Braking system for a vehicle and method for operating a braking system

Abstract

A method for operating a braking system of a vehicle includes detecting a stroke of a brake operating device, generating a braking request signal representative of a target braking torque based on the detected stroke of the brake operating device or based on an external brake operating signal, generating a braking torque corresponding to the target braking torque based on the braking request signal by operating an electric machine exclusively as a generator, identifying a change in the stroke of the brake operating device, and implementing a pressure build-up and prevention process in at least one wheel brake in response to a decrease in the stroke of the brake operating device, the wheel brake being connected via a supply line to a master brake cylinder operable by an actuator based on the braking request signal and connected via a discharge line to a pressure accumulator. In this case, the following steps are carried out: closing an outlet valve arranged in the discharge line between the wheel brake and the pressure accumulator, or, if the outlet valve is in a closed state, keeping the outlet valve closed; opening a high-pressure switching valve arranged in the return line connecting the pressure accumulator to the master brake cylinder; and returning hydraulic fluid from the pressure accumulator through the return line into the master brake cylinder, and the actuator operates the master brake cylinder based on a braking request signal as a result of a reduction in the stroke of the brake operating device so that the pressure in the master brake cylinder is reduced.

Description

[Technical Field]

[0001] The present invention relates to a braking system for a vehicle and to a method for operating a braking system. [Background technology]

[0002] Background technology In fully or partially electrically powered vehicles, the electric machine is also commonly used to brake the vehicle by operating it as a generator. The electrical energy thus produced is typically fed into the vehicle battery. This process is also referred to as regenerative braking.

[0003] To achieve more powerful deceleration, hydraulic braking systems are generally used, in which braking pressure is applied to the wheel brakes by hydraulic or brake fluid. Systems with brake boosters have proven effective, in which the force generated by the driver on a brake operating device, such as a brake pedal, is boosted by a pneumatic or electromechanical actuator. In particular, electromechanical actuators can generate hydraulic braking pressure independently of the operation of the brake operating device, and it is usually desirable for this to be imperceptible to the brake operating device.

[0004] When the brake operating device is operated for moderate deceleration, which can be achieved exclusively by an electric machine, the actuator is operated, which also causes brake fluid to be transferred into the wheel brake. This generates a braking torque that reduces the energy that can be recovered by regenerative braking. Therefore, in such cases, the brake fluid is usually directed into a pressure accumulator connected to the wheel brake via an outlet valve. The pressure accumulator acts as an elastic spring for the brake fluid, which can reduce the braking torque at the wheel brake, but cannot completely eliminate it.

[0005] Against this background, DE 10 2014 205 645 A1 discloses a method for reducing the residual braking torque in wheel brakes, in which the brake fluid is displaced into a pressure accumulator, the drain valve is closed, and by operating an electromechanical actuator, the fluid is sucked from the wheel brake back into the master brake cylinder for pressure reduction. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] German Patent Application Publication No. 102014205645 Summary of the Invention [Problem to be solved by the invention]

[0007] Disclosure of the Invention According to the invention, a method for operating a braking system of a vehicle is envisaged with the features of claim 1 and a braking system for a vehicle with the features of claim 7. [Means for solving the problem]

[0008] According to a first aspect of the present invention, a method for operating a braking system of a vehicle includes detecting a stroke of a brake operating device, for example by using a sensor, generating a braking request signal representing a target braking torque based on the detected stroke of the brake operating device or based on an external brake operating signal, and generating a braking torque corresponding to the target braking torque based on the braking request signal by operating an electric machine exclusively as a generator, particularly when the braking request signal satisfies a predetermined condition, for example, when the braking request signal represents a deceleration of less than 0.3 g. Furthermore, the method further includes determining a change in the stroke of the brake operating device based on the detected stroke, and implementing a pressure build-up / blocking process in at least one wheel brake in response to a decrease in the stroke of the brake operating device, the wheel brake being connected via a supply line to a master brake cylinder operable by an actuator based on the braking request signal and via a discharge line to a pressure accumulator.

[0009] The pressure build-up and blocking process includes closing an outlet valve located in the discharge line between the wheel brake and the pressure accumulator, or, if the outlet valve is in a closed state, keeping the outlet valve closed, opening a high-pressure switching valve located in the return line connecting the pressure accumulator to the master brake cylinder, and returning hydraulic fluid from the pressure accumulator through the return line into the master brake cylinder, and an actuator operating the master brake cylinder based on a braking request signal as a result of a reduction in the stroke of the brake operating device, such that the pressure in the master brake cylinder is reduced.

