Wheeled vehicle steering system for steering the rear wheels of a vehicle

Claim 1

A steering system for steering the rear wheels of a vehicle having rear wheels with a controllable variable steering angle comprising: an actuator for steering the rear wheels; sensor means for measuring vehicle parameters and generating signals indicative thereof, the sensor means including a sensor which measures displacement of a manually operable steering element of the vehicle, a sensor which measures vehicle speed, a lateral accelerometer, and a yaw rate gyrometer which measures yaw rate of the vehicle; and a control unit for controlling the actuator to steer the rear wheels comprising a processor which processes the signals generated by the sensor means and controls the steering angle of the controllable variable rear wheels accordingly, the processor determining a desired yaw rate as a function of the measured steering element displacement and the measured vehicle velocity, wherein the control unit further comprises detection means to detect when the signals generated by sensor means indicate that the vehicle is sliding, override means which modifies the operation of the control system when sliding is detected, a closed yaw rate error control loop feedback means for controlling the steer angle of the rear wheels having means for generating an error signal from a difference between the measured yaw rate and the determined desired yaw rate, and means for multiplying the error signal by a variable gain to generate a signal for controlling the controllable variable steering angle of the rear wheels, and wherein the detection means detects sliding by determining whether the lateral acceleration measured by the lateral accelerometer and the measured steering element displacement are of opposite sense, the detection means combines the signals indicative of yaw rate, steering angle and lateral acceleration according to a preprogrammed algorithm, to generate a control variable which is compared with a preprogrammed value, the override means only altering the variable gain of the control system when the detection means detects that the control variable exceeds the preprogrammed value, and the override means alters the variable gain in the yaw rate error control loop when sliding is detected.