METHOD FOR OPERATING A VEHICLE'S BRAKE SYSTEM

Description

[0001] The invention relates to a method for operating a braking system of a vehicle with electromechanically actuated wheel brakes.

[0002] In hydraulic braking systems with electric brake force amplification, a brake pressure is typically set centrally in a hydraulic control unit to apply the brakes to the wheel brakes. For example, an electric motor, such as a brushless DC motor, may drive a hydraulic cylinder or a tandem master cylinder, which in turn pressurizes the brake fluid. The resulting brake pressure is then transmitted via hydraulic valves to the pressure cylinders of the wheel brakes. The brake pressure is adjusted proportionally to the actuation of the brake pedal, and the wheels are braked accordingly.

[0003] Document DE 100 12 448 A1 describes a method for distributing braking force in a vehicle with at least two braked axles, wherein overheating of the wheel brakes is determined during a braking process based on certain criteria and, if overheating is detected, the wheel brakes on a front axle and on a rear axle are alternately subjected to braking force in order to cool the wheel brakes of an axle that are not currently subjected to braking force.

[0004] To maintain high brake pressures, the electric motor must generate high torque to sustain the pressure. This requires correspondingly high currents to flow in the electric motor's supply lines. Under prolonged conditions of this type of load with high motor current, the electric motor can overheat. To prevent damage from overheating, the electric motor is relieved of this load by closing the wheel brake inlet valves, thus trapping the applied brake pressure within the wheel brakes. The wheel brakes then continue to operate at the previously set brake pressure, while the electrical pressure supply, and therefore the electric motor, is decoupled from the wheel cylinders, allowing the motor current to be reduced. This effectively prevents the electric motor from overheating.

[0005] In dry braking systems with electromechanically actuated wheel brakes, the pressure cylinders in the wheel brakes are replaced by electromechanical actuators or force adjusters. Together with appropriate control electronics at the wheel (especially wheel brake control units, WCUs), this creates independent electric wheel brakes. In such electromechanical wheel brakes, an electric motor of a force adjuster typically drives an actuating device via a gearbox. This device applies a clamping force to the brake linings in the direction of a friction partner, such as a brake disc or a brake drum. To maintain high clamping forces and thus high braking torques, the electric motor must apply a high torque to the gearbox and consequently be driven with a high motor current.Consequently, in a braking system with electromechanical wheel brakes, the problem is that during prolonged braking maneuvers with high clamping forces, the electric motor of the wheel brake is constantly under heavy load and can overheat.

[0006] Situations in which such overheating of the wheel brake electric motors can occur include long, very steep downhill stretches at low speed, especially with a fully loaded vehicle and possibly a trailer attached, or very slow downhill stretches on loose surfaces. In both cases, while the friction surfaces of the wheel brakes would only absorb a small amount of heat due to the low speeds, the electric drive of the wheel brakes would have to be operated continuously with a high motor current and would therefore overheat quickly. The consequence would be that, once the electric motors reach a certain temperature, the vehicle would have to be immobilized, as safe continued driving could not be guaranteed.

[0007] Against this background, the present invention is based on the technical problem of providing a method for operating a braking system of a vehicle with electromechanically actuated wheel brakes, which ensures a continuous provision of a high braking torque by electromechanical force actuators of the wheel brakes while simultaneously avoiding overheating of the force actuators.

[0008] This problem is solved by the method according to claim 1. Preferred embodiments of the invention are the subject of the dependent claims. The invention relates to a method for operating a vehicle's braking system with electromechanically actuated wheel brakes and a control unit, wherein the wheel brakes each have an electromechanical force actuator for applying a braking torque to a vehicle wheel associated with the respective wheel brake. The method includes detecting a braking request, determining a braking torque to be applied by the force actuators of the wheel brakes to implement the braking request, determining the current temperatures of the wheel brakes and / or components of the wheel brakes, and determining the vehicle's speed.If the determined temperature of at least one wheel brake and / or the components of the wheel brake is above a first limit value and the determined speed is below a second limit value, the force actuator of at least one wheel brake is activated to provide part of a braking torque corresponding to the braking request, and the force actuator of the at least one wheel brake is mechanically locked in an actuation position corresponding to the set braking torque. The remaining wheel brakes are then activated to provide the braking torque remaining to fully implement the braking request, with the aforementioned steps being carried out by the control unit.

