DE102021205745A1 - Braking system for motor vehicles with an extended fallback level - Google Patents

Abstract

The invention relates to a braking system for motor vehicles, having a hydraulic braking system with a first hydraulic sub-circuit, which includes at least one front wheel brake of the motor vehicle, and with a second hydraulic sub-circuit, which includes at least one rear wheel brake of the motor vehicle and a linear actuator, the first hydraulic sub-circuit and the second hydraulic sub-circuit are connected via an electrically controllable circuit separating valve, and an electromechanical parking brake, which is set up to actuate the at least one rear wheel brake. To improve the redundancy of the system, it is provided that the brake system has a computing unit that is set up to detect a hydraulic leak in the at least one rear wheel brake and, in the event of a leak in the rear wheel brake, to hydraulically decouple the front wheel brakes from the rear wheel brakes, wherein when braking is requested, only the front wheel brakes are actuated by applying hydraulic pressure and the rear wheel brakes are actuated exclusively by means of the electromechanical parking brake.

Description

The invention relates to a braking system for motor vehicles, having a hydraulic braking system with a first hydraulic sub-circuit, which includes at least one front wheel brake of the motor vehicle, and with a second hydraulic sub-circuit, which includes at least one rear wheel brake of the motor vehicle and a linear actuator, the first hydraulic sub-circuit and the second hydraulic sub-circuit are connected via an electrically controllable circuit separating valve, and an electromechanical parking brake, which is set up to actuate the at least one rear wheel brake.

As a safety-critical device in a motor vehicle, braking systems require at least one fallback level, which ensures that the motor vehicle is braked in the event of a fault. From the WO 2017/055152 A1 a braking system is known which, in addition to a hydraulic service brake, includes an electromechanical parking brake. The hydraulic brake works in a normal operating mode according to the brake-by-wire method. In a fall-back mode, the driver is able to intervene manually on the hydraulic service brakes, with the electromechanical parking brake also being actuated. To prevent them from being overloaded, the entry threshold of the electromechanical parking brake is successively increased with each braking operation.

A particularly critical hazardous situation exists when hydraulic leaks occur in the hydraulic braking system. Over time, more and more brake fluid is lost through the leakage until the remaining brake fluid is insufficient to maintain at least a minimum braking force needed to brake the vehicle to a safe speed range. If the brake fluid level has fallen below a critical value, hydraulic braking power is no longer available.

It is therefore the object of the invention to specify a brake system which has increased operational readiness in the event of a hydraulic leak.

The object is achieved by a brake system for motor vehicles according to claim 1, having a hydraulic brake system with a first hydraulic sub-circuit, which includes at least one front wheel brake of the motor vehicle, and with a second hydraulic sub-circuit, which includes at least one rear wheel brake of the motor vehicle and a linear actuator, wherein the first hydraulic sub-circuit and the second hydraulic sub-circuit are connected via an electrically controllable circuit separating valve. The braking system also has an electromechanical parking brake, which is set up to actuate the at least one rear wheel brake. In addition, the braking system includes a computing unit that is set up to detect a hydraulic leak in the at least one rear wheel brake and, in the event of a leak in the rear wheel brake, to hydraulically decouple the front wheel brakes from the rear wheel brakes, with only the front wheel brakes being activated when there is a braking request a hydraulic pressure and the rear wheel brakes are operated exclusively by means of the electromechanical parking brake.

The complete separation of the controls for the rear wheel brakes and the front wheel brakes in two physically separate ways, namely the hydraulic actuation and the electromechanical actuation, prevents further loss of brake fluid. At the same time, it is ensured that there is no need to do without brake support on the rear axle. The vehicle can thus continue to be braked over a long period of time with a high deceleration.

In a linear actuator, to build up pressure, a piston is pushed axially into a hydraulic pressure chamber that is built in series with a rotation-translation gear. The motor shaft of an electric motor is converted into an axial displacement of the piston by the rotation-translation gear.

A movement of the linear actuator from its resting position forwards into the pressure chamber shifts brake fluid volume from the linear actuator via the open valves into the wheel brakes and thus causes pressure to build up. In the opposite case, the movement of the linear actuator back towards its rest position leads to a pressure reduction in the wheel brakes. A required system pressure is set using a suitable pressure regulator or a suitable pressure control system.

