Technology for operating a vehicle braking system with an electric parking brake

Abstract

A method for operating a vehicle braking system is described. The vehicle braking system comprises an electric parking brake with an actuating element capable of acting on a wheel brake piston. Furthermore, the vehicle braking system comprises a service brake with a cylinder and a piston contained therein, which is configured to pressurize a hydraulic fluid in the cylinder to actuate the wheel brake piston. The vehicle braking system also includes a first sensor configured to generate a first signal indicating pressure in the cylinder of the service brake, and a second sensor configured to generate a second signal indicating movement of the piston of the service brake.The procedure comprises the following steps: pressurizing the hydraulic fluid in the cylinder to actuate the wheel brake piston; determining, based on the first signal, whether the pressure has reached a target pressure or whether a change in the pressure of the pressurized hydraulic fluid over time has fallen below a first threshold or is zero; determining, based on the second signal, whether a change in the piston's movement over time has fallen below a second threshold or is zero; and generating a control signal for the electric parking brake if both the determination based on the first signal and the determination based on the second signal are positive. Furthermore, a computer program, a control unit, and a vehicle braking system are presented.

Description

Technical field

[0001] The present disclosure relates generally to the technical field of vehicle braking systems, which include an electric parking brake, also called a handbrake. The present disclosure relates in particular to a method for operating a vehicle braking system, a vehicle braking system with a control unit for executing such a method, as well as a computer program product and a control unit. State of the art

[0002] Vehicle braking systems comprising both a hydraulically actuated wheel brake and an electric parking brake are known and already widely used in vehicles. The wheel brake includes a piston that can be moved under the influence of hydraulic pressure. When the piston is in an actuated position, it typically rests against a friction lining and presses this lining against a rotor coupled to a wheel of the vehicle, such as a brake disc. The hydraulic pressure can be generated by the driver, for example, by pedal actuation. Furthermore, it is known to generate the hydraulic pressure, at least partially, independently of the driver by means of additional electro-hydraulic components, or to amplify pressure generated by the driver.

[0003] It is also known to provide an electric parking brake that continuously applies braking force, particularly when the vehicle temporarily assumes a stationary state (parked, hill start, etc.). For this purpose, the electric parking brake can be designed to hold the wheel brake piston in a braking force-generating position and lock it mechanically.

[0004] German patent application DE 10 2011 007 772 A1 discloses a method for adjusting the braking force applied by an electric parking brake and, if necessary, additionally by a hydraulic braking device. The build-up of hydraulic pre-pressure prior to a parking brake application is subject to dynamic influences, meaning that the actual pressure at the brake piston is unknown without the use of a pressure sensor on the wheel brake. To prevent this uncertainty, the pre-pressure of the hydraulic braking device is only taken into account once it has remained within a permissible range for a defined period. The clamping force of the electric parking brake is therefore only made available after the defined period, and not when the dynamic influences have actually subsided. Depending on the chosen time period, this can result in the parking brake being applied later than necessary or too early. Brief overview

[0005] It is a technique for operating a vehicle braking system with an electric parking brake that enables quick and efficient locking of the electric parking brake.

[0006] According to a first aspect, a method for operating a vehicle braking system is provided, wherein the vehicle braking system comprises an electric parking brake with an actuating element capable of acting on a wheel brake piston. Furthermore, the vehicle braking system comprises a service brake with a cylinder and a piston contained therein, which is configured to pressurize a hydraulic fluid in the cylinder for actuating the wheel brake piston. The vehicle braking system also comprises a first sensor configured to generate a first signal indicating pressure in the cylinder of the service brake, and a second sensor configured to generate a second signal indicating movement of the piston of the service brake.The procedure comprises the following steps: pressurizing the hydraulic fluid in the cylinder to actuate the wheel brake piston; determining, based on the first signal, whether the pressure has reached a target pressure or whether a time-dependent pressure change of the pressurized hydraulic fluid has fallen below a first threshold or is equal to zero; determining, based on the second signal, whether a time-dependent movement of the piston has fallen below a second threshold or is equal to zero; and generating a control signal for the electric parking brake if both the determination based on the first signal and the determination based on the second signal are positive.

