METHOD AND APPARATUS FOR COMPENSATING A YAW MOMENT ACTING ON A VEHICLE

Abstract

The disclosure relates to a method for compensating a yaw moment acting on a vehicle which is caused by asymmetrical braking forces on at least one vehicle axle. In the method, at least one vehicle-related condition is queried after initiation of a braking operation, a yaw variable present on the vehicle is detected, the value of the detected yaw variable is compared with a yaw variable limit value, a corrective steering angle is determined depending on the difference and/or the change in the difference between the value of the detected yaw variable and the yaw variable limit value, taking into account the sign of the yaw variable, and, lastly, a corrective steering angle is automatically set on at least one vehicle wheel of a steered vehicle axle. The disclosure also relates to an apparatus for compensating a yaw moment acting on a vehicle.

Claims

1. A method for compensating a yaw moment acting on a vehicle, the yaw moment being caused by asymmetrical braking forces on at least one vehicle axle, the method comprising the steps of: a) querying at least one vehicle-related condition after initiating a braking operation; b) detecting a yaw variable present at the vehicle (10); c) comparing a value of the detected yaw variable with a yaw variable limit value; d) determining a corrective steering angle depending on a difference and/or a change in the difference between the value of the detected yaw variable and the yaw variable limit value, taking into account a sign of the yaw variable; and e) automatically adjusting a corrective steering angle on at least one vehicle wheel of a steered vehicle axle.

2. The method according to claim 1, wherein steps a) to e) are performed continuously by a control system integrated in the vehicle as long as the braking operation continues and/or as long as a value of the yaw variable exceeds that of the yaw variable limit value.

3. The method according to claim 1, wherein the at least one condition in step a) comprises one or more of the following conditions: a steering angle is below a predefined limit steering angle, a brake pedal travel is below a predefined limit value, a brake pedal force is below a predefined limit force, a value of a braking force difference between opposing vehicle wheels is above a predefined limit and all driving safety systems are inactive.

4. The method according to claim 1, wherein the yaw variable in step b) is the yaw rate and/or the yaw moment.

5. The method according to claim 4, wherein the yaw rate is determined via a yaw rate sensor and/or the yaw moment is determined on the basis of the braking forces.

6. The method according to claim 5, wherein the braking forces are determined via the position of the wheel brake actuators and/or clamping force sensors on brakes.

7. The method according to claim 1, wherein the corrective steering angle is cancelled in step e) if the value of the yaw variable in step d) is below that of the yaw variable limit value.

8. The method according to claim 1, wherein the corrective steering angle is determined in step d) by calculation using a formula or using empirical data in the form of stored curves, matrices or tables and/or iteratively using a change in the difference determined in step e).

9. The method according to claim 1, wherein in step d) a corrective steering angle is applied to each of two vehicle wheels of the steered vehicle axle.

10. An apparatus for compensating for a yaw moment acting on a vehicle and for carrying out the method according to claim 1, the apparatus comprising a braking system, a sensor device for sensing a yaw variable, at least one further sensor for sensing vehicle-related data, at least one steered vehicle axle with a vehicle wheel, wherein the vehicle wheel is adjustable by a superimposed steering system and/or with an electric motor, and a vehicle-integrated controller which is set up to continuously query conditions on the basis of the determined vehicle-related data and to compare between the detected yaw variable and a yaw variable limit value, wherein the control system determines a corrective steering angle depending thereon and fixes the steering angle of the vehicle wheel of the steered vehicle axle to the corrective steering angle.

11. The apparatus according to claim 10, wherein the braking system comprises at least two individual independent brakes with electric wheel brake actuators.

12. The apparatus according to claim 11, wherein the sensor device comprises a yaw rate sensor for detecting the yaw rate and/or a travel sensor for determining the position of the wheel brake actuators by which a braking force at each vehicle wheel can be estimated and/or a clamping force sensor at each brake to determine the braking force at each vehicle wheel.

13. The apparatus according to claim 10, wherein the at least one sensor for detecting vehicle-related data comprises a steering angle sensor and/or a travel sensor for measuring brake pedal travel and/or a force sensor for measuring brake pedal force and/or sensors for determining the value of the braking force difference between opposing vehicle wheels.

14. The method according to claim 2, wherein the at least one condition in step a) comprises one or more of the following conditions: a steering angle is below a predefined limit steering angle, a brake pedal travel is below a predefined limit value, a brake pedal force is below a predefined limit force, a value of a braking force difference between opposing vehicle wheels is above a predefined limit and all driving safety systems are inactive.

15. The method according to claim 14, wherein the yaw variable in step b) is the yaw rate and/or the yaw moment.

16. The method according to claim 15, wherein the corrective steering angle is cancelled in step e) if the value of the yaw variable in step d) is below that of the yaw variable limit value.

17. The method according to claim 16, wherein the corrective steering angle is determined in step d) by calculation using a formula or using empirical data in the form of stored curves, matrices or tables and/or iteratively using a change in the difference determined in step e).

18. The method according to claim 17, wherein in step d) a corrective steering angle is applied to each of two vehicle wheels of the steered vehicle axle.

19. The apparatus according to claim 12, wherein the at least one sensor for detecting vehicle-related data comprises a steering angle sensor and/or a travel sensor for measuring brake pedal travel and/or a force sensor for measuring brake pedal force and/or sensors for determining the value of the braking force difference between opposing vehicle wheels.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0054] The disclosure is described below with reference to various exemplary arrangements illustrated in the accompanying drawings. In the figures:

[0055] FIG. 1 shows a schematic drawing of a vehicle equipped with the apparatus according to the disclosure in plan view, representative of various exemplary arrangements by which the method according to the disclosure is carried out;

[0056] FIG. 2 shows a schematic drawing of the vehicle in plan view during a braking operation;

[0057] FIG. 3 shows a schematic drawing of the vehicle in plan view during the braking operation, in which a corrective steering angle is applied to vehicle wheels;

[0058] FIG. 4 shows a schematic detailed view of a brake in section; and

[0059] FIG. 5 shows a schematic detailed view of a vehicle wheel during a braking operation.

DETAILED DESCRIPTION

[0060] FIG. 1 shows a vehicle 10 with two vehicle axles 12 and a braking system 14 as part of an apparatus for compensating a yaw moment.

[0061] The two vehicle axles 12 each have two vehicle wheels 16 and are divided into a steered front axle 18, which forms a front axle, and an unsteered vehicle axle 20, which forms a rear axle

[0062] According to a further exemplary variant, it is also conceivable that the steered vehicle axle forms the rear axle, and the unsteered vehicle axle 20 forms the front axle.

[0063] Furthermore, an exemplary variant is also possible in which both the front axle and the rear axle form a steered vehicle axle 18.

[0064] The steered front axle 18 comprises a steering system 22 by which a steering angle can be applied to the vehicle wheels 16 of the steered front axle 18.

[0065] The steering system 22 is, for example, a so-called steer-by-wire steering system comprising two electric motors 24 by which the vehicle wheels 16 of the steered vehicle axle 18 can each be individually adjusted.

[0066] The steering system 22 additionally comprises steering angle sensors 26 which can be used to detect the steering angles of the two vehicle wheels 16.

[0067] According to a further option, it is also possible for the steer-by-wire steering system to comprise only one electric motor, which adjusts both vehicle wheels 16 of the steered front axle 18 simultaneously, for example via a toothed rack.

[0068] According to a third option, the steering system 22 can also be a superimposed steering system, which enables driver-independent steering interventions, for example if the superimposed steering system comprises a harmonic gearing.

[0069] The braking system 14 of the vehicle 10 comprises four, for example electromechanical brakes 28, the wheel brake actuators 30 of which can be adjusted electrically, so that all vehicle wheels 16 can be individually braked via the braking system 14. Thus, the braking system 14 can be a brake-by-wire system.

[0070] In addition, the braking system 14 comprises travel sensors 32 on each brake 28 to enable the position of the electric wheel brake actuators 30 to be determined.

[0071] In addition, or instead of the travel sensors 32, the braking system 14 comprises clamping force sensors 34 that are used to determine the force applied by the electric wheel brake actuator 30 at each brake 28.

[0072] The brakes 28 of the braking system 14 are actuated by the driver via a brake pedal 36.

[0073] A force sensor 38 is provided on the brake pedal 36 and is used to detect the brake pedal force applied by the driver.

[0074] Furthermore, a travel sensor 40 is arranged on the brake pedal and is used to determine the brake pedal travel.

[0075] The aforementioned sensor device 33 can comprise, in addition to the travel sensor 32 and the clamping force sensor 34, or instead of these sensors, a yaw rate sensor 42, which is used to determine the yaw rate present when the vehicle 10 is yawing.

[0076] The sensors 26, 32, 34, 38, 40, 42 and the sensor device 33 are part of the aforementioned apparatus.

[0077] Furthermore, the apparatus comprises a control system 44 integrated in the vehicle. The controller 44 integrated in the vehicle is coupled here to the sensors 26, 32, 34, 38, 40, 42. Further, the control system 44 integrated in the vehicle is also coupled to the sensor device 33 as well as to the electric motors 24 and the electric wheel brake actuators 30.

[0078] A method for compensating the yaw moment using the apparatus is explained below with reference to FIGS. 2-5.

[0079] In order to improve clarity, FIGS. 2 to 3 largely do not show the components and parts explained above.

[0080] A yaw moment 45 during braking operations when driving straight ahead can occur when there are asymmetrical braking forces 50, 52 at one vehicle axle, so that the braking forces are unequal and there is a braking force difference (see FIG. 2).

