Method for operating brake system

Abstract

The invention relates to a method for operating a brake system (1), which comprises a booster brake (4) and a driving dynamics regulator (8), after a detection of a level below a minimum level in a brake fluid reservoir (12) of the brake system (1). The method comprises the following steps: detecting (D) a brake signal for advancing the external force piston (64) into the forward position (F), in which a necessary brake pressure (psoll) is achieved, determining (G) a stroke amount ([Delta] sF) of the external force piston (64) from the original position into the current position, and maintaining (H) the brake pressure (p), and in the event that the brake pressure (p) drops during the maintaining (H), determining (G) the brake pressure ([Delta] sF) of the external force piston (64) from the original position into the current position. If so, the external force piston (64) is further advanced (I) by a correction stroke (sK). The method additionally comprises the following steps: after the end of the braking process, the external force piston (64) is moved back (J) by the determined stroke amount ([Delta] sF), and the brake system (1) is degraded (K) if a limit value (sMax) of the forward position of the external force piston (64) is reached.

Description

Technical Field

[0001] The invention relates to a method for operating a brake system comprising a power-assisted brake and a vehicle dynamics control system after detecting a drop below a minimum fluid level in a brake fluid reservoir of the brake system. The invention also relates to a brake system with which the method can be implemented. Background Art

[0002] The characteristic feature of today's power-assisted brake systems is that they are connected to the driver mechanically and / or hydraulically. This is achieved by connecting the brake pedal to an input rod. The driver's braking desire is detected in the entire system via this input rod and the pedal feel is realized in the form of a force-displacement characteristic curve. The pressure is then generated by a piston that is hydraulically decoupled from the driver's foot. As a fallback, the driver's foot is connected to the wheel brake cylinders by means of a brake cylinder actuated by muscle force, which enables the driver to apply brake pressure with his foot. This allows the vehicle to still be braked in the event of a malfunction.

[0003] DE 10 2018 222 488 A1 discloses an electro-hydraulic assisted vehicle brake system for an autonomously driven land vehicle. The electro-hydraulic assisted vehicle brake system is provided with two redundant assisted brake pressure generators, so that in the event of failure of the self-driven and externally driven brake pressure generators, the other assisted brake pressure generator can brake the vehicle without driver intervention.

[0004] The object of the present invention is to specify a method with which the brake system can be kept in a safe state for a longer period of time when a minimum fluid level in a brake fluid reservoir is detected.

[0005] This object is achieved by a method for operating a brake system having the features of claim 1. Preferred embodiments are revealed in the dependent claims. Summary of the invention

[0006] The invention provides a method for operating a brake system having a power-assisted brake and a driving dynamics controller after detecting a drop below a minimum fluid level in a brake fluid reservoir of the brake system. When a drop below the minimum fluid level is detected, it is assumed that there is a leak in the brake system, through which part of the brake fluid has been lost. The method comprises the following steps: detecting a brake signal that advances an external force piston into an advanced position, in which the required brake pressure is achieved, and determining the stroke value of the external force piston from an original position to the current position. The stroke value is the value of the feed stroke of the external force piston, which value can be given, for example, in millimeters. The original position is the position of the external force piston in which no brake pressure is generated.

[0007] In another step, the brake pressure is maintained and, if the brake pressure drops during the maintenance period, the external force piston is further moved forward by a correction stroke. In this case, the brake pressure drops due to the loss of brake fluid due to leakage. In order to keep the brake pressure constant despite the leakage, the external force piston is correspondingly moved toward the outlet of the external force cylinder to compensate for the loss of brake fluid.

[0008] After the braking process is over, the external force piston moves back by the determined stroke value. Therefore, the external force piston moves back by the same stroke value as the stroke value used to reach the braking pressure for the first time. Therefore, the external force piston does not additionally move back by the correction stroke. Accordingly, before the next braking, the external force piston does not reach the previous original position. Since the brake fluid may be lost due to leakage during the braking process, air is prevented from entering the brake system. As a result, the brake system can be kept in a safe state for a longer time without having to be downgraded when a drop below the minimum fluid level is detected.

[0009] If the limit value of the forward position of the external force piston is reached, the brake system is downgraded. The limit value can be fixedly set or variably adapted. The downgrade of the brake system should be understood as the transition of the brake system to emergency operation, in which the full functional capacity is no longer provided. As a result, the brake system continues to operate in a degraded state in a timely manner before the braking capacity fails, so that limited braking can still be achieved. As a result, it can be ensured that the brake system runs into the degraded mode in a timely manner despite the long operation of the brake system. Therefore, the safety of such a brake system can still be guaranteed.

