Method for checking the functionality of a braking system, and braking system

Abstract

A method for checking functionality of a motor vehicle braking system. The braking system has a main module, including: hydraulically actuatable wheel brakes, pairs being assigned to respective brake circuits; at least one electrically actuatable wheel valve per wheel brake sets wheel-specific brake pressures; a pressure provision device actively builds up pressure in the wheel brakes; a pressure-medium reservoir at atmospheric pressure, and an auxiliary module, which has for each of two wheel brakes: a pressure sensor for measuring pressure in a wheel brake line; an open when deenergized isolating valve in the wheel brake line; a pump. At least one variable is measured to assess functionality of the braking system. Using least one acceptance criterion, and checked whether the variable satisfies the acceptance criterion. Determining at least one variable representing the viscosity of the brake fluid, and the at least one acceptance criterion depends on a variable representing viscosity.

Claims

1. A method for checking the functionality of a braking system for motor vehicles, which braking system comprises a main module, comprising: hydraulically actuatable wheel brakes, wherein pairs of said wheel brakes are assigned to respective brake circuits; at least one electrically actuatable wheel valve per wheel brake for setting wheel-specific brake pressures; a pressure provision device for actively building up pressure in the wheel brakes; a pressure-medium reservoir at atmospheric pressure, and furthermore comprising an auxiliary module, which comprises for each of two wheel brakes: a pressure sensor for measuring the pressure in a wheel brake line leading to the respective wheel brake; an isolating valve, which is arranged in the respective wheel brake line and which is open when deenergized; and a pump, the method comprising: measuring at least one variable in order to assess the functionality of the braking system, wherein at least one acceptance criterion is used, checking whether the variable satisfies said acceptance criterion, and determining at least one variable representing viscosity of brake fluid, wherein the at least one acceptance criterion depends on the variable representing the viscosity.

2. The method as claimed in claim 1, wherein a temperature of the brake fluid is measured by at least one temperature sensor.

3. The method as claimed in claim 2, wherein the temperature is measured with the aid of a multiplicity of temperature sensors, from the measured at least one variable of which a mean value is then formed.

4. The method as claimed in claim 2, wherein at least one pressure sensor is arranged in the main module and at least one pressure sensor is arranged in the auxiliary module.

5. The method as claimed in claim 4, wherein the at least one temperature sensor is formed by a respective pressure sensor of integrated design.

6. The method as claimed in claim 1, wherein the viscosity of the brake fluid is determined from a temperature by a characteristic.

7. A braking system for motor vehicles, comprising a main module, comprising: hydraulically actuatable wheel brakes, wherein pairs of said wheel brakes are assigned to respective brake circuits; at least one electrically actuatable wheel valve per wheel brake for setting wheel-specific brake pressures; a pressure provision device for actively building up pressure in the wheel brakes; a pressure-medium reservoir at atmospheric pressure, and furthermore comprising an auxiliary module, which comprises for each of two wheel brakes: a pressure sensor for measuring the pressure in a wheel brake line leading to the wheel brake; an isolating valve, which is arranged in the wheel brake line and which is open when deenergized; and a pump, having an open-loop and closed-loop control unit, in which a method as claimed in claim 1 is implemented in hardware and/or software.

8. The braking system as claimed in claim 7, wherein the pressure provision device is a linear actuator.

9. A method for checking the functionality of a braking system for motor vehicles, which braking system comprises a main module, comprising: hydraulically actuatable wheel brakes, wherein pairs of said wheel brakes are assigned to respective brake circuits; at least one electrically actuatable wheel valve per wheel brake for setting wheel-specific brake pressures; a pressure provision device for actively building up pressure in the wheel brakes; a pressure-medium reservoir at atmospheric pressure, and furthermore comprising an auxiliary module, which comprises for each of two wheel brakes: a pressure sensor for measuring the pressure in a wheel brake line leading to the respective wheel brake; an isolating valve, which is arranged in the respective wheel brake line and which is open when deenergized; and a pump, the method comprising: measuring at least one variable in order to assess the functionality of the braking system, wherein at least one acceptance criterion is used, checking whether the variable satisfies said acceptance criterion, and determining at least one variable representing viscosity of the brake fluid, wherein the at least one acceptance criterion depends on the variable representing the viscosity; and carrying out a purging process, in which brake fluid is displaced into the pressure-medium reservoir by the pressure provision device, wherein a first system pressure is measured, and wherein a second system pressure is measured, and the acceptance criterion counts as satisfied if the first system pressure is lower than an acceptance pressure, which depends on the pressure difference between the first and second system pressures.