In normal operation, with such a power brake system, the driver actuates a pedal simulator, this pedal actuation being detected by pedal sensors and a corresponding desired pressure value for the linear actuator for actuating the wheel brakes being determined. The pedal simulator can, for example, be part of an electronic brake pedal without a hydraulic connection. Alternatively, the brake pedal can be integrated into the hydraulic system via a master brake cylinder and can be hydraulically connected to the pedal simulator in normal operation.

In a preferred embodiment of the invention, the at least one rear wheel brake and the at least one front wheel brake are hydraulically decoupled by closing the circuit isolating valve. The first hydraulic sub-circuit and the second hydraulic sub-circuit can thus regulate the hydraulic pressure completely independently of one another.

In a further preferred embodiment of the invention, the first hydraulic partial circuit has a brake master cylinder that can be actuated by a brake pedal, and the computing unit is set up to establish a flow-open connection between the brake master cylinder and the front wheel brakes in the event of a leak in the at least one rear wheel brake. This can be done by appropriately controlling the hydraulic valves between the master brake cylinder and the front wheel brakes. Due to the connection of the master brake cylinder to the front wheel brakes and simultaneous hydraulic decoupling to the rear wheel brakes, the driver only has to actuate a subset of the wheel brakes with muscle power. As a result, the braking force on the front wheel brakes can be greater.

In a particularly preferred embodiment of the invention, the flow-open connection is established by opening a master cylinder valve, which is arranged at an outlet of the master brake cylinder, and inlet valves of the at least one front wheel brake.

In an alternative preferred embodiment of the invention, the computing unit is set up to establish a flow-open connection between the linear actuator and the at least one front wheel brake in the event of a leak in the at least one rear wheel brake, with the hydraulic decoupling of the at least one rear wheel brake and the at least one front wheel brake being activated by closing at least one inlet valve of the at least one rear wheel brake is carried out. This means that full braking power is still available on the front axle.

In a particularly preferred embodiment of the invention, the master brake cylinder is hydraulically decoupled from the at least one front wheel brake. The braking force on the front wheel brake is thus built up by the linear actuator without influencing the brake pedal. As a result, brake-by-wire operation is still possible on the front axle, in which brake assistance functions can be implemented with a high level of comfort.

In an alternative preferred embodiment, an electronic brake pedal is provided without a hydraulic connection for detecting the driver's braking request.

In a preferred embodiment of the invention, the hydraulic brake system includes a secondary pressurization device, which is set up to build up hydraulic pressure in the at least one front wheel brake. In this case, the computing unit is set up to build up hydraulic pressure in the front wheel brakes by means of the secondary pressurization device in the event of a leak in the at least one rear wheel brake when braking is requested.

In a particularly preferred embodiment of the invention, the first hydraulic sub-circuit has a master brake cylinder that can be actuated by a brake pedal, and the processing unit is set up to hydraulically decouple the master brake cylinder from the front wheel brakes in the event of a leak in the at least one rear wheel brake.

In a further preferred embodiment of the invention, the electromechanical parking brake is activated only when a braking request is greater than a threshold value. Since electromechanical parking brakes are generally not designed for permanent deceleration of the motor vehicle, this ensures that the electromechanical parking brake is not overloaded.

In a further preferred embodiment of the invention, the clamping forces of the electromechanical parking brake are limited based on the steering angle. This prevents the vehicle from swerving in curves and thus dangerous situations are avoided.

In a further preferred embodiment of the invention, the processing unit is also set up to determine a complete loss of brake fluid and in this case to activate only the electromechanical parking brake of the rear axle when braking is requested. Thus, at least a limited braking function is guaranteed even if the braking system is completely drained.

In a particularly preferred embodiment of the invention, the vehicle speed is limited to a limit speed, in particular less than 10 km/h. This means that the vehicle remains at least a little manoeuvrable even if the brake system fails completely, for example to remove the vehicle from a dangerous position or to maneuver it onto a tow truck. The restriction prevents higher speeds and thus further danger situation for the vehicle itself and other road users.

