Method for pointing out fading of brake under thermal load

Abstract

To provide a method for pointing out fading of a brake under a thermal load.SOLUTION: According to the present invention, a method controls an automobile brake device of a hydraulic brake-by-wire automobile brake system including a sensor system for detecting a brake pedal operation amount and an actuator for providing hydraulic pressure to a wheel brake, and a control value of the actuator is generated on the basis of the brake pedal operation amount and a stored pedal characteristic curve. A fading phenomenon is detected to avoid a dangerous state during the fading phenomenon, and a volume monitoring part determines a present pressure-volume characteristic curve to determine a quantitative fading value, compares the present pressure-volume characteristic curve with a stored pressure-volume characteristic curve, generates a control value adjusted for the actuator on the basis of the brake pedal operation amount, the stored pedal characteristic curve, and a quantitative fading value of when the fading phenomenon is confirmed, and controls the actuator by using the adjusted control value.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The invention relates to a hydraulic brake-by-wire braking system for a vehicle and to a method for controlling such a system, which comprises a sensor system for detecting at least one brake pedal actuation quantity and at least one actuator for supplying hydraulic pressure to at least one wheel brake, in which method a control value for the at least one actuator is generated on the basis of the brake pedal actuation quantity and a stored pedal characteristic curve. [Background technology]

[0002] Hydraulic braking systems for automobiles are typically implemented in a dual circuit, permanent or switchable, to have some degree of failure redundancy. The two circuits are connected in an X-pipe manner or in a front-to-rear split. In a brake-by-wire system, an electric actuator is responsible for the actual pressure setting, and the driver is connected to a simulator via a brake pedal. The brake pressure demand is generated based on the brake pedal operation, which the actuator translates. The brake pressures on the front and rear axles are generally equal, but may differ from each other depending on various circumstances.

[0003] In the case of a brake-by-wire system, the driver is thus decoupled from the wheel brakes. The simulator gives the driver a constant pedal feeling, independent of the actual wheel brake state and method. However, in the case of a conventional hydraulic wheel brake system, heavy brake use leads to a reduction in braking effect, which is called fade. The brake friction coefficient decreases and the brake volume absorption increases. The driver immediately notices this, since more force is required via the brake pedal to achieve the same level of deceleration. In a brake-by-wire braking system, by decoupling the driver from the wheel brakes, this feedback is absent. Therefore, due to the lack of direct feedback, the driver may unknowingly overuse the wheel brakes.

[0004] In racetrack driving, the wheel brakes may not be cooled properly or may not have enough cooling air flowing around them, which can cause the brake fluid in the brake caliper to reach its boiling point when it is hot, causing the brake fluid to degas and create bubbles or air in the brake caliper.

[0005] US Pat. No. 5,399,633 discloses a method for monitoring brake fluid volume, whereby a volumetric displacement is established which is useful for indicating leaks.

[0006] From the non-published US Pat. No. 5,399,433 a method is known for determining an air estimate in the case of an inaccurate pV curve, where a real pV curve is determined which differs from the nominal pV curve due to heavy braking.

[0007] From DE 10 200 03 133 A method for operating a hydraulic vehicle braking system is known, in which the ratio between the driver's brake actuation and the resulting wheel brake pressure is modified in the event of a fault. The aim here is to provide feedback to the driver that the braking force of the brake system has been reduced. This is achieved by modifying a predefined pedal characteristic. In a first embodiment, the progression of the pedal characteristic is modified in such a way that, upon actuation, the wheel pressure initially increases relatively little and then increases to a greater extent. The curves of the different pedal characteristics are selected such that they intersect. In a further embodiment, the change in the pedal characteristic is combined with a warning (visual, acoustic, etc.). In this case, the pedal characteristic is modified in such a way that the vehicle deceleration is smaller. At an actuation point of a pedal force F of 500 N, the vehicle deceleration of 2.44 m / s 2 The progression of the curve is made flatter so that a vehicle deceleration of 0.05 mm can be achieved. In that case, however, the extent of the actual fade phenomenon is not taken into account. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] German Patent Application Publication No. 102019215418A1 [Patent Document 2] DE 102022209170 [Patent Document 3] German Patent Application Publication No. 102018212852A1 Summary of the Invention [Problem to be solved by the invention]

