control of switching valve SV4 or PD1 valve.

[0168] For the many functions of pressure reduction p.sub.ab in one brake circuit and pressure build-up p.sub.auf in the other brake circuit, it is possible for the floating piston SK of the master brake cylinder HZE to move. To prevent this, a blocking element SE may be arranged in BK1 or BK2 which acts directly on the SK as mechanical blocking. The blocking element may also be part of the HZV.

[0169] With this pressure regulation system, the functions described in 5b and 5c of p.sub.auf in one brake circuit and p.sub.ab in the other can be implemented independently of the pressure level of the brake circuits.

[0170] FIG. 5d shows as an example the pressure reduction in brake circuit I and brake circuit II which is implemented on pressure reduction from high pressures. With isolating valves TV1 and TV2 open, the following is carried out: [0171] controlled pressure build-up in RB1 by time control of valves SV1 and PD3 via pressure control of the pressure reduction by pressure estimation based on phase current measurement in brake circuit I; [0172] controlled pressure build-up in RB2 by time control of valves SV2 and PD3 via pressure control of the pressure reduction by pressure estimation based on phase current measurement in brake circuit I; [0173] controlled pressure build-up in RB3 by time control of valves SV3 and PD3 via pressure control of the pressure reduction by pressure measurement based on the pressure sensor in brake circuit II; [0174] controlled pressure build-up in RB4 by time control of valves SV4 and PD3 via pressure control of the pressure reduction by pressure measurement based on the pressure sensor in brake circuit II.

[0175] For pressure reduction for individual wheels, in the same way as shown in FIG. 5a, the switching valves SV1-SV4 may be switched with a temporal offset.

[0176] One possibility (not shown) is that of pressure reduction via PD1 valve which is similar to the process for PD3 valve. The pressure reduction may take place for all brake circuits via the PD1 valve. The pressure reduction may also take place via PD3 and PD1 valve. This is similar to FIG. 5a with the difference that the pressure of all wheel brakes is reduced via the pressure supply unit, and hence the advantages of pressure reduction in the closed brake circuit are obtained, which has safety advantages in particular after completion of a braking process (e.g. after ABS operation).

[0177] FIG. 6 describes a system with double-stroke piston in advantageous dual circuit design. The structure of THZ, DE and the valve circuits for pressure control ABS with MUX and AV is identical to FIG. 5c.

[0178] in contrast to FIG. 5c, the pressure supply in the advance stroke acts on brake circuit II and the back of the floating piston SK. This transmits the volume and pressure to brake circuit I. If the double-stroke piston has travelled close to the end position, it is reversed and operated in return stroke and acts on BK I. Then via the return stroke, pressure acts on the front side of the floating piston SK. This transmits the pressure again to brake circuit BK II. The SK piston is always active with its seals, as in the current THZ.

[0179] The double-stroke piston 3 also has a bypass valve ShV, which is switched substantially under three conditions: [0180] a) at high pressure, to reduce the piston force, the volume of the advance stroke is also conducted to the back of the double-stroke piston 3 to balance the pressure; [0181] b) in ABS regulation and also MUX regulation, the double-stroke piston 3 is switched in a single circuit via the ShV valve; [0182] c) pressure reduction p.sub.ab from high pressure level takes place simultaneously in both brake circuits BK I and BK II.

[0183] This valve circuit has the consequence for the floating piston position that the return spring 1 moves the floating piston SK to the right stop or locks it in the middle position. The pressure sensor 9 measures the pressure in BK II and, with a single circuit arrangement for the regulation and control functions, can evaluate the pressure in both brake circuits.

[0184] For special functions with pressure build-up P.sub.auf in BK I and pressure reduction p.sub.ab in BK II and vice versa, it is advantageous to place in the connection to THZ in BK II, or a blocking element SE in brake circuit BK 1 which prevents the movement of the floating piston SK. The blocking valve SE may also be part of the HVZ.

[0185] This system contains the additional potential for reducing the pressure in BK II via the DHK piston 3, and separately from BK I via the valves TV2b and PD1. This solution has advantages in use for different pressure level activation on the two axles in recuperation. For this, the blocking element SE must then be used at SK or in BK I.

[0186] The functions described in FIGS. 5-6 and the additional overlaid time control via outlet valve(s) give the MUX system according to the invention a very good performance with high regulation dynamic and precision, with significantly lower cost than individual wheel regulation with inlet and outlet valves.