[0010] According to a second aspect of the present invention, a braking system for a vehicle includes a brake operating device configured to perform a stroke as a result of manual operation, a sensor for detecting the stroke of the brake operating device, a brake booster device kinematically coupled to the brake operating device, the brake booster device comprising an actuator and a master brake cylinder operable by the actuator, at least one braking circuit with at least one wheel brake connected to the master brake cylinder via a supply line, a pressure accumulator connected to the wheel brake via a discharge line and to the master brake cylinder via a return line, an outlet valve arranged in the discharge line, a high-pressure switching valve arranged in the return line, and a control device, the control device being connected to transmit signals to the sensor device, the actuator, the outlet valve and the high-pressure switching valve and having an interface configured to send signals to and / or receive signals from an electric machine, the electric machine being operable as a generator to generate a braking torque, the control device being configured to cause the braking system to perform a method as claimed in the preceding claims.

[0011] The idea behind the present invention is to maintain a reduced brake pressure at the wheel brakes even when the stroke of the brake operating device is reduced or the actuator is operated in the opposite direction by closing or keeping the outlet valves closed during purely regenerative braking with the electric machine and directing the brake fluid not through the supply line, and thus not through a path directly connected to the wheel brakes, but through the return line connecting the pressure accumulator and the master brake cylinder. For this purpose, a high-pressure switching valve arranged in the return line is opened.

[0012] The advantage of this approach is that brake fluid can be returned from the pressure accumulator without the need to open the outlet valve. This allows brake fluid from the brake circuit to be sucked into the master brake cylinder without activating the wheel brakes and thus without causing residual braking torque during regenerative braking. A further advantage is that the high-pressure switching valve is always available for return stroke, preventing an increase in brake pressure even in the case of actuators that are not electrically operated, such as pneumatic or vacuum actuators.

[0013] Advantageous embodiments and developments are evident from the further dependent claims and the description with reference to the drawings.

[0014] According to some embodiments, it may be envisaged that the actuator is an electromechanical actuator, which is configured to operate the master brake cylinder for pressure build-up or pressure reduction based on a higher-level control signal when the stroke of the brake operating device is constant, and the method further comprises: in response to a constant stroke of the brake operating device, closing the outlet valve, closing or keeping closed the high-pressure switching valve, and based on the higher-level control signal, reducing the braking pressure in the wheel brake by operating the master brake cylinder for pressure reduction using the actuator, this sequence being upstream of the pressure build-up and avoidance process and advantageously reducing the residual braking torque of the hydraulic wheel brake.

[0015] According to some embodiments, it may be assumed that the actuator is configured to operate the master brake cylinder for a pressure increase or pressure decrease based on an external brake signal when the stroke of the brake operating device is constant. The external brake operating signal may be generated, for example, by a distance sensor measuring the distance from the vehicle to objects present in the vehicle's surroundings and converted into a braking request signal. This therefore corresponds to autonomous braking. In this case, the actuator is operated to increase or decrease the pressure in the master brake cylinder accordingly. This has the advantage that, for example, when a pedal operation is performed during autonomous braking, the brake operating device always produces a predetermined characteristic between the stroke, the operating force, and the deceleration of the vehicle for the driver. During autonomous braking, while the external brake operating signal is constant, i.e., while the position of the master brake cylinder is constant, the procedure described above for the case of a constant stroke can be followed. In response to a decrease in the external brake operating signal, the procedure can be followed in the same way as in the case of a stroke decrease.

[0016] According to some embodiments, it may be envisaged that the method further comprises, in response to an increase in the stroke of the brake operating device, operating the master brake cylinder for a pressure increase based on a braking demand signal by means of an actuator, closing or keeping closed the high-pressure switching valve, and opening the outlet valve to bypass hydraulic fluid into the accumulator. The same can be done in response to an increase in the external brake operating signal, whereby the residual braking torque is advantageously reduced to the return pressure of the accumulator during the start of braking or during increased deceleration.

