Method for detecting an increased probability of fading in a wheel brake system of a motor vehicle, control unit, wheel brake system

Abstract

The invention relates to a method for detecting an increased probability of brake fade in a wheel brake assembly (1) of a motor vehicle, wherein the wheel brake assembly (1) is configured to decelerate the motor vehicle by generating a clamping force (F) acting on a brake disc (3), and wherein the wheel brake assembly comprises a brake piston (7) with a controllable electrical actuator (5) associated with the brake piston (7). According to the invention, during a deceleration process, the clamping force (F) and a displacement position (s) of the brake piston (7) are determined, an actual profile of the clamping force (F) and the displacement position (s) during the deceleration process is compared with a target profile, and an increased probability of brake fade is detected if the actual profile deviates from the target profile.

Description

[0001] The invention relates to a method for detecting an increased probability of fading of a wheel brake device of a motor vehicle, wherein the wheel brake device is designed to decelerate the motor vehicle by generating a clamping force acting on a brake disc, and wherein the wheel brake device has a brake piston with a controllable electrical actuator associated with the brake piston.

[0002] Furthermore, the invention relates to a control device and a wheel brake device of a motor vehicle. State of the art

[0003] Wheel brake systems in motor vehicles are exposed to high temperatures during operation. If very aggressive braking leads to extremely high temperatures on the brake discs and pads, this can result in a significant reduction in the frictional torque between the pad and disc, and thus a drop in braking torque. This effect is also known as "fading." To compensate for this drop, the braking force or pressure must be increased dramatically, for example, by the driver applying more pressure to the brake pedal and / or by activating an actuator in the wheel brake system's braking force generator. Only after the pads and disc have cooled down does the frictional torque return to its original level.

[0004] To detect potential brake fade situations early and initiate appropriate countermeasures, disc and pad temperatures in hydraulic brake systems are typically estimated using physical models. Because these models are generally not supported by any additional temperature sensors, their accuracy is limited.

[0005] Methods for detecting faults in a wheel brake system without using temperature models are also known in the prior art. For example, German patent application DE 10 2010 040 573 A1 discloses a method for detecting a fault in a service or parking brake that includes an electromechanical brake device comprising an electric brake motor and a hydraulic brake device for generating a clamping force. A fault in the hydraulic brake device is detected if a motor parameter of the electric brake motor, or a correlated parameter, lies outside a permissible range. A hydraulic clamping force can also be generated in the event of wheel brake fade, where the vehicle deceleration does not correspond to the driver's braking request.Fading is determined from a previous braking process by querying whether the vehicle deceleration from a previous braking process did not correspond to the driver's braking request. Disclosure of the invention

[0006] The method according to the invention, with the features of claim 1, is characterized in that the clamping force and the displacement position of the brake piston are determined during a deceleration process, that the actual profile of the clamping force and the displacement position during the deceleration process is compared with a target profile, and that an increased probability of fading is detected if the actual profile deviates from the target profile. By comparing the profiles, an impending fading situation is advantageously detected early and with high reliability, without, as mentioned above, using temperature models with limited accuracy, thus providing sufficient time to take appropriate countermeasures, such as degrading the system.The comparison of the actual behavior to the target behavior is preferably carried out over a predetermined, in particular a first, period and a large number of braking operations in order to detect any change in the deviation over time and to extrapolate when fading is likely to occur. To distinguish fading from normal wear, the deviation or change must occur or be detected within a sufficiently short time. Preferably, an increased probability of fading is therefore only detected if the deviation reaches, and in particular exceeds, a predetermined value within a predetermined period, in particular a second period, which lies within the first. As mentioned at the outset, fading is understood to mean that, due to overheating of the brake disc and brake pad, the frictional torque is reduced, and the resulting braking torque or the clamping force achieved with constant actuation is decreased.The clamping force is generated, in particular, by actuating the actuator to displace the brake piston, whereby a brake pad coupled to the brake piston, especially one arranged at its end face, is preferably displaced towards the brake disc. The desired profile is defined, in particular, statically, for example as a typical profile, or depending on the wear condition of the wheel brake system. In contrast, for example, to the prior art mentioned above, in which only previously existing fading from a preceding braking process is subsequently detected if the vehicle deceleration did not correspond to the driver's braking request, according to the invention, a noticeably deviating actual profile of clamping force and displacement position from an expected target profile is used as an indication or warning sign of an increased probability of fading.The invention ensures improved and therefore more robust early detection of brake fade by continuously monitoring the displacement position and clamping force. If the brake caliper temperature rises, the expansion of the brake pad and brake disc leads to a change in the piston position-to-clamping force characteristic. This allows a brake fade to be detected in advance, before it actually occurs. By detecting a spontaneous change in the characteristic during individual braking events, conclusions can be drawn about the component temperatures on which the brake fade depends. If the change is observed over a certain period, as described above, the risk of a brake fade can be particularly advantageously estimated in advance through extrapolation.In contrast to known algorithms, no further environmental conditions, such as the ambient temperature, which usually require the use of rather crude estimation models, need to be known. This makes the inventive method more accurate and robust than known algorithms.

