Electrical power steering system

An electric power steering apparatus comprising:

• a steering mechanism which operatively connects a steering wheel to road wheels of a vehicle;
• an electric motor operatively connected to the steering mechanism;
• a torque signal generator adapted to produce a torque signal indicative of a torque carried by a portion of the steering mechanism;
• a column angular position signal generator for producing a column angle signal indicative of an angular position of the steering wheel or steering column;
• and
• a signal processing unit adapted to receive the column torque signal and the column angle signal 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 column 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 generates a friction compensating torque signal, the value of the friction compensating torque signal being derived from observations of both the column torque signal and the column angle signal over time;
• and
• a torque demand signal generating circuit adapted to produce a torque demand signal as a function of both the friction compensating torque signal and the assistance torque signal;
• and
• in which the friction compensating torque signal at least partially assists the driver in overcoming the effects of static and/or kinetic friction in the steering mechanism;
• the apparatus further comprising:
• a friction estimator which generates an estimate of the static friction in the steering mechanism;
• and
• in which the friction compensating torque signal generated by the second signal processing circuit is adapted as a function of the estimate of friction.