Vehicle brake system and method of control

1. A method of controlling a vehicle brake system comprising:

applying a friction brake to slow rotation of a vehicle wheel; and

pulsing a secondary brake that provides a secondary brake torque that slows rotation of the vehicle wheel by decreasing torque that is provided to the vehicle wheel to rotate the vehicle wheel when a wheel slip condition is detected.

2. The method of claim 1 wherein the secondary brake is pulsed at a constant rate when the wheel slip condition is detected.

3. The method of claim 2 wherein pulsing of the secondary brake at the constant rate is terminated when the wheel slip condition is no longer detected.

4. The method of claim 2 wherein pulsing of the secondary brake occurs in response to a constant frequency control pulse that is generated by a controller.

5. The method of claim 2 wherein the secondary brake is not pulsed at the constant rate when the wheel slip condition is not detected.

6. The method of claim 2 wherein the secondary brake is not pulsed at the constant rate when the friction brake is not applied.

7. The method of claim 1 wherein the secondary brake is applied in proportion to a driver brake command when the wheel slip condition is not detected.

8. A method of controlling a vehicle brake system comprising:

applying a friction brake torque with a friction brake to slow rotation of a vehicle wheel;

determining whether a wheel slip condition is present; and

applying a secondary brake torque with a secondary brake in proportion to a wheel slip error when a wheel slip condition is present, wherein the secondary brake torque slows rotation of the vehicle wheel by decreasing torque that is provided to the vehicle wheel to rotate the vehicle wheel.

9. The method of claim 8 wherein the wheel slip condition is present when a wheel slip ratio of the vehicle wheel exceeds a slip reference value.

10. The method of claim 9 wherein the wheel slip ratio is based on a wheel speed and a vehicle speed.

11. The method of claim 9 wherein the slip reference value is a constant.

12. The method of claim 9 wherein the wheel slip error is based on a difference between wheel slip ratio and the slip reference value.

13. The method of claim 8 wherein the wheel slip condition is detected while braking the vehicle.

14. The method of claim 8 wherein the secondary brake is pulsed at a variable rate when the wheel slip condition is detected.

15. A vehicle brake system comprising:

a friction brake that is disposed proximate a vehicle wheel and that is configured to provide a friction brake torque to slow rotation the vehicle wheel;

a secondary brake that is configured to provide a secondary brake torque that slows rotation of the vehicle wheel by decreasing torque that is provided to the vehicle wheel to rotate the vehicle wheel; and

a controller that controls the friction brake and the secondary brake such that the friction brake torque and the secondary brake torque are applied when a wheel slip condition is detected.

16. The system of claim 15 wherein the secondary brake is activated and deactivated by the controller at a constant rate when the wheel slip condition is detected to provide the secondary brake torque.

17. The system of claim 15 wherein the secondary brake torque is based on a wheel slip error that is based on data from a vehicle speed sensor and a wheel speed sensor and a predetermined slip reference value.

18. The system of claim 15 wherein the friction brake torque and the secondary brake torque are provided by pulsing at least one of the friction brake and the secondary brake, respectively, when the wheel slip condition is detected.

19. The system of claim 15 wherein the secondary brake torque is provided by a driveline retarder.

20. The system of claim 15 wherein the friction brake is configured as a drum brake or a disc brake.