Hydraulic self-test for leak localization

Abstract

The invention relates to a method for controlling a hydraulic brake system for motor vehicles having an electrical pressure supply device, wherein a hydraulic brake pressure is built up by means of the electrical pressure supply device in order to carry out a braking operation based on a brake pressure request, wherein a leakage is detected during the braking operation.In order to improve braking performance when such a leak occurs, the invention provides for determining the position of the leak during braking by carrying out a circuit separation into two separate brake circuits during braking and, after the end of braking with the braking demand dropping to zero, the hydraulic pressure in the wheel brakes is not reduced to zero but is set to a test value greater than zero, and pressure monitoring is carried out in at least one, preferably in both, brake circuits, wherein, in the event of a pressure drop in one brake circuit in this brake circuit, an inlet valve of a first wheel brake is closed and an inlet valve of a second wheel brake is kept open and further pressure monitoring is carried out, wherein, in the event of a renewed pressure drop, the position of the leak in the second wheel brake is determined.

Description

[0001] The invention relates to a method for controlling a hydraulic brake system having an electrical pressure supply device. In order to perform a braking operation based on a brake pressure request, a hydraulic brake pressure is built up by means of the electrical pressure supply device, wherein a leak is detected during braking. The invention also relates to such a hydraulic brake system.

[0002] Such by-wire braking systems have two separate brake circuits for redundancy reasons, into which the wheel brakes are permanently or switchably divided. In the event of a leak, a circuit isolation valve is closed. However, the brake circuit with the leak very quickly loses its braking effect, so that effective braking can only be achieved with two wheels. Without localizing the leak, depending on the system, it can happen that the wrong brake circuit - i.e. two intact wheels - are disconnected from the electrical pressure supply device, leaving only one defective wheel brake and one unamplified wheel brake available. Even with the correct brake circuit selection, only two intact wheels are connected to the pressure source. One intact wheel is disconnected and does not contribute to deceleration. In order to keep this situation as short as possible, a quick decision must be made.This time constraint compromises robustness and can lead to incorrect results. It could cause the defective circuit to be reconnected to the pressure source.

[0003] From DE102015219905 A1 it is known to monitor the volume of brake fluid delivered during braking and the pressure built up in a brake circuit in order to conclude that there is a leak in the respective brake circuit in the event of a deviation from an expected behavior, while a predetermined pressure is generated in the other brake circuit.

[0004] DE 102018212016 A1 also discloses determining a relationship between the pressure generated and the volume delivered during braking in order to check the tightness of the braking system. The test can be performed during each service braking, at specified intervals, after a specified number of braking operations, or at a standstill.

[0005] It is therefore an object of the present invention to provide a method for a braking system and a braking system which ensures a high deceleration of the motor vehicle even in the event of a leak.

[0006] The object is achieved by a method according to claim 1, wherein the position of the leak is additionally determined during braking. For this purpose, the circuit is separated into two separate brake circuits during braking, i.e. the wheel brakes are divided into two groups that are fluidly separated. After braking has ended, i.e. when the braking demand drops to zero, the hydraulic pressure in the wheel brakes is not reduced to zero in accordance with the demand, but is set to a test value greater than zero, and pressure monitoring is carried out in at least one, preferably in both, brake circuits. If the pressure drops in one of the brake circuits, an inlet valve of a first wheel brake in this brake circuit is closed and an inlet valve of a second wheel brake is kept open. This performs further pressure monitoring, and if the pressure drops again, the position of the leak in the second wheel brake is determined.This enables leak localization with wheel brake precision.

[0007] By implementing the localization outside of the pressure demand according to the invention, the driver, in particular, cannot disrupt or falsify the localization by applying the brake pedal. Localization is thus performed during active operation, i.e., not at start-up, but after the vehicle has already started moving after the ignition is switched on. Furthermore, localization does not have to wait until the end of the ignition cycle, so localization can be performed promptly.

[0008] In a preferred embodiment of the invention, to detect a leak, i.e., to determine whether a leak is present, the fill level of a brake fluid reservoir is monitored by a sensor. If the brake fluid level is lower than a threshold value, a leak is indicated. Additionally or alternatively, a comparison is made between the volume introduced into the wheel brakes and the resulting pressure with a target value, in particular a corresponding characteristic curve.

[0009] In a preferred embodiment of the invention, the circuit separation is carried out by closing at least one circuit separation valve, wherein after the circuit separation has taken place, only one brake circuit is connected to the electrical pressure supply device.