In a preferred embodiment of the invention, the second hydraulic sub-circuit does not include a front wheel brake and the first hydraulic sub-circuit does not include a rear wheel brake. Accordingly, there is a black/white division of the wheel brakes of the vehicle. All front wheel brakes are arranged together in the first hydraulic circuit and all rear wheel brakes in the second circuit.

The object is also achieved by a method for controlling a brake system according to Claim 13, in which a hydraulic leak in the at least one rear wheel brake is monitored and, when a leak is detected on the rear axle, at least one front wheel brake is hydraulically decoupled from at least one rear wheel brake, with one Braking request exclusively the at least one front wheel brake is actuated by applying a hydraulic pressure and the at least one rear wheel brake is actuated exclusively by means of the electromechanical parking brake.

• 1 zeigt schematisch eine erfindungsgemäße Bremsanlage mit Leckage an der Hinterachse,
• 2 zeigt schematisch eine erfindungsgemäße Bremsanlage erster Ausführungsform bei der Durchführung der Rückfallebene,
• 3 zeigt schematisch eine erfindungsgemäße Bremsanlage bei der Durchführung der Rückfallebene in einer zweiten Ausführungsform,
• 4 zeigt schematisch eine weitere erfindungsgemäße Bremsanlage bei der Durchführung der Rückfallebene,

Further features, advantages and possible applications of the invention also result from the following description of exemplary embodiments and the drawings. All of the described and/or illustrated features belong to the subject matter of the invention, both individually and in any combination, even independently of their summary in the claims or their back-references.

• 1 shows schematically a brake system according to the invention with leakage on the rear axle,
• 2 shows schematically a first embodiment of a brake system according to the invention during implementation of the fallback level,
• 3 shows schematically a brake system according to the invention in the implementation of the fallback level in a second embodiment,
• 4 shows schematically another brake system according to the invention in the implementation of the fallback level,

This in 1 The brake system shown for a motor vehicle includes four hydraulically actuated wheel brakes 8a-8d. The brake system comprises a master brake cylinder 2 that can be actuated by means of an actuating or brake pedal 1, a displacement simulator or a simulation device 3 that interacts with the master brake cylinder 2, a pressure medium reservoir 4 that is at atmospheric pressure, an electrically controllable pressure supply device 5, and wheel-specific brake pressure modulation valves, which, for example, are used as inlet valves 6a-6d and outlet valves 7a-7d are executed. Furthermore, the brake system includes at least one electronic control and regulation unit 12 for controlling the electrically actuable components of the brake system.

According to the example, the wheel brake 8a is assigned to the left front wheel (FL), the wheel brake 8b to the right front wheel (FR), the wheel brake 8c to the left rear wheel (RL) and the wheel brake 8d to the right rear wheel (RR).

The master brake cylinder 2 has a master brake cylinder piston 15 in a housing 16, which delimits a hydraulic pressure chamber 17, and represents a single-circuit master brake cylinder 2. The pressure chamber 17 accommodates a return spring 9, which positions the piston 15 in an initial position when the master brake cylinder 2 is not actuated. The pressure chamber 17 is connected to the pressure medium reservoir 4 via radial bores formed in the piston 15 and a corresponding pressure compensation line 41 , which can be shut off by a relative movement of the piston 15 in the housing 16 . The pressure chamber 17 is on the other hand connected by means of a hydraulic line section (also referred to as the first supply line) 22 to a brake supply line 13 to which the input ports of the inlet valves 6a-6d are connected. Thus, the pressure chamber 17 of the master brake cylinder 2 is connected to all inlet valves 6a-6d.

In the pressure compensation line 41 or in the connection between the pressure chamber 17 and the pressure medium reservoir 4, for example, no hydraulic valve, in particular no electrically or hydraulically actuated valve and no check valve, is arranged.

Alternatively, a diagnostic valve, in particular one that is normally open, can be contained in the pressure compensation line 41 or between the master brake cylinder 2 and the pressure medium reservoir 4, preferably a parallel connection of a normally open diagnostic valve with a check valve that closes toward the pressure medium reservoir 4.