[0009] SUMMARY OF THE PRESENT EMBODIMENT It is therefore an object of the present invention to provide a brake device and a method for controlling the brake device that suppresses problems caused by the fade phenomenon. [Means for solving the problem]

[0010] This problem is solved by the volume monitoring unit determining a current pressure-volume characteristic curve, comparing the current pressure-volume characteristic curve with a stored pressure-volume characteristic curve in order to detect a fade phenomenon and determine a quantitative fade value, and when a fade phenomenon is identified, generating an adjusted control value for at least one actuator based on the brake pedal actuation amount, the stored pedal characteristic curve and the quantitative fade value, and controlling the actuator with the adjusted control value to supply pressure to at least one wheel brake. Thereby, the behavior of the braking system is gradually adjusted as the fade phenomenon increases, so that the driver can receive accurate feedback on the state of the braking system. This allows the driver to adjust his driving behavior accordingly.

[0011] In a preferred embodiment of the invention, the sensor system comprises at least one sensor for the pedal stroke, the brake pedal force and / or the brake pedal pressure. The brake pedal pressure is in this case in particular the master cylinder pressure, i.e. the pressure generated by the driver's muscle forces via the brake pedal. In particular, by combining several of these quantities, it is possible to draw accurate and reliable inductive inferences about the driver's braking wishes.

[0012] In a further preferred embodiment of the invention the actuator is a linear actuator and / or a piston pump. In particular, a linear actuator allows very precise control of both pressure increase and pressure reduction.

[0013] In a further preferred embodiment of the invention, the volume monitoring section is formed by a leak monitoring unit, which detects a leak if the difference between the current pressure-volume characteristic curve and the stored pressure-volume characteristic curve is greater than a threshold value, and switches to a fallback mode when a leak is detected.

[0014] In a further preferred embodiment of the invention, the fade value comprises a volume difference, in particular an offset between the current pressure-volume characteristic curve and a nominal pressure-volume characteristic curve, a slope of the current pressure-volume characteristic curve and / or a coefficient between a stored pressure-volume characteristic curve and the current pressure-volume characteristic curve.

[0015] In a further preferred embodiment of the invention, at least one intermediate value is determined from the brake pedal actuation quantity and the stored pedal characteristic curves in order to determine the adjusted control value, and the intermediate value is modified by a quantitative fade value. For example, a deceleration demand or a pressure demand can be calculated as the intermediate value. For this reason, it is also possible to use a pedal characteristic curve, which can be selected depending on the driver's input, for example in a sport mode.

[0016] In a further preferred embodiment of the invention, the adjusted control value provides a lower pressure to the wheel brakes than the control value that would be used without the fade phenomenon being checked, so that the reduced braking effect is more clearly visible to the driver.

[0017] In a further preferred embodiment of the invention, the fade value is multiplied to the intermediate value as a factor, in particular as a factor less than one.

[0018] In a further preferred embodiment of the present invention, the fade value is subtracted from the intermediate value as an offset.

[0019] In a further preferred embodiment of the invention, when a fade phenomenon is identified, an audible and / or visual warning message is additionally output to the driver, the warning message being output immediately upon detection of the fade phenomenon and / or after the fade value exceeds a threshold value, so that the driver can adjust his / her driving behavior at an early stage.

[0020] In a further preferred embodiment of the invention, the adjustment of the control value in the event of a fade phenomenon is carried out only if the method is activated via the driver interface, so that such a function can be used in particular for test drives.

[0021] The problem is further solved by a hydraulic brake-by-wire vehicle braking system, which comprises a sensor system for detecting at least one brake pedal actuation quantity, at least one actuator for supplying hydraulic pressure to at least one wheel brake, and a control device configured to generate a control value for the at least one actuator based on the brake pedal actuation quantity and a pedal characteristic curve stored in a storage device of the control device, the control device being further configured to perform a volume monitoring, which determines a current pressure-volume characteristic curve and compares the current pressure-volume characteristic curve with the stored pressure-volume characteristic curve in order to detect a fade phenomenon and determine a quantitative fade value, whereby, when a fade phenomenon is identified, an adjusted control value for the at least one actuator is generated based on the brake pedal actuation quantity, the stored pedal characteristic curve and the quantitative fade value, and the actuator is controlled by the adjusted control value in order to supply pressure to the at least one wheel brake.

