Optimized wheel brake slip control of a wheel brake for motor vehicles, and motor vehicle brake of this type

Abstract

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

Description

[0001] Optimized wheel brake slip control of a wheel brake for motor vehicles and such motor vehicle brake

[0002] Description

[0003] 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.

[0004] 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.

[0005] The electric drive unit can comprise an electrically driven motor, also referred to below 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.

[0006] This actuator or force controller can exert a force on a friction partner – a spreading force in the case of a drum brake and a clamping force in the case of a disc brake – 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. To control such a wheel brake, actuation information that can correspond to a requested braking force is usually recorded by an actuating device, such as an electronic brake pedal. 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.

[0007] 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, the 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 102008 036 546 A1.

[0008] 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.

[0009] 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.

[0010] The inventors have addressed this problem. 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:

[0014] - Providing a central brake force request ForiverReq to the local brake control unit under consideration,

[0015] - Providing a vehicle reference speed VRef or the wheel speeds Vx of the other wheels to the local brake control unit,

[0016] - Providing the current wheel speed VRad of the wheel in question to the local brake control unit,

[0017] - Providing the current wheel braking force FRad of the wheel in question, and

[0018] - Providing a limit value Vmin for the wheel speed to the local brake control unit,

[0019] - Generation of a local braking force request FCMD and / or a local controller state information ZustandRBv by the local brake control unit on the basis of the information provided as described above, - Actuation of the force actuator of the local wheel brake of the wheel in question by applying the local braking force component FCMD.

[0020] 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.

[0021] 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 performed directly locally at the wheel brakes. For example, control runthrough 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 locally shifted to the wheel brake. Furthermore, wheel slip can be improved by specifying a limit value Vmin for the wheel speed.

[0022] 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.

[0023] 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 carry out the assigned functions or control routines independently of one another. This can significantly increase the reliability of the braking system because 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.

[0024] 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 to form 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.

[0025] The method considered in the context of the invention for controlling the

[0026] The wheel brake 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 FüriverReq (“Driver Request”), but in the sense of the invention refers not only to driver braking requests, but also, for example, braking requests from a higher-level vehicle or on-board system. From the centrally specified braking request FüriverReq, a local braking force component FCMD (“CMD” = “Command”) can be determined according to the invention, 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 braking pressure for a hydraulically actuated brake cylinder, or as clamping or spreading force for an electromechanically actuated wheel brake.For simplicity, this is also referred to as actuating the force actuator of the local wheel brake by applying the local braking force component FCMD.

[0027] 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.

[0028] The method is designed to detect important status information of the associated wheel brakes, for example the current wheel speed VRad or the current wheel braking force FRad, using appropriate sensors, or to determine it based on empirically determined models stored in non-volatile memory, preferably within the local brake control units. In addition, the method according to the invention can provide for the local brake control unit to 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 or integrated into a higher-level vehicle computer.The central control unit can therefore provide functions and / or information for a plurality of wheels of the motor vehicle.

[0029] 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.

[0030] According to a preferred embodiment of the invention, the central control unit can, for example, provide the central brake request (Fiver Request) to the local brake control unit. The brake request (Fiver Request) can be detected by an actuating device, for example, an electronically actuated brake pedal. However, it can also be provided, for example, by a higher-level vehicle computer.

[0031] Furthermore, according to a preferred embodiment of the invention, the central control unit can cyclically provide the determined vehicle speed, hereinafter referred to as the reference speed VRet, or also the wheel speeds of the remaining wheels of the motor vehicle, 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.

[0032] For this purpose, according to a further preferred embodiment of the invention, the local brake control units can transmit wheel state variables such as the current wheel speed of the respective wheel VRad or the current wheel braking force FRad to the central control unit. Based on this wheel speed information, the central control unit can determine the vehicle reference speed VRef. The determination of the reference vehicle speed VRef can be carried out using methods known from ABS control systems.

