METHOD FOR CHECKING AN ELECTROHYDRAULIC VEHICLE POWER BRAKE SYSTEM OF AN AUTONOMOUSLY DRIVING LAND VEHICLE FOR THE PRESENCE OF AIR IN THE BRAKE FLUID

Abstract

To check an electrohydraulic power vehicle brake system of a passenger car traveling autonomously on public roads for air in the brake fluid, the method provides for a consecutive generation of a brake pressure using a first power brake-pressure generator and a redundant, second power brake-pressure generator. The brake pressure is intermittently lowered to ambient pressure. In the method, a comparison of the brake fluid volumes required for generating the brake pressure is provided. If the brake fluid volume required for generating the brake pressure using the second power brake-pressure generator is greater, then the presence of air in a brake fluid of the vehicle brake system is inferred.

Claims

1-6. (canceled)

7. A method for checking an electrohydraulic power vehicle brake system of a land vehicle autonomously traveling on public roads for air in brake fluid, the vehicle brake system including a first power brake-pressure generator configured to generate a brake pressure, to which a hydraulic wheel brake is connected via a first valve, a second, redundant power brake-pressure generator to which the hydraulic wheel brake is connected via a second valve, a manual brake master cylinder, which is connected to a brake fluid reservoir and to which the second power brake-pressure generator is connected via a suction valve and a return valve, the method comprising the following steps: generating, with the first valve open and the second valve closed, a first brake pressure by the first power brake-pressure generator; lowering the brake pressure; with the first valve open, opening the second valve, closing the suction valve and the return valve, and generating a second brake pressure by the first power brake-pressure generator; opening the return valve so that brake fluid flows through the brake master cylinder into the brake fluid reservoir and the brake pressure is lowered again; closing the return valve, opening the suction valve, and generating a third brake pressure by the second power brake-pressure generator; and comparing a second brake fluid volume, which is supplied by the first power brake-pressure generator for generating the second brake pressure with a third brake fluid volume, which is supplied by the second brake-pressure generator for generating the third brake pressure.

8. The method as recited in claim 7, wherein the first power brake-pressure generator has a piston-cylinder unit whose piston is moved in a first direction in a cylinder of the piston-cylinder unit to generate the first brake pressure, and which is moved in an opposite direction for a subsequent lowering of the brake pressure and moved anew in the first direction in the cylinder to generate the second brake pressure, and then is no longer moved until the end of the method.

9. The method as recited in claim 7, wherein: (i) the first brake pressure is greater than the second brake pressure, and/or (ii) the third brake pressure and/or the second brake pressure is as high as the third brake pressure.

10. The method as recited in claim 7, wherein the brake pressure during the method is always so high that a friction brake pad of the wheel brake is applied at a brake element.

11. The method as recited in claim 7, wherein the brake master cylinder is connected to the brake fluid reservoir without an interconnected valve.

12. The method as recited in claim 7, wherein the brake fluid reservoir is pressureless.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0018] Below, the present invention will be described in greater detail with the aid of an embodiment illustrated in the figure.

[0019] FIG. 1 shows a hydraulic circuit diagram of an electrohydraulic power vehicle brake system, based on which the method according to an example embodiment of the present invention will be described.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0020] Electrohydraulic power vehicle brake system 1 shown in FIG. 1 is intended for a land vehicle, i.e., a passenger car, which travels autonomously up to level 4 or 5 on public roads. Level 4 refers to autonomous driving in which the driver may be prompted to intervene, and level 5, the highest level, refers to autonomous driving that does not require any driver intervention. Vehicle brake system 1 is a dual-circuit vehicle brake system.

[0021] Power vehicle brake system 1 has a first electromechanical power brake-pressure generator 2, a second, redundant electromechanical power brake-pressure generator 3 and a manual brake master cylinder 4. In the standard case, a brake pressure is generated by the first power brake-pressure generator 2, and when an interruption or a malfunction of first power brake-pressure generator 2 has occurred, the brake pressure is generated by the redundant second power brake-pressure generator 3 and/or brake master cylinder 4. In non-autonomous driving, brake master cylinder 4 is used as a setpoint adjuster for the brake pressure to be generated. In the exemplary embodiment, a pedal travel of brake pedal 16 is measured with the aid of a pedal travel sensor 24.

