Motor driven power steering system and method of controlling the same

A motor driven power steering system comprising:

• a steering angle sensor configured to sense a steering angle of a steering wheel;
• a motor current sensor configured to sense a current of a motor;
• an encoder configured to sense a position of the motor;
• a vehicle speed sensor configured to sense a speed of a vehicle;
• a virtual torque estimation unit configured to estimate, when an error of a torque sensor is detected, a virtual torque using the position of the motor, the steering angle of the steering wheel, and the current of the motor that are respectively sensed by the encoder, the steering angle sensor, and the motor current sensor;
• an assist torque calculation unit configured to calculate an assist torque for operation of the motor using the steering angle of the steering wheel, the speed of the vehicle, and the virtual torque that are respectively inputted from the steering angle sensor, the vehicle speed sensor, and the virtual torque estimation unit;
• and
• a torque sensor error detection unit configured to compare the virtual torque with a detected torque detected by the torque sensor, and further configured to determine an error of the torque sensor if a difference between the virtual torque and the detected torque exceeds a reference value,
• wherein the virtual torque estimation unit is further configured to detect a steering angle speed, wherein as the steering angle speed increases, the virtual torque estimation unit is configured to estimate the virtual torque by reducing a damping gain value.

A method of controlling a motor driven power steering system, comprising:

• estimating, by a virtual torque estimation unit, a virtual torque using a position of a motor, a steering angle of a steering wheel, and a current of a motor when an error of a torque sensor is detected;
• calculating, by an assist torque calculation unit, an assist torque for operation of the motor using the steering angle of the steering wheel, a speed of a vehicle, and the virtual torque estimated by the virtual torque estimation unit;
• and
• controlling, by a motor controller, a current to be applied to the motor depending on the assist torque calculated by the assist torque calculation unit,
• wherein estimating the virtual torque comprises detecting a steering angle speed of the steering wheel, wherein as the steering angle speed increases, the virtual torque is estimated by reducing a damping gain value,
• wherein detecting the assist torque comprises amplifying a high-frequency signal of the virtual torque at a preset amplification rate or disable the high-frequency signal, and restrain an oscillation phenomenon due to a noise of the virtual torque.