A Study On Return-To-Center Control For Automotive Electric Power Steering

In recent years, more and more vehicles have been equipped with electric power steering system(EPS)for its energy saving, environmental protection, active safety and good manipulation stability. The control mode of EPS developed from the previous assistant control to return-to-center control, maximum current control and compensating control etc.The control functions have been further enhanced.The vehicle is driven under complex condition and the alignment torque changes significantly on different road. it leads to insufficient/overshoot return ability, which has an important influence on driving safety and manipulation stability. In view of this situation, the thesis chose pinion-assist type EPS as the research object, mainly to study the return-to-center control algorithm.Return-to-center control based on the estimation of actual aligning moment is discussed to overcome the negative effects of adhesion coefficients on return ability.The EPS dynamic model was established. The ideal aligning moment is determined according to speed and steering wheel angle. The motor speed is estimated based on motor model with variable resistance and back-electromotive force(EMF) coefficient,and the actual aligning moment is obtained by vehicle dynamics equations with the motor current, estimated motor angular velocity and measured steering torque. With the difference between the actual and ideal aligning moment, the return to center compensation is implemented by a segmented proportional-integral-derivative(PID)feedback controller.The hardware of EPS ECU consisted of core circuit of MCU, power circuit,speed signal processing circuit, motor driving circuit and communication circuit of CAN. Hardware in loop(HIL) simulation bench was developed, which consisted of Carsim, Labview and real steering equipment. Based on the developed hardware in loop simulation bench, a contrast experiment were conducted, which consisted of the control algorithm based on the estimation of actual aligning moment and the control algorithm based on the steering wheel angle. Experimental results show that the control algorithm based on the estimation of actual aligning moment aids the steering wheel returning to center position accurately and quickly, which improve steering-wheel return ability and driving safety.