Brake system and method for operating a motor vehicle

Abstract

The invention relates to a brake system (2) for a motor vehicle (1), wherein the brake system (2) has at least two controllable wheel brakes (8, 9) assigned to drivable wheels (6, 7) of an axle of the motor vehicle (1) and at least one brake controller (12), wherein the brake controller (12) is designed to control the wheel brakes (8, 9) depending on current friction values of the driven wheels (6, 7), in particular in the event of a coefficient of friction difference of the current friction values that exceeds a predefinable limit value. According to the invention, the brake controller (12) has at least one first brake sub-controller (15) which monitors a vehicle actual acceleration (aist) and is designed to control the wheel brakes (8, 9) depending on a comparison of the vehicle actual acceleration (aist) with a vehicle target acceleration (asoll).

Description

[0001] R. 411708

[0002] - 1 -

[0003] Description

[0004] title

[0005]

[0006] The present invention relates to a braking system for a motor vehicle, wherein the braking system comprises wheel brakes assigned to at least two driven wheels of an axle of the motor vehicle and at least one brake controller, wherein the brake controller is configured to control the wheel brakes depending on the current friction coefficients of the driven wheels, in particular in the case of a friction coefficient difference of the current friction coefficients exceeding a predefinable limit value.

[0007] Furthermore, the invention relates to a method for operating such a braking system, wherein the wheel brakes are controlled depending on at least current friction values ​​of the driven wheels, in particular in the case of a friction value difference of the current friction values ​​exceeding a predefinable limit value.

[0008] State of the art

[0009] Braking systems and methods of the type mentioned above are known from the prior art. To increase driving safety, a so-called traction control system is generally used, the task of which is to prevent the driven wheels of a motor vehicle from spinning and thus to meet safety requirements, in particular with regard to stability, steering, and traction. A traction control system typically comprises a brake controller, for example in the form of an electronic differential lock, and an engine controller, which work together to prevent the driven wheels from spinning during driving. The brake controller has, in particular, the task of maintaining traction. R. 411708

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[0011] The braking system's purpose is to ensure and, if necessary, optimize the vehicle's traction without compromising its stability. Typically, the driven wheels are synchronized by the brake controller to guarantee maximum traction on the road surface. This involves first monitoring the friction coefficients of the driven wheels to detect, for example, whether one of the wheels is spinning or at risk of spinning. This would result in a loss of drive torque due to the spinning wheel, thus reducing the vehicle's overall traction. If a so-called p-split situation occurs, where the friction coefficients of the driven wheels differ significantly because the road surface differs between the two wheels, the brake controller usually uses the wheel speed or rotational speed as a control variable to achieve the desired target behavior.For this to work, the differential speed of the two wheels must be specified, from which a differential locking factor for an electronic limited-slip differential between the two wheels is derived. This locking factor allows torque to be transferred to the high-friction wheel, i.e., the wheel with the higher coefficient of friction with the road surface, in order to generate drive slip. Both control loops, that of the motor controller and the brake controller, regulate the wheel speed. While the motor controller reduces the drive torque, the brake controller increases traction at the high-friction wheel. Both control loops must therefore be very precisely coordinated to ensure a constant locking factor and thus constant vehicle acceleration.

[0012] Disclosure of the invention

[0013] The braking system according to the invention, with the features of claim 1, has the advantage of offering improved controller performance with reduced effort and increased controller robustness. Thus, the braking system according to the invention enables the advantageous implementation of requirements for the target behavior and performance of the motor vehicle. In particular, the braking system according to the invention offers the advantage that the application of the brake controller with regard to the desired target behavior is possible with direct parameter access and no control engineering R. 411708

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[0015] Lateral dependencies with other driving situations arise. This allows the brake controller to be specifically optimized for controlling selected driving situations and therefore enables robust behavior in the face of external boundary conditions and system dependencies. Preferably, the braking system has one such brake controller for each controllable wheel brake of the vehicle.

