A Development of the Model Based Torque Feedback Control with Disturbance Observer for Electric Power Steering System

Electric Power Steering (EPS) needs to meet both functional and stability requirements, it plays significant role in controlling vehicle motion. In the meantime, customers emphasizes natural steering feel which can reflect vehicle motion and road surface information while isolate unwanted external disturbances. In general, conventional EPS control algorithms exert assist torque according to driver torque measured from torque sensor, while maintaining stability using stabilizing compensator. However, there exist significant trade-off between steering feel and stability, because the performances of assist torque control and stabilizing compensator are strongly coupled. In this paper a torque feedback control algorithm for EPS system is proposed in order to overcome the trade-off, and to achieve more natural, robust steering feel. The torque feedback algorithm consists of two main components; target torque generator, target torque following controller which can decouple steering feel and stability control performances. Target torque generator defines desired driver torque based on estimated road reaction torque; the road reaction torque estimation is carried out using EPS system dynamics model. Target torque following controller based on lead-lag compensator controls motor torque to achieve desired driver torque, while maintaining overall control stability. Disturbance observer (DOB) is introduced to improve target torque following performance and control robustness against unwanted external disturbances. Subjective assessment and objective measurement of vehicle steering performance shows that proposed algorithm can achieve natural steering feel and it can efficiently feedback vehicle driving condition such as road surface information.