METHOD FOR MONITORING THE RELEASE BEHAVIOR OF AN ELECTROMEMIC WHEEL BRAKE OF A VEHICLE

Description

[0001] The invention relates to a method for monitoring the release behavior of an electromechanical wheel brake of a vehicle.

[0002] Electromechanical wheel brakes as service brakes for motor vehicles are known in a wide variety of variants in the prior art. One such electromechanical brake for motor vehicles is described, for example, in DE 10 2017 206 798 A1. The described electromechanical brake comprises a brake caliper housing and a brake pad holder, a drive unit for generating torque, a rotation-translation gear, and a transmission unit for transmitting the torque to the rotation-translation gear. The torque of the drive unit is converted by the transmission unit and the rotation-translation gear into a translational movement of an application element, which leads to the pressing of brake pads against a brake disc.

[0003] Another electromechanical brake for motor vehicles is described in EP 1 085 240 A1. For this purpose, an actuator is provided at each brake, which is controlled by a controller. The controller can control the motor within a braking operating range so that a braking force is generated in accordance with an input variable from a brake pedal.

[0004] Such electromechanical wheel brakes are often characterized by a high mechanical transmission ratio between the drive unit and the brake application element. Due to this high mechanical transmission ratio, the mechanical efficiency is <100%, so that passive, independent release of the brake after the drive unit has been activated is not automatically guaranteed. Such independent release is usually highly dependent on the stress on the wheel brake components over their service life and on the lubrication status of the components of the transmission and the wheel brake drive.

[0005] At the same time, the automatic release of such an electromechanical brake in a passive, i.e., de-energized, state is a safety-critical function. For example, a tensioned electromechanical wheel brake could lead to an unstable driving condition of the vehicle in the event of a power failure if the wheel brake, which is then no longer controllable, does not release automatically and completely.

[0006] It is also known from the prior art to support the passive release of an electromechanical brake, i.e., a reduction of the application force without electronic control of the actuator, using mechanical systems, such as springs on the wheel brake's bearing elements. One such system is described, for example, in DE 10 2019 219 331 A1. While such a system increases the probability of automatic release, it cannot always guarantee the intended release behavior of the wheel brake.

[0007] Against this background, the object of the present application is to provide a method which enables monitoring of the release behaviour of an electromechanical wheel brake of a vehicle and thus a warning to the driver should the release behaviour of the wheel brake reach a critical level.

[0008] The invention relates to a method for monitoring the release behavior of an electromechanical wheel brake of a vehicle, wherein the electromechanical wheel brake has at least one electrically controllable force actuator, wherein the force actuator is designed to apply an application force to friction partners of a wheel brake in order to decelerate the vehicle.The method comprises controlling the force actuator according to a defined control pattern, wherein the control pattern depends on a current operating state of the wheel brake and / or a current operating state of other wheel brakes of the vehicle and / or a current operating state of the vehicle, interrupting the control of the force actuator at a defined point in time, monitoring operating parameters of the wheel brake for a defined period of time after the interruption of the control and determining the release behavior of the wheel brake by determining a reaction of the force actuator to the interruption of the control based on the determined operating parameters.

[0009] A "force actuator" is understood to mean a device that can apply a controllable force to an element of the wheel brake, in this case the friction partners of the friction brake, which presses the friction partners together in such a way that a braking force that decelerates the rotation of a vehicle wheel acts on the vehicle wheel connected to the wheel brake. The friction partners can be, for example, a brake disc rigidly connected to the vehicle wheel and friction pads acting on the brake disc. The friction pads can be subjected to a force in the direction of the brake disc by the force actuator. The force acting on the friction pads in the direction of the brake disc is understood to be the "application force." However, the invention is not limited to the described design of the wheel brake as a disc brake.Rather, the inventive concept can also be implemented in another type of brake, such as an electromechanical drum brake.