[0016] The braking system according to the invention is characterized in that the brake controller has at least one first brake sub-controller which monitors the actual vehicle acceleration as a controlled variable and is configured to actuate the wheel brakes based on a comparison of the actual vehicle acceleration with a target vehicle acceleration. The brake sub-controller is thus configured to actuate the wheel brakes depending on the vehicle acceleration. In contrast to previous methods, in which the vehicle speed or wheel speed was considered as the controlled variable, the vehicle acceleration is now used as the controlled variable, ensuring advantageous actuation of the wheel brakes in a p-split situation. If the braking system detects a difference in the coefficient of friction that exceeds the predefinable limit, the existence of a p-split situation is first recognized.The wheel brakes are then controlled based on a comparison of the vehicle's actual acceleration with the target acceleration, in order to optimize the vehicle's traction during acceleration. This results in the advantages already mentioned above.

[0017] Preferably, the first brake control unit is configured to increase the braking torque of at least one of the wheel brakes when, due to a friction coefficient difference exceeding a predefinable limit, the actual vehicle acceleration falls below the target vehicle acceleration by a predetermined limit value. This ensures that if the actual vehicle acceleration does not reach the expected target vehicle acceleration, the braking torque at one of the friction brakes, particularly the friction brake assigned to the driven high-friction wheel (i.e., the wheel with the higher friction coefficient), is increased. 411708

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[0019] Improved distribution of the available torque to the two wheels of the same axle is achieved.

[0020] According to a preferred embodiment of the invention, the brake controller comprises at least a second brake sub-controller and a selector, wherein at least the second brake sub-controller is configured to actuate the wheel brakes depending on a speed difference of the driven wheels, and wherein the selector is configured to select one of the brake sub-controllers for actuating the wheel brakes depending on the coefficient of friction difference. Thus, the brake controller includes not only the first brake sub-controller, which operates depending on the vehicle acceleration, but also a second brake sub-controller, which operates depending on the wheel speed. Optionally, the brake controller comprises at least one further brake sub-controller.The multiple brake control units allow the brake controller's tasks to be performed independently, enabling a rapid response to different driving situations through simple switching between them. The selector ensures that one or the other brake control unit is activated depending on the current driving situation. Specifically, the selector activates one or the other brake control unit based on the current driving situation, particularly the difference in friction coefficient and / or speed difference between the driven wheels on the same axle. Preferably, the brake control units are integrated into a brake system control unit or a central vehicle control unit.Furthermore, the controlled variable of the second brake control unit is, in particular, the deviation of the wheel speed or wheel rotational speed from a target speed or wheel rotational speed of the driven wheels. Specifically, the instability of a wheel can be detected based on the rotational speed profile, for example, by comparing an expected speed or rotational speed gradient with the actual speed or rotational speed gradient.

[0021] According to a preferred embodiment of the invention, the selector is configured to select the first brake component controller when the R. 411708

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[0023] The friction coefficient difference exceeds the limit value. This means that, in the event of a p-split situation, the selector automatically selects the first brake sub-controller to control the brakes in order to achieve the aforementioned advantages.

[0024] Furthermore, the selector is preferably configured to select the second brake control unit when the difference in the coefficient of friction is less than or equal to the limit value. In such a case, wheel synchronization is enabled because it is assumed that both wheels are moving on the same or a similar surface, resulting in the same or a similar coefficient of friction.

[0025] Preferably, the first brake sub-controller is designed as an integral (I) controller. Its controlled variable is the deviation of the actual vehicle acceleration from the target vehicle acceleration. A single, optional feedforward control to reach the operating point is therefore sufficient. The implementation of further requirements for the first brake sub-controller through constraints that can determine the I-controller is advantageously enabled by the selector and / or the first brake sub-controller itself.

[0026] Preferably, the second brake controller is designed as a PID controller. Preferably, the second brake controller has a parameter set that depends on the road gradient. Thus, a road gradient is preferably continuously monitored by means of a tilt sensor and taken into account when controlling the PID controller.

[0027] Preferably, the brake controller is configured to monitor the engine speed of an internal combustion engine drive unit of the motor vehicle and, with the first brake component controller selected, to actuate the wheel brakes of the driven wheels or to influence the actuation of the first brake component controller when the engine speed falls below a predetermined limit. This particularly prevents the internal combustion engine from stalling at excessively low engine speeds. If the engine speed threatens to fall below the critically low speed range, R. 411708

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[0029] To achieve this, the braking torque on at least one of the wheel brakes is reduced to prevent a further reduction in engine speed.