[0017] According to some embodiments, a check valve is arranged in the return line between the pressure accumulator and the high-pressure switching valve. The check valve may be designed to open when returning hydraulic fluid from the accumulator through the return line to the master brake cylinder as soon as a pressure difference across the check valve exceeds a predetermined threshold value as a result of a pressure reduction in the master brake cylinder. The predetermined threshold value may be, for example, less than 1 bar. Thus, if the pressure in the return line when the high-pressure switching valve between the check valve and the master brake cylinder is open is less than the pressure in the return line between the check valve and the accumulator by more than the threshold value, the check valve opens to allow the flow of brake fluid from the accumulator into the master brake cylinder. Optionally, the accumulator can be filled to at least a minimum return pressure during braking exclusively using the electric machine, which is less than the predetermined threshold value for opening the return valve by a predetermined difference. This predetermined difference may be, for example, in the range of 0.1 bar.

[0018] According to some embodiments, it may be assumed that the actuator is configured as an electromechanical actuator and is configured to operate the master brake cylinder for pressure increase or pressure decrease based on a higher-level control signal when the stroke of the brake operating device is constant.

[0019] According to some embodiments, it may be assumed that the actuator is configured as a pneumatic actuator and is configured to apply an additional force to the master brake cylinder for pressure increase in addition to the force applied to the master brake cylinder by the brake operating device based on the detected stroke when the stroke of the brake operating device increases.

[0020] In the following, the invention will be explained with reference to the illustrations in the drawings. [Brief explanation of the drawings]

[0021] [Figure 1]1 is a schematic diagram of a braking system according to one embodiment of the present invention; [Figure 2] 1 is a flowchart of a method according to an embodiment of the present invention. [Figure 3] 3 is a diagram showing the progression over time of the braking torque, the residual braking torque at the wheel brakes of the braking system and the switching state of the valves of the braking system during the implementation of a method according to an embodiment of the invention; [Figure 4] 5 is a diagram showing the progression over time of the braking torque, the residual braking torque at the wheel brakes of the braking system and the switching state of the valves of the braking system during the implementation of a method according to a further embodiment of the invention; [Figure 5] 5 is a diagram showing the progression over time of the braking torque, the residual braking torque at the wheel brakes of the braking system and the switching state of the valves of the braking system during the implementation of a method according to a further embodiment of the invention;

[0022] The same reference numbers in multiple drawings indicate identical or functionally equivalent components, unless stated otherwise. DETAILED DESCRIPTION OF THE INVENTION

[0023] 1 shows a schematic representation of a braking system 100 for a vehicle, for example a road vehicle such as a car, a truck or a bus. In principle, the braking system 100 can also be used in two-wheeled vehicles, for example a motorcycle.

[0024] 1, the braking system 100 includes a brake operating device 1, a brake booster 2 with an actuator 2A and a master brake cylinder 2B, a sensor 3, a control device 5, a first braking circuit 10 with a first wheel brake 11A and a second wheel brake 11B, and a second braking circuit 20 with a first wheel brake 21A and a second wheel brake 21B. However, the present invention is not limited to a braking system 100 with two braking circuits 10, 20, but rather the braking system 100 includes at least one braking circuit 10, 20. More or less than two wheel brakes 11A, 11B, 21A, 21B may also be provided for each braking circuit 10, 20, and in particular at least one wheel brake 11, 21 may be provided. As further shown schematically in Figure 1, the electric machine 6 may form part of a braking system 100, the electric machine 6 being coupled to at least one wheel of the vehicle and operable both as a motor and as a generator. At least the braking system 100 or the controller 5 has an interface 51 for communicating with the electric machine 6, as shown schematically in Figure 1.

[0025] The brake operating device 1 may be configured as, for example, a brake pedal that can be manually operated by a driver, as exemplarily shown in Fig. 1. The brake operating device 1 may perform a stroke as a result of operation, and this stroke is measured or detected using the sensor 3. Therefore, the sensor 3 may be configured as a stroke sensor.

[0026] The actuator 2A of the brake booster 2 may be configured, for example, as an electromechanical actuator with a transmission (not shown) and an electric motor (not shown), as shown schematically in FIG. 1 . The actuator 2A is coupled to a master brake cylinder 2B and operates the master brake cylinder 2B for pressure increase or pressure reduction by linearly moving a piston (not shown) in the internal volume of the brake cylinder. The actuator 2A is further kinematically coupled to a brake operating device 1, the displacement or movement of which causes stroke-dependent operation of the actuator 2A. In particular, the stroke of the brake operating device 1 is detected using a sensor 3, and a control signal is generated, for example, using a control device 5, based on the detected stroke and output to the actuator 2A. The electromechanical actuator 2A may be configured, in particular, to operate the master brake cylinder 2B for pressure increase or pressure reduction based on a higher-level control signal generated, for example, by the control device 5, when the stroke of the brake operating device 1 is constant.