[0010] In a preferred embodiment of the invention, the forward end position of the external force piston is selected as a limit value, in which no further pressure buildup can be achieved by the external force piston. The forward end position of the external force piston is here the position at which the maximum displacement stroke of the external force piston is reached. This can be, for example, the position in which the external force piston reaches the external force cylinder at the bottom. In this case, there is no brake fluid in the external force cylinder that could increase the brake pressure. By selecting this limit value, the brake system can be kept in a safe, non-degraded state for the maximum time, so that the full braking capacity is available.

[0011] In another preferred embodiment of the present invention, the demotion of the brake system is directly implemented when the limit value is reached. When the limit value is reached, it may not be possible to generate additional brake pressure. In order to be able to continue to brake the vehicle when the brake pressure decreases due to leakage, the brake system is immediately degraded. This provides sufficient safety for braking the vehicle.

[0012] Preferably, the travel before reaching the end position is selected as the limit value for the forward position, so that after reaching the limit value, the braking process can be ended before reaching the end position taking into account the correction travel. Therefore, after reaching the limit value, despite the correction path, there is still enough brake fluid in the external brake cylinder for the current braking process, so that the braking process can be safely ended. For the correction travel, a certain value is advantageously assumed. Therefore, the braking system does not have to be degraded during the braking process. Since the degradation during the braking process would lead to a loss of braking force, the current braking process can be ended with full braking force. The safety of the braking process is thereby increased.

[0013] In an advantageous embodiment, the correction path from the previous braking is used for determining the limit value. Since the correction path depends on the type and size of the leak, a fixedly assumed correction path may be too small. However, several braking cycles are usually performed before the limit value is reached. Based on the correction path required during these braking cycles, the correction path for determining the limit value can be better estimated, so that the brake system can be kept in a safe state for a longer period of time by this method.

[0014] Advantageously, the degrading of the brake system is performed after the braking process is completed. Therefore, the degrading of the brake system does not have to be performed during braking. Thus, the safety of the brake system is improved.

[0015] In a further advantageous embodiment, the circuit separation valve for separating the brake circuit of the driving dynamics control is closed in the unbraked state. The two brake circuits are separated from each other by the circuit separation valve. This prevents the other brake circuit from running unloaded in addition to the circuit with the leak. Thus, at least in the degraded state there is a brake circuit with which a braking force can be applied to the vehicle brake.

[0016] According to an advantageous embodiment, after the detection of a drop below a minimum fluid level in a brake fluid reservoir of the brake system, a warning is issued to the user. Thus, the driver is informed of a problem with the brake system at an early stage, before the brake system degrades, so that he can still drive safely to the nearest repair station. The warning is advantageously issued visually, acoustically and / or haptically.

[0017] Preferably, after the demotion of the brake system, the brake pressure is applied by the driving dynamics controller and / or the master brake cylinder. Therefore, after the demotion, no brake pressure is applied by the external force cylinder. Therefore, in emergency operation, the brake pressure continues to be applied despite the positioning of the external force piston in the forward end position, so that the safety of the brake system is ensured.

[0018] Advantageously, after the downgrade of the brake system, the brake circuit is separated by a circuit separation valve for separating the brake circuit for driving dynamics control. With the downgrade, the pressure generation of the piston due to external force is omitted, so that the circuit separation valve does not need to be opened. As a result, the influence of leakage on the brake circuit is limited, so that braking can be achieved using at least one brake circuit in emergency operation. This improves the failure reliability of the brake system.

[0019] In addition, the object on which the present invention is based is also achieved by a brake system having a power-assisted brake and a driving dynamics control and a control unit which is configured to execute a method for operating a brake system when a minimum fluid level in a brake fluid reservoir of the brake system is detected to have fallen below. With such a brake system, the advantages mentioned for the method are achieved. As a result, such a brake system can remain in a safe state for a longer period of time. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Embodiments of the present invention are shown in the drawings and explained in detail in the following description. The drawings show:

[0021] Figure 1 An exemplary embodiment of a braking system for carrying out the method during braking is shown.

[0022] Figure 2 A method for operating a brake system according to an embodiment of the invention is shown, and

[0023] Figure 3 A time diagram showing the travel of the external force piston in conjunction with the brake pressure. DETAILED DESCRIPTION

[0024] Figure 1 An exemplary embodiment of a brake system 1 for carrying out the method is shown. The brake system 1 is shown here during braking. The brake system 1 comprises an externally-powered brake 4 and a driving dynamics controller 8. The externally-powered brake 4 comprises a brake fluid reservoir 12, the fluid level of which can be determined using a fluid level sensor (not shown). In the brake system 1 shown here, a signal 16 is present to indicate that the fluid level is below a minimum fluid level.

