Design Of Four-wheel Independent Steering System For Autonomous Vehicles

In recent years,with the rapid development of autonomous vehicles,it puts forward higher requirements for the accuracy and flexibility of control,and the four-wheel independent steering vehicles with strong mobility and quick response provide an effective solution to meet the above requirements.However,due to the high nonlinearity of four-wheel independent steering vehicles,there are complicated coupling constraints such as "lateral-longitudinal-vertical",which makes it difficult to coordinate steering maneuverability and stability of this type of vehicle.In this paper,the four-wheel independent steering vehicle was taken as the research object,the whole vehicle model was established according to its four-wheel independent controllable characteristics,steering stability control system and optimal torque distribution were designed,which improves the flexibility of four-wheel independent steering vehicle at low speed and stability at high speed,and made it more suitable for the development of autonomous vehicles.Specific research contents are as follows:(1)A dynamic model of four-wheel independent steering vehicle based on Carsim and Simulink is designed.In order to restore the dynamic characteristics of the real vehicle,the car body model was established in Carsim according to the vehicle mass,wheelbase and other parameters,and the power system model was established in Simulink considering the mechanical characteristics of the hub motor.At the same time,a four-wheel steering model based on Ackerman principle was built in Simulink,and made different steering strategies for high and low vehicle speeds.The whole vehicle model was established through the joint simulation of Carsim and Simulink,which provided the model basis for the subsequent steering control research.(2)Steering stability control strategy based on yaw moment and steering stability judgment based on phase plane method are designed.Because the four-wheel independent steering vehicle is easy to lose stability when cornering at high speed,the steering stability control strategy based on yaw moment was formulated by analyzing the influencing factors of vehicle stability control.At the same time,the ideal four-wheel steering vehicle model was established as the ideal value,by comparing the deviation between the ideal value and the actual value,the instability of the vehicle was corrected.The phase plane method was used to judge the vehicle steering stability,which provided the best time for the steering stability control system to intervene.(3)A distributed strategy of four-wheel independent steering torque control is designed by adopting hierarchical design.Reasonable control and distribution of fourwheel torque is the key to improve steering stability.The hierarchical design method was adopted as a whole,and the upper controller was used to decide the driving torque,additional yaw torque and correction torque needed to keep the vehicle running stably.The lower distributor used the optimal torque distribution method to distribute the torque to each wheel reasonably.The effectiveness of the designed four-wheel independent steering system was verified by the vehicle simulation test under different working conditions.Finally,a semi-real test platform for automatic driving was built,and the simulation test of automatic driving trajectory tracking was completed through data acquisition,decision calculation and simulation,which proved the practical performance of fourwheel independent steering system in the field of automatic driving.