Optimized wheel brake slip control of a wheel brake for motor vehicles and such motor vehicle brake

Abstract

The present invention generally relates to a method for optimized wheel brake slip control of a motor vehicle brake and to such a motor vehicle brake.

Description

[0001] The present invention generally relates to a method for optimized wheel brake slip control of a wheel brake for motor vehicles and to such a motor vehicle brake.

[0002] In addition to well-known hydraulically actuated wheel brakes, purely electrically actuated wheel brakes, also known as electromechanical wheel brakes ("EMB"), are increasingly being used as braking systems for motor vehicles. Such electromechanical wheel brakes typically have an electric or electronic drive unit that interacts with a mechanism or transmission. A brake unit can then be arranged on the output side, which can comprise a friction lining that can be pressed against a brake disc or drum by means of translational movement. This can cause deceleration during operation of the wheel brake. An electromechanical wheel brake is described, for example, in the document DE 10 2017 206 798 A1.

[0003] The electric drive unit can comprise an electrically driven motor, also referred to as an actuator. The associated functions for controlling or regulating the actuators are typically stored in an electronic control device or a brake control unit.

[0004] This actuator or force adjuster can exert a force on a friction partner - in the case of a drum brake a spreading force and in the case of a disc brake a clamping force - so that a vehicle wheel that is non-rotatably connected to the friction partner can be subjected to a braking torque that decelerates the rotation of the vehicle wheel.

[0005] To control such a wheel brake, actuation information is typically detected by an actuation device, such as an electronic brake pedal, which can correspond to a requested braking force. Corresponding braking signals can also be provided, for example, by a higher-level central controller. The requested braking force can be interpreted by a brake control unit and converted into control commands, which can then be transmitted to the individual wheel brakes, where they can be converted accordingly.

[0006] If the requested braking force exceeds a certain limit, the respective wheel may tend to lock. Locking wheels lead to instability or unsteerability of a vehicle and should be avoided as much as possible. In the prior art, this problem is usually addressed and solved with a central controller or a central control unit equipped with corresponding ABS control modules for all wheels of the vehicle. One method for ABS control is described, for example, in the applicant's document DE 10 2008 036 546 A1.

[0007] The time delays for data exchange between the central control unit or a higher-level vehicle computer and the local controllers can be considered unfavorable. Control run-through or loop times can be 10 ms or more. This limits the control accuracy.

[0008] Therefore, methods for controlling the braking force of a wheel of a motor vehicle that do not exhibit the aforementioned disadvantages or at least mitigate them are desirable. A braking system for a motor vehicle that is suitable for implementing the method is also desirable.

[0009] The inventors have taken on this task.

[0010] This problem is solved surprisingly simply by a method for controlling a wheel brake, in particular for a motor vehicle, and a braking system according to one of the independent claims. Preferred embodiments and further developments of the invention can be found in the respective subclaims.

[0011] The present invention therefore relates, in a first aspect, to a method for controlling a wheel brake, in particular for a motor vehicle. The wheel brake can be assigned to a wheel of the motor vehicle that is to be controlled according to the invention. It is understood that the method for controlling a wheel brake described below is presented as an example for only one wheel, but the method can be implemented for all relevant wheels of the motor vehicle.

[0012] The wheel brake of a wheel of the motor vehicle is preferably controlled by a local brake control unit (“WCU” = “Wheel Control Unit”), which is locally assigned to a wheel brake.

[0013] The procedure may include at least the following steps: - Providing a central braking force request F DriverReq at the local brake control unit under consideration, - Providing a vehicle reference speed V Ref or the wheel speeds V x of the other wheels to the local brake control unit, - Providing the current wheel speed V Rad of the wheel in question at the local brake control unit, - Providing the current wheel braking force F Rad of the wheel in question, and - Providing a limit value V min for the wheel speed at the local brake control unit, - Generating a local braking force request F CMD and / or a local controller state information state RBV by the local brake control unit based on the information provided above, - Actuate the force actuator of the local wheel brake of the wheel in question by applying the local braking force component F CMD .

[0014] The method offers the possibility of executing certain functions of a service brake for a motor vehicle locally, or directly at the respective wheel brakes, by means of a correspondingly designed local brake control unit. Shifting relevant functions of the service brake away from a higher-level, central brake control unit or central control unit to brake control units locally assigned to the individual wheel brakes offers several advantages. In the context of the invention, "local" means that the associated component or function can be individually assigned to a wheel of the motor vehicle and / or located in the immediate vicinity of the wheel. In other words, certain functions of the service brake can be executed directly at the wheel brake or the brake control unit directly assigned to the wheel brake.

[0015] In this way, time delays for data exchange between the central brake control unit and the local controllers can be minimized, especially when time-critical functions or controls are carried out directly locally at the wheel brakes. For example, control run-through or loop times for data exchange, which can be 10 ms or more in braking systems with a central brake control unit, can be reduced to less than 10 ms, preferably less than 5 ms, and most preferably to 1 ms or even less, especially for those functions that are shifted locally to the wheel brake. Furthermore, by specifying a limit value V min for the wheel speed the wheel slip can be improved.

