Motor-driven power steering system and method of controlling same

A motor-driven power steering system comprising:

• a steering wheel;
• a steerable wheel;
• a steering mechanism interconnecting said steering wheel and said steerable wheel;
• an electric motor applying a steering assistive force to said steering mechanism;
• a plurality of frictional elements of said electric motor and said steering mechanism each being provided with a lubricating material having a temperature-dependent viscosity;
• steering torque detecting means for detecting a steering torque applied by the steering wheel to said steering mechanism and for generating a signal indicative of the steering torque;
• temperature detecting means for detecting the temperature of at least one of said frictional elements and for generating a signal indicative of the temperature;
• motor control signal generating means for generating a motor control signal based on at least the signals from said steering torque detecting means and said temperature detecting means, wherein said motor control signal generating means includes
• road load control signal producing means for determining and producing a road load control signal indicative of a road load based on the signal from said steering torque detecting means,
• temperature-compensated-value producing means for determining and producing a temperature-compensated value corresponding to a loss of assistive power from said electric motor which is caused by a change brought about by the temperature in the friction of said one frictional element, based on the signal from said temperature detecting means, and
• processing means for adding said road load control signal and said temperature-compensate value;
• and
• motor driver means for controlling said electric motor based upon the motor control signal.