METHOD AND DEVICE FOR OPERATING A MOTOR VEHICLE, AND MOTOR VEHICLE

Abstract

A method for operating a motor vehicle which includes a primary braking system and a secondary braking system, each wheel of the motor vehicle being assigned a wheel brake which is actuatable by the braking systems, the secondary braking system being activated during an emergency braking operation in such a way that each of the wheel brakes generates the same brake force. An instantaneous steering angle of the motor vehicle is detected during the emergency braking operation and the secondary braking system is activated as a function of the detected steering angle.

Claims

1-9. (canceled)

10. A method for operating a motor vehicle which includes a primary braking system and a secondary braking system, each wheel of the motor vehicle being assigned a wheel brake which is hydraulically actuatable by the primary braking system and the secondary braking system, the secondary braking system being activated during an emergency braking operation in such a way that each of the wheel brakes generates the same brake force, the method comprising: detecting an instantaneous steering angle of the motor vehicle during the emergency braking operation; and activating the secondary braking system as a function of the detected steering angle.

11. The method as recited in claim 10, further comprising: detecting an instantaneous velocity of the motor vehicle in the emergency braking operation, wherein the secondary braking system is also activated as a function of the instantaneous velocity.

12. The method as recited in claim 10, wherein a brake force to be adjusted is determined with the aid of the Kamm circle.

13. The method as recited in claim 12, wherein the adjusted brake force is reduced with increasing steering angle.

14. The method as recited in claim 10, wherein an actual steering angle between a vehicle longitudinal axis of the motor vehicle and a longitudinal axis of a steerable wheel of the motor vehicle is detected as the steering angle.

15. The method as recited in claim 10, wherein a setpoint steering angle which is predefinable by a steering wheel of the motor vehicle is detected as the steering angle (α).

16. The method as recited in claim 10, wherein during the emergency braking operation, the brake force is adjusted to be oscillating at least from time to time.

17. A device for operating a motor vehicle which includes a primary braking system, a secondary braking system, and at least one drive unit, each wheel of the motor vehicle being assigned a wheel brake which is hydraulically actuatable by the primary braking system and the secondary braking system, the device comprising: a control unit configured to activate the secondary braking system, in the case of failure of the primary braking system, during an emergency braking operation in such a way that the wheel brakes generate the same brake force, the control unit configured to detect an instantaneous steering angle of the motor vehicle during the emergency braking operation, and activate the secondary braking system as a function of the detected steering angle.

18. A motor vehicle, comprising: a front wheel axle; a rear wheel axle; a primary braking system; a secondary braking system, wherein each wheel of the wheel axles is assigned a wheel brake which is hydraulically actuatable by the primary braking system and the secondary braking system; at least one drive unit; and a device including a control unit configured to activate the secondary braking system, in the case of failure of the primary braking system, during an emergency braking operation in such a way that the wheel brakes generate the same brake force, the control unit configured to detect an instantaneous steering angle of the motor vehicle during the emergency braking operation, and activate the secondary braking system as a function of the detected steering angle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 shows a motor vehicle in a simplified top view.

[0017] FIG. 2 shows a simplified method for operating the motor vehicle.

[0018] FIG. 3 shows a brake force-time-diagram to explain an advantageous method for operating the motor vehicle.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0019] FIG. 1 shows a simplified top view of a motor vehicle 1. Motor vehicle 1 includes a front wheel axle 2 and a rear wheel axle 3 each of which has two wheels 4 and 5, respectively, which are in contact with the roadway for the purpose of subjecting motor vehicle 1 to an acceleration torque or a deceleration torque.

[0020] In order to accelerate vehicle 1, the latter includes a drive system 6 having two drive units 7 and 8 in the present case. First drive unit 7 is designed as an internal combustion engine and is or may be operatively connected to wheels 4 of wheel axle 2 through a transmission 9 and a clutch (not illustrated). Second drive unit 8 is designed as an electric machine which is operatively connected to wheels 5 of rear wheel axle 3 through a transmission 10. Drive units 7 and 8 may thus be used to generate different positive or negative torques to be applied to wheel axles 2 and 3. Motor vehicle 1 may also include only one of drive units 7, 8.

