Method for indicating fading in thermally stressed brakes

Abstract

The invention relates to a method for controlling a hydraulic brake-by-wire motor vehicle braking system comprising sensors for detecting at least one brake pedal actuation parameter, and at least one actuator for providing hydraulic pressure in at least one wheel brake. where, based on the brake pedal actuation magnitude and a stored pedal characteristic curve, a control signal is generated for at least one actuator. To prevent dangerous situations during brake fade, a current pressure-volume characteristic curve is determined by volume monitoring and compared with a stored pressure-volume characteristic curve to detect fading and determine a quantitative fading magnitude. wherein, when fading is detected, an adapted control parameter is generated for the at least one actuator based on the brake pedal actuation magnitude, the stored pedal characteristic curve and the quantitative fading magnitude, and the actuator is controlled with this adapted control parameter to provide pressure in the at least one wheel brake.

Description

[0001] The invention relates to a hydraulic brake-by-wire motor vehicle brake system and a method for controlling a hydraulic brake-by-wire motor vehicle brake system comprising sensors for detecting at least one brake pedal actuation variable, and at least one actuator for providing a hydraulic pressure in at least one wheel brake, wherein a control variable for the at least one actuator is generated based on the brake pedal actuation variable and a stored pedal characteristic curve.

[0002] Hydraulic braking systems for motor vehicles are typically designed with permanent or switchable dual circuits to provide a degree of fault redundancy. These two circuits can be connected diagonally or in a black-and-white configuration. In a brake-by-wire system, an electric actuator takes over the actual pressure setting, while the driver is connected to a simulator via the brake pedal. The brake pressure request is generated based on the brake pedal actuation, and the actuator implements this. The brake pressure on the front and rear axles is generally equivalent, but can differ in different situations.

[0003] In a brake-by-wire system, the driver is thus decoupled from the wheel brake. The simulator gives the driver a constant pedal feel regardless of the actual condition and type of wheel brake. However, with conventional hydraulic wheel brake systems, a reduction in braking effect occurs when the brake load is high. This is known as fading. The friction coefficient of the brake decreases and the volume absorption of the brake increases. The driver notices this immediately, as greater force is required via the brake pedal to achieve the same level of deceleration. Because the driver is decoupled from the wheel brake in brake-by-wire braking systems, this feedback is missing. It is therefore possible that the driver unknowingly overstresses the wheel brake due to the lack of direct feedback.

[0004] In racetrack operation, there are situations where the wheel brakes are not sufficiently cooled or not sufficiently supplied with cooling air. This can cause the brake fluid in the brake caliper to reach its boiling point at high temperatures. As a result, the brake fluid degasses, and vapor bubbles or air form in the brake caliper.

[0005] DE102019215418A1 discloses a method for monitoring brake fluid volume. This method determines the effective volume displacement to indicate a leak.

[0006] The unpublished patent application DE 10 2022 209 170 discloses a method for determining air pressure estimation when the pV curve is inaccurate. This method determines the actual pV curve, which deviates from the nominal pV curve due to high brake load.

[0007] DE10 2018 212 852 A1 discloses a method for operating a hydraulic vehicle brake system which modifies the relationship between the driver's brake application and the resulting wheel brake pressure in the event of a fault. The goal is to provide the driver with feedback that the braking system is experiencing reduced braking performance. This is achieved by changing the intended pedal characteristic. In a first embodiment, the progression of the pedal characteristic is modified so that upon application, the wheel pressure increases less initially and then more strongly. The curves of different pedal characteristics are selected so that they overlap. In another embodiment, the change in the pedal characteristic is combined with a warning (visual, acoustic, or similar). In this case, the pedal characteristic is modified so that this results in lower vehicle deceleration.The curve is made flatter so that at the operating point a pedal force F of 500 N results in a vehicle deceleration of 2.44 m / s. 2 can be achieved. However, the degree of actual fading is not taken into account.

[0008] It is therefore an object of the present invention to provide a braking system and a method for controlling a braking system which overcomes the problems caused by fading.

[0009] The problem is solved by a volume monitor that determines a current pressure-volume characteristic curve and compares it with a stored pressure-volume characteristic curve to detect fading and determine a quantitative fading variable. Upon detection of fading, an adapted control variable is generated for the at least one actuator based on the brake pedal actuation variable, the stored pedal characteristic curve, and the quantitative fading variable. The actuator is controlled with this adapted control variable to provide pressure in the at least one wheel brake. Thus, the behavior of the braking system is gradually adapted as fading increases, so that the driver receives precise feedback on the condition of the braking system. They can thus adapt their driving behavior accordingly.

[0010] In a preferred embodiment of the invention, the sensor system comprises at least one sensor for pedal travel, brake pedal force, and / or brake pedal pressure. The brake pedal pressure is, in particular, a master cylinder pressure, i.e., the pressure built up by the driver's muscle power via the brake pedal. A combination of several of these variables, in particular, allows precise and reliable conclusions to be drawn about the driver's braking request.

