Study On Control Strategy For Electric Power Steering System And Its Influences On Vehicle Handling And Stability Based On Bifurcation Theory

With the high development of the expressway, the speed of modern vehicles is becoming faster and faster. The people's demand on the whole performance of vehicle is also becoming higher and higher. The steering system is effecting on the ride performance of vehicle more and more, such as handling and stability, ride comfort etc. Nowadays, as a most important safety system in the vehicle, EPS system technology has the trend of putting into production. The work is supported in part by grants from the project "Study and Development on the Electric Power Steering System of automobile" sponsored by Jiangsu Province Six Kinds Talents Summit for providing financial support. This thesis focuses on the EPS system as investigated subject by using computer technique based on nonlinear dynamic theory foundation. The virtual co-simulation frame of vehicle equipped with EPS system handling and stability performance test is established. And research work is carried out to the vehicle of nonlinear dynamic equipped with EPS system, the control strategy of EPS system and its influences on vehicle both in theories and in experiments. The principal contents are as following:First, it has been established that driver-vehicle equipped with EPS system closed loop nonlinear handling dynamics simulation model by considering dry-friction simulink model of EPS system and Magic Formula of tire, which has been carried out on nonlinear dynamic characteristic analysis and numerical simulation under autonomous case. The influencing laws of bifurcation vehicle speed on motion stability of closed loop nonlinear system are on discussed. The influence of parameters of the automobile on bifurcation vehicle speed is also being studied. The influence of parameters of the EPS system on bifurcation vehicle speed is investigated on in details.Secondly, based on the basic target and familiar control strategy of EPS system, design methods of the classical PI controlling the current loop of the electromotor are researched and robust H_∞controller of EPS system by H_∞mixed sensitivity is put forward. The simulation results indicate that H_∞mixed sensitivity controller with PI current loop can improve the capability of low frequency noise enhancement and high frequency enervation in wide frequency band, and attenuate noise of torque sensor. The designed control system for EPS system exhibits good robust performances in assist tracking and anti-disturbance, greatly enhances the returnability, driver's steering feel, handling characteristic.Thirdly, the energy-saving method is proposed with PWM control mode for electromagnetic clutch of EPS system, which controls the current of the electromagnetic clutch's coil along with the torque from steering wheel, so that the torque transmitted can be changed as required. Quantity of heat drops because of the current of the electromagnetic clutch's coil diminishing and accordingly reducing the diameter of coil, the weight, the cost. Bench-test results show the proposed method can work well, and at the same time, it can save greatly energy, satisfy the use of requirements, and prevent the pollution of the environment. The configuration schemes about mesh sleeve of electromagnetic clutch based on existing structure have been formed, and the patent has been authorized.Fourthly, driver-vehicle closed loop nonlinear handling dynamics simulation model has been established using MATLAB/Simulik, based on the mathematics model of steering movement of EPS system. The simulation of handling and stability for the vehicle equipped with EPS system is studied. The influence of the structure parameters and control methods of EPS system on handling and stability of the automobile are discussed, such as reduction ratio, twisting bar stiffness of the torque sensor, assist gain and so on.Finally, taking a type of car as an example, the virtual prototype model of the vehicle equipped with EPS system under the environment of ADAMS/Car is assembled, and the controller models of EPS system are set up by using MATLAB/Simulink. The handling and stability of whole vehicle is analyzed based on the co-simulation method with ADAMS/Car and MATLAB/Simulink. The real experiments of handling and stability (such as the steering maneuverability test, the steering returnability, the pylon course slalom test, and so on) are carried out. Compared with the virtual experiment, the result shows that EPS system with robust H_∞mixed sensitivity has a good effect on the handling and stability test. Therefore, the virtual experiment can reflect real experiment.The research methods and study results are useful to develop EPS system with independent intellectual property, settle the match relation with EPS system and handling and stability of whole vehicle for guidance purpose and reference merit, and facilitate the development of EPS system in our country.