[0021] In order to decelerate motor vehicle 1, the latter includes a vehicle braking system including a brake pedal 12 which is actuatable by the driver and which is connected to an electromechanical brake booster 13 which boosts the foot power applied to brake pedal 12 and converts it into a hydraulic pressure of a primary braking system 14. Primary braking system 14 includes hydraulically actuatable wheel brakes 15 and 16 for each of wheels 4 and 5, respectively. In order to hydraulically actuate wheel brakes 15, 16, primary braking system 14 includes an ABS/ESP module 17 which controls the hydraulic pressure individually for each wheel and thus allows for a wheel-individual brake force adjustment. In this case, module 17 may intervene to stabilize the vehicle, for example, and may apply a brake force to individual wheels 4, 5, to maintain the driving stability of motor vehicle 1 in a critical driving situation.

[0022] Furthermore, the vehicle braking system includes a secondary braking system 11 which is also designed to work hydraulically in the present exemplary embodiment and which is activated when primary braking system 14, in particular module 17, fails. Secondary braking system 11 uses brake booster 13 as the actuator. In this case, the hydraulic pressure in the brake circuit which is adjusted with the aid of electromechanical brake booster 13 in an automated manner is variable, but it is distributed equally among wheel brakes 15, 16 due to failure of module 17, so that the same brake force is adjusted at all wheel brakes 15, 16. As a result, the longitudinal stabilization of motor vehicle 1 is carried out in an automated manner or in a semi-automated manner even in the case of failure of primary braking system 14.

[0023] A control unit 19 which is, for example, a control unit of the vehicle braking system, in particular of electromechanical brake booster 13, monitors the operability of primary braking system 14 during the operation of the motor vehicle. If control unit 19 establishes that primary braking system 14 functions erroneously or no longer functions at all, it is fully deactivated by control unit 19 and secondary braking system 11 is instead activated to decelerate motor vehicle 1 in a longitudinally stabilized manner.

[0024] Secondary braking system 11 increases the hydraulic pressure at all wheel brakes 15, 16 or hydraulically adjusts a brake force with the aid of electromechanical brake booster 13. As already explained above, the resulting brake force is the same at all wheels 4, 5.

[0025] According to the present exemplary embodiment, wheels 4 of wheel axle 2 are designed to be steerable. For this purpose, longitudinal axis 4′ is plotted as a dashed line at one of wheels 4 in FIG. 1 during a curve negotiation. During the curve negotiation, an angle results between longitudinal axis 4′ of wheel 4 and a vehicle longitudinal axis 1′ which is also plotted in FIG. 1 as a dashed line. This angle is referred to in the following as the steering angle a of motor vehicle 1. There are different ways to ascertain steering angle α. It is conceivable, for example, to assign wheel 4 a sensor system which detects the instantaneous actual steering angle of wheel 4. Alternatively, a setpoint steering angle which is predefined by the driver or by the motor vehicle itself, during autonomous driving operation, may be ascertained and established as steering angle α.

[0026] Control unit 19 is designed to adjust the brake force or the hydraulic pressure with the aid of secondary braking system 11 as a function of this steering angle α.

[0027] For this purpose, FIG. 2 shows a simplified illustration of the advantageous method for operating motor vehicle 1 in which control unit 19 receives as input signals a piece of information about the vehicle longitudinal deceleration, such as in particular setpoint deceleration L, steering angle α as well as a piece of information about instantaneous vehicle velocity v. As a function of these input signals, control unit 19 ascertains target pressure p which is to be adjusted by secondary braking system 11, in particular with the aid of electromechanical brake booster 13, in order to generate the desired brake force at wheel brakes 15, 16.

