CN111348024A

CN111348024A - Brake system for a vehicle, vehicle and method for operating a brake system

Info

Publication number: CN111348024A
Application number: CN201911318268.9A
Authority: CN
Inventors: B.海因茨; C.韦尔茨; J.瓦格纳; 姚迅
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2018-12-20
Filing date: 2019-12-19
Publication date: 2020-06-30
Also published as: DE102018222517A1

Abstract

A braking system for a vehicle includes: a master brake cylinder for generating a brake signal; a pressure generating device; a first wheel brake circuit and a second wheel brake circuit, wherein each wheel brake circuit has at least one wheel brake cylinder and is coupled to the pressure generating device, and wherein the pressure generating device is designed to generate a brake pressure in the wheel brake circuit in accordance with the brake signal; an inlet valve arranged in the hydraulic path of the respective wheel brake circuit, which inlet valve separates a section of the hydraulic path on the brake cylinder side from a section of the hydraulic path on the pressure generator side in the closed state and is connected to said section in the open state; and an outlet valve which separates a brake cylinder-side section of the hydraulic path from the hydraulic accumulator in a closed state and is connected to the hydraulic accumulator in an open state, wherein a flow cross section of the outlet valve can be changed in the open state in order to regulate the throughflow.

Description

Brake system for a vehicle, vehicle and method for operating a brake system

Background

Brake systems for vehicles, in particular for motor vehicles such as passenger cars or trucks, are usually realized as electrohydraulic brake systems. Such brake systems typically have a master brake cylinder with an actuator piston. The master brake cylinder can be actuated either directly manually or by means of an over-brake booster.

So-called brake-by-wire systems are also increasingly used. Such a system is described, for example, in US 2015/0021978 a 1. In such a brake system, a hydraulic pressure is generated in the simulator device by actuating the master brake cylinder. The brake signal is generated by actuating the master brake cylinder. The wheel brake circuits each have two wheel brake cylinders, wherein a switchable inlet valve and a switchable outlet valve are provided for each wheel brake cylinder. By means of a pressure generating device coupled to the wheel brake circuit, a brake pressure is generated in the wheel brakes in accordance with the brake signal. In parallel with each inlet valve, a respective non-return valve is provided which allows a return flow of hydraulic fluid from the wheel brake cylinders into the wheel brake circuit in order to prevent a pressure in the wheel brake cylinders which is greater than the pressure generated by the pressure generating device. In order to achieve a wheel-specific pressure regulation, in particular a pressure reduction, in the wheel brake cylinders, the outlet valve is periodically opened and closed in a pulsed manner (getakted).

Disclosure of Invention

According to a first aspect of the invention, there is provided a braking system according to claim 1. According to a second aspect of the invention, there is provided a vehicle according to claim 8. According to a third aspect of the invention, a method for operating a brake system according to claim 9 is provided.

The brake system according to the first aspect of the invention comprises a master brake cylinder for generating a brake signal, a pressure generating device and a first and a second wheel brake circuit, wherein each wheel brake circuit has at least one wheel brake cylinder and is coupled to the pressure generating device, and wherein the pressure generating device is set up for: brake pressure is generated in the wheel brake circuit in accordance with the brake signal. Thus, by actuating the master brake cylinder, a signal representing the braking request can be generated, for example, by means of a pressure sensor and/or a control distance sensor. The brake system also has an inlet valve arranged in the hydraulic path of the respective wheel brake circuit, which in the closed state separates a brake-cylinder-side section of the hydraulic path extending between the inlet valve and the wheel brake cylinder from a pressure-generator-side section of the hydraulic path coupled to the pressure generating device and in the open state connects the pressure-generator-side section to the brake-cylinder-side section. Furthermore, according to the invention, an outlet valve is also provided which separates a brake cylinder-side section of the hydraulic path from the hydraulic accumulator in a closed state and connects the brake cylinder to the hydraulic accumulator in an open state, wherein the flow cross section of the outlet valve can be changed in order to adjust the throughflow.

A vehicle according to a second aspect of the invention includes: at least one first axle and one second axle, wherein at least one wheel is arranged on each axle; an electric drive motor which is kinematically coupled to one or more of the wheels and which is operable in a motoring mode for driving the respective wheel and in a generator mode for braking the respective wheel; and a brake system according to the first aspect of the invention. At least one wheel of the first axle can be braked by means of a wheel brake cylinder of the first wheel brake circuit. At least one wheel of the second axle can be braked by means of a wheel brake cylinder of the second wheel brake circuit. In particular, a respective wheel brake cylinder of at least one of the wheel brake circuits is arranged on an axle in order to brake the wheels of the axle.