[0009] The invention is based on the idea that heat generation in a braking system with electromechanical wheel brakes can be reduced by generating a constant braking torque level not by continuously energizing an electromechanical force actuator, but rather by selectively mechanically locking the force actuator in an operating position corresponding to that braking torque level. Once such a locking has occurred, the force actuator no longer needs to be energized to the previously required extent to constantly generate the required braking torque, thus reducing heat generation in the force actuator. The portion of the braking torque provided by the wheel brake with the force actuator locked is preferably selected such that the locked force actuator generates as large a proportion as possible of the total braking torque required to fulfill the braking demand.The remaining braking torque required to fully meet the braking demand is then generated by the other wheel brakes. In this process, deviations from an ideal brake force distribution are possible, such that, for example, 90% of the required braking power is applied by the rear axle wheel brakes.

[0010] The temperature limits used for the wheel brakes or their components, particularly the temperature of the wheel brake force actuator, can either be predefined as fixed parameters or dynamically adjusted depending on the driving situation. The wheel brake could be, for example, a disc brake or a drum brake. In principle, however, the described approach is applicable to any brake design that can apply a braking torque to a vehicle wheel by controlling an electric force actuator.

[0011] When selecting the braking torque level to be provided by the wheel brake with a locked force actuator, it is also important to ensure that the vehicle wheel associated with the wheel brake with the locked force actuator does not lock. According to one embodiment, unintentional locking of a vehicle wheel, which is subjected to a braking torque by a wheel brake with a locked force actuator, is therefore avoided by setting the braking torque provided by the locked force actuator of the at least one wheel brake a defined value below a braking torque that would lead to locking of the vehicle wheel associated with the wheel brake. For example, the selected braking torque level can be set at approximately 70% to 90% of the braking torque at which, based on current operating information of the vehicle, wheel locking would be expected.

[0012] The system continuously monitors wheel slip to detect any risk of wheel lock-up. As soon as it is determined that the wheels are about to lock or are already locked, the force actuator's locking mechanism is immediately released, allowing the wheel brakes to release and the wheels to return to an acceptable slip range. The force actuator can then be re-locked at a lower braking torque level.

[0013] Besides preventing wheel lock-up due to a locked force actuator, when selecting the braking torque level provided by the locked force actuator, it is also important to consider that the braking demand, and thus the braking torque required for its implementation, can change continuously. In the previously described scenario of slow downhill driving on a slippery surface, a driver will repeatedly release the brake pedal slightly and then, once the vehicle speed has increased somewhat, apply it more firmly again. The braking torque level provided by the locked force actuator is therefore preferably set so that the remaining braking torque required to fully implement the braking demand is sufficient to accommodate fluctuations in the driver's braking input.

[0014] In a further embodiment, the relief of the electromechanical wheel brakes by selectively locking individual wheel brakes is improved by making the proportion of the braking torque provided by the locked force controller of at least one wheel brake, relative to the total braking torque required to implement the braking request, inversely proportional to the rate of change of the braking request. Thus, the less the braking request fluctuates over time, the greater the proportion of the braking request that can be implemented by the locked force controller. The lower the braking torques that must be generated by the actively energized wheel brakes, the lower the thermal load on the wheel brakes and their components, particularly the electromechanical force controller.

[0015] According to a preferred embodiment, it is further provided that if the measured temperature of at least one wheel brake is above a first limit value and the measured speed is below a second limit value, the force actuators of the wheel brakes of a vehicle axle are activated to provide a portion of a braking torque corresponding to the braking requirement, and the force actuators of the wheel brakes of the vehicle axle are locked in an actuation position corresponding to the set braking torque. Locking the force actuators of the respective wheel brakes axle by axle has the advantage that the corresponding set braking forces generate only a small, ideally no, yaw moment acting on the vehicle. A yaw moment would only occur in this configuration if the vehicle wheels had different degrees of slip, for example, due to different road surfaces or different coefficients of friction of the tires.