[0025] The brake system 1 additionally comprises a master brake cylinder 20 which can be actuated by the driver via a brake pedal 24 and is supplied with brake fluid from a brake fluid reservoir 12. The travel of the brake pedal is measured by a travel sensor 28. The master brake cylinder 20 has a first and a second brake piston 32, 36, with which separate brake circuits 40a, 40b can be actuated, respectively. The pressure generated in the master brake cylinder 20 is measured by means of a master brake cylinder pressure sensor 4.

[0026] In control operation of the brake system 1, the master brake cylinder isolating valves 48a and 48b are closed. The master brake cylinder isolating valves 48a, 48b can be used to connect the master cylinder 20 to the brake circuits 40a, 40b of the driving dynamics control 8 in order to apply brake pressure to the vehicle brakes 52a, 52b, 52c, 52d of the brake system 1. In the case shown here, the pressure of the master brake cylinder 20 is used to actuate the brake feel simulator 56, by which the brake feel is generated for the driver.

[0027] The external force brake 4 additionally comprises an external force cylinder 60, in which an external force piston 64 is arranged, which can be moved in the axial direction of the external force cylinder 60 by a motor 68 to generate a brake pressure. In the figure shown here, the external force piston 64 is in the forward end position. The external force cylinder 60 is fluidically connected to the brake fluid reservoir 12 via an external force cylinder valve 72. The brake pressure generated by the external force piston 64 is measured by a brake pressure sensor 76. The external force cylinder 60 is connected to the two brake circuits 40a, 40b of the driving dynamics regulator 8 via two circuit separation valves 80a, 80b.

[0028] In order to supply brake fluid to the brake circuits 40a, 40b of the driving dynamics controller 8, these brake circuits 40a, 40b are directly connected to the brake fluid reservoir 12 via non-return valves 84a, 84b, respectively. Via these non-return valves 84a, 84, the driving dynamics controller 8 draws fluid from the brake fluid reservoir 12 when necessary. In order to control the driving dynamics and to apply and release the brake pressure to the vehicle brakes 52a, 52b, 52c, 52d, the driving dynamics controller 8 has a variety of inlet and outlet valves and a brake fluid pump 92 driven by a pump motor 88.

[0029] In addition, the brake system 1 has a control unit 94 which is connected to the external power brake 4 and the vehicle dynamics control system 8 in terms of signal technology. The control unit 94 receives the measured values ​​of the sensors and controls the valves of the external power brake 4 and the vehicle dynamics control system 8 as well as the movement of the external power piston 64. Figure 2 The method shown according to the invention runs on control unit 94 .

[0030] Figure 2A method for operating a brake system 1 according to an embodiment of the present invention is shown. In a first step A, a drop below the minimum fluid level in the brake fluid reservoir 12 is detected. In a subsequent step B, a warning is issued to the driver. The warning is configured in the form of an illuminated warning light. The brake system 1 is operated accordingly in a corresponding leakage mode. Since this leakage leads to a loss of brake fluid, in the next step, the circuit separation valves 80a and 80b are closed during the unbraked state in order to separate the brake circuits 40a, 40b. This prevents the two brake circuits 40a, 40b from running unloaded due to leakage, so that at least one brake circuit 40a, 40b can still be used for braking the vehicle.

[0031] In the next step D, it is detected whether a brake signal is present. If a brake signal is present, the circuit separation valves 80a, 80b are opened again in the next step E. This ensures that the brake pressure generated by the external force piston 64 can be applied to the brake circuits 40a, 40b and thus to the vehicle brakes 52a, 52b, 52c, 52d. Accordingly, in the subsequent step F, the external force piston 64 is pushed into the forward position in accordance with the brake signal, so that the necessary brake pressure p is generated. soll .

[0032] Then, in step G, the stroke value Δs of the external force piston 64 from the original position to the current position is obtained. F This can be determined by means of a rotation sensor 96 attached to the motor 68 for driving the external force piston 64. When the required brake pressure p soll This brake pressure is then maintained in the next step H. If the brake pressure p measured by the master brake cylinder pressure sensor 44 should drop during the maintenance period due to leakage, the external force piston 64 is advanced further until the required brake pressure p is present again. soll This results in a correction stroke s of the external force piston 64 K .

[0033] At the end of the braking process B END Then, the external force piston 64 is moved back in step J by the stroke value Δs determined in step G. F to an initial position in which there is no brake pressure p. The new initial position can be due to the correction travel s K Finally, check whether the forward position of the external force piston 64 has reached the limit value s Max , so that no further braking can be performed. If this is not the case, the method can be performed further. Here, it starts again with step C, wherein the circular separating valves 80a, 80b are first closed.