[0033] The method may further provide the current wheel speed VRad of the wheel in question to 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.

[0034] The method may further provide for the provision of the current wheel braking force FRad of the wheel in question to 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.

[0035] According to a particularly preferred embodiment of the invention, the central control unit can further provide a limit value Vmin for the wheel speed as a further requirement for the wheel brake slip control of the local brake control unit. This limit value Vmin can represent a threshold, preferably a lower threshold, and thus an additional condition for the wheel speed control. By adding a lower limit for the current wheel speed, which accordingly defines a speed below which the vehicle should not fall during a braking operation, excessive brake slip can be very advantageously avoided.

[0036] The reason for this is that a braked wheel rotates more slowly than an unbraked wheel. The difference in wheel rotation between braked and unbraked can also be referred to as slip speed.

[0037] From a control perspective, an optimum balance between maximum braking force or maximum braking torque on the one hand and sufficient driving stability on the other may be desired to improve braking efficiency. Shifting the corresponding control functions to the local brake control units, combined with the short loop times for data exchange at the wheel level, enables particularly effective local wheel brake slip control. Reducing brake slip according to the invention can increase driving stability during braking.

[0038] According to one embodiment of the invention, this limit value Vmin can be fixed, for example, by the central control unit. It can be determined, for example, based on the vehicle type and stored in a memory of the central control unit.

[0039] According to another embodiment of the invention, it can be provided that the limit value Vmin can be set variably. This can mean that the limit value Vmin can be changed, for example, during operation of the motor vehicle or even during a braking operation. This makes it possible to adapt the limit value Vmin, for example, depending on the current vehicle speed and / or the estimated road surface condition, and thus improve braking stability. In this way, for example, a larger limit value Vmin can be set at higher speeds and a smaller limit value Vmin at lower speeds. In other words, the method according to the invention can further comprise the following method step:

[0040] - Adjusting the limit value Vmin depending on the current vehicle speed VRef and / or depending on the road surface condition.

[0041] For example, during braking, the limit value Vmin can be adjusted to the current vehicle speed VRef in such a way that a speed limit is taken into account.

[0042] According to a particularly preferred embodiment of the invention, the limit value Vmin can comprise an absolute value for the wheel speed, which defines the wheel speed below which the vehicle should not be undercut. 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.

[0043] According to a further development of the invention, it can also be provided that the value of the maximum wheel slip is specified as the limit value Vmin. The control system can then be configured in such a way that, based on this value, the lower limit for the wheel speed to be considered can be determined and used for further calculations.

[0044] As an output variable, the local brake control unit can generate a local brake force request (FCMD) and / or a local controller status information (RBV) based on the information provided above. The local brake control units can have one or more appropriately configured control modules to determine these variables. The respective force actuator(s) of the local wheel brake can then be supplied with the local brake force component (FCMD).

[0045] During a braking operation, at least the process steps mentioned above, which can represent a corresponding control cycle,

[0046] - Providing a central braking force request ForiverReq,

[0047] - Providing a vehicle reference speed VRef or the wheel speeds Vx of the other wheels,

[0048] - Providing the current wheel speed VRad,

[0049] - Providing the current wheel braking force FRad, and / or

[0050] - Providing a limit value Vmin for the slip speed and

[0051] - Generating the local brake force request FCMD and / or the local controller status information ZustandRBv by the local brake control unit is performed multiple times. 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 period for one run of the aforementioned method steps.

[0052] 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.

[0053] 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.

[0054] The signal processing submodule can, for example, be configured to calculate the vehicle's reference speed VRef from the wheel speeds of the motor vehicle's wheels Vi ... Vx, if these are not provided by the central control unit. Furthermore, the vehicle deceleration AccRef and the wheel deceleration AccRad can be derived from the corresponding wheel speeds VRad or the change in the wheel speed VRad from one control run to the next.

