Method for monitoring brake fluid volume

Abstract

The invention relates to a method for monitoring a brake fluid volume in a hydraulic brake system of a vehicle, wherein the brake system comprises a brake fluid reservoir (4) with at least one analog level sensor (50) and a brake circuit hydraulically connected to the brake fluid reservoir (4), the brake circuit comprising a pressure supply device (5) and a plurality of wheel brakes (8a-8d) hydraulically connected to the pressure supply device (5). The method comprises pressurizing the brake fluid in the brake circuit by means of the pressure supply device (5), determining the brake fluid volume displaced by the pressure supply device (5) to generate the pressure, and determining a change in the brake fluid volume in the brake fluid reservoir (4) during pressurization by means of the level sensor (50).Determining the effective volume displacement of the pressure supply device (5) from the determined brake fluid volume displaced by the pressure supply device (5) and the change in the brake fluid volume in the brake fluid reservoir (4), determining the pressure prevailing in the brake circuit, comparing the pair of values ​​for effective volume displacement and prevailing pressure with a reference, and signaling a leak if the prevailing pressure for the effective volume displacement is lower than a pressure stored in the reference for the effective volume displacement.

Description

[0001] The invention relates to a method for monitoring a brake fluid volume in a hydraulic brake system of a vehicle.

[0002] Modern hydraulic braking systems increasingly feature monitoring functions that continuously monitor the system's functionality and, if necessary, warn of a safety-critical condition. In addition to monitoring the wear of individual components, it is also common practice to monitor the available volume of brake fluid. This ensures that the braking system remains effective and meets the minimum deceleration requirements.

[0003] Such monitoring is particularly relevant for braking systems designed as brake-by-wire systems. In normal operation, the brake pedal is decoupled from the wheel brakes and is only connected to a pedal simulator. This simulator provides a counterforce to any pedal press, giving the driver the sensation of operating a conventional hydraulic brake. Pedal presses are detected by sensors, generating a signal that identifies a braking request. Based on this signal, a pressure supply unit, usually driven by an electric motor, is activated, building up hydraulic pressure in the brake circuit corresponding to the braking request.

[0004] Methods for checking the condition or functionality of hydraulic brake systems are known from DE 10 2013 018 072 A, DE 10 2016112971 A and DE 10 2016 218 022 A.

[0005] In a hydraulic fallback system, which is activated, for example, in the event of a failure of the control unit for the by-wire functionality, the brake pedal is directly connected to the wheel brakes again, allowing the driver to continue operating the wheel brakes. In this case, a portion of the available brake fluid volume remains in the part of the brake circuit responsible for providing pressure in by-wire operation and is therefore unavailable for operation in the fallback system. Consequently, it must be ensured that even when switching from by-wire operation to the fallback system, a sufficient volume of brake fluid remains available to fulfill a braking request.

[0006] In addition to simply monitoring the available brake fluid volume, there is also a need to detect leaks in a brake system through which brake fluid escapes. Such leaks can affect the pressure setting in the wheel brakes and consequently cause malfunctions in control functions.

[0007] Simply monitoring the fluid level in the brake system's reservoir is insufficient to determine the actual volume of brake fluid in the brake circuit. Even in the absence of leaks, the fluid level in the reservoir changes continuously due to varying states of the individual components during operation. For example, the position of the pressure supply device or the switching states of the wheel valves significantly influence the volume of brake fluid in the reservoir.

[0008] Against this background, the present invention aims to provide a method for monitoring a brake fluid volume in a hydraulic brake system that allows an accurate statement about the total available volume and the presence of leaks at any time.

[0009] This problem is solved by the method according to claim 1. Advantageous embodiments are the subject of the dependent claims.

[0010] The invention relates to a method for monitoring a brake fluid volume in a hydraulic brake system of a vehicle, wherein the brake system comprises a brake fluid reservoir with at least one analog level sensor and a brake circuit hydraulically connected to the brake fluid reservoir. The brake circuit further comprises a pressure supply device and a plurality of wheel brakes hydraulically connected to the pressure supply device. The method comprises pressurizing the brake fluid in the brake circuit by means of the pressure supply device, determining the brake fluid volume displaced by the pressure supply device to generate the pressure, and determining a change in the brake fluid volume in the brake fluid reservoir during pressurization by means of the level sensor.Determining the effective volume displacement of the pressure supply device from the determined brake fluid volume displaced by the pressure supply device and the change in the brake fluid volume in the brake fluid reservoir, determining the pressure prevailing in the brake circuit, comparing the pair of values ​​for effective volume displacement and prevailing pressure with a reference, and signaling a leak if the prevailing pressure for the effective volume displacement is lower than a pressure stored in the reference for the effective volume displacement.