A separating valve 23 is arranged between the supply line 22 connected to the pressure chamber 17 and the brake supply line 13 or the pressure chamber 17 is connected to the brake supply line 13 via the first supply line 22 with a separating valve 23 . The isolating valve 23 is designed as an electrically actuable, preferably de-energized open (SO), 2/2-way valve designed. The hydraulic connection between the pressure chamber 17 and the brake supply line 13 can be shut off by the isolating valve 23 .

A piston rod 24 couples the pivoting movement of the brake pedal 1 as a result of a pedal actuation with the translational movement of the master brake cylinder piston 15, the actuation path of which is detected by a displacement sensor 25, which is preferably designed redundantly. As a result, the corresponding piston travel signal is a measure of the brake pedal actuation angle. It represents a driver's braking request.

A pressure sensor 20 connected to the first supply line 22 detects the pressure built up in the pressure chamber 17 by a displacement of the piston 15 .

This pressure value can also be evaluated to characterize or determine the driver's braking request. As an alternative to a pressure sensor 20, a force sensor 20 can also be used to determine the driver's braking request.

According to the example, the simulation device 3 is designed hydraulically and is hydraulically coupled to the master brake cylinder 2 . The simulation device 3 essentially has, for example, a simulator chamber 29, a simulator rear chamber 30 and a simulator piston 31 separating the two chambers 29, 30 from one another. The simulator piston 31 is supported on a housing by an elastic element 33 (eg simulator spring) arranged in the (for example dry) simulator rear chamber 30 . According to the example, the hydraulic simulator chamber 29 is connected to the pressure chamber 17 of the master brake cylinder 2 by means of a simulator release valve 32 that can preferably be actuated electrically and is preferably closed when de-energized.

The braking system or the braking system comprises an inlet valve 6a-6d and an outlet valve 7a-7d for each hydraulically actuated wheel brake 8a-8d, which are hydraulically interconnected in pairs via central connections and connected to the wheel brake 8a-8d. The inlet valves 6a-6d are each connected in parallel with a non-return valve, which opens towards the brake supply line 13. The outlet connections of the outlet valves 7a-7d are connected to the pressure medium reservoir 4 via a common return line 14.

The electrically controllable pressure supply device
5 is designed as a hydraulic cylinder-piston arrangement (or a single-circuit, electro-hydraulic actuator (linear actuator)), the piston 36 of which can be actuated by a schematically indicated electric motor 35 with the interposition of a rotation-translation gear 39 also shown schematically. The piston 36 delimits the single pressure chamber 37 of the pressure supply device 5.

A rotor position sensor, indicated only schematically, which serves to detect the rotor position of the electric motor 35 is denoted by the reference number 44 .

A line section (also referred to as the second supply line) 38 is connected to the pressure chamber 37 of the electrically controllable pressure supply device 5 . The supply line 38 is connected to the brake supply line 13 via an electrically actuable, preferably normally closed, sequence valve 26 . The hydraulic connection between the pressure chamber 37 of the electrically controllable pressure supply device 5 and the brake supply line 13 (and thus the input connections of the inlet valves 6a-6d) can be opened and shut off in a controlled manner by the switching valve 26 .

The actuator pressure generated by the force of the piston 36 on the pressure medium enclosed in the pressure chamber 37 is fed into the second supply line 38 . In a “brake-by-wire” operating mode, in particular when the brake system is in a fault-free state, the supply line 38 is connected to the brake supply line 13 via the switching valve 26 . In this way, during normal braking, wheel brake pressure is built up and reduced for all wheel brakes 8a-8d by moving the piston 36 forwards and backwards.

When the pressure is reduced by moving the piston 36 back, the pressure medium previously displaced from the pressure chamber 37 of the pressure supply device 5 into the wheel brakes 8a-8d flows back into the pressure chamber 37 in the same way.

Alternatively, different wheel brake pressures can be adjusted individually for each wheel simply by means of the inlet and outlet valves 6a-6d, 7a-7d. With a corresponding reduction in pressure, the proportion of pressure medium released via the outlet valves 7a-7d flows via the return line 14 into the pressure medium reservoir 4.

