Electric power steering control system and control method thereof

An electric power steering control system arranged to control a drive current of a brushless motor that generates a steering assist force on the basis of a steering torque acting on a steering shaft and a vehicle speed, comprising:

• a d-q-axis current command section for computing a d- and q-axis current command value for generating a steering assist force in response to a predetermined characteristic on the basis of said steering torque and said vehicle speed;
• an electrical angle computing section for computing an electrical angle signal on the basis of a rotational position signal of said brushless motor;
• a d-q coordinate transformation section for performing a d-q coordinate transformation on the basis of a detection current signal in response to a three-phase AC current to be inputted to said brushless motor and of said electrical angle signal, thereby obtaining a d- and q-axis detection current value;
• a d-q axis voltage command section for computing a d- and q-axis voltage command value in response to a deviation between said d- and q-axis current command value and said d- and q-axis detection current value;
• a d-q coordinate inverse transformation section for performing a d-q coordinate inverse transformation on the basis of said d- and q-axis voltage command value and said electrical angle signal thereby computing a three-phase voltage command value;
• a three-phase voltage command compensation section for performing compensation with respect to said three-phase voltage command value according to a computing expression for improving efficiency of utilizing a previously set voltage, thereby computing a three-phase voltage command compensation value;
• and
• a PWM output section for generating a PWM waveform for PWM drive of said brushless motor in response to said three-phase voltage command compensation value.