[0022] Further features, advantages and possible applications of the invention are evident from the following description of examples and from the drawings, in which all the features described and / or shown belong to the subject matter of the invention both individually and in any combination, independently of the claims or their abstracts in the references thereto. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a diagram showing a brake device operating in a normal operating mode using a brake-by-wire system. [Diagram 2] FIG. 13 is a diagram illustrating driver's request detection. [Diagram 3] 1 is a graph showing a pressure-volume characteristic curve. [Figure 4] FIG. 13 is a diagram showing a calculation logic for detecting a driver's request. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] FIG. 1 shows a brake system 1 operating in a normal operating mode with a brake-by-wire system. Brake fluid is stored in a reservoir or brake fluid tank 2, which is fed to the individual wheel brakes 6, 7, 8, 9 by a master brake cylinder 3 or a linear actuator 5. In normal driving mode, the master brake cylinder 3 is separated from the individual wheel brakes 6, 7, 8, 9 by a shut-off valve (MCV) 11 or master brake cylinder valve and is used only for detecting the driver's braking request. Instead, a simulator valve 10 is open and a connection is established between the master brake cylinder 3 and the simulator 4 via the simulator valve 10. When a brake pedal connected to the master brake cylinder 3 is actuated, brake fluid is transferred from the master brake cylinder 3 to the simulator 4 in order to simulate a suitable pedal feeling. The master brake cylinder pressure generated by the driver via the master brake cylinder 3 is measured by a pressure measuring device 17. Furthermore, the position of the brake pedal is detected by a suitable measuring device, in particular by a pedal stroke measurement. Based on the measured master brake cylinder pressure and / or the brake pedal position, a linear actuator 5 is controlled, which is connected to each wheel brake 6, 7, 8, 9 via an open supply valve (PFV) 12. The brake circuit of the hydraulic brake device 1 is divided or can be divided into two partial circuits by a circuit isolation valve (CSV) 13. The wheel brakes 8, 9 of the rear axle are assigned to a first partial circuit and the wheel brakes 6, 7 of the front axle are assigned to a second partial circuit. Furthermore, the circuit isolation valve 13 isolates the second partial circuit from the linear actuator 5. Each of the wheel brakes 6, 7, 8, 9 has an inlet valve 14 and an outlet valve 15. In normal driving mode, the circuit isolation valve 13 is open, whereby the hydraulic pressure supplied via the linear actuator 5 is supplied to all four wheel brakes 6, 7, 8, 9.

[0025] 2 next shows the driver's request detection, which determines a first deceleration request on the basis of the pedal stroke sensor 21 and a first pedal characteristic curve 22. A second deceleration request 26 is determined on the basis of the master cylinder pressure sensor 17 and a second pedal characteristic curve 25. The first deceleration request 23 and the second deceleration request 26 are processed to form a joint deceleration request 27 which is used as the basis for the subsequent control of the linear actuator 5. In addition to this, further conversion cascades can subsequently take place, including, for example, the pressure to be delivered, the required volume, the torque, the motor current, etc.

[0026] Heavy braking during driving leads to a thermal load on the brakes (brake fade phenomenon) which increases the volumetric absorption. Since the brake pedal and the brake system are decoupled, the driver does not receive immediate feedback from the system about the reduced braking effect. The volume monitoring function (VDM) in the control and regulation unit (ECU) of the brake system detects the increase in volumetric consumption caused by the deviation of the actual pressure-volume characteristic curve (pV curve) 32 or from a stored or nominal pressure-volume characteristic curve 31 (Figure 3).

[0027] By analyzing the pressure-volume characteristic curves 31, 32, several pieces of information regarding the fade phenomenon can be determined: in particular the volume difference (offset), the slope of the actual pressure-volume characteristic curve 32, and the coefficient between the actual pressure-volume characteristic curve 32 and the nominal pressure-volume characteristic curve 31.

[0028] If the difference (coefficient or slope) between the actual and nominal pressure-volume characteristic curves increases, then according to the invention feedback is given to the driver regarding the deceleration behavior. For this purpose, information regarding the state of the brakes and the state of the volumetric absorption is provided by a monitoring function (VDM), as shown in FIG.

[0029] To achieve a gradual adjustment of the deceleration demand depending on the degree of the fade phenomenon, a correction factor in the driver's request detection can be used for the adjustment. For this purpose, different input values ​​are detected by the volume monitoring function (VDM), which influences the correction factor.

