Research On Control Strategy Of Vehicle Active Front Wheel Steering

Steering system is an important system to control the running direction of the vehicle. Active front wheel steering is the research hotspot and development trend of vehicle steering system. This paper focus on the research of control strategies about active obstacle avoidance、lane keeping and lateral stability for active front wheel steering vehicle, and the effect of Active front wheel steering on electric power steering(EPS) has also been researched. The main works are as following:(1)Based on the original electric power steering, a novel active front wheel steering system is designed. The system contains a set of double planetary gear mechanism and a motor. The motor can produce a corrective angle to realize the active obstacle avoidance, lane-keeping and lateral stability control. According to the torque of steering wheel and vehicle states, the EPS motor can produce a torque to improve the driver’s steering feeling.(2)In order to realize active obstacle avoidance and lane-keeping, based on model predictive control theory, a nonlinear model predictive control strategy is established. The performance of the control strategy is demonstrated in the scenarios of active obstacle avoidance and continues obstacle avoidance on ice and snow road by MATLAB/Simulink software. The simulation results show when the obstacle is not observed by the distracted driver, the proposed controller can produce a corrective steering angle to realize the obstacle avoidance. After the process of avoidance, the controller do not produce corrective angle any more, the vehicle is controlled by the driver.(3) In order to select the optimal route for obstacle avoidance, a two-layer model predictive controller is established. The upper controller is used to calculate the optimal route to avoid the obstacle, and pass the optimal route to the lower controller. The lower controller calculates the optimal steering angle of the front wheel and brake torque of the rear wheel, and the optimal value is applied to the actual vehicle, the optimal obstacle avoidance path tracking is achieved.(4)The reason for losing yaw stability of the vehicle is analyzed. Based on BP neural network PID theory, a yaw stability control strategy is established. The simulation results show that using the BP neural network PID control strategy, the vehicle can track the desired yaw rate accurately. At the same time, the sideslip angle is less than that of the vehicle without yaw stability control.(5)Based on model predictive control theory, an integrated control strategy of chassis is designed, which cooporated Active Front Steering(AFS) and Direct Yaw Moment Control(DYC). Simulation results show the integrated controller can guarantee the vehicle to avoid the obstacle on both sides of the road, and the yaw stability of the vehicle is improved.(6)The variable steering ratio for active front wheel steering vehicle is designed. The effects of variable steering ratio on power mode, return mode and damping modes of the electric power steering system are analyzed by simulation. Considering the increasing of friction and inertia in the steering system, the strategies for friction compensation and inertia compensation are designed, and verified in simulation.(7)A test bench of active front wheel steering system with electric power steering system is designed. And the variable steering ratio function of active front wheel steering system is verified in the test bench. The performance of the nonlinear model predictive control strategy is verified in test vehicle at the Hyundai research center of the University of California, Berkeley.