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

Abstract

The invention relates to a method for operating a brake system of a vehicle, comprising: detecting a travel of a brake actuation device; generating a brake desired signal on the basis of the detected stroke of the brake actuation device or on the basis of an external brake actuation signal, the brake desired signal representing a target brake torque; generating a braking torque corresponding to the target braking torque only by operation of a motor as a generator based on the braking desired signal; determining a change in the travel of the brake actuation device; and in response to a decrease in travel of the brake actuation device, performing a pressure increase avoidance process on at least one wheel brake of the brake circuit, which is connected via a supply line to a brake master cylinder actuatable by an actuator on the basis of the brake desired signal and via a discharge line to a pressure accumulator. A discharge valve arranged between the wheel brake and the pressure accumulator in the discharge line is closed, or the discharge valve is kept closed if the discharge valve is in a closed state; opening a high-pressure switching valve, which is arranged in a return line connecting the pressure accumulator to the hydraulic pressure generating device; and conducting hydraulic fluid from the pressure accumulator back into the master brake cylinder via the return line, the actuator actuating the master brake cylinder on the basis of the brake desired signal as a result of a decrease in the stroke of the brake actuating device, such that the pressure in the master brake cylinder drops.

Description

Technical Field

[0001] The invention relates to a brake system for a vehicle and to a method for operating a brake system. Background Art

[0002] In vehicles with full or partial electric drive, the electric motor is often also used as a generator to brake the vehicle. The electrical energy generated in this way is usually fed into the vehicle battery. This process is also known as regeneration or regenerative braking.

[0003] Regenerative braking is usually used for decelerations of up to 0.3 G. For greater decelerations, hydraulic brake systems are usually used, in which the wheel brakes apply the braking force by hydraulic pressure or brake fluid. Systems with brake boosters have been proven in this regard, in which the force generated by the driver on the brake device, such as the brake pedal, is boosted by means of pneumatic or electromechanical actuators. In particular, in the case of electromechanical actuators, it is possible to generate hydraulic brake pressure independently of the actuation of the brake actuation device, wherein it is usually desirable that this not be perceived at the brake actuation device.

[0004] If the brake actuation device is actuated for a moderate deceleration that can only be produced with the aid of the electric motor, this also results in the actuation of the actuator and thus in the inflow of brake fluid into the wheel brakes. This generates a braking torque that reduces the energy recovered by regenerative braking. Therefore, the brake fluid is usually introduced into a pressure reservoir that is connected to the wheel brakes via an outlet valve in this case. Since the pressure reservoir acts as an elastic spring for the brake fluid, the braking torque on the wheel brakes can be reduced, but not completely eliminated.

[0005] Against this background, DE 10 2014 205 645 A1 discloses a method for reducing residual braking torques at wheel brakes, wherein after the brake fluid has entered a pressure accumulator, an outlet valve is closed and fluid is sucked back from the wheel brakes into the master brake cylinder by actuating an electromechanical actuator of the master brake cylinder for reducing the pressure. Summary of the invention

[0006] According to the invention, a method for operating a vehicle brake system having the features of claim 1 and a brake system of a vehicle having the features of claim 7 are provided.

[0007] According to a first aspect of the invention, a method for operating a brake system of a vehicle comprises detecting a travel of a brake actuating device, for example by means of a sensor, generating a brake request signal representing a target braking torque based on the detected travel of the brake actuating device or based on an external brake actuation signal, and generating a braking torque corresponding to the target braking torque based on the brake request signal only by operating an electric machine as a generator, in particular if the brake request signal satisfies a predetermined condition, for example representing a deceleration of less than 0.3 G. Furthermore, a change in the travel of the brake actuating device is determined, in particular based on the detected travel, and in response to a reduction in the travel of the brake actuating device, a pressure increase avoidance process is performed on at least one wheel brake of a brake circuit, the wheel brake being connected via a supply line to a master brake cylinder that can be actuated by an actuator based on the brake request signal and being connected via a discharge line to a pressure accumulator.

[0008] The pressure increase avoidance process includes closing a discharge valve arranged in a discharge pipe between the wheel brake and the pressure storage device, or if the discharge valve is in a closed state, keeping the discharge valve closed, opening a high-pressure switching valve arranged in a return pipe connecting the pressure storage device to a hydraulic pressure generating device, and returning hydraulic fluid from the pressure storage device through the return pipe to the master brake cylinder, wherein, due to the reduction in the stroke of the brake actuating device based on a brake desired signal, an actuator actuates the master brake cylinder so that the pressure in the master brake cylinder is reduced.