[0028] During the emergency braking operation, i.e. when primary braking system 14 has failed, instantaneous steering angle a of motor vehicle 1 is taken into account when determining the brake force or when activating secondary braking system 11. With the aid of the Kamm circle, in particular, the lateral cornering force or the lateral cornering forces of wheels 4 is/are determined as a function of steering angle α. For this purpose, the driving speed of the motor vehicle as well as a friction coefficient between wheels 4 and the roadway is taken into account. The friction coefficient or μ value is preferably ascertained prior to initiating the curve negotiation with the aid of a one-channel pressure modulation in which the brake force or the hydraulic pressure is adjusted to be oscillating in order to ascertain on the basis of the thus resulting overall vehicle reaction what torque the wheels of the motor vehicle are capable of generating or whether the wheels lock, for the purpose of determining the instantaneous friction coefficient therefrom. Methods of this type are conventional in general so they will not be discussed in greater detail at this point. The lateral cornering force of wheels 4 which is transferable from wheels 4 to the roadway may be determined as a function of the friction coefficient, vehicle velocity v as well as steering angle α.

[0029] Control unit 19 now activates secondary braking system 11 in such a way that the adjusted brake force ensures that the lateral cornering force, which allows for motor vehicle 1 to be steered during deceleration, is maintained during deceleration. In this way, the driving stability of motor vehicle 1 is also reliably ensured during a curve negotiation during the emergency braking operation.

[0030] If the friction coefficient has not been ascertained prior to initiating the curve negotiation, it is preferably estimated, for example on the basis of previously ascertained friction coefficients, with the aid of sensors, friction coefficient maps or the like. Since the instantaneous actual friction coefficient is unknown during the curve negotiation itself, it may be estimated as a function of the friction coefficient by taking into account the occurrence probability of coupled events (severity classification: coupling of 1, for example; high degree of deceleration and necessary adaptation during curve negotiation: coupling of 2; sudden friction coefficient change during curve negotiation: coupling of 3).

[0031] Control unit 19 reduces the hydraulic pressure or the brake force of secondary braking system 11 with increasing steering angle α, in order to ensure the lateral cornering forces of wheels 4 which are necessary in the particular situation.

[0032] FIG. 3 shows a possible functional illustration of how steering angle α is taken into account for the one-channel stabilizing function of secondary braking system 11. In the one-channel pressure modulation for longitudinally stabilizing the vehicle, which is based on the overall vehicle longitudinal deceleration information as the stability indicator, stimulation is typically used in a way which makes it possible to evaluate the vehicle reaction. For this purpose, the brake force, as already mentioned above, is in particular adjusted to be oscillating. This is illustrated in FIG. 3 which shows the hydraulic pressure of secondary braking system 11 p.sub.11 against time t. Due to the oscillating hydraulic pressure having a defined or predefinable frequency, an effective hydraulic pressure results for stable vehicle deceleration p.sub.11_e. As the evaluation criterion, the response to the stimulation on the vehicle level or a sudden deceleration on the motor vehicle level is used, this response being establishable by an acceleration sensor as a sudden longitudinal acceleration of the motor vehicle. By taking into account the stability criteria with regard to the response to the vehicle reaction, mean effective pressure p.sub.11-e is adjusted for stable vehicle deceleration by the one-channel longitudinally stabilizing function with the aid of control unit 19. By taking into account steering angle α and, potentially, vehicle velocity v, effective pressure p.sub.11_e is adapted in such a way by adjusting the brake force that the lateral cornering forces may be made available for a curve negotiation. In FIG. 3, this is apparent in that pressure p hardly or no longer increases starting from point in time t.sub.1 at which a curve negotiation is initiated.

[0033] The advantageous method described above may be carried out not only on a backup level, when the primary actuator fails, but also when the motor vehicle has no wheel-specific measured variables available for the braking system in general, so that the brake force or the brake pressure adjusted in each case is also affected or predefined as a function of a detected steering angle. This allows for the lateral cornering forces to be improved for one-channel as well as for multi-channel braking systems.