Vehicle steering system comprising a degraded operating mode in case of breakdown of a wheel steering actuator

Abstract

Steering control system for a land vehicle with at least four steered wheels (AvG,AvD,ArG,ArD), the system comprising one actuator per steered wheel (AvG,AvD,ArG,ArD). The system comprises a control member () available to a driver and delivering a demanded steering angle (θ), a steering control unit () which, by way of input variable, uses at least the said demanded steering angel (θ) to determine a steering control angle for operating the said actuator, and means () for detecting a lock-up of one of the steered-wheel actuators, which are capable of delivering an alarm signal identifying an actuator that has locked up. The steering control unit has at least one normal mode and at least one degraded mode in the event of an actuator becoming locked up, the degraded mode calculating the steering control angle (αp) of the non-locked wheels by imposing a first degraded-mode strategy whereby the instantaneous center of rotation (CIR) of the vehicle lies more or less on the straight line perpendicular to the plane of the locked wheel and passing through the center of the ground-contact area of the locked wheel, in a position that is dependent on the demanded steering angle (θ).