[0022] First power brake-pressure generator 2 has a piston-cylinder unit 6 including a piston 7, which is displaceable in a cylinder 10 of piston-cylinder unit 6 by an electric motor 8 via a mechanical reduction gear unit and a helical gear 9 in order to generate a hydraulic brake pressure. Cylinder 10 is connected to two brake circuits of vehicle brake system 1 via a first valve 11 in each case.

[0023] Second power brake-pressure generator 3 has a hydraulic pump 12, e.g., a piston pump, for each brake circuit, which can be jointly driven by an electric motor 13 in order to generate a brake pressure. In each brake circuit, intake sides of hydraulic pumps 12 are connected to brake master cylinder 4 via a suction valve 14, and pressure sides of hydraulic pumps 12 in each brake circuit are connected to brake master cylinder 4 via a return valve 15.

[0024] Brake master cylinder 4 is a dual circuit brake master cylinder, which is manually operable using a brake pedal 16. It has a non-pressurized brake fluid reservoir 17 to which brake master cylinder 4 is directly connected, that is to say, especially without an interconnected valve.

[0025] In a brake circuit, a piston-cylinder unit having a piston, acted upon by a spring, is connected to brake master cylinder 4 via a simulator valve 18 as a pedal travel simulator 19.

[0026] In the exemplary embodiment, vehicle brake system 1 has four hydraulic wheel brakes 20, of which two in each case are assigned to a brake circuit.

[0027] Each wheel brake 20 has an intake valve 21 and a discharge valve 22. Via intake valves 21, wheel brakes 20 are connected to first valves 11 of first power brake-pressure generator 2, and they are connected to the pressure sides of hydraulic pumps 12 of second power brake-pressure generator 3 via a second valve 23 in each brake circuit. By way of first valves 11, second valves 23 and return valves 15 of second power brake-pressure generator 3, first power brake-pressure generator 2 is connectable to brake master cylinder 4. Via discharge valves 22, wheel brakes 20 are connected to brake fluid reservoir 17.

[0028] Intake valves 21 and discharge valves 22 form wheel brake pressure-control valve systems by which wheel-brake pressures in each wheel brake 20 are able to be individually regulated, a regulation also denoting a control. Slip-traction controls such as an anti-lock braking control, a drive-traction control and a driving dynamics/electronic stability program, for which the abbreviations ABS, ATC and ESP are commonly used, are realizable with the aid of intake valves 21 and discharge valves 22. Such traction controls are conventional and will not be described here.

[0029] First valves 11, suction valves 14, return valves 15, simulator valve 18, intake valves 21, discharge valves 22 and second valves 23 are 2/2 way solenoid valves, return valves 15, intake valves 21 and second valves 23 being open in their currentless basic positions and first valves 11, suction valves 14, simulator valve 18 and discharge valves 22 being closed in their currentless basic positions. Other embodiments are possible.

[0030] The method according to the present invention for checking power brake system 1 of a vehicle for the presence of air in the brake fluid is preferably carried out exclusively in stationary vehicles and with a non-actuated brake master cylinder 4, so that brake master cylinder 4 communicates with brake fluid reservoir 17. With open first valves 11 and closed second valves 23, a first brake pressure p.sub.1 of up to approximately 100 bar, for instance, is then generated by first power brake-pressure generator 2. The brake pressure is generated by displacing piston 7 in cylinder 10 of piston-cylinder unit 6 of first power brake-pressure generator 2 in a first direction without return travel. Because of open intake valves 21, first brake pressure p.sub.1 acts on hydraulic wheel brakes 20 so that their brake pads rest against their brake elements such as their brake disks or brake drums. This overcomes an air gap in wheel brakes 20 so that wheel brakes 20 do not take on brake fluid by overcoming the air gap between the brake pads and the brake elements during the further check. Discharge valves 22 of wheel brakes 20 remain closed.

[0031] Next, the brake pressure is lowered again by a return travel of piston 7 in cylinder 10 counter to the first direction. In particular, the brake pressure is lowered to the ambient pressure or a slightly higher pressure, which ensures that the brake pads of wheel brakes 20 remain applied to the brake elements.