[0010] The invention is based on the idea of ​​simulating a situation in which targeted control of the force actuator is no longer possible, for example due to a power failure, by controlling the force actuator according to a defined and therefore replicable pattern and then interrupting the control. The reaction of the force actuator to this interruption, which can be determined from observing the brake's operating parameters, can then be used to check whether the wheel brake would release automatically to the necessary extent even in the event of a fault. If it is determined that self-release would not occur sufficiently, a warning can be issued to the driver, for example. Furthermore, the control of the affected wheel brake can be adjusted accordingly so that, for example, by changing the brake force distribution, the wheel brake is less heavily involved in implementing a braking request.

[0011] According to one embodiment, a first control pattern includes setting and maintaining a defined application force by the force controller of the wheel brake, wherein the first control pattern is only carried out when the vehicle is stationary. This control pattern simulates the case in which the control of the wheel brake suddenly fails when the application force is applied. In this case, ideally, the application force of the wheel brake should reduce automatically to such an extent that any remaining residual application force does not significantly influence the driving behavior of the vehicle. However, if it is determined during the course of the method that the application force does not decrease sufficiently after the control is interrupted, this can be an indication of a safety-critical wear or lubrication condition of the wheel brake components.

[0012] The implementation of the first control pattern when the vehicle is stationary is intended to prevent driving behavior that is unexpected for the driver from occurring when the method is carried out. According to a preferred embodiment, the standstill of the vehicle for implementation of the first control pattern is ensured in that the vehicle is held stationary by the execution of a parking brake function by at least some of the vehicle's wheel brakes. During the parking brake function, it can be provided that at least a subset of the wheel brakes is controlled in such a way that the respective force actuators as a whole set an application force that is sufficient to prevent the vehicle from rolling away. In this case, a parking brake function can in particular also be implemented by only some of the wheel brakes, for example by the wheel brakes of a rear axle of the vehicle.The first control pattern can in principle be carried out on those wheel brakes that are not involved in the implementation of the parking brake function, as well as on the wheel brakes that hold the vehicle in the parking position.

[0013] In order to ensure that the vehicle continues to be held securely in the parking brake position when the parking brake function is activated, despite the interruption of the control of a wheel brake involved in the implementation of the parking brake function within the scope of the method according to the invention, it is provided according to a further embodiment that, before the interruption of the control of the wheel brake in the first control pattern, it is determined whether the other wheel brakes of the vehicle as a whole apply a braking force that is sufficient to hold the vehicle at a standstill, wherein the defined application force of the wheel brake is set to a value that is greater than an application force for implementing the parking brake function if the braking force applied by the other wheel brakes is not sufficient to hold the vehicle at a standstill.

[0014] Essentially, two cases must be distinguished. In the first case, the first control pattern is executed on a wheel brake that is not involved in implementing the parking brake function. In this case, the method according to the invention can be implemented without restriction on the wheel brake in question, since the parking brake situation remains unchanged.

[0015] In the second case, the method is carried out on a wheel brake that is involved in the execution of the parking brake function. In this case, it must be ensured that the interruption in the control of the corresponding force actuator does not result in the applied braking force dropping so far that the vehicle can no longer be held safely in its parking position. According to the embodiment, this is achieved by determining in advance whether a reduction in the application force of the wheel brake under investigation could become problematic for maintaining the parking brake position. If this is the case, the application force of the wheel brake is first increased to such an extent that an interruption in the control and thus a partial drop in the application force does not result in the total braking force applied by the vehicle's wheel brakes falling below a braking force level that is necessary to maintain the parking brake function.

[0016] According to a further embodiment, it is further provided that the force actuator comprises an electric motor-operated pressure piston or an electric motor-operated clamping element, wherein an application force provided by the force actuator is generated by the force actuator applying a force along an application direction of the wheel brake to the pressure piston or clamping element. Preferably, a friction lining is arranged on the pressure piston, so that upon displacement of the pressure piston or clamping element in the application direction, the friction lining is brought into contact with a brake disc and subjected to a force in the direction of the brake disc.