[0055] The target wheel speed generation submodule can, for example, be configured to determine an optimal target wheel speed Vset for wheel speed control. This should achieve optimal wheel brake slip for both braking efficiency and wheel stability. When specifying a limit value Vmin, it may be advantageous to set the target wheel speed slightly higher than this limit value so that this additional condition is also met by the wheel speed control.

[0056] 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:

[0057] - to detect the braking condition of the wheel,

[0058] - to calculate the braking force requirement for the corresponding conditions and / or

[0059] - to initiate the transitions between the different states.

[0060] Different wheel braking states can be defined for the state controller in order 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 following four braking states or

[0061] State information ZustandRBv can be defined as: “inactive”, “unstable”, “halt” and “stable”.

[0062] The "inactive" state (stateRBv = "inactive") can mean that the braking force request FüirverReq is relatively small and its magnitude does not endanger wheel stability at all if this braking force request FüirverReq is fully adjusted at the wheel brake. Accordingly, a corresponding limitation of the braking force request FüirverReq is not necessary. For the control strategy, this can mean that the central braking force request FüirverReq can be forwarded directly to the wheel brake. In this case, the braking force request FRBV from the state controller or the wheel lock prevention system can be identical to the braking force request FüirverReq. The "unstable" state (stateRBv = "unstable") can mean that the wheel dynamics indicate a tendency for the wheel to lock due to strong wheel deceleration AccRad and / or significant wheel slip.For the control strategy, this can mean the following: If the "unstable" state is detected for the first time, the current wheel braking force Fwheel can be assumed as a locking limit Flock. To prevent wheel locking, a force reduction, i.e., a reduction of the current braking force, can be implemented immediately.

[0063] FRBV = Flock - Foffset.

[0064] The offset Fowset can be dependent on the wheel dynamics and can represent the force reduction component for the first step of the braking force reduction. The offset Fowset can be applied once during the first control run, in which a wheel locking tendency is detected, to reduce the braking force component from the FRBV state controller in order to quickly reduce the braking force and thus prevent wheel locking.

[0065] After that, the braking force reduction can be continued step by step with

[0066] FRVB = FRVB.OId - Fab, as long as the "unstable" state is still active. FRVB.O refers to the previously requested braking force component from the state controller during the last control cycle. During a further braking force reduction, the Fab component of the reduction, i.e., the current force reduction component, can be recalculated for each control cycle depending on the wheel dynamics.

[0067] The "stable" state (ZustandRBv = "stable") can mean that the wheel dynamics, particularly the wheel deceleration AccRad and / or the wheel slip, are in a relatively stable range. A tendency for the wheel to lock is not detectable. If the "stable" state is detected, a force build-up for the FRVB braking force request can be executed according to the following rule: FRVB = FRVB.OId + Fauf.

[0068] The proportion Fauf of the braking force buildup for each control cycle depends on the wheel dynamics. Fauf therefore refers to the force buildup component for force buildup steps for the braking force.

[0069] The "halt" state (stateRBv = "halt") can mean that the braking state is between "unstable" and "stable." Neither the conditions for "unstable" nor the conditions for "stable" are met. If the "halt" state is present, the braking force demand FRVB can be kept constant.

[0070] The transition from one state to another can occur according to the following rules:

[0071] 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."

[0072] 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.

[0073] 3) The state change to “halt” is only possible from the “unstable” state.

[0074] 4) The state “stable” can be changed from the state “halt” or “unstable”.

[0075] 5) The state controller always returns to the "inactive" state if a. the vehicle speed falls below a certain threshold, or b. the RBV brake force request FRVB is greater than the brake force request FüirverReq, or c. there is no brake force request FüirverReq. The wheel speed control sub-module can, for example, be designed for speed control to determine the deviation between the wheel speed and the optimal target wheel speed Vsoii. A PID controller, for example, can be provided for this purpose. In this way, the desired wheel slip can be achieved very well through continuous and precise control.