[0027] Instead of an electromechanical actuator, the actuator 2A can also be configured as a pneumatic actuator. For example, the actuator 2A can be configured 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 actuators in the brake booster 2 are known to those skilled in the art, and therefore will not be described in further detail.

[0028] At least one wheel brake 11, 21 of each brake circuit 10, 20 is connected to the master brake cylinder 2B via a supply line 12, 22. As shown schematically in Figure 1, the wheel brakes 11, 21 of each one of the brake circuits 10, 20 can be connected to the master brake cylinder 2B via a common supply line 12, 22. As shown in Figure 1, a respective isolation valve 19, 29 can be arranged in the supply line 12, 22, via which the wheel brakes 11, 21 of each one of the brake circuits 10, 20 can be hydraulically connected to and hydraulically isolated from the master brake cylinder 2B. The supply lines 12, 22 branch between the isolation valves 19, 20 and the wheel brakes 11A, 11B, 21A, 21B in such a way that dedicated supply paths 12A, 12B, 22A, 22B lead to the respective wheel brakes 11A, 11B, 21A, 21B of the respective braking circuits 10, 20. A respective supply valve 30A, 30B, 40A, 40B may further be arranged in each supply path 12A, 12B, 22A, 22B.

[0029] As further shown in Figure 1, each wheel brake 11, 21 of each brake circuit 10, 20 is connected via a drain line 13, 23 to one pressure accumulator 14, 24 for each brake circuit 10, 20. As shown in Figure 1, each wheel brake 11A, 11B of the first brake circuit 10 can be connected to the first pressure accumulator 14 via lines 13A, 13B that merge into a common drain line 13. In this case, an outlet valve 15A, 15B is arranged in each line 13A, 13B between the respective wheel brake 11A, 11B and the pressure accumulator 14, by means of which the respective wheel brake 11A, 11B is selectively connected to or disconnected from the pressure accumulator 14. Similarly, each wheel brake 21A, 21B of the second braking circuit 20 may be connected to the second pressure accumulator 24 via lines 23A, 23B that merge into a common discharge line 23. In this case, an outlet valve 25A, 25B is arranged in each line 23A, 23B between the respective wheel brake 21A, 21B and the pressure accumulator 24, by means of which the respective wheel brake 21A, 21B is selectively connected to or disconnected from the pressure accumulator 24.

[0030] 1, optionally, one pump 81, 82 can be provided for each brake circuit 10, 20, which pump 81, 82 is connected by a suction connection to the respective pressure accumulator 14, 24 and by a pressure connection to the respective 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 driven, for example, by the control device 5, so that they perform a function, such as an ABS function, for varying the brake pressure at each individual wheel.

[0031] The accumulators 14, 24 of each brake circuit 10, 20 are further connected to the master brake cylinder 2B via respective return lines 17, 27. As shown in Figure 1, a high-pressure switching valve 16, 26 is disposed in each return line 17, 27, which, when open, allows the flow of brake fluid between the accumulators 14, 24 and the master brake cylinder 2B and closes or blocks the return line 17, 27 when closed.

[0032] As shown in Figure 1, a check valve 18, 28 may be further disposed in the return line 17, 27 between the pressure accumulator 14, 24 and the high-pressure switching valve 16, 26. The check valve 18, 28 is biased to a closed position and is configured to open as soon as the pressure difference across the check valve 18, 28 exceeds a predetermined threshold, e.g., an opening threshold. To open the check valve 18, 28, the pressure between the pressure accumulator 14, 24 and the check valve 18, 28 must be greater than the pressure between the check valve 18, 28 and the high-pressure switching valve 16, 26 by more than the opening threshold. The predetermined opening threshold may be, for example, less than 1 bar.

[0033] The control device 5 may in particular be an electronic control device. For example, the control device 5 may have a processor, for example in the form of a CPU, ASIC, FPGA, etc., and a data memory, in particular a non-volatile data memory, for example an SD memory, a flash memory, etc. The data memory is readable by the processor and may store software that is executable by the processor and causes the processor to perform calculation steps, in particular to process input signals and to output output signals based on the input signals.

