Research On Instability Mechanism And Handling Stability Control Of Distributed Drive Electric Vehicles Under Combined Slip Condition

In order to deal with the problem of fossil energy consumption and environmental pollution brought by the development of the automobile industry,the electric vehicle has become an important direction of the automobile technology.Due to the advantages of high transmission efficiency and a high degree of controllable freedom,the distributed drive electric vehicle based on in-wheel motors(IEVs)has become a research hotspot in recent years.The rapid and accurate torque response of the in-wheel motor distributed drive electric vehicle is conducive to better four-wheel torque distribution,which provides a new opportunity for the improvement of vehicle handling and stability.However,at present,the research of the mechanism of vehicle handling and stability control is mainly focused on the pure sideslip condition,and the influence of vehicle dynamics characteristics change on vehicle control under the combined slip condition is not deeply analyzed.Under the combined slip condition,the control of vehicle handling and stability is faced with a series of challenges: The change of vehicle dynamics results in the change of vehicle stability region and increases the risk of vehicle instability;The nonlinear characteristics of tire change significantly,which makes the sideslip and longitudinal slip of tire present complex coupling mechanism;How to coordinate the control weights of maneuverability and stability to obtain the best comprehensive control performance of vehicle needs further study.Therefore,based on the research platform of IEV and the combined slip vehicle dynamics mechanism,this paper studies the theory and application from two aspects of vehicle “stability judgment” and “cooperative control of lateral and longitudinal motion of the vehicle”,aiming to improve the control effect of IEVs under combined slip condition.The specific research contents are as follows:Firstly,according to the requirements of vehicle dynamics simulation,the fourteen degrees of freedom(14-DOF)vehicle dynamics model,the UniTire tire model,the in-wheel motor model,and the driver model are built,which provide the model basis and simulation platform for the later vehicle stability analysis and control algorithm design.The accuracy of the model is verified by the real vehicle parameters and test data.Secondly,to solve the problem of vehicle state estimation,a sideslip angle observer based on the fusion of kinematics-based method and model-based method is proposed.The extended Kalman filter(EKF)is designed based on the kinematics equation to estimate the dynamic value,and the steady value and small value results are calculated by the model-based method.The acceleration offset compensation structure is introduced to eliminate the influence of the signal offset.In order to combine the advantages of the two observers,a fusion strategy based on the dynamic characteristics of the vehicle is designed,which can effectively improve the observation accuracy of the sideslip angle in the real vehicle environment.besides,observers are also designed to observe the longitudinal speed and tire lateral and longitudinal force,which meets the requirements of the controller for vehicle states.Thirdly,to cope with the problem of vehicle stability judgment,the mechanism of vehicle instability under the limit condition is analyzed comprehensively.Based on Lyapunov stability theory and handling diagram method the stability of the vehicle under the pure side slip condition is analyzed from the control theory and dynamics point of view respectively;the influence of the input condition change on the equilibrium point position and stability area is analyzed by using the handling diagram method;considering the “lateral-longitudinal-vertical” coupling effect of the vehicle under the combined slip condition,with the simplified combined slip tire model,the dynamics mechanism of vehicle instability under the limit combined condition is revealed.Based on the analysis results,the phase plane saddle point position equation of front and rear tire slip angle and the quantitative vehicle stability index are proposed,which can quantitatively present the vehicle stability margin according to the vehicle states,solving the problem of vehicle stability evaluation under the limit condition.Then,an integrated chassis control system considering the coupling effect of “lateral-longitudinal-vertical” is proposed to improve the control performance of the distributed drive electric vehicle under the combined slip condition.(1)A control-oriented combined Uni Tire model is proposed,which can describe the strong coupling and nonlinear mechanical characteristics of the tire under combined slip condition more accurately.By concentrating the lateral and longitudinal coupling and nonlinear characteristics of tire mechanics in the expression of effective cornering stiffness,the tire model can be more convenient for controller design.(2)Based on the control-oriented combined slip UniTire model and 7-DOF vehicle model,the state-space presentation of the vehicle is established,which can improve the prediction accuracy of vehicle states under combined slip condition.On this basis,the model predictive controller(MPC)is designed,which can centralize the maneuverability,stability,and four-wheel slip rate as the control objective for centralized optimization.(3)In order to improve the adaptability of the controller to different driving conditions,an adaptive weight scheme based on the stability index and tire slip rate is proposed.When the state is stable,take the maneuverability as the main control objective to improve the driving performance of the vehicle;when the vehicle is going to lose stability,take the stability as the control objective to ensure safety.As a result,the controllability and stability of the distributed drive electric vehicle under the combined slip condition can be effectively improved.Finally,the effectiveness of the algorithm is verified by simulation and real vehicle test.(1)The simulation results of double lane change(DLC)show that compared with the fixed weight scheme,the proposed adaptive weight scheme is conducive to improving the vehicle's maneuverability and reducing the driver's steering load;at the same time,it can restrict the vehicle sideslip angle to a smaller range,making the vehicle more stable;it is also conducive to reducing the yaw moment demand and reducing the tire slip rate.The simulation results of the brake-steering combined slip condition show that the control-oriented UniTire tire model in the MPC controller is conducive to more reasonable four-wheel torque distribution under the brake steering combined slip condition.So that the additional yaw moment is reduced and the tire slip rate is restricted in a smaller value under the same states control effect.When considering signal offset,noise,and vehicle parameter deviation,the robustness of the algorithm is verified by simulation as well.The results show that the algorithm can still effectively ensure the stability of the vehicle under extremely harsh conditions.(2)Based on the test platform of Dongfeng E70,the proposed sideslip angle estimation algorithm and the MPC algorithm are verified.The results show that the estimation algorithm can accurately estimate the sideslip angle under high-?,low-? and limit conditions;the MPC controller can realize the driving assistance function under combined slip condition,so as to improve the control performance of the vehicle.