Method for detecting a leak in a power brake system
The method improves leak detection in power-assisted braking systems by continuously monitoring volume-pressure values, adjusting for various factors, to accurately identify leaks and maintain system reliability.
Patent Information
- Application Number
- PCT/EP2025/062231
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-07-12
- Filing Date
- 2025-05-05
- Publication Date
- 2026-01-15
AI Technical Summary
Existing methods for detecting leaks in power-assisted braking systems are not sufficiently accurate, particularly in systems with open hydraulics, leading to potential safety risks due to unreliable volume balance verification.
A method that continuously determines the volume-pressure value during braking, accounting for time-dependent and pressure-dependent brake fluid loss, including adjustments for wear, ABS activation, and parking brake usage, to accurately detect leaks by comparing actual and tolerated fluid loss against predetermined limits.
Enhances the certainty of leak detection, reducing false alarms and ensuring reliable system functionality by accurately identifying leaks, thus maintaining vehicle safety and functionality.
Smart Images
Figure EP2025062231_15012026_PF_FP_ABST
Abstract
Description
[0001] Description
[0002] Title:
[0003] Method for detecting a leak in an external power braking system
[0004] The present invention relates to a method for detecting a leak in an external power braking system and an external power braking system with which such a method can be carried out.
[0005] State of the art
[0006] Modern power-assisted braking systems are characterized by their mechanical and / or hydraulic decoupling from the driver. This is achieved via a brake pedal connected to an input rod. In the complete system, this input rod detects the driver's braking request, and a pedal feel is generated via a simulator in the form of a force-displacement curve. Pressure is then built up by a piston that is hydraulically decoupled from the driver's foot. As a fallback, the driver's foot is then coupled to the wheel brake cylinders via a manually operated brake cylinder, allowing the driver to apply braking pressure with their foot. This ensures that the vehicle remains brakeable in the event of a failure.
[0007] Brake systems can operate with closed hydraulics, meaning that a reservoir containing hydraulic fluid serves only to compensate for leakage and temperature fluctuations, thus maintaining a constant available hydraulic volume. Examples include classic brake systems such as vacuum brake boosters, electromechanical brake boosters like the iBooster, or a decoupled power brake (DPB) combined with an ESP system. Alternatively, brake systems can operate with open hydraulics, such as IPB systems (integrated power brake). In this case, a reservoir containing hydraulic fluid can be used during normal operation to temporarily store hydraulic volume. Therefore, the available hydraulic volume of the brake system can change during braking.Different braking systems have different characteristics. For example, closed-loop hydraulic systems have the problem that, depending on the operating conditions, the suction of an ESP system in the relevant area of the braking system—i.e., below the master cylinder and down to the wheel brake cylinders—results in a higher hydraulic volume than should be present during normal operation. Therefore, directly verifying the volume balance using a pressure-volume curve is only possible with limitations.
[0008] WO 2024 / 052009 A1 describes a method for operating a braking system that includes an electric assist brake and a vehicle dynamics control system. The method aims to maintain safety when the brake fluid level in the system's reservoir falls below the minimum required level. Upon detection of a brake signal, the electric assist piston is moved to a forward position to generate the necessary brake pressure. If the brake pressure drops due to a leak, the electric assist piston is moved further forward to correct the situation. The braking system also includes a control unit connected to the electric assist brake and the vehicle dynamics control system to manage the valves and movements.
[0009] German patent DE 10 2010 043 887 A1 discloses a system for brake circuit failure detection. This system in vehicles is designed to detect sudden and discontinuous changes in the pressure-volume characteristic of the brake system, which could indicate a system defect. By comparing target and actual pressure values in the brake circuits, the system can detect a circuit failure and react accordingly. It includes a hydraulic control unit that adjusts the control functions of driver-assisted braking systems such as ABS and ESP to respond appropriately to emergency situations. The circuit failure detection threshold is based on the displacement-pressure characteristic and allows the system to differentiate between a functioning brake circuit and a circuit failure. The system thus offers a reliable way to detect potential defects in the brake system at an early stage and ensure vehicle safety.
[0010] The object underlying the invention is to provide a method by which a leakage in an external power braking system can be detected with greater certainty.