[0016] The inventors have discovered that even safety-critical functions such as ABS control can be performed locally in a local brake control unit, which is particularly advantageous. The accuracy of the brake control can thus be significantly increased, since shorter loop times can lead to greater precision and responsiveness of the individual wheel brakes. The method according to the invention can be used particularly advantageously for wheel slip control and the control functions or routines required for this. By shifting the wheel slip control to the local brake control units, the wheel slip control can be executed faster, more precisely, and on a wheel-by-wheel basis, so that the overall braking time can be reduced and / or the braking distance can be further shortened.

[0017] According to a preferred embodiment of the invention, a motor vehicle can comprise a local brake control unit on each of the wheels 1 ... x (x = number of wheels of the motor vehicle), for example, on wheels 1, 2, 3, and 4 in the case of four wheels. These local brake control units can operate individually and perform the assigned functions or control routines independently of one another. This can significantly increase the reliability of the braking system, as this provides redundancy on all wheels. If, for example, one local brake control unit fails, the remaining three local brake control units remain operational. However, it is also possible to apply the method according to the invention only to wheels, for example, on one axle of the motor vehicle. The method described above represents a control run or loop.

[0018] According to a further, likewise preferred embodiment of the invention, the local brake control units of the wheel brakes of an axle can be combined in terms of their functions into a local, axle-specific brake control unit. For the sake of simplicity, the term "local brake control unit" will be used in the context of this invention, but it should be understood that this also includes a local, axle-specific brake control unit. The method according to the invention can therefore also be executed on a local, axle-specific brake control unit for the wheel brakes of the wheels of an axle of the motor vehicle.

[0019] The method for controlling the wheel brake considered in the context of the invention can be responsible for setting a requested braking torque or the braking force of the wheel. The centrally specified braking request is also referred to below as F DriverReq("Driver Request" = driver braking force request), but in the sense of the invention means not only driver braking requests, but also, for example, braking requests from a higher-level vehicle or on-board system. From the centrally specified braking request F DriverReq According to the invention, a local braking force component F CMD ("CMD" = "Command", brake command) can be determined, which can apply to the respective wheel brake. Depending on the type of wheel brake, this braking force component can then be made available as brake pressure for a hydraulically actuated brake cylinder, or as clamping or spreading force for an electromechanically actuated wheel brake. For simplification, this is also referred to as actuating the force actuator of the local wheel brake by applying the local braking force component F CMD designated.

[0020] Preferably, the wheel brake for the invention can be designed as an electromechanically actuated wheel brake (EMB), such as an electromechanical disc brake or an electromechanical drum brake. The advantages of the invention, such as short response times, are particularly effective in this case. The associated wheel brakes can be designed, in particular, as service brakes, although parking brake functions can also be integrated.

[0021] The procedure is intended to provide important status information of the associated wheel brakes, for example the current wheel speed V Rad or the current wheel braking force F Rad, to be recorded with appropriate sensors, or to be determined on the basis of empirically determined models stored in non-volatile memories, preferably within the local brake control units. In addition, the method according to the invention can be provided so that the local brake control unit can continuously exchange important information or signals with a higher-level, central control unit (VCU = “Vehicle Control Unit”) during operation. This central control unit can, for example, comprise a central brake controller or another higher-level control system of the motor vehicle. The central control unit can also be connected to a higher-level vehicle computer or integrated into it. The central control unit can therefore provide functions and / or information for a plurality of wheels of the motor vehicle.

[0022] For data exchange, at least one data connection can be provided between the local brake control unit and the central control unit. This connection can be designed redundantly to increase reliability. According to one embodiment of the invention, a data connection can also be provided between at least two, preferably several, local brake control units.

[0023] According to a preferred embodiment of the invention, the central control unit can, for example, control the central braking request F Driver Request the local brake control unit. The brake request F Driver Request This can be detected using an actuating device, such as an electronically actuated brake pedal. However, it can also be provided, for example, by a higher-level vehicle computer.

[0024] Furthermore, according to a preferred embodiment of the invention, the central control unit can determine the determined vehicle speed, hereinafter referred to as reference speed V Ref , or the wheel speeds of the remaining wheels of the motor vehicle, are cyclically made available to the local brake control unit. In the case of a first wheel under consideration (x = 1) and a total of four wheels of a motor vehicle, the wheel speeds V2, V3, and V4 of the other wheels (x = 2, 3, 4) can be provided when considering wheel 1.

[0025] For this purpose, according to a further preferred embodiment of the invention, it can be provided that the local brake control units wheel state variables such as the current wheel speed of the respective wheel V Rad or the current wheel braking force F Radto the central control unit. Based on this wheel speed information, the central control unit can determine the vehicle reference speed V Ref Determining the reference driving speed V Ref can be done using procedures known from ABS regulations.

[0026] The method may further comprise providing the current wheel speed V Rad of the wheel in question at the local brake control unit. This can be provided, for example, by appropriate wheel speed sensors, which can be connected directly to the local brake control unit.

[0027] The method may further comprise providing the current wheel braking force F Radof the wheel in question at the local brake control unit. This can be provided, for example, by appropriate force sensors on the wheel brakes, which can be connected directly to the local brake control unit.