10. The method as claimed in claim 9, wherein the acceptance pressure is formed from the sum of a first constant and a product of the pressure difference between the first and the second system pressure and a second constant.

11. The method as claimed in claim 10, wherein the first system pressure is measured in a hydraulic line in the main module, and wherein the second system pressure is measured in a hydraulic line in the auxiliary module.

12. The method as claimed in claim 9, wherein the first system pressure is measured in a hydraulic line in the main module, and wherein the second system pressure is measured in a hydraulic line in the auxiliary module.

13. The method as claimed in claim 12, wherein a pressure sensor, by which the second system pressure is measured, is arranged in a first brake circuit in the auxiliary module, and wherein the value of the system pressure measured in the first brake circuit is also used in the purging process for the second brake circuit.

14. A method for checking the functionality of a braking system for motor vehicles, which braking system comprises a main module, comprising: hydraulically actuatable wheel brakes, wherein pairs of said wheel brakes are assigned to respective brake circuits; at least one electrically actuatable wheel valve per wheel brake for setting wheel-specific brake pressures; a pressure provision device for actively building up pressure in the wheel brakes; a pressure-medium reservoir at atmospheric pressure, and furthermore comprising an auxiliary module, which comprises for each of two wheel brakes: a pressure sensor for measuring the pressure in a wheel brake line leading to the respective wheel brake; an isolating valve, which is arranged in the respective wheel brake line and which is open when deenergized; and a pump, the method comprising: measuring at least one variable in order to assess the functionality of the braking system, wherein at least one acceptance criterion is used, checking whether the variable satisfies said acceptance criterion, and determining at least one variable representing viscosity of the brake fluid, wherein the at least one acceptance criterion depends on the variable representing the viscosity, and wherein a pressure build-up process is carried out with the aid of the pressure provision device, wherein the brake fluid is delivered into at least one wheel brake by the pressure provision device for a predetermined time period, and wherein a system pressure that is present after the predetermined time period is measured, and wherein the acceptance criterion counts as satisfied if the measured pressure is greater than an expected pressure, which depends on the time period and an expected delivery rate of the pressure provision device.

15. The method as claimed in claim 14, wherein an expected delivery volume is calculated from the product of a delivery rate, which is dependent on the viscosity and/or temperature, and the predetermined time period, and wherein the expected pressure is calculated from the expected delivery volume with the aid of a predetermined pressure-volume characteristic.

16. The method as claimed in claim 15, wherein the expected delivery volume is corrected by a release clearance volume.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An exemplary embodiment of the invention will be discussed in more detail on the basis of a drawing. In the drawing, in a highly schematic illustration:

(2) FIG. 1 shows a braking system having a main module and an auxiliary module in a preferred embodiment;

(3) FIG. 2 shows the braking system according to FIG. 1 in a first state; and

(4) FIG. 3 shows the braking system according to FIG. 1 in a second state.

(5) In all of the figures, identical parts are denoted by the same reference designations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) A braking system 2 illustrated in FIG. 1 comprises a brake pedal 6, by means of which a tandem brake master cylinder 10 is actuated. The tandem brake master cylinder 10 has a first pressure chamber or primary pressure chamber 16, into which a primary pressure piston 20 that is acted upon by a first elastic element 24 can be displaced. The tandem brake master cylinder 10 has a second pressure chamber or secondary chamber 30, into which a secondary piston 34, in particular a floating secondary piston, that is acted upon by a second elastic element 40 can be displaced. In the unactuated state of the primary pressure piston 20, the primary pressure chamber 16 is connected via a hydraulic line 46 to a pressure-medium reservoir 18. In the unactuated state of the secondary pressure piston 34, the secondary pressure chamber 16 is connected via a hydraulic line 52 to the pressure-medium reservoir 18.

(7) The braking installation or braking system 2 comprises four hydraulically actuatable wheel brakes 60, 62, 64, 66. The braking system 2 has a main module 70, which is used for pressure control during the normal operation of the braking installation, and an auxiliary module 74, which is used if the main module 70 malfunctions or fails.