[0016] Furthermore, it may be provided that, after locking the force actuators of the wheel brakes on an axle, it is checked whether a yaw movement of the vehicle occurs. Should it be detected that locking the force actuators causes a yaw moment, the braking torques set by the respective force actuators of the wheel brakes can be adjusted so that the yaw moment is compensated for.

[0017] In a preferred embodiment, the vehicle axle is further provided to be a rear axle of the vehicle. In this way, the front axle used for steering the vehicle is only minimally affected by a deceleration torque and ideally only has to transmit small forces in the longitudinal direction of the vehicle.

[0018] According to a further embodiment, each force actuator has an electric motor drive and a gearbox downstream of the drive, wherein the locking of a force actuator is achieved by a mechanical blocking of the electric motor drive or the gearbox. For this purpose, for example, a gear can be arranged non-rotatably on a drive shaft connected to the electric motor drive, wherein a locking tooth engages the gear to lock the force actuator and prevents further rotation of the gear and thus of the drive. Alternatively, such a locking tooth can also engage directly with a gear of the gearbox.In addition to such or similar blockage by a positive locking mechanism, it may also be provided that the drive or the transmission is blocked by a frictional locking mechanism, for example by a braking device that acts on the rotor of the motor, the drive shaft or a transmission element.

[0019] Direct temperature measurement within a wheel brake is often difficult and requires considerable assembly effort. Therefore, according to a further embodiment, the temperature of the wheel brakes and / or wheel brake components is estimated from the wheel brake operating parameters. For example, the duration of a braking process, in conjunction with the operating parameters of the force controller, can be used to estimate the amount of heat generated in the motor during that time. From this information, and optionally taking into account other boundary conditions such as ambient temperature, vehicle speed, or similar factors, the temperature of the force controller or its electronic components can then be estimated. This eliminates the need for direct temperature measurement using sensors.

[0020] According to a further embodiment, it is also provided that the at least one wheel brake, whose force actuator is mechanically locked, is the wheel brake of the braking system with the highest temperature.

[0021] Occasionally, wheel brakes can heat up to high operating temperatures due to a brief but intense load, which can also result in a corresponding temperature increase in the force actuator or its components. To prevent incorrect activation of the wheel brakes to lock the force actuator, a further embodiment provides that, in addition to the temperature of the wheel brakes and / or wheel brake components and the vehicle speed, the vehicle's tilt angle is also determined. The force actuator of at least one wheel brake is only locked if the vehicle's tilt angle exceeds a defined limit. This allows a temporary heating of a force actuator due to a short but very strong deceleration of the vehicle to be distinguished from a continuous load on the wheel brakes due to a downhill drive.The determined inclination angle can also be validated using a longitudinal acceleration sensor or location information from the vehicle (GPS) and knowledge of the road surface conditions.

[0022] Furthermore, according to a preferred embodiment, the at least one wheel brake has a parking brake function, wherein the locking of the force actuator of the at least one wheel brake is effected by activating the parking brake function. The braking torque to be generated by the force actuator is provided by the parking brake function. In this way, an existing infrastructure for the permanent adjustment of a braking torque can be used, so that a separate adjustment of a wheel brake in the sense of a service brake is not necessary.

[0023] In the previously described procedure, the steps were always specified in a particular order. However, this order is not to be interpreted as restrictive, as the individual steps can be changed in sequence if such a change is logically sound or technically feasible.

[0024] Preferred embodiments of the invention are explained in more detail below with reference to the drawings. These show: Figure 1 is a schematic representation of a vehicle with an electromechanical braking system and Figure 2 is a flowchart of an exemplary procedure.

[0025] In the following, similar or identical features are marked with the same reference symbols.

[0026] The Figure 1Figure 1 shows a schematic representation of a vehicle 100 with an exemplary braking system comprising four electromechanically actuated wheel brakes 102, 104, 106, and 108. The braking system shown here serves only to illustrate the invention and is not to be understood as limiting. Thus, the method can also be implemented in a braking system that, for example, comprises only two electromechanically actuated wheel brakes and two hydraulically actuated wheel brakes.