[0076] The state controller for wheel lock prevention can ensure that the speed controller resets and reinitializes the speed 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 additional braking force demand FvRegier calculated in this way can then contribute to eliminating the speed deviation between the actual wheel speed and the optimal target wheel speed Vsoii.

[0077] By superimposing the two braking force requirements FRVB and FvRegier from the state controller for wheel lock prevention and the speed controller for wheel speed control, the final, local braking force requirement FCMD of the wheel brake slip control can be determined.

[0078] The final brake force request FCMD of the wheel brake slip control can be limited by the brake force request FoirverReq as an upper limit. At the same time, this final brake force request FCMD should not be negative.

[0079] 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.

[0080] Further details of the invention emerge from the description of the illustrated embodiments and the appended claims.

[0081] The drawings show: Fig. 1 a standard interface of the wheel brake slip control,

[0082] Fig. 2 shows the essential input and output variables of the wheel brake slip control according to the invention,

[0083] Fig. 3 the four main modules of the wheel brake slip control,

[0084] Fig. 4 the input and output variables for signal processing,

[0085] Fig. 5 the input and output variables for the target wheel speed generation,

[0086] Fig. 6 the input and output variables for the state controller,

[0087] Fig. 7 the four braking states and their possible transitions for the

[0088] state controller,

[0089] Fig. 8 the input and output variables for the wheel speed control, and

[0090] Fig. 9 the relationship between the two sub-controllers and the final output variables of the wheel brake slip control.

[0091] 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.

[0092] Fig. 1 shows a purely schematic view of 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.

[0093] Fig. 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.

[0094] 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:

[0095] - Providing a central brake force request ForiverReq to the local brake control unit,

[0096] - Providing a vehicle reference speed VRef or the wheel speeds Vx of the other wheels of the motor vehicle to the local brake control unit 21,

[0097] - Providing the current wheel speed VRad of the wheel in question to the local brake control unit 21,

[0098] - Providing the current wheel braking force FRad of the wheel in question, and

[0099] - Providing a limit value Vmin for the wheel speed to the local brake control unit 21 .

[0100] In the case of four vehicle wheels as in the exemplary embodiment mentioned, the motor vehicle comprises four wheels with four wheel brakes, wherein each wheel brake comprises a local brake control unit 21, so that instead of the vehicle reference speed VRef, 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.

[0101] The local brake control unit 21 generates a local braking force request FCMD and / or a local controller state information ZustandRBv based on this input information.

[0102] A force controller of the local wheel brake 40 can then be acted upon with the local braking force component FCMD. The braking force request FüriverReq is provided 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 even an on-board computer of the motor vehicle.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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 adverse environmental influences. Therefore, a local brake control unit 21 within the scope of the invention can also 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.

[0109] According to the invention, safety-critical functions such as ABS control or wheel slip control are performed 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 brakes.

[0110] 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.

[0111] The local brake force component FCMD determined by the method according to the invention provides, during operation, a brake pressure for a hydraulically actuated brake cylinder, or a clamping or spreading force for an electromechanically actuated wheel brake.

[0112] The current wheel speed VRad and the current wheel braking force FRad are recorded using appropriate sensors, using force sensors, for example those based on strain gauges, or wheel speed sensors.

[0113] 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.

[0114] The central control unit 30 provides the central braking request F Driver Request to the local brake control unit. The braking request F Driver Request can be detected by an actuating device, for example, an electronically actuated brake pedal. However, it can also be provided, for example, by a higher-level vehicle computer.

[0115] Furthermore, according to a preferred embodiment of the invention, the central control unit 30 provides the reference speed VRet and / or the wheel speeds of the remaining wheels of the motor vehicle to the local brake control unit 21.

[0116] For this purpose, the local brake control units 21 transmit wheel state variables such as the current wheel speed of the respective wheel VRad or the current wheel braking force FRad to the central control unit 30. Based on this wheel speed information, the central control unit 30 can determine the vehicle reference speed VRef. The determination of the reference vehicle speed VRef can be carried out using methods known from ABS control systems.