[0009] According to a second aspect of the invention, a braking system for a vehicle comprises: a brake operating device, which is constructed to perform a stroke due to manual operation; a sensor for detecting the stroke of the brake operating device; a brake booster coupled to the brake operating device, which has an actuator and a master brake cylinder that can be operated by the actuator; at least one brake circuit, which has at least one wheel brake connected to the master brake cylinder via a supply pipe; a pressure storage device, which is connected to the wheel brake via a discharge pipe and to the master brake cylinder via a return pipe; a discharge valve arranged in the discharge pipe; a high-pressure switching valve arranged in the return pipe; and a control device, which is signal-conductingly connected to the sensor device, the actuator, the discharge valve and the high-pressure switching valve, and has an interface, which is configured to send signals to and / or receive signals from an electric motor, wherein the electric motor can be operated as a generator to generate a braking torque, and wherein the control device is configured to cause the braking system to perform a method according to any one of the preceding claims.

[0010] The invention is based on the idea that during purely regenerative braking, even when the stroke of the brake actuation device is reduced or the actuator is actuated backwards, the reduced brake pressure at the wheel brakes can be maintained by means of an electric machine by closing the outlet valve or keeping it closed and draining the brake fluid not through a supply line and therefore not directly connected to the wheel brakes, but through a return line that connects the pressure accumulator to the master brake cylinder. For this purpose, the high-pressure switching valve arranged in the return line is opened.

[0011] The advantage of this procedure is that brake fluid can be conducted back from the pressure accumulator without opening the outlet valve. As a result, brake fluid can be drawn from the brake circuit into the master brake cylinder without this leading to an actuation of the wheel brakes and thus to the generation of residual braking torques during regenerative braking. Another advantage is that the path through the high-pressure switching valve is always available during the return stroke and a build-up of brake pressure can be prevented even in the case of non-electrically actuated actuators, for example pneumatic or vacuum actuators.

[0012] Advantageous embodiments and refinements emerge from the further dependent claims and from the description with reference to the drawings.

[0013] According to some embodiments, it can be provided that the actuator is an electromechanical actuator which is designed to actuate the master brake cylinder for pressure increase or pressure reduction based on a superior control signal with a constant travel of the brake actuating device, wherein the method additionally comprises: in response to the constant travel of the brake actuating device, closing the outlet valve, closing or keeping the high-pressure switching valve closed, and reducing the brake pressure in the wheel brakes by actuating the master brake cylinder for pressure reduction based on the superior control signal by means of the actuator. This sequence is carried out before the above-mentioned pressure increase avoidance process and advantageously reduces the residual braking torque of the hydraulic wheel brakes.

[0014] According to some embodiments, it can be provided that the actuator is constructed to operate the master brake cylinder based on an external brake actuation signal for pressure increase or pressure reduction when the stroke of the brake actuation device is constant. The external brake desired signal can be generated, for example, by a distance sensor that measures the distance from the vehicle to an object in the vehicle's surroundings, and is converted or transformed into a brake desired signal. This is equivalent to automatic braking. The actuator is correspondingly actuated here for the pressure increase or decrease in the master brake cylinder. This brings the following advantage, the brake actuation device always provides the driver with a predetermined characteristic between the vehicle's stroke, actuation force and deceleration, for example when the pedal is actuated during automatic braking. During autonomous braking, during the period when the external actuation signal is constant, that is, during the period when the position of the master brake cylinder is constant, the above-mentioned procedure for constant stroke can be carried out accordingly. In response to a reduction in the external brake actuation signal, a process similar to that in response to a reduction in stroke can be carried out.

[0015] According to some embodiments, it can be provided that the method, in response to an increase in the stroke of the brake actuating device, comprises actuating the master brake cylinder for pressure increase by means of an actuator based on a brake request signal, closing or keeping the high-pressure switching valve closed, and opening the discharge valve to guide the hydraulic fluid into the pressure reservoir. In the same way, it can be handled in response to an increase in the external brake actuating signal. Therefore, during the initiation of braking or during the increase in deceleration, the residual braking torque is advantageously reduced to the reset pressure of the pressure reservoir.

[0016] According to some embodiments, it can be provided that a check valve is arranged in the return line between the pressure reservoir and the high-pressure switching valve, and when the pressure drop in the master brake cylinder causes the pressure difference across the check valve to exceed a predetermined threshold value, the check valve is opened when the hydraulic fluid is returned from the pressure reservoir through the return line to the master brake cylinder. The predetermined threshold value can be less than 1 bar, for example. If the pressure in the return line is less than the pressure in the return line between the check valve and the brake master cylinder by more than a threshold value when the high-pressure switching valve between the check valve and the brake master cylinder is open, the check valve opens and allows the brake fluid to flow from the pressure reservoir into the brake master cylinder. Optionally, during braking only with the aid of the electric motor, the pressure reservoir can be filled at least to a minimum reset pressure, wherein the minimum reset pressure is less than the predetermined threshold value for opening the reset valve by a predetermined difference. The predetermined difference can be, for example, in the range of 0.1 bar.

