Research On EPS System Assist Control Strategy Based On Fuzzy Theory

Steering system is an important part of the vehicle and steering performance is an important indicator to evaluate the overall performance of the vehicle. The electric power steering system, which is abbreviated as EPS, is developed on the basis of the traditional hydraulic power steering. It can not only guarantee the steering portability and handling stability, but also has the advantages of high safety, good comfort, energy saving and environmental protection, so it is more and more concerned by people. Power assist characteristic is the main control target of EPS system, which has a direct influence on the steering performance of the vehicle. Therefore, this thesis will study the power assist characteristic of the EPS system, and the brushless DC motor was used as the power assist motor. In this thesis, the system parameters are optimized based on the study of the relationship between the structural parameters and the steering performance. The fuzzy theory was used to fit the power curve of the EPS system to get a good steering performance.Firstly, the classification, structure and working principle of EPS system were studied, and the design methods of straight linear, broken linear and curved linear three traditional linear assist characteristic curves of EPS system were analyzed in this thesis. The structure model of EPS system was simplified based on the analysis of the moment distribution of each part of EPS system. And then derives the mathematical models of steering column, gear, DC motor, the vehicle with two degrees freedom and tire. The state space equation of the system was also derived, which laid the foundation for further research of the steering performance.Secondly, this thesis had analyzed the three aspects of performance evaluation indicators, including the steering feel, steering sensitivity and steering stability of EPS system. According to the state space equation of EPS system, the correlative transfer function was deduced. The system parameters that affect the steering performance of the steering feel were optimized by analyzing the rotation inertia of assisted motor, steering shaft stiffness, the power gain and the steering transmission ratio, after optimization the steering feel was good.Thirdly, the fuzzy control theory had been used in this paper to fit the assist characteristic curve. According to vehicle speed and steering wheel torque signal, this paper had designed fuzzy rules which were consistent with the performance of the steering system with the target current signal for the output. And the area bisection method was used to solve fuzzy control and the smoothness of the power curve was verified to be better, there was no sense of jumping when the steering wheel was manipulated. Fuzzy assist characteristic curve was designed by using the fuzzy toolbox in MATLAB and combining with the current closed-loop PID control to guarantee the rapidity, accuracy and stability of the assist current tracking.Finally, the simulation model was established in MATLAB/Simulink environment to validate the power assisted effect and the road feel after the parameter optimization. Results showed that the fuzzy control of assist characteristic could meet the requirements of steering portability, steering sensitivity and steering feel under different steering wheel target input and vehicle speed. The steering feel of fuzzy assist characteristics was obviously better than before the parameter optimization to achieve the desired effect after the parameter optimization.