[0011] 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 for very precise control of both pressure buildup and pressure reduction.

[0012] In a further preferred embodiment of the invention, the volume monitoring is implemented by a leakage monitoring unit, which detects a leak if the current pressure-volume characteristic curve and the stored pressure-volume characteristic curve deviate by more than a threshold value. If a leak is detected, the system then switches to a fallback mode.

[0013] In a further preferred embodiment of the invention, the fading variable comprises a difference volume, in particular an offset between the current and nominal pressure volume characteristic curve, a gradient of the current pressure volume characteristic curve and / or a factor between the stored pressure volume characteristic curve and the current pressure volume characteristic curve.

[0014] In a further preferred embodiment of the invention, at least one intermediate value is determined from the brake pedal actuation value and the stored pedal characteristic curve to determine the adjusted control variable, and the intermediate value is modified using the quantitative fading variable. For example, a deceleration request or pressure request can be calculated as an intermediate variable. Thus, a pedal characteristic curve can also be used, which can be selected, for example, depending on a driver input such as a sport mode.

[0015] In a further preferred embodiment of the invention, the adjusted control variable provides a lower pressure in the wheel brakes than the control variable used without detected fading. This makes the decreasing braking effect clear to the driver.

[0016] In a further preferred embodiment of the invention, the fading size is multiplied as a factor, in particular less than 1, to the intermediate size.

[0017] In a further preferred embodiment of the invention, the fading size is subtracted as an offset from the intermediate size.

[0018] In a further preferred embodiment of the invention, an acoustic and / or visual warning is additionally issued to the driver upon detection of fading. The warning is issued immediately upon detection of fading and / or only when the fading magnitude exceeds a threshold. This allows the driver to adjust their driving behavior in good time.

[0019] In a further preferred embodiment of the invention, the adjustment of the control variable in the event of fading is only performed if this has been activated via a driver interface. Thus, such a function can be used particularly for test drives.

[0020] The object is further achieved by a hydraulic brake-by-wire motor vehicle brake system comprising sensors for detecting at least one brake pedal actuation variable, and at least one actuator for providing a hydraulic pressure in at least one wheel brake, as well as a control device configured to generate a control variable for the at least one actuator based on the brake pedal actuation variable and a pedal characteristic stored in a memory device of the control device, wherein the control device is further configured to carry out volume monitoring, which determines a current pressure-volume characteristic and compares this with a stored pressure-volume characteristic in order to detect fading and to determine a quantitative fading variable, wherein when fading is detected based on the brake pedal actuation variable,the stored pedal characteristic curve and the quantitative fading variable, an adapted control variable for the at least one actuator is generated and the actuator is controlled with this adapted control variable to provide a pressure in the at least one wheel brake.,

[0021] Further features, advantages, and possible applications of the invention will become apparent from the following description of exemplary embodiments and the drawings. All described and / or illustrated features, both individually and in any combination, are part of the subject matter of the invention, regardless of their summary in the claims or their references.

[0022] In Fig. 1 shows a braking system 1 which, in normal operating mode, operates according to the brake-by-wire principle. Brake fluid is stored in a reservoir or brake fluid container 2 and can be pumped to individual wheel brakes 6, 7, 8, 9 by a master brake cylinder 3 or a linear actuator 5. In normal operating mode, the master brake cylinder 3 is separated from the individual wheel brakes 6, 7, 8, 9 by an isolation valve (MCV) 11 or master brake cylinder valve and serves solely to detect the driver's braking command. Instead, a simulator valve 10 is open, via which a connection is established between the master brake cylinder 3 and a simulator 4. If a brake pedal connected to the master brake cylinder 3 is actuated, brake fluid is shifted from the master brake cylinder 3 to the simulator 4 in order to simulate a suitable pedal feel.The master cylinder pressure generated by the driver via the master brake cylinder 3 is measured by a pressure measuring device 17. In addition, the brake pedal position is detected using a suitable measuring device, in particular a pedal travel measurement. Based on the measured master cylinder pressure and / or the brake pedal position, a linear actuator 5 is controlled, which is connected to the individual wheel brakes 6, 7, 8, 9 via an open supply valve (PFV) 12. The brake circuit of the hydraulic brake system 1 is divided or can be divided into two sub-circuits by a circuit separation valve (CSV) 13. The wheel brakes 8, 9 of the rear axle are assigned to a first sub-circuit, and the wheel brakes 6, 7 of the front axle are assigned to a second sub-circuit. The circuit separation valve 13 also separates the second sub-circuit from the linear actuator 5. The wheel brakes 6, 7, 8, 9 each have an inlet valve 14 and an outlet valve 15.In a normal operating mode, the circuit isolation valve 13 is open so that the hydraulic pressure provided by the linear actuator 5 is directed to all four wheel brakes 6, 7, 8, 9.

