Simulation Research On Control Strategies For Active 4WS Vehicle Based On The Steer-by-Wire Technology

The four-wheel steering (4WS) system is an effective active control technology for improving maneuverability and safety of vehicle in common use. In order to take full advantage of 4WS vehicle, on the basis of the development of steer-by-wire (SBW) technology, the characteristics of vehicle lateral dynamics and its control strategies are regarded as the research direction by this thesis, and the influences of linearity, uncertainty and nonlinear on vehicle dynamics are also fully considered by this thesis, then, a systematic research on control strategies for active 4WS vehicle is carried out.In the beginning, the dynamics analysis of 4WS vehicle is carried out and its nonlinear differential equation is established, then the calculation method of this nonlinear model is studied and implemented by using the MATLAB/Simulink software. All above works provide a real-time simulation platform of chassis control system with higher accuracy and quicker runtime that can verify different control strategies and control effects for 4WS vehicle. Furthermore, a virtual prototype of 4WS vehicle is founded by using the ADAMS software, and the viewable simulation of this virtual prototype combined with the controllers designed by MATLAB/Simulink is also realized by using ADAMS/Controls block.By means of the SBW technology and optimum control theory, a control strategy which need follow an ideal model is proposed for active 4WS vehicle, and this ideal vehicle model followed is determined too. Subsequently, the optimum controller that is used to control the angles of front and rear wheels of 4WS vehicle is designed. The open loop simulation indicates that the optimum controller can decrease the sideslip angle, and can keep the desired yaw rate at the same time during a steering process. The close loop simulation after introducing driver model shows that the 4WS vehicle under optimum control has better accuracy in path deviation and better evaluation index of active safety, and has an intelligently assistant function for manipulate of driver. So, the maneuverability when vehicle is driving at higher speed is improved.A sliding mode variable structure control strategy (SMC) is studied when 4WS vehicles are disturbed by some uncertain elements. The SMC controller is designed for active 4WS vehicle by treating the cornering stiffness of the front and rear tires and outer disturbance as uncertain parameters, but their variance in a limited range, and by using a linearity model as an ideal target followed. This SMC controller can maintain the vehicle with near zero sideslip angles, and track desired yaw rate, the controlled vehicle system behaves favorable robustness: The simulation test of a closed-loop system indicates that the SMC controller can overcome the effect of parameters perturbations and outer disturbances on system stability, and can adapt variance of the road adhesion condition to a certain extent. Furthermore, a state estimate method for sideslip angle and yaw rate of vehicle is discussed by using the measurable lateral acceleration signal based on the Kalman filtering principle.Because the 4WS system has limited effect on stability of vehicle when the sum of all tyre side force arrive maximum, so, a vehicle stability control strategy (VSC) is proposed for 4WS vehicle, a fuzzy controller of VSC system with multi-input and multi-output variables is designed by combining active rear-wheel steering with differential braking technology. Furthermore, the distributing of membership function and scale parameters of fuzzy controller are optimized by using Genetic Algorithms and the optimized results indicate that the transient response of vehicle is improved. The open loop simulation shows that the VSC system can ensure effectively the stability, and increase the active safety during critical steering process of vehicle.A kind of coordination control project between the 4WS steering controller and VSC stability controller is researched. Firstly, whether or not the vehicle lost its stability is estimated by using phase plane of state variables. Secondly, the valid action ranges of different controller are determined, and the control signals of different controller are also assigned by using fuzzy method. With all of above works, the conflict of control goal between the two controllers and the hard switch action of rear wheels are avoided accordingly. At last, a simulation test shows that this coordination control strategy is feasible and effective.