Research On Four-wheel Steer By Wire System Integrated By Suspension System

With the development of intelligent vehicles,the traditional automobile chassis is transitioning to the chassis by wire with high degree of integration and modularity.Four-wheel steer by wire technology can improve the mobility at low-speed and the stability at high-speed with fast response,which is one of the important development directions of chassis by wire technology.The distributed drive electric vehicle directly installs the drive motor inside the wheel,which increases the unsprung mass and leads to poor handling stability and ride comfort.The integrated design of the steer system and suspension system can reduce the unsprung mass,and improve the vehicle handling stability and ride comfort,and improve chassis integration and modularization,and give play to the advantages of steer by wire technology in fast response.In this paper,the structure and control of the four-wheel steer by wire system was researched based on the distributed drive electric vehicle,and the main research contents were as follows:(1)To improve the integration and modularization of chassis,a four-wheel steer by wire actuator scheme integrated by suspension system was proposed,and wheel positioning parameters were designed based on the demand for automatic wheel return;the parameters of the suspension system were matched,and the suspension stiffness and damper damping coefficients were optimized based on particle swarm algorithm;the steer motor and reduction mechanism were selected and matched with parameters,and the vehicle was analyzed for forces based on three typical operating conditions.The topology optimization was carried out with the goal of mass reduction by equally distributing the weights of the three typical working conditions,and the optimized 3D model was reconstructed and analyzed statically.(2)A co-simulation platform was built based on Carsim and MATLAB/Simulink,and signals were carried out interaction through the data interaction interface;a four-wheel independent drive and four-wheel independent steer vehicle model was built and parameterized based on Carsim;a steer by wire actuator model and steer motor model was built based on MATLAB/Simulink,and a three-closed-loop control system was adopted of turning angle,speed and current tocontrol the steer motor,the steer motor response time was verified.The driver model and drive motor model were built,and the correctness of the co-simulation platform is verified by selecting the angle step condition and the sinusoidal condition.(3)The front wheel variable transmission ratio of the steer system was designed based on the method of constant steady state yaw rate gain,and the rear wheel angle was calculated based on the feedforward proportional control and the yaw rate feedback proportional control,and the co-control method was developed to calculate the rear wheel angle based on the analysis of the two control methods,which targeted the minimum amplitude of the sideslip angle;the rear wheel variable transmission ratio of the steer system was designed based on the front wheel variable transmission ratio of the steer system and the co-control method,and the variable transmission ratio of the four-wheel steering system was obtained based on the Ackerman steering principle.The linear two-degree-of-freedom model of the four-wheel independent steer vehicle was built,and the correctness of the four-wheel steer angle control strategy was verified under high-speed steer wheel angle step condition and low-speed steer wheel angle step condition.(4)A sliding mode controller for vehicle lateral stability was designed to calculatethe front wheel feedback angle according to the difference between the actual yaw rate and the ideal yaw rate,and the simulation verification is carried out in the high-speed steer wheel angle step condition,the sinusoidal condition,the double lane change condition and the low-speed steering wheel angle step condition and the sinusoidal condition;the results showed that the difference of yaw rate,lateral acceleration and sideslip angle from the ideal values was reduced under the above conditions by using sliding mode control and four-wheel variable transmission ratio control method,and the sliding mode controller can effectively improve the vehicle lateral stability.