A suction of pressure medium into the pressure chamber 37 is possible by retracting the piston 36 when the sequence valve 26 is closed, by pressure medium from the container 4 via the line 42 with a flow direction to the actuator
5 opening check valve 53 can flow into the Akuatordruckraum or pressure chamber 37.
According to the example, the pressure chamber 37 is also connected to the pressure medium reservoir 4 via one or more snifting holes when the piston 36 is not actuated. This connection between the pressure chamber 37 and the pressure medium reservoir 4 is separated when the piston 36 is actuated (sufficiently) in the direction of actuation 27 .

In the brake supply line 13, an electrically actuable, normally open circuit separating valve 40 is arranged, through which the brake system is divided into two hydraulic sub-circuits. Brake supply line 13 is divided into a first line section 13a, which is connected to master brake cylinder 2 (via separating valve 23), and a second line section 13b in the second hydraulic sub-circuit, which is connected to pressure supply device 5 (via switching valve 26). The first line section 13a is connected to the inlet valves 6a, 6b of the wheel brakes 8a, 8b and the second line section 13b is connected to the inlet valves 6c, 6d of the wheel brakes 8c, 8d.

When the circuit separating valve 40 is open, the brake system is designed as a single circuit. By closing the circuit separating valve 40, the brake system can be separated or divided into two hydraulic sub-circuits, brake circuits I and II, in particular controlled according to the situation. In the first brake circuit I, the master brake cylinder 2 (via the separating valve 23) is only connected to the inlet valves 6a, 6b of the wheel brakes 8a, 8b of the front axle VA, and in the second brake circuit II the pressure supply device 5 (with the connection valve 26 open) is only connected to the inlet valves 6a, 6b connected to the wheel brakes 8c and 8d of the rear axle HA.

When the circuit separating valve 40 is open, the input connections of all inlet valves 6a-6d can be supplied with a pressure by means of the brake supply line 13 which, in a first operating mode (e.g. “brake-by-wire” operating mode), corresponds to the brake pressure generated by the pressure supply device 5 is provided. The brake supply line 13
can be acted upon by the pressure of the pressure chamber 17 of the master brake cylinder 2 in a second operating mode (e.g. in a de-energized fallback operating mode).

Advantageously, the brake system includes a level measuring device 50 for determining a pressure medium level/status in the pressure medium reservoir 4. Advantageously, the situation for circuit separation is detected by means of the circuit separating valve 40 via the level measuring device 50.

For example, the hydraulic components, namely the master brake cylinder 2, the simulation device 3, the pressure supply device 5, the valves 6a-6d, 7a-7d, 23, 26, 40 and 32 and the hydraulic connections including the brake supply line 13, together in a hydraulic control - and control unit 60 (HCU) arranged. The electronic control and regulation unit (ECU) 12 is assigned to the hydraulic control and regulation unit 60 . Hydraulic and electronic control and regulation units 60, 12 are preferably designed as one unit (HECU).

The brake system includes a pressure sensor 19 or system pressure sensor for detecting the pressure provided by the pressure supply device 5 .
The pressure sensor 19 is arranged behind the sequence valve 26 as viewed from the pressure chamber 37 of the pressure supply device 5 .

In addition to the hydraulic actuation, the two rear wheel brakes 8c, 8d are each equipped with an integrated parking brake 48c, 48d, which are designed as electromechanical parking brakes.

In the 1 the case is now shown that a leak occurs at the right rear wheel brake 8d in the sleeping state of the brake system. The line sections affected by this leakage, which lose the brake fluid contained therein, are represented by a dotted line 50 . This applies to the complete connection of the wheel brake 8d to the brake fluid reservoir 4 via the master brake cylinder 2. The chamber of the brake fluid reservoir 4, which is connected to the master brake cylinder 2, runs completely empty in this case. To compensate for this loss, the control unit can move brake fluid from the linear actuator 5 into the brake fluid reservoir 4 when it wakes up.

In 2 the fallback level according to the invention is now shown in a first embodiment. In this variant, the braking force on the front axle 8a, 8b is provided by the master brake cylinder 2 and thus the brake pedal and the muscle power of the driver. For this purpose, the simulator valve 32 is closed and the isolating valve 23 is opened. The circuit separating valve 40 and the supply valve 26 are also closed in order to avoid loss of brake fluid at the rear axle. The lines and components which are pressurized to provide braking force to the front axle wheel brakes 8a, 8b are shown by the dashed line 52. FIG.

