Electrical Power Assisted Steering System

An electric power assisted steering system comprising a steering mechanism which operatively connects a steering wheel to road wheels of a vehicle, an electric motor operatively connected to the steering mechanism;

• a torque sensor adapted to produce a torque signal indicative of torque carried by a portion of the steering mechanism, and a signal processing means operable in a normal mode when an output signal of the torque sensor is reliably available and a fault mode when it is not, in which the signal processing means includes:
• a signal processing means which in the normal mode is adapted to generate an assistance torque signal that is dependent on the torque applied by the driver, and during a fault operation is adapted to generate an assistance torque signal that is independent of the driver applied torque, the signal processing means including a first sub-unit which in the fault mode generates a first term of the assistance torque signal that has a value that varies as a function of an angular position of the steering relative to a variable datum, the variable datum being reset when the direction of the rotation of steering reverses.