[0023] Fig. Figure 2 now shows the driver request detection, which determines a first deceleration request based on the pedal travel sensor 21 and a first pedal characteristic curve 22. A second deceleration request 26 is determined based on 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 into a common deceleration request 27, which serves as the basis for the subsequent control of the linear actuator 5. This can be followed by a further conversion cascade, which includes, for example, the pressure to be provided, the required volume, torque, motor current, and other factors.

[0024] Due to high brake loads during driving, the brakes are subjected to thermal stress (brake fading), which increases the volume consumption. Due to the decoupling of the brake pedal and the braking system, the driver initially receives no feedback from the system regarding the diminishing braking effect. A volume monitoring function (VDM) in the control and regulation unit (ECU) of the braking system records the actual pressure-volume characteristic curve (pV curve) 32 or the increase in volume consumption, as it results from the deviation from the stored or nominal pressure-volume characteristic curve 31 ( Fig. 3).

[0025] By analyzing the pressure-volume curves 31, 32, information about fading can be determined. This includes, in particular, a differential volume (offset), the slope of the actual pressure-volume curve 32, and a factor between the actual pressure-volume curve 32 and the nominal pressure-volume curve 31.

[0026] In the case of an increasing deviation from the real and the nominal pV curve (factor or gradient), the driver is given feedback about the deceleration behavior. For this purpose, as in Fig. 4, information about the status of the brake and the volume intake is transferred from the monitoring function (VDM).

[0027] To gradually adjust the deceleration request to the degree of fading, the adjustment can be made using a correction factor in the driver request detection system. For this purpose, the volume monitoring function (VDM) records various input variables, which influence the correction factor.

[0028] For example, the correction factor can be based on the ratio between real and nominal volume absorption. akorr=anom∗1 / Fpv

[0029] With the corrected delay requirement a korr, the nominal delay requirement a nom and the ratio between real and nominal volume absorption F pv .

[0030] The pV factor F pv Experience has shown that values ​​between 1.0 and 2.0 are achieved, with a pV factor of 1.0 corresponding to an ideal match between the nominal and real pV curve.

[0031] In addition, an offset can also be provided, which follows, for example, from a table of values: This allows an offset for the delay request to be determined based on the current volume deviation: dV [ccm] a x [m / s 2 ] 0 0,0 2 0,6 4 1,2 6 1,8 8 2,4 10 3,0

[0032] The nominal delay requirement can then be corrected by this offset: akorr=anom−ax

[0033] The overall adjustment can be limited in its effect and must not, on the one hand, lead to under-braking, while, on the other, the changes between braking applications must not be so great that they irritate the driver. For example, changing the deceleration requirement cannot result in the service brake performance of 6.43 m / s being reduced. 2 is exceeded by the correction values ​​(e.g. during full deceleration). At the same time, the adjustment should be limited so that a deviation of more than 3 m / s 2 is not achieved.

[0034] The information from the monitoring function (VDM) is used to influence the vehicle's deceleration behavior. This influence is implemented as a change within the calculation logic of the driver request detection, as described in Fig.4. This does not change the pedal characteristic curves. Rather, the downstream calculation of the deceleration request is intervened. This enables essentially continuous adaptation despite the limited computing and storage capacities of vehicle control units. Furthermore, other functions that involve selecting the characteristic curve, such as a driving mode selector, are not affected. According to the invention, the volume monitoring function (VDM) provides information about the presence and severity of fading.

[0035] The downstream calculation within the driver request detection system can adjust the deceleration request based on the fading information and thus influence the deceleration behavior. For example, a correction factor in the calculation algorithm is used to adjust the deceleration request to the degree of fading, resulting in a lower deceleration as fading increases. Here, the volume monitoring function outputs a (pV) factor which represents the degree of fading by relating the actual and nominal volume consumption. Reducing or adjusting the deceleration request to the degree of fading results in a lower deceleration request as fading increases. This indicates to the driver a decrease in braking performance. The driver can then adapt their driving style to the system. The stored pedal characteristics remain unchanged.To compensate for the reduced deceleration, which is dependent on fading, the driver must control a higher operating point via the pedal. If the changed deceleration behavior indicates to the driver that the braking effect is decreasing, this can be supplemented by a visual or audible warning. For example, a text message can be shown on the display (e.g. "high brake load"), which can be supplemented by a warning light. Fading is usually a reversible system condition. As the wheel brake cools down, the degree of fading decreases, i.e. the volume requirement of the wheel brake decreases again. The system should then rehabilitate itself and the correction value reduced again.