The integrated parking brakes 48c, 48d implement the deceleration on the rear axle 8c, 8d. In the event of emergency braking, the linear actuator 5 can also be switched on.

In the 3 an alternative variant is shown, in which the linear actuator 5 is the pressure source for the front axle 8a. 8b represents. The integrated parking brake 48c, 48d in turn provides the braking force on the rear axle 8c, 8d. In order to prevent the linear actuator 5 on the rear axle wheel brake 8d, which has the leak, from losing brake fluid, the inlet valves 6c, 6d of the rear axle are closed. The supply valve 26 and the circuit separating valve 40 are open in order to establish a connection between the linear actuator 5 and the wheel brakes 8a, 8b of the front axle. A separation between the wheel brakes of the front axle 8a, 8b and the master brake cylinder 2 is ensured via the closed isolating valve 23. The lines and components which are pressurized to provide braking force to the front axle wheel brakes 8a, 8b are shown by the dashed line 53. FIG. In an alternative embodiment, instead of the master brake cylinder 2 and the simulator device 3, a dry electronic brake pedal can be provided for detecting the driver's braking request. In this case there is a direct connection between the isolating valve 23 and the brake fluid reservoir 4.

In the 4 a further embodiment of a brake system is now shown. The connection to the front wheel brakes 8a, 8b is routed through a further brake unit, each with a switching valve 59a, 59b, which is open during normal operation. For reasons of redundancy, the further brake unit comprises a further pressurization device 57, which is designed as two hydraulic pumps 57a, 57b with a common motor. The hydraulic pumps 57a, 57b are connected on the suction side via a normally closed pump isolating valve 58a, 58b to an associated low-pressure accumulator 55a, 55b, which in turn has a connection to the brake fluid reservoir 4. The low-pressure accumulators 55a, 55b are also connected to the associated wheel brakes 8a, 8b via a further valve 56a, 56b.

The hydraulic pump 57 and the linear actuator 5 can be controlled by two separate control devices.

In the fallback level according to the invention, as in 2 the supply valve 26 and the circuit separating valve 40 are closed. Furthermore, master brake cylinder 2 is separated from further brake unit 54 either by closing isolating valve 23 and/or by closing switchover valves 59a and 59b. The pressure buildup in the front wheel brakes 8a, 8b then takes place by activating the hydraulic pumps 57a, 57b. The rear wheel brakes are in turn braked by means of the electromechanical parking brake 48c, 48d. In an alternative embodiment, as in 3 instead of the master brake cylinder 2 and the simulator device 3, a dry electronic brake pedal can be provided for detecting the driver's braking request. In this case, there is again a direct connection between the isolating valve 23 and the brake fluid reservoir 4.

Reference List

1

brake pedal

2

master cylinder

3

simulation facility

4

pressure fluid reservoir

5

pressurization device

6

a to d inlet valves

7

a to d exhaust valves

8th

a to d wheel brake

9

return spring

12

control system

13

brake supply line

14

return line

16

Housing

17

pressure chamber

19

system pressure sensor

20

master cylinder pressure sensor

22

first supply line

23

isolation valve

24

piston rod

25

displacement sensor

26

sequence valve

29

simulator chamber

30

simulator rear chamber

31

simulator piston

32

simulator release valve

33

elastic element

35

Pistons

36

electric motor

37

pressure room

38

supply line

39

Rotation-translation gear

40

circuit isolation valve

41

pressure compensation line

42

Management

44

rotor position sensor

50

level sensor

51

Drained lines

52

lines under pressure

53

lines under pressure

54

switching valve

55

low-pressure accumulator

56

anti-cavitation valve

57

hydraulic pump

58

pump isolation valve

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2017/055152 A1 [0002]

Claims (15)

