Four-Wheel Steering And Direct Yaw Moment Control

With the rapid development of automobile industry technology and the improvement of traffic conditions,the driving speed of vehicle is also getting higher and higher.People pay more and more attention to the safety of driving and the stability of driving operations under high speed extreme conditions.Four-wheel steering(4WS)technology improves vehicle stability by actively controlling the front and rear wheel angles of the vehicle,the direct yaw moment control overcomes oversteering or understeering by controlling the driving force or braking force of the wheels to generate an additional yaw moment,thereby actively controlling the steering stability of the vehicle.Therefore,this paper takes the four-wheel steering vehicle as the reserach object and deeply studies the strategy based on the combination of four-wheel steering and direct yaw moment control.Firstly,seven degrees of freedom(7DOF)nonlinear 4WS vehicle dynamics model is set up by using vehicle standard coordinate system At the same time,a wheel dynamics model is established to analyze the rotational force state of the wheel.Considering the influence of the tire's nonlinear characteristics,a nonlinear dugoff tire model is introduced.They offer a favorable simulation modeling platform for verifying the effectiveness of the controller.Secondly,to improve the stability of the vehicle under high speed operation,a control strategy based on the combination of four-wheel steering and direct yaw moment is designed.Taking the yaw rate and side slip angle as the control variables,the controller of the sliding mode controller is designed to generate the additional yaw moment and the rear wheel angle required by the vehicle steering,and the yaw moment is distributed to the four wheels of vehicle according to the one-side braking method.Aiming at the seven degrees of freedom model,the time-domain simulation analysis is performed under sinusoidal conditions with high and low surface coefficients.The results of simulation show that the control strategy based on the combination of direct yaw moment and four-wheel steering is effective on reduction of sideslip angle and tracking the ideal yaw rate,improves the handling stability and verifies the effectiveness of yaw moment distribution.Finally,when use the one-side braking method,the longitudinal speed of vehicle is reduced greatly and the additional torque required for each wheel is large,so that the required yaw moment can not be provided under emergency conditions.So the improved method of coordinated drive braking is used to distribute the yaw moment generated by the controller to four wheels.Under the same simulation conditions,compared with the one-side braking method,the simulation results show that the improved distribution method has a small change in the longitudinal speed of vehicle,the additional yaw moment required by each wheel is reduced,and the yaw moment output capability of vehicle is improved,at the same time the yaw rate and side slip angle also track the ideal reference model well,which verifies the effectiveness of the improved distribution method and improves the steering stability of the vehicle furtherly.