[0011] The problem is solved by a method with the subject matter of claim 1. Preferred embodiments can be found in the dependent claims.
[0012] Disclosure of the invention
[0013] The invention discloses a method for detecting a leak in a power-assisted braking system of a motor vehicle. The power-assisted braking system comprises a power-assisted brake cylinder with a brake piston for generating brake pressure. The method includes the step of continuously determining a volume-pressure value based on a measured brake pressure and a volume of brake fluid delivered by the power-assisted brake cylinder during braking.In the event that the volume-pressure value exceeds a predetermined volume-pressure limit, the following steps are performed, comprising summing a tolerated and time- and pressure-dependent brake fluid loss from the start of braking until a first refilling of the external power brake cylinder and before at least one further refilling, determining a brake fluid volume taken up during each refilling, reducing the brake fluid loss by the brake fluid volume taken up during the refilling, and determining an error if the brake fluid loss calculated in this way before a refilling is below a volume limit.
[0014] The volumetric pressure value describes the volume of brake fluid required to build up a specific pressure. If the brake fluid volume is significantly higher, a leak is initially suspected. This is then investigated through further steps. The tolerated and time-dependent brake fluid loss indicates the amount of brake fluid lost during a braking process, for example, through seals. This amount is considered acceptable and corresponds to the proper operation of the power-assisted braking system. This loss occurs as long as brake pressure is applied and can be modeled as a pressure-dependent factor. Therefore, the tolerated brake fluid loss is higher during prolonged braking than during short braking, making this amount time-dependent.
[0015] The refilling process typically compensates for brake fluid loss. The tolerated, time-dependent brake fluid loss must therefore at least equal the amount of brake fluid added during refilling. However, if the amount of brake fluid added is greater than the tolerated loss, it is assumed that the actual brake fluid loss is greater than the assumed loss. If the brake fluid loss, reduced by the volume of fuel added during refilling, falls below the volume limit, a leak is assumed. By monitoring brake fluid loss, false leakage reports, caused by factors such as long stops at level crossings, can be ruled out. A leak in the power-assisted braking system can thus be detected with greater certainty.
[0016] In a preferred embodiment of the invention, a predetermined wear-related brake fluid loss is factored in to compensate for dead volume and the effects of brake pad wear, such as uneven wear. This wear-related brake fluid loss corresponds to the amount of brake fluid required to overcome the air gap between the brake pad and the brake disc. By factoring in such a brake fluid loss, a false leakage warning due to brake pad wear can be avoided. The actual brake fluid loss can thus be determined more accurately, allowing leaks to be detected with greater certainty. In a further preferred embodiment of the invention, a predetermined ABS brake fluid loss is factored in when an ABS system is activated during braking.When the ABS system is activated, an unknown quantity of brake fluid is temporarily stored in the low-pressure storage chambers of the ESP (Electronic Stability Program). To prevent this quantity from being incorrectly identified as a leak, an estimated amount of brake fluid required by the ABS system is factored into the brake fluid loss calculation upon activation. This allows for a more accurate calculation of any actual leakage.
[0017] Preferably, the power-assisted braking system is degraded in the event of a fault. Degradation reduces the system's functionality. This ensures that the vehicle remains capable of braking, allowing it to reach a repair shop. This can increase the vehicle's safety.
[0018] In a beneficial advanced system, a predetermined amount of brake fluid loss is factored in when the parking brake is activated. Activating a parking brake, for example, when stopped at a traffic light on a hill, requires a specific quantity of brake fluid. To prevent this amount from being falsely detected as a leak, this brake fluid loss is taken into account when the parking brake is engaged. This allows for more accurate and reliable detection of leaks in the power-assisted braking system.
[0019] Advantageously, the tolerated and time-dependent brake fluid loss is calculated as a function of the brake pressure. The tolerated and time-dependent brake fluid loss is greater at higher brake pressures than at lower brake pressures. This is because seals perform less effectively under high pressure than under low pressure. To account for this, the tolerated and time- and pressure-dependent brake fluid loss is adjusted accordingly. This prevents leaks from being falsely detected under high pressure. A leak can therefore be detected more accurately and with greater certainty.
