METHOD AND CONTROL UNIT FOR OPERATING A HYDRAULIC BRAKING SYSTEM, BRAKING SYSTEM, AND MOTOR VEHICLE

Abstract

A method for operating a hydraulic braking system, which includes at least one actuatable actuator for generating a hydraulic brake pressure using brake fluid. A first leakage loss of the brake fluid in the braking system is ascertained as a function of a volume of a pressure chamber of the actuator at a starting pressure at the beginning of a braking process and the volume of the pressure chamber when the starting pressure is reached at the conclusion of the braking process. A second leakage loss of the braking fluid is continuously calculated while the braking process is carried out. The first leakage loss is compared to the second leakage loss for the plausibility check after a braking process was carried out.

Claims

1-9. (canceled)

10. A method for operating a hydraulic braking system, which includes at least one actuatable actuator configured to generate a hydraulic brake pressure using a brake fluid in the braking system, the actuator including a pressure chamber for the brake fluid and a displaceable piston configured to decrease or increase a volume of the pressure chamber, and the actuator, for carrying out a braking process, being activated to increase the hydraulic pressure in the braking system from a starting pressure to a brake pressure by decreasing the volume, and, for concluding the braking process, to reduce the hydraulic pressure to the starting pressure by increasing the volume, the method comprising: ascertaining a first leakage loss of the brake fluid in the braking system as a function of the volume of the pressure chamber at the starting pressure at the beginning of the braking process and the volume of the pressure chamber when the starting pressure is reached at the conclusion of the braking process; continuously calculating a second leakage loss of the brake fluid while the braking process is carried out; and comparing the first leakage loss to the second leakage loss for a plausibility check after the braking process has been carried out.

11. The method as recited in claim 10, wherein the first leakage loss is determined as a function of a duration of the braking process.

12. The method as recited in claim 10, wherein the second leakage loss is calculated only in phases with increasing hydraulic pressure.

13. The method as recited in claim 10, wherein an erroneous first leakage loss is recognized when the second leakage loss is lower than the first leakage loss.

14. The method as recited in claim 13, wherein the erroneous first leakage loss is recognized only when a difference between the first leakage loss and the second leakage loss exceeds a predefinable tolerance value.

15. The method as recited in claim 10, wherein a warning message is output when a critical leakage loss is recognized.

16. A control unit for operating a braking system for a motor vehicle, which includes at least one actuatable actuator configured to generate a hydraulic brake pressure using a brake fluid in the braking system, the actuator including a pressure chamber for the brake fluid and a displaceable piston configured to decrease or increase a volume of the pressure chamber, and the actuator, for carrying out a braking process, being activated to increase the hydraulic pressure in the braking system from a starting pressure to a brake pressure by decreasing the volume, and, for concluding the braking process, to reduce the hydraulic pressure to the starting pressure by increasing the volume, the control unit configured to: ascertain a first leakage loss of the brake fluid in the braking system as a function of the volume of the pressure chamber at the starting pressure at the beginning of the braking process and the volume of the pressure chamber when the starting pressure is reached at the conclusion of the braking process; continuous calculating a second leakage loss of the brake fluid while the braking process is carried out; and compare the first leakage loss to the second leakage loss for a plausibility check after the braking process has been carried out.

17. A braking system for a motor vehicle, comprising: at least one actuatable actuator configured to generate a hydraulic brake pressure using a brake fluid, the actuator including a pressure chamber for the brake fluid and a displaceable piston configured to decrease or increase a volume of the pressure chamber, and the actuator, for carrying out a braking process, being activated to increase the hydraulic pressure in the braking system from a starting pressure to a brake pressure by decreasing the volume, and, for concluding the braking process, to reduce the hydraulic pressure to the starting pressure by increasing the volume; one or multiple hydraulically actuatable wheel brakes; and a control unit configured to: ascertain a first leakage loss of the brake fluid in the braking system as a function of the volume of the pressure chamber at the starting pressure at the beginning of the braking process and the volume of the pressure chamber when the starting pressure is reached at the conclusion of the braking process; continuous calculating a second leakage loss of the brake fluid while the braking process is carried out; and compare the first leakage loss to the second leakage loss for a plausibility check after the braking process has been carried out.

18. A motor vehicle, comprising: a braking system including: at least one actuatable actuator configured to generate a hydraulic brake pressure using a brake fluid, the actuator including a pressure chamber for the brake fluid and a displaceable piston configured to decrease or increase a volume of the pressure chamber, and the actuator, for carrying out a braking process, being activated to increase the hydraulic pressure in the braking system from a starting pressure to a brake pressure by decreasing the volume, and, for concluding the braking process, to reduce the hydraulic pressure to the starting pressure by increasing the volume; one or multiple hydraulically actuatable wheel brakes; and a control unit configured to: ascertain a first leakage loss of the brake fluid in the braking system as a function of the volume of the pressure chamber at the starting pressure at the beginning of the braking process and the volume of the pressure chamber when the starting pressure is reached at the conclusion of the braking process; continuous calculating a second leakage loss of the brake fluid while the braking process is carried out; and compare the first leakage loss to the second leakage loss for a plausibility check after the braking process has been carried out.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 shows an advantageous braking system of a motor vehicle.

[0018] FIG. 2 shows a diagram to explain an advantageous method for operating the braking system.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0019] FIG. 1 shows, in a simplified illustration, an advantageous braking system 1 of a motor vehicle which is not shown in greater detail here. Braking system 1 is designed as a hydraulically operating braking system and includes multiple wheel brakes LR, RF, LF and RR, which are each assigned to a wheel of the motor vehicle and are hydraulically actuatable. For this purpose, braking system 1 includes a master brake cylinder 2, which is actuatable by a driver of the motor vehicle with the aid of a brake pedal 3. In the present example, master brake cylinder 2 is designed as a tandem cylinder including two hydraulic chambers 4 and 5, a design of the braking system including a single master brake cylinder also being possible. Hydraulic chambers 4, 5 are each hydraulically connectable by a switching valve 6, 7 to a pair of wheel brakes LR, RF and LF, RR, respectively. In the present example, wheel brakes LR and RF are connectable to switching valve 6, and wheel brakes LF and RR are connectable to switching valve 7, a particular volume control valve 8 being interconnected in each case between the particular switching valve 6, 7 and the particular wheel brake LR, RF, LF and RR, which regulates the hydraulic pressure in each case.

[0020] With the aid of a respective discharge valve 9, the particular wheel brake LR, RF, LF and RR is additionally hydraulically connectable to a tank 10 of braking system 1 storing the brake fluid. To reduce the hydraulic pressure at the particular wheel brake, the associated discharge valve 9 is actuated, for example, so that the hydraulic pressure drops until a desired braking force or a release of the particular wheel brake overall is achieved.

[0021] Braking system 1 moreover includes an activatable actuator 11, which may increase the hydraulic pressure in braking system 1 regardless of a brake pedal actuation. For this purpose, actuator 11 includes a displaceable piston 12, which delimits a pressure chamber 18 and is displaceable, in particular linearly shiftable, by an electric motor 13, to vary the hydraulic pressure in braking system 1 by varying the volume of pressure chamber 18. Actuator 11 is hydraulically or fluidically connected by two dedicated switching valves 14, 15 to the outlets of the particular switching valve 6 or 7, so that the actuator is connectable to the particular pairs of wheel brakes by valves 15, 14, as master brake cylinder 2 is connectable by switching valves 6, 7.

[0022] In an automated driving state, valves 6, 7 are preferably closed, and actuator 11 is activated to carry out a braking process as needed. Optionally, a braking process may also be carried out by actuator 11 as a function of a brake pedal actuation 3. Valves 6, 7 are also closed for this purpose, and optionally a braking force sensation simulator 16 is connected to master brake cylinder 2, which provides the driver at brake pedal 3 with haptic feedback corresponding to the braking force, so that the driving sensation is preserved for the driver in the customary manner, even though the braking force is actually generated by actuator 11 with the aid of an electric motor.

[0023] In particular when braking system 1 operates by power operation, i.e., by the activation of actuator 11, the monitoring of braking system 1 for a leakage loss is advantageous. For this purpose, braking system 1 advantageously includes a control unit 17, which carries out the method described hereafter.

[0024] To explain the method, FIG. 2, by way of example, shows hydraulic pressure p prevailing in braking system 1 plotted over the liquid volume displaced by piston 12, which corresponds to the volume or a volume change of pressure chamber 18.

[0025] The method begins at a point in time t.sub.1, at which starting pressure p.sub.s prevailing in the system already has a value of p.sub.s=6 bar. In the present example, this shall be understood to be purely by way of example. This starting pressure is present at a displaced volume of piston 12 or of actuator 11 of V.sub.s, the position of piston 12 being regulated as a function of the monitored hydraulic pressure. For this purpose, preferably one or multiple pressure sensor(s) 19 is/are assigned in braking system 1. If actuator 11 is now activated, the displaced volume is increased according to the arrow until piston 12 comes to a halt at maximal displaced volume V.sub.max. At this point in time, maximal pressure P.sub.max is reached. The maximal pressure or maximal volume does not have to be the maximal achievable pressure in terms of geometry or design or the maximal volume; in this case, it is rather the locally or instantaneously maximal achieved volume or the maximal achieved pressure of the particular braking process. To hold the pressure during an extended braking duration, it may become necessary due to a leakage loss to displace piston 12 further, and to reduce the volume of chamber 18, or to increase the displaced volume of piston 12, as shown by an arrow V.sub.x. The further displacement of piston 12 depends on the braking duration and the extent of the leakage loss. If thereafter, upon conclusion of the braking process, the pressure is reduced again, for example in that piston 12 is retracted, the volume of pressure chamber 18 in braking system 1 changes accordingly, until it reaches a final value of V.sub.E at a point in time at which the starting pressure is reached. Due to leakage loss V.sub.x, brake fluid is missing when starting pressure p.sub.s is reached and must be compensated for by actuator 11.

[0026] As a result of a comparison of volume V.sub.s at the beginning of the braking process to volume V.sub.E at the end of the braking process, a first leakage loss of braking system 1 is thus ascertainable.

[0027] Leakage loss V.sub.x is thus advantageously calculated as a function of the duration of the braking process as well as the difference between the volume at the beginning and at the end of the braking process. In addition, at the same time the leakage loss is continuously calculated during the braking process. For this purpose, the leakage loss is continuously calculated during the phases with increasing hydraulic pressure in braking system 1, and an overall leakage loss is determined. After the braking process has been concluded, when value V.sub.E is also known, the first leakage loss is compared to the second leakage loss to carry out a plausibility check of the first leakage loss. If it is found in the process that the second leakage loss is lower than the first leakage loss, it is inferred that the calculated first leakage loss is not trustworthy and thus cannot be decisive for the output of a warning message or the like. In this way, unnecessary warning messages and/or premature warning messages are avoided.

[0028] However, if it is recognized that the first leakage loss and the second leakage loss at least essentially correspond to one another, for example when the leakage losses do not deviate from one another beyond a predefinable tolerance value, a warning message is generated in the case of a critical leakage loss and is indicated, for example visually or acoustically, to the driver of the motor vehicle, so that he or she drives to a repair shop, for example to replenish brake fluid and/or have braking system 1 checked.

[0029] If it is recognized that the first leakage loss was erroneously calculated, the result is initially discarded, and the method is carried out again during the next braking process to confirm the leakage loss and/or to confirm the erroneously calculated first leakage loss. When it is established that the first leakage loss was correctly calculated, i.e., the result of the first measurement is confirmed by the subsequent measurement, and the leakage loss is assessed as being critical, a warning message is preferably output to the driver of the vehicle to draw his or her attention to the fact that a critical leakage loss is present. In this way, an overall robust method and a robust braking system are offered, whose probability of failure is reduced due to the identification of an erroneous monitoring.