Research On Performance Analysis And Control Simulation Of Active Front Steering System

Aiming at solving the problems of low steering operation controllability and vehicle control stability caused by fixed gear ratio of traditional steering system, the Active Front Steering (AFS) system structure integrated with EPS is designed and the control methods are analyzed.To design the Active Front Steering system structure, the relationships of three different assistant steering forms with the rated front axle load are discussed. The structural features and transmission characteristics of two variable gear ratio mechanisms, which are single and double planetary gear trains, are compared. The comparison results indicate that double planetary gear train is advantageous for high transmission efficiency and little disturbance on steering characteristics. In addition, the double planetary gear train doesn’t affect the driver’s perception of road information and can be used to realize the decoupling control of angular displacement and torque transmission characteristics of steering system.The dynamic model of AFS system integrated with two degrees of freedom vehicle is established, and the influences of the control parameters and gear ratio on steering performance are analyzed. The analysis results indicate that the value of proportion coefficient should be small and the value of differential coefficient should be large. The gear ratio doesn’t affect the transmission of steering feel, and the steering sensitivity will increase with decreased gear ratio. The influences of total gear ratio of steering system on vehicle steering gain are analyzed, and the analysis results indicate that the yaw rate gain and lateral acceleration gain increase with the decreased gear ratio and increased vehicle speed. In order to realize sensitivity at low vehicle speed and control stability at high vehicle speed comprehensively, the design criteria of AFS gear ratio should be that small gear ratio value at low vehicle speed and large gear ratio at high vehicle speed.The ideal variable gear ratio curve of AFS system is determined according to constant yaw rate gain and lateral acceleration gain criteria. The simulation results show that the steering gains are within the allowable range and meet the requirements of steering performance requirements. To solve the shake of steering wheel during the interference of active steering, a corrected assisting electric current feedback control method is presented, and the calculation model of corrected current value is established to guarantee good steering feel during active steering.Three kinds of steering wheel inputs, which are step angle input, single sine input and double sine input, are simulated to obtain the steering response curves of vehicle, by using the established simulation model of AFS system. The simulation results of different driving conditions indicate that the sideslip angle and yaw rate can reach a large level at low vehicle speed and the vehicle mobility will increase; the sideslip angle and yaw rate will be lower at high vehicle speed and the stability and security will increase. The research results of this paper can provide the basis for the engineering design and stability control of AFS system and can be used to improve vehicle steering dynamic performance.