[0010] In a particularly preferred embodiment of the invention, the other brake circuit is connected to a brake pedal-coupled master cylinder. Thus, each of the two brake circuits has its own pressure source.

[0011] In another particularly preferred embodiment of the invention, the other brake circuit is connected to a second electric pressure supply device. This also allows increased braking of the wheels in the second brake circuit, exceeding the muscle power of an average driver.

[0012] In a particularly preferred embodiment of the invention, a pressure sensor is used for pressure monitoring. This can indicate the current hydraulic pressure very accurately and reliably.

[0013] In a particularly preferred embodiment of the invention, motor variables of the pressure supply device are used for pressure monitoring. This can, in particular, be a motor torque or a motor current or its power. A motor position sensor can also be used.

[0014] In a particularly preferred embodiment of the invention, the pressure supply device is torque-based for pressure monitoring. It is controlled to provide a precisely specified torque. This should build up a certain pressure in the system, depending on the geometry. If there is no leak, no further brake fluid is pumped into the system once this pressure is reached.

[0015] In a further preferred embodiment of the invention, the second wheel brake, i.e., the leaking wheel brake, is isolated by closing the associated inlet valve and kept isolated for subsequent braking applications. The vehicle can then be braked with three functional brakes without further loss of brake fluid.

[0016] In a further preferred embodiment of the invention, the test value to which the pressure in the wheel brakes is set is selected such that it corresponds to a deceleration of less than 2.5 m / s^2. In particular, a test value of less than 50 bar, preferably 20 to 30 bar, can be selected. This prevents safety-critical overbraking.

[0017] In a further preferred embodiment of the invention, the test value is set to the pressure in the wheel brakes that corresponds to a deceleration greater than 2.5 m / s^2, with the pressure being released after a test time of less than 1 second. This also prevents safety-critical overbraking.

[0018] The object is also achieved by a hydraulic motor vehicle brake system having at least one electrical pressure supply device for supplying a plurality of wheel brakes, as well as a control unit for regulating the at least one electrical pressure supply device, wherein the control unit is configured to carry out an above-mentioned method.

[0019] Further features, advantages and possible applications of the invention will become apparent from the following description of embodiments and the drawings. Fig. 1 shows a braking system according to the invention, Fig. 2 shows a flow chart of the method according to the invention, Fig. 3 shows quantities of a torque-based control, Fig. 4 shows the sequence of the method according to the invention;

[0020] In Fig. Figure 1 shows a redundant hydraulic braking system for motor vehicles. The braking system is designed to operate four hydraulically actuated wheel brakes 8, and expansion to more wheel brakes is easily possible. For example, the two upper wheel brakes (HL, HR) in the figure are assigned to the rear axle, and the wheel brakes (VL, VR) to the front axle of the vehicle.

[0021] The brake system comprises a first structural unit, which is embodied, for example, as a first electrohydraulic brake control unit with a valve block and a first electronic control unit ECU1, and a second structural unit, which is embodied, for example, as a second electrohydraulic brake control unit with a valve block and a second electronic control unit ECU2. The valve blocks each form a separate housing.

[0022] A pressure fluid reservoir 4 with two chambers is arranged on the first structural unit, with a first reservoir connection associated with the first chamber and a second reservoir connection associated with the second chamber. Reservoirs with three or more chambers are also possible.

[0023] A first electrically actuatable pressure source 5 or pressure supply device is arranged in the first structural unit.

[0024] In the second structural unit, a second electrically actuated pressure source 2 and wheel-individual brake pressure modulation valves are arranged, which are designed as an electrically actuated inlet valve 6 and an electrically actuated outlet valve 7 for each wheel brake 8.

[0025] The first pressure source 5 and the second pressure source 2 are connected on the pressure side to a brake supply line to which the four inlet valves 6 are connected. Thus, all four wheel brakes 8 can be actuated by means of the first pressure source 5 and / or the second pressure source 2.