[0028] According to a particularly preferred embodiment of the invention, the central control unit can furthermore set a limit value V min for the wheel speed as a further requirement for the wheel brake slip control of the local brake control unit. This limit value V min can represent a threshold, preferably a lower threshold, and thus an additional condition for wheel speed control. By adding a lower limit for the current wheel speed, which thus defines a speed below which the vehicle should not fall during braking, excessive brake slip can be advantageously avoided.

[0029] The reason for this is that a braked wheel rotates more slowly than a non-braked wheel. The difference in wheel rotation between braked and unbraked can also be referred to as slip speed. From a control engineering perspective, an optimum between maximum braking force or maximum braking torque on the one hand and sufficient driving stability on the other may be desired in order to improve braking efficiency. The relocation of the corresponding control functions to the local brake control units, in conjunction with the short loop times during data exchange at the wheel level, enables particularly effective local wheel brake slip control. If the brake slip is reduced according to the invention, this can increase driving stability during braking.

[0030] This limit value V minAccording to one embodiment of the invention, it can be predefined, for example, by the central control unit. It can, for example, be determined based on the vehicle type and stored in a memory of the central control unit.

[0031] According to another embodiment of the invention, it can be provided that the limit value V min can be set variably. This can mean that the limit value V min for example, during operation of the vehicle or even during braking. This allows the limit value V min For example, depending on the current vehicle speed and / or the estimated road conditions, and thus improve braking stability. In this way, for example, a larger limit value V min at higher speeds and a smaller limit value V minat lower speeds. In other words, the method according to the invention may further comprise the following method step: - Adjusting the limit value V min depending on the current vehicle speed V Ref and / or depending on the road conditions.

[0032] Thus, the limit value V min for example during braking to the current vehicle speed V Ref be adjusted to take a speed limit into account.

[0033] The limit value V minAccording to a particularly preferred embodiment of the invention, the wheel speed can include an absolute value for the wheel speed, which specifies the wheel speed below which the vehicle should not be driven. The control system can then consider this value as the lower limit for the wheel speed of the wheel in question. These values result in a maximum wheel slip, which is then not exceeded during operation or braking. If the road surface condition deteriorates, the value can be reduced, for example, while maintaining the same driving speed, so that a lower maximum wheel slip can result.

[0034] According to a further development of the invention, it can also be provided that the limit value V minthe magnitude of the maximum wheel slip is specified. The control system can then be designed in such a way that, based on this, the lower limit for the wheel speed to be considered can be determined and used for further calculations.

[0035] As an output variable, the local brake control unit can provide a local brake force request F CMD and / or a local controller state information state RBV based on the information provided above. The local brake control units can have one or more appropriately designed control modules to determine these variables. The respective force controller(s) of the local wheel brake can then be controlled with the local brake force component F CMD be applied.

[0036] During a braking operation, at least the process steps mentioned above, which can represent a corresponding control cycle, - Providing a central braking force request F DriverReq , - Providing a vehicle reference speed V Ref or the wheel speeds V x the other wheels, - Providing the current wheel speed V Rad , - Providing the current wheel braking force F Rad , and / or - Providing a limit value V min for the slip speed and - Generating the local braking force request F CMD and / or the local controller state information RBVbe executed multiple times by the local brake control unit. The time for such a control run, or loop time, is intended to be less than 10 ms, preferably 5 ms or less, particularly preferably 1 ms or even less. This enables particularly precise and fine tuning of the wheel brakes. The control run, or loop time, refers to the time required for one run of the aforementioned method steps.

[0037] The local brake control unit can be understood in particular as a functional unit designed to implement the aforementioned functions. For this purpose, it can comprise corresponding control algorithms and, for example, be modular in design. The control algorithms can be implemented, for example, in hardware or software. The local brake control unit can be designed, for example, as a microcontroller, microprocessor, application-specific integrated circuit (ASIC), programmable logic controller, or as another programmable or hard-wired unit. In particular, it can comprise processor means and memory means, wherein program code is stored in the memory means, upon execution of which program code the processor means performs a functionality as specified herein.

[0038] According to a preferred embodiment of the invention, the wheel brake slip control system within the local brake control unit can comprise four submodules. These can be assigned the functions of signal processing, wheel lock prevention, target wheel speed generation, and / or wheel speed control.

[0039] The signal processing sub-module can, for example, be designed to calculate the reference speed V Ref of the vehicle from the wheel speeds of the wheels of the motor vehicle V1 ... V x to be calculated if these are not provided by the central control unit. Furthermore, the vehicle deceleration Acc Re f and the wheel deceleration Acc Rad from the corresponding wheel speeds V Rad or the change in wheel speed V Rad be derived from one control run to the following control run.

[0040] The target wheel speed generation sub-module can, for example, be designed to generate an optimal target wheel speed V Soll for wheel speed control. The goal was to achieve optimal wheel brake slip for both braking efficiency and wheel stability. When specifying the limit value V min It may be advantageous to set the target wheel speed somewhat higher than this limit value so that this additional condition is also fulfilled by the wheel speed control at the same time.

[0041] The state controller submodule can, for example, be configured for the wheel lock prevention (RBV) function. This module can perform the following control tasks, for example: - to detect the braking condition of the wheel, - to calculate the braking force requirement for the corresponding conditions and / or - to initiate the transitions between the different states.

[0042] Different braking states of the wheel can be defined for the state controller in order to be able to adapt the corresponding control strategy even more efficiently and quickly to the respective operating state of the motor vehicle and the wheel brake. According to a preferred embodiment of the invention, for example, the four following braking states or state information: RBV defined as: “inactive”, “unstable”, “halt” and “stable”.

[0043] The state “inactive” (state RBV = “inactive”) can mean that the braking force requirement F DirverReq is relatively small and whose size does not endanger the wheel stability at all if this braking force requirement F DirverReq is fully adjusted at the wheel brake. A corresponding limitation of the braking force requirement F DirverReqis therefore not required. For the control strategy, this may mean that the central braking force requirement F DirverReq can be transmitted directly to the wheel brake. In this case, the braking force requirement F RBV from the state controller or the wheel lock prevention system is identical to the braking force request F DirverReq be.

[0044] The state “unstable” (state RBV = "unstable") can mean that the wheel dynamics are affected by a strong wheel deceleration Acc Rad and / or indicates a tendency of the wheel to lock due to a large wheel slip. For the control strategy, this can mean the following: If the "unstable" condition is detected for the first time, the current wheel braking force F Rad as a blocking limit F Lock To avoid wheel locking, a power reduction, i.e. a reduction of the current braking force, can be carried out immediately with FRBV=FLock−FOffset.

[0045] The offset F Offset be dependent on the wheel dynamics and represent the force reduction component for the first reduction step of the braking force. The offset F Offset can be used once in the first control run, in which a wheel locking tendency is detected, to reduce the braking force component from the state controller F RB v can be used to quickly reduce braking force and thus avoid wheel locking.

[0046] After that, the braking force reduction can be continued step by step with FRVB=FRVB,Old−Fab, as long as the "unstable" state is still active. F RVB,Old refers to the previously requested braking force component from the state controller from the last control cycle. In the course of a further reduction in braking force, the component F abof the reduction, i.e. the current power reduction component, is recalculated for each control run depending on the wheel dynamics.

[0047] The state “stable” (state RBV = "stable") can mean that the wheel dynamics, especially the wheel deceleration Acc Rad and / or wheel slip are in a relatively stable range. A tendency of the wheel to lock is not detected. If the "stable" state is detected, a force build-up for the braking force request F RVB be carried out according to the following rule: FRVB=FRVB,Old+Fauf.

[0048] The proportion F auf of the braking force build-up for each control cycle depending on the wheel dynamics. F auf therefore refers to the force build-up component for force build-up steps for the braking force.

[0049] The state “halt” (state RBV= "halt") can mean that the braking condition lies between "unstable" and "stable." Neither the conditions for "unstable" nor the conditions for "stable" are met. If the condition "halt" exists, the braking force requirement F RVB be kept constant.

[0050] The transition from one state to another can occur according to the following rules: 1) The "inactive" state is the initial state. It is set at the beginning of a braking operation and remains in this state until wheel instability is detected. When wheel instability is detected, the wheel braking state changes from "inactive" to "unstable." 2) The "unstable" state can be entered from both the "inactive" and "stable" states. A state change from "stop" to "unstable" is also possible. A state change from "unstable" to "stop" or "stable" can be achieved by actively reducing the braking force. 3) The state change to “halt” is only possible from the “unstable” state. 4) The state “stable” can be changed from the state “halt” or “unstable”. 5) The state controller always returns to the “inactive” state when a. the driving speed falls below a certain threshold or b. the RBV braking force requirement F RVB is greater than the braking force requirement F DirverReq or c. no braking force request F DirverReq is present.

[0051] The wheel speed control sub-module can, for example, be designed for speed control in order to compensate the deviation between the wheel speed and the optimal target wheel speed V Soll To determine this, a PID controller, for example, can be used. This allows the desired wheel slip to be achieved very effectively through continuous and precise control.

[0052] The state controller for wheel lock prevention can ensure that the cruise control performs a reset and reinitialization of the V-controller in the "inactive" and "unstable" braking states. The wheel speed control can continue to operate continuously in the "stop" and "stable" braking states. The resulting additional braking force requirement F VRegler can then be used to eliminate the speed deviation between the wheel speed and the optimal target wheel speed V Sollcontribute.

[0053] By superimposing the two braking force requirements F RVB and F VRegler The final, local braking force requirement F can be calculated from the state controller for wheel lock prevention and the speed controller for wheel speed control. CMD the wheel brake slip control.

[0054] The final braking force requirement F CMD the wheel brake slip control by the brake force request F DirverReq as an upper limit. At the same time, this final braking force requirement F CMD not be negative.

[0055] According to a further aspect, the invention also relates to a braking system, in particular for a motor vehicle, which is designed to carry out a method for controlling a wheel brake as described above.

[0056] Further details of the invention emerge from the description of the illustrated embodiments and the appended claims.

[0057] The drawings show: Fig. 1 a standard interface for wheel brake slip control, Fig. 2 the essential input and output variables of the wheel brake slip control according to the invention, Fig. 3 the four main modules of the wheel brake slip control, Fig. 4 the input and output variables for signal processing, Fig. 5 the input and output variables for the target wheel speed generation, Fig. 6 the input and output variables for the state controller, Fig. 7 the four braking states and their possible transitions for the state controller, Fig. 8 the input and output variables for the wheel speed control, and Fig. 9 the relationship between the two sub-controllers and the final output variables of the wheel brake slip control.