[0011] An "analog level sensor" is understood to be a sensor whose measuring range is not limited to discrete level values. Rather, an analog level sensor is designed to continuously, i.e., steplessly, determine the current level of the brake fluid across its available measuring range. Consequently, with such a level sensor and knowledge of the brake fluid reservoir's geometry, it is theoretically possible to accurately determine the volume of brake fluid contained within the reservoir.

[0012] The method according to the invention is based on the consideration that, in a brake-by-wire braking system during normal operation and in the absence of control functions such as an anti-lock braking system, the pressure supply device and the wheel brakes form a closed hydraulic system when pressurized. Consequently, in this case, the ratio between the pressure prevailing in the braking system and the volume displaced by the pressure supply device is generally predictable or known.

[0013] This changes, however, if, for example, a control intervention results in some of the built-up pressure being released by opening the wheel brake outlet valves. In this case, the volume of brake fluid available in the hydraulic system, consisting of the pressure supply unit and wheel brakes, is reduced. Consequently, a measured pressure-volume characteristic curve in the hydraulic system will deviate from an expected curve, since the total volume is reduced. This would, in turn, be identified as a potential leak.

[0014] However, such a misinterpretation is avoided according to the invention by continuously monitoring the brake fluid volume in the brake fluid reservoir, particularly during pressure build-up in the brake system. The brake fluid volume that escapes from the wheel brakes as a result of the opening of the outlet valves flows back into the brake fluid reservoir and is registered there as a rising fluid level. From this rise, it can be determined how much brake fluid volume escaped from the hydraulic system during the pressure phase, which in turn can be used to correct the determined pressure-volume characteristic curve in the hydraulic system.

[0015] Consequently, the inventive method enables monitoring of the brake fluid volume even during the execution of control functions. Only if a loss of brake fluid is still detected, even after taking the previously described correction into account, can a leak in the system be assumed. The inventive method functions independently of the switching state of the hydraulic components of the brake system.

[0016] The reference value for the ratio of displaced brake fluid volume to pressure can be determined, for example, shortly after brake system maintenance, as leaks can then be ruled out. This determined reference value can then be stored in a brake control unit and read from a corresponding memory area when the procedure is carried out.

[0017] In one embodiment, the pressure supply device consists of a pressure cylinder with a pressure chamber and a cylinder piston that can be displaced by the linear actuator, actuated by an electrically driven linear actuator. The volume of brake fluid displaced to generate the pressure is determined from the displacement of the cylinder piston and the geometry of the pressure chamber. The linear actuator is essentially a rotary-translational drive connected to an electric motor such that the rotation of the motor shaft of the electric motor is converted into a linear motion or a thrust of a pressure cylinder connected to the linear actuator. Preferably, a ball screw drive is used as the rotary-translational drive, which is characterized by particularly low friction.The displacement of the cylinder piston can, for example, be easily determined from the operating parameters of the electric motor and the translation of the linear actuator, i.e., the drive per revolution of the electric motor.

[0018] Besides the loss of some brake fluid due to the opening of the wheel brake outlet valves, other factors can also influence the ratio of pressure to displaced brake fluid volume in the hydraulic system. According to a further embodiment, the temperature of the brake fluid and / or the wheel brakes is taken into account when determining the pressure prevailing in the brake circuit and / or when determining the effective volume displacement. For example, a brake fluid temperature higher than the temperature at which the reference was determined can lead to a higher pressure for the same volume displacement or to a lower volume absorption for the same pressure, since the brake fluid can expand with increasing temperature.Furthermore, the temperature of the braking system, and especially the wheel brakes, can also influence the expansion of the mechanical elements of the braking system, which in turn affects the measured volume displacement. To account for the temperature, a table of correction values ​​can be used, for example, which is read out together with the reference value.

[0019] Another potential source of error in the system lies in an incorrectly determined brake fluid volume in the brake fluid reservoir, for example, due to the vehicle's current driving situation. One embodiment provides that the vehicle's tilt angle is taken into account when determining a change in the brake fluid volume during pressurization. Depending on the position of the level sensor within the brake fluid reservoir, different brake fluid levels can be determined for different vehicle tilt angles, even if the actual brake fluid volume in the reservoir is identical. However, such an inaccurate level reading can be compensated for if the vehicle's tilt angle is known.