[0017] According to some specific embodiments, it can be provided that the actuator is designed as an electromechanical actuator and is designed to actuate the master brake cylinder based on a superordinate control signal for a pressure increase or a pressure reduction with a constant travel of the brake actuating device.

[0018] According to some specific embodiments, it may be provided that the actuator is designed as a pneumatic actuator and is designed to apply an additional force to the master brake cylinder for the purpose of increasing the pressure in addition to the force applied to the master brake cylinder by the brake actuating device based on the detected travel. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be explained below with reference to the accompanying drawings.

[0020] Figure 1 is a schematic diagram of a braking system according to an embodiment of the present invention;

[0021] Figure 2 is a flowchart of a method according to an embodiment of the present invention;

[0022] Figure 3 is a graph of the time variation of the braking torque at the wheel brakes of the braking system, the residual braking torque and the switching state of the valve of the braking system during the method according to an embodiment of the present invention;

[0023] Figure 4is a graph of the time variation of the braking torque at the wheel brakes of the braking system, the residual braking torque and the switching state of the valves of the braking system during a method according to another embodiment of the present invention; and

[0024] Figure 5 1 is a graph showing the time profile of the braking torque at the wheel brakes of the braking system, the residual braking torque and the switching state of the valves of the braking system during a method according to another exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0025] In the figures, unless otherwise indicated, the same reference signs denote identical or functionally identical components.

[0026] Figure 1 Schematically shown is a brake system 100 for a vehicle, for example a road vehicle, such as a passenger car, a truck or a bus. In principle, the brake system 100 can also be used on a single track vehicle, such as a motorcycle.

[0027] like Figure 1 As shown, the brake system 100 includes a brake operating device 1, a brake booster 2 having an actuator 2A and a master brake cylinder 2B, a sensor 3, a control device 5, a first brake circuit 10 having a first and a second wheel brake 11A, 11B, and a second brake circuit 20 having a first and a second wheel brake 21A, 21B. However, the present invention is not limited to a brake system 100 having two brake circuits 10, 20, but the brake system 100 includes at least one brake circuit 10, 20. It is also possible to provide more or less than two wheel brakes 11A, 11B, 21A, 21B for each brake circuit 10, 20, in particular to provide at least one wheel brake 11, 21. As shown in FIG. Figure 1 As further schematically shown in FIG. 1 , the electric machine 6 can form part of the brake system 100, which is connected to at least one wheel of the vehicle and can be operated not only as a motor but also as a generator. At least the brake system 100 or the control device 5 has an interface 51 for communicating with the electric machine 6, which is Figure 1 As shown schematically in FIG.

[0028] like Figure 1 As shown by way of example in , the brake actuating device 1 can be designed as a brake pedal that can be manually actuated by the driver, for example. Due to the actuation, the brake actuating device 1 executes a travel that is measured or detected by means of the sensor 3. Therefore, the sensor 3 can be implemented as a displacement sensor.

[0029] like Figure 1As schematically shown in , the actuator 2A of the brake booster 2 can be constructed as an electromechanical actuator, for example, having a transmission (not shown) and an electric motor (not shown). The actuator 2A is connected to the master brake cylinder 2B and actuates the master brake cylinder to increase or reduce the pressure by linearly moving a piston (not shown) in the inner volume of the brake cylinder. The actuator 2A is also coupled to the brake operating device 1 in motion, wherein a deflection or movement of the brake operating device 1 causes a stroke-dependent actuation of the actuator 2A. In particular, the stroke of the brake operating device 1 is detected by means of a sensor 3, and based on the detected stroke, a control signal is generated, for example, by means of a control device 5 and output to the actuator 2A. The electromechanical actuator 2A can be constructed in particular for actuating the master brake cylinder 2B for pressure increase or pressure reduction when the stroke of the brake operating device 1 is constant, based on a superior control signal, for example, generated by the control device 5.

[0030] Instead of an electromechanical actuator, the actuator 2A may also be configured as a pneumatic actuator. For example, the actuator 2A may be implemented as a vacuum actuator, which is configured to apply an additional force to the master brake cylinder 2B for pressure increase in addition to the force applied to the master brake cylinder 2B by the brake operating device 1 based on the detected stroke when the stroke of the brake operating device 1 increases. The structure and function of such an actuator in the brake booster 2 are known to those skilled in the art, so further details will not be explained in detail.