[0034] The method can be performed frequently until the limit value s of the forward position of the external force piston 64 is detected. Max In this case, the brake system 1 is degraded so that the brake pressure p is applied via the driving dynamics controller 8 and / or the master brake cylinder 20 .

[0035] Figure 3 1 shows a travel time diagram s / t of the external force piston 64 in relation to the brake pressure p / t. In this figure, a plurality of brake cycles 100 are shown. The upper curve shows the travel s of the external force piston 64, while the lower curve shows the brake pressure p. It can be seen here that the brake pressure p increases as the external force piston 64 moves forward until the required brake pressure p is reached. soll Although the brake pressure p soll The stroke s of the external force piston 64 changes further. This is because the external force piston 64 compensates for the pressure loss caused by the leakage by advancing further. In the absence of leakage, when the required brake pressure p is reached soll , the stroke of the external force piston will not change.

[0036] The leakage results in a loss of brake fluid. Accordingly, after a minimum fluid level in the brake fluid reservoir 12, a warning is issued at point FLI and the leakage mode is activated. Accordingly, in a subsequent braking cycle 100 after the braking process has ended, the external force piston 64 is only moved back by the amount of the brake pressure p that is required for the first time to be reached. soll Required stroke value Δs F Accordingly, in order to maintain the brake pressure p soll Required correction stroke K Therefore, the new original position is moved forward by the correction stroke s K .

[0037] In the leakage mode, more braking cycles can be performed until the external force piston 64 reaches the limit value s of the forward position. Max .like Figure 3 As shown in , this can be in the forward end position. Max From the point in time CFM, the braking system continues to operate only in degraded mode.

Claims

1. A method for operating a brake system (1) after detecting that a minimum fluid level in a brake fluid reservoir (12) of the brake system (1) has fallen below the minimum fluid level, the brake system comprising a power-assisted brake (4) and a driving dynamics control (8), the method comprising the following steps: - detect (D) brake signals, - pushing (F) the external force piston (64) into the forward position, in which the required braking pressure (p soll ), - Calculate (G) the stroke value (Δs) of the external force piston (64) from the original position to the current position F ), - Maintaining (H) the brake pressure (p) and, if the brake pressure (p) decreases during the maintaining (H) period, further advancing (I) the external force piston (64) by a correction stroke (s K ), After the braking process is completed, the external force piston (64) is retracted (J) by the determined stroke value (Δs F ),as well as - If the limit value (s) of the forward position of the external force piston (64) is reached Max ), the braking system (1) is downgraded (K).

2. The method according to claim 1, characterized in that The forward end position of the external force piston (64) is selected as the limit value (s Max ), in which advanced end position no further pressure buildup (p) can be achieved by the external force piston (64).

3. The method according to claim 2, characterized in that When the limit value (s Max ), the braking system (1) is directly downgraded (K).

4. The method according to claim 1, characterized in that: For the forward position, the distance (s) before reaching the end position is selected as the limit value (s Max ), so that when the limit value (s Max ) after taking into account the corrected stroke (s Max ), the braking process can be ended before reaching the end position.

5. The method according to claim 4, characterized in that For the limit value (s Max ) is obtained using the correction distance (s) from the previous braking K ).

6. The method according to claim 4 or 5, characterized in that: After the braking process is completed, the brake system (1) is degraded (K).

7. The method according to any one of the preceding claims, characterized in that Circuit separation valves (80a, 80b) for separating brake circuits (40a, 40b) of the vehicle dynamics control system (8) are closed (C) in an unbraked state.

8. The method according to any one of the preceding claims, characterized in that After detecting that a minimum fluid level in a brake fluid reservoir (12) of the brake system (1) has fallen below, a warning (B) is issued to a user.

9. The method according to claim 8, characterized in that The warning is issued visually, audibly and / or tactilely.

10. The method according to any one of the preceding claims, characterized in that After the reduction (K) of the brake system, the brake pressure (p) is applied via the vehicle dynamics control system (8) and / or the master brake cylinder (20).

11. The method according to any one of the preceding claims, characterized in that After degrading (K) of the brake system (1), the brake circuits (40a, 40b) are separated by circuit separation valves (80a, 80b) for separating the brake circuits (40a, 40b) of the vehicle dynamics control (8).

12. A braking system (1) having an externally powered brake (4) and a driving dynamics controller (8) and a controller (94), which is configured to execute a method according to any of the preceding claims when a minimum fluid level in a brake fluid reservoir of the braking system is detected.

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