Trajectory Tracking Control Based On Road Adhesion Coefficient Identification

In this paper,a four-wheel distributed independent drive electric vehicle is taken as the research object,and a state observer and a trajectory tracking controller are designed based on the object and the Uni Tire nonlinear tire model.The observer estimates the vehicle’s state parameters,such as its speed and side slip angle,and adhesion coefficient between the tire and road surface.The purpose of this article is to propose the strategy of improving controllers by analyzing parameters such as vehicle speed and adhesion coefficient on the tracking controller and the impact of vehicle driving stability.The main work includes:1.Dynamics modeling of distributed drive vehicle and tire.To reduce the computational complexity as much as possible under the premise of ensuring the algorithm’s accuracy,this paper establishes a seven-degree-of-freedom vehicle dynamics model and a two-degree-of-freedom vehicle lateral dynamics model used for designing the vehicle trajectory tracking controller and analyzing the vehicle’s stability of the lateral motion,respectively.Meanwhile,the Uni Tire model is introduced to calculate the nonlinear tire force,and its dynamic friction coefficient expression is simplified to design an adhesion coefficient observer based on the Kalman filter algorithm.The Uni Tire and its simplified model are identified based on the experimental data of tire mechanical properties.2.Design of vehicle state observer and phase plane-based analysis of vehicle stability domain and its boundary design.The observer estimates the state parameters of the vehicle that cannot be obtained directly by sensors,such as vehicle’s speed and side slip angle,and the tire-road adhesion coefficient.At the same time,based on the phase plane of the tire slip angle and the vehicle lateral dynamics model,the influence of speed and adhesion coefficient on system stability is analyzed,and the results showed that both the increase in speed and the decrease of adhesion coefficient would make the vehicle stability worse,while the effect of the latter is more pronounced.Based on the above conclusions,through many simulation experiments,the mathematical model of the boundary of the phase plane stability region concerning vehicle speed and adhesion coefficient is fitted.3.Design a horizontal and vertical decoupled hierarchical trajectory tracking controller.The longitudinal speed tracking of the controller’s upper layer adopts a sliding mode control algorithm to calculate the total longitudinal force of four wheels;the lateral trajectory tracking adopts the model predictive control algorithm to optimize and solve the front wheel angle and additional yaw moment control values.The lower control allocator considers the constraints of road adhesion conditions and solves the optimal four-wheel torque distribution based on convex quadratic programming.The Car Sim and Matlab/Simulink co-simulation platform simulation results show that the controller can achieve good reference trajectory and vehicle speed tracking performance.The influence on trajectory tracking accuracy and vehicle stability is analyzed by setting the simulation conditions of different vehicle speeds and adhesion conditions.The results show that increased speed and deterioration of adhesion conditions will worsen the trajectory tracking accuracy and vehicle stability.4.Trajectory tracking controllers’ accuracy and stability are improved using a stability margin model.Based on the boundary model of the stable region,the stable area of the tire slip angle phase plane is divided into three regions: the stable region,the weakly stable region and the unstable region,and a vehicle stability margin is established that comprehensively considers the value and the speed of the tire slip angle.Based on the above model,two control strategies are proposed.The first is adjusting the trajectory tracking and stability control’s weight of the trajectory tracking controller,and the second is correcting the target vehicle speed.The simulation results show that the tracking accuracy and driving stability of the vehicle are improved no matter driving on the low or high adhesion road.Therefore,the strategy of coordinating trajectory tracking and stability control based on the stability margin model and target vehicle speed correction strategy can improve the trajectory tracking accuracy and driving stability of the controller.