Method for operating a braking system, and braking system

Abstract

The invention relates to a braking system (100), comprising a primary brake actuator (1) and a secondary brake actuator (2), wherein a target braking pressure (p_target) is to be set (603) by means of the primary brake actuator. In a first monitoring phase, the secondary brake actuator (2) monitors (604) the primary brake actuator (1) to ascertain whether the target braking pressure is present. If the target braking pressure is not present, the secondary brake actuator (2) hydraulically disconnects (605) at least one hydraulic connection (22) between the primary and secondary brake actuator (1, 2).

Description

[0001] Description

[0002] title

[0003] Method for operating a braking system and braking system

[0004] State of the art

[0005] DE 10 2009 001 135 A1 discloses a method for actuating a hydraulic vehicle brake system having an electromechanical brake booster and a wheel slip control system. The disclosure proposes actuating the vehicle brake system with the brake booster in situations where a brake pedal is not depressed, for example, to limit vehicle speed or to maintain a distance to a vehicle ahead, or when parking.

[0006] Disclosure of the invention

[0007] The invention is based on a method for operating a braking system comprising a primary brake actuator and a secondary brake actuator. A target brake pressure is to be set using the primary brake actuator.

[0008] In an initial monitoring phase, the secondary brake actuator monitors the primary brake actuator to determine whether the target brake pressure is present, i.e., whether this target brake pressure is actually being applied. If the target brake pressure is not present, the secondary brake actuator hydraulically interrupts at least one hydraulic connection between the primary and secondary brake actuators.

[0009] The primary brake actuator can be a by-wire actuator, and the secondary brake actuator can be an ESP hydraulic unit hydraulically connected to the by-wire actuator. Together, the primary and secondary brake actuators form a by-wire braking system, in which there is no hydraulic or mechanical connection between the brake pedal / brake actuation unit and the primary and secondary actuators. In other words, the brake pedal / brake actuation unit and the brake actuators are hydraulically and mechanically independent of each other. The primary and secondary brake actuators are both connected to a brake actuation unit, either to a control unit of the brake actuation unit or directly to corresponding sensors (e.g., a position sensor, force sensor, etc.).The by-wire connection of the brake actuation unit allows a braking command from a driver or other vehicle systems to be transmitted quickly and efficiently to the primary and secondary actuators. To ensure redundancy and reliability, the braking command can be easily converted into a target brake pressure by both actuators. By monitoring the primary brake actuator's function from the secondary brake actuator, the secondary brake actuator can react appropriately if the primary brake actuator malfunctions.

[0010] One possible response may be to first close the ESP hydraulic unit's switching valves to lock in the existing pressure in the ESP system and prevent an unwanted pressure drop in the ESP system, i.e., in the secondary brake actuator. Since the switching valves have parallel check valves that allow hydraulic fluid to flow toward the wheel brakes, this can continue to cause pressure buildup even if the primary brake actuator is still functional.

[0011] In a refinement of the method, the secondary brake actuator continues to monitor the primary brake actuator in a second monitoring phase to determine whether the target brake pressure is still not present. If the target brake pressure is still not present, the secondary brake actuator takes over the braking functions of the primary brake actuator. This increases the safety of the overall system. Because the secondary actuator takes over, the takeover and continuation of braking can occur essentially unnoticed by the driver. If pressurized brake fluid is trapped by closing the valves and braking is continued with the second actuator, there is no or only an imperceptible fluctuation in the target brake pressure, and thus in the braking effect.

[0012] It is also advantageous that in the first monitoring phase a non-existence is determined if the target brake pressure is not reached for a first period of time, in particular if it is not reached by a first amount. Thus, after the end of the first time phase, if the target brake pressure is not reached, a defect in the primary brake actuator can be detected promptly and an initial measure can be taken promptly. By monitoring not only for the undershoot and the first period of time, but also for the extent of the undershoot, the initiation of the measure (closing the changeover valves) can be adapted with regard to the time or the severity of the undershoot. It is also advantageous that in the second monitoring phase a non-existence is determined if the target brake pressure is not reached for a second period of time, in particular if it is not reached by a second amount.

