Electric power steering apparatus

An electric power steering system comprising:

• a shaft angle sensor configured to output a signal indicating a rotation angle of a shaft through which a force of a turning operation of a steering wheel is transmitted to a steering mechanism;
• a motor angle sensor configured to output a signal indicating a rotation angle of a motor that generates a steering assist force;
• a temperature sensor configured to detect a temperature near the steering mechanism;
• and
• a control section configured to:
• (1) thermally correct the output of the shaft angle sensor and the output of the motor angle sensor, based on the temperature detected by the temperature sensor, (2) determine the number of turns of the steering wheel based on both the output of the shaft angle sensor and the output of the motor angle sensor, and (3) maintain a fixed speed reduction ratio between the output of the shaft angle sensor and the output of the motor angle sensor regardless of temperature changes, wherein:
• when determining the number of turns, the control section detects an angle error between the output of the shaft angle sensor and the output of the motor angle sensor such that;
• (i) when the angle error falls within a predetermined allowable range, the control section determines the number of turns, and (ii) when the angle error is out of the predetermined allowable range, the control section avoids determining the number of turns, and
• the thermal correction of the output of the shaft angle sensor includes correcting an actual detected pinion angle output based on mapped data that provides a correlation between a change in the shaft angle and a change in temperature.