[0034] As shown schematically in FIG. 1 , the control device 5 has a first interface 51 configured to transmit signals to and / or receive signals from the electric machine 6. The control device 5 is connected to the electric machine 6 via the first interface 51. The control device 5 is connected to transmit signals to the sensor device 3, the actuator 2A, the outlet valves 15 and 25, and the high-pressure switching valves 16 and 26 via a second interface 52. The first interface 51 and the second interface 52 may be physically separate interfaces or may be configured as one common interface, for example, as a bus interface. Optionally, the control device 5 may further be connected to a sensor system (not shown) that generates an external brake actuation signal, for example, using a distance sensor that measures the distance from the vehicle to other objects.

[0035] The control device 5 is configured to cause the braking system 100 to implement the following method M, which will be explained below on the basis of FIG. 2 with reference to the braking system 100 shown in FIG.

[0036] In step M1, the stroke of the brake operating device 1 is detected using the sensor 3. An external brake operating signal can also be detected in step M1. In step M2, the control device 5 generates a braking request signal representing a target braking torque based on the detected stroke of the brake operating device 1 or based on the external brake operating signal.

[0037] While the target braking torque does not exceed a predetermined threshold, e.g., less than 0.3 G, in step M3, a braking torque corresponding to the target braking torque is generated based on the braking request signal by operating the electric machine 6 exclusively as a generator. This can occur, for example, as a result of a manually initiated stroke increase detected in step M1 or as a result of a detected external brake actuation signal. In both cases, the control device 5 can drive the actuator 2A to actuate the master brake cylinder 2B. In the case of an external brake actuation signal, this also results in a stroke change of the brake actuation device 1, at least once the predetermined actuation threshold has been exceeded, due to the kinematic coupling of the brake actuation device 1 to the actuator 2A. Therefore, in the following, an increase in stroke can be equated with an increase in the external brake actuation signal, a decrease in stroke can be equated with a decrease in the external brake actuation signal, and a constant stroke can be equated with a constant external brake actuation signal.

[0038] In step M4, determination of the change in the stroke of the brake operating device 1 is performed using the control device 5. In step M41, the control device 5 evaluates whether the stroke is increasing, as indicated by the symbol "+" in Figure 2, or whether the stroke is decreasing, as indicated by the symbol "-" in Figure 2, or whether the stroke remains constant, as indicated by the symbol "#" in Figure 2.

[0039] The control device drives the actuator 2A in response to an increase in the stroke of the brake operating device 1, and the actuator 2A operates the master brake cylinder 2B to increase pressure based on the braking request signal. To this end, the isolation valves 19, 29 and the inlet valves 30, 40 are opened. In step M6, the high-pressure switching valves 16, 26 are closed or kept closed. Furthermore, in step M7, the outlet valves 15, 25 are opened. As a result, the brake fluid or hydraulic fluid is bypassed directly into the pressure accumulators 14, 24 by operation of the master brake cylinder 2B. The pressure accumulators 14, 24 exert a return force on the brake fluid, generating residual brake pressure in the wheel brakes 11, 21 that is proportional to this return force.

[0040] If the actuator 2A is configured as an electromechanical actuator as shown in FIG. 1 , the electromechanical actuator can operate the master brake cylinder 2B for pressure increase or pressure reduction based on a control signal generated by the control device 5 that is higher than the braking request signal when the stroke of the brake operating device 1 is constant. In method M, steps M8 to M10 are performed in response to a constant stroke of the brake operating device 1. In step M8, the control device 5 switches the outlet valves 14, 24 to their closed positions, thereby hydraulically isolating the pressure accumulators from the wheel brakes 11, 21. In step M9, the high-pressure switching valves 16, 26 are closed or kept closed by the control device 5. Furthermore, based on the higher-level control signal, the brake pressure in the wheel brakes 11, 21 is reduced by operating the master brake cylinder 2B for pressure reduction using the actuator 2A (step M10). This causes brake fluid to be pumped from the wheel brakes 11, 21 through the supply lines 12, 22 back to the master brake cylinder 2B, and the residual braking torque generated by the respective wheel brakes 11, 21 is set to zero or at least reduced. Furthermore, in step M10, the isolation valves 19, 29 are opened or kept open.

