Method of controlling motor driven power steering system

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

• determining failure of a torque sensor according to existence of a torque sensor signal input from the torque sensor;
• when it is determined that the torque sensor has failure, receiving signals from a steering angle sensor and a motor angle sensor and calculating one or more values relevant to a motor torque command based on computations done by controllers;
• and
• driving a motor of the MDPS based on the one or more values relevant to the motor torque command calculated by the controllers,
• wherein the one or more values relevant to the motor torque command comprise a motor output value,
• wherein the controllers receive a steering angle sensor signal from the steering angle sensor and a motor angular speed sensor signal from the motor angle sensor, and calculate the motor output value,
• wherein the motor output value is calculated by multiplying an integral value of a difference between a steering angular speed output value and a value obtained by multiplying a reduction gear ratio and a motor angular speed by a predetermined hardness value of a torsion bar, and then multiplying that result by a predetermined first tuning value,
• wherein the one or more values relevant to the motor torque command further comprise an assist maintaining output value that is to be added to or subtracted from the motor output value,
• wherein the controllers receive the steering angle sensor signal, a lateral acceleration signal, and a yaw rate signal, and calculate the assist maintaining output value based on the steering angle sensor signal, the lateral acceleration signal, and the yaw rate signal,
• wherein the assist maintaining output value is calculated further based on a predetermined second tuning value that is a weighting factor for a vehicle speed and to be applied to the steering angle sensor signal, the lateral acceleration signal, or the yaw rate signal.