[0117] The current wheel speed VRad is determined by local wheel speed sensors which are connected to the local brake control unit 21.

[0118] The current wheel braking force FRad 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.

[0119] The invention provides that the central control unit 30 additionally provides a limit value Vmin for the wheel speed as a further requirement for the wheel brake slip control of the local brake control unit 21. This limit value Vmin represents an additional threshold and thus an additional condition for the wheel speed control. By adding a further limit for the current wheel speed, which accordingly defines a speed below which the vehicle should not fall during a braking operation, excessive brake slip can be advantageously avoided.

[0120] According to one embodiment of the invention, this limit value Vmin is fixedly specified by the central control unit 30.

[0121] According to the aforementioned embodiment of the invention, the limit value Vmin is variably adjustable. This means that the limit value Vmin is adjusted during operation of the motor vehicle, as is the case here, and in particular during a braking operation. Thus, the limit value Vmin can be adjusted depending on the current vehicle speed and / or the estimated road conditions during a braking operation. In this way, it is also possible to react to changes during a braking operation, such as a change in the road surface or a change in the speed limit, and the method according to the invention can be adjusted accordingly.

[0122] In the exemplary embodiment, the limit value Vmin represents an absolute value for the wheel speed, which specifies the wheel speed below which the wheel in question must not be undercut. Assuming a driving speed VRef = 100 km / h, for example, a limit value Vmin = 90 km / h can be set as the lower limit. The control system can then use 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 set to Vmin = 95 km / h, for example, at the same driving speed VRef = 100 km / h, which can result in a maximum wheel slip of 5%.

[0123] According to a further development of the invention, it can also be provided that the value of the maximum wheel slip is specified as the limit value Vmin. The control system can then be configured in such a way that, based on this value, the lower limit for the wheel speed to be considered can be determined and used for further calculations.

[0124] As an output variable, the local brake control unit 21 generates the local braking force request FCMD and, in the exemplary embodiment, the local controller state information RBV on the basis of the input information.

[0125] 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 in more detail below in connection with Fig. 3.

[0126] 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.

[0127] 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

[0128] - Providing a central braking force request FüriverReq,

[0129] - Providing a vehicle reference speed VRef or the wheel speeds Vx of the other wheels,

[0130] - Providing the current wheel speed VRad,

[0131] - Providing the current wheel braking force FRad,

[0132] - Providing a limit value Vmin for the slip speed,

[0133] - generating the local braking force request FCMD and / or the local controller state information ZustandRBv by the local brake control unit, and / or outputting actuation information to the force actuator of the local wheel brake 40 or actuating the force actuator of the local wheel brake 40 by applying the local braking force component FCMD.

[0134] The following procedural step may also be included:

[0135] - Adjusting the limit value Vmin depending on the current vehicle speed VRef and / or depending on the road condition, before generating a local brake force request FCMD and / or a local controller state information ZustandRBv

[0136] 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 shown in the exemplary embodiment in Fig. 3.

[0137] The signal processing submodule 22 is designed to calculate the reference speed VRef of the vehicle from the wheel speeds of the motor vehicle's wheels Vi ... Vx, if this is not provided by the central control unit 30. Furthermore, the vehicle deceleration AccRef and the wheel deceleration AccRad are derived from the corresponding wheel speeds VRad and the change in the wheel speed VRad from one control run to the subsequent control run, respectively. Fig. 4 schematically shows the input and output variables for the signal processing 22.

[0138] The target wheel speed generation submodule 24 is designed to determine an optimal target wheel speed Vset 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 Vmin according to the invention, it is advantageous to set the target wheel speed slightly higher than this limit value so that this additional condition is also simultaneously met by the wheel speed control. Fig. 5 shows the input and output variables for the target wheel speed generation 24.

