Controlling motors in electric power assisted steering systems

Abstract

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, in which the method comprises calculating a force demand indicative of an assistance force dependent upon the torque applied by a user (T) to the steering mechanism that it 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 (T) indicative of a force which it is desired to overlay over the assistance force, to generate a combined assistance force (T); and controlling the motor according to the combined assistance force, such that together with the force applied by the user, a total force is applied to the steering mechanism, in which, in order to generate the combined assistance force the torque applied by the user is subjected to a non-linear boost function and in which the overlay force demand is combined with the force demand such that, for an arbitrary change in the overlay force demand the change in the total force is independent of the torque applied by the user over a range of torques applied by the user.