[0020] In a further advantageous embodiment, a volume limit of 10-20% of the externally driven brake cylinder volume is assumed. If this value is not reached, it can be assumed that a leak must be present.
[0021] According to a suitable design, brake fluid loss is limited to a maximum value. This limit applies to both negative and positive brake fluid loss. This prevents the assumption of implausible and unrealistic values for brake fluid loss. As a result, the reliability of leak detection in the power-assisted braking system is improved.
[0022] According to another practical implementation, a negative brake fluid loss at the end of braking is factored into the brake fluid loss calculation for a subsequent braking event. By including a negative fuel loss in the calculation, even small leaks that only fall below the volume limit after a second or third braking event can be detected. This improves the detection of leaks, especially small ones.
[0023] Preferably, a positive brake fluid loss at the end of braking is set to zero. With a positive brake fluid loss, the assumed brake fluid loss is greater than the actual brake fluid loss. Therefore, if a positive brake fluid loss is present at the end of braking, it is assumed that the power-assisted braking system is functioning correctly and that no leakage is present. To ensure that a leak can still be detected with a high degree of certainty during subsequent braking, the positive brake fluid loss is set to zero. This prevents a negative brake fluid loss from canceling out the previously positive one, thus delaying the detection of a leak. The problem underlying the invention is further solved by a power-assisted braking system for a motor vehicle with which such a method can be implemented.The power-assisted braking system comprises a power-assisted brake cylinder with a brake piston to generate brake pressure, a brake pressure sensor to detect the brake pressure, a means of measuring the volume of brake fluid absorbed during the refilling process, and a control unit to determine the theoretical brake fluid loss and to detect a fault. With such a power-assisted braking system, the properties described above are achieved, making leaks in the system detectable with greater certainty.
[0024] Furthermore, a motor vehicle equipped with such an external power braking system is specified. Such a motor vehicle achieves the previously described characteristics and advantages.
[0025] Exemplary embodiments of the invention are shown in the drawing and explained in more detail in the following description. It shows:
[0026] Figure 1 shows an exemplary embodiment of a
[0027] External power braking system for carrying out a procedure for detecting a leakage in the external power braking system,
[0028] Figure 2 shows an embodiment of the method for
[0029] Detection of a leak in the external power braking system,
[0030] Figure 3 shows a volume-pressure diagram with the volume-pressure limit value marked.
[0031] Figure 4 illustrates the brake fluid loss and the resulting
[0032] External force on the brake cylinder displaced brake fluid loss as well as brake pressure in the event of a failure, and Figure 5 illustrates the brake fluid loss and the from
[0033] External force on the brake cylinder displaced brake fluid loss and brake pressure during normal operation.
[0034] Figure 1 shows an embodiment of an assisted braking system 10 for carrying out a method for detecting a leak in the assisted braking system 10. The assisted braking system 10 comprises a brake pressure generation unit 14 and an ESP unit 18. The ESP unit 18 can be constructed in a known manner. The wheel brakes 22 are supplied with brake pressure via the ESP unit 18. The brake pressure generation unit 14 comprises a master brake cylinder 26 in which two pistons 30a, 30b are arranged. A first piston 30a is connected to a brake pedal 34 for applying a braking force. A position sensor 38 is arranged on the brake pedal 34, via which the actuation travel of the brake pedal 34 can be measured.
[0035] The master brake cylinder 26 forms two master brake chambers 42a, 42b, which are supplied with brake fluid via a brake fluid reservoir 46. To simulate pedal feel for the driver, the master brake cylinder 26 is hydraulically connected to a pedal feel simulator 54 via a pedal feel valve 50. Compression of a spring 58 located in the pedal feel simulator 54 provides the driver with a braking sensation. The brake pressure generation unit 14 additionally includes an externally driven brake pressure generator 62, which comprises an externally driven brake cylinder 66 and a brake piston 74 that is displaceable therein by a motor 70. A rotor position sensor 78 is arranged on the motor 70, which allows the position of a rotor and thus also the displacement of the brake piston 74 to be determined.