[0031] At least one wheel brake 11, 21 of each brake circuit 10, 20 is connected to the master brake cylinder 2B via a supply pipe 12, 22. Figure 1 As schematically shown in FIG. 1 , the wheel brakes 10 , 20 of each brake circuit 10 , 20 can be connected to the master brake cylinder 2B via a common supply line 12 , 22 . Figure 1 As shown in FIG. 1 , a separation valve 19, 29 can be arranged in the supply pipe 12, 22, through which the wheel brakes 10, 20 of the respective brake circuits 10, 20 can be hydraulically connected to the master brake cylinder 2B or hydraulically separated from the master brake cylinder. The supply pipe 12, 22 branches between the separation valve 19, 20 and the wheel brakes 11A, 11B, 21A, 21B, so that a separate supply path 12A, 12B, 22A, 22B leads to each wheel brake 11A, 11B, 21A, 21B of the corresponding brake circuit 10, 20. In the corresponding supply path 12A, 12B, 22A, 22B, a supply valve 30A, 30B, 40A, 40B can be additionally arranged.

[0032] like Figure 1 As further shown, each wheel brake 11, 21 of the corresponding brake circuit 10, 20 is connected to a pressure reservoir 14, 24 of each brake circuit 10, 20 via a discharge line 13, 23. Figure 1 As shown in , each wheel brake 11A, 11B of first brake circuit 10 can be connected to first pressure reservoir 14 via pipes 13A, 13B that merge into a common discharge pipe 13. Here, in each pipe 13A, 13B, an outlet valve 15A, 15B is arranged between the corresponding wheel brake 11A, 11B and pressure reservoir 14, so as to selectively connect or disconnect the corresponding wheel brake 11A, 11B to pressure reservoir 14. In the same way, each wheel brake 21A, 21B of second brake circuit 20 is connected to second pressure reservoir 24 via pipes 23A, 23B that merge into a common discharge pipe 23. Here, in each pipe 23A, 23B, an outlet valve 25A, 25B is arranged between the corresponding wheel brake 21A, 21B and pressure reservoir 24, so as to selectively connect or disconnect the corresponding wheel brake 21A, 21B to pressure reservoir 24.

[0033] like Figure 1 As further shown in FIG. 1 , a pump 81, 82 can be optionally provided for each brake circuit 10, 20, which is connected with a suction connection to the corresponding pressure reservoir 14, 24 and with a pressure connection to the corresponding supply line 12, 22. The pumps 81, 82 can be driven by a common motor 8. The pumps 81, 82, the inlet valves 30, 40 and the outlet valves 15, 25 can be controlled, for example, by the control device 5, so that these functions for wheel-specific brake pressure changes, such as ABS functions, are performed.

[0034] The pressure storage devices 14, 24 of the corresponding brake circuits 10, 20 are also connected to the master brake cylinder 2B via corresponding return lines 17, 27. Figure 1 As shown in FIG, a high-pressure switching valve 16, 26 is arranged in each return line 17, 27, which allows brake fluid to flow between pressure storage device 14, 24 and master brake cylinder 2B in an open state and closes or blocks return line 17, 27 in a closed state.

[0035] In the return line 17, 27, a non-return valve 18, 28 may be arranged between the pressure accumulator 14, 24 and the high-pressure switching valve 16, 26. Figure 1 . The check valves 18, 28 are preloaded into the closed position and are designed to open when the pressure difference across the check valves 18, 28 exceeds a threshold value, for example, an opening threshold value. In order to open the check valves 18, 28, the pressure between the pressure reservoir 14, 24 and the check valves 18, 28 must exceed the opening threshold value if it is greater than the pressure between the check valves 18, 28 and the high-pressure switching valves 16, 26. The predetermined opening threshold value can be, for example, less than 1 bar.

[0036] The control device 5 can be an electronic control device in particular. For example, the control device 5 can have a processor, for example in the form of a CPU, an ASIC, an FPGA, etc., and a data memory, in particular a non-volatile data memory, such as an SD memory, a flash memory, etc. The data memory can be read by the processor and can store software that can be executed by the processor and cause it to perform calculation steps, in particular for processing input signals and for outputting output signals based on the input signals.

[0037] like Figure 1 As schematically shown in FIG. 1 , the control device 5 has a first interface 51, which is configured to send signals to the motor 6 and / or receive signals from the motor 6. The control device 5 is connected to the motor 6 via the first interface 51. The control device 5 is connected to the sensor device 3, the actuator 2A, the outlet valves 15, 25 and the high-pressure switching valves 16, 26 via the second interface 52. The first and second interfaces 51, 52 can be physically separated interfaces or can also be implemented as a common interface, for example as a bus interface. Optionally, the control device 5 can be additionally connected to a sensor system (not shown) that generates an external brake actuation signal, for example, by means of a distance sensor that measures the distance between the vehicle and other objects.

