METHOD FOR OPERATING A BRAKE SYSTEM

Abstract

A method for operating a brake system, which includes a power brake and a vehicle dynamics control, after detecting that the level in a brake fluid reservoir of the brake system has fallen below a minimum fill level. The method includes detecting a brake signal, advancing a power piston to a forward position in which the required brake pressure is achieved, ascertaining the amount of travel of the power piston from the home position to the current position, holding the brake pressure and further advancing the power piston by a correction distance if the brake pressure decreases during the holding. The method also includes retracting the power piston by the ascertained amount of travel after the braking process has ended and degrading the brake system when a limit value for the forward position of the power piston has been reached.

Claims

1-12. (canceled)

13. A method for operating a brake system, which includes a power brake and a vehicle dynamics control, after detecting that a level in a brake fluid reservoir of the brake system has fallen below a minimum fill level, the method comprising the following steps: detecting a brake signal; advancing a power piston to a forward position in which a required brake pressure is achieved; after the advancing, ascertaining an amount of travel of the power piston from a home position to a current position; holding a brake pressure and further advancing the power piston by a correction distance when the brake pressure decreases during the holding; retracting the power piston by the ascertained amount of travel after a braking process has ended; and degrading the brake system when a limit value for a forward position of the power piston has been reached.

14. The method according to claim 13, wherein a front end position of the power piston at which no further pressure build-up by the power piston is possible is the limit value.

15. The method according to claim 14, wherein the degradation of the brake system is carried out immediately upon reaching the limit value.

16. The method according to claim 13, wherein a distance before reaching an end position is the limit value for the forward position, so that, after reaching the limit value, the braking operation can be ended before reaching the end position by including a correction distance.

17. The method according to claim 16, wherein a correction distance from previous braking operations is used to ascertain the limit value.

18. The method according to claim 16, wherein the degradation of the brake system is carried out after the braking operation is complete.

19. The method according to claim 13, wherein circuit separating valves are closed in an unbraked state to separate brake circuits of the vehicle dynamics control.

20. The method according to claim 13, wherein, after detecting that the level in a brake fluid reservoir of the brake system has fallen below the minimum fill level, a warning is output to a user.

21. The method according to claim 20, wherein the warning is output optically and/or acoustically and/or haptically.

22. The method according to claim 13, wherein, after the degradation of the brake system, the brake pressure is applied via the vehicle dynamics control and/or a master brake cylinder.

23. The method according to claim 13, wherein, after the degradation of the brake system, brake circuits are separated via circuit separating valves to separate the brake circuits of the vehicle dynamics control.

24. A brake system, comprising: a power brake; a vehicle dynamics control; and a control unit configured to, when it is detected that the level in a brake fluid reservoir of the brake system has fallen below a minimum fill level: detect a brake signal, advance a power piston to a forward position in which a required brake pressure is achieved, after the advancing, ascertain an amount of travel of the power piston from a home position to a current position, hold a brake pressure and further advancing the power piston by a correction distance when the brake pressure decreases during the holding, retract the power piston by the ascertained amount of travel after a braking process has ended, and degrade the brake system when a limit value for a forward position of the power piston has been reached.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 shows an embodiment example of a brake system for carrying out the method during a braking operation, according to the present invention.

[0022] FIG. 2 shows an illustration of a method for operating the brake system according to an embodiment example of the present invention.

[0023] FIG. 3 shows an illustration of a distance/time graph for the power piston in conjunction with the brake pressure.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0024] FIG. 1 shows an embodiment example of a brake system 1 for carrying out the method. The brake system 1 is shown during a braking operation. The brake system 1 comprises a power brake 4 and a vehicle dynamics control 8. The power brake 4 comprises a brake fluid reservoir 12, the fill level of which can be ascertained using a not depicted fill level sensor. In the brake system 1 shown here, there is a signal 16 that indicates that the fill level is below a minimum fill level.

[0025] The brake system 1 also comprises a master brake cylinder 20, which can be actuated by a driver via a brake pedal 24 and is supplied with brake fluid from the brake fluid reservoir 12. The travel of the brake pedal is measured using a travel sensor 28. The master brake cylinder 20 comprises a first and a second brake piston 32, 36, each of which can be used to control a separate brake circuit 40a, 40b. The pressure generated in the master brake cylinder 20 is measured by means of a master brake cylinder pressure sensor 44.

[0026] In normal operation of the brake system 1, the master brake cylinder separating valves 48a, 48b are closed. The master brake cylinder separating valves 48a, 48b can be used to connect the master cylinder 20 to the brake circuits 40a, 40b of the vehicle dynamics control 8 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 actuate a brake feel simulator 56, via which a braking feeling is created for the driver.

[0027] The power brake 4 also comprises a power cylinder 60, in which a power piston 64 is disposed that can be moved via a motor 68 in the axial direction of the power cylinder 60 to generate brake pressure. In the figure shown here, the power piston 64 is shown in a front end position. The power cylinder 60 is fluidically connected to the brake fluid reservoir 12 via a power cylinder valve 72. The brake pressure generated via the power piston 64 is measured using a brake pressure sensor 76. The power cylinder 60 is connected to the two brake circuits 40a, 40b of the vehicle dynamics control 8 via two circuit separating valves 80a, 80b.

