Research On Dynamics And Robust Control Of Four-wheel Steering Vehicle

Four wheel steering (4WS) control system allows greater maneuverability atlow speed and by means of counter-phase rear steering and improves stability athigh speed through same-phase rear steering. The primary research on dynamicsand robust control of four-wheel steering vehicle is done in this dissertation. Theoutline of the dissertation is described as follows.Firstly, this dissertation comprehensively expatiate 4WS system on itsworking theory, development courses, control method and control principle.Then,the development of 4WS vehicle simulation techniques are introduced. And itbriefly states the main content of the dissertation.In chapter two, the steering response characteristic of 4WS is sdudied in detail.Firstly, the steering characteristic of 4WS in low speed and high speed isintroduced, and then the performance of 4WS vehicle is compared with that oftraditional 2WS (front wheel steering) vehicle. A mathematical model with twodegrees (yaw rate and sideslip angle) of 4WS vehicle is derived. Another mathmodel with three degrees (yaw rate, sideslip angle and roll angle) is also described.Taken front tire turning angle as step input, high speed controllability and stabilityin response to both front and rear steering input can be theoretically discussed by2DOF model with the help of the feedforward and feedback control style. Thedynamics of sideslip angle, yaw rate and lateral acceleration is analyzed. Moreover,according to control target of 4WS system, active rear wheel steering is selected asthe control principle.Chapter three researches H ∞ robust control method. The development andcharacteristic H ∞ robust control theory are reviewed. Then, the method of linearMatrix Inequality (LMI) and its theorem are introduced. A two degree of freedom(2DOF) 4WS vehicle model with external disturbance is presented, and the vehicledynamic characteristic analysis is done. Moreover, the optimization controllingdesign is accomplished with the help of the method of H ∞ robust control theoryand the feedforward and feedback control style. And the H ∞ optimal controller isdesigned by the LMI control tool box in MATLAB. The effectiveness of theproposed control strategies is verified by the simulation software MATLAB. Greatcontrol effect is gained.In chapter four, the problem of H 2 /H∞ mixed control with the output as thefeedback is described. Based on the theory mentioned above, the optimizationcontrolling design is accomplished with the help of the method of H 2 /H∞ mixedcontrol theory and the feedforward and feedback control style. The H 2 /H∞mixed optimal dynamic controller with the output as the feedback is designed bythe LMI control tool box in MATLAB. The effectiveness of the proposed controlstrategies is verified by the simulation software MATLAB. Great control effect isgained.In chapter five, the problem of H 2 /H∞ mixed control with the state as thefeedback is described. Based on the theory mentioned above, the optimizationcontrolling design is accomplished with the help of the method of H 2 /H∞ mixedcontrol theory and the feedforward and feedback control style. The H 2 /H∞mixed optimal static controller with the state as the feedback is designed by theLMI control tool box in MATLAB. The effectiveness of the proposed controlstrategies is verified by the simulation software MATLAB. Great control effect isgained. Compared with the simulation results of H ∞ robust control menthod andH 2 /H∞ mixed control method with the state as the feedback, the simulation resultof H 2 /H∞ mixed control method with the output as the feedback shows greatrobust and performance of 4WS system.The last chapter summarizes the research work in this dissertation, and lists thefuture works in the area.In conclusion, based on the external disturbance, in this paper, we do thecontrol design with modern robust theory. Excellent performance is obtained byH 2 control. External disturbance is attenuated perfectly and the system robustperformance is enhanced effectively by H ∞ control.