[0020] In a further embodiment, it is provided that the geometry of the brake fluid reservoir is taken into account when determining a change in the brake fluid volume during pressurization. Thus, depending on the geometry of the brake fluid reservoir, a vehicle's inclination can have a different effect on the determined fill level and therefore on the determined brake fluid volume in the reservoir.

[0021] Furthermore, according to another embodiment, when determining a change in the brake fluid volume in the brake fluid reservoir during pressurization, a lateral acceleration and / or a longitudinal acceleration of the vehicle is taken into account. Here, too, the geometry of the brake fluid reservoir is preferably considered simultaneously.

[0022] In another embodiment, the lateral acceleration of the vehicle is determined in a simple manner from the steering angle and the vehicle speed. Consequently, an acceleration sensor is not required.

[0023] Similarly, according to another embodiment, the longitudinal acceleration of the vehicle can also be determined without acceleration sensors by calculating the longitudinal acceleration from at least one wheel rotation speed of the vehicle. In particular, a change in the wheel rotation speed, given the wheel radius, provides information about the longitudinal acceleration of the vehicle.

[0024] According to a further embodiment, the inaccuracies described above in determining the brake fluid level in the brake fluid reservoir are compensated for by the brake fluid reservoir having at least two analog level sensors. A fluid level determined by a first level sensor during the determination of the change in the brake fluid volume in the reservoir is validated by a fluid level determined by a second level sensor. A determined fluid level is preferably only used in the inventive method if the fluid level determined by the first level sensor lies within a defined tolerance around the fluid level determined by the second level sensor.

[0025] According to a further embodiment, the plausibility check takes into account the relative position of the level sensors to each other and additionally a geometry of the brake fluid reservoir and / or an inclination angle of the vehicle and / or a lateral acceleration of the vehicle and / or a longitudinal acceleration of the vehicle.

[0026] Preferred embodiments of the method according to the invention are explained in more detail below with reference to the drawings. These show: Fig. 1 an embodiment of a hydraulic braking system, Fig. 2 a flowchart of the process, Fig. 3 different fluid level configurations in a brake fluid reservoir with a level sensor and Fig. 4 different fluid level configurations in a brake fluid reservoir with two level sensors.

[0027] In the following, similar or identical features are marked with the same reference symbols.

[0028] In Fig. Figure 1 schematically illustrates a first embodiment of a hydraulic braking system for a motor vehicle for carrying out the method according to the invention. The braking system comprises four hydraulically actuated wheel brakes 8a-8d. The braking system includes a master brake cylinder 2 actuated by means of an actuating or brake pedal 1, a displacement simulator or simulation device 3 interacting with the master brake cylinder 2, a brake fluid reservoir 4 at atmospheric pressure, an electrically controlled pressure supply device 5, and a valve arrangement 90 with wheel-specific brake pressure modulation valves, which are, for example, configured as inlet valves 6a-6d and outlet valves 7a-7d.

[0029] For example, wheel brake 8a is assigned to the left front wheel (FL), wheel brake 8b to the right front wheel (FR), wheel brake 8c to the left rear wheel (RL) and wheel brake 8d to the right rear wheel (RR).

[0030] The master brake cylinder 2 comprises a master brake cylinder piston 15 within a housing 16, which defines a hydraulic pressure chamber 17, and is a single-circuit master brake cylinder. The pressure chamber 17 accommodates a return spring 9, which positions the master brake cylinder piston 15 in its initial position when the master brake cylinder 2 is not actuated. The pressure chamber 17 is connected to the brake fluid reservoir 4 via radial bores formed in the master brake cylinder piston 15 and a corresponding pressure equalization line 41. These bores can be closed off by a relative movement of the master brake cylinder piston 15 within the housing 16. The pressure chamber 17 is also connected to a brake supply line 13 via a hydraulic line section 22 (also referred to as the first supply line), to which the inlet ports of the inlet valves 6a-6d are connected.Thus, the pressure chamber 17 of the master brake cylinder 2 is connected to all inlet valves 6a-6d.

[0031] For example, no valve, in particular no electrically or hydraulically actuated valve and no check valve, is arranged in the pressure equalization line 41 or in the connection between the pressure chamber 17 and the brake fluid reservoir 4.