The method for operating a brake system according to the third aspect of the invention can be used in particular for operating a brake system according to the first aspect of the invention. According to the invention, the brake signal is generated by means of the master brake cylinder. In addition, brake pressures are generated in the first and second wheel brake circuits by means of a pressure generating device in accordance with the brake signals, wherein at least one inlet valve associated with the respective wheel brake cylinder is opened and the outlet valve associated with the wheel brake cylinders is closed. Thus, when the brake pressure is generated, at least one inlet valve on the wheel brake cylinder is opened, or the inlet valves of the respective wheel brake circuits are opened, or all inlet valves of both wheel brake circuits are opened. In this case, the same inlet valve or inlet valves that are opened are assigned to the same wheel brake cylinder or the respective outlet valve assigned to the same wheel brake cylinder is closed. Furthermore, the inlet valve associated with the wheel brake cylinder is closed and the opening state of the outlet valve associated with the wheel brake cylinder is adjusted in order to reduce the brake pressure at the respective wheel brake cylinder by a predetermined value, wherein the flow cross section of the outlet valve is adjusted as a function of the predetermined value. The adjustment of the flow cross section can also be carried out in particular as a function of the pressure reduction characteristic, i.e. as a function of the desired temporal profile of the pressure reduction.

The idea underlying the invention is to configure the outlet valve as a valve which can adjust the throughflow, that is to say as a valve having a variably adjustable flow cross section. In the open state of the outlet valve, in which the outlet valve allows a through-flow of fluid, the flow cross-section can be reduced or enlarged without interrupting the through-flow of fluid. In this way, the temporal pressure profile or pressure reduction characteristic can be flexibly adjusted. In particular, the number of switching processes for switching the outlet valve between the open state and the closed state during pressure reduction can be reduced, thereby reducing switching noise and vibration.

A further idea of the invention is that, for electrically driven vehicles, a wheel-specific or axle-specific brake force distribution is improved by the brake system during the operation of the electric motor in the generator operating mode and thus braking of one or more axles of the vehicle with a specific brake torque. By configuring the outlet valve as a valve which can regulate the throughflow, it is possible in a simple manner to regulate a plurality of brake pressures and thus the brake torque per axle or per wheel.

Advantageous embodiments and developments emerge from the dependent claims with reference to the independent claims in conjunction with the description.

The brake system optionally includes a check valve connected in parallel with the respective inlet valve, the check valve allowing backflow from the brake cylinder side section of the hydraulic path into the pressure generator side section. The check valve is therefore designed such that it opens when a predetermined pressure difference between the brake-cylinder-side section and the pressure-generator-side section is reached, as a result of which hydraulic fluid can flow from the brake-cylinder-side section into the pressure-generator-side section.

According to one embodiment of the brake system, it is provided that the flow cross section of the outlet valve in the open state can be varied steplessly. This has the advantage that no switching between the open state and the closed state is required during the pressure reduction. In addition, the desired pressure profile can be set in virtually any manner during the pressure reduction.

According to another embodiment, it is provided that the flow cross section of the outlet valve in the open state can be changed in at least two different steps (Stufe). Thus, in the open state, for example, at least two discrete open positions of the outlet valve can be produced, such as a first open position and a second open position, wherein in the first open position 50% of the maximum possible flow cross section is released and wherein in the second open position 100% of the maximum possible flow cross section is released. By means of the stepped change, a particularly easy actuation of the outlet valve can be used to achieve a plurality of desired pressure profiles.

According to another embodiment, it is provided that the hydraulic accumulator is coupled to the pressure generating device. In particular, a hydraulic line can be provided between the pressure generating device and the hydraulic accumulator, via which hydraulic fluid from the accumulator can be supplied to the pressure generating device. As a result, the hydraulic fluid drawn out of the respective wheel brake circuit can be fed back into the brake circuit via the outlet valve during the pressure reduction.

According to a further embodiment, the brake system has a first switching valve and a second switching valve, wherein the first switching valve separates the master brake cylinder from the first wheel brake circuit in the closed state and connects the master brake cylinder to the first wheel brake circuit in the open state, and wherein the second switching valve separates the master brake cylinder from the second wheel brake circuit in the closed state and connects the master brake cylinder to the second wheel brake circuit in the open state. In the event of a failure of the pressure generating device, the master brake cylinder can thereby be hydraulically coupled to the wheel brake circuit in order to regulate the wheel brake pressure.