[0058] In the following detailed description of preferred embodiments, for the sake of clarity, like reference numerals designate substantially similar parts in or on these embodiments. However, to better illustrate the invention, the preferred embodiments illustrated in the figures are not always drawn to scale.

[0059] Fig. Figure 1 shows, purely schematically, a standard interface of a braking system 10 with a local brake control unit 11, which is suitable for controlling a wheel brake 40, in particular for a motor vehicle. The local brake control unit 11 can be configured to implement wheel brake slip control 11. In the illustrated embodiment, the braking system 10 further comprises a central control unit 30.

[0060] Fig. Figure 2 shows, purely schematically, the essential input and output variables of a wheel brake slip control system according to the invention, which is implemented in a local brake control unit 21 of a braking system 20. The braking system 20 is also configured to control a wheel brake 40, which is not shown for the sake of clarity. The braking system 20 further includes a central control unit 30, which is also not shown for the sake of clarity.

[0061] The method according to the invention for controlling the wheel brake 40, in particular for a motor vehicle, is therefore carried out by the local brake control unit 21, which is locally assigned to the wheel brake 40. It comprises the following steps for each wheel to be controlled: - Providing a central braking force request F DriverReq on the local brake control unit, - Providing a vehicle reference speed VRef or the wheel speeds V x the other wheels of the motor vehicle to the local brake control unit 21, - Providing the current wheel speed V Rad of the wheel in question at the local brake control unit 21, - Providing the current wheel braking force F Rad of the wheel in question, and - Providing a limit value V min for the wheel speed at the local brake control unit 21.

[0062] In the case of four vehicle wheels as in the exemplary embodiment mentioned, the motor vehicle comprises four wheels with four wheel brakes, each wheel brake comprising a local brake control unit 21, so that instead of the vehicle reference speed V Ref the wheel speeds V2, V3, V4 of the remaining wheels can also be provided to the local brake control unit 21 if the wheel in question is the first wheel x=1.

[0063] The local brake control unit 21 generates a local brake force request F CMD and / or a local controller state information state RBV generated based on this input information.

[0064] A force controller of the local wheel brake 40 can then be controlled with the local braking force component F CMD be applied.

[0065] Providing the braking force requirement F DriverReq is carried out by the higher-level, central control unit 30. This can represent a central brake controller or another higher-level control system of the motor vehicle or also an on-board computer of the motor vehicle.

[0066] At least one data connection is provided between the local brake control unit 21 and the central control unit 30 in order to be able to transmit data, signals or information.

[0067] According to a further embodiment of the invention, a data connection can also be provided between the local brake control units. This allows information, for example, concerning the respective wheel brake or the speed of the respective wheel, to be exchanged even more quickly between the local brake control units.

[0068] In this case, the wheel brake 40 is designed as a service brake. This does not affect the fact that additional functions, such as a parking brake, can be integrated into the wheel brake.

[0069] The wheel brake 40 is designed as an electromechanically actuated wheel brake (EMB). This can be, for example, an electromechanical disc brake or an electromechanical drum brake.

[0070] The method according to the invention offers the possibility of shifting relevant functions of the service brake away from the higher-level, central control unit 30 to brake control units 21 assigned locally to the individual wheel brakes.

[0071] In the context of the invention, local means that the local brake control unit 21 is individually assigned to a wheel of the motor vehicle and / or is located in the immediate vicinity of the wheel. This can be the case, for example, directly on or in the wheel brake or on or in a surrounding housing of the wheel brake. However, this can mean that the local brake control unit 21 is part of the unsprung masses and can be exposed to unfavorable environmental influences. In the context of the invention, a local brake control unit 21 can therefore also be understood to mean a local brake control unit 21 which is arranged, for example, on or in the wheel housing, preferably in the immediate vicinity of the wheel brake, and thus advantageously belongs to the sprung masses. According to the invention, safety-critical functions such as ABS control or wheel slip control are carried out locally in the local brake control unit 20.This can significantly increase the accuracy of the brake control, since shorter control cycle or loop times lead to greater precision and responsiveness of the individual wheel brake.

[0072] According to a further, likewise preferred embodiment of the invention, the local brake control units 21 of the wheel brakes 40 of an axle can be combined into a single local brake control unit. This allows the number of local brake control units 21 to be reduced, which can result in a cost advantage. Even with this embodiment, the loop times can still be kept very short. A local axle brake control unit combined in this way also offers the advantage that the associated component belongs to the sprung masses.

[0073] The local braking force component F determined by the method according to the invention CMDDuring operation, it provides a brake pressure for a hydraulically actuated brake cylinder, or a clamping or spreading force for an electromechanically actuated wheel brake.

[0074] The current wheel speed V Rad and the current wheel braking force F Rad is recorded using appropriate sensors, using force sensors, for example those based on strain gauges, or wheel speed sensors.