[0081] For example, there can be a braking force 50 at the left vehicle wheel 16 of the front axle that is greater than the braking force 52 that is present at the right vehicle wheel 16 of the front axle. This results in a braking force difference at the front axle 18, wherein both braking forces have the same lever arm in relation to the vehicle centre of gravity 56. There is no torque compensation, and therefore a yaw moment 45 is created. This causes the vehicle 10 to yaw, which is induced by the yaw moment 45 (see FIGS. 2 and 3).

[0082] In a first step of the method, the vehicle-related conditions are queried. Here it is ensured that the steering angle of the two vehicle wheels 16 of the steered vehicle axle 18 is below a predefined limit angle, In addition, it is checked whether the brake pedal travel of the brake pedal 36 recorded by the travel sensor 40 is below a predefined limit value and the brake pedal force recorded by the force sensor 38 is below a predefined limit force. This means that there is no emergency braking or very heavy braking. Optionally, the value of the braking force difference between two opposing vehicle wheels, i.e. two vehicle wheels 16 arranged on a vehicle axle 12, can also be determined and it can be checked whether this is above a predefined limit value.

[0083] These vehicle-related conditions are intended to ensure that the method is only carried out when the vehicle is travelling approximately in a straight line and the braking operation itself represents a typical braking operation in normal operation, with neither an overall very high nor an overall very low braking force.

[0084] In the next step of the method, the yaw variable present in the vehicle 10 is detected.

[0085] According to a first exemplary arrangement of the method, the yaw variable is the yaw rate of the vehicle, which is detected by the yaw rate sensor 42 of the sensor device 33.

[0086] According to a further exemplary arrangement, the yaw variable is the yaw moment present, which is determined on the basis of the braking forces present at the vehicle wheels 16.

[0087] The determination of the braking forces at all four vehicle wheels 16 can be carried out according to different exemplary variants, which are explained below with reference to FIGS. 4 and 5. All of these variants are based on determining a clamping force 58 which us applied by the electric wheel brake actuator 30 and with which brake pads 60 press on a brake disc 62. The clamping force 58 and the coefficient of friction present between the brake pads 60 and the brake disc 62 can be used to determine the circumferential force 66 acting on the effective brake disc radius 64. Subsequently, a braking force 78 acting between the vehicle wheel 16 and the road surface can be concluded via the product of the circumferential force 66 with the ratio of the effective brake disc radius 64 and a dynamic tyre radius 70.

[0088] According to a first exemplary variant, the determination of the braking force 68 via the clamping force 58 is carried out by the clamping force sensors 34, which are arranged on the brakes 28.

[0089] According to a second exemplary variant, the clamping force is determined via the position of the wheel brake actuators 30, which is detected by the travel sensors 32.

[0090] According to a further exemplary variant, the current consumption of the individual electric wheel brake actuators 30 is used to infer the clamping force 58 generated, from which the braking force 68 can be derived.

[0091] If the yaw rate is known in the form of the yaw rate and/or the yaw moment, the next step of the method is to compare the value of the detected yaw rate(s) with (in each case) a predefined, stored yaw variable limit value. This (in each case one) yaw variable limit value represents a value below which a yaw variable is negligible and above which a yaw variable must be compensated by the method.

[0092] A corrective steering angle 72 is determined on the basis of the difference between the value of the detected yaw variable(s) and the yaw variable limit value, The direction of rotation of the yaw moment 45 is also taken into account. The corrective steering angle 72 can be determined mathematically using a formula with a known chassis geometry and known braking forces 68.

[0093] The corrective steering angle 72 refers here to the steering angle of both vehicle wheels 16 of the steered vehicle axle 18 (here front axle). By applying a corrective steering angle 72 to the vehicle wheels, a lateral force 76 is generated at each of the two vehicle wheels 16 and results in a compensating torque 74 that counteracts the yaw moment 45. This has the effect of decreasing the value of the yaw variable and stabilizing the vehicle 10.

[0094] Alternatively, the corrective steering angle 72 can also be determined on the basis of empirical data which, for example, was collected during the testing process and is available in the form of stored curves, matrices or tables. In this data, the control system 44 integrated in the vehicle seeks out a braking operation that is similar to the present one, so that the corrective steering angle 72 can be set analogously to that of the empirical data.

[0095] Lastly, it is also possible to first determine the corrective steering angle 72 mathematically and/or on the basis of the empirical data and then to determine it iteratively depending on the change in the determined difference between the yaw variable and the yaw variable limit value. The corrective steering angle 72 is continuously adjusted via such a closed control loop.

[0096] In all of these approaches, the corrective steering angle 72 is cancelled as soon as the value of the yaw variable falls below the yaw variable limit value, indicating that the vehicle 10 has returned to a stable driving state.

[0097] With the corrective steering angle 72 present, the vehicle wheels 16 on the steered front axle 18 are adjusted by the control system 44 integrated in the vehicle, which is coupled to the steering system 22.

[0098] In addition, the steering angle is adjusted via the two electric motors 24.