According to a further embodiment, the brake system has a first separating valve and a second separating valve, wherein the first separating valve connects the pressure generating device to the first wheel brake circuit in the open state and separates it from the first wheel brake circuit in the closed state, and wherein the second separating valve connects the pressure generating device to the second wheel brake circuit in the open state and separates it from the second wheel brake circuit in the closed state.

According to another embodiment, it is provided that the pressure generating device has a plunger which can be moved in translation, for example by means of an electric motor.

According to one specific embodiment of the method, a wheel brake signal is generated which represents a target wheel brake pressure in one of the wheel brake cylinders. The wheel brake signal can be provided, for example, by a brake system or a control device of the vehicle. If the target wheel brake pressure for the respective wheel brake cylinder, which is provided in the wheel brake circuit and is greater than the brake pressure in the respective wheel brake circuit, the wheel brake pressure is generated in the respective wheel brake cylinder by means of the pressure generating device with the inlet valve open and the outlet valve closed. If the target wheel brake pressure is lower than the brake pressure in the respective wheel brake circuit, the inlet valve associated with the respective wheel brake cylinder is closed and the wheel brake pressure is generated in the respective wheel brake cylinder by adjusting the opening state of the outlet valve associated with the respective wheel brake cylinder. It is therefore provided that the wheel-specific pressure increase is carried out by the pressure generating device with the inlet valve open and the wheel-specific pressure decrease is carried out by changing the flow cross section of the outlet valve with the inlet valve closed. In this way, different pressures for the respective wheel brake cylinders can be set particularly easily.

Drawings

The invention is explained below with reference to the drawings. Shown in the drawings are:

FIG. 1 shows a schematic view of a braking system according to an embodiment of the invention; and is

FIG. 2 shows a schematic view of a vehicle according to an embodiment of the invention.

In the drawings, the same reference numbers indicate identical or functionally identical components, unless stated to the contrary.

Detailed Description

Fig. 1 shows an exemplary brake system 1 for a vehicle 100. The brake system 1 comprises a master brake cylinder 2, a pressure generating device 3, a first wheel brake circuit 4 and a second wheel brake circuit 5, and a hydraulic accumulator 10.

As is shown in fig. 1 by way of example, master brake cylinder 2 can be designed in particular as a tandem cylinder having two axially

displaceable pistons

21, 22. The master brake cylinder 2 can be actuated by means of an actuating device, such as, for example, a brake pedal 23, as is schematically illustrated in fig. 1. Furthermore, as shown in fig. 1, an optional simulator circuit 15 can be coupled to the master brake cylinder 2. The simulator circuit 15 comprises, in particular, a reset simulator 16, which is hydraulically, i.e., fluid-guided, coupled to the master brake cylinder 2 via a hydraulic line 17. The actuation of the master brake cylinder 2 involves displacing the

pistons

21, 22 by means of the actuating device, thereby displacing hydraulic fluid, for example oil, against the restoring force generated by the restoring simulator 16 and thereby generating a hydraulic pressure in the simulator circuit 15. The hydraulic pressure can be detected, for example, by means of a pressure sensor (not shown). The axial adjustment travel, over which the

pistons

21, 22 have moved, can be detected by means of the adjustment travel sensor 18. Both the hydraulic pressure in the simulator circuit and the control path detected by means of the control path sensor 18 represent the braking request, i.e. the desired braking force, and can therefore be used as a braking signal. As a rule, master brake cylinder 2 is therefore set up to generate a brake signal.

As is shown in fig. 1 by way of example, the pressure generating device 3 can have, in particular, a translationally movable plunger 30, which is guided in a pressure cylinder 31. For moving the plunger 30, an electric motor 32 can be provided, which is kinematically coupled to the plunger 30, for example by a gear mechanism. The pressure generating device 3 is functionally coupled to the master brake cylinder 2, for example, by a control device (not shown). In particular, the pressure generating device 3 is set up to apply pressure to the hydraulic fluid, wherein the level of the pressure is regulated in accordance with the brake signal.