[0026] An electrically actuated circuit isolation valve 40 is arranged in the brake supply line and thus in the second structural unit, so that when the circuit isolation valve 40 is closed, the brake supply line is separated into a first line section, to which the inlet valves 6 or the wheel brakes 8 of the rear axle are connected, and a second line section, to which the inlet valves 6 or the wheel brakes 8 of the front axle are connected. The second pressure source 2 is hydraulically connected to the first line section, and the first pressure source 5 is hydraulically connected to the second line section. When the circuit isolation valve 40 is closed, the brake system is thus separated or divided into two hydraulic brake circuits I and II, or a first partial circuit and a second partial circuit.In the first brake circuit I, pressure source 2 is connected (via the first line section) only to the wheel brakes 8 of the rear axle, and in the second brake circuit II, the first pressure source 5 is connected (via the second line section) only to the wheel brakes 8 of the front axle. The circuit separation valve 40 is advantageously designed to be open when de-energized. Such an operating mode can be called circuit separation mode or ACS for "Active Circuit Separation."

[0027] As already mentioned, the braking system comprises an inlet valve 6 and an outlet valve 7 for each hydraulically actuated wheel brake 8, which are hydraulically interconnected in pairs via central connections and each connected to a hydraulic wheel connection of the second structural unit to which the corresponding wheel brake 8 is connected. A check valve opening towards the brake supply line is connected in parallel to each of the inlet valves 6. The output connections of the outlet valves 7 are connected to the pressure fluid reservoir 4 or its second chamber via a common return line. The input connections of all inlet valves 6 can be supplied with a pressure provided by the first pressure source 5 or, for example, if the first pressure source 5 fails, by the second pressure source 2.

[0028] The first electrically controllable pressure source 5 of the valve block is designed as a hydraulic cylinder-piston arrangement (or a single-circuit electro-hydraulic actuator (linear actuator)), the piston of which can be actuated by a schematically indicated electric motor with the interposition of a rotation-translation gear, also shown schematically, in particular can be moved forwards and backwards in order to build up and reduce pressure in a pressure chamber. The piston delimits the pressure chamber of the pressure source 5. To control the electric motor, a rotor position sensor (only schematically indicated) is provided that detects the rotor position of the electric motor. By means of this sensor, the piston position and speed, and thus the released or absorbed brake fluid volume, can be determined.

[0029] A system pressure line section is connected to the pressure chamber of the first electrically controllable pressure source 5. By means of this line section, the pressure source 5 or its pressure chamber is connected to a hydraulic connection of the first structural unit, which is connected to a hydraulic connection of the second structural unit via a hydraulic connecting element. This connection represents the only hydraulic pressure connection between the first and second structural units. It is a hydraulic connection for transmitting brake pressure for actuating the wheel brakes 8.

[0030] The pressure chamber is connected to the pressure fluid reservoir 4 via a (suction) line, regardless of the actuation state of the piston. A check valve 53, closing in the direction of the pressure fluid reservoir 4, is arranged in the line in connection with the second chamber. An electrically switchable valve 23 forms a further connection to this line, which is connected to the output port of the linear actuator 5. This isolating valve 23 is designed to be open when de-energized, so that when de-energized, the wheel brakes 8 are connected to the brake fluid reservoir 4. The cylinder-piston arrangement 5, for example, has no sniffing holes.

[0031] The second electrically controllable pressure source 2 of the second assembly is designed, for example, as a two-piston pump whose two pressure sides are interconnected. The suction sides are connected to the return line and thus to the pressure fluid reservoir 4. The pressure sides are connected to the first line section of the brake supply line.

[0032] In addition to the pressure source 2 and the brake pressure modulation valves 6, 7, an electrically actuated, preferably normally open, isolation valve 26 is arranged in the second structural unit, for example. Isolation valve 26 is hydraulically arranged between the connection and the second line section of the brake supply line. Thus, the first pressure source 5, and thus the entire first structural unit, is separably connected to the second line section or the brake supply line via the isolation valve 26.

[0033] For example, the braking system includes a pressure sensor in brake circuit I, which is thus assigned to the second pressure source 2. This is advantageous for burst protection during active circuit separation, i.e., when the circuit separation valve 40 is closed. However, the pressure sensor can also be located in brake circuit II.

[0034] Each valve block is assigned an electronic control unit (ECU1 and ECU2). Each electronic control unit comprises electrical and / or electronic elements (e.g., microcontrollers, power units, valve drivers, other electronic components, etc.) for controlling the electrically actuated components of the associated valve block and, if applicable, the associated sensors—thus, the entire respective assembly. The valve block and electronic control unit are advantageously designed as an electrohydraulic unit, as is known.

[0035] The first electronic control device controls the first pressure source 5. For example, the first pressure source 5 is supplied with energy (from a first electrical energy source) via the first electronic control device.