[0032] Alternatively, a diagnostic valve, preferably a normally open (if de-energized) diagnostic valve, or preferably a parallel connection of a normally open (if de-energized) diagnostic valve with a check valve closing towards the brake fluid reservoir 4, can be included in the pressure equalization line 41 or between the master brake cylinder 2 and the brake fluid reservoir 4.

[0033] A shut-off valve 23 is arranged between the supply line 22 connected to the pressure chamber 17 and the brake supply line 13. Alternatively, the pressure chamber 17 is connected to the brake supply line 13 via the first supply line 22 and a shut-off valve 23. The shut-off valve 23 is designed as an electrically actuated, preferably normally open (SO), 2 / 2-way valve. The hydraulic connection between the pressure chamber 17 and the brake supply line 13 can be shut off by the shut-off valve 23.

[0034] Piston rod 24 couples the pivoting movement of the brake pedal 1 resulting from pedal actuation with the translational movement of the master brake cylinder piston 15, whose actuation travel is detected by a preferably redundant displacement sensor 25. The corresponding piston travel signal is thus a measure of the brake pedal actuation angle. It represents a driver's braking request.

[0035] A pressure sensor 20 connected to the first supply line 22 detects the pressure built up in the pressure chamber 17 by the movement of the master brake cylinder piston 15. This pressure value can also be evaluated to characterize or determine the driver's braking request. Alternatively, a force sensor 20 can also be used to determine the driver's braking request.

[0036] The simulation device 3 is, for example, hydraulically designed and hydraulically coupled to the master brake cylinder 2. Simulation device 3 essentially comprises, for example, a simulator chamber 29, a simulator return chamber 30, and a simulator piston 31 separating the simulator chamber 29 and the simulator return chamber 30. The simulator piston 31 is supported against a housing by an elastic element 33 (e.g., a simulator spring) arranged in the (for example, dry) simulator return chamber 30. The hydraulic simulator chamber 29 is, for example, connected to the pressure chamber 17 of the master brake cylinder 2 by means of a preferably electrically actuated, preferably normally closed simulator release valve 32.A check valve 34 arranged hydraulically antiparallel to the simulator release valve 32 allows the brake fluid to flow back from the simulator chamber 29 to the pressure chamber 17 largely unimpeded, regardless of the switching state of the simulator release valve 32.

[0037] As already mentioned, the braking system comprises, for each hydraulically actuated wheel brake 8a-8d, an inlet valve 6a-6d and an outlet valve 7a-7d, which are hydraulically connected in pairs via center connections and linked to the wheel brake 8a-8d. Each inlet valve 6a-6d is connected in parallel to an unspecified check valve opening towards the brake supply line 13. The outlet ports of the outlet valves 7a-7d are connected to the brake fluid reservoir 4 via a common return line 14.

[0038] The electrically controlled pressure supply device 5 is designed as a hydraulic cylinder-piston arrangement (or a single-circuit electrohydraulic actuator (linear actuator)), whose cylinder piston 36 can be actuated by a schematically indicated electric motor 35 via an interposed rotary-translational transmission 39, also schematically depicted. The cylinder piston 36 defines the single pressure chamber 37 of the pressure supply device 5.

[0039] A rotor position sensor, which serves to detect the rotor position of the electric motor 35 and is only schematically indicated, is designated by reference numeral 44.

[0040] A line section (also referred to as the second supply line) 38 is connected to the pressure chamber 37 of the electrically controlled pressure supply device 5. Supply line 38 is connected to the brake supply line 13 via an electrically actuated, preferably normally closed, switching valve 26. The switching valve 26 allows the hydraulic connection between the pressure chamber 37 of the electrically controlled pressure supply device 5 and the brake supply line 13 (and thus the inlet ports of the inlet valves 6a-6d) to be opened and closed in a controlled manner.

[0041] The actuator pressure generated by the force exerted by the cylinder piston 36 on the brake fluid enclosed in the pressure chamber 37 is fed into the second supply line 38. In a "brake-by-wire" operating mode, particularly when the brake system is functioning correctly, supply line 38 is connected to the brake supply line 13 via the switching valve 26. During normal braking, this results in the build-up and release of wheel brake pressure for all wheel brakes 8a-8d by the forward and retraction of the cylinder piston 36.

[0042] When pressure is reduced by retracting the cylinder piston 36, the brake fluid previously moved from the pressure chamber 37 of the pressure supply device 5 into the wheel brakes 8a-8d flows back into the pressure chamber 37 via the same route.