[0028] To supply the brake circuits 40a, 40b of the vehicle dynamics control 8 with brake fluid, the brake circuits 40a, 40b are connected directly to the brake fluid reservoir 12 via a respective check valve 84a, 84b. This check valve 84a, 84b allows the vehicle dynamics control 8 to draw brake fluid out of the brake fluid reservoir 12 as needed. To regulate the vehicle dynamics and to apply and release brake pressure to the vehicle brakes 52a, 52b, 52c, 52d, the vehicle dynamics control 8 comprises various inlet and outlet valves, as well as a brake fluid pump 92 driven by a pump motor 88.

[0029] The brake system 1 also comprises a control unit 94, which is connected to the power brake 4 and to the vehicle dynamics control 8 in such a way that signals can be transmitted. This control unit 94 receives the measured values of the sensors and controls the valves of the power brake 4 and the vehicle dynamics control 8, as well as the movement of the power piston 64. The method according to the present invention as shown in FIG. 2 runs on this control unit 94.

[0030] FIG. 2 shows an illustration of a method for operating the brake system 1 according to an embodiment example of the present invention. In a first step A, it is detected that the level in the brake fluid reservoir 12 has fallen below a minimum fill level. In a following step B, a warning is output to a driver. The warning can be in the form of a warning lamp that lights up. The brake system 1 is then operated in a corresponding leak mode. Since such a leak causes a loss of brake fluid, the circuit separating valves 80a, 80b are closed in a next step (step C) during the unbraked state to separate the brake circuits 40a, 40b. This prevents both brake circuits 40a, 40b from running dry due to the leak, so that at least one of the brake circuits 40a, 40b remains usable for braking the vehicle.

[0031] In a next step D, it is detected whether a brake signal is present. If a brake signal is present, the circuit separating valves 80a, 80b are opened again in a next step E. This ensures that the brake pressure generated via the power piston 64 can be applied to the brake circuits 40a, 40b and thus to the vehicle brakes 52a, 52b, 52c, 52d. In a following step F, the power piston 64 is accordingly advanced to a forward position corresponding to the brake signal, so that a required brake pressure p.sub.target is generated.

[0032] Then, in step G, the amount of travel s.sub.F of the power piston 64 from the home position to the current position is ascertained. This can be ascertained using a rotation sensor 96 attached to the motor 68 for driving the power piston 64. After the required brake pressure p.sub.target is achieved, it is held in a next step H. If the brake pressure p measured via the master brake cylinder pressure sensor 44 drops during holding due to the leak, the power piston 64 is advanced further (step I) until the required brake pressure p.sub.target is present again. This provides a correction distance s.sub.K of the power piston 64.

[0033] After the braking process B.sub.END has ended, the power piston 64 is retracted in step J by the amount of travel s.sub.F ascertained in step G to a home position in which there is no brake pressure p. Due to the correction distance s.sub.K, the new home position can be further forward. Lastly, there is a check to see whether a limit value s.sub.Max for the forward position of the power piston 64 has been reached, so that no further braking can be carried out. If this is not the case, the method can continue. The method starts again with step C, in which the circuit separating valves 80a, 80b are closed first.

[0034] The method can be carried out as many times as necessary until the check indicates that a limit value s.sub.Max for the forward position of the power piston 64 has been reached. If this is the case, the brake system 1 is degraded (step K) so that brake pressure p is applied via the vehicle dynamics control 8 and/or the master brake cylinder 20.

[0035] FIG. 3 shows an illustration of a distance/time graph s/t for the power piston 64 together with the brake pressure p/t. This graph shows several braking cycles 100. The upper curve shows the distance s of the power piston 64, while the lower curve shows the brake pressure p. It can be seen that advancing the power piston 64 increases the brake pressure p until the required brake pressure p.sub.target is achieved. Even though the brake pressure p.sub.target remains constant, the distance s of the power piston 64 continues to change. This is because the power piston 64 compensates the pressure loss caused by a leak by advancing further. Without a leak, the distance of the power piston would not change once the required brake pressure p.sub.target is achieved.

[0036] The leak causes a loss of brake fluid. Then, when the level in the brake fluid reservoir 12 falls below a minimum fill level, a warning is output at point FLI and leak mode is activated. In subsequent braking cycles 100, the power piston 64 is retracted after the braking process has ended by only the amount of travel s.sub.F that was needed to achieve the required brake pressure p.sub.target. in the first place. The piston is accordingly not retracted by the correction distance s.sub.K required to hold the brake pressure p.sub.target. The new home position is therefore shifted forward by this correction distance s.sub.K.

[0037] Consequently, numerous braking cycles can be carried out in leak mode until the power piston 64 has reached the limit value s.sub.Max for the forward position. As shown in FIG. 3, said limit value can be at the front end position. From the time CFM at which the limit value s.sub.Max is reached, the brake system is operated only in a degraded mode.