Method of sensing excessive slip in a wheel slip control system

#This invention relates to a method of sensing an excessive slip condition for a vehicle slip control system.

#When a torque is applied to a vehicle wheel for braking or driving the wheel, a force is generated between the wheel and the road surface tending to decelerate or accelerate the vehicle. This force is dependent upon various parameters, including the road surface conditions and the amount of slip between the wheel and the road surface. As the torque input to the wheel is increased, the force between the tire and the road surface increases as wheel slip increases, until a critical slip value is surpassed. When the slip exceeds this critical slip value, the force between the wheel and the road surface decreases. Further, as wheel slip increases, the lateral forces acting on the slipping wheel decrease.

#Various methods have been proposed for preventing this excessive slip condition of the wheels of a vehicle. In the case of acceleration slip, these methods include a traction controller that adjusts engine torque and/or the application of the brakes of the driven wheels when an excessive slip condition is sensed. In the case of vehicle braking, a wheel-lock controller limits brake pressure when an excessive slip condition is sensed. Whatever method is selected for controlling wheel slip, it is an objective of these systems to (a) maximize the longitudinal driving or braking forces and (b) to continuously maintain lateral

#The invention may be best understood by reference to the following description of a preferred embodiment and the drawings in which:

#FIG. 1 is a schematic diagram of a vehicle traction control system; and

#FIGS. 2-15 are flow diagrams illustrating the operation of the traction controller of FIG. 1.

#The invention is described as applied to a traction control system. However, it is also applicable to anti-lock braking control systems.

#A traction control system for a front wheel drive vehicle is illustrated in FIG. 1. The vehicle has left and right front driven wheels 10 and 12 and left and right non-driven wheels 14 and 16. The front wheels 10 and 12 have respective hydraulic actuated brakes 18 and 20 actuated by manual operation of a conventional master cylinder 22 through a pair of traction control pressure actuators 24 and 26. When the actuators 24 and 26 are inactive, the hydraulic fluid from the master cylinder 22 passes through the actuators 24 and 26 to the brakes 18 and 20 of the wheels 10 and 12. Thus, the actuators 24 and 26 are transparent to the braking system during normal braking of the wheels 10 and 12. The actuators 24 and 26 may each take the form of a motor driven piston for regulating the braking pressure during traction control as illustrated in the U.S. Pat. No. 5,025,882 issued Jun. 25, 1991 and assigned to the assignee of this invention.

#The rear wheels 14 and 16 also include a pair of hydraulic actuated brakes 28 and 30 operated by hydraulic fluid under pressure from the master cylinder 22 in response to manual actuation of the brakes.