(8) The braking system 2 also has the following further components in the main module 70. A pressure provision device 80 is designed as a linear actuator, having an electric motor 82, a downstream rotation-translation mechanism 84, which converts the rotation of the motor shaft into a translation of a pressure piston 86 delimiting a pressure space 88. The pressure space 88 is connected to the pressure-medium reservoir 18 via a replenishment line 90, into which a check valve 92 is inserted. The rotation-translation mechanism 84 is preferably designed as a ball screw.

(9) The primary pressure chamber 16 is connected hydraulically to a simulator 96, wherein the connection between the primary pressure chamber 16 and the simulator 16 can be divided by a simulator valve 98. Each of the wheel brakes 60-66 is assigned an outlet valve 100, 102, 104, 106, which is preferably closed when deenergized, and an inlet valve 110, 112, 114, 116, which is preferably open when deenergized. A respective check valve is connected in parallel with the respective inlet valve 110-116.

(10) The wheel brakes 60, 62 are assigned to a first brake circuit I and are connected hydraulically in a separable manner to the secondary chamber 30 by an isolating valve 120. The wheel brakes 64, 66 are assigned to a second brake circuit II and are connected hydraulically in a separable manner to the primary chamber 16 by an isolating valve 126.

(11) The pressure provision device 80 can be connected hydraulically to wheel brakes 60, 62 by an activation valve 130 and can be connected hydraulically to wheel brakes 64, 66 by an activation valve 136.

(12) A pressure sensor 140, preferably of redundant design, measures the pressure in the secondary chamber 30. A pressure sensor 150, preferably of redundant design, measures the pressure in the pressure chamber 88, which is referred to as system pressure.

(13) When valve 160 is switched and valve 136 is switched, a further self test can be carried out to check the capacity of the pressure provision device to provide pressure. Valve 162 prevents a vacuum in the actuating unit if, for example, the pressure provision device moves back into the rear end position as part of this self test. The motor position or rotation angle is measured with the aid of a sensor 170. A further sensor 172 is preferably provided for measuring the motor winding temperature. The wheel brakes are split diagonally, wherein brakes 60 and 64 are the front wheel brakes.

(14) An auxiliary module 74 of the braking system 2 is designed, when required, particularly when there is a fault or a total failure of the main module 70, to set a wheel-specific brake pressure in wheel brakes 60, 64 in a first operating mode, when required. Wheel brakes 62, 66 are directly connected hydraulically to pressure provision device 80 or brake master cylinder 10, and no wheel brake pressure is set in these brakes by the auxiliary module 74. In another operating mode, the auxiliary module 74 of the braking system 2 is designed to set a brake pressure at all the wheels 60 to 66 if, should there be a partial failure of the main module 70, the valves 100-162 thereof are still available for operation.

(15) When viewed from wheel brake 60, a pressure sensor 184, a pump 190 with a first feed pump 200, an isolating valve 194 which is open when deenergized and with which a check valve 210 is connected in parallel, and a pressure sensor 198 are arranged in a hydraulic line 180, by means of which wheel brake 60 can be connected to the brake master cylinder 10 or the pressure provision device 80 when required. On the suction side, the pump 200 is connected via a hydraulic suction line 204 to a volume reservoir 208 for pressure medium, which is connected to the pressure-medium reservoir 18 via a line. A changeover valve 212, which is used to throttle the pump and which is closed when deenergized, is inserted into the suction line 204. The volume reservoir 208 is furthermore connected to wheel brake 60 via an activation valve 220, which is preferably closed when deenergized.

(16) The auxiliary module 74 is designed in the same way in respect of wheel brake 64, and the corresponding components are provided with the same reference signs. Here, however, in contrast to the wheel brake 60, no pressure sensor 198 that measures the pressure in the line 180 is provided. In an alternative embodiment, a further pressure sensor 198 can be provided in this case too. In the preferred embodiment, at least one pressure sensor is formed in combination with a temperature sensor 242 (shown by way of example in the case of pressure sensor 198). It is thereby possible also to measure the temperature of the brake fluid at those points at which the pressure is measured. All the pressure sensors are preferably combined with temperature sensors, or both sensors are formed in an integrated design.

