Research On Control Of Active Front Steering System For Automotive

With the maturely and widely used of the Electric Power Steering System(EPS),Active Steering System has become a hot spot in this field.Active steering provides a better control method and performance for vehicle handling and stability control.It will provide the possibility for the future development of automatic driving,automatic collision avoidance and automatic navigation.Active Front Steering System(AFS)input the additional steering angle basis on the steering wheel angle,in this way,the independence of the steering intervention can be achieved,the vehicle response to driver input will be optimized,and it also can improve the stability of vehicle in case of emergency.Actually,the Active Front Steering System is a kind of steering system between traditional power steering and steer by wire.It is based on the structure of the traditional power steering system,and has the advantages of the line transmission system,also,it can control the vehicle actively.Active front wheel steering system can achieve variable transmission ratio and stability control.The variable transmission ratio control can be achieved through input the additional steering angle basis on the steering wheel angle which could optimize the vehicle response of driver input.Active Steering System also can improve vehicle stability in case of emergency or at high speed,and the steering sensitivity at low speed.Therefore,it has become a hot spot of the Electric Power Steering System research.This paper has studied the ideal transmission ratio control law of active front steering system based on the established linear vehicle model with two degree freedom.According to the design requirements of the ideal transmission ratio control,the ideal transmission ratio control curve of active front steering system has been designed under the fixed yaw angular velocity gain.Considering the gain of yaw velocity and lateral the acceleration,the ideal transmission ratio control curve was optimized.And the inflection point mutation is eliminated by fitting curve using S function to make the curve of ideal variable transmission ratio become smoothly.Further,jitter of the front wheel angle caused by the mutation transmission ratio is eliminated.To some extent,the steering wheel torque characteristic is also improved.At the same time,this paper analyzed and compared the influence of the rule of the different ideal transmission ratio control for vehicle handling stability.In order to increasing the vehicle's anti-interference ability of the lateral wind,And eliminate the disturbance of vehicle road surface condition on the front wheel angle as well as remove the error of active steering control system,so that it can make up for the insufficient of the vehicle dynamics in the open loop active control system.The additionalangle closed-loop control strategy of active front steering sliding mode has been presented.It is based on feedback control.It established on the design of the ideal transmission ratio curve and linear model of two degrees freedom.The control system of sliding mode feedback is denounced by thickness function,at the same time,through the integral part to eliminate the static error of system.Based on the Matlab/Simulink and Carsim's simulation environment,the control strategy is simulated and verified,and the vehicle stability control is analyzed by phase plane method.In summary,the content of this paper provide the theoretical basis for the development and design of active front wheel steering system.