[0007] According to a preferred embodiment of the invention, the target behavior is defined by means of a target characteristic curve, and the actual behavior is compared with the target characteristic curve. The corresponding characteristic curve advantageously ensures that the detection of increased fading probability is particularly simple and robust. In particular, the corresponding values ​​of clamping force and displacement position are continuously determined, so that a deviation is advantageously detected at an early stage.

[0008] It is particularly advantageous to determine and specify the target behavior based on at least one historical actual behavior from a previous deceleration process. This method of mediating and specifying the target behavior offers the advantage that it is dependent on a previous actual behavior, thus advantageously representing, for example, a wear condition.

[0009] According to a preferred embodiment of the invention, the clamping force is determined by means of a sensor arrangement comprising at least one force sensor and / or torque sensor. By appropriately using the sensor arrangement, it is advantageously ensured that the clamping force is determined particularly easily and accurately. The torque sensor is specifically designed as a brake torque sensor for detecting a braking torque.

[0010] Preferably, the clamping force is determined as a function of at least one parameter relating to the vehicle's condition, in particular as a function of the inclination angle of the road surface on which the vehicle is traveling, and / or the vehicle's speed. Taking the vehicle's condition into account in this way offers the advantage of further improving the accuracy of the clamping force determination.

[0011] According to a preferred embodiment of the invention, the actuator is designed as an electric machine, and the clamping force is determined as a function of the current value of the actuator's electric motor current. Determining the clamping force via the current value advantageously ensures that the clamping force is determined particularly robustly without the need for additional sensors. In particular, the clamping force is additionally determined using the aforementioned sensor arrangement, thus providing redundancy.

[0012] Preferably, the actuator is designed as an electric machine and has a rotor shaft, and the displacement position is determined as a function of the rotor shaft's position. Determining the displacement position based on the rotor position offers the advantage of simple and precise rotor position determination. For example, the rotor position is determined using a suitable rotor position sensor. Electromechanical wheel brake systems with electrically commutated machines typically incorporate rotor position sensors, which are necessary for the commutation and control of the machine.

[0013] According to a preferred embodiment of the invention, the brake piston is operatively connected to the actuator via a gear arrangement, and the displacement position is determined as a function of the gear ratio of the gear arrangement. By appropriately considering the gear ratio or the gear stages used, particularly in combination with the aforementioned determination as a function of the rotor position, the displacement position is advantageously determined particularly simply and precisely.

[0014] Particularly advantageous is the detection of an increased probability of fading only when the actual behavior deviates from the target behavior by at least a predetermined tolerance. This results in the advantage of further improving the robustness of the method according to the invention.

[0015] According to a preferred embodiment of the invention, at least one threshold value, preferably several threshold values, is specified depending on an expected braking behavior and / or coefficient of friction, and the tolerance is specified depending on the threshold value(s). By taking the corresponding threshold value into account, it is advantageously ensured that the accuracy of detecting the increased probability of fading is further improved.

[0016] Particularly preferred is the issuance of a visual, audible, and / or haptic warning message, especially on a vehicle display device, when an increased probability of fading is detected. Issuing this warning message has the advantage of immediately informing the driver of the vehicle and allowing them to take appropriate countermeasures.

[0017] The control device with the features of claim 12 is characterized in that it is specifically designed to carry out the method according to the invention. This results in the advantages already mentioned. In particular, the control device is designed as a computer device assigned to, and preferably arranged in, a motor vehicle. For example, the control device is assigned to a wheel brake system of the motor vehicle.

[0018] The wheel brake device of a motor vehicle with the features of claim 13 is designed to decelerate the motor vehicle by generating a clamping force acting on a brake disc and comprises a brake piston with a controllable electrical actuator associated with the brake piston. It is characterized by the control device according to the invention. The advantages already mentioned also result from this. The wheel brake device is designed, in particular, as a purely electromechanical wheel brake device.

[0019] Further preferred features and combinations of features will become apparent from the foregoing and from the claims. The invention will now be explained in more detail with reference to the drawings. These drawings show... Fig. 1 a wheel braking device, Fig. 2 a method for detecting an increased probability of fading of the wheel braking device, and Fig. 3 Force-displacement characteristics of the wheel brake system.