(17) A first electronic open-loop and closed-loop control unit 250 in the main module is used to detect the braking demand, in particular with the aid of a travel sensor 264 and the activation of the pressure provision device 80 and of the valves 100-106, 110-16, 120, 126, 130, 136.

(18) A second electronic open-loop and closed-loop control unit 260 in the auxiliary module 74 is used to control the pump 200 and the valves 194, 220, 212. As an input signal, the electronic open-loop and closed-loop control unit 260 receives the driver braking demand detected by the sensor 264. There is a signal link between the two open-loop and closed-loop control units 250, 260 at least in one direction.

(19) The braking system 2 is illustrated in an operating position corresponding to a normal braking process in FIG. 2. The driver is actuating the brake pedal 6 and displacing pressure medium from the primary chamber in the direction of the simulator 96. The two chambers 16, 30 of the brake master cylinder 10 are separated hydraulically from the wheel brakes 60-66 by the closed isolating valves 120, 126. The activation valves 130, 136 are open, and therefore pressure can be built up actively in the wheel brakes by the pressure provision device 80. For this purpose, the inlet valves 110-116 are open and the outlet valves 100-106 are closed. For wheel-specific pressure build-up, pressure can be built up selectively in a wheel brake 60-66 by opening the respective outlet valve 100-106. The valves 194 in the auxiliary module 74 are open. To enable the pressure medium flow to take place in this way, valves 120, 126 must be closed and valves 130, 136 must be open, unlike the situation illustrated in FIG. 2.

(20) In FIG. 3, the braking system 2 is illustrated during a two-phase purging cycle or purging process, in which the system checks whether the braking system 2 can discharge pressure medium into the pressure-medium reservoir 18. For this purpose, a check is made in a first phase as to whether pressure medium can be discharged in brake circuit I. For this purpose, activation valve 130 is opened, while activation valve 136 and isolating valves 120, 126 are closed. Only the inlet valve 110 assigned to wheel brake 60 is opened, while inlet valves 112, 114, 116 are closed. Outlet valve 100 is closed and valve 194 is opened. The pressure accumulator 208 is connected hydraulically to wheel brake 60. With the aid of the linear actuator or pressure provision device 80, brake fluid is now displaced into brake circuit I, and therefore, owing to the switching of the valves, brake fluid is displaced into the pressure-medium reservoir 18.

(21) Pressure sensor 150 measures a first system pressure P.sub.sys,1, which corresponds substantially to the pressure prevailing in the pressure space 88. Pressure sensor 198 measures a second system pressure P.sub.sys,2 in the line 180 in the auxiliary module 74. At the volume flow determined by the linear actuator, system pressures P.sub.sys,1 and P.sub.sys,2 will now be established, especially if the brake fluid is highly viscous. It is preferably the mean values of the system pressures P.sub.sys,1 and P.sub.sys,2 over the time period in which the predetermined volume flow is set which are evaluated.

(22) In the case of a clogged line between the auxiliary module 74 and the pressure-medium reservoir 18 or reservoir, the two pressures P.sub.sys,1 and P.sub.sys,2 will have almost the same (relatively high) value. If this is not the case, i.e. the hydraulic connection to the reservoir is open, a pressure gradient from P.sub.sys,1 to P.sub.sys,2 is established. An acceptance criterion is therefore preferably formulated
P.sub.sys,1<(P.sub.akzept=c+r*(P.sub.sys,1−P.sub.sys,2)).

(23) Here, r is a numerical constant derived from the relationship between the line resistances. R.sub.1 determined by the line and the valves on the way from P.sub.sys,1 to P.sub.sys,2 and R.sub.2 determined by the line and the valve on the way from P.sub.sys,2 to the reservoir 18. The following applies: r=(R.sub.1+R.sub.2)/R.sub.2

(24) r>1 and c is a constant with a unit of pressure, where c>0.

(25) After this purging process, the system checks whether it is also possible in the second brake circuit II to displace pressure medium into the reservoir. For this purpose, activation valve 136 is opened, while activation valve 130 and isolating valves 120, 126 are closed. As described above, valve 194 is opened in the auxiliary module. The pressure accumulator 208 is connected hydraulically to wheel brake 64. Since there is no pressure sensor in brake circuit II, the value measured there for the purging process in brake circuit II is used for the application of the above-described acceptance criterion.