[0075] During operation, relevant information is exchanged between the local brake control units 21 of the motor vehicle and the higher-level, central control unit 30, which provides functions and / or information for all wheels of the motor vehicle. The central control unit can also be connected to a higher-level vehicle computer.

[0076] The central control unit 30 provides the central braking request F Driver Requestthe local brake control unit. The brake request F Driver Request This can be detected using an actuating device, such as an electronically actuated brake pedal. However, it can also be provided, for example, by a higher-level vehicle computer.

[0077] Furthermore, according to a preferred embodiment of the invention, the central control unit 30 sets the reference speed V Ref and / or the wheel speeds of the remaining wheels of the motor vehicle to the local brake control unit 21.

[0078] For this purpose, the local brake control units transmit 21 wheel state variables such as the current wheel speed of the respective wheel V Rad or the current wheel braking force F Rad to the central control unit 30. Based on this wheel speed information, the central control unit 30 can determine the vehicle reference speed V RefDetermining the reference driving speed V Ref can be done using procedures known from ABS regulations.

[0079] The current wheel speed V Rad is determined by local wheel speed sensors connected to the local brake control unit 21.

[0080] The current wheel braking force F Rad is made available to the local brake control unit 21 by corresponding force sensors on the wheel brakes, which are directly connected to the local brake control unit.

[0081] The invention provides that the central control unit 30 additionally sets a limit value V min for the wheel speed as a further requirement for the wheel brake slip control of the local brake control unit 21. This limit value V minrepresents an additional threshold and thus an additional condition for the wheel speed control. By adding a further limit for the current wheel speed, which therefore defines a speed below which the vehicle should not brake, it is very advantageous to avoid excessive brake slip.

[0082] This limit value V min is, according to one embodiment of the invention, fixedly predetermined by the central control unit 30.

[0083] According to the said embodiment of the invention, the limit value V min variably adjustable. This means that the limit value V min during operation of the motor vehicle as in the present case and especially during braking. Thus, limit value V mindepending on the current vehicle speed and / or the estimated road conditions during a braking maneuver. In this way, changes during a braking maneuver, such as a change in the road surface or a change in the speed limit, can be reacted to, and the method according to the invention can be adapted accordingly.

[0084] The limit value V min In the exemplary embodiment, represents an absolute value for the wheel speed, which determines the wheel speed that should not be exceeded on the wheel in question. Assuming a driving speed V Ref = 100 km / h, for example, a limit value V min= 90 km / h as the lower limit. The control system can then consider this value as the lower limit for the wheel speed. These example values result in a maximum wheel slip of 10%, which can be a practical value for good road conditions at the specified driving speed. If the road condition deteriorates or is already less good, the value can be adjusted, for example, at the same driving speed V Ref = 100 km / h on V min = 95 km / h, so that a maximum wheel slip of 5% can result.

[0085] According to a further development of the invention, it can also be provided that the limit value V minthe magnitude of the maximum wheel slip is specified. The control system can then be designed in such a way that, based on this, the lower limit for the wheel speed to be considered can be determined and used for further calculations.

[0086] The local brake control unit 21 provides the local brake force requirement F as the output variable. CMD and in the embodiment the local controller state information state RBV generated based on the input information.

[0087] The local brake control units 21 have appropriately designed control modules to determine these variables. According to the illustrated embodiment of the invention, four control modules are provided for this purpose, which will be discussed further below in connection with Fig. 3 will be discussed in more detail.

[0088] Carrying out at least the wheel slip control at the level of local brake control units 21 offers a great speed advantage when running through the required control steps.

[0089] The method according to the invention thus offers the great advantage that at least the following method steps can be completed in less than 10 ms, preferably 5 ms or less, particularly preferably 1 ms or even less. This very short control run-through or loop time enables particularly precise and fine tuning of the wheel brakes. The method steps include - Providing a central braking force request F DriverReq , - Providing a vehicle reference speed V Ref or the wheel speeds V x the other wheels, - Providing the current wheel speed V Rad , - Providing the current wheel braking force F Rad , - Providing a limit value V min for the slip speed, - Generating the local braking force request F CMD and / or the local controller state information RBV by the local brake control unit, and / or - Output of actuation information to the force actuator of the local wheel brake 40 or actuation of the force actuator of the local wheel brake 40 by applying the local braking force component F CMD .

[0090] The following procedural step may also be included: - Adjusting the limit value V min depending on the current vehicle speed V Ref and / or depending on the road surface condition, before generating a local braking force request F CMD and / or a local controller state information state RBV

[0091] The local brake control unit 21 comprises corresponding control algorithms or is designed to execute these method steps. The control algorithms are preferably implemented in corresponding hardware and / or software. The wheel brake slip control according to the invention within the local brake control unit 21 is divided into four control sub-modules or modules 22, 23, 24 and 25, as in the exemplary embodiment of the Fig. 3 shown.

[0092] The signal processing sub-module 22 is designed to calculate the reference speed V Ref of the vehicle from the wheel speeds of the wheels of the motor vehicle V1 ... V x to be calculated if this is not provided by the central control unit 30. Furthermore, the vehicle deceleration Acc Re f and the wheel deceleration Acc Rad from the corresponding wheel speeds V Rador the change in wheel speed V Rad derived from one control run to the following control run. Fig. Figure 4 shows schematically the input and output variables for the signal processing 22.