The brake system 1 shown by way of example in fig. 1 has a first wheel brake circuit 4 and a second wheel brake circuit 5. As can be seen in fig. 1, these wheel brake circuits 4, 5 can be correspondingly identically designed, so that they are explained in common below. Each wheel brake circuit 4, 5 has at least one

wheel brake cylinder

9A, 9B, 9C, 9D, each

wheel brake cylinder

9A, 9B, 9C, 9D has an

inlet valve

6A, 6B, 6C, 6D and an

outlet valve

7A, 7B, 7C, 7D, and each

inlet valve

6A, 6B, 6C, 6D has a

non-return valve

8A, 8B, 8C, 8D. Furthermore, each wheel brake circuit 4, 5 comprises an

optional separating valve

12A, 12B.

As schematically shown in fig. 1, the first wheel brake circuit 4 and the second wheel brake circuit 5 are hydraulically coupled to the pressure generating device 3. In the brake system 1 shown in fig. 1 by way of example, the first wheel brake circuit 4 has two

wheel brake cylinders

9A, 9B and the second wheel brake circuit 5 has two

wheel brake cylinders

9C, 9D. The

wheel brake cylinders

9A, 9B of the first wheel brake circuit 4 are coupled to the pressure generating device 3 via a first hydraulic path 40. The

wheel brake cylinders

9C, 9D of the second wheel brake circuit 5 are coupled to the pressure generating device 3 via a second hydraulic path 50. The

wheel brake cylinders

9A, 9B, 9C, 9D are provided for applying a frictional force to a frictional surface coupled to a wheel of the vehicle as a function of a brake pressure prevailing in the respective wheel brake circuits 4, 5. The brake pressure can be set by means of the pressure generating device 3 as a function of the brake signal generated at the master brake cylinder 2, for example by axially displacing the piston 30 by means of the electric motor 32.

The

inlet valve

6A, 6B, 6C, 6D can be designed in particular as a switching valve (Schaltventil) which can be switched between a closed state in which it blocks the flow cross section and thus prevents the through-flow of fluid, and an open state in which it releases the flow cross section and thus allows the through-flow of fluid. For example, the

inlet valves

6A, 6B, 6C, 6D can be designed as solenoid valves which open without current flow. As shown in fig. 1 by way of example, the

inlet valves

6A, 6B, 6C, 6D are arranged in the respective hydraulic paths of the respective wheel brake circuits 4, 5 and divide these into a brake cylinder-

side section

41, 51 and a pressure generator-

side section

42, 52. The brake-cylinder-

side sections

41, 51 of the

hydraulic paths

40, 50 extend between the

respective inlet valves

6A, 6B, 6C, 6D and the respective

wheel brake cylinders

9A, 9B, 9C, 9D. The pressure generator-

side sections

42, 52 extend between the

respective inlet valves

6A, 6B, 6C, 6D and the pressure generating device 3. In the open state, the

respective inlet valve

6A, 6B, 6C, 6D fluidically connects the brake-cylinder-

side section

41, 51 and thus the respective

wheel brake cylinder

9A, 9B, 9C, 9D to the pressure-generator-

side section

42, 52 of the

hydraulic path

40, 50. In the closed state, the

respective inlet valve

6A, 6B, 6C, 6D separates the brake-cylinder-

side section

41, 51 and thus the respective

wheel brake cylinder

9A, 9B, 9C, 9D from the pressure-generator-

side section

42, 52 of the

hydraulic path

40, 50.

As schematically shown in fig. 1, a

check valve

8A, 8B, 8C, 8D may optionally be connected in parallel with each

inlet valve

6A, 6B, 6C, 6D, respectively. The

check valves

8A, 8B, 8C, 8D allow a return flow from the brake cylinder-

side section

41, 51 of the

hydraulic path

40, 50 into the pressure generator-

side section

42, 52 and thereby prevent the pressure in the brake cylinder-

side section

41, 51 from exceeding the pressure in the pressure generator-

side section

42, 52 of the

hydraulic path

40, 50 by an impermissible value.

Furthermore, as schematically shown in fig. 1,

outlet valves

7A, 7B, 7C, 7D are provided for each

wheel brake cylinder

9A, 9B, 9C, 9D, respectively. The

outlet valve

7A, 7B, 7C, 7D can be realized in particular as a solenoid valve which can be switched between a closed state, in which it blocks the flow cross section and thus prevents the throughflow of fluid, and an open state, in which it releases the flow cross section and thus allows the throughflow of fluid. In particular, it is provided that the flow cross section of the

respective outlet valve

7A, 7B, 7C, 7D can be changed in the open state, i.e. the throughflow rate of the hydraulic valve can be influenced by a change in the flow cross section. In particular, it can be provided that the flow cross section of the

outlet valves

7A, 7B, 7C, 7D can be varied steplessly in the open state or that the flow cross section can be varied in at least two different steps in the open state.

