| On the icy-snowy road,the vehicle handling stability will deteriorate sharply,making the possibility of traffic accidents significantly increase.The vehicle stability control also faces a series of challenges at this time and still needs further research: the vehicle’s lateral and longitudinal dynamics will interact with each other,bringing the nonlinear characteristics of the longitudinal slip of the tires-side deflection presents a complex mutual coupling mechanism;due to the changes in vehicle dynamics,it is necessary to consider the vehicle transverse and longitudinal stability of multiple control objectives and the vehicle stability control mechanism in an integrated manner,and the coordination and balance between multiple control objectives and multiple system constraints;the vehicle state changes drastically under snow and ice,how to reduce the risk of vehicle instability and improve the comprehensive control performance of vehicle driving stability and safety.To this end,this paper takes the four-wheel hub-driven electric vehicle as the research object.It develops the theoretical and applied research around the collaborative control of transverse and longitudinal stability of the vehicle under the ice and snow road,specifically.Firstly,to meet the requirements of vehicle stability control strategy development,a seven-degree-of-freedom dynamics model for the control strategy is established,specifically including the longitudinal,lateral and yaw motion of the vehicle body and wheel rotation,and a combined-slip tire model is used to describe the longitudinal sliplateral deflection coupling characteristics of the tire force on the icy-snowy road.Based on the lateral and yaw motion characteristics and Lyapunov stability theory,the stability/critical stability/unstable region is identified and used as a criterion to assess the stability of the vehicle.This will provide the basis for the design and validation of the vehicle transverse-longitudinal stability control strategy in the later paper.Secondly,a hierarchical coordinate control strategy for vehicle longitudinal and lateral stability is proposed for strongly coupled nonlinearities in the dynamics of vehicle motion control on the icy-snowy road.In order to satisfy the vehicle stability control requirements,the upper level designs a controller based on a three-degree-offreedom model of the vehicle with a nonlinear combined-slip tire model in the framework of nonlinear model predictive control and takes the tire slip ratio and tire slip angle as virtual control inputs.The lower level converts the virtual control inputs to obtain the additional torques acting on the independent motors by combining the longitudinal-lateral slip coupling characteristics of the tires on the icy-snowy road.The control strategy is able to meet the stability control objectives with high computational efficiency and reduces the complexity of the control structure by omitting parameter adjustment in the form of numerical resolution in the lower level torque calculation.Then,an integrated coordinate control strategy for vehicle longitudinal-lateral stability is proposed for the problem of actuator saturation constraints in vehicle stability control systems on the icy-snowy road.The control-oriented combined-slip Lu Gre tire model is derived to express the coupled nonlinearity of the vehicle’s longitudinal-lateral motion on the icy-snowy road at the tire force level centrally,capturing the longitudinal-lateral slip state of the tire and facilitating the controller design.Based on the model predictive control theory,the controller is designed to solve the multi-objective and multi-constraint problem related to vehