Electrical Power Assisted Steering System

An electric power assisted steering apparatus comprising a steering mechanism which operatively connects a steering wheel to road wheels of a vehicle, the steering wheel having a centre position corresponding to a movement of the vehicle straight ahead, an electric motor operatively connected to the steering mechanism;

• a torque signal generator adapted to produce a torque signal indicative of the torque carried by a portion of the steering mechanism, a column position signal generator for producing a column position signal indicative of an angular position of the steering wheel or steering column and a signal processing unit adapted to receive the column position signal produced by the column position signal generator and to produce therefrom a torque demand signal representative of a torque to be applied to the steering mechanism by the motor, in which the signal processing unit includes:
• a first signal processing circuit which receives an input signal dependent on the torque signal and outputs an assistance torque signal, the relationship between the input and the output of the first signal processing circuit being defined by a boost curve,
• a second signal processing circuit which in use applies an additional offset torque to at least one of the input signal and the output signal of the first processing circuit in the event that the steering wheel of the vehicle is moving back towards the centre position following a period of movement of the steering wheel away from the centre position, the additional offset torque being reduced or not being applied in the event that the steering wheel moves away from the centre position;
• and
• a torque demand signal generating circuit adapted to produce the torque demand signal from the assistance torque signal.