As is schematically shown in fig. 1, the cylinder-

side sections

41, 51 of the

hydraulic paths

40, 50 of the wheel brake circuits 4, 5 are hydraulically connected to the hydraulic accumulator 10 via a common recirculation line 19. The hydraulic accumulator 10 can be realized in particular as a container in which hydraulic fluid at ambient pressure is contained. Furthermore, the hydraulic accumulator 10 can also be coupled to the pressure generating device 3 via the supply line 20. As is schematically shown in fig. 1, the supply line 20 can, for example, open into a pressure cylinder 31 of the pressure generating device 3.

Fig. 1 also shows that the

outlet valves

7A, 7B, 7C, 7D each couple a

section

41, 51 of the

hydraulic paths

40, 50 on the brake cylinder side to the recirculation line 19. In the open state, the

respective outlet valve

7A, 7B, 7C, 7D fluidically connects the brake-cylinder-

side section

41, 51 of the

hydraulic path

40, 50 and thus the respective

wheel brake cylinder

9A, 9B, 9C, 9D to the hydraulic accumulator 10. In the closed state, the

respective outlet valve

7A, 7B, 7C, 7D separates the fluid connection between the cylinder-

side sections

41, 51 of the

hydraulic paths

40, 50 and thus the respective

wheel brake cylinder

9A, 9B, 9C, 9D from the hydraulic accumulator 10.

The

optional separating valve

12A, 12B can be designed in particular as a switching valve which can be switched between a closed state, in which it blocks the flow cross section and thus prevents the through-flow of fluid, and an open state, in which it releases the flow cross section and thus allows the through-flow of fluid. For example, the separating

valves

12A, 12B can be embodied as solenoid valves which are closed without current.

As schematically shown in fig. 1, the first separation valve 12A is arranged in a section 42 on the pressure generator side of the first hydraulic path 40. In the open state, the first separating valve 12A fluidically connects the pressure generating device 3 to the first wheel brake circuit 4. In the closed state, the first separating valve 12A separates the fluid connection between the pressure generating device 3 and the first wheel brake circuit 4. The second separation valve 12B is arranged in a section 52 on the pressure generator side of the second hydraulic path 50. In the open state, the second separating valve 12B connects the pressure generating device 3 in a fluid-conducting manner to the second wheel brake circuit 5. In the closed state, the second isolation valve 12B isolates the fluid connection between the pressure generating device 3 and the second wheel brake circuit 5.

Furthermore, as exemplarily shown in fig. 1, an optional first switching valve 11A and an optional second switching valve 11B can be provided. The

optional switching valves

11A, 11B can be designed in particular as on-off valves which can be switched between a closed state, in which they block the flow cross section and thus prevent the through-flow of fluid, and an open state, in which they release the flow cross section and thus allow the through-flow of fluid. For example, the switching

valves

11A, 11B can be configured as solenoid valves that open without current.

In the open state, first switching valve 11A fluidically connects master brake cylinder 2 to first wheel brake circuit 4. In the closed state, first switching valve 11A separates the fluid connection between master brake cylinder 2 and first wheel brake circuit 4. In the open state, second switching valve 11B fluidically connects master brake cylinder 2 to second wheel brake circuit 5. In the closed state, second switching valve 11B separates the fluid connection between master brake cylinder 2 and second wheel brake circuit 5. If the pressure generating device 3 fails or has a fault, it can be disconnected from the wheel brake circuit by way of the separating

valves

12A, 12B and the master brake cylinder 2 is used to generate brake pressure and is connected to the wheel brake circuits 4, 5 by way of the switching

valves

12A, 12B.

Furthermore, the brake system 1 can have a control device (not shown) which is set up to generate control signals for the

inlet valves

6A, 6B, 6C, 6D and the

outlet valves

7A, 7B, 7C, 7D and is functionally connected to these inlet and outlet valves. For data exchange, the control device can also be functionally connected to an adjustment travel sensor 18 and/or other sensors that generate a braking signal. In addition, the control device can also be designed to generate a control signal for actuating the pressure generating device 3 on the basis of the brake signal. For this purpose, the control device can be functionally connected to the pressure generating device 3 for data exchange.

Fig. 2 shows an exemplary vehicle 100 with a brake system 1 explained with reference to fig. 1. The vehicle 100, which is shown in fig. 2 by way of example and only schematically, has a first shaft 101, a second shaft 102, an electric drive motor 105 and an optional electrical energy storage device 110.