[0030] Furthermore, the selector is preferably designed to reduce the braking torque for the wheel with the higher coefficient of friction of the driven wheels, depending on its rotational speed profile, if the rotational speed gradient of this wheel exceeds a predetermined limit. Thus, if it is detected that the wheel with the higher coefficient of friction is becoming unstable, the braking torque is selectively reduced to bring the wheel slip of the unstable wheel back into the stable range. Subsequently, the braking torque is preferably slowly increased again to support the restoration of traction.

[0031] Furthermore, the selector is specifically designed to reduce the braking torque at one of the wheel brakes during testing in order to determine a difference in friction coefficients. When both wheels of the axle are running stably, particularly during acceleration, the selector selectively reduces the braking torque at one of the wheels to determine whether the wheels have the same or different friction coefficients. This allows a p-split situation to be tested using simple means.

[0032] The method according to the invention, with the features of claim 11, is characterized in that the actual acceleration of the motor vehicle is monitored and compared with a target acceleration, and that the wheel brakes are controlled depending on the comparison. This results in the advantages already mentioned above. The method is implemented in particular according to the invention. Preferably, the braking torque at the wheel with the lower coefficient of friction is increased when, with a difference in the coefficient of friction exceeding a predefinable limit value, the actual acceleration of the vehicle falls below the target acceleration by a predetermined limit value. This results in the advantages already mentioned above. Further optional method steps result from the preceding

[0033] Described.R. 411708

[0034] - 7 -

[0035] Further advantages and preferred features and combinations of features will become apparent in particular from the foregoing and the claims. The invention will now be explained in more detail with reference to the drawing. To this end, we show...

[0036] Figure 1 shows a motor vehicle in a simplified top view.

[0037] Figure 2 shows a simplified representation of a motor vehicle brake regulator.

[0038] Figure 3 shows the behavior of a conventional brake control unit,

[0039] Figure 4 shows the behavior of an advantageous brake partial controller, and

[0040] Figures 5A to D show further procedural steps.

[0041] Figure 1 shows a simplified top view of a motor vehicle 1, which has a braking system 2 and a drive system 3.

[0042] The drive system 3 comprises an internal combustion engine drive unit 4, which is operatively connected or operatively connectable to two wheels 6, 7 of a front axle of the motor vehicle 1 via a transmission 5. The brake system 2 comprises at least wheel brakes 8, 9 assigned to the wheels 6, 7 driven by the drive system 3, each of which, for example, has an electromechanical brake actuator and / or is connected to a hydraulic brake circuit of the brake system 2.

[0043] Furthermore, the motor vehicle 1 advantageously has a control unit 10 which includes a drive controller 11 and a brake controller 12, which control the drive system 3 on the one hand and the wheel brakes 8 on the other hand, in order to ensure safe operation of the motor vehicle 1.

[0044] For example, the motor controller 11 is designed to control the drive unit 4, depending on a requested drive torque, to generate the torque that is then transmitted to the drive wheels 6, 7R. 411708

[0045] - 8 -

[0046] is transmitted. In this case, the motor vehicle 1 accelerates. The brake regulator 12 is designed to ensure the driving safety of the motor vehicle even during acceleration, for example by preventing the wheels 6, 7 from spinning and thus ensuring that both lateral and longitudinal forces used to accelerate the motor vehicle can be optimally implemented.

[0047] Figure 2 shows a simplified representation of the control unit 10, which has a brake regulator 12 for each of the four wheel brakes. According to the present embodiment, only wheels 6 and 7 are driven; according to another embodiment, all wheels of the vehicle 1 are driven by the drive unit. Thus, at least one brake regulator 12 is provided for each of the wheel brakes 8 and 9 of the vehicle 1, which in this case has a total of four wheel brakes. In Figure 2, one of the brake regulators 12 is shown enlarged to illustrate its construction, which is identical for the other brake regulators 12.

[0048] Each brake controller 12 has a selector 13 upstream of two brake sub-controllers 14 and 15. The brake sub-controllers 14 and 15 are configured differently to perform different tasks, which are explained in more detail below. Optionally, a coordinator 16 is connected downstream of the brake sub-controllers 14 and 15.