[0038] The control device 5 is configured to cause the brake system 100 to execute a method M, which will be referred to below. Figure 2 And refer to Figure 1 The brake system 100 shown is explained.

[0039] In step M1, the travel of the brake actuating device 1 is detected by means of the sensor 3. In step M1, an external brake actuation signal may also be detected in addition. In step M2, the control device 5 generates a brake request signal representing a target braking torque based on the detected travel of the brake actuating device 1 or based on the external brake actuation signal.

[0040] As long as the target braking torque does not exceed a certain threshold value, for example, less than 0.3G, in step M3, a braking torque corresponding to the target braking torque is generated based on the brake demand signal, only by operating the electric machine 6 as a generator. This may occur, for example, due to a manually caused increase in stroke detected in step M1 or due to a detected external brake actuation signal. In both cases, the control device 5 can control the actuator 2A to actuate the master brake cylinder 2B. In the case of an external brake actuation signal, due to the motion coupling of the brake actuation device 1 with the actuator 2A, this will lead to a change in the stroke of the brake actuation device 1, at least once a certain actuation threshold value is exceeded. Therefore, in the following, an increased stroke can be equivalent to an increased external brake actuation signal, a reduced stroke can be equivalent to a reduced external brake actuation signal, and a constant stroke can be equivalent to a constant external brake actuation signal.

[0041] In step M4, the control device 5 determines the change in the travel of the brake actuating device 1. In step M41, the control device 5 evaluates whether the travel has increased, which is Figure 2 The symbol "+" indicates whether the stroke is reduced. Figure 2 Indicated by the symbol "-"; or whether the stroke remains unchanged, as shown in Figure 2 Indicated by the symbol “#”.

[0042] In response to the increase in the stroke of the brake actuating device 1, the control device controls the actuator 2A, which actuates the master brake cylinder 2B for pressure increase based on the brake request signal. For this purpose, the separation valves 19, 29 and the inlet valves 30, 40 are opened. In step M6, the high-pressure switching valves 16, 26 are closed or remain closed. In addition, in step M7, the outlet valves 15, 25 are opened. Therefore, by actuating the master brake cylinder 2B, the brake fluid or hydraulic fluid is directly guided into the pressure storage device 14, 24. Since the pressure storage device 14, 24 exerts a reset force on the brake fluid, a residual brake pressure proportional to the reset force is generated at the wheel brakes 11, 21.

[0043] If the actuator 2A is constructed as an electromechanical actuator, such as Figure 1 As shown in , this can cause the actuator 2A to actuate the master brake cylinder 2B for pressure increase or pressure reduction based on the control signal of the brake desired signal and generated by the control device 5, when the stroke of the brake actuating device 1 is constant. In response to the constant stroke of the brake actuating device 1, steps M8-M10 are performed in method M. In step M8, the control device 5 switches the outlet valves 14, 24 to their closed position. The pressure storage device is thereby hydraulically separated from the wheel brakes 11, 21. In step M8, the high-pressure switching valves 16, 26 are closed or kept closed by means of the control device 5. In addition, the brake pressure in the wheel brakes 11, 21 is reduced by actuating the master brake cylinder 2B for pressure reduction based on the superior control signal by means of the actuator 2A (step M10). As a result, the brake fluid from the wheel brakes 11, 21 is transported back to the master brake cylinder 2B via the supply pipes 12, 22, and the pressure generated by the corresponding wheel brakes 11, 21 is set to zero or at least reduced. In step M10 , the isolation valves 19 , 29 are also opened or remain open.

[0044] In response to the reduction in the travel of the brake actuating device 1, a pressure increase avoidance process is performed at least on the wheel brakes 11, 21, such as by Figure 2, as shown in the step sequence M11 in . In this case, in step M111, the outlet valves 15, 25 are closed, or if the outlet valves 15, 25 are in the closed state, for example after step M10, the outlet valves 15, 25 remain closed. Furthermore, in step M112, the high-pressure switching valves 16, 26 are opened, so that a hydraulic connection is established between the master brake cylinder 2B and the pressure storage device 14, 24, which hydraulic connection can still be interrupted by the non-return valves 18, 28 if necessary. In step M113, the brake fluid is returned from the pressure storage device 14, 24 to the master brake cylinder 2B via the return line 17, 27. For this purpose, the control device 5 controls the actuator 2A so as to actuate the master brake cylinder 2B based on the brake request signal due to the reduction in the stroke of the brake actuating device 1 and reduce the pressure in the master brake cylinder 2B. As soon as the pressure difference between the pressure storage device 14, 24 and the master brake cylinder 2B exceeds the opening threshold of the optional non-return valves 18, 28, the brake fluid flows back from the pressure storage device 14, 24 to the master brake cylinder 2B.