[0027] The wheel brakes 102, 104, 106, and 108 are each assigned to vehicle wheels 110, 112, 114, and 116, respectively, and are designed to apply a braking torque to the vehicle wheels 110, 112, 114, and 116. A "braking torque" is understood to be a torque that opposes the rotation of the vehicle wheel. For this purpose, the wheel brakes 102, 104, 106, and 108 each have electromechanical force actuators 120, 122, 124, and 126, which, when appropriately controlled by a control unit 118 of the braking system, apply a frictional force to a friction element 130, 132, 134, or 136 that is fixedly connected to the respective vehicle wheel.

[0028] In particular, the wheel brakes 102, 104, 106, and 108 can be disc brakes in which a brake disc, non-rotatably connected to the respective vehicle wheel 110, 112, 114, or 116, is arranged as a friction element 130, 132, 134, or 136 between two friction linings. The friction linings are subjected to a force in the direction of the brake disc by the respective force actuator 120, 122, 124, or 126. The resulting friction between the friction lining and the brake disc generates a braking torque on the brake disc and thus on the vehicle wheel 110, 112, 114, or 116.

[0029] The electromechanical force actuators 120, 122, 124, and 126, for example, can be an arrangement consisting of an electric motor drive and a gearbox downstream of the drive. The gearbox is a rotary-translation gearbox that converts the rotation of a drive shaft of the electric motor drive into the translation of a pressure piston of the wheel brake 102, 104, 106, or 108. A friction lining arranged on the pressure piston can thus be selectively subjected to a force in the direction of the brake disc. The magnitude of this force, also called clamping force, is proportional to the motor current applied to the respective electric motor drive.

[0030] The control unit 118, which subsequently transmits corresponding control signals to the wheel brakes 102, 104, 106, and 108 following brake application, is responsible for controlling the braking system described above. The brake application is detected by a brake actuation unit 138, for example, a brake pedal.

[0031] As previously explained, in such a configuration, prolonged heavy braking can lead to significant heat generation due to the high motor currents applied to the force actuators in 120, 122, 124, and 126. To prevent an emergency shutdown of the braking system or immobilization of the vehicle due to overheating of the braking system, one embodiment of the invention provides for mechanically locking the electric motor drive or the transmission of at least one of the electric motor force actuators 120, 122, 124, or 126 after a suitable braking torque has been set, so that the electric motor force actuator 120, 122, 124, or 126 is no longer energized and consequently is not heated further.

[0032] Such a locking mechanism can be configured in such a way that further actuation of the electromechanical force actuator 120, 122, 124, or 126 in the force-increasing direction of the associated wheel brake 102, 104, 106, or 108 to increase the clamping force remains possible, while only actuation in the force-reducing direction, i.e., a reduction of the clamping force, is prevented. For example, retightening of a locked wheel brake 102, 104, 106, or 108, i.e., a subsequent increase of the applied clamping force, can still be carried out without having to release the locking mechanism. In principle, however, a locking mechanism can affect both the force-increasing and force-decreasing directions.

[0033] This will be discussed below with reference to the Figure 2 An exemplary embodiment of a method for operating the braking system of the previously described vehicle 100 is described.

[0034] In the first process step 200, a braking request is detected. This can occur, for example, as a result of the driver actuating the brake actuation unit 138. The corresponding actuation signal is then forwarded to the control unit 118.

[0035] In a subsequent process step 202, the control unit 118 determines the braking torque that the force actuators 120, 122, 124, and 126 of the wheel brakes 102, 104, 106, and 108 must apply to implement the received braking request. For this purpose, a characteristic curve can be used, for example, which assigns a braking torque to an incoming brake actuation signal. Furthermore, it can also be provided that a braking force required to implement the braking request is first determined, and then the braking torques to be applied by the wheel brakes 102, 104, 106, and 108 are determined from this braking force. When deriving the braking torques from the braking force, specific characteristics of the wheel brakes, such as their position in the vehicle 100 or their dimensions, can be taken into account.