[0036] The second electronic control device controls the second pressure source 2. For example, the second pressure source 2 is supplied with energy (from a second electrical energy source) via the second electronic control device.

[0037] For example, the first pressure source or primary pressure source 5 can be or is controlled exclusively by the first electronic control device and the second pressure source or secondary pressure source 2 can be or is controlled exclusively by the second electronic control device.

[0038] During normal operation, the pressure in the wheel brakes is built up by primary pressure source 5. The pressure in primary pressure source 5 is released by retracting its piston. The pressure is modulated for each wheel as needed by the inlet and outlet valves. If necessary, isolation valve 26 is closed so that primary pressure source 5 can draw in additional volume.

[0039] If a particularly high flow rate is required, both pressure sources 5 and 2 operate in parallel. If a particularly high pressure is required, the isolation valve 26 is closed, and the secondary pressure source 2 increases the pressure above the pressure of the primary pressure source 5. Outside of braking situations, atmospheric pressure equalization can be permanently ensured via the isolation valve 23 and the isolation valve 26.

[0040] The control units ECU1 and ECU2 are configured to implement a pressure buildup using the linear actuator 5 and / or the piston pump 2 based on pressure requests. These can originate from automatic functions of the assistance program and, in particular, from the driver, who transmits the driver's command via the brake pedal. This is transmitted to the control units as a brake pedal actuation variable. The brake pedal actuation variable can be a brake pedal travel, a brake pedal force, and / or similar variables.

[0041] Isolation valve 26 is preferably controlled by the secondary ECU. The following description of operation in the event of a fault refers to this valve assignment.

[0042] If the secondary system fails electrically, specifically the secondary ECU or its voltage source, the pressure is increased and decreased via the primary pressure source 5, as in normal operation. Individual wheel pressure control is not required, but joint modulation of the wheel pressures remains possible to prevent the vehicle from being destabilized by locking wheels.

[0043] If the primary system fails electrically, particularly the primary system or its power supply, the secondary ECU switches to a fallback mode in which the second component is the sole pressure source for the vehicle's wheel brakes 8. To do so, it closes the isolation valve 26 to allow pressure to build up via the secondary pressure source 2. Without the isolation valve 26 closed, the brake fluid would continue to flow into the reservoir via the normally open isolating valve 23, thus preventing pressure from building up. Pressure is released via the outlet valves 7. Preferably, the inlet and outlet valves are controlled by the secondary ECU so that the pressure can be modulated for each wheel.

[0044] For example, the brake system includes a level measuring device for leak monitoring to determine the pressure fluid level in the pressure fluid reservoir 4. If the pressure fluid level falls below a predetermined value, a leak is considered detected. Leakage monitoring can also be implemented using other monitors. A Volume Deviation Monitor (VDM) is a leak monitoring device that can detect a hydraulic leak based on the pressure-volume ratio (PV ratio). The volume required to build up a certain pressure is determined by the sensed linear actuator or pedal position. The expected PV behavior can be stored using functions or stored characteristic curves. In other words, the building up pressure is compared with the pumped volume using a characteristic curve. A low brake fluid sensor level (BFLS) in the reservoir or excessively low pressure can also lead to leak detection.However, leak detection alone still doesn't reveal the exact location of the leak. Therefore, specific countermeasures aren't possible.

[0045] In the event of a leak in the brake system, typically only the circuit isolation valve 40 is closed, thereby dividing the system into two independent brake circuits I and II.

[0046] Thus, the effect of the leak is limited to one of the brake circuits, while the other brake circuit remains fully functional. In order to be able to use at least one wheel brake in the leaking brake circuit, a more precise localization of the leak is necessary. For this purpose, the method described in Fig. 2 is shown.

[0047] During braking, a leak is detected by detecting a volume deviation using Volume Deviation Monitoring, or if the brake fluid level drops below a threshold. A circuit split is then immediately implemented by closing circuit isolation valve 40. The current braking is continued in such a way that the pressure for the wheel brakes of the front axle is provided by linear actuator 5 and the pressure for the rear axle is provided by pump 2.