Brake system for motor vehicles, having a hydraulic brake system with a first hydraulic sub-circuit, which includes at least one front wheel brake (8a, 8b) of the motor vehicle, and with a second hydraulic sub-circuit, which includes at least one rear wheel brake (8c, 8d) of the motor vehicle and a linear actuator (5) comprises, wherein the first hydraulic sub-circuit and the second hydraulic sub-circuit are connected via an electrically controllable circuit separating valve (40), and an electromechanical parking brake (48c, 48d), which is set up to actuate the at least one rear wheel brake (8c, 8d), thereby characterized in that the brake system has a computing unit (12) which is set up to detect a hydraulic leak in the at least one rear wheel brake (8d), and in the event of a leak in the rear wheel brake (8d) the front wheel brakes (8a, 8b) hydraulically to decouple from the rear wheel brakes (8c, 8d), wherein at a braking request only the front wheel brakes (8a, 8b) are actuated by applying hydraulic pressure and the rear wheel brakes (8c, 8d) are actuated exclusively by means of the electromechanical parking brake (48c, 48d). braking system after claim 1 , characterized in that the hydraulic decoupling of the at least one rear wheel brake (8c, 8d) and the at least one front wheel brake (8a, 8b) is carried out by closing the circuit separating valve (40). braking system after claim 1 or 2 , characterized in that the first hydraulic sub-circuit comprises a master brake cylinder (2) that can be actuated by a brake pedal (1) and the computing unit (12) is set up to open a flow connection between the at least one rear wheel brake (8c, 8d) in the event of a leak the master brake cylinder (2) and the front wheel brakes (8a, 8b). braking system after claim 3 , characterized in that the open-flow connection is established by opening a master cylinder valve (23) which is arranged at an outlet of the master brake cylinder (2) and inlet valves (6a, 6b) of the at least one front wheel brake (8a, 8b). braking system after claim 1 or 2 , characterized in that the computing unit (12) is set up to establish a flow-open connection between the linear actuator (5) and the at least one front wheel brake (8a, 8b) in the event of a leak in the at least one rear wheel brake (8c, 8d), wherein the at least one rear wheel brake (8c, 8d) and the at least one front wheel brake (8a, 8b) are hydraulically decoupled by closing an inlet valve (6c, 6d) of the at least one rear wheel brake (8c, 8d). braking system after claim 5 , characterized in that the master brake cylinder (2) is hydraulically decoupled from the at least one front wheel brake (8a, 8b). braking system after claim 5 , characterized in that an electronic brake pedal is provided without a hydraulic connection to the driver braking request detection. braking system after claim 1 , characterized in that the hydraulic brake system has a secondary pressurization device (57) which is set up to build up hydraulic pressure in the at least one front wheel brake (8a, 8b), the computing unit (12) being set up to do this in the event of a leak at the at least one rear wheel brake (8c, 8d) to build up hydraulic pressure in the front wheel brakes (8a, 8b) by means of the secondary pressurization device (57) when braking is required braking system after claim 8 , characterized in that the first hydraulic partial circuit comprises a master brake cylinder (2) which can be actuated by a brake pedal (1) and the computing unit (12) is set up to activate the master brake cylinder (2nd ) hydraulically decoupled from the front wheel brakes (8a, 8b). Braking system according to one of the preceding claims, characterized in that the electromechanical parking brake (48c, 48d) is activated only when a braking request is greater than a threshold value. Braking system according to one of the preceding claims, characterized in that the clamping forces of the electromechanical parking brake (48c, 48d) are limited based on a steering angle. Braking system according to one of the preceding claims, characterized in that the computing unit (12) is also set up to determine a complete loss of brake fluid and in this case to activate only the electromechanical parking brake (48c, 48d) of the rear axle when braking is requested. braking system after claim 12 , characterized in that a signal is sent to the motor vehicle to limit the vehicle speed to a limit speed, in particular less than 10 km/h. Braking system according to one of the preceding claims, characterized in that the second hydraulic sub-circuit does not include a front wheel brake (8a, 8b) and the first hydraulic sub-circuit does not include a rear wheel brake (8c, 8d). Method for controlling a braking system, characterized in that a hydraulic leakage at the at least one rear wheel brake (8c, 8d) is monitored and, upon detection of a leakage at the rear axle (8c, 8d), at least one front wheel brake (8a, 8b) is activated hydraulically by at least one rear wheel brake (8c, 8d) is decoupled, wherein when braking is requested, only the at least one front wheel brake (8a, 8b) is actuated by applying hydraulic pressure and the at least one rear wheel brake (8c, 8d) is actuated exclusively by means of the electromechanical parking brake.

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