[0017] With such a configuration of the wheel brake, a further embodiment provides that a second control pattern includes accelerating the pressure piston or tensioning element along the application direction without generating an application force. The control pattern is therefore preferably carried out as long as there is sufficient clearance between the friction partners of the wheel brake, i.e. the friction partners are not touching. The pressure piston is preferably accelerated to a defined speed or to a defined position along the possible displacement path. When the speed or position is reached, the control of the force actuator is interrupted and the acceleration of the pressure piston by the force actuator is thus stopped. From the deceleration of the pressure piston after the interruption of the control, it is possible to estimate how smooth the mechanism of the wheel brake is, which in turn gives an indication of how strong orhow quickly the applied wheel brake application force would be reduced in the event of a control or power supply failure.

[0018] It is not necessary to wait until the pressure piston or clamping element comes to a complete stop. Rather, it can also be determined how long it takes for the pressure piston or clamping element to decelerate from an initial speed to a target speed after acceleration has ended. Alternatively, after acceleration has occurred, it can also be determined how long the pressure piston or clamping element requires in freewheel mode to reach a defined position along the application direction.

[0019] According to a further embodiment, it is further provided that a third control pattern includes the reduction of an application force applied as a result of a brake application. This control pattern can be used, for example, when a braking request is ended, for example by removing the actuation of a brake pedal, so that the wheel brakes are activated to reduce an applied application force. In this case, the application force of the wheel brake initially decreases in a controlled manner from an initial value. Within the framework of the third control pattern, it is provided that at least part of this actually controlled release process takes place in an uncontrolled manner. In this way, it can be determined during the active operation of the wheel brake whether the brake would continue to be released sufficiently even in the event of a control failure.

[0020] To determine the release behavior, a further embodiment further provides that the operating parameters of the wheel brake monitored after an interruption in control include the temporal progression of an applied application force and / or the temporal progression of an actuation travel of the wheel brake. The application force, as well as the actuation travel, can be determined, for example, using existing sensors for controlling the force actuator and provide direct information about the behavior of the force actuator in freewheel mode. The "actuation travel" is understood in particular to mean a displacement travel of the pressure piston or clamping element along an application direction. Such an actuation travel can either be measured directly by corresponding travel sensors or determined from operating parameters of the force actuator, for example, the motor position of an electric motor drive and the transmission position of a transmission connected downstream of the motor.

[0021] According to a further embodiment, the release behavior is determined from a difference in the actuation travel or from a difference in the application force over a time difference. The release behavior is recognized as inadequate if the determined difference in the actuation travel or the determined difference in the application force falls below a threshold value. Falling below the threshold value indicates that the wheel brake mechanism is slower than it should be, so that reliable release behavior of the wheel brake can no longer be assumed. Furthermore, the difference in the application force over the difference in the actuation travel can also be considered here.

[0022] According to a further embodiment, it is provided that the release behavior is determined from a temporal gradient of the actuating path or the clamping force, wherein the release behavior is recognized as insufficient if the determined gradient of the actuating path or the determined gradient of the clamping force falls below a threshold value.

[0023] In the variants described above for determining the release behavior of the wheel brake, in addition to a direct observation of the behavior of the force actuator, it can also be provided that other parameters that influence the release behavior of the force actuator are taken into account. In particular, the temperature of the wheel brake or components of the wheel brake, in particular the force actuator, can be taken into account. For example, the temperature influences the viscosity of lubricants used in the wheel brake, which can have a direct impact on the release behavior of the wheel brake. By observing and taking the temperature into account, it is therefore possible to distinguish whether a release behavior identified as inadequate is simply due to a temperature that is too low, or whether there is actually a critical state of wear or a defect in the wheel brake.