[0045] The advantages of this method are particularly Figures 3 to 5 As can be seen in the accompanying drawings, these figures respectively show graphs in which time is plotted on the horizontal axis x and the braking torque is plotted on the first vertical axis y1, wherein line L1 represents the change curve of the target braking torque, line L2 represents the change curve of the braking torque generated by the motor 6, and line L3 represents the residual braking torque generated by the wheel brakes 11, 21. On the second vertical axis y2, the state of the exhaust valves 15, 25 between "0" and "1" is plotted, where "0" represents "closed" and "1" represents "open". The time change curve is represented by line L4. On the third vertical axis y3, the state of the high-pressure switching valves 16, 26 between "0" and "1" is plotted, where "0" represents "closed" and "1" represents "open". The time change curve is represented by line L5. Figure 4 and 5 In FIG. 1 , the states of the separation valves 19 , 29 are plotted on a fourth vertical axis y4 between “0” and “1”, where “0” represents “open” and “1” represents “closed”.

[0046] Figure 3 The curve of the brake system 100 is shown as it operates only based on the external brake actuation signal. From time t0 to time t1, the external brake actuation signal increases, which corresponds to an increase in the stroke of the brake actuation device 1. Figure 2Block M41 of method M in the above method triggers the step sequence M5-M7. This means that, when the isolating valves 19, 29 are open and the high-pressure switching valves 16, 26 are closed, the outlet valves 15, 25 are opened at time t0, so that brake fluid is delivered to the pressure reservoirs 14, 24. From time t0a, a constant residual brake pressure (line L3) is established, which is caused by the reset force of the pressure reservoirs 14, 24. From time t1 onwards, there is a constant external brake actuation signal, which corresponds to a constant travel. Therefore, at time t1a, the step sequence M5-M7 is triggered. Figure 2 . That is, at time t1a, the outlet valves 15, 25 are closed (line L4), the high-pressure switching valves 16, 26 remain closed and the master brake cylinder 2B is returned by means of the actuator 2A in order to suck the brake fluid out of the wheel brakes 11, 21. The isolating valves 19, 29 of one wheel brake circuit 10, 20 can be closed for a short time (line L6), for example, while the isolating valves 19, 29 in the other wheel brake circuit 10, 20 are opened. From time t1b, the residual braking torque is reduced to zero. From time t2, the external brake actuation device or the travel is reduced until time t3. Correspondingly, from time t2, method steps M111-M113 are initiated, wherein the outlet valves 15, 25 (line L4) and the isolating valves 19, 29 (line L6) remain closed, while the high-pressure switching valves 16, 26 (line L5) are opened. As a result, the low pressure at the wheel brakes 11, 21 is maintained (line L3). From time t3 to time t4, the external brake actuation signal increases again, which corresponds to an increase in the stroke of the brake actuation device 1. Figure 2 Block M41 of method M in the above example again triggers the sequence of steps M5-M7. Figure 3 As can be seen in FIG. 1 , the target braking torque and the actual braking torque (lines L1, L2) exceed the threshold value J. As described above, the target braking torque is proportional to the travel of the brake operating device 1. From the threshold value J, the electromechanical actuator 2A can no longer move unless a reaction is exerted on the brake operating device 1. Figure 3 As can be seen, steps M5-M7 can be set to be executed in the same way, and steps M8-M10 can be executed in the same way starting from time point t4 (from which time point the external brake operation signal is constant again), and starting from time point t5 (from which time point the external brake operation signal decreases again).