[0013] In an advantageous embodiment, the second time period is longer than the first time period. This allows a shorter first time period to quickly respond to a detected pressure drop and thus a suspected defect in the primary brake actuator, while a longer time period can then be used to observe whether the primary brake actuator is actually defective. Continued operation of the primary brake actuator via the parallel-connected check valves of the switching valves is still known to be possible.

[0014] In one embodiment, the hydraulic interruption between the primary brake actuator and the secondary brake actuator is achieved by means of at least one switching valve of the secondary brake actuator. This allows a brake pressure to be easily maintained using means already present in the secondary brake actuator and used as a basis for potential continued operation by the secondary brake actuator.

[0015] In a further embodiment of the method, the primary brake actuator and the secondary brake actuator are controlled with an identical braking command. A braking command is understood to be a braking request or braking intention, which can be specified by a driver or originate from another vehicle system. The braking request, braking command, or braking intention can each include the fact that braking is desired, as well as the desired braking force. Likewise, another vehicle system (such as traffic jam following, adaptive cruise control) can specify the braking force to the braking system.It is advantageous if both brake actuators receive the same braking command, because then the secondary brake actuator can take over the braking function from the primary brake actuator without the secondary brake actuator having to derive, estimate, or calculate the braking command from other variables, which could lead to an unwanted delay. This allows a braking command to be implemented quickly and easily in a by-wire braking system, while ensuring increased reliability. The brake actuators are controlled electronically via appropriate signal lines or communication networks available in the vehicle.

[0016] In a further development of the method, the target brake pressure to be set by the primary and / or secondary brake actuator for a braking command is stored in the primary and secondary brake actuators based on a brake command-target brake pressure relationship. The brake command-target brake pressure relationship can be identical in both brake actuators. As mentioned above, this has the advantage that the secondary brake actuator can assume braking functions reliably and quickly.

[0017] When the braking functions of the primary brake actuator are taken over by the secondary brake actuator, at least one high-pressure switching valve of the secondary brake actuator is opened to supply hydraulic fluid. This allows the secondary brake actuator to receive the required hydraulic fluid, which will be used to build up pressure.

[0018] The invention further comprises a braking system comprising a primary brake actuator and a secondary brake actuator and means for carrying out a method as described. The invention also comprises a computer program configured to carry out the steps of the method and a machine-readable storage medium on which the computer program is stored.

[0019] Figures

[0020] Figure 1 shows a braking system in a first operating situation.

[0021] Figure 2 shows a target brake pressure brake specification relationship with entered values.

[0022] Figure 3 shows the braking system in a second operating situation.

[0023] Figure 4 shows the braking system in a third operating situation.

[0024] Figure 5 shows the braking system in a fourth operating situation.

[0025] Figure 6 shows a process flow for operating a braking system.

[0026] Embodiments of the invention

[0027] Figure 1 shows a schematic representation of a braking system 100, which comprises a primary brake actuator 1 and a secondary brake actuator 2. Primary brake actuator 1 and secondary brake actuator 2 are controlled by an actuating unit 5 with signals 6 and 7. Actuating unit 5 can detect the actuation of the braking system by a driver via a brake actuating element 20 of a vehicle, for example, with a sensor unit 19. Signals 6 and 7 thus correspond to an existing braking request.

[0028] Braking commands can also be provided by other vehicle systems, such as automated driving systems, automatic follow-through systems, or emergency braking systems. Such systems are not shown here for simplicity.

[0029] Primary brake actuator 1 and secondary brake actuator 2 are both capable of generating brake pressure in connected hydraulic brake circuits 3 and 4 at the respective hydraulic wheel brakes 21, thus generating a braking effect for the vehicle in a known manner, for example, via brake pads and brake discs. Both brake actuators 1 and 2 are capable of independently generating brake pressure at the wheel brakes.