[0024] Furthermore, the different control patterns are preferably carried out multiple times with different parameters so that an actual release behavior can be interpolated from the partial results for the different parameters. For example, the first control pattern can be designed to use different levels of application force so that the release behavior can be evaluated based on different starting conditions. Furthermore, different pressure piston speeds can be tested in the second control pattern. In the third control pattern, the point of the release process, represented by an applied residual clamping force or a time since the start of the reduction in the application force, from which point the brake should be released automatically, can be varied.

[0025] Furthermore, it can be provided that, before performing the actuation pattern, the force actuator of the wheel brake to be tested is moved once or repeatedly by a defined stroke. This can be particularly advantageous if the force actuator has been held in a specific actuation position for an extended period, which can increase the force actuator's breakaway torque. This changed breakaway torque, in turn, influences the determination of the wheel brake's self-release behavior. By moving the force actuator slightly back and forth once or repeatedly, the force actuator's breakaway torque can be leveled out, thus improving the accuracy of a subsequent investigation of the release behavior.

[0026] Preferred embodiments of the invention are explained in more detail below with reference to the drawings. In the drawings: Figure 1 shows a flow diagram of an exemplary method, Figure 2 shows a schematic representation of an exemplary force-displacement curve of an electromechanical wheel brake, Figure 3 shows a further schematic representation of the exemplary force-displacement curve of an electromechanical wheel brake and Figure 4 shows a schematic representation of the behavior of a force controller when carrying out an exemplary control pattern.

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

[0028] The Figure 1shows a flow diagram of an exemplary embodiment of a method according to the invention. In a first method step 100, the force actuator of an electromechanical wheel brake is controlled according to a defined control pattern. The force actuator can, for example, be an arrangement with an electric motor drive, followed by a gear arrangement with a rotation-translation gear, so that a rotation of a drive shaft of the drive is converted into a translational movement. A pressure piston or a tensioning element of a wheel brake is preferably coupled to the gear, so that a rotation of the drive shaft of the drive or a torque acting on the drive shaft is converted into a translational movement of the pressure piston or tensioning element or a force acting on the pressure piston or the tensioning element along an application direction.

[0029] By moving the pressure piston or clamping element, or by applying a force to these elements, the friction partners of a friction brake are preferably brought into contact with each other or pressed against each other with a force that can be controlled by the electric motor drive. The resulting friction force exerts a braking torque on a vehicle wheel operatively connected to the wheel brake, which either slows or prevents rotation of the vehicle wheel.

[0030] The control pattern according to which the force actuator is activated depends on the current operating state of the wheel brake in question. The operating state of the wheel brake describes, for example, whether the wheel brake is currently exerting a braking force on the assigned vehicle wheel, or whether the wheel brake is in the open state, i.e. the friction partners of the wheel brake are not in contact. Furthermore, it can also influence whether the wheel brake is currently being used to decelerate the vehicle, or whether the wheel brake is implementing a parking brake function that is intended to prevent the vehicle from rolling away from a parking brake position. Consequently, the operating state of the vehicle can also influence which type of control pattern is used within the method. The operating state of the vehicle can, for example, be determined from the wheel speed of a wheel assigned to the wheel brake in question.Finally, when selecting the control pattern used, the current operating status of the vehicle's other wheel brakes can also be taken into account, i.e. whether braking forces are currently being generated by the other wheel brakes and how high these braking forces are, if any.

[0031] Subsequently, in step 102, the control of the force actuator, i.e., the execution of the control pattern, is interrupted at a defined time. The interruption preferably occurs in such a way that the control of the force actuator, previously controlled within the scope of the control pattern, is suspended, i.e., no further control commands are issued to the force actuator. In this way, for example, an interruption in the power supply to the wheel brake or a control failure due to an interruption in signal transmission or a control unit error can be simulated.

[0032] Subsequently, in step 104, it is determined how the wheel brake, and in particular the force actuator with the elements downstream of the force actuator in the chain of action (gearbox and friction partners), behaves after the interruption of the control. For this purpose, the operating parameters of the wheel brake are monitored for a defined period of time after the interruption of the control. Preferably, a curve of the position of a pressure piston or clamping element, or a clamping force applied via the pressure piston or clamping element, is recorded over the elapsed time since the interruption of the control.

