Research On Control Strategy Of Active Four Wheel Steering Vehicle Based On Steering-by-wire Technology

The four-wheel steering technology is a direct,fast and efficient vehicle active safety technology to improve the handling and stability of the vehicle.With the development of the X-by-wire technology in the field of automobile,four-wheel steering technology has developed from the traditional active rear wheel steering to active four-wheel steering by-wire,and has realized the active steering control of the front and rear wheels.Furthermore,it has optimized the lateral dynamic response characteristic of the car.This thesis focuses on control strategy of active four-wheel steering by-wire,so as to deal with the uncertainty factors which affect the maneuverability of the vehicle.Firstly,the nolinear eight DOF(degree-of-freedom)active four-wheel steering dynamic al model whih the motion of longitudinal,lateral,yaw,roll and four-wheels rotation is established.Dugoff tire model is used to analyze the force situation of the tire.A simulation platform whit the nolinear eight-DOF active four-wheel steering model is built in Matlab/Simulink.Finally,A sixteen-DOF active four-wheel steering model is constructed under the AMEsim simulation platform.In this paper,the accuracy of the nonlinear eight-DOF model is tested by the comparison experiment.In order to improve the handing stability of active four wheel steering vehicle based on the Steer-by-Wire technology,a nonlinear triple-step method is adopted to the linear active four wheel steering systems so as to track the output of the ideal reference model.The proposed scheme consists of three parts: steady-state-like control,feedforward control which considers the variation of the reference signal,state-dependent error feedback control.The method guarantees that the actual sideslip angle and yaw rate can track the ideal sideslip angle and the ideal yaw rate by the control of the front wheel angle and rear wheel angle of active four wheel systems.The nonlinear eight-DOF vehicle model is used to verify the effectiveness of the proposed scheme.Simulation results show that the designed controller can track the output of ideal reference model,and improve the handing stability of the active four-wheel steering system.Aiming at the influence of uncertainty factors on vehicle handling stability and active safety,an active four-wheel steering model by-wire is established which considers tire cornering stiffness perturbation and lateral wind disturbance.According to the uncertainty model,a global sliding mode controller is designed to suppress and eliminate the influence of uncertainties.The robustness of the closed-loop system is analyzed and verified.The simulation results show that the active four-wheels steering vehicle by-wire under the control of global sliding mode can track the ideal reference model accurately,and has strong robustness.It effectively overcomes the influence of tire nonlinear variation,lateral wind burst and road adhesion reduction on the lateral dynamic characteristics of the vehicle,and the performance of controller is better than the active four-wheels steering vehicle by-wire under the control of LQR(linear quadratic regulation).The preview optimal curvature fuzzy PID driver model is established,and realizes the skilled driving operation of the active four-wheels steering vehicle by-wire.A human-vehicle-road closed loop system is built,It makes a evaluation for the ability to improve the handling stability,and active safety of both the triple-step method controller and the global sliding mode controller.The closed-loop simulation results show that both of the two controllers designed in this paper are effective to improve the maneuverability of the active four-wheels steering vehicle by-wire.