[0047] Figure 4 The curve in which the brake system 100 operates only based on the travel generated at the brake actuating device 1 is shown. From the time t0 to the time t1, the travel increases and thus the target braking torque increases. Figure 2Block M41 of method M in the above method triggers the step sequence M5-M7. This means that, when the isolating valves 19, 29 (line L6) are open and the high-pressure switching valves 16, 26 (line L5) are closed, the outlet valves 15, 25 are opened at time t0, so that brake fluid is delivered to the pressure reservoirs 14, 24. From time t0a, a constant residual brake pressure (line L3) is established, which is generated by the reset force of the pressure reservoirs 14, 24. From time t1, there is a constant stroke. Therefore, at time t1a, the step sequence M5-M7 is triggered. Figure 2 1 . That is, at time t1a, outlet valves 15, 25 are closed (line L4), high-pressure switching valves 16, 26 remain closed, and master brake cylinder 2B is moved back by means of actuator 2A in order to suck brake fluid out of wheel brakes 11, 21. Isolation valves 19, 29 of one wheel brake circuit 10, 20 can be closed briefly here (line L6), for example, while isolation valves 19, 29 in the other wheel brake circuit 10, 20 are open. From time t1b, the residual braking torque decreases to zero. From time t2, the travel is reduced until time t3. Correspondingly, from time t2, method steps M111-M113 are initiated, wherein outlet valves 15, 25 (line L4) and isolation valves 19, 29 (line L6) remain closed, while high-pressure switching valves 16, 26 (line L5) are opened. As a result, the low pressure at the wheel brakes 11, 21 is maintained (line L3). From time t3 to time t4, the stroke of the brake actuating device 1 increases again. Figure 2 Block M41 of method M in the above example again triggers the sequence of steps M5-M7. Figure 4 As can be seen from the figure, the target and actual braking torques (lines L1, L2) exceed the threshold value J, from which the electromechanical actuator 2A can no longer be moved without reacting on the brake actuating device 1. Therefore, in the time period t4 to t5, during which the travel is constant, the residual braking torque (line L3) will not decrease according to steps M8-M10. If the travel decreases again from the time t5, steps M111-M113 are executed again, as described above.

[0048] Figure 5 The curve shows a brake system 100 which is operated solely on the basis of the travel generated by the brake actuating device 1 and has a pneumatic actuator as actuator 2A which cannot operate the master brake cylinder 2B independently of the travel, in particular cannot be used for pressure reduction at a constant travel. From time t0 to time t1, the travel and therefore also the target braking torque increase. Accordingly, according to Figure 2 Block M41 of method M in the above method triggers the sequence of steps M5-M7. This means that when the separation valves 19, 29 ( Figure 5When the outlet valves 15, 25 (not shown in the figure) are opened and the high-pressure switching valves 16, 26 (line L5) are closed, the outlet valves 15, 25 are opened at time t0 so that the brake fluid is delivered to the pressure accumulators 14, 24. From time t0a, a constant residual brake pressure (line L3) is established, which is generated by the reset force of the pressure accumulators 14, 24. From time t1, there is a constant stroke. The outlet valves 15, 25 remain open so that the residual brake pressure remains constant. From time t2, the stroke decreases until time t3. Therefore, from time t2, method steps M111-M113 are triggered, whereby the outlet valves 15, 25 (line L4) are closed and the high-pressure switching valves 16, 26 (line L5) are opened. The separation valves 19, 29 remain open at all times. As a result, the pressure at the wheel brakes 11, 21 (line L3) drops to zero and at the same time the hydraulic fluid can be delivered from the pressure accumulators 14, 24 back to the master brake cylinder 2B. If the stroke remains constant from time point t3 to time point t4 , the outlet valves 15 , 25 (line L4 ) and the high-pressure switching valves 16 , 26 (line L5 ) may remain closed to maintain a low residual brake pressure.

[0049] Although the invention has been described above by way of example using exemplary embodiments, the invention is not limited thereto but can be modified in many ways. In particular, combinations of the above-described embodiments are also conceivable.

Claims

1. A method (M) for operating a braking system (100) of a vehicle, include: Detecting (M1) the travel of a brake operating device (1); Generating (M2) a braking desired signal based on the detected travel of the brake operating device (1) or based on an external brake operating signal, wherein the braking desired signal represents a target braking torque; generating (M3) a braking torque corresponding to the target braking torque based on the braking request signal only by operating the electric machine as a generator; Determining (M4) a change in the stroke of the brake operating device (1); and In response to a reduction in the stroke of the brake operating device (1), a pressure increase avoidance process (M11) is performed on at least one wheel brake (11, 21) of a brake circuit (10, 20), wherein the at least one wheel brake is connected to a brake master cylinder (2B) operable by an actuator (2A) based on the brake desired signal via a supply pipe (12, 22) and is connected to a pressure storage device (14, 24) via a discharge pipe (13, 23), wherein the pressure increase avoidance process comprises: closing (M111) an outlet valve (15, 25) arranged in the outlet line (13, 23) between the wheel brake (11, 21) and the pressure accumulator (14, 24), or, if the outlet valve (15, 25) is in a closed state, keeping the outlet valve (15, 25) closed; opening (M112) a high-pressure switching valve (16, 26) which is arranged in a return line (17, 27) connecting the pressure accumulator (14, 24) to the hydraulic pressure generating device (2); and The hydraulic fluid is returned (M113) from the pressure storage device (14, 24) to the master brake cylinder (2B) via the return line (17, 27), wherein the actuator (2A) actuates the master brake cylinder (2B) based on the brake desired signal due to a reduction in the stroke of the brake actuating device (1), so that the pressure in the master brake cylinder (2B) decreases.