[0043] Alternatively, different wheel brake pressures can be easily set using the inlet and outlet valves 6a-6d and 7a-7d. When the pressure drops accordingly, the brake fluid released via the outlet valves 7a-7d flows through the return line 14 into the brake fluid reservoir 4.

[0044] Brake fluid can be drawn into pressure chamber 37 by retracting the cylinder piston 36 with the switching valve 26 closed. This allows brake fluid to flow from the brake fluid reservoir 4 into pressure chamber 37 via line 42, through a check valve 53 that opens in the flow direction towards the actuator 5. This re-priming of brake fluid may be necessary, for example, if, due to a control function, so much brake fluid has flowed back into brake fluid reservoir 4 via open outlet valves 7a-7d that a subsequent pressure build-up is no longer possible.

[0045] For example, pressure chamber 37 is also connected to the brake fluid reservoir 4 via one or more vent holes when the cylinder piston 36 is not actuated. This connection between pressure chamber 37 and brake fluid reservoir 4 is severed when the cylinder piston 36 is (sufficiently) actuated in the actuation direction 27.

[0046] An electrically actuated, normally open circuit isolating valve 40 is arranged in the brake supply line 13. This valve allows the brake supply line 13 to be separated into a first line section 13a, which is connected (via the isolating valve 23) to the master brake cylinder 2, and a second line section 13b, which is connected (via the switching valve 26) to the pressure supply device 5. The first line section 13a is connected to the inlet valves 6a, 6b of the wheel brakes 8a, 8b, and the second line section 13b is connected to the inlet valves 6c, 6d of the wheel brakes 8c, 8d. When the circuit isolating valve 40 is open, the brake system is a single-circuit system. By closing the circuit isolating valve 40, the brake system can be separated or divided into two partial brake circuits, I and II, particularly when controlled according to the situation.In the first partial brake circuit I, the master brake cylinder 2 (via the isolating valve 23) is connected only to the inlet valves 6a, 6b of the wheel brakes 8a, 8b of the front axle VA, and in the second partial brake circuit II, the pressure supply device 5 (with the switching valve 26 open) is connected only to the wheel brakes 8c and 8d of the rear axle HA.

[0047] With the circuit separator valve 40 open, the inlet ports of all inlet valves 6a-6d can be supplied with pressure via the brake supply line 13. In a first operating mode (e.g., "brake-by-wire" mode), this pressure corresponds to the brake pressure provided by the pressure supply device 5. In a second operating mode ("fallback mode"), the brake supply line 13 can be pressurized with the pressure from the pressure chamber 17 of the master brake cylinder 2.

[0048] The braking system includes a pressure sensor 19 for each partial brake circuit I or II. This is preferably arranged upstream of one of the wheel brakes of partial brake circuit I or II, e.g. wheel brakes 8a or 8d, e.g. between inlet valve and wheel brake.

[0049] The brake system also includes an analog level sensor 50 for determining the fill level in the brake fluid reservoir 4. The level sensor 50 is designed to determine the fill level of the brake fluid in the brake fluid reservoir 4 over a continuous measuring range.

[0050] Based on the one in Fig. The braking system shown in section 1 will now be described below with reference to Fig. 2 one embodiment of the method according to the invention is described.

[0051] In a first process step 100, the brake fluid in a brake circuit consisting of pressure supply device 5, brake supply line 13 and wheel brakes 8a-8d is pressurized by the pressure supply device 5. The pressure is set, for example, on the basis of an actuation signal determined by the displacement sensor 25 or the pressure sensor 20.

[0052] Subsequently, in step 110, the brake fluid volume displaced by the pressure supply device 5 to generate the pressure is determined. For this purpose, the thrust of the cylinder piston 36 into the pressure chamber 37, and thus the brake fluid volume displaced by the cylinder piston 36, can be determined, for example, using the rotor position sensor 44 and knowledge of the transmission ratio of the rotary-translational gear 39.

[0053] In the absence of a control function for the wheel brake pressures, i.e., permanently closed outlet valves 7a-7d, the pressure supply device 5, the brake supply line 13, and the wheel brakes 8a-8d form a self-contained hydraulic system. The brake fluid volume enclosed in this hydraulic system is constant in the absence of a leak. Consequently, in this case, the ratio between the displaced brake fluid volume and the pressure prevailing in the brake circuit would be clearly predictable. Therefore, a leak could be detected simply by examining a corresponding pressure-volume characteristic curve.