[0139] The state controller submodule 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:

[0140] - to detect the braking condition of the wheel,

[0141] - to calculate the braking force requirement for the corresponding conditions and / or

[0142] - to initiate the transitions between the different states.

[0143] According to the described embodiment of the invention, different wheel braking states 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 following four braking states or state information (StatusRBv) are distinguished: "inactive," "unstable," "halt," and "stable."

[0144] The "inactive" state (ZustandRBv = "inactive") means that the braking force request FüirverReq is relatively small and does not, or does not significantly, endanger wheel stability if this braking force request FüirverReq is fully applied to the wheel brake. Accordingly, a corresponding limitation of the braking force request FüirverReq is not required. For the control strategy, this means that the central braking force request FüirverReq is forwarded directly to the wheel brake. In this case, the braking force request FRBV from the state controller is therefore identical to the braking force request FDirverReq.

[0145] The "unstable" state (stateRBv = "unstable") means that the wheel dynamics indicate a tendency to lock due to a strong wheel deceleration AccRad and / or a large wheel slip. The following applies to the control strategy: If the "unstable" state is detected for the first time, the current wheel braking force FRad is assumed to be a locking limit Flock. To prevent wheel locking, an immediate force reduction is carried out with

[0146] FRBV = Flock - Foffset.

[0147] In this way, a safe operating condition can be restored as quickly as possible. According to the exemplary embodiment, the offset Foffset depends on the wheel dynamics. The offset Foffset is applied once during the first control run, in which a wheel locking tendency is detected, to reduce the braking force component from the FRBV state controller in order to quickly reduce the braking force and thus prevent wheel locking.

[0148] Afterwards, i.e. in the following control runs, the braking force reduction is carried out step by step with

[0149] FRVB = FRVB.OId - Fab, as long as the "unstable" state is still active. During further brake force reduction, the Fab portion of the reduction, i.e., the current force reduction component, can be recalculated for each control cycle depending on the wheel dynamics.

[0150] The "stable" state (ZustandRBv = "stable") means that the wheel dynamics, particularly the wheel deceleration AccRad and / or wheel slip, are in a relatively stable range. A tendency for the wheel to lock is not detectable. If the "stable" state is detected, a force buildup for the brake force request FRVB is executed according to the following rule:

[0151] FRVB = FRVB.OId + Fauf. The proportion Fauf of the braking force buildup for each control cycle depends on the wheel dynamics.

[0152] The "halt" state (stateRBv = "halt") means that the braking state is between "unstable" and "stable." Neither the conditions for "unstable" nor the conditions for "stable" are met. If the "halt" state is present, the RBV braking force request FRBV remains unchanged.

[0153] Fig. 7 shows the four braking states or state information ZustandRBv and their possible transitions for the state controller 23. The transition from one state to another occurs according to the following rules:

[0154] 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."

[0155] 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.

[0156] 3) The state change to “halt” is only possible from the “unstable” state.

[0157] 4) The state “stable” can be changed from the state “halt” or “unstable”.

[0158] 5) The state controller always returns to the “inactive” state if a. the vehicle speed falls below a certain threshold or b. the RBV braking force request FRVB is greater than the braking force request FüirverReq or c. there is no braking force request FüirverReq.

[0159] The wheel speed control submodule 25 is designed for speed control to determine the deviation between the wheel speed and the optimal target wheel speed Vset. A PID controller is provided for this purpose in the exemplary embodiment. Fig. 8 shows the input and output variables for the wheel speed control 25.

[0160] 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 FvRegier calculated in this way is used to eliminate the speed deviation between the wheel speed and the optimal target wheel speed Vsoii.

[0161] The final, local brake force request FCMD of the wheel brake slip control is determined by superimposing the two brake force requests FRBv and FvRegier from the state controller for wheel lock prevention and the speed controller for wheel speed control. This brake force request FCMD is applied to the force controller of the wheel brake 40. The final brake force request FCMD of the wheel brake slip control is limited by the brake force request FoirverReq as the upper limit. At the same time, this final brake force request FCMD should not be negative.