The vehicle 100 exemplarily shown in fig. 2 has four

wheels

101A, 102B, 102C, 101D. As can be seen from fig. 2, two

wheels

101A, 101D are associated with the first shaft 101 and two

wheels

102B, 102C are associated with the second shaft 102 or are coupled to the

respective shafts

101, 102.

As exemplarily shown in fig. 2, the drive motor 105 can be kinematically coupled to the first drive shaft 101, for example, directly or via a transmission. Thus, the first shaft 101 and thus the

wheels

101A, 101D coupled to the first shaft 101 can be driven by the drive motor 105. Of course, the drive motor 105 can also be coupled to the second shaft 102 or to both

shafts

101, 102. Furthermore, it is also conceivable to couple the drive motor 105 directly to the

respective wheel

101A, 102B, 102C, 101D or to provide a plurality of drive motors, one drive motor being associated with each wheel.

The drive motor 105 is electrically connected to an energy storage device 110, which can be realized, for example, as a rechargeable battery. Furthermore, the drive motor 105 can be operated as a motor and as a generator. In the motoring mode, the drive motor 105 receives electrical energy from the energy storage device 110 and converts it into a mechanical torque, which drives the first shaft 101 or the

wheels

101A, 101D coupled to the first shaft 101. In the generator operating mode, the drive motor 105 absorbs a torque from the first shaft 101, as a result of which the first shaft 101 or the

wheels

101A, 101D coupled to the first shaft 101 are braked and the torque is converted into electrical energy, which is fed back into the energy storage device 110.

Fig. 2 shows an exemplary manner in which wheel brake cylinders 9A of first wheel brake circuit 4 brake a first wheel 101A of first axle 101, and wheel brake cylinders 9D of second wheel brake circuit 5 brake a second wheel 101D of first axle 101. Furthermore, wheel brake cylinders 9B of first wheel brake circuit 4 brake first wheel 102B of second axle 102, and wheel brake cylinders 9C of second wheel brake circuit 5 brake second wheel 102C of second axle 101. In general, it can be provided that the first shaft 101 can be braked by means of the wheel brake cylinders of the first wheel brake circuit 4, and that the second shaft 102 can be braked by means of the wheel brake cylinders of the second wheel brake circuit 5. It can also be provided more generally that at least one

wheel

101A, 101D of the first axle 101 can be braked by means of the

wheel brake cylinders

9A, 9B, 9C, 9D of the first wheel brake circuit 4, and at least one

wheel

102B, 102C of the second axle 102 can be braked by means of the

wheel brake cylinders

9A, 9B, 9C, 9D of the second wheel brake circuit 5.

The method for operating the brake system 1 is described below with reference to fig. 1 and 2. For braking the vehicle 100, a brake signal is generated by actuating the master brake cylinder 2, for example by means of the control distance sensor 18. When the brake system 1 is used in the vehicle 100 shown in fig. 2, it can optionally be provided that the drive motor 105 is switched into the generator operating mode by means of a brake signal and that one or both of the

shafts

101, 102 or one of the wheels arranged on the

respective shaft

101, 102 is/are thereby braked as a function of the coupling to the

shaft

101, 102.

In general, in the first and second wheel brake circuits 4, 5, brake pressure is generated in accordance with a brake signal by means of the pressure generating device 3, for example, by axial movement of the plunger 30. One or more of the

inlet valves

6A, 6B, 6C, 6D are opened in this case, and the

outlet valves

7A, 7B, 7C, 7D associated with the same

wheel brake cylinders

9A, 9B, 9C, 9D as the opened

inlet valves

6A, 6B, 6C, 6D are closed. Further, as shown in fig. 2, the

optional separation valves

12A, 12B are opened and the switching

valves

11A, 11B are closed. In order to reduce the brake pressure in a specific

wheel brake cylinder

9A, 9B, 9C, 9D, the

inlet valve

6A, 6B, 6C, 6D associated with this

wheel brake cylinder

9A, 9B, 9C, 9D is closed, and the

outlet valve

7A, 7B, 7C, 7D associated with this

wheel brake cylinder

9A, 9B, 9C, 9D is switched into the open state, wherein the flow cross section of the

outlet valve

7A, 7B, 7C, 7D is adjusted according to a predetermined value, with which the wheel brake pressure is to be reduced in magnitude. The

outlet valves

7A, 7B, 7C, 7D, which can be varied in the flow cross section, allow flexible adjustment of the temporal profile of the pressure drop in this case. In this case, the change in the flow cross section can be effected, for example, by a corresponding control signal generated by a control device of the brake system.