[0048] If the braking demand now drops to zero because the driver releases the brake pedal and / or an assistance function no longer requests deceleration, the pressure is not reduced to zero. Instead, the pressure is reduced to a predetermined test value of 25 bar. This happens on the rear axle by stopping pump 2 and opening outlet valves 7 until a pressure of 25 bar is reached, after which the outlet valves are closed again. The pressure sensor is now used to monitor the pressure. If a drop in pressure is detected, in particular if the pressure drops by more than a certain value, in particular 5 bar, in a specified time or a pressure drop gradient exceeds a specified value, in particular 10 bar / s, this brake circuit is regarded as having a leak. If no drop in pressure is detected, it can be assumed directly that the other brake circuit is having a leak.Alternatively, pressure monitoring is also performed in the other brake circuit. Since no pressure sensor is available in this brake circuit, linear actuator 5 is used for pressure monitoring. This involves monitoring its motor torque, which is related to the hydraulic pressure via its geometry and transmission ratio, as well as its position, which is related to the pumped volume. As shown in . Fig. As shown in Figure 3, the linear actuator advances to 1 Nm in the example torque control, which is reflected in the increasing LAC position. The motor torque increases to 1 Nm. If there is no leakage, the LAC now stops. However, if there is a leak, the LAC must continue to advance to maintain the set torque.

[0049] Once the leaking circuit has been identified, one of the inlet valves is closed, isolating a wheel brake to pinpoint the leak. If closing the inlet valve stops the pressure drop, the associated wheel brake is identified as the leaking one. If the pressure drop persists, the inlet valve is reopened and the other inlet valve is closed. Once the leak has been located, the corresponding inlet valve is kept closed.

[0050] By implementing leak localization according to the invention directly at the connection point of the braking process in which the leak was detected, by not completely reducing the braking pressure, the availability of three wheel brakes can be ensured very quickly, thus providing greater deceleration performance. Disadvantages caused by a subsequent active pressure buildup specifically for localization are avoided.

Claims

[1] Method for controlling a hydraulic brake system for motor vehicles comprising an electrical pressure supply device, wherein a hydraulic brake pressure is built up by means of the electrical pressure supply device in order to carry out a braking operation based on a brake pressure request, wherein a leakage is detected during the braking operation, characterized bythat the position of the leak is determined during braking by separating the circuit into two separate brake circuits and, after the end of braking with the brake demand dropping to zero, the hydraulic pressure in the wheel brakes is not reduced to zero but is set to a test value greater than zero, and pressure monitoring is carried out in at least one, preferably in both, brake circuits, wherein in the event of a pressure drop in one brake circuit in this brake circuit, an inlet valve of a first wheel brake is closed and an inlet valve of a second wheel brake is kept open and further pressure monitoring is carried out, wherein in the event of a renewed pressure drop the position of the leak in the second wheel brake is determined. [2] Method according to claim 1, characterized bythat in order to detect the leakage, a fill level of a brake fluid reservoir is monitored by means of a sensor and, if the brake fluid level is lower than a threshold value, a leakage is indicated and / or a comparison is made between the volume introduced into the wheel brakes and the resulting pressure with a target value. [3] Method according to claim 1 or 2, characterized by that the circuit separation is carried out by closing at least one circuit separation valve, wherein after the circuit separation has taken place only one brake circuit is connected to the electrical pressure supply device. [4] Method according to claim 3, characterized by that the other brake circuit is connected to a brake pedal-coupled master cylinder. [5] Method according to claim 3, characterized by that the other brake circuit is connected to a second electrical pressure supply device. [6] Method according to one of the preceding claims, characterized by that a pressure sensor is used for pressure monitoring. [7] Method according to one of the preceding claims, characterized by that motor sizes of the pressure supply device are used for pressure monitoring. [8] Method according to claim 7, characterized by that the pressure supply device is controlled on a torque-based basis for pressure monitoring. [9] Method according to one of the preceding claims, characterized by that the second wheel brake is disconnected by closing the corresponding inlet valve and is kept disconnected for subsequent braking operations. [10] Method according to one of the preceding claims, characterized by that the test value to which the pressure in the wheel brakes is set corresponds to a deceleration of less than 2.5 m / s^2. [11] Method according to one of the preceding claims, characterized bythat the test value to which the pressure in the wheel brakes is set corresponds to a deceleration greater than 2.5 m / s^2, with the pressure being reduced after a test time of less than 1 second. [12] Hydraulic motor vehicle brake system comprising at least one electrical pressure supply device for supplying a plurality of wheel brakes, and a control unit for regulating the at least one electrical pressure supply device, characterized by that the control unit is configured to carry out a method according to one of claims 1 to 11.

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