[0041] As shown by step M11 in FIG. 2, in response to a reduction in the stroke of the brake actuation device 1, a pressure build-up / blocking process is performed in at least one wheel brake 11, 21. In this case, in step M111, the outlet valves 15, 25 are closed or, if they were closed after step M10, are kept closed. In step M112, the high-pressure switching valves 16, 26 are opened, thereby establishing a hydraulic connection between the master brake cylinder 2B and the pressure accumulators 14, 24, which may still be interrupted by the check valves 18, 28. In step M113, brake fluid from the pressure accumulators 14, 24 is returned to the master brake cylinder 2B through the return lines 17, 27. For this purpose, the control device 5 operates the master brake cylinder 2B based on the braking request signal as a result of a stroke reduction of the brake operating device 1, driving the actuator 2A to reduce the pressure in the master brake cylinder 2B. As soon as the pressure difference between the pressure accumulators 14, 24 and the master brake cylinder 2B exceeds the opening threshold of the optional check valves 18, 28, the brake fluid from the pressure accumulators 14, 24 flows back into the master brake cylinder 2B.

[0042] The advantages of this method are particularly apparent from FIGS. 3 to 5, which each show a diagram where time is plotted on the horizontal axis x and braking torque is plotted on a first vertical axis y1. Line L1 represents the progression of the target braking torque, line L2 represents the progression of the braking torque generated by the electric machine 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 outlet valves 15, 25 is plotted between "0" and "1," where "0" represents "closed" and "1" represents "open." The progression over time is shown by line L4. On the third vertical axis y3, the state of the high-pressure switching valves 16, 26 is plotted between "0" and "1," where "0" represents "closed" and "1" represents "open." The progression over time is shown by line L5. 3 and 4 also plot the state of the isolation valves 19, 29 on a fourth vertical axis y4 between "0" and "1", where "0" represents "open" and "1" represents "closed".

[0043] FIG. 3 shows a progression when the braking system 100 is operated solely based on an external brake actuation signal. From time t0 to time t1, the external brake actuation signal increases, which should be equated with an increase in the stroke of the brake actuation device 1. Accordingly, step sequence M5 to M7 is initiated according to block M41 of method M in FIG. 2. That is, with the isolation valves 19 and 29 open and the high-pressure switching valves 16 and 26 closed, the outlet valves 15 and 25 are opened at time t0, thereby pumping brake fluid into the pressure accumulators 14 and 24. As a result, from time t0a, a constant residual brake pressure results from the return force of the pressure accumulators 14 and 24 (line L3). From time t1, a constant external brake actuation signal is present, which corresponds to a constant stroke. Therefore, at time t1a, steps M8 to M10 of method M in FIG. 2 are initiated. That is, at time t1a, the outlet valves 15, 25 are closed (line L4), the high-pressure switching valves 16, 26 are kept closed, and the master brake cylinder 2B is returned by the actuator 2A to suck brake fluid from the wheel brakes 11, 21. In this case, the isolation valves 19, 29 of one wheel brake circuit 10, 20 can be closed for a short time while the isolation valves 19, 29 of the other wheel brake circuit 10, 20 are open (line L6). From time t1b, the residual braking torque is reduced to zero. From time t2 to time t3, the external brake actuation signal or stroke is reduced. Therefore, from time t2, method steps M111 to M113 are initiated, in which the outlet valves 15, 25 (line L4) and the isolation valves 19, 29 (line L6) are kept closed, and the high-pressure switching valves 16, 26 (line L5) are opened. This allows a low pressure to be maintained in the wheel brakes 11, 21 (line L3). From time t3 to time t4, the external brake actuation signal increases again, which should be equated with an increase in the stroke of the brake actuation device 1. Correspondingly, the sequence of steps M5 to M7 is started again in accordance with block M41 of method M in FIG. 2.However, as can be seen in Fig. 3, the target braking torque and the actual braking torque (lines L1, L2) exceed the threshold value J. As mentioned above, the target braking torque is proportional to the stroke of the brake operating device 1. Once this threshold value J is exceeded, the electromechanical actuator 2A can no longer be moved without a reaction on the brake operating device 1. As can be seen in Fig. 3, it can be assumed that steps M5 to M7 are also performed, as well as steps M8 to M10 from time t4, when the external brake operating signal becomes constant again, and steps M111 to M113 from time t5, when the external brake operating signal decreases again.

