Electric power assisted steering system with friction compensation and method for controlling the system

An electric power assisted steering system for a vehicle, the electric power assisted steering system comprising:

• a steering gear having a rack bar and a housing, the rack bar being movable linearly relative to the housing for turning steerable wheels of the vehicle, movement of the rack bar being resisted by friction;
• an electric motor being connected with the rack bar of the steering gear, energization of the electric motor causing linear movement of the rack bar relative to the housing;
• a vehicle speed sensor for monitoring speed of the vehicle and generating a signal indicative of the vehicle speed;
• a torsion sensor for monitoring torsion across a torsion bar of the steering gear and generating a signal indicative of the torsion;
• and
• a controller for receiving the vehicle speed and torsion signals and, in response to the received signals, for generating a motor control signal for controlling the electric motor,
• the controller storing data indicative of the friction resisting movement of the rack bar relative to the housing of the steering gear, the controller modifying the motor control signal to compensate for the friction.