[0054] However, if a control function is present, some of the brake fluid can flow back from the brake circuit into the brake fluid reservoir 4 due to the opening of one or more outlet valves 7a-7d. The result would be a pressure-volume characteristic curve that indicates a leak, even though no leak actually exists.

[0055] However, according to the invention, this is prevented by using the level sensor 50 in step 120 to detect a change in the brake fluid volume in the brake fluid reservoir during pressurization.

[0056] The change in brake fluid volume thus determined corresponds precisely to the volume fraction that flowed back from the brake circuit via the outlet valves 7a-7d into the brake fluid reservoir 4. Consequently, knowing this change in brake fluid volume, the effective volume displacement of the pressure supply device 5 can be determined in step 130 from the calculated brake fluid volume displaced by the pressure supply device 5 and the change in the brake fluid volume in the brake fluid reservoir 4.

[0057] In addition to the effective volume displacement, the pressure prevailing in the brake circuit is also determined in step 140, for example by one of the pressure sensors 19. Process step 140 can also be carried out before step 130.

[0058] The resulting pair of values ​​for effective volume displacement and pressure in the brake circuit is then compared in step 150 with a reference, which preferably specifies a target value for the ratio of pressure to displaced volume in the brake circuit. If a significant deviation from the behavior specified by the reference is detected, a leak is signaled in step 160, for example, by activating a corresponding warning light in the vehicle interior. A significant deviation exists if the pressure for effective volume displacement is lower than a pressure stored in the reference for effective volume displacement, or if the effective volume displacement determined for the prevailing pressure is greater than would be expected according to the reference.

[0059] The above determination of effective volume displacement and prevailing pressure can also be carried out continuously during a pressure setting, so that a leak is detected early.

[0060] For the method described above, it is particularly advantageous if a change in the brake fluid volume in the brake fluid reservoir 4 is detected as accurately as possible. According to embodiments of the method, a multitude of influencing factors are taken into account when determining the change in the brake fluid volume in the brake fluid reservoir. These include, among other things, the vehicle's tilt angle, longitudinal and / or lateral acceleration, and the geometry of the brake fluid reservoir 4.

[0061] The following will refer to Fig. 3. Several situations are described as examples to further explain the effect of the aforementioned influencing factors.

[0062] This is in the Fig. Figure 3 shows a brake fluid reservoir 4 with an analog level sensor 50 in different situations. The brake fluid reservoir 4 has a partition 200 that divides it into a first section 202 and a second section 204. The partition 200 does not extend over the entire height of the brake fluid reservoir 4, so that the sections 202 and 204 are connected above the partition 200. The respective fluid level 206 of the brake fluid is indicated by the dashed line. The level sensor 50 is positioned in the brake fluid reservoir 4 such that only the fluid level in the first section 202 can be directly determined.

[0063] In the Fig. Figure 3 a) illustrates the situation in which the fluid level 206 is above the partition and the fluid level is exactly horizontal, meaning the vehicle is neither tilted nor subject to lateral or longitudinal acceleration. In this case, the fill level can be determined directly by the fill level sensor 50.

[0064] In Fig. 3 b) The fluid level 206 is below the partition, with the fill level being the same in both sub-sections 202 and 204. Since the level sensor can only determine the fill level in the first sub-section 202, the fill level in the second sub-section 204 must be estimated. For example, the possible volume present in the second sub-section 204 can be assumed as an error or tolerance for the determined total volume. Alternatively, it can be assumed that the two sub-sections 202 and 204 empty evenly, so that the same fill level is assumed for both sub-sections. This can be made dependent on previous conditions of the vehicle. For example, if there was a previous strong lateral acceleration that accelerated the fluid to the right, it can be assumed that less brake fluid remained in the first sub-section 202 than in the second sub-section 204.This is the case, for example, in . Fig. 3 d) shown.

[0065] Finally, in the Fig. 3 c) A situation is depicted in which the fluid level 206 is not horizontal, but rather slopes downwards to the left. This can be caused, for example, by a tilt or lateral acceleration of the vehicle. In this case, however, the fluid level 206 is above the partition 200. Consequently, the expected slope of the fluid level 206 can be determined directly from the knowledge of the lateral acceleration. Knowing the geometry of the brake fluid reservoir 4, the actual volume of brake fluid in the brake fluid reservoir 4 can be deduced from the determined slope of the fluid level 206 and the fill level determined by the level sensor 50.