[0093] The target wheel speed generation sub-module 24 is designed to generate an optimal target wheel speed V Soll for wheel speed control. The goal is to achieve optimal wheel brake slip for both braking efficiency and wheel stability. When specifying the limit value V according to the invention, min It is advisable to set the target wheel speed slightly higher than this limit value so that this additional condition is also fulfilled by the wheel speed control at the same time. Fig. 5 shows the input and output variables for the target wheel speed generation 24.

[0094] The sub-module state controller 23 is designed for the wheel lock prevention (RBV) function. Fig. 6 shows the input and output variables for the state controller 23. This module 23 is designed to perform at least the following control tasks: - to detect the braking condition of the wheel, - to calculate the braking force requirement for the corresponding conditions and / or - to initiate the transitions between the different states.

[0095] According to the described embodiment of the invention, different braking states of the wheel are defined for the state controller 23 in order to be able to adapt the corresponding control strategy even more efficiently and quickly to the respective operating state of the motor vehicle and the wheel brake. According to the described embodiment of the invention, the four following braking states or state information states RBVdifferentiated: “inactive”, “unstable”, “halt” and “stable”.

[0096] The state “inactive” (state RBV = “inactive”) means that the braking force requirement F DirverReq is relatively small and whose size does not or does not significantly endanger the wheel stability, if this braking force requirement F DirverReq is fully adjusted at the wheel brake. A corresponding limitation of the braking force requirement F DirverReq is therefore not required. For the control strategy, this means that the central braking force requirement F DirverReq is transmitted directly to the wheel brake. In this case, brake force requirement F RB v from the state controller is identical to the braking force requirement F DirverReq .

[0097] The state “unstable” (state RBV = "unstable") means that the wheel dynamics are affected by a strong wheel deceleration Acc Radand / or a large wheel slip indicates a tendency of the wheel to lock. The following applies to the control strategy: If the "unstable" condition is detected for the first time, the current wheel braking force F Rad as a blocking limit F Lock To avoid wheel locking, an immediate power reduction is carried out with FRBV=FLock−FOffset.

[0098] In this way, a safe operating condition can be restored as quickly as possible. According to the exemplary embodiment, the offset F Offset depends on the wheel dynamics. The offset F Offset In the first control run, in which a wheel locking tendency is detected, the braking force component is reduced once from the state controller F RB v is used to quickly reduce braking force and thus prevent wheel locking.

[0099] Afterwards, i.e. in the following control runs, the braking force reduction is carried out step by step with FRVB=FRVB,Old−Fab, as long as the “unstable” state is still active. As a result of further braking force reduction, the proportion F ab of the reduction, i.e. the current power reduction component, is recalculated for each control run depending on the wheel dynamics.

[0100] The state “stable” (state RBV = "stable") means that the wheel dynamics, especially the wheel deceleration Acc Rad and / or wheel slip are in a relatively stable range. A tendency of the wheel to lock is not detected. If the "stable" state is detected, a force build-up for the braking force request F RVB carried out according to the following rule: FRVB=FRVB,Old+Fauf.

[0101] The proportion F aufof the braking force build-up for each control cycle depending on the wheel dynamics.

[0102] The state “halt” (state RBV = "halt") means that the braking state lies between "unstable" and "stable." Neither the conditions for "unstable" nor the conditions for "stable" are met. If the state "halt" exists, the RBV braking force request F remains RBV unchanged.

[0103] Fig. 7 shows the four braking states or state information state RBV and their possible transitions for the state controller 23. The transition from one state to another occurs according to the following rules: 1) The "inactive" state is the initial state ("start"). It is set at the beginning of a braking operation and remains in this state until wheel instability is detected. When wheel instability is detected, the wheel braking state changes from "inactive" to "unstable." 2) The "unstable" state can be entered from both the "inactive" and "stable" states. A state change from "stop" to "unstable" is also possible. A state change from "unstable" to "stop" or "stable" can be achieved by actively reducing the braking force. 3) The state change to “halt” is only possible from the “unstable” state. 4) The state “stable” can be changed from the state “halt” or “unstable”. 5) The state controller always returns to the “inactive” state when a. the driving speed falls below a certain threshold or b. the RBV braking force requirement F RVB is greater than the braking force requirement F DirverReq or c. no braking force request F DirverReq is present.

[0104] The sub-module wheel speed control 25 is designed for speed control in order to compensate the deviation between the wheel speed and the optimal target wheel speed V Soll to determine.

[0105] For this purpose, a PID controller is provided in the embodiment. Fig. 8 shows the input and output variables for the wheel speed control 25.

[0106] The state controller 23 for wheel lock prevention ensures that the speed controller resets and reinitializes the speed controller 25 in the "inactive" and "unstable" braking states. The wheel speed control continues to operate continuously in the "stop" and "stable" braking states. The additional braking force requirement F calculated in this way VRegler is used to eliminate the speed deviation between the wheel speed and the optimal target wheel speed V Soll.