[0030] The primary brake actuator 1 is designed as a by-wire brake actuator that is controlled by signals, for example signal 6 from the actuation unit 5. Such a primary brake actuator 1 can comprise a control unit that controls a motor unit. Via a gear, the motor unit can displace an input piston of a master brake cylinder, directly or via intermediate components such as springs or additional pistons, which delimits a hydraulic chamber of the master brake cylinder. By displacing the input piston, hydraulic fluid located in the chamber is subjected to force and thus displaced in the direction of the output lines 22, which ultimately allows a hydraulic pressure to be set in the braking system. Other designs of the primary brake actuator 1 are conceivable, for example pumps or accumulators.

[0031] The secondary brake actuator 2 can be connected to output lines 22. In Figure 1, this is shown as a classic brake control system, the elements of which are briefly discussed below. Like the primary brake actuator 1, the secondary brake actuator 2 can also be controlled based solely on signals, such as signal 7 from the actuating unit 5, and can set a brake pressure. For this purpose, the secondary brake actuator 2 has a hydraulic pump 12 in each brake circuit 3 and 4, which can be driven by a motor 13. If the pump 12 pumps hydraulic fluid towards one or more wheel brakes 21, the brake pressure can be generated at these. Targeted brake pressure generation at individual ones of the multiple wheel brakes 21 can be achieved using existing hydraulic valves.In the hydraulic lines leading to each wheel brake 21 there are inlet valves 8, for example controllable, normally open valves, with a check valve in parallel, which prevents hydraulic flow to the wheel brakes. Leading away from the wheel brakes there are outlet valves 9, in the form of switchable, normally closed valves without a check valve. The outlet valves 9 connect the wheel brakes to a hydraulic accumulator 10, which, together with the outlet valves 9, is connected to the suction side of the pump 12 via a check valve 11. The check valve 11 allows hydraulic flow towards the pump. The outlet valves 9 are normally closed valves. Also connected to the suction side of the pump 12 is a hydraulic connection to a high-pressure switching valve 16, which is switchable, normally closed and has no check valve and is connected with its other side to one of the hydraulic lines 22 of the primary brake actuator 1.

[0032] On the pressure side of the pump there are elements 14 which influence the hydraulic flow from the pump 12, such as a damper, and a check valve which allows hydraulic flow away from the pump.

[0033] The pressure side of pump 12 is connected to one of the hydraulic lines 22 of the primary brake actuator 1 via a normally open valve 15, which includes a check valve that allows hydraulic flow from the primary brake actuator 1 toward the inlet valves 8. The pressure side is also connected to the associated inlet valves 8. The valves 15 are so-called changeover valves, which are controllable and include said check valve when normally open.

[0034] In situations that require brake control intervention, such as for the implementation of anti-lock functions, traction control or skid protection, brake pressure can be increased or decreased for each wheel individually, either directly via the primary brake actuator 1 or with the involvement of the hydraulic pumps 12, the accumulator chamber 10 and correspondingly positioned valves in a known manner.

[0035] Normal braking is typically performed by the primary brake actuator 1, which can generate brake pressure at the wheel brakes 21 via the hydraulic lines 22, the valves 15, and the inlet valves 8. The outlet valves 9 are then closed. The hydraulic lines 23 highlighted in Figure 1 (also including the lines 22) show where the corresponding brake pressure is present in the secondary brake actuator 2 when pressure is generated by the primary brake actuator 1. It can be seen that the brake pressure reaches the wheel brakes 21 and cannot pass through them due to the closed valves 16 and the closed outlet valves 9.

[0036] The prevailing brake pressure p_actual can be determined by means of a pressure sensor 17.