[0049] The selector 13 is configured to enable one of the two brake control units 14, 15 depending on the current driving situation, so that only this unit is selected for controlling a braking torque. In particular, the selector 13 is configured to select one of the two brake control units 14, 15 depending on a difference in the coefficient of friction of the driven wheels 6, 7 and the current driving situation.

[0050] First, the rotational speeds or velocities of the driven wheels 6 and 7 are recorded and monitored. Based on these recorded wheel rotational speeds, the coefficients of friction acting on wheels 6 and 7, resulting from their interaction with the road surface, are then determined.

[0051] In particular, the coefficients of friction are compared. Are the determined values ​​R. 411708

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[0053] If the coefficients of friction are equal or nearly equal, it is determined that the vehicle 1 is located with both wheels 6, 7 on the same road surface or on the same road surface, so that a drive torque can be advantageously distributed equally between both driven wheels 6, 7 to achieve maximum traction of the vehicle 1. In this case, the selector 13 activates the brake control unit 14, which performs wheel synchronization depending on the wheel speeds, with the aim of ensuring that the driven wheels have the same rotational speed.

[0054] However, if the comparison of the friction coefficients reveals that the difference in friction coefficients exceeds a predefined limit, it is determined that the driven wheels 6 and 7 are located on different road surface areas with different friction properties. This is also referred to as the so-called p-split situation, in which one of the driven wheels has a significantly higher coefficient of friction with the road surface than the other of the two driven wheels on the same axle.

[0055] The advantageous braking system and the advantageous method are explained in more detail with reference to Figures 3 to 5.

[0056] As a first step, motor vehicle 1 is put into operation.

[0057] Subsequently, or in response to an acceleration request, the brake control unit 14 (synchronization controller) is activated in a second step by selector 13. The vehicle's behavior, particularly the behavior of the driven wheels 6 and 7, is then monitored to determine the current driving situation. This monitoring focuses on the speeds, especially the rotational speeds, of the driven wheels 6 and 7.

[0058] Figure 3 shows, in a first diagram, the wheel speeds ne and ni of wheels 6 and 7, and the braking torque MB that can be generated by the wheel brakes 8 and 9, each plotted against time t. A first characteristic curve vi shows the target speed of the motor vehicle 1. A second characteristic curve nei shows the rotational speed or rotational speed of the driven wheel 6, which is located in the R. 411708

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[0060] The present embodiment exhibits stable driving behavior. A dashed line shows the target speed n?, s of the wheel 7 which has become unstable in this embodiment, and the characteristic curve n?,i is the actual rotational speed or actual velocity of the driven wheel 7. According to the present embodiment, the wheel 7 becomes unstable at a time t1, such that the actual rotational speed n?,i is the target rotational speed n?, s significantly exceeds the set speed. This indicates that the driven wheel 7 has lost traction and is spinning. The synchronization controller, or brake control unit 14, reacts by increasing the braking torque MB, 7 at the unstable wheel 7 by applying the wheel brake 9 until the wheel speed n?,i approaches the target speed N. n 7, s approximates or corresponds to it again.

[0061] The wheel speeds or rotational speeds are also checked by the selector 13 and / or the control unit 10 to determine whether a p-split situation exists. A p-split situation is characterized by differing coefficients of friction between the unstable wheel 7 and the road surface and the stable wheel 6 and the road surface, which deviate from each other beyond a predefined limit. The coefficients of friction can be determined using known methods. If it is determined that the difference in coefficients of friction exceeds the limit, a p-split situation is detected, and the selector 13 activates the brake control unit 15 or switches to the brake control unit 15, the function of which will be explained in more detail below with reference to Figure 4.

[0062] Figure 4 shows, in a further diagram, the vehicle acceleration a and the braking torque MB exerted on the unstable wheel, plotted against time t. The target acceleration a is derived from the acceleration request of the driver or a vehicle computer. S0 n, which is shown with a dashed line in Figure 4. Depending on at least the reaction speed and the forces that can be generated by the drive unit 4, and taking into account friction losses and inertia, the actual acceleration a^ of the motor vehicle 1 is obtained.

[0063] If it is now determined that the measured friction coefficients of the driven wheels 6, 7 differ so greatly from each other that the specified limit value is exceeded by R. 411708

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[0065] If the friction coefficient difference is exceeded, for example at time t2, the brake control unit 15 controls the wheel brake 9 assigned to the unstable wheel 7 to increase the braking torque MB, 7.