2. The method (M) according to claim 1, wherein the actuator (2A) is an electromechanical actuator which is designed to actuate the master brake cylinder (2B) at a constant stroke of the brake actuating device (1) based on a superior control signal for a pressure increase or a pressure decrease, wherein the method (M) additionally include: In response to the constant travel of the brake operating device (1): Close (M8) the discharge valve (14, 24) closing or maintaining (M9) the high-pressure switching valve (16, 26) closed; and Based on the superior control signal, the brake pressure in the wheel brakes (11, 21) is reduced (M10) by means of the actuator (2A) by actuating the master brake cylinder (2B) for a pressure reduction.

3. The method (M) according to claim 2, wherein the actuator (2A) is designed to actuate the master brake cylinder (2B) for pressure increase or pressure reduction based on the external brake signal when the stroke of the brake actuating device (1) is constant.

4. The method (M) according to any one of the preceding claims, further comprising: include: In response to an increase in the travel of the brake operating device (1): Based on the brake desired signal, the brake master cylinder (2B) is actuated (M5) by means of the actuator (2A) to increase the pressure; Closing or keeping the high-pressure switching valve (16, 26) closed (M6); as well as The outlet valve (14, 24) is opened (M7) to conduct hydraulic fluid into the pressure reservoir (14, 24).

5. A method (M) according to any one of the preceding claims, wherein a check valve (18, 28) is arranged in the return line (17, 27) between the pressure storage device (14, 24) and the high-pressure switching valve (16, 26), and the check valve is opened when the hydraulic fluid is returned (M103) from the pressure storage device (14, 24) through the return line (17, 27) to the master brake cylinder (2B), as long as the pressure difference across the check valve (18, 28) due to a pressure drop in the master brake cylinder (2B) exceeds a predetermined threshold value.

6. Method (M) according to claim 5, wherein the predetermined threshold value is less than 1 bar.

7. A braking system (100) for a vehicle, comprising: A brake operating device (1), the brake operating device being designed to execute a stroke due to manual operation; A sensor (3) for detecting the travel of the brake operating device (1) a brake booster (2) motion-coupled to the brake actuating device (1), the brake booster having an actuator (2A) and a master brake cylinder (2B) actuable by the actuator (2A); at least one brake circuit (10, 20) having at least one wheel brake (11, 21), the at least one wheel brake being connected to the master brake cylinder (2B) via a supply line (12, 22); A pressure storage device (14, 24), which is connected to the wheel brakes (11, 21) via a discharge pipe (13, 23) and is connected to the master brake cylinder (2B) via a return pipe (17, 27); a discharge valve (15, 25) arranged in the discharge pipe (13, 23); a high-pressure switching valve (16, 26) arranged in the return pipe (17, 27); as well as A control device (5) which is signal-conductingly connected to the sensor device (3), the actuator (2A), the outlet valve (15, 25) and the high-pressure switching valve (16, 26), and has an interface (51) which is configured to send signals to an electric motor (6) and / or receive signals from the electric motor (6), wherein the electric motor (6) can be operated as a generator to generate a braking torque, and wherein the control device (5) is configured to cause the braking system (100) to perform a method (M) according to any one of the preceding claims.

8. A braking system (100) according to claim 7, wherein a check valve (18, 28) is arranged in the return pipe (17, 27) between the pressure storage device (14, 24) and the high-pressure switching valve (16, 26), and the check valve is constructed so that the check valve (18, 28) is opened as long as the pressure difference across the check valve (18, 28) exceeds a predetermined threshold value due to a pressure drop in the master brake cylinder (2B) when the high-pressure switching valve (16, 26) is opened.

9. The braking system (100) according to claim 7, wherein the predetermined threshold value is less than 1 bar.

10. The brake system (100) according to any one of claims 6 to 8, in: The actuator (2A) is designed as an electromechanical actuator and is designed to actuate the master brake cylinder (2B) with a constant stroke of the brake actuating device (1) based on a superior control signal for pressure increase or pressure reduction; or The actuator (2A) is constructed as a pneumatic actuator and is constructed to apply an additional force to the master brake cylinder (2B) for pressure increase in addition to the force applied to the master brake cylinder (2B) by means of the brake operating device (1) based on the detected stroke when the stroke of the brake operating device (1) increases.

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