[0030] For example, the correction factor may be based on the ratio of actual volumetric absorption to nominal volumetric absorption. a korr =a nom *1 / F pv Here, a korr is the corrected deceleration demand, a nom is the nominal deceleration demand, and F pv is the ratio of actual volumetric absorption to nominal volumetric absorption.

[0031] From experience, pV coefficient F pv reaches values ​​between 1.0 and 2.0, where a pV factor of 1.0 corresponds to an ideal match between the nominal and real pV curves.

[0032] Additionally, an offset may also be provided, which may be inferred, for example, from a table of values. In this manner, an offset in the deceleration demand may be determined based on the current volume difference.

[0033] [Table 1]

[0034] The nominal deceleration demand is corrected by this offset. a korr =a nom -a x In this case, the overall adjustment may have a limited effect, and on the one hand, it must not result in under-braking, and on the other hand, the change during the braking operation must not be so large that it irritates the driver. For example, the change in the deceleration demand amount is 6.43 m / s 2It cannot be less than 3m / s (for example, at full deceleration). 2 The adjustment needs to be limited so that the difference does not exceed

[0035] The information from the monitoring function (VDM) is used to influence the deceleration behavior of the vehicle. The influence is given as a change in the calculation logic of the driver's request detection, as shown in FIG. 4. The change is not made to the characteristic curve of the pedal characteristic. Rather, an intervention is made in the downstream calculation of the deceleration demand. This allows an essentially infinitely infinite adaptation, despite the limited computing power and memory capacity of the vehicle control device. Furthermore, other functions, including the selection of the characteristic curve, such as the driving mode selection, are not affected. According to the invention, the volume monitoring function (VDM) provides information about the presence and the extent of the fade phenomenon.

[0036] The downstream calculation in the driver's wish detection can adjust the deceleration demand based on the fade phenomenon information and can have such an influence on the deceleration behavior. For example, a correction factor in the calculation algorithm is used to adapt the deceleration demand to the degree of the fade phenomenon, so that the more the fade phenomenon increases, the smaller the deceleration. In this embodiment, the volume monitoring function outputs a (pV) factor, which represents the degree of the fade phenomenon by the relationship between the actual volume absorption and the nominal volume absorption. The reduction or adjustment of the deceleration demand to the degree of the fade phenomenon results in a smaller deceleration demand as the fade phenomenon increases. This indicates to the driver a decrease in the braking performance. The driver can then adapt his driving style to the system. The stored pedal characteristic is not changed in this case. To compensate for the reduced deceleration according to the fade phenomenon, the driver must control the pedal to a higher operating point. The reduced braking effect due to the changed deceleration behavior can be complemented by a visual or audible warning message. For example, a text message (e.g. "heavy braking") can be shown on the display, which can be complemented by a warning light. Fade is usually a reversible system state. As the wheel brakes cool down, the degree of fade decreases, i.e. the wheel brake volume requirement decreases again. The system must then be restored and the correction value reduced again.

[0037] As in conventional braking systems where the driver has direct access to the wheel brakes, the driver in this case experiences a gradual change in the deceleration behavior and can adapt his driving style. If the fade phenomenon decreases due to an adjusted driving style or a reduction in the use of the brakes, the correction factor for the deceleration demand is reduced again. The fade phenomenon itself is then generally not interpreted as a failure or a fault condition of the brake system. However, if the driver does not recognize at an early stage that the system behavior is changing, the fade phenomenon can lead to failure and deterioration of the brake system.

[0038] Taking the quantitative fade phenomenon into account makes it possible to improve racetrack suitability through early warning measures in the event of brake fade and to increase the driving safety of the vehicle, especially during test runs, thereby preventing any potential risks of premature system degradation due to increased volumetric consumption.

[0039] In an alternative embodiment, the modification of the deceleration behavior is not performed in the driver's request detection, but directly in the control logic of the actuator (pressure regulator, SPC), i.e. the pressure demand is modified instead of the deceleration demand due to the fade phenomenon. As in the above embodiment, the modification is performed depending on the manifestation of the fade phenomenon. This has the same effect at the system level.

[0040] The current volumetric absorption of the system is detected and normalized to the actual volumetric absorption of the system. A normalization factor can be derived therefrom and used in the pressure regulator. The control strategy can then be adjusted or changed. With the normalization factor a control differential is set which is proportional to the system volume difference. The higher the actual volumetric absorption relative to the nominal volumetric absorption, the higher the control differential. The control differential causes the driver to press harder on the pedal in order to obtain a higher actual pressure (the control differential causes the actual pressure not to reach the target pressure). Due to the increasing volume requirement the driver must constantly adjust his operation to achieve the desired deceleration. In this way the fade phenomenon is "simulated" or indicated to the driver. [Explanation of symbols]

[0041] 1 Brake device 5. Linear Actuators 6, 7, 8, 9 Wheel brakes 17 Master cylinder pressure sensor 21 Pedal stroke sensor 22 Pedal characteristic curve 31 Stored pressure-volume characteristic curve 32 Actual (current) pressure-volume characteristic curve

Claims

1. 1. A method for controlling a hydraulic brake-by-wire vehicle braking system comprising a sensor system for detecting at least one brake pedal actuation quantity and at least one actuator for supplying hydraulic pressure to at least one wheel brake, the method comprising: generating a control value for the at least one actuator on the basis of the brake pedal actuation quantity and a stored pedal characteristic curve, the method comprising: a volume monitoring unit for determining a current pressure-volume characteristic curve and comparing the current pressure-volume characteristic curve with a stored pressure-volume characteristic curve to detect a fade event and determine a quantitative fade value; generating an adjusted control value for the at least one actuator based on the brake pedal depression amount, the stored pedal characteristic curve, and the quantitative fade value when a fade phenomenon is identified; controlling said actuator with said adjusted control value to supply pressure to at least one wheel brake.

2. 2. The method according to claim 1, characterized in that the sensor system comprises at least one sensor for pedal travel, brake pedal force and / or brake pedal pressure.

3. 3. The method according to claim 1 or 2, characterized in that the actuator is a linear actuator and / or a piston pump.

4. 4. The method according to claim 1, wherein the volume monitoring unit forms a leak monitoring unit, which detects a leak if a difference between the current pressure-volume characteristic curve and the stored pressure-volume characteristic curve is greater than a threshold value.

5. 5. The method according to claim 1, wherein the fade values ​​include a leakage value, a volume difference and / or a slope of the current pressure-volume characteristic curve and / or a coefficient between the stored pressure-volume characteristic curve and the current pressure-volume characteristic curve.

6. 6. The method according to claim 1, further comprising the step of determining at least one intermediate value for determining the adjusted control value from the brake pedal actuation quantity and the stored pedal characteristic curve, and modifying the intermediate value by the quantitative fade value.

7. 7. A method according to claim 1, wherein the adjusted control value provides a lower pressure to the wheel brakes than the control value used without checking for fade phenomena.

8. Method according to any one of the preceding claims, characterized in that the intermediate value is multiplied by the fade value as a factor, in particular as a factor less than one.

9. A method according to any one of claims 1 to 8, characterized in that the fade value is subtracted from the intermediate value as an offset.

10. When a fade phenomenon is confirmed, an audio and / or visual warning message is additionally output to the driver; Method according to any one of the preceding claims, characterized in that the warning message is output immediately upon detection of a fade phenomenon and / or once the fade value exceeds a threshold value.

11. 11. The method according to claim 1, wherein the adjustment of the control value in the event of a fade phenomenon is only performed if the method is activated via a driver interface.

12. A hydraulic brake-by-wire type automobile brake device, A vehicle brake system comprising a sensor system for detecting at least one brake pedal actuation quantity, at least one actuator for supplying hydraulic pressure to at least one wheel brake, and a control device configured to generate a control value for the at least one actuator on the basis of the brake pedal actuation quantity and a pedal characteristic curve stored in a memory device of the control device, the control device is further configured to perform a volume monitor that determines a current pressure-volume characteristic curve and compares the current pressure-volume characteristic curve with a stored pressure-volume characteristic curve in order to detect a fade event and determine a quantitative fade value; generating an adjusted control value for the at least one actuator based on the brake pedal actuation amount, the stored pedal characteristic curve, and the quantitative fade value when a fade phenomenon is identified; 4. A hydraulic brake-by-wire vehicle braking system, characterized in that said actuator is controlled by said adjusted control value for supplying pressure to said at least one wheel brake.

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