[0036] In a further process step 204, the current temperature of the wheel brakes 102, 104, 106, and 108, or of components of the wheel brakes 102, 104, 106, and 108, is determined. These components may include, in particular, the force adjusters 120, 122, 124, and 126. To determine the current temperature, temperature sensors within the wheel brakes 102, 104, 106, and 108 can be used, for example. Alternatively, the temperature can also be estimated from operating parameters of the wheel brakes 102, 104, 106, and 108 during previous braking operations. For example, the applied clamping force for a given duration of the braking operation provides information about the amount of heat generated in the wheel brake 102, 104, 106, or 108.Furthermore, operating parameters of the force controllers 120, 122, 124 and 126, in particular applied motor currents, can also provide information about the amount of heat generated in the force controllers 120, 122, 124 and 126 during previous braking maneuvers.

[0037] In addition to the temperature, the speed of the vehicle 100 is also determined in a further process step 206, for example from a consideration of the wheel speeds of the vehicle 100.

[0038] In process step 208, it is checked whether the determined temperature is above a first limit value and the determined speed is below a second limit value. The limit values ​​used can be predefined parameters or dynamically adjusted depending on the driving situation of the vehicle 100. Furthermore, in addition to temperature and speed, the inclination of the vehicle 100 can also be considered in this process step, which indicates whether the vehicle 100 is traveling downhill. If it is determined that the temperature is below the first limit value or the vehicle speed is above the second limit value, the braking request is implemented in the known manner, and the process returns to process step 204, in which the temperature of the wheel brakes 102, 104, 106, and 108 and / or components of the wheel brakes 102, 104, 106, and 108 is determined.

[0039] If, however, it is determined that the temperature of the wheel brakes 102, 104, 106, and 108 is above the first limit value and the speed of the vehicle 100 is below the second limit value, then in step 210, the force actuators 124 and 126 of the rear axle wheel brakes 106 and 108 of the vehicle 100 are controlled such that the wheel brakes 106 and 108 provide as large a proportion as possible of the braking torque required to fulfill the braking request. For example, it may be provided that the rear axle wheel brakes 106 and 108 are subjected to a braking torque that generates as large a proportion as possible of the braking force required to fulfill the braking request, for example, approximately 70% to 90% of the required braking force. However, it is also taken into account that the set braking torques of the wheel brakes 106 and 108 rear axle of the vehicle 100 do not lead to a blockage of the vehicle wheels 114 and 116.For example, it can be estimated at what braking torque the vehicle wheels 114 and 116 would lock up, whereby the set braking torque is adjusted below this braking torque by a defined amount or a defined percentage.

[0040] The rear axle wheel brakes 106 and 108, and the force actuators 124 and 126, are then mechanically locked. This means that the previously set braking torque continues to be generated by the wheel brakes 106 and 108, but the force actuators 124 and 126 are no longer actively energized to the extent previously required to set the braking torque. As a result, heat generation in these wheel brakes 106 and 108 and their components is limited, thus preventing a forced shutdown of the braking system and a shutdown of the vehicle 100.

[0041] In process step 212, the wheel brakes 102 and 104 of the front axle of vehicle 100 are further controlled such that each wheel brake 102 and 104 applies a braking torque which, in conjunction with the braking force provided by the wheel brakes 106 and 108 of the rear axle of vehicle 100, ensures the complete implementation of the received braking request. Consequently, as large a portion as possible of the braking force required to implement the braking request is provided by the wheel brakes 106 and 108 of the rear axle of vehicle 100, while a smaller portion is implemented by the wheel brakes 102 and 104 of the front axle. In particular, the wheel brakes 102 and 104 of the front axle of the vehicle can also reflect changes in the braking request resulting from altered actuation of the brake control unit 138 by the driver.

[0042] Despite the mechanical locking of the force actuators 124 and 126 of the wheel brakes 106 and 108 on the rear axle of vehicle 100, the system continuously monitors whether the rear wheels 114 and 116 are at risk of locking. For this purpose, the wheel speeds of wheels 114 and 116 can be compared with the wheel speeds of the front wheels 110 and 112. If it is detected that the vehicle wheels 114 or 116 are at risk of locking, the locking mechanism of the force actuator 124 or 126 of the respective vehicle wheel 114 or 116 is released, so that the respective wheel brake 106 or 108 relaxes and the vehicle wheel 114 or 116 can be returned to an acceptable slip range.

[0043] The process steps described above represent an exemplary embodiment of the process. Modifications are possible within the scope of protection defined by the claims below.