[0033] From the data thus obtained, it is then determined in step 106 whether the release behavior of the wheel brake is sufficient in the event of an interruption in the control, so that sufficient self-release is still ensured in the event of a failure of the control or power supply to the wheel brake. If it is determined that the release behavior is inadequate, i.e., if the applied braking force is reduced too slowly in the event of a fault, or if the wheel brake remains stuck at an excessively high application force, it can further be provided that the wheel brake is no longer used or only used to a limited extent to implement a braking request, and a corresponding warning is issued to the driver.

[0034] Different variants for control patterns are described below with reference to the Figure 2 , 3 and 4 described as an example.

[0035] The Figure 2An exemplary curve of the application force F z of an electromechanical wheel brake over the displacement s K of a pressure piston or a clamping element of the electromechanical wheel brake. The curve of the application force differs when the wheel brake is applied, i.e., an increase in the application force 200, from the curve of the application force when the wheel brake is released, i.e., a reduction in the application force 202.

[0036] A first variant of a control pattern can be such that the force controller sets a defined application force A and maintains it over a longer period of time. This can be provided, for example, when implementing a parking brake function; in this case, the corresponding application force is usually selected such that the sum of the wheel brakes involved in implementing the parking brake function provides a braking force that prevents the vehicle from rolling away.

[0037] If the control of the force actuator is interrupted, the pressure piston or the clamping element is subjected to a force due to deformation energy of the wheel brake, in particular the brake caliper, which is usually stored in the applied wheel brake. This force leads to a release of the brake, i.e. a movement of the pressure piston or clamping element against the application direction. This also reduces the applied application force, whereby the extent to which the application force changes when the pressure piston or clamping element moves over a defined distance provides information about whether the wheel brake would release itself sufficiently in the event of a fault. In addition to considering the applied application force F z over the displacement distance s K, the applied application force F z over the time t since the interruption of the control can alternatively or additionally be determined and considered.Furthermore, differences in application forces ΔF z over different time differences Δt or differences in the displacement distance Δs K can be considered or corresponding gradients can be formed in order to quantify the release behavior of the wheel brake.

[0038] An example of a second control pattern is given below with reference to the Figure 3 and 4 described. The Figure 3 shows again the already mentioned with reference to Figure 2described exemplary curve of the application force F z of an electromechanical wheel brake over the displacement s K of a pressure piston or a clamping element of the electromechanical wheel brake. In this case, however, the area of ​​the displacement s K that is to the left of the vertical coordinate axis is relevant. In this area, the pressure piston or clamping element is displaced without any application force building up. Consequently, the origin of the coordinate system describes exactly the point along the displacement s K at which the friction partners of the wheel brake come into contact. The area to the left of the coordinate origin describes the clearance area of ​​the wheel brake, i.e. the area in which the friction partners do not come into contact and therefore no application force initially builds up when the friction partners are displaced.

[0039] The second control pattern in this area is designed to accelerate the pressure piston or clamping element from a point C along the displacement path s K . As soon as the pressure piston or clamping element reaches point C', the control of the force actuator is interrupted. The time it takes for the pressure piston or clamping element to reach point D along the displacement path s K is then observed.

[0040] This is exemplified in the Figure 4the position of the pressure piston or tensioning element s K along the displacement path over time t during execution of the second control pattern. Starting from a position C at time t, an accelerated movement of the pressure piston or tensioning element initially starts. At point C' or at the corresponding time t' c, the control of the force actuator is interrupted so that the pressure piston or tensioning element runs out and the speed drops again until the speed falls below a threshold value at point D at a corresponding time t D. From the duration Δt = t D - t' c it can then be determined whether the freewheel of the force actuator is sufficiently light so that a safe release behavior of the electromechanical wheel brake can be assumed.

