Patent ID: 8434583

Claim:
An electric power assisted steering system comprising: a steering shaft which connects a handwheel through a steering gear to a pair of roadwheels of a vehicle; a torque sensor producing at least one output signal indicative of a torque applied to the handwheel by a driver; a motor which is connected to a part of the steering system to provide a motor torque to the steering system in order to reduce the torque that the driver needs to apply to the handwheel to steer the vehicle; and a motor controller which is adapted to receive the at least one output signal and to generate a motor torque demand signal whose value is indicative of the motor torque to be applied by the motor to the steering system, the motor torque demand signal being formed from at least: a driver demand component with a value dependent upon the torque applied by the driver; and a diagnostic component with a value independent of the torque applied by the driver and which varies in value over time in a predetermined manner; and in which the steering system further comprises: a monitor for monitoring the at least one output signal to determine whether the at least one output signal is indicative of a fault by correlating the diagnostic component of the motor torque demand signal with the at least one output signal, the monitor generating a fault indicating signal indicating that the at least one output signal is faulty if the correlation between the diagnostic component of the motor torque demand signal and the at least one output signal falls outside of a predefined range.