[0066] This results in an increase in the torque that can be transmitted by the more stable wheel 6, thereby improving the overall acceleration of the vehicle 1.

[0067] In particular, the braking torque MB, 7 for wheel 7 is increased until the actual acceleration aist corresponds at least substantially to the target acceleration as0n. The braking torque MB, 7 can then be reduced again. Preferably, however, it is only reduced again when it is determined that the p-split situation no longer exists and both driven wheels 6, 7 have the same or nearly the same coefficient of friction. Thus, as long as the p-split situation exists, the brake control unit 15 advantageously regulates the braking torque for the driven wheel 7 to improve the acceleration of the vehicle 1. The selector then preferably switches the brake control unit 12 back to the brake control unit 14.

[0068] The brake control unit 15 thus regulates the vehicle acceleration and not, as before, the wheel speed. The brake control unit 14 is preferably configured as a PID controller whose controlled variable is the deviation of the wheel speed from the target speed of the unstable wheel 7. The brake control unit 15 is preferably configured as an I-controller whose controlled variable is the deviation of the vehicle acceleration from the target acceleration a. S0 n is.

[0069] By using different brake component controllers, these can be optimally configured and applied to their respective tasks. The selector 13, in particular the state machine, enables and activates the individual brake component controllers 14, 15. In addition to the two brake component controllers 14, 15 described above, one or more further brake component controllers may also be present. Direct control of the vehicle acceleration by means of the brake component controller 15 simplifies the control process and advantageously reduces the application effort. R. 411708

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[0071] This results in a particularly robust control concept, as the control is based on a less dynamic variable, thus further reducing the risk of control instabilities.

[0072] Furthermore, the brake control unit 15 allows secondary conditions to override its control behavior. In particular, during the acceleration of the vehicle 1, when it is active via an acceleration controller of the control unit, the driving situation is monitored in order to make further interventions in the brake control if necessary, as described below.

[0073] Figures 5A to D show several procedure options that are additionally carried out by the braking system, preferably depending on the current driving situation.

[0074] Figure 5A shows a diagram in which the rotational speed n4 of the drive unit 4 and the braking torque MB, 7 are recorded over time t. The actual rotational speed N4 of the internal combustion engine is continuously monitored and compared with a minimum rotational speed n4,min. The minimum rotational speed n4,min is selected such that it ensures the operation of the drive unit 4 without it being unable to perform work due to an excessively low rotational speed and thus being stalled. Typically, an idle speed controller is used in the control unit 10 for this purpose, which ensures that the drive unit 4 does not run below the minimum rotational speed.If it is detected that the rotational speed is approaching and falling below the limit speed, then, in addition to increasing the torque of the drive motor 4 to maintain the rotational speed, the wheel brake 9, which is assigned to the unstable wheel 7, is activated to reduce the braking torque MB, 7 acting on it. Thus, the brake regulator 12 itself advantageously prevents the internal combustion engine from stalling.

[0075] According to the example in Figure 5B, in which the wheel speeds ne, m and the braking torque MB, 7 are plotted against time t, the braking torque MB, 7 is reduced or decreased when it is detected that the previously stable wheel 6 is itself becoming unstable. To prevent instability of the R. 411708

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[0077] To avoid vehicle collisions, the braking torque at the high-friction wheel is therefore reduced, particularly quickly or abruptly, to ensure driving stability. As soon as it is determined that the high-friction wheel is running stably again, the braking torque MB, 7 is preferably increased again, particularly slowly, to support traction build-up.

[0078] According to the example in Figure 5C, in which the wheel speeds ne, n? and the braking torque MB, 7 are plotted against time t, the braking torque MB, 7 is reduced when the braked wheel 7 is at risk of locking up and being overbraked, resulting in brake slip. This controlled reduction of torque advantageously prevents brake slip.