[0036] The external brake cylinder 66 is hydraulically connected to the ESP unit 18 via two shut-off valves 82a, 82b. A brake pressure sensor 86 is arranged downstream of the external brake cylinder 66 and upstream of the shut-off valves 82a, 82b, which measures the brake pressure PB generated by the external brake cylinder 66. A check valve 90a, 90b is arranged between the brake fluid reservoir 46 and the shut-off valves 82a, 82b, so that the external brake cylinder 66 can draw brake fluid from the brake fluid reservoir 46 via these check valves 90a, 90b during a refilling process.
[0037] The power-assisted braking system 10 additionally includes a control unit 94, which is electrically connected to both the brake pressure generation unit 14 and the ESP unit 18. The control unit 94 receives signals from the sensors 38, 78, and 86 and controls the solenoid valves and the motor 70. Furthermore, the control unit 94 is configured to perform the leak detection procedure in the power-assisted braking system 10.
[0038] An embodiment of such a method for detecting a leak in the external power braking system 10 is shown in Figure 2. In a first step A of the method, a volumetric pressure value p(V) is continuously determined from the brake pressure PB measured by the brake pressure sensor 86 and the displacement determined from the rotor position sensor 78 during braking. Subsequently, it is checked whether the volumetric pressure value p(V) exceeds a predetermined volumetric pressure limit value PG(V). If the volumetric pressure value p(V), as shown in Figure 3, is smaller than the dashed area, the volumetric pressure value p(V) continues to be continuously determined.
[0039] However, if the volume-pressure value p(V) lies within the dashed area shown in Figure 3, the volume-pressure limit value PG(V) is exceeded. Subsequently, in step B, a tolerated and time-dependent brake fluid loss VB is calculated from the start of braking until the first refill Ni of the external brake cylinder 66 and before at least one further refill N. n summed up. This is illustrated again in Figures 4 and 5. The upper graph shows a volume-time diagram, in which the brake fluid loss VB is plotted against time t. The lower diagram shows the corresponding brake pressure PB and the displaced brake fluid volume VFZ of the externally driven brake cylinder 66 against time t. The dashed vertical lines indicate the time of the refilling process Ni, N nAt this point, the brake piston 74 has reached its front stop and thus displaced the maximum brake fluid volume Vpzmax, which is represented by the horizontal line.
[0040] The tolerated and time-dependent brake fluid loss VB is calculated based on the applied brake pressure PB. In addition, a wear-dependent brake fluid loss AVv is added to the brake fluid loss VB. The wear brake fluid loss AVv corresponds to the amount required to overcome the air gap between a brake disc and a brake pad. The system also checks whether an ABS system was activated during braking. If so, an ABS brake fluid loss AVABS is added to the brake fluid loss VB. Furthermore, the system checks whether a parking brake PB was activated during braking. If so, a predefined parking brake brake fluid loss AVPB ZU is added to the brake fluid loss VB.The pressure in the wheel brake cylinders is maintained, as far as possible given the leakage, by valve actuation in the ESP.
[0041] The system then checks whether the calculated brake fluid loss VB has a maximum value V max exceeds the maximum value V. max If the calculated brake fluid loss VB is exceeded, the maximum value V will be used instead. max The calculation continues for the brake fluid loss VB. Otherwise, it continues with the calculated brake fluid loss VB. In the next step C, this is done with each refill N. n The volume of brake fluid absorbed, AVN, is determined. In a subsequent step D, the brake fluid loss, VB, is reduced by the volume of brake fluid AVN absorbed during the refilling process. This can be seen in the upper diagram of Figures 4 and 5, where the brake fluid loss, VB, during the refilling process is N.n The brake fluid volume AVN is reduced by the amount of brake fluid absorbed. Correspondingly, the volume VFZ displaced by the external brake cylinder 66 decreases. The brake pressure PB applied by the external brake cylinder 66 is therefore briefly zero. Subsequently, it is checked whether the calculated brake fluid loss VB is reached before a further refill. n If the volume falls below a limit value VG shown in Figures 4 and 5 (as is the case in the upper graph of Figure 4), an error is displayed in step E. In the next step F, the external power braking system 10 is degraded. The functionality of the external power braking system 10 is thus reduced to ensure continued braking for a limited duration.
[0042] If, during the check whether the calculated brake fluid loss VB is sufficient before a refilling process N nIf, as shown in Figure 5, the volume limit VG is above the limit value, no error is detected. The system then checks whether the braking B r was completed. In the event that the braking B r If the process is not completed, the current brake fluid loss VB is saved to be factored in again in step B. The process then restarts with step A. If the volumetric pressure p(V) remains above the volumetric pressure limit PG(V), the brake fluid loss VB recorded so far is included in step B. Steps B to D are performed until either an error is detected or the volumetric pressure p(V) falls below the volumetric pressure limit PG(V).
[0043] If it should be determined that the braking B rOnce the braking process has been completed, a subsequent step checks whether the measured brake fluid loss VB, as shown in Figure 5, is greater than zero. If so, the measured brake fluid loss VB is not included in the calculation for a subsequent braking event. However, if the brake fluid loss is less than zero, this value is stored and included in the calculation for the fuel loss VB during the next braking event. The process then restarts from step A.
Claims
Claims 1. Method for detecting a leak in an external power braking system (10) of a motor vehicle, wherein the external power braking system (10) comprises an external power brake cylinder (66) with a brake piston (74) for generating a brake pressure (PB), and the method comprises the steps: Continuous determination (A) of a volume-pressure value (p(V)) based on a measured brake pressure (PB) and a brake fluid volume (VFZ) displaced by the external brake cylinder (66) during a braking operation (B) r ), In the event that the volume-pressure value (p(V)) exceeds a predefined volume-pressure limit (PG(V)), the following steps are performed: Summing (B) of a tolerated and time- and pressure-dependent brake fluid loss (VB) from the beginning of braking (B r ) up to a first refilling process (Ni) of the external power brake cylinder (66) and before at least one further refilling process (Nn ), Determination (C) of a (Ni, N) during each refilling process n ) absorbed brake fluid volume (AVN), reducing (D) the brake fluid loss (VB) by the amount lost during the refilling process (Ni, N) n ) absorbed brake fluid volume (VN), Determining (E) a fault if the calculated brake fluid loss (VB) occurs before a refilling operation (Ni, N) n ) below a volume limit (VL).
2. Method according to claim 1, characterized in that a predetermined wear-related brake fluid loss (AVv) is taken into account to compensate for brake pad wear.
3. Method according to claim 1 or 2, characterized in that when an ABS system is activated during braking (B r ) a predetermined ABS brake fluid loss (AVABS) is taken into account.
4. Method according to one of the preceding claims, characterized in that the external force braking system (10) is degraded (F) in the event of a fault.
5. Method according to one of the preceding claims, characterized in that a predetermined parking brake fluid loss (AVPB) is taken into account when a parking brake is activated.
6. Method according to one of the preceding claims, characterized in that the tolerated and time-dependent brake fluid loss (VB) is calculated as a function of the brake pressure (PB).
7. Method according to one of the preceding claims, characterized in that a volume limit value (VV) of 10-20% of the external force brake cylinder volume is assumed.
8. Method according to one of the preceding claims, characterized in that the brake fluid loss (VB) is reduced to a maximum value (V). max ) is limited.
9. Method according to one of the preceding claims, characterized in that a negative brake fluid loss (VB) occurs at the end of the braking (B). r ) as negative brake fluid loss (VB) into a subsequent braking action (B) r ) is included in the brake fluid loss (VB).
10. Method according to one of the preceding claims, characterized in that a positive brake fluid loss (VB) occurs at the end of the braking (B). r ) is set to zero.
11. External power braking system (10) for a motor vehicle for carrying out a method according to one of the preceding claims, comprising: an external power brake cylinder (66) with a brake piston (74) for generating a brake pressure (PB), a brake pressure sensor (86) for detecting the brake pressure (PB), Means (78) for detecting a (Ni, N) during the refilling process n) recorded brake fluid volume (AVN), and a control unit (94) for determining the theoretical brake fluid loss (VB) and for detecting a fault.
12. Motor vehicle comprising an external power braking system (10) according to claim 11.