[0044] FIG. 4 shows the progression when the braking system 100 is operated solely on the basis of the stroke generated by the brake actuation device 1. From time t0 to time t1, the stroke, and thus the target braking torque, increases. Accordingly, step sequence M5 to M7 is initiated according to block M41 of method M of FIG. 2. That is, if the isolation valves 19, 29 are open (line L6) and the high-pressure switching valves 16, 26 are closed (line L5), the outlet valves 15, 25 are opened at time t0, thereby pumping brake fluid into the pressure accumulators 14, 24. As a result, from time t0a, a constant residual brake pressure results from the return force of the pressure accumulators 14, 24 (line L3). From time t1, a constant stroke exists. Therefore, at time t1a, steps M8 to M10 of method M of FIG. 2 are initiated. That is, at time t1a, the outlet valves 15, 25 are closed (line L4), the high-pressure switching valves 16, 26 are kept closed, and the master brake cylinder 2B is returned by the actuator 2A to suck brake fluid from the wheel brakes 11, 21. In this case, the isolation valves 19, 29 of one wheel brake circuit 10, 20 can be closed for a short time (line L6), for example, while the isolation valves 19, 29 of the other wheel brake circuit 10, 20 are open. From time t1b, the residual braking torque is reduced to zero. From time t2 to time t3, the stroke decreases. Therefore, from time t2, method steps M111 to M113 are initiated, in which the outlet valves 15, 25 (line L4) and the isolation valves 19, 29 (line L6) are kept closed, and the high-pressure switching valves 16, 26 (line L5) are opened. This allows a low pressure to be maintained in the wheel brakes 11, 21 (line L3). From time t3 to time t4, the stroke of the brake actuator 1 increases again. Correspondingly, the step sequence M5 to M7 is started again in accordance with block M41 of method M in Fig. 2. However, as can be seen in Fig. 4, the target and actual braking torques (lines L1, L2) exceed a threshold value J, above which the electromechanical actuator 2A can no longer be actuated without a counter-influence on the brake actuator 1.Therefore, during the period t4 to t5 when the stroke is constant, the reduction of the residual braking torque (line L3) according to steps M8 to M10 is not performed. When the stroke decreases again from time t5, steps M111 to M113 are performed again as described above.

[0045] FIG. 5 shows a progression when the braking system 100 is operated solely based on the stroke generated by the brake actuation device 1 and includes a pneumatic actuator 2A that is unable to operate the master brake cylinder 2B independently of the stroke, particularly due to pressure reduction when the stroke is constant. From time t0 to time t1, the stroke, and thus the target brake torque, increases. Accordingly, step sequence M5 to M7 is initiated according to block M41 of method M in FIG. 2. That is, with the isolation valves 19 and 29 open (not shown in FIG. 5) and the high-pressure switching valves 16 and 26 closed (line L5), the outlet valves 15 and 25 are opened at time t0, thereby pumping brake fluid into the pressure accumulators 14 and 24. As a result, a constant residual brake pressure resulting from the return force of the pressure accumulators 14 and 24 is generated from time t0a (line L3). From time t1, a constant stroke exists. In this case, the outlet valves 15, 25 remain open, thereby maintaining a constant residual brake pressure. From time t2 to time t3, the stroke decreases. Therefore, from time t2, method steps M111 to M113 are initiated, in which the outlet valves 15, 25 (line L4) are closed and the high-pressure switching valves 16, 26 (line L5) are opened. The isolation valves 19, 29 remain open throughout this period. This allows the pressure in the wheel brakes 11, 21 (line L3) to be reduced to zero, while simultaneously pumping hydraulic fluid from the pressure accumulators 14, 24 back into the master brake cylinder 2B. If the stroke remains constant from time t3 to time t4, the outlet valves 15, 25 (line L4) and the high-pressure switching valves 16, 26 (line L5) can remain closed to maintain a low residual brake pressure.

[0046] Although the present invention has been described above by way of example only, it is to be understood that the present invention is not limited to these examples and can be modified in many different ways. In particular, combinations of the above-described examples are also contemplated.

Claims

1. A method (M) for operating a braking system (100) of a vehicle, comprising: The method (M) Detecting a stroke of a brake operating device (1) (M1); generating a braking request signal representing a target braking torque based on the detected stroke of the brake operating device (1) or based on an external brake operating signal (M2); generating a braking torque corresponding to the target braking torque based on the braking request signal by operating an electric machine exclusively as a generator (M3); Identifying a change in the stroke of the brake operating device (1) (M4); In response to a stroke reduction of the brake operating device (1), a pressure build-up and blocking process is carried out (M11) in at least one wheel brake (11, 21) of a braking circuit (10, 20), the wheel brake (11, 21) being connected via a supply line (12, 22) to a master brake cylinder (2B) operable by an actuator (2A) based on the braking request signal, and via a discharge line (13, 23) to a pressure accumulator (14, 24); Including, The pressure increase and prevention process includes: Closing (M111) an outlet valve (15, 25) arranged in the discharge 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 (M111); Opening (M112) a high-pressure switching valve (16, 26) disposed in a return line (17, 27) connecting the pressure accumulator (14, 24) to a hydraulic pressure generating device (2); Returning (M113) hydraulic fluid from the pressure accumulator (14, 24) through the return line (17, 27) into the master brake cylinder (2B), wherein the actuator (2A) operates the master brake cylinder (2B) based on the braking request signal as a result of the stroke reduction of the brake operating device (1) so that the pressure in the master brake cylinder (2B) is reduced (M113); The method (M) comprises:

2. The actuator (2A) is an electromechanical actuator, the electromechanical actuator is configured to operate the master brake cylinder (2B) for increasing or decreasing pressure based on a higher-level control signal when the stroke of the brake operating device (1) is constant; The method (M) In response to a certain stroke of the brake operating device (1), Closing the outlet valves (14, 24) (M8); Closing the high pressure switching valve (16, 26) or keeping it closed (M9); reducing (M10) the braking pressure in the wheel brakes (11, 21) by operating the master brake cylinder (2B) for pressure reduction using the actuator (2A) based on the higher-level control signal; 10. The method (M) of claim 1, further comprising:

3. The actuator (2A) is configured to operate the master brake cylinder (2B) to increase or decrease pressure based on the external brake signal when the stroke of the brake operating device (1) is constant.

3. The method (M) according to claim 2.

4. The method (M) In response to an increase in the stroke of the brake operating device (1), operating (M5) the master brake cylinder (2B) for pressure increase based on the braking request signal using the actuator (2A); Closing the high pressure switching valve (16, 26) or keeping it closed (M6); Opening (M7) the outlet valve (14, 24) to divert hydraulic fluid into the accumulator (14, 24); The method (M) according to any one of claims 1 to 3, further comprising:

5. a check valve (18, 28) is disposed in the return line (17, 27) between the pressure accumulator (14, 24) and the high-pressure switching valve (16, 26); the check valves (18, 28) open as soon as a pressure difference across the check valves (18, 28) exceeds a predetermined threshold value as a result of a pressure reduction in the master brake cylinder (2B) when the hydraulic fluid from the pressure accumulator (14, 24) is returned (M103) to the master brake cylinder (2B) through the return line (17, 27).

5. The method (M) according to any one of claims 1 to 4.

6. the predetermined threshold is less than 1 bar; The method (M) according to claim 5.

7. A braking system (100) for a vehicle, comprising: The braking system (100) comprises: a brake operating device (1) configured to perform a stroke as a result of a manual operation; a sensor (3) for detecting the stroke of the brake operating device (1); a brake booster (2) kinematically coupled to the brake operating device (1), the brake booster (2) including an actuator (2A) and a master brake cylinder (2B) operable by the actuator (2A); at least one braking circuit (10, 20) with at least one wheel brake (11, 21) connected to said master brake cylinder (2B) via a supply line (12, 22); an accumulator (14, 24) connected to the wheel brake (11, 21) via a discharge line (13, 23) and connected to the master brake cylinder (2B) via a return line (17, 27); an outlet valve (15, 25) disposed in the discharge line (13, 23); a high-pressure switching valve (16, 26) disposed in the return line (17, 27); A control device (5); and the control device (5) is connected to transmit signals 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) configured to send signals to and / or receive signals from the electric machine (6); the electric machine (6) being operable as a generator to generate a braking torque; The control device (5) is configured to cause the braking system (100) to implement a method (M) according to any one of claims 1 to 6. A braking system (100).

8. a check valve (18, 28) is disposed in the return line (17, 27) between the pressure accumulator (14, 24) and the high-pressure switching valve (16, 26); the check valves (18, 28) are configured to open as soon as a pressure difference across the check valves (18, 28) exceeds a predetermined threshold as a result of a pressure reduction in the master brake cylinder (2B) when the high pressure switching valve (16, 26) is open. The braking system (100) of claim 7.

9. the predetermined threshold is less than 1 bar; The braking system (100) of claim 7.

10. The actuator (2A) is configured as an electromechanical actuator and is configured to operate the master brake cylinder (2B) for pressure increase or pressure decrease based on a higher-level control signal when the stroke of the brake operating device (1) is constant, or The actuator (2A) is configured as a pneumatic actuator and is configured to apply an additional force to the master brake cylinder (2B) to increase pressure 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. A braking system (100) according to any one of claims 6 to 8.

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