[0066] Is the liquid level 206 located below the partition wall 200, as shown in the Fig. 3 e) is shown, as previously mentioned with reference to Fig. As described in 3 b) and d), it can be estimated what brake fluid volume is to be expected in the second sub-area 204.

[0067] In the Fig. The four situations depicted are essentially the same as those previously described in relation to Fig. 3 were discussed. However, in the case of the one in Fig. In the brake fluid reservoir 4 shown in Figure 4, a second level sensor 50' is provided next to the level sensor 50 in the first sub-area 202, which is arranged so that it can directly determine the fill level in the second sub-area 204. In this case, a fill level determined by the first level sensor 50 can be verified by means of the second level sensor 50'. This is particularly useful in the following scenario: Fig.4 c) In the situation shown, the slope of the fluid level 206 can be reconstructed from the combined view of the determined fill levels, thus enabling a plausibility check of a determined brake fluid volume. Furthermore, if a second fill level sensor 50' is present, an estimation of the fill level in the second sub-area 204 is no longer necessary when the fluid level 206 is below the partition 200.

Claims

[1] Method for monitoring a brake fluid volume in a hydraulic brake system of a vehicle, wherein the brake system comprises a brake fluid reservoir (4) with at least one analog level sensor (50) and a brake circuit hydraulically connected to the brake fluid reservoir (4), wherein the brake circuit comprises a pressure supply device (5) and a plurality of wheel brakes (8a-8d) hydraulically connected to the pressure supply (5), wherein the method comprises the following steps: • Pressurizing the brake fluid in the brake circuit by means of the pressure supply device (5), • Determining the volume of brake fluid displaced by the pressure supply device (5) to generate the pressure, • Determining a change in the brake fluid volume in the brake fluid reservoir (4) during pressurization using the level sensor (50), • Determining the effective volume displacement of the pressure supply device (5) from the determined brake fluid volume displaced by the pressure supply device (5) and the change in the brake fluid volume in the brake fluid reservoir (4), • Determining the pressure prevailing in the brake circuit, • Comparing the pair of values ​​for effective volume displacement and prevailing pressure with a reference, and • Signaling a leak when the pressure prevailing for effective volume displacement is lower than a pressure stored in the reference for effective volume displacement. [2] Method according to claim 1, characterized by, that the pressure supply device (5) is a pressure cylinder actuated by an electrically driven linear actuator with a pressure chamber (37) and a cylinder piston (36) which can be displaced by the linear actuator, wherein the volume of brake fluid displaced to generate the pressure is determined from a displacement of the cylinder piston (36) and the geometry of the pressure chamber (37). [3] Method according to claim 1 or 2, characterized by , that when determining the pressure prevailing in the brake circuit and / or when determining the effective volume displacement, the temperature of the brake fluid and / or the wheel brakes (8a-8d) is taken into account. [4] Method according to any one of the preceding claims, characterized by , that when determining a change in the brake fluid volume in the brake fluid reservoir (4) during pressurization, an inclination angle of the vehicle is taken into account. [5] Method according to any one of the preceding claims, characterized by , that when determining a change in the brake fluid volume contained in the brake fluid reservoir during pressurization, a geometry of the brake fluid reservoir (4) is taken into account. [6] Method according to any one of the preceding claims, characterized by , that when determining a change in the volume of brake fluid in the brake fluid reservoir (4) during pressurization, a lateral acceleration and / or a longitudinal acceleration of the vehicle is taken into account. [7] Method according to claim 6, characterized by , that the lateral acceleration of the vehicle is determined from a steering angle of the vehicle steering system and a speed of the vehicle. [8] Method according to claim 6 or 7, characterized by , that the longitudinal acceleration is determined from at least one wheel rotation speed of a wheel of the vehicle. [9] Method according to any one of the preceding claims, characterized by , that the brake fluid reservoir (4) has at least two analog level sensors (50, 50'), wherein a fluid level (206) determined by a first level sensor (50) in the brake fluid reservoir (4) during the determination of the change in the brake fluid volume is verified by a fluid level (206) determined by a second level sensor (50'). [10] Method according to claim 9, characterized by , that during plausibility checks the relative position of the level sensors (50, 50') to each other and additionally a geometry of the brake fluid reservoir (4) and / or an inclination angle of the vehicle and / or a lateral acceleration of the vehicle and / or a longitudinal acceleration of the vehicle is taken into account.

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