Claims

1. A method for operating a brake system of a vehicle (100) having electromechanically actuated wheel brakes (102, 104, 106, 108) and a control unit (118), wherein the wheel brakes (102, 104, 106, 108) each have an electromechanical force actuator (120, 122, 124, 126) for applying a braking torque to a vehicle wheel (110, 112, 114, 116) assigned to the respective wheel brake (102, 104, 106, 108), wherein the method comprises the execution of the following steps by the control unit (118): • detecting a braking request, • determining a braking torque to be applied by the force actuators (120, 122, 124, 126) of the wheel brakes (102, 104, 106, 108) in order to implement the braking request, • determining current temperatures of the wheel brakes (102, 104, 106, 108) and / or of components of the wheel brakes (102, 104, 106, 108), • determining a speed of the vehicle (100), • if the determined temperature of at least one wheel brake (102, 104, 106, 108) and / or of the components of the wheel brake (102, 104, 106, 108) is above a first limiting value and the determined speed is below a second limiting value, actuating the force actuator (120, 122, 124, 126) of at least one wheel brake (102, 104, 106, 108) to provide a part of a braking torque corresponding to the braking request, and mechanically locking the force actuator (120, 122, 124, 126) of the at least one wheel brake (102, 104, 106, 108) in an actuation position corresponding to the set braking torque, • actuating the other wheel brakes (102, 104, 106, 108) to provide the braking torque remaining for complete implementation of the braking request.

2. The method as claimed in claim 1, characterized in that the braking torque provided by the locked force actuator (120, 122, 124, 126) of the at least one wheel brake (102, 104, 106, 108) is, by a defined value, below a braking torque which would result in locking of the vehicle wheel (110, 112, 114, 116) assigned to the wheel brake (102, 104, 106, 108).

3. The method as claimed in claim 1 or 2, characterized in that the proportion of the braking torque provided by the locked force actuator (120, 122, 124, 126) of the at least one wheel brake (102, 104, 106, 108) to the total braking torque required to implement the braking request is inversely proportional to a rate of change of the braking request.

4. The method as claimed in any one of the preceding claims, characterized in that, if the determined temperature of at least one wheel brake (102, 104, 106, 108) is above a first limiting value and the determined speed is below a second limiting value, the force actuator (120, 122, 124, 126) of the wheel brakes (102, 104, 106, 108) of a vehicle axle is actuated to provide a part of a braking torque corresponding to the braking request, and the force actuators (120, 122, 124, 126) of the wheel brakes (102, 104, 106, 108) of the vehicle axle are locked in an actuation position corresponding to the set braking torque.

5. The method as claimed in claim 4, characterized in that the vehicle axle is a rear axle of the vehicle (100).

6. The method as claimed in any one of the preceding claims, characterized in that the force actuators (120, 122, 124, 126) each have an electromotive drive and a transmission connected downstream of the drive, wherein the locking of a force actuator (120, 122, 124, 126) is carried out by a mechanical blockade of the electromotive drive or of the transmission.

7. The method as claimed in any one of the preceding claims, characterized in that the temperature of the wheel brakes (102, 104, 106, 108) and / or of components of the wheel brakes (102, 104, 106, 108) is estimated from operating parameters of the wheel brakes (102, 104, 106, 108).

8. The method as claimed in any one of the preceding claims, characterized in that the at least one wheel brake (102, 104, 106, 108), the force actuator (120, 122, 124, 126) of which is mechanically locked, is the wheel brake (102, 104, 106, 108) of the brake system having the highest temperature.

9. The method as claimed in any of the preceding claims, characterized in that, in addition to the temperature of the wheel brakes (102, 104, 106, 108) and / or of components of the wheel brakes (102, 104, 106, 108) and the speed, an inclination angle of the vehicle (100) is determined, wherein the force actuator (120, 122, 124, 126) of the at least one wheel brake (102, 104, 106, 108) is locked only if the inclination angle of the vehicle (100) is above a defined limiting value.

10. The method as claimed in any one of the preceding claims, characterized in that the at least one wheel brake (102, 104, 106, 108) has a parking brake function, wherein the force actuator (120, 122, 124, 126) of the at least one wheel brake (102, 104, 106, 108) is locked by activating the parking brake function.