[0107] By superimposing the two braking force requirements F RBV and F VRegler The final, local braking force requirement F is calculated from the state controller for wheel lock prevention and the speed controller for wheel speed control. CMD of the wheel brake slip control. With this braking force requirement F CMD the force actuator of the wheel brake 40 is actuated. The final braking force requirement F CMD the wheel brake slip control by the brake force request F DirverReq as an upper limit. At the same time, this final braking force requirement F CMD not be negative.

[0108] Fig.Finally, Figure 9 shows the relationship between the two control modules 25, 26 and the determination of the final output variables of the wheel brake slip control according to the invention. The outputs of the two control modules 25, 26 are fed through a limiter 26, which limits the determined braking force F Sum of the two control modules 25, 26 to the central braking force request F DriverReq makes.

[0109] According to a further aspect, the invention also relates to a braking system 20, in particular for a motor vehicle, which is designed to carry out a method for controlling a wheel brake 40 as described above. List of reference symbols: 10 Braking system 11 local brake control unit 20 Braking system 21 local brake control unit 22 Signal processing module 23 Wheel lock prevention module 24 Module for target wheel speed generation 25 Wheel speed control module 26 limiters 30 central control unit 40 Wheel brake Acc Ref Vehicle deceleration Acc Rad Wheel deceleration F DriverReq central braking force request F Rad current wheel braking force F CMD local braking force component F RB v requested braking force from the state controller F RVB,Old previously requested braking force component from the state controller F Lock Blocking limit F Offset Force reduction component for the first reduction step F ab Force reduction component for further force reduction step F auf Strength building component for strength building steps F VRegler Demand share from the speed controller V Rad current wheel speed V RefVehicle reference speed V min Limit value for slip speed V Soll optimal target wheel speed V x Wheel speed x Wheels of the motor vehicle VCU “Vehicle Control Unit” = central control unit WCU “Wheel Control Unit” = local brake control unit Condition RBV Controller status information 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 10 2017 206 798 A1

[0002] DE 10 2008 036 546 A1

[0006]

Claims

[1] Method for controlling a wheel brake (40), in particular for a motor vehicle, by a local brake control unit (21) which is locally assigned to the wheel brake (40), comprising at least the following steps: - Providing a central braking force request (F DriverReq ) on the local brake control unit (21), - Providing a vehicle reference speed (V Ref ) or the wheel speeds (V x ) of the other wheels of the motor vehicle to the local brake control unit (21), - Providing the current wheel speed (V Rad ) of the wheel in question at the local brake control unit (21), - Providing the current wheel braking force (F Rad ) of the wheel in question, and - Providing a limit value (V min ) for the wheel speed at the local brake control unit (21), - Generating a local braking force request (F CMD ) and / or local controller status information (status RBV ) by the local brake control unit (21) on the basis of the above-mentioned information provided, - Actuating a force actuator of the wheel brake (40) of the wheel in question by applying the local braking force component (F CMD ). [2] Method according to the preceding claim, characterized by that the provision of the braking force request (F DriverReq ) by a higher-level, central control unit (30), in particular a central brake controller or another higher-level control system of the motor vehicle. [3] Method according to one of the preceding claims, characterized by that there is at least one data connection between the local brake control unit (21) and the central control unit (30). [4] Method according to one of the preceding claims, characterized by that a data connection exists between the local brake control units (21). [5] Method according to one of the preceding claims, characterized by that the wheel brake (40) is designed as a service brake. [6] Method according to one of the preceding claims, characterized by that the wheel brake (40) is designed as an electromechanically actuated wheel brake, preferably as an electromechanical disc brake or as an electromechanical drum brake. [7] Method according to one of the preceding claims, characterized by that the limit value (V min ) includes a minimum value for the wheel speed. [8] Method according to one of the preceding claims, characterized by that the limit value (V min ) is fixed by the central control unit (30). [9] Method according to one of the preceding claims 1-7, comprising the following step: - Adjusting the limit value (V min) depending on the current vehicle speed (V Ref ) and / or depending on the road surface condition, preferably before generating a local braking force request (F CMD ) and / or local controller status information (status RBV ). [10] Method according to one of the preceding claims, characterized by that at least the procedural steps - Providing a braking force request (F DriverReq ), - Providing a vehicle reference speed (V Ref ) or the wheel speeds (V x ) of the other wheels, - Providing the wheel speed (V Rad ), - Providing the wheel braking force (F Rad ), - Providing a limit value (V min ) for the wheel speed, and / or - Generating the local braking force request (F CMD ) and / or the local controller state information (state RBV) by the local brake control unit represent a control cycle and can be carried out repeatedly during a braking operation, wherein the time for a control cycle is less than 10 ms, preferably 5 ms or less, particularly preferably 1 ms or less. [11] Method according to one of the preceding claims, characterized by that the local brake control unit (21) comprises at least one of the following control modules: a module for signal processing (22), a module for wheel locking prevention (23), a module for target wheel speed formation (24) or a module for wheel speed control (25). [12] Method according to the preceding claim, characterized bythat the wheel lock prevention module (23) defines different braking states of the wheel, in particular four different braking states "inactive", "unstable", "halt", "stable", wherein the selection of the control strategy for determining the local braking force requirement (F RBV ) depending on the braking conditions. [13] Braking system (20), in particular for a motor vehicle, designed to carry out a method for controlling a wheel brake (40) according to one of the preceding claims.

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