| Intelligent vehicle is an effective way to liberate drivers,improve driving safety,comfort and reduce traffic accidents.Unmanned driving technology can promote safety,comfort and convenience in intelligent transportation systems.The trajectory planning and tracking control of autonomous vehicles is an important part of realizing autonomous driving of vehicles,and it is the core module of autonomous driving.As a highly redundant system with overdrive,the full x-by-wire electric vehicle has the advantage that the steering angle and motor torque of the four wheels can be accurately controlled.Compared with the active front steering vehicles,the full x-by-wire electric vehicles have a higher degree of controllable freedom and flexibility.This paper is based on the Science and Technology Project of Jilin Province Education Department “Distributed Electric Vehicle Dynamic Modeling and Collaborative Control Based on Chassis-by-Wire System”(Project Number: JJKH20200963KJ),takes full x-by-wire electric vehicles as the research carrier to study the trajectory planning and tracking control algorithm in the on-road driving environment.Trajectory planning algorithm uses a planning algorithm that decouples path planning and speed planning,and trajectory tracking control algorithm uses a hierarchical tracking control architecture.The main research contents of this paper can be described as follows:(1)In this paper,a vehicle trajectory planning algorithm that decouples path planning and speed planning is proposed.The improved A* algorithm is used to perform behavior selection and path planning simultaneously in spatial-temporal space,and an optimization-based method is used to convert the path smoothing problem into a quadratic programming problem to solve,and obtain a smooth path that satisfies the vehicle curvature constraints.Based on the predicted obstacle trajectory information and the planned path,a path-time graph,namely ST graph,is constructed,and the graph-based search algorithm is used to select and plan the coarse speed;the optimization-based method is adopted,and the acceleration constraint and lateral direction are considered.Acceleration constraints,maximum vehicle speed constraints and collision safety constraints transform the speed smoothing problem into an optimization problem with nonlinear constraints.By solving the optimization problem,a continuous jerk speed plan can be obtained.(2)The trajectory tracking control module of the full x-by-wire electric vehicle is designed in this paper.A trajectory tracking control algorithm,which is based on a hierarchical control architecture,is designed to carry out effective trajectory tracking control for full x-by-wire electric vehicles.The hierarchical control algorithm includes trajectory tracking layer,tire force distribution layer and actuator control layer.The trajectory tracking layer uses longitudinal force,lateral force and yaw moment as the control variables,and uses model predictive control algorithms to control the vehicle to follow the desired trajectory;the tire force distribution layer distributes the desired resultant force/moment to each vehicle tire.The force distribution problem is transformed into a constrained quadratic programming problem to solve,and the longitudinal force and lateral force of each tire in the vehicle coordinate system are obtained;in order to obtain the vehicle steering angle and the vehicle driving/braking torque,the actuator control layer uses the coordinate system conversion to obtain the longitudinal force and the lateral force in the tire coordinate system,and the arctangent function tire model is used to obtain the desired tire slip angle,and then the wheel steering angle and wheel torque.(3)In order to verify the effectiveness of the trajectory planning algorithm,this paper designs different driving scenarios and validates the proposed trajectory planning algorithm.The results indicate that the proposed planning algorithm can plan feasible trajectories that satisfy vehicle constraints in different driving scenarios.Based on the Matlab/Simulink and CarSim joint simulation platform,the effectiveness of the proposed control algorithm is verified.The results show that the control algorithm proposed in this paper can follow the desired trajectory well.At the same time,this papr conducts a tracking simulation test on the planned trajectory,and the results indicate that the trajectory planned by the planning algorithm can be well tracked by the vehicle. |