[0037] In order to adequately implement a braking command, for example a driver's braking request, the braking request must be determined. This can be recorded, for example, using an actuating travel s of a brake pedal 20, which is detected by a sensor 19. This actuating travel s can be fed to the primary and secondary brake actuators 1, 2 as signal 6 or 7. Pre-processing of the signal is also conceivable. Likewise, instead of an actuating travel, an actuating force, an actuating pressure, an actuating speed, or correlated variables can be recorded. For the sake of simplicity, the following assumes a braking request s that is present in a certain size or to a certain extent.

[0038] Likewise, a braking command can be issued directly by other vehicle systems that do not necessarily rely on a driver command.

[0039] An expected brake pressure p_target, which is set by the primary brake actuator 1, can be assigned to the brake command s by means of a characteristic curve 205. With the primary brake actuator 1 intact, a brake pressure p_target according to marking 201 results in Figure 2 for a brake command s1.

[0040] Characteristic curve 205 is stored or present in both brake actuators 1, 2. Thus, both brake actuators 1, 2 know which brake pressure p_target should be set for the respective brake command s, transmitted as signals 6, 7. In the following, it is assumed that primary brake actuator 1 can no longer, or no longer completely, generate the brake pressure p_target that matches the brake command s1. This can manifest itself in a lower brake pressure being set according to marking 202, although a value according to 201 is expected.

[0041] This can be determined in the secondary brake actuator 2 by means of the control unit 18, to which the brake pressure sensor 17 is connected.

[0042] Figure 3 shows the brake pressure 23 prevailing in the system, which essentially corresponds to the brake pressure distribution in Figure 1, but is lower according to the marking 202 in Figure 2. For example, if the brake pressure in Figure 1 was 60 bar, in Figure 3 only 50 bar may be present.

[0043] If the lower brake pressure 202 persists for longer than a first time period t1, the switching valves 15 in the secondary brake actuator 2 are closed. This locks in the remaining pressure in the hydraulic lines of the secondary brake actuator 2. An example time period could be 10 milliseconds, after which the switching valves 15 are closed. The switching valves 15 have parallel-connected check valves that allow hydraulic fluid to flow toward the wheel brakes. Thus, in the event that the primary brake actuator 1 is still functional, it can continue to cause pressure to build up at the wheel brakes.

[0044] In Figure 4, line 23a shows how the brake pressure generated by the primary brake actuator continues to drop, corresponding to Figure 2, to a value of 203. This further drop can occur, for example, to 40 bar. In the section marked by line 23b, the brake pressure of Figure 3 continues to be present due to the closed valves 15, corresponding to mark 202 in Figure 2.

[0045] If the secondary brake actuator 2 determines by means of the pressure sensor 17 and the control unit 18 that the brake pressure is lower than the expected brake pressure (201 in Figure 2) for a second time period t2, it is assumed that the primary brake actuator is defective. The secondary brake actuator takes over the tasks of the primary brake actuator and continues braking based on the brake command s1. The brake pressure expected according to characteristic curve 205 is then readjusted by means of the pumps 12, with the corresponding valve position (shown in Figure 5). The changeover valves 15 remain closed, the valves 16 are opened so that the pumps 12 can suck in hydraulic fluid. The section of the hydraulic lines marked with line 23b is then back at the initial pressure level 201, which is to be adjusted based on the braking command s1 using the characteristic curve 205.The pressure level maintained by the primary brake actuator 1 may have dropped further, possibly even to zero (Figure 2, mark 204).

[0046] Once a defect in the primary brake actuator 1 is detected, all braking functions are taken over and implemented by the secondary brake actuator 2.

[0047] The procedure described is as follows.

[0048] In the initial step 601, a starting point is assumed in which both brake actuators, primary 1 and secondary 2, are functional.

[0049] In the following step 602, the brake command s1 is detected and made available as signal 6,7 to the brake actuators 1,2.

[0050] In the following step 603, the brake pressure 201 is set by the primary brake actuator 1 according to the brake specification s1 using the characteristic curve 205.