[0162] Finally, Fig. 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 system according to the invention. The outputs of the two control modules 25, 26 pass through a limiter 26, which limits the determined braking force Fsum of the two control modules 25, 26 to the central braking force request ForiverReq.

[0163] 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:

[0164] 10 Braking system

[0165] 11 local brake control unit

[0166] 20 Braking system

[0167] 21 local brake control unit

[0168] 22 Signal processing module

[0169] 23 Wheel lock prevention module

[0170] 24 Module for target wheel speed generation

[0171] 25 Wheel speed control module

[0172] 26 limiters

[0173] 30 central control unit

[0174] 40 Wheel brake

[0175] AccRef vehicle deceleration

[0176] AccRad wheel deceleration

[0177] FüriverReq central brake force request

[0178] FRad current wheel braking force

[0179] FCMD local braking force component

[0180] FRBV requested braking force component from the state controller

[0181] FRVB.OW previously requested braking force component from the state controller

[0182] Flock blocking limit

[0183] Foffset force reduction component for the first reduction step

[0184] Fab force reduction component for further force reduction step

[0185] Fauf strength building component for strength building steps

[0186] FvRegier Request share from the speed controller VRad current wheel speed

[0187] VRef vehicle reference speed

[0188] Vmin limit for slip speed

[0189] Vsoii optimal target wheel speed Vx wheel speed x wheels of the vehicle

[0190] VCU “Vehicle Control Unit” = central control unit

[0191] WCU “Wheel Control Unit” = local brake control unit

[0192] StateRBv Controller state information

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 brake force request (FüriverReq) to the local brake control unit (21), - Providing a vehicle reference speed (VRef) or the wheel speeds (Vx) of the other wheels of the motor vehicle to the local brake control unit (21), - Providing the current wheel speed (VRad) of the wheel in question to the local brake control unit (21), - Providing the current wheel braking force (FRad) of the wheel in question, and - providing a limit value (Vmin) for the wheel speed to the local brake control unit (21), - generating a local brake force request (FCMD) and / or a local controller status information (StatusRBv) by the local brake control unit (21) on the basis of the above-mentioned provided information, - Actuating a force actuator of the wheel brake (40) of the wheel in question by applying the local braking force component (FCMD).

2. Method according to the preceding claim, characterized in that the provision of the braking force request (FüriverReq) is carried out 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 in 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 in that a data connection exists between the local brake control units (21).

5. Method according to one of the preceding claims, characterized in that the wheel brake (40) is designed as a service brake.

6. Method according to one of the preceding claims, characterized in 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 in that the limit value (Vmin) comprises a minimum value for the wheel speed.

8. Method according to one of the preceding claims, characterized in that the limit value (Vmin) is fixedly predetermined 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 (Vmin) depending on the current vehicle speed (VRef) and / or depending on the road surface condition, preferably before generating a local Brake force request (FCMD) and / or local controller state information (StatusRBv).

10. Method according to one of the preceding claims, characterized in that at least the method steps - Providing a braking force request (ForiverReq), - Providing a vehicle reference speed (VRef) or the wheel speeds (Vx) of the other wheels, - Providing the wheel speed (VRad), - Providing the wheel braking force (FRad), - Providing a limit value (Vmin) for the wheel speed, and / or - Generation of the local braking force request (FCMD) and / or the local controller state information (ZustandRBv) by the local brake control unit represents 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 in 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 lock 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 in that the wheel locking prevention module (23) is able to detect different Braking states of the wheel, in particular four different braking states “inactive”, “unstable”, “stop”, “stable” are defined, whereby the selection of the control strategy for determining the local braking force requirement (FRBV) is dependent on the braking states.

13. Brake 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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