[0041] Again, based on the presentation of the Figure 2A third control pattern can also be described as an example. In this case, the force actuator is first controlled such that a defined application force is applied at point A. This can, for example, be an application force resulting from a braking request, which can be triggered, for example, by a vehicle driver pressing a brake pedal. Subsequently, due to the braking request being terminated, the application force F z is reduced in a controlled manner by correspondingly controlling the force actuator along curve 202. At a defined point B, however, the control of the force actuator is interrupted, so that part of the force reduction is carried out automatically by the wheel brake. If, for example, the gradient of the application force F z over the displacement distance s K or the time t elapsed since the interruption of the control is considered, the ability of the wheel brake to self-release can again be determined.

[0042] The control patterns described above can be combined as desired to form a monitoring profile for the electromechanical wheel brake, so that the release behavior of the wheel brake can be determined in different ways in different situations.

Claims

1. Method for monitoring the release behaviour of an electromechanical wheel brake of a vehicle, the electromechanical wheel brake having at least one electrically controllable force actuator, the force actuator being configured to apply a brake-application force to friction partners of a wheel brake for the purposes of decelerating the vehicle, the method having the following steps: • activating the force actuator in accordance with a defined control pattern, the control pattern being dependent on a present operating state of the wheel brake and / or a present operating state of further wheel brakes of the vehicle and / or a present operating state of the vehicle (100), • interrupting the activation of the force actuator at a defined point in time (102), • monitoring operating parameters of the wheel brake for a defined period of time after the interruption of the activation (104), and • determining the release behaviour of the wheel brake by ascertaining a reaction of the force actuator to the interruption of the activation on the basis of the ascertained operating parameters (106).

2. Method according to Claim 1, characterized in that a first control pattern comprises the setting and holding of a defined brake-application force by the force actuator of the wheel brake, the first control pattern being implemented only when the vehicle is stationary.

3. Method according to Claim 2, characterized in that the implementation of a parking brake function causes the vehicle to be held stationary by at least a proportion of the wheel brakes of the vehicle.

4. Method according to Claim 3, characterized in that, before the interruption of the activation of the wheel brake in the first control pattern, it is ascertained whether the further wheel brakes of the vehicle are collectively imparting a braking force that is sufficient to hold the vehicle stationary, the defined brake-application force of the wheel brake being set to a value higher than a brake-application force for implementing the parking brake function if the braking force imparted by the further wheel brakes is not sufficient to hold the vehicle stationary.

5. Method according to any one of the preceding claims, characterized in that the force actuator has a pressure piston that is actuatable by electric motor or an application element that is actuatable by electric motor, a brake-application force provided by the force actuator being generated by application of a force to the pressure piston or application element along a brake-application direction of the wheel brake by the force actuator.

6. Method according to Claim 5, characterized in that a second control pattern comprises the acceleration of the pressure piston or application element along the brake-application direction without generation of a brake-application force.

7. Method according to any one of the preceding claims, characterized in that a third control pattern comprises the reduction of a brake-application force that is acting owing to a brake actuation.

8. Method according to any one of the preceding claims, characterized in that the operating parameters of the wheel brake that are monitored after an interruption of the activation include the profile with respect to time of an acting brake-application force and / or the profile with respect to time of an actuating travel of the wheel brake.

9. Method according to Claim 8, characterized in that the release behaviour is ascertained from a difference in the actuating travel or from a difference in the brake-application force over a time difference, the release behaviour being identified as being insufficient if the ascertained difference in the actuating travel or the ascertained difference in the brake-application force is below a threshold value.

10. Method according to Claim 8 or 9, characterized in that the release behaviour is ascertained from a gradient with respect to time of the actuating travel or of the brake-application force, the release behaviour being identified as being insufficient if the ascertained gradient of the actuating travel or the ascertained gradient of the brake-application force is below a threshold value.