[0007] The service brake cylinder can be a master cylinder or the cylinder of a cylinder-piston arrangement provided in addition to or as an alternative to the master cylinder. The piston housed in the service brake cylinder can be electrically actuated, for example according to the principle of brake-by-wire (BBW) or electro-hydraulic brake force amplification ("e-boost").

[0008] The first threshold value can be less than 3 bar / s, less than 2 bar / s, or less than 1 bar / s. The first threshold value can be chosen depending on the stiffness of the wheel brake and / or the wear level of the wheel brake's friction lining.

[0009] The second threshold can refer to a piston speed or be convertible to a piston speed. The second threshold can be less than 1 mm / s, less than 0.5 mm / s, or less than 0.1 mm / s. The second threshold can be chosen based on the stiffness of the wheel brake and / or the wear level of a brake pad in the wheel brake.

[0010] The determination based on the second signal can only be performed if the determination based on the first signal was previously positive. In this case, the determination based on the second signal can be interrupted if a subsequent determination based on the first signal is negative.

[0011] The hydraulic fluid in the cylinder can be pressurized to set a target pressure in the service brake cylinder and / or a wheel brake cylinder. Achieving the target pressure may be a prerequisite for the success of the determinations based on the second signal and / or first signal.

[0012] The control signal can cause the electric parking brake to bring the actuator into contact with the wheel brake piston and / or lock it in place. Furthermore, following the actuation and / or locking of the actuator, the pressurized hydraulic fluid can be released.

[0013] At the start of the process, the pressure change over time may be above the first threshold or greater than zero. Additionally or alternatively, the piston's movement over time may be above the second threshold or greater than zero.

[0014] The second sensor allows the temporal movement behavior of the piston to be inferred from its signal. This second sensor can detect at least one parameter from the pedal travel of a brake pedal, an operating parameter (e.g., current draw or speed) of an electric motor used to actuate the piston, and a parameter (e.g., speed or travel) of a gearbox located between the electric motor and the piston.

[0015] The first sensor can be located in the area of ​​the service brake cylinder or near an outlet of the cylinder (e.g., spaced away from the wheel brakes). The first sensor can be a pressure sensor.

[0016] The method can include recording the time interval between an initial point in time, at which pressure builds up in the cylinder, and a second point in time, at which both determinations based on the first and second signals are positive. In this case, the method can further include determining the wear state of a friction lining of the vehicle brake based on the recorded time interval. As the wear state increases, the time interval will also increase, since more hydraulic fluid volume needs to be pumped. Based on the recorded time interval, a signal indicating the wear state can be generated (e.g., a warning signal to the driver).

[0017] According to a second aspect, a computer program product is provided, comprising program code means to perform a procedure as described herein when the computer program product is executed on a processor.

[0018] According to a third aspect, a control unit is provided, comprising a processor and the computer program product as described herein.

[0019] According to a fourth aspect, a vehicle braking system is provided for a vehicle. The vehicle braking system comprises an electric parking brake with an actuating element capable of acting on a wheel brake piston. Furthermore, the vehicle braking system comprises a service brake with a cylinder and a piston contained therein, which is configured to pressurize a hydraulic fluid in the cylinder to actuate the wheel brake piston. The vehicle braking system also comprises a first sensor configured to detect a first signal indicating pressure in the cylinder, and a second sensor configured to detect a second signal indicating movement of the service brake piston. The vehicle braking system further comprises a control unit configured to cause the vehicle braking system to perform a procedure as described herein. Brief description of the characters

[0020] Further advantages, details, and features of the solution described here will become apparent from the following description of exemplary embodiments and from the figures. These show: Fig. 1 a schematic view of part of a vehicle braking system according to an exemplary embodiment; Fig. 2 a cross-sectional view of a wheel brake with an electric parking brake according to an exemplary embodiment; Fig. 3 a flowchart of a method for operating a vehicle braking system according to an exemplary embodiment; and Fig. 4 A schematic diagram for pressure and piston stroke profiles during a pressure increase. Detailed description

[0021] In the following description of exemplary embodiments, the same reference numerals denote the same or similar components.