[0032] After the brake pressure has been lowered by the return travel of piston 7 in cylinder 10 of piston-cylinder unit 6 of first power brake-pressure generator 2, first valves 11 remain open and second valves 23 are also opened. Suction valves 14 and return valves 15 of second power brake-pressure generator 3 remain open or are closed so that no brake fluid is able to be displaced into brake master cylinder 4. The renewed displacement of piston 7 in cylinder 10 of piston-cylinder unit 6 of first power brake-pressure generator 2, once again in the first direction, raises the brake pressure again, which means that a second brake pressure p.sub.2 is generated. Second brake pressure p.sub.2 may have the same magnitude or a higher magnitude than first brake pressure p.sub.1, but second brake pressure p.sub.2 is preferably lower than first brake pressure p.sub.1. In the exemplary embodiment, second brake pressure p.sub.2 amounts to approximately 40 bar or, expressed in more general terms, amounts to approximately one-half the first brake pressure p.sub.1 or less.

[0033] A brake fluid volume V.sub.2 required for generating second brake pressure p.sub.2 and supplied by first power brake-pressure generator 2 or displaced from cylinder 10 is measured or ascertained. In the exemplary embodiment, second brake fluid volume V.sub.2 required for generating second brake pressure p.sub.2 is calculated as the product of the travel of piston 7 in cylinder 10 in the first direction, multiplied by a piston surface area.

[0034] Then, return valves 15 are opened so that brake fluid flows from cylinder 10 of piston-cylinder unit 6 of first power brake-pressure generator 2 and from wheel brakes 20 into brake master cylinder 4 and through non-actuated brake master cylinder 4 into non-pressurized brake fluid reservoir 17, so that brake pressure p drops to ambient pressure again or to a level slightly above ambient pressure.

[0035] After the renewed lowering of brake pressure p, return valves 15 are closed again and suction valves 14 are opened instead. Return valves 15 are closed before the friction brake pads of hydraulic wheel brakes 20 lift off the brake elements. Piston 7 in cylinder 10 of piston-cylinder unit 6 of first power brake-pressure generator 2 is no longer moved until the end of the check method. Using second power brake-pressure generator 3, a third brake pressure p.sub.3 is now generated, which is higher, equal to or lower than first brake pressure p.sub.1 or second brake pressure p.sub.2. In the exemplary embodiment, third brake pressure p.sub.3 is approximately as high as second brake pressure p.sub.2. Measured or ascertained is a third brake fluid volume V.sub.3, which is required for raising the brake pressure to third brake pressure p.sub.3 or for generating third brake pressure p.sub.3. In the exemplary embodiment, this is accomplished in that the number of revolutions of electric motor 13 of the two hydraulic pumps 12, which are piston pumps in the exemplary embodiment, or the number of piston travels is multiplied by the piston travels, which means the piston travels and the piston surface areas.

[0036] Second brake fluid volume V.sub.2 is compared with third brake fluid volume V.sub.3. If the brake fluid is incompressible, then second brake fluid volume V.sub.2 and third brake fluid volume V.sub.3 are equal when second brake pressure p.sub.2 and third brake pressure p.sub.3 are equal and the lowered brake pressures p are equal prior to the generation of second brake pressure p.sub.2 by first power brake-pressure generator 2 and prior to the generation of third brake pressure p.sub.3 by second power brake-pressure generator 3.

[0037] If brake pressures p, p.sub.2, p.sub.3 differ, then this must be taken into account when comparing brake fluid volumes V.sub.2, V.sub.3. In practice, small deviations of brake fluid volumes V.sub.2, V.sub.3 are produced by elasticities of vehicle brake system 1. If third brake fluid volume V.sub.3 is considerably greater than second brake fluid volume V.sub.2, then air in the brake fluid in the region of second power brake-pressure generator 3 has to be assumed.

[0038] If air is present in the brake fluid in the region of first power brake-pressure generator 2 and/or wheel brakes 20, then this increases brake fluid volume V.sub.2 required for generating second brake pressure p.sub.2 and also third brake fluid volume V.sub.3 required for generating third brake pressure p.sub.3. A second brake fluid volume V.sub.2 increased because of air in the brake fluid, for instance, is determined when second brake pressure p.sub.2 is generated by first power brake-pressure generator 2 in that second brake fluid volume V.sub.2 required for this purpose and supplied or displaced by first power brake-pressure generator 2 is compared with the generated second brake pressure p.sub.2. If generated second brake pressure p.sub.2 is too low in comparison with brake fluid volume V.sub.2 required for its generation, then the brake fluid is compressible, which means that it contains air or that some other error has occurred in vehicle brake system 1.

[0039] A check of a compressibility of the brake fluid in vehicle brake system 1 is carried out, which may lead to the conclusion that air is present in the brake fluid.