[0020] Fig. Figure 1 shows, schematically only, components of a wheel brake assembly 1 for an electromechanical braking system of a motor vehicle (not shown in detail). The wheel brake assembly 1 has a brake caliper 2 with at least one brake disc 3 and a brake pad 4. Preferably, the wheel brake assembly 1 has two brake pads 4 on the brake caliper 2, the brake pads being assigned, in particular, to the opposite side surfaces of the brake disc 3.

[0021] Furthermore, the wheel brake device 1 comprises an electromechanical actuator 5, in this case designed as an electric machine, a gear arrangement 6, and a brake piston 7. The brake pad 4 is coupled to the brake piston 7, in particular arranged at its end face.

[0022] The actuator 5 is assigned to the brake piston 7, in this case coupled to the brake piston 7 and thus to the brake pad 4 via the gear arrangement 6, in order to move the latter in the direction of the brake disc 3 within a displacement path s and to decelerate the motor vehicle by generating a clamping force acting on a brake disc 3.

[0023] The actuator is associated with a control device 8. The clamping force is generated by actuating the actuator 5 by means of the control device 8 to displace the brake piston 7. The gear arrangement 6 translates, in particular, a rotary movement of a rotor shaft (not shown) of the machine 5 into a translational movement of the brake pad 4, wherein the gear arrangement 6 comprises, for example, a spindle gear arrangement or a ball screw drive.

[0024] The following refers to Fig. 2 describes an advantageous method for detecting an increased probability of fading of the wheel brake device 1. For this purpose, the Fig. 2. The procedure is described using a flowchart. In particular, the procedure ensures that an increased probability of fading is detected at an early stage. The procedure is preferably carried out using the control unit 8.

[0025] In step S1, the process begins with the detection of a braking request and the initiation of a deceleration process. The actuator 6 is then controlled, as described above, to move the brake piston 7. During the deceleration process, the clamping force F and the displacement position s of the brake piston 7, as described above, are determined.

[0026] The clamping force F can be determined in various ways. In particular, it is determined by means of a sensor arrangement comprising at least one force sensor and / or torque sensor, and / or as a function of a current value of an electric motor current of the actuator 5.

[0027] Particularly preferably, the clamping force F is determined, alternatively or additionally, as a function of at least one quantity relating to a vehicle condition of the motor vehicle, in particular as a function of an inclination angle of a roadway on which the motor vehicle travels, and / or a driving speed of the motor vehicle.

[0028] The displacement position s is determined in particular as a function of the rotor position of the rotor shaft of the actuator 5, which is designed as an electric machine as described above, and / or as a function of the gear ratio of the gear arrangement 6.

[0029] In step S2, the actual profile of the clamping force F and the displacement position s during the deceleration process is compared with a target profile. The target profile is determined and specified based on at least one historical actual profile from a previous deceleration process.

[0030] The target curve is defined here using a target characteristic curve K. S specified, and the actual course as actual characteristic curve K I compared to the target characteristic curve. This is in Fig. 3 shown, where the actual characteristic curve K I and the target characteristic curve K S as corresponding force-displacement characteristics of the wheel brake device 1, with the clamping force F on the vertical axis, and the displacement position s on the horizontal axis.

[0031] Furthermore, as an extreme case, a fading characteristic K F applied, where corresponding fading occurs. The actual characteristic curve K Ilies in this respect between the target characteristic curve K S and the fading characteristic K F , where there is usually a smooth transition from the target characteristic curve K S on the fading characteristic K F This has been done.

[0032] Additionally, a point is marked on the respective characteristic curve at which the braked wheel locks, i.e., the maximum possible deceleration performance is reached or exceeded.

[0033] The force required for each of these increases from a corresponding first point P1 on the target characteristic curve K. S via a second point P2 on the actual characteristic curve K I up to a third point P3 on the fading characteristic curve K F steadily, as visualized by a dashed connecting line.

[0034] The comparison of the actual curve to the target curve or the corresponding characteristic curve is preferably carried out over a predetermined initial period and a large number of braking operations in order to detect a change in the deviation over time and to extrapolate when fading is likely to occur.

[0035] If the actual curve deviates from the target curve, an increased probability of fading is detected, and the process continues in step S3.

[0036] For example, in the Fig. 3. It is apparent that the actual characteristic curve K I already significantly deviating from the target characteristic curve K S deviates, but not yet from the fading characteristic K F This corresponds to the robustness of the method. To increase the robustness of the method, an increased fading probability is preferably only detected if the actual curve, in this case the actual characteristic curve K, is within the specified parameters. I, beyond at least a specified tolerance from the target curve, in this case the target characteristic curve K S , differs.

[0037] For this purpose, preferably at least one, in particular temperature-dependent, threshold value, preferably several threshold values, are specified depending on an expected braking behavior and / or coefficient of friction, and the tolerance is specified depending on the threshold value(s).

