Patent ID: 8285451

Claim:
A method for controlling an electric power steering system wherein a d-axis current command value Id and a q-axis current command value Iq are calculated based on a steering assist current command value Iref calculated from a steering torque, a motor is controlled by an inverter with a vector control based on the d-axis current command value Id and the q-axis current command value Iq and is driven to provide a steering assist power to a vehicle steering system, the method comprising: carrying out a field-weakening control on a d-axis of the vector control in accordance with a following equation; Id=−|Iref |·sin(cos −1 (ω b /ω e )) where, ω e is an angular speed of the motor and ω b is a base angular speed, obtaining a first limit value of the steering assist current command value Iref and a first base limit value based on the angular speed ω e and a rated output characteristic of the motor; obtaining a second limit value of the steering assist current command value Iref based on the steering assist current command value Iref and the first limit value; obtaining a second base limit value based on the steering assist current command value Iref and the first base limit value; calculating the d-axis current command value Id based on the angular speed ω e and the second limit value; switching the second base limit value to an output when the angular speed ω e is equal to or smaller than the base angular speed ω b and the second limit value to an output when the angular speed ω e exceeds the base angular speed ω b ; and limiting the steering assist current command value Iref by the switching of the second base limit value and the second limit value.