Fig. 2, for example, shows that the brake pressure should be reduced at the

wheels

102B, 102C of the second axle 102. During the pressure reduction, the

inlet valves

6B, 6C are closed and the

outlet valves

7B, 7C are opened, wherein their flow cross-sections are adjusted according to the desired pressure reduction. As is symbolically illustrated by the dashed lines in fig. 2, a lower brake pressure is therefore present at the

wheel brake cylinders

9B, 9C in the wheel-brake-cylinder-

side sections

41, 51 than in the pressure-generator-

side sections

41, 51. The hydraulic fluid from the

wheel brake cylinders

9B, 9C is conducted via a line 19 to a hydraulic accumulator. The

inlet valves

6A, 6D associated with the

wheel brake cylinders

9A, 9D acting on the wheels 101A, 101C of the first axle 101 are opened, while the associated

outlet valves

7A, 7D are closed. Thus, the brake pressure can be increased in the

wheel brake cylinders

9A, 9D by means of the pressure generating device 3, while a controlled pressure reduction takes place in the

wheel brake cylinders

9B, 9C. This may lead to an undesired pressure build-up if hydraulic fluid enters the

wheel brake cylinders

9B, 9C due to a non-seal in the

inlet valves

6B, 6C. The pressure build-up can be reduced in a controlled manner by the

outlet valves

7B, 7C, which can be varied in their flow cross section, in a simple manner.

Furthermore, the method optionally comprises the generation of a wheel brake signal, which represents a target wheel brake pressure in one of the

wheel brake cylinders

9A, 9B, 9C, 9D. The wheel brake signals can be axle-specific or wheel-specific signals which are provided, for example, by a control device of the brake system 1 and which are generated, for example, on the basis of characteristic values of the driving dynamics, such as the acceleration of the vehicle 100, the wheel rotational speed, etc. If the wheel brake pressure is greater than the brake pressure in the respective wheel brake circuit 4, 5, a target wheel brake pressure is generated in the respective

wheel brake cylinder

9A, 9B, 9C, 9D by means of the pressure generating device 3 with the

inlet valve

6A, 6B, 6C, 6D open and the

outlet valve

7A, 7B, 7C, 7D closed. If the target wheel brake pressure is lower than the brake pressure in the respective wheel brake circuit 4, 5, the

inlet valve

6A, 6B, 6C, 6D associated with the respective

wheel brake cylinder

9A, 9B, 9C, 9D is closed and the target wheel brake pressure is generated in the respective

wheel brake cylinder

9A, 9B, 9C, 9D by adjusting the open state of the

outlet valve

9A, 9B, 9C, 9D associated with the respective

wheel brake cylinder

9A, 9B, 9C, 9D.

Although the invention has been exemplarily explained above by means of embodiments, it is not limited thereto but can be modified in various ways. Combinations of the aforementioned embodiments are also conceivable in particular.

Claims

1. Brake system (1) for a vehicle (100), having

A master brake cylinder (2) for generating a brake signal;

a pressure generating device (3);

a first wheel brake circuit (4) and a second wheel brake circuit (5), wherein each wheel brake circuit (4; 5) has at least one wheel brake cylinder (9A; 9B; 9C; 9D) and is coupled to the pressure generating device (3), and wherein the pressure generating device (3) is designed to generate a brake pressure in the wheel brake circuit (4; 5) in accordance with a brake signal;

an inlet valve (6A; 6B; 6C; 6D) which is arranged in a hydraulic path (40; 50) of the respective wheel brake circuit (4; 5) and which, in the closed state, separates a brake-cylinder-side section (41; 51) of the hydraulic path (40; 50) which extends between the inlet valve (6A; 6B; 6C; 6D) and the wheel brake cylinder (9A; 9B; 9C; 9D) from a pressure-generator-side section (42; 52) of the hydraulic path (40; 50) which is coupled to the pressure generating device (3) and, in the open state, connects said pressure-generator-side section;

and

an outlet valve (7A; 7B; 7C; 7D) which separates a brake-cylinder-side section (41; 51) of the hydraulic path (40; 50) from a hydraulic accumulator (10) in a closed state and is connected to the hydraulic accumulator (10) in an open state, wherein a flow cross section of the outlet valve (7A; 7B; 7C; 7D) can be varied in order to regulate the throughflow.

