Method for applying torque overlay during split-mu braking conditions

1. A method for providing vehicle stability control for a vehicle, said vehicle including a slip control braking system for applying independent braking pressure to respective vehicle brakes of said vehicle, said vehicle further including a steering system for applying steering intervention for stability control, said method comprising the steps of:

detecting a split-mu road surface;

applying a steering assist torque to said steering system in response to detecting the split-mu road surface, the steering assist torque being independent of a steering torque applied by a driver of said vehicle;

determining whether a steering wheel angle condition is within a predetermined threshold, said steering wheel angle condition comprising a steering wheel angle and a yaw angle of said vehicle; and

applying an aggressive braking strategy if said steering wheel angle condition is within said predetermined threshold, else applying a non-aggressive braking strategy.

2. The method of claim 1 wherein said step of detecting a split-mu road surface includes applying a braking torque to said respective vehicle wheels and detecting a deceleration rate slower than a predetermined deceleration rate threshold for a first front wheel and a first rear wheel, and detecting a deceleration faster than said predetermined deceleration rate threshold for a second front wheel and a second rear wheel.

3. The method of claim 2 wherein said aggressive braking strategy includes applying a braking pressure greater than a predetermined braking pressure to a first front vehicle brake and applying a braking pressure less than said predetermined braking pressure to a second front vehicle brake and to a first rear vehicle brake and a second rear vehicle brake.

4. The method of claim 2 wherein said aggressive braking strategy includes applying a braking pressure greater than a predetermined braking pressure to a first front vehicle brake and a second rear vehicle brake and applying a braking pressure less than said predetermined braking pressure to a second front vehicle brake and a second rear vehicle brake.

5. The method of claim 1 wherein said non-aggressive braking strategy includes applying a braking pressure less than a predetermined braking pressure to said respective vehicle brakes.

6. The method of claim 1 wherein applying a steering assist torque includes applying a torque overlay to said steering system.

7. The method of claim 1 wherein determining a steering wheel angle condition includes determining whether a steering wheel condition is within a predetermined steering wheel angle threshold.

8. The method of claim 1 wherein determining a steering wheel angle condition includes determining whether a first directional vehicle yaw force produced by a driver in response to said applied steering assist torque is in opposition to second directional vehicle yaw force produced by said slip control braking system.

9. The method of claim 8 wherein said first directional yaw force and said second directional yaw force produce a net yaw force of substantially zero.

10. The method of claim 9 wherein said first directional yaw force and said second directional yaw force produce a yaw angle of substantially zero.

11. A vehicle stability control system of a vehicle comprising:

a slip control braking system for applying independent braking pressure to respective vehicle brakes of said vehicle; and

a steering system for applying steering intervention for stability control;

wherein said slip control braking system detects a split-mu road surface and said steering system applies a steering assist torque in response to detecting said split-mu road surface, said steering assist torque being independent of a steering torque applied by a driver of said vehicle; and

wherein said slip control braking system determines whether a steering wheel angle condition is within a predetermined threshold and applies an aggressive braking strategy if said steering wheel angle condition is within said predetermined threshold, else applying a non-aggressive braking strategy, said steering wheel angle condition comprising a steering wheel angle and a yaw angle of said vehicle.

12. The vehicle stability control system of claim 11 wherein said slip control braking system includes a slip control braking unit and a head control unit, said slip control braking unit controls said head control unit for applying said independent braking pressure to said respective vehicle brakes.

13. The vehicle stability control system of claim 11 wherein said steering system includes an electric assist steering control unit and an electric assist steering motor, said electric assist steering control unit controls said electric assist steering motor for applying said steering intervention for stability control.

14. The vehicle stability control system of claim 11 further comprising a torque and angle sensor for determining a steering wheel torque applied and steering wheel angle of said vehicle.

15. The vehicle stability control system of claim 11 further comprising a speed sensor and a yaw sensor for sensing vehicle speed and yaw of said vehicle.

16. The method of claim 1 wherein said steering system includes an electric steering system.

17. The method of claim 16 wherein said electric steering system includes an electric assist steering system having an electric assist steering motor for providing an assisted steering torque.