[0022] Fig. Figure 1 shows a schematic view of part of a vehicle braking system 10 according to an exemplary embodiment. The vehicle braking system 10 comprises a service brake 12 with a cylinder 14 and a piston 16 housed therein. In this exemplary embodiment, the cylinder 14 is the master brake cylinder. The master brake cylinder can be actuated by means of a brake pedal 20, which may be equipped with an electric or other brake force assist 22. The master brake cylinder may also be part of a brake booster system.

[0023] In other embodiments, the cylinder 14 can be part of an electrically actuated cylinder-piston arrangement, which is provided in addition to or as an alternative to the master brake cylinder.

[0024] The piston 16 is configured to pressurize hydraulic fluid 18 in the cylinder 14. The piston 16 is coupled to the brake pedal 20. When the pedal 20 is actuated, the piston 16 moves within the cylinder 14, thereby changing the pressure of the hydraulic fluid 18. Actuation of the pedal 20 is assisted by the electric brake booster 22. Alternatively or additionally to actuation by means of the pedal 20, an electric motor (not shown) can be provided, which is configured to pressurize the hydraulic fluid 18 according to the BBW principle.

[0025] The vehicle braking system 10 further comprises a first sensor 24, which is configured to generate a first signal indicating pressure in the cylinder 14 of the service brake 12. The first sensor 24 is arranged in a hydraulic line 26 at a short distance from the outlet side of the cylinder 14. The first sensor 24 can be a pressure sensor, in particular a diaphragm pressure sensor.

[0026] The vehicle braking system 10 further comprises a second sensor 30, which is configured to generate a second signal indicating movement of the piston 16 of the service brake 12. The second sensor 30 detects the pedal travel of the pedal 20. If an electric motor (not shown) is provided for actuating the piston 16, the second sensor 30 can be configured to detect at least one operating parameter of the electric motor. This at least one operating parameter can be a motor rotation angle, an angular velocity, a power input, a current drawn by the electric motor, or an electrical voltage drop across the electric motor. Two or more second sensors 30, based on different operating principles, can also be provided, the signals of which are evaluated redundantly as described below.

[0027] The vehicle braking system 10 further comprises at least one wheel brake 32 assigned to a vehicle wheel 36, which is connected to the cylinder 14 via the hydraulic line 26. Valves (not shown) are provided in the line 26, which are designed to regulate the fluid communication between the wheel brake 32 and the cylinder 14. The wheel brake 32 comprises a wheel brake piston (in Fig. 1 (not shown), which is designed to be actuated by means of a pressure change in the hydraulic fluid in cylinder 14. The wheel brake 32 comprises at least one friction lining (in Fig. 1 (also not shown), which is designed to be pressed against a rotating, mounted brake disc 34 when the wheel brake 32 is applied. This brakes the vehicle wheel 36, which is rotationally fixed to the brake disc 34.

[0028] The vehicle braking system 10 also includes an electric parking brake 38 with an actuating element (in Fig. 1 not shown). The actuating element is designed to be operated by means of an electric motor (in Fig. (1 not shown) of the electric parking brake 38 to be moved relative to the wheel brake piston. The actuating element can be moved in such a way as to come into contact with a region of the wheel brake piston, such that the actuating element presses the wheel brake piston against a friction lining (in Fig. (1 not shown) presses, which then interacts with the brake disc 34. The actuating element thus allows the wheel brake 32 to be brought into an actuated position and / or held in the actuated position, even if there is no hydraulic pressure in the wheel brake 32.

[0029] The vehicle braking system 10 further comprises a control unit 31 (e.g., in the form of a control device, also called an Electric Control Unit, ECU). The control unit 31 is configured to receive the first signal and the second signal. Furthermore, the control unit 31 is configured to actuate the electric parking brake 38. The reception of the signals and the actuation of the electric parking brake 38 are effected via electrical lines (not shown). Alternatively, at least one connection can be wireless. The control unit 31 consists of Fig. 1 can be one or more additional, in Fig. 1. Control vehicle brakes (not shown) with associated electric parking brakes 38. In particular, the control unit 31 can perform this task for two vehicle brakes on a rear axle.

[0030] The control unit 31 comprises a processor 31A and a memory 31B and is configured to perform the procedure described here for operating the vehicle braking system 10. For this purpose, corresponding instructions are stored in the memory 31A of the control unit 31, which cause the processor 31A of the control unit 31 to carry out the procedure described here. The control unit 31 can, for example, be a central control unit of the vehicle, which also serves to control other functions of the vehicle. Alternatively, the control unit 31 can also be a separate control unit for the vehicle braking system 10.

[0031] Although the Fig. Although Figure 1 shows a vehicle with only one wheel 36 and thus with only one wheel brake 32, the present disclosure is not limited to this. A vehicle braking system according to the present disclosure can include further wheel brakes, each with or without an electric parking brake 38, for example, for further wheels of the vehicle. As a rule, the vehicle will have four wheel brakes 32 distributed between a front axle and a rear axle, with only the two wheel brakes 32 of the rear axle each being equipped with an electric parking brake 38.

[0032] Fig. Figure 2 shows a cross-sectional view of a wheel brake 32 with an electric parking brake 38 and a brake disc 34, as also shown in the embodiment according to Figure 2. Fig. 1. The wheel brake 32 is mechanically designed as a generally known floating caliper brake, with only selected components of the wheel brake 32 being shown.

[0033] The wheel brake 32 comprises a brake housing 40 in the form of a known brake caliper. Furthermore, it is rotationally fixed to a vehicle wheel (see reference numeral 36 in [reference number]). Fig. 1) A coupled brake disc 34 is provided. Friction pads 42 are located on both sides of the brake disc 34, which can be brought into contact with the brake disc 34 to achieve a braking force. For this purpose, the wheel brake 32 includes a movable wheel brake piston 46, which is received in a bore 44 in the brake housing 40. This piston is designed as a hollow piston and, together with the bore 44, defines a hydraulic chamber 47. By introducing and releasing hydraulic fluid into the hydraulic chamber 47, the hydraulic pressure in the chamber 47 can be varied, and the wheel brake piston 46 can be moved along a displacement axis V in a known manner. Movement along the displacement axis V in Fig. 2 to the left corresponds to a movement in a clamping direction Z. Overall, the friction linings 42 can thus be brought into contact with the brake disc 34 to achieve a braking force and, when the hydraulic pressure is released, can be released from it again to ensure a service brake function in a known manner.

[0034] To achieve the desired return movement of the wheel brake piston 46 to its initial position after the hydraulic pressure is released, the wheel brake 32 includes a seal 48, shown schematically. This seal is received in a groove 50 extending from the bore 44 and rests against an outer wall of the wheel brake piston 46. The seal 48 provides a so-called "rollback" function in a known manner, which assists in pushing the wheel brake piston 46 back into its initial position when the hydraulic pressure is released.

[0035] Furthermore, one can recognize in Fig. 2, that an electric parking brake 38 is provided for the parking brake function, which includes a component that can also move along the displacement axis V. The electric parking brake 38 is designed mechanically according to known solutions and comprises an actuator unit 52, which in the present embodiment is designed as a nut / spindle arrangement. More precisely, the actuator unit 52 comprises an actuating element 54, designed in the exemplary embodiment as a nut, which can be moved translationally along the displacement axis V by rotating a spindle 56. The actuating element 54 can also be brought into contact with a piston base 58, which is designed as an inner end wall region of the wheel brake piston 46 opposite the actuating element 54 and delimiting the hydraulic chamber 47.

[0036] The actuator unit 52 is connected to the brake housing 40 via a coupling area 60, wherein an electromechanical drive and gearbox unit (not shown separately) is flanged to the brake housing 40 from the outside at the coupling area 60. The electromechanical drive and gearbox unit drives the spindle 56 rotationally to achieve the desired displacement movement of the actuating element 54 along the axis V.

[0037] In Fig. 2. Furthermore, the gap dimensions S present when the wheel brake function is not activated, which must be bridged to achieve a braking force, are shown. These relate to (in Fig. 2 from left to right): a gap S between the brake housing 40 and the in Fig. 2 left friction lining 42, a gap S between this left friction lining 42 and the brake disc 34, a gap S between the in Fig. 2 right friction lining 42 and the brake disc 34 and a gap S between the wheel brake piston 46 and the right friction lining 42. The electric parking brake 38 must additionally overcome a gap S between the actuating element 54 and the piston base 58 of the wheel brake piston 46 in order to generate braking forces.

