Patent ID: 8666597

Claim:
An electric power assisted steering system for a vehicle, comprising: a steering mechanism which operatively connects a steering wheel to road wheels of the vehicle; a torque sensor arranged to, in use, produce a torque signal indicative of the torque carried by a portion of the steering mechanism; an electric motor operatively connected to the steering mechanism; a signal processing unit arranged to, in use, produce from the torque signal a torque demand signal representative of a torque to be applied to the steering mechanism by the motor, the signal processing unit comprising a first calculating unit arranged to calculate an initial torque demand signal dependent on the torque signal, and a second calculating unit arranged to, in use, calculate a torque damping signal indicative of an amount the initial torque signal is to be damped in order to generate the torque demand signal; a motor drive stage arranged to provide, in use, a drive current to the motor responsive to the torque demand signal; wherein the second calculating unit comprises an input for a signal dependent upon the torque demand signal, a differentiator, arranged to, in use, differentiate the input signal dependent upon the torque signal and a high pass filter arranged to filter the input signal to reduce low frequency components thereof; wherein the signal processing unit is arranged to, in use, combine the amount of torque indicated by the torque damping signal with the amount of torque indicated by the initial torque demand signal in order to generate the torque demand signal; and wherein the amounts axe combined in an additive manner so as to add, the amount of torque indicated by the torque damping signal to the amount of torque indicated by the initial torque demand signal.