Assist characteristics design is one of the key technologies of ECHPS system.For the problem of poor steering safety in high speed caused by HPS with fixed assist characteristics in heavy commercial vehicle, design requirement of alterable assist characteristics is analyzed, and simulation models of ECHPS system, including mechanical model ,hydraulic model and three degrees freedom vehicle dynamic model are established by taking the bypass flow ECHPS of heavy commercial vehicle as the research object. Assist characteristics of ECHPS under different speeds are achieved by simulation models.Simulation results show that, ECHPS system can not only improve the steering easiness at low speed but also keep good road feel and handling stability at high speed.

As a tool of modeling the system of multiple energy, bond-graph theory is applied to the modeling for electric power steering system which is electromechanical hybrid. Bond graph models for C type, P type and R type of EPS dynamic system are built respectively. The CH7140 type of car equipped with C type EPS is taken as a prototype and its maneuverable performance is simulated under typical working conditions. A PID control method with time delay transfer function is adopted to imitate the driver’s operation. Steering maneuverability test of the CH7140 car is conducted. The simulation result is consistent with the test result. It is shown that the bond-graph model of EPS is rational, and simulation model derived from bond-graph is effective which can be applied to EPS and full vehicle steering maneuverability simulation research.

In order to realize the curve-type assistance characteristic of the electric power steering system , the parabola-type assistance characteristic is brought out, and realizing it by designing the electric power steering system which is controlled by computer. And then math model of computer control system is constructed, and adopting Z-transform to analysis the stabilization of the system. The result shows that computer control system can realize the curve-type assistance characteristic.

Most Automotbile Electric Power Steering (EPS) controller designs are based on a simplified accurate model,however,EPS controller is affected by many nonlinear friction and damping easily, such as road condition,sensor noises and the lateral wind disturbance.These uncertainties affect the accuracy of assist current, the EPS performance and the driving safety. Aimed at the nonlinear MIMO system of electric power steering system,the mechanism and dynamic characteristic of EPS is analysed,and EPS model is developed. Then the fuzzy-neural network controller is designed and the corresponding simulation is performed.The results show that the proposed EPS control strategy can provide good performance of stability and controllability and can increase the anti-jamming capability of vehicle.

Automotive EPS controller is easy to be affected by the parameter perturbations, road agitations, sensor noises and the lateral wind disturbance in the process of vehicle operation. These uncertainties affect the accuracy of assist current, the EPS performance and the driving safety. The mathematical model of EPS system was established including the internal uncertainty and external disturbance. The LFT linear fractional transform representing model and M matrix with stripped uncertainty were designed. The robust H∞ controller and weight function were designed according to the H∞ control theory and performance indexes of EPS system. The comparative performance with PD controller was made. The results showed that the H∞ controller had better robustness and robust stability than the PD controller. It also had great anti-disturbance performance to satisfy the performance requirements of EPS system.