Study On Active Front Steering System Of Vehicle

Study on Active Front Steering System of vehicleBoth steering portability and steering sensitivity should be considered to coordinate in steering system design. Steering portability requires the steering force that the driver act on steering wheel should be small and the total number of steering wheel turns be less, and steering sensitivity requires the vehicle dynamic response to driver's operation rapidly and reasonably. Steering work can be expressed as steering torque by steering angle; thereby there are two ways to reduce steering work. Firstly, provide assist steering torque to reduce driver's steering torque; secondly, provide a superimposed steering angle to reduce the need of steering angle that driver should act on the steering wheel. The active front steering system provides a superposition angle to change the steering work.In this paper, several kinds of variable gear ratio structure are presented such as single planetary gear set, double planetary gear set, harmonic gear and so on. This paper also introduces their structure characteristics and work principle. Because single planetary gear set has low efficiency and harmonic gear demands higher process requirements. So double planetary gear set is a better choice. At the same time, its kinematic analysis, variable gear ratio modeling and transmission ratio characteristics also are studied.The basic knowledge of tire mechanics was introduced, the vehicle motion in vehicle fixed coordinate system and ground fixed coordinate system was analyzed in theory. And the influence of vehicle parameters on the yaw rate gain and lateral acceleration gain was studied. The steady-state response of vehicle in different gear ratio was researched. From the simulation curve we can conclude that vehicle with small steering ratio low speed steering, the car response quickly and yaw rate gain in a suitable range. With the increase of gear ratio, when steering in low speed,the response slow and sensitivity reduces; the vehicle with small steering ratio high speed steering, lateral acceleration gain may become out of control, with the increase of gear ratio yaw rate gain and lateral acceleration gain become small, vehicle safety improve. Conventional vehicle steering system can not be taking into requirements of low-high speed steering. So it's necessary to study variable gear ratio steering system.From the relationship between steady-state steering response and steering gear ratio, a variable steering ratio have been presented, and the torque control of a BLDC was also introduced. In determining the upper and lower gear ratio, car operating condition should be considered, when running on low speed in the city, in order to satisfy the driver's steering sensitivity, gear ratio shouldn't be too big; while on high speed, consider the safety of vehicle, the gear ratio shouldn't be too small. Consider both the yaw rate and the lateral acceleration an ideal gear ratio curve was obtained, and adding upper-lower limits ratio and by curve fitting approach, the variable gear ratio curve was generated finally.According to the equivalence principle, the inertia of wheels and steering column, and steering rack mass were equivalent to shaft of motor. The superposition angle model is established; the desired motor rotate angle can be calculated through the vehicle running state. The low-high speed multi-condition was simulated using Matlab/Simulink. Compared the simulation result with the traditional car, the result shows that, on low speed steering the vehicle with active front steering can get a larger lateral displace than the traditional one, yaw rate response more rapidly. In the low speed U-turn the steering wheel total turning angle reduces; in high speed steering the vehicle with active front steering can get a smaller lateral displace than the traditional one, lateral acceleration got low, the path change caused by drive misoperation will be smaller, which reduces the risk of collision with other vehicles, high speed security was increased.