Study On Handling Performance And Control Strategy For Electric Power Steering System

With the increasing number of EPS systems installed in vehicles, people pay more attention to the handling performance of the EPS system, which challenges designing such controller with high performance.In this paper, a two-lay structure controller is proposed to improve handling performances in the steering and returning process with uncertain influencing factors. Simulations for the study of the controller have been carried out in the EPS model and verified in the experiments of test bench and vehicle. The content and results are summarized as follows.Firstly, EPS dynamics and simplified vehicle model are established to analysis the influencing factors. This paper developed a two-lay structure controller with main and servo loops to improve the handling performance of EPS system. The main loop consists of steering assist control strategy and return control strategy according to the driving conditions.Secondly, in the steering assist control strategy, a new method to define boost curve in form of piecewise multinomial is proposed, which can provide a good"steering feel"to the driver. However, the boost curve is also a nonlinear factor to the EPS system. In this paper, a time delay-dependent H∞robust fuzzy state feedback controller is adopted to achieve the handling performances in the steering process. Simulation results show that the steering assist control strategy proposed in this paper can achieve the handling performances in the steering process, including steering easiness and road feel, tracking performance, steering smoothness and robust stability.Thirdly, in the return control strategy, a sliding mode controller based on a reference model is adopted to deal with the parameters uncertainty, external disturbance and nonlinear frictions. A sliding mode observer is developed to estimate the immeasurable states. Simulation results show that the return control strategy proposed in this paper can achieve fast and accurate return with low vehicle speed and handling stability of the vehicle with high speed.Fourthly, a PID controller with disturbance observer designed in servo loop to weaken the effect of the frictions and disturbance. Simulation results show that the proposed controller in servo loop can suppress the disturbance well and the tracking performance to the target current is improved.Lastly, a two-lay structure controller with the proposed control strategies is development, including the hardware and software. A test bench is established to debug the controller. Experiments for steering easiness and tracking performance are carried out in the test bench. An EPS system is assembled with the designed controller and installed in a car. Steering and returnability maneuver experiments are carried out in the car. Experimental results validate the EPS system and vehicle models and prove that the proposed control strategies can get better handling performance.In this paper, a two-lay structure controller is studied and realized to achieve the handling performances of EPS system with uncertain influencing factors. Simulations and experiments have been carried out and show that the control strategies proposed in this paper could meet the requirement of the handling performances better than that of the traditional one-lay structure PID controller.