[0036] Similar to conventional braking systems, where the driver has direct control over the wheel brake, they experience a gradual change in the deceleration behavior and can adjust their driving style. If fading decreases due to an adjusted driving style or reduced brake load, the correction factor in the deceleration request will decrease again. Fading itself is not considered a fault or error condition of the braking system in general. However, fading can lead to errors and degradation of the braking system if the driver is not alerted to the changing system behavior early on.

[0037] By quantitatively incorporating fading, racetrack suitability can be improved through early warning measures for brake fade, as well as increasing vehicle operational reliability, especially during test operations. This prevents the latent risk of premature system degradation caused by increasing volume consumption.

[0038] In an alternative embodiment, the change in deceleration behavior is not made in the driver request detection, but directly in the actuator control logic (pressure regulator, SPC). This means that due to fading, the pressure request is changed instead of the deceleration request. As in the previously mentioned embodiment, the change is made depending on the severity of the fading. At the system level, this has the same effect.

[0039] The current volume consumption of the system is recorded and normalized to the actual volume consumption of the system. From this, a standard factor can be derived, which is made available to the pressure controller. This can be used to adapt or change the control strategy. The standard factor is used to set a control deviation which is proportional to the deviation in the system volume: the higher the actual volume consumption is compared to the nominal volume consumption, the greater the control deviation. The control deviation will cause the driver to press the pedal harder in order to achieve a higher actual pressure (the actual pressure does not reach the target pressure due to the control deviation). As the volume requirement increases, the driver must constantly adjust their pressure in order to achieve the desired deceleration. In this way, fading is “simulated” or demonstrated to the driver. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 102019215418A1

[0005] DE 10 2022 209 170

[0006] DE 10 2018 212 852 A1

[0007]

Claims

[1] Method for controlling a hydraulic brake-by-wire motor vehicle brake system comprising sensors for detecting at least one brake pedal actuation variable, and at least one actuator for providing a hydraulic pressure in at least one wheel brake, wherein a control variable for the at least one actuator is generated based on the brake pedal actuation variable and a stored pedal characteristic curve, characterized by that a volume monitor determines a current pressure volume characteristic and compares this with a stored pressure volume characteristic in order to detect fading and determine a quantitative fading value, wherein, when fading is detected, an adapted control variable for the at least one actuator is generated based on the brake pedal actuation variable, the stored pedal characteristic curve and the quantitative fading variable, and the actuator is controlled with this adapted control variable in order to provide a pressure in the at least one wheel brake. [2] Method according to claim 1 characterized by that the sensor system comprises at least one sensor for the pedal travel, a brake pedal force and / or a brake pedal pressure. [3] Method according to one of the preceding claims characterized by that the actuator is a linear actuator and / or a piston pump. [4] Method according to one of the preceding claims characterized by that the volume monitoring forms a leakage monitoring unit which detects a leak if the deviation between the current pressure volume characteristic curve and the stored pressure volume characteristic curve is greater than a threshold value. [5] Method according to one of the preceding claims characterized by that the fading variable includes a leakage value, a differential volume and / or a gradient of the current pressure volume characteristic curve and / or a factor between the stored pressure volume characteristic curve and the current pressure volume characteristic curve. [6] Method according to one of the preceding claims characterized by that to determine the adjusted control variable from the brake pedal actuation variable and the stored pedal characteristic curve, at least one intermediate value is determined and the intermediate value is modified with the quantitative fading variable. [7] Method according to one of the preceding claims characterized by that the adjusted control variable provides a lower pressure in the wheel brakes than the control variable which is used without detected fading. [8] Method according to one of the preceding claims characterized bythat the fading size is multiplied as a factor, especially less than 1, to the intermediate size. [9] Method according to one of the preceding claims characterized by that the fading size is subtracted as an offset from the intermediate size. [10] Method according to one of the preceding claims characterized by that when fading is detected, an acoustic and / or visual warning message is also issued to the driver, whereby the warning message is issued immediately when fading is detected and / or only when the fading size exceeds a threshold value. [11] Method according to one of the preceding claims characterized by that the adjustment of the control variable in case of fading is only carried out if this has been activated via a driver interface. [12] Hydraulic brake-by-wire motor vehicle brake system comprising sensors for detecting at least one brake pedal actuation variable, and at least one actuator for providing a hydraulic pressure in at least one wheel brake, as well as a control device configured to generate a control variable for the at least one actuator based on the brake pedal actuation variable and a pedal characteristic stored in a memory device of the control device, characterized bythat the control device is further configured to carry out volume monitoring, which determines a current pressure-volume characteristic curve and compares this with a stored pressure-volume characteristic curve in order to detect fading and to determine a quantitative fading variable, wherein, when fading is detected, an adapted control variable is generated for the at least one actuator based on the brake pedal actuation variable, the stored pedal characteristic curve and the quantitative fading variable, and the actuator is controlled with this adapted control variable to provide a pressure in the at least one wheel brake.

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