Motor-driven power steering apparatus

A motor-driven power steering apparatus driving an electric motor by a motor driving structure, converting rotation of the electric motor into a linear stroke of a rack shaft by a power transmission mechanism, and steering and assisting a wheel coupled to the rack shaft, comprising;

• a correction coefficient memory previously storing an output correction coefficient to be multiplied by a current control signal applied to the electric motor for at least one point in a range of temperatures in a such a manner as to correct a temperature dependent output torque which changes in dependence upon an environmental temperature of the electric motor to a predetermined demand output torque required in the electric motor;
• wherein values for the temperature dependent output torque are calculated and established as a function of temperature;
• a temperature sensor detecting the environmental temperature of the electric motor;
• a correction coefficient computing circuit reading the output correction coefficient corresponding to the temperature detected by the temperature sensor from the correction coefficient memory;
• and a current control computing element calculating a corrected current control signal by multiplying the current control signal of the electric motor computed on the basis of a steered condition of a vehicle by the output correction coefficient read from the correction coefficient computing circuit, and outputting a drive control signal based on the corrected current control signal to the motor driving structure, wherein the temperature dependent output torque is obtained by feeding a constant current to each of the electric motors of the same standard product, changing the environmental temperature between a low temperature range and a high temperature range, measuring the motor output torque at each of the temperatures, and averaging a plurality of output torques obtained at the respective temperatures per the temperatures.