[0051] In a monitoring step 604, the secondary brake actuator monitors whether the currently prevailing brake pressure p_actual is more than, for example, 5 bar below the target brake pressure p_target provided for the brake specification s1, and this for a first time period t > t1. If this is the case, the method continues with step 605; otherwise, monitoring for undershooting continues for a first time period t1, and the pressure is further adjusted according to step 603 with the primary brake actuator 1.

[0052] In step 605, a monitoring function is activated in the secondary brake actuator 2 to check whether another criterion 606 is met. In addition, the valves 15 are closed to lock in the brake pressure already provided by the primary brake actuator 1, as described above.

[0053] In step 606, it is checked whether the current brake pressure p_actual falls below the target brake pressure p_target by, for example, more than 5 bar for a second time period t > t2.

[0054] If this is the case, the takeover of the braking functions by the secondary brake actuator 2 is enabled in step 607. The valves 15 are kept closed, the valves 16 are opened, and the pumps 12 are controlled so that the target brake pressure according to the brake specification s1 based on characteristic curve 205 is again reached by operating the secondary brake actuator 2.

[0055] In addition, in step 608, a warning signal (acoustic, visual, ...) can be issued in the vehicle to indicate the defect of the primary brake actuator.

[0056] In step 609, the braking requests are then taken over solely by the secondary brake actuator 2, for example to keep the vehicle stationary, to accelerate the vehicle again, or to carry out driver-induced and system-induced braking.

Claims

Claims 1. Method for operating a braking system (100), comprising a primary brake actuator (1) and a secondary brake actuator (2), wherein a target braking pressure (p_target) is to be set (603) by means of the primary brake actuator (1), and wherein the secondary brake actuator (2) monitors (604) the primary brake actuator (1) in a first monitoring phase to determine whether the target braking pressure is present, wherein if the target braking pressure is not present, the secondary brake actuator (2) hydraulically interrupts (605) at least one hydraulic connection (22) between the primary and secondary brake actuators (1, 2).

2. Method according to claim 1, wherein the secondary brake actuator (2) in a second monitoring phase further monitors (606) the primary brake actuator (1) to determine whether the target brake pressure is still not present, wherein if not present, the secondary brake actuator (2) takes over the braking functions of the primary brake actuator (1) (607).

3. Method according to claim 1, characterized in that in the first monitoring phase, a non-existence is determined if the target brake pressure is undershot for a first period of time, in particular by a first amount.

4. Method according to claim 2, characterized in that in the second monitoring phase, a non-existence is determined if the target brake pressure (p_target) is undershot for a second period of time, in particular by a second amount.

5. The method according to claim 3 and 4, wherein the second time period is greater than the first time period.

6. Method according to one of the preceding claims, wherein the hydraulic interruption between the primary brake actuator (1) and the secondary brake actuator (2) is effected by means of at least one changeover valve (15) of the secondary brake actuator (2).

7. Method according to one of the preceding claims, wherein the primary brake actuator (1) and the secondary brake actuator (2) are controlled (602), in particular electronically, with an identical braking specification (6, 7, s).

8. The method according to claim 7, wherein the target brake pressure (p_target) to be set by the primary and / or secondary brake actuator (1, 2) for a brake specification (6, 7, s) is stored in the primary and secondary brake actuator (1, 2) on the basis of a brake specification - target brake pressure relationship (205), in particular on the basis of a brake specification - target brake pressure relationship (205) that is identical in both brake actuators (1, 2).

9. The method according to claim 2, wherein, in order to take over the braking functions of the primary brake actuator (1) by operating the secondary brake actuator (2), at least one high-pressure switching valve (16) of the secondary brake actuator (2) is opened during its operation to supply it with hydraulic fluid.

10. Braking system comprising a primary brake actuator (1) and a secondary brake actuator (2), and comprising means (18,19) for carrying out a method according to one of the preceding claims.

11. A computer program configured to carry out the steps of the method according to any one of claims 1 to 9.

12. A machine-readable storage medium on which the computer program according to claim 11 is stored.

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