[0079] As shown in Figure 5D, where the wheel speeds ne, m and the braking torque MB, 7 are plotted against time t, it is checked whether the wheels 6, 7 still have different coefficients of friction or have the same or nearly the same coefficients of friction, i.e., in particular, whether a p-split situation still exists or has already resolved itself. For this purpose, a braking torque MB, 6, MB, 7 is preferably reduced at both driven wheels 6, 7 in order to determine the coefficients of friction acting on the wheels 6, 7. The controlled reduction of the braking torque allows the p-split situation to be clearly identified, especially considering the difference in coefficients of friction between the two wheels 6, 7.

[0080] The options shown in Figures 5A to 5D and described above are advantageously implemented by the control unit 10, in particular by the brake controller 12.

Claims

R. 411708 - 14 - Claims 1. Braking system (2) for a motor vehicle (1), wherein the braking system (2) comprises at least two driven wheels (6, 7) of an axle of the motor vehicle (1) and controllable wheel brakes (8, 9) and at least one brake controller (12), wherein the brake controller (12) is configured to control the wheel brakes (8, 9) depending on the current friction coefficients of the driven wheels (6, 7), in particular when the difference in the current friction coefficients exceeds a predefinable limit value, characterized in that the brake controller (12) comprises at least one first brake sub-controller (15) which provides a vehicle actual acceleration (ai St ) is monitored and trained to, depending on a comparison of the vehicle's actual acceleration (ai St ) with a target vehicle acceleration (a S0 n) to control the wheel brakes (8,9).

2. Braking system according to claim 1, characterized in that the first brake component controller (15) is configured to increase the braking torque of at least one of the wheel brakes (8, 9) when, in the case of a friction coefficient difference exceeding the predefinable limit value, the vehicle actual acceleration (ai S t) the vehicle target acceleration (a S0 n) falls below a predetermined limit acceleration value.

3. Braking system according to one of the preceding claims, characterized in that the brake controller (12) has at least a second brake sub-controller (14) and a selector (13), wherein at least the second brake sub-controller (13) is configured to actuate the wheel brakes (8, 9) depending on a speed difference of the driven wheels (6, 7), and wherein the selector (13) is configured to select one of the brake sub-controllers (15, 15) for actuating the wheel brakes (8, 9) depending on the coefficient of friction difference. R. 411708 - 15 - 4. Braking system according to one of the preceding claims, characterized in that the selector (13) is configured to select the first brake component controller (15) when the friction coefficient difference exceeds the limit value.

5. Braking system according to one of the preceding claims, characterized in that the selector (13) is configured to select the second brake component controller (14) when the friction coefficient difference is less than or corresponds to the limit value.

6. Braking system according to one of the preceding claims, characterized in that the first brake component controller (15) is designed as an I-controller.

7. Braking system according to one of the preceding claims, characterized in that the second brake component controller (14) is designed as a PID controller.

8. Braking system according to one of the preceding claims, characterized in that the brake controller 0 is configured to monitor an engine speed (n4) of an internal combustion engine drive unit (4) of the motor vehicle (1) and, when the first brake component controller (15) is selected, to actuate the wheel brakes (8,9) of the driven wheels (6,7) or to influence the actuation of the first brake component controller (15) when the engine speed (04) falls below a predetermined limit value.

9. Braking system according to one of the preceding claims, characterized in that the selector (13) is configured to reduce a braking torque (MB, / ) for the wheel (6,7) with the higher coefficient of friction of the driven wheels (6,7) depending on the rotational speed profile of the wheel (6,7) with the higher coefficient of friction of the driven wheels (6,7) when a speed gradient exceeds a predetermined limit value. R. 411708 - 16 - 10. Braking system according to one of the preceding claims, characterized in that the selector (13) is configured to reduce the braking torque (MB, 6, MB, 7) on one of the wheel brakes (8, 9) for a test operation in order to determine a friction coefficient difference.

11. Method for operating a braking system (2) for a motor vehicle (1), wherein the braking system (2) has at least two driven wheels (8, 9) of an axle of the motor vehicle (1) assigned and controllable wheel brakes (8, 9), and wherein the wheel brakes (8, 9) are controlled depending on the current friction coefficients of the driven wheels (6, 7), in particular when the difference in the current friction coefficients exceeds a predefinable limit value, characterized in that a vehicle actual acceleration (ai St ) of the motor vehicle is monitored and compared with a target vehicle acceleration (a S0n) is compared, and that the wheel brakes (8,9) are controlled depending on the comparison.

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