2. The braking system (1) according to claim 1, wherein the flow cross section of the outlet valve (7A; 7B; 7C; 7D) is steplessly changeable in the open state.

3. The brake system (1) according to claim 1, wherein the flow cross section of the outlet valve (7A; 7B; 7C; 7D) is changeable in at least two different steps in the open state.

4. The brake system (1) according to claim 1 or 2, wherein the hydraulic reservoir is coupled to the pressure generating device (3).

5. Braking system (1) according to any one of the preceding claims, additionally having:

a first switching valve (11A) which, in a closed state, separates the master brake cylinder (2) from the first wheel brake circuit (4) and, in an open state, is connected to the first wheel brake circuit (4); and

a second switching valve (11B) which, in a closed state, separates the master brake cylinder (2) from the second wheel brake circuit (5) and, in an open state, is connected to the second wheel brake circuit (5).

6. Braking system (1) according to any one of the preceding claims, additionally having:

a first separating valve (12A) which connects the pressure generating device (3) to the first wheel brake circuit (4) in an open state and separates it from the first wheel brake circuit in a closed state; and

a second separating valve (12B) which connects the pressure generating device (3) to the second wheel brake circuit (5) in an open state and separates from the second wheel brake circuit in a closed state.

7. The braking system (1) according to any one of the preceding claims, wherein the pressure generating device (3) has a translationally movable plunger (30).

8. A vehicle (100) having

At least one first axle (101) and one second axle (102), wherein at least one wheel (101A; 102B; 102C; 101D) is arranged on each axle (101; 102);

an electric drive motor (105) which is kinematically coupled to one or more of the wheels (101A; 102B; 102C; 101D) and can be operated in a motor operating mode for driving the respective wheel (101A; 102B; 102C; 101D) and in a generator operating mode for braking the respective wheel (101A; 102B; 102C; 101D); and

the braking system (1) according to any one of the preceding claims;

wherein at least one wheel (101A; 101D) of the first axle (101) can be braked by means of a wheel brake cylinder (9A; 9B; 9C; 9D) of the first wheel brake circuit (4); and is

Wherein at least one wheel (102B; 102C) of the second shaft (102) can be braked by means of a wheel brake cylinder (9A; 9B; 9C; 9D) of the second wheel brake circuit (5).

9. Method for operating a brake system (1) according to one of claims 1 to 7, comprising:

generating a brake signal by means of the master brake cylinder (2);

according to the braking signal, a braking pressure is generated in the first and second wheel brake circuits (4; 5) by means of the pressure generating device (3), wherein at least one inlet valve (6A; 6B; 6C; 6D) associated with the respective wheel brake cylinder (9A; 9B; 9C; 9D) is opened and an outlet valve (7A; 7B; 7C; 7D) associated with the wheel brake cylinders (9A; 9B; 9C; 9D) is closed;

closing an inlet valve (6A; 6B; 6C; 6D) associated with the wheel brake cylinder (9A; 9B; 9C; 9D); and is

The opening state of an outlet valve (7A; 7B; 7C; 7D) associated with the wheel brake cylinder (9A; 9B; 9C; 9D) is adjusted in order to reduce the brake pressure at the respective wheel brake cylinder (9A; 9B; 9C; 9D) by a predetermined value, wherein the flow cross section of the outlet valve (7A; 7B; 7C; 7D) is adjusted as a function of the predetermined value.

10. The method of claim 9, additionally comprising:

generating a wheel brake signal representing a target wheel brake pressure in one of the wheel brake cylinders (9A; 9B; 9C; 9D);

wherein, if the wheel brake pressure is greater than the brake pressure in the respective wheel brake circuit (4; 5), the target wheel brake pressure is generated in the respective wheel brake cylinder (9A; 9B; 9C; 9D) by means of the pressure generating device (3) with the inlet valve (6A; 6B; 6C; 6D) open and the outlet valve (7A; 7B; 7C; 7D) closed; and is

Wherein, if the target wheel brake pressure is lower than the brake pressure in the respective wheel brake circuit (4; 5), the inlet valve (6A; 6B; 6C; 6D) associated with the respective wheel brake cylinder (9A; 9B; 9C; 5B) is closed and the target wheel brake pressure is generated in the respective wheel brake cylinder (9A; 9B; 9C; 9D) by adjusting the opening state of the outlet valve (7A; 7B; 7C; 7D) associated with the respective wheel brake cylinder (9A; 9B; 9C; 9D).