Patent ID: 8751111

Claim:
A method of controlling an electric motor in an electric power assisted steering system, the motor being arranged so as to apply a force to part of a steering mechanism of a vehicle, the method comprising the steps of: calculating a force demand indicative of an assistance force dependent upon a torque applied by a user to the steering mechanism that is to be applied to the steering mechanism to reduce the exertion required by the user to steer the vehicle; combining the force demand with an overlay force demand, the overlay force demand being indicative of a force which is desired to overlay over the assistance force, to generate a combined assistance force; and controlling the electric motor according to the combined assistance force, such that together with a force applied by the user, a total force is applied to the steering mechanism, wherein, in order to generate the combined assistance force, the torque applied by the user is subjected to a non-linear boost function, and further wherein the overlay force demand is combined with the force demand such that, for an arbitrary change in the overlay force demand, a change in the total force is independent of the torque applied by the user over a range of torques applied by the user.