[0038] Preferably, an increased probability of fading is only detected if the deviation within a predetermined second period, which lies within the first period, at least reaches, and in particular exceeds, a predetermined value.

[0039] In the specific example of the Fig. Figure 3 shows threshold values ​​in the form of a first threshold line G1 and a second threshold line G2. The actual characteristic curve K IIt lies between the two lines G1 and G2. At every point, it deviates from the target characteristic curve K beyond the threshold values ​​defined by the first threshold line G1. S in the direction of the fading characteristic curve K F away.

[0040] The straight lines G1 and G2 are defined in particular as a function of the temperature presumably to be expected at the brake disc 3 and / or brake pad 4 under such braking behavior and / or friction coefficient of the wheel brake assembly 1, such that the temperature of the wheel brake assembly 1 is, for example, in the present case, in a range that is still acceptable but elevated above a normal range. In this respect, it serves as a reliable indicator of an increased probability of brake fade.

[0041] If, for example, the thresholds defined by the second threshold line G2 were also exceeded, it can be assumed that an unacceptably high fading is imminent.

[0042] In step S3, after the increased probability of fading is detected, an optical, acoustic and / or haptic warning message is issued, in particular on a display device of the motor vehicle.

[0043] Alternatively or additionally, a substitute reaction is carried out, in particular a performance of the wheel braking device 1, for example a maximum deceleration performance, is limited in order to at least slow down a further temperature increase and thus prevent, or at least postpone, the actual occurrence of a critical fading situation. QUOTES INCLUDED IN THE DESCRIPTION

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

[0000] DE 10 2010 040 573 A1

[0005]

Claims

[1] Method for detecting an increased probability of fading of a wheel brake device (1) of a motor vehicle, - wherein the wheel brake device (1) is designed to decelerate the motor vehicle by generating a clamping force (F) acting on a brake disc (3), and - wherein the wheel brake device has a brake piston (7) with a controllable electrical actuator (5) associated with the brake piston (7), characterized by , - that during a deceleration process the clamping force (F) and a displacement position (s) of the brake piston (7) are determined, - that an actual profile of the clamping force (F) and the displacement position (s) during the deceleration process is compared with a target profile, and - that if the actual curve deviates from the target curve, an increased probability of fading is detected. [2] Method according to claim 1, characterized by, that the target curve is determined by means of a target characteristic curve (K S ) is specified, and that the actual course is defined as the actual characteristic curve (K I ) with the target characteristic curve (K S ) is compared. [3] Method according to any one of the preceding claims, characterized by , that the target curve is determined and specified depending on at least one historical actual curve of a previous delay process. [4] Method according to any one of the preceding claims, characterized by that the clamping force (F) is determined by means of a sensor arrangement comprising at least one force sensor and / or torque sensor. [5] Method according to any one of the preceding claims, characterized by, that the clamping force (F) is determined as a function of at least one quantity relating to a vehicle condition of the motor vehicle, in particular as a function of an inclination angle of a roadway on which the motor vehicle travels, and / or a driving speed of the motor vehicle. [6] Method according to any one of the preceding claims, characterized by , that the actuator (5) is designed as an electrical machine, and that the clamping force (F) is determined as a function of a current value of an electrical motor current of the actuator (5). [7] Method according to any one of the preceding claims, characterized by , that the actuator (5) is designed as an electric machine and has a rotor shaft, and that the displacement position is determined as a function of a rotor position of the rotor shaft. [8] Method according to any one of the preceding claims, characterized by, that the brake piston (7) is operatively connected to the actuator (5) by a gear arrangement (6), and that the displacement position is determined as a function of a gear ratio of the gear arrangement. [9] Method according to any one of the preceding claims, characterized by , that an increased probability of fading is only detected if the actual curve deviates from the target curve by at least a specified tolerance. [10] Method according to claim 9, characterized by , that at least one threshold value, preferably several threshold values, is specified depending on an expected braking behavior and / or coefficient of friction, and that the tolerance is specified depending on the threshold value(s). [11] Method according to any one of the preceding claims, characterized by, that if an increased probability of fading is detected, an optical, acoustic and / or haptic warning message is issued, in particular on a display device of the motor vehicle. [12] Control device (8), characterized by that the control device (8) is specifically designed to carry out the procedure according to one of the preceding claims. [13] Wheel brake device (1) of a motor vehicle, wherein the wheel brake device (1) is designed to decelerate the motor vehicle by generating a clamping force acting on a brake disc (3), and wherein the wheel brake device has a brake piston (7) with a controllable electrical actuator (5) associated with the brake piston (7), characterized by a control device (8) according to claim 12.

Images