[0038] The gap S between the friction linings 42 and the brake disc 34 is generally referred to as the "clearance" or "brake clearance," which is why this gap S is additionally designated with the reference symbol L. The clearance L should have a predetermined minimum value to prevent residual friction torques in the form of unwanted contact between the friction linings 42 and the brake disc 34 when the wheel brake 32 is not applied.

[0039] The gap S between the actuating element 54 and the piston base 58 of the wheel brake piston 46 is a safety clearance, which is why this gap S is additionally designated with the reference symbol X. For system safety reasons, the safety clearance X assumes a predetermined minimum value to ensure proper service brake function even if the electric parking brake 38 is not activated.

[0040] During a normal driver-controlled service braking maneuver, hydraulic pressure is built up in the hydraulic chamber 47, and the wheel brake piston 46 is moved along the clamping direction Z into a braking force-generating actuation position. In doing so, it comes into contact with the right friction lining 42, which in turn comes into contact with the brake disc 34, thus clamping the wheel brake 32 in a known manner according to the floating caliper design. All clearance dimensions S, including the air clearance L, are bridged, with the exception of the safety distance X between the actuating element 54 and the piston crown 58. To release the braking force, the wheel brake piston 46 moves against the clamping direction Z as a result of a reduction in hydraulic pressure and with the "rollback" assistance of the seal 48, whereupon the initially bridged clearance dimensions S and L are readjusted.

[0041] The electric parking brake 38 can generally be activated in the presence or absence of hydraulic pressure to move the wheel brake piston 46 into its actuating position and / or to lock it mechanically. For this purpose, the actuating element 54 is moved along the axis V in the manner described above and is supported (at least when moving in the clamping direction Z and after overcoming the safety distance X) against the piston base 58.

[0042] Fig. Figure 3 shows a flowchart 100 of an exemplary embodiment of a method for operating a vehicle braking system. The vehicle braking system could, for example, be the vehicle braking system 10 shown above (see Figure 3). Fig. 1 and Fig. 2) act, whereby the procedure is then executed by the control unit 31. Generally, this is referred to as... Fig. The methods shown in Figure 3 are executed by a control unit of a vehicle braking system. The following describes the method for a single wheel brake 32, although the method can also be performed for a plurality of wheel brakes 32 with associated electric parking brakes 38.

[0043] The procedure includes, in step 102, pressurizing the hydraulic fluid in cylinder 14 to actuate the wheel brake piston 46. The hydraulic fluid is supplied by actuating the piston 16 (see figure). Fig. 1) Pressurized. The hydraulic pressure build-up can be assisted or initiated by the electric brake booster 22 or an electric motor of a BBW system. The hydraulic fluid in cylinder 14 is pressurized to set a target pressure for a parking brake operation. The target pressure is selected such that the vehicle is prevented from rolling away. The target pressure can be selected depending on the road gradient.

[0044] In step 104, the procedure includes determining, based on an initial signal from pressure sensor 24 and by the control unit 31, whether the pressure has reached a target pressure or whether a pressure change over time of the pressurized hydraulic fluid in the area of ​​cylinder 14 has fallen below a first threshold value or is equal to zero. Three different criteria are therefore possible for the determination according to step 104, which can be checked individually, in pairs, or cumulatively. The determination based on the first signal can be positive if the pressure has reached a predetermined target pressure. Alternatively or additionally, the determination based on the first signal can be positive if a pressure change over time of the pressurized hydraulic fluid in the area of ​​cylinder 14 has fallen below a first threshold value.Alternatively or additionally, the determination based on the first signal can be positive if the pressure change over time of the pressurized hydraulic fluid in the area of ​​cylinder 14 is zero. Typically, only one of these three criteria will be stored in the control unit 31.

[0045] In step 106, the procedure includes determining, based on a signal from the pedal travel sensor 30 or another sensor 30 that detects a parameter (e.g. motor speed) of an electric motor / gearbox unit actuating the piston 16 of the service brake, whether a temporal movement behavior of the piston 16 has fallen below an associated threshold value or is equal to zero.

[0046] Even if no significant pressure change is detected in cylinder 14, hydraulic fluid can still be transported from cylinder 14 to wheel brake 32. This is due, for example, to hydraulic resistance (e.g., throttling points in the area of ​​valves). The resulting decrease in the hydraulic fluid volume in cylinder 14 causes the piston 16 to move (and vice versa). However, as soon as the pressure in cylinder 14 and wheel brake 32 is the same, the hydraulic fluid transport ceases and the piston movement stops. The end of the piston movement, or at least the point at which the pressure falls below the assigned threshold (e.g., 0.1 mm / s), can be determined using the signal from sensor 30.

[0047] Direct pressure measurement of the pressure in the wheel brake piston 46 using a sensor in the wheel brake piston 46 would have disadvantages. For example, a pressure sensor would have to be provided for each wheel brake, which would entail additional costs. This disadvantage can be circumvented in step 106, because determining whether pressure equalization has occurred between the cylinder 14 and the wheel brake 32 can be carried out using the first and second sensors 24 and 30, respectively, without requiring additional pressure sensors on the wheel brakes 32.

[0048] The determination based on the signal from sensor 30 according to step 106 can optionally only be performed if the determination based on the signal from pressure sensor 24 according to step 104 has been successful. In other words, step 106 follows step 104. For many vehicle brake systems, the piston movement only ends after the pressure change over time reaches zero or falls below the first threshold value. Since a successful determination based on the signal from pressure sensor 24 is then a prerequisite for the determination based on the signal from sensor 30, unnecessary evaluations of sensor 30 can be avoided. However, it would also be conceivable to perform both steps 104 and 106 simultaneously or in reverse order.

[0049] The procedure further includes, in step 108, generating a control signal for the electric parking brake 36 if both the determination based on the signal from pressure sensor 24 and the determination based on the signal from sensor 30 are positive. If a pressure change of zero and a piston velocity of zero are used as criteria for both determinations, and both determinations are positive, the pressure in cylinder 14 is static and no hydraulic fluid flows from cylinder 14 to wheel brake 32. Thus, the same pressure exists in cylinder 14 and wheel brake 32. If a threshold value is used as a criterion for at least one determination, the pressure equalization between cylinder 14 and wheel brake 32 is approximately complete. The degree of pressure equalization depends on the choice of threshold values.The smaller at least one of the two threshold values ​​is chosen, the closer the pressures in cylinder 14 and wheel brake 32 will be to each other if both determinations based on the signal from pressure sensor 24 and the signal from sensor 30 are positive. If, for at least one of the two determinations, the criterion is chosen to be falling below the threshold value instead of reaching zero, the corresponding determination will be positive sooner. This reduces the duration of the procedure.

[0050] The control signal causes the electric parking brake 38 to bring the actuating element 54 into contact with the wheel brake piston 46 and / or to lock it in contact with the wheel brake piston 46. The actuating element 54 locks the wheel brake piston 46 in such a way that the wheel brake piston 46 is essentially held in the position to which it was moved due to the pressure build-up in the cylinder 14 and in the wheel brake 32.

[0051] The pressure in cylinder 14 increased until a target pressure was reached. That this target pressure was also applied, at least substantially, to the wheel brake 32 was determined by verifying that both tests were successful. Since the electric parking brake 38 was activated after both tests were successful, the actuating element 54 locks the wheel brake piston 46, at least substantially, in the position it would have been in when the target pressure was applied. The activation can occur immediately after the tests are successful. Therefore, the electric parking brake 38 can be activated promptly.

[0052] The procedure may optionally include one or more further steps. For example, step 110 may involve relieving the pressurized hydraulic fluid. This relieving can be achieved by retracting the piston 16. Alternatively or additionally, at least some of the hydraulic fluid can be directed into a pressureless or slightly pressurized container. Maintaining the pressure generally requires the vehicle to be running. If the braking force for the wheel brake 32 is applied by the electric parking brake 38, the pressure can now be relieved and the vehicle switched off. The parking brake force is then maintained solely by the electric parking brake 38.

[0053] In step 112, the procedure can record the time interval between a first point in time, from which a pressure build-up occurs in cylinder 14, and a second point in time, at which both determinations based on the signal from pressure sensor 24 and the signal from sensor 30 are positive. The benefit of the recorded time interval becomes apparent from Fig. 4 and the subsequent consideration of the pressure profile in the wheel brake 32 along a time axis.

[0054] Fig. Figure 4 shows a schematic diagram 70 for pressure and piston displacement profiles during a pressure increase in cylinder 14. The horizontal axis labeled “t” represents the time axis. The vertical axis labeled “p” and “x” represents both the pressures present in cylinder 14 and wheel brake 32, respectively, and the displacement of piston 16. Diagram 70 includes a horizontal line 82 that represents the target pressure to be achieved in cylinder 14 (and wheel brake 32). Fig. Diagram 4, example shown, describes the case in which the determination based on the first signal is positive when the hydraulic pressure has reached a target pressure.

[0055] Graph 72 describes the pressure in cylinder 14. The pressure in cylinder 14 increases at time t0. Correspondingly, the pressure in wheel brake 32 also increases. At time t1, the pressure exceeds the target pressure, meaning the determination based on the first signal is positive. From time t1 onwards, it is checked whether the determination based on the second signal is also positive. Alternatively, the determination based on the second signal can also be checked earlier, without being triggered by a positive result from the first determination.

[0056] The pressure profile depends on the volume of hydraulic fluid being transported. Therefore, two different cases are now explained: a pressure increase with a small volume of hydraulic fluid being transported and a pressure increase with a large volume of hydraulic fluid being transported. Accordingly, the pressure in the wheel brake 32 and the piston velocities behave differently. Graphs 74 and 76 represent the case with small volumes and are shown with solid lines. Graphs 78 and 70 represent the case with large volumes and are shown with dashed lines. The small volume can correspond to a small wheel brake 32, high brake stiffness, and / or a wear-free friction lining 42. The large volume can correspond to a large wheel brake 32, low brake stiffness, and / or a worn friction lining 42.When the friction lining 42 is worn, the wheel brake piston 46 must travel a greater distance, requiring a larger volume of hydraulic fluid to be delivered to the wheel brake 32. Simultaneously, a worn friction lining 42 increases the brake stiffness.

[0057] Graph 74 depicts the pressure profile in the wheel brake 32 with a small hydraulic fluid volume. The hydraulic fluid is transported into the wheel brake 32 from time t0 until, at time t2, the pressure at the wheel brake is the same as in the cylinder 14. The difference Δt a is the time interval between times t0 and t2.

[0058] Graph 76 represents the displacement of piston 16 with a small volume of hydraulic fluid. Since piston 16 pumps hydraulic fluid into the wheel brake 32 between times t0 and t2, the piston displacement changes, and the piston velocity is greater than zero. From time t2 onwards, the hydraulic fluid transport is complete, and the velocity of piston 16 is zero.

[0059] Graph 78 depicts the pressure profile in the wheel brake 32 with a large hydraulic fluid volume. Due to the larger volume to be transported, the pressure in the wheel brake 32 only reaches the target pressure at time t3, which is later than time t2. Thus, the time duration Δt b between times t0 and t3 greater than the time duration Δt a .

[0060] Based on the diagram in Fig.As can be seen in Figure 4, the time duration Δt is an indicator of how long (and how much) the hydraulic fluid must be transported until a pressure equilibrium is reached between the cylinder 14 and the wheel brake 32. Generally, the volume to be transported increases with the increasing wear of the friction lining 42 of the wheel brake 32. Consequently, with increasing wear of the friction lining 42, the recorded time duration Δt increases. Conversely, the recorded time duration allows the wear state of the friction lining to be determined. Based on the recorded time duration Δt, a warning signal can therefore be generated and issued to the driver to alert them to a required friction lining replacement.

[0061] As explained in the preceding description of exemplary embodiments, the sensors 24 and 30 make it possible to determine the point in time at which the pressures in the wheel brake 32 and in the area of ​​the cylinder 14 have at least substantially equalized. Since this point in time can be reliably determined, the electric parking brake 38 can be activated without unnecessary waiting time. The sensors 24 and 30 required for this only need to record data about the pressure in the area of ​​the cylinder 14 of the service brake and the movement of the piston 16 in the cylinder 14. Therefore, a separate pressure sensor is not required for each wheel brake 32. Instead, it is sufficient to provide the sensors 24 and 30 centrally in the area of ​​the cylinder 14. This reduces the complexity of the vehicle braking system 10.

Claims

[1] Method (100) for operating a vehicle braking system (10), wherein the vehicle braking system (10) comprises the following: an electric parking brake (38) with an actuating element (54) that is able to act on a wheel brake piston (46); a service brake (12) with a cylinder (14) and a piston (16) contained therein, which is designed to pressurize a hydraulic fluid (18) in the cylinder (14) for actuating the wheel brake piston (46); a first sensor (24) configured to generate a first signal indicating pressure in the cylinder (14) of the service brake (12); and a second sensor (30) which is configured to generate a second signal indicating a movement of the piston (16) of the service brake (12); wherein the procedure (100) comprises the following steps: Pressurizing (102) the hydraulic fluid (18) in the cylinder (14) to actuate the wheel brake piston (46) Determine (104), based on the first signal, whether the pressure has reached a target pressure or whether a time-dependent pressure change of the pressurized hydraulic fluid (18) has fallen below a first threshold or is equal to zero; Determine (106), based on the second signal, whether the temporal motion behavior of the piston (16) has fallen below a second threshold or is equal to zero; and Generating (108) a control signal for the electric parking brake (38) when the determination based on the first signal and the determination based on the second signal are both positive. [2] Method (100) according to claim 1, wherein the determination based on the second signal is only carried out if the determination based on the first signal has been positive. [3] Method (100) according to claim 1 or 2, wherein the hydraulic fluid (18) in the cylinder (14) is pressurized to set a target pressure. [4] Method (100) according to one of the preceding claims, wherein the control signal causes the electric parking brake (38) to bring the actuating member (54) into contact with the wheel brake piston (46) and / or to lock it in contact with the wheel brake piston (46). [5] Method (100) according to claim 4, wherein, following the application of the actuating element (54), the pressurized hydraulic fluid (18) is again relieved (110). [6] Method (100) according to one of the preceding claims, wherein at the beginning of the method (100) the pressure has reached a target pressure or the pressure change over time is above the first threshold or is greater than zero and / or the movement behavior over time of the piston (16) is above the second threshold or is greater than zero. [7] Method (100) according to one of the preceding claims, wherein the second sensor (30) detects at least one parameter from a pedal travel of a brake pedal (20), an operating parameter of an electric motor for actuating the piston (16) and a parameter of a transmission provided between the electric motor and the piston (16). [8] Method (100) according to one of the preceding claims, wherein the first sensor (24) is provided in the area of ​​the cylinder (14) of the service brake or near an outlet of the cylinder (14). [9] Method (100) according to any of the preceding claims, wherein the method (100) further comprises recording (112) a time interval between a first time point from which a pressure build-up occurs in the cylinder (14) and a second time point at which both determinations based on the first signal and the second signal are positive. [10] Method (100) according to claim 9, wherein the method (100) further comprises determining a wear state of a friction lining (42) of a wheel brake (32) se based on the recorded time period. [11] Computer program product comprising program code means for carrying out a method (100) comprising the steps according to any one of claims 1 to 10 when the computer program product is executed on a processor (31A). [12] Control unit (31) comprising a processor (31A) and the computer program product according to claim 11. [13] Vehicle braking system (10) for a vehicle, wherein the vehicle braking system (10) comprises an electric parking brake (38) with an actuating element (54) that is able to act on a wheel brake piston (46); a service brake (12) with a cylinder (14) and a piston (16) contained therein, which is designed to pressurize a hydraulic fluid (18) in the cylinder (14) for actuating the wheel brake piston (46); a first sensor (24) configured to detect a first signal indicating a pressure in the cylinder (14); and a second sensor (30) which is designed to detect a second signal indicating a movement of the piston (16) of the service brake (12); wherein the vehicle braking system (10) further comprises a control unit (31) which is configured to cause the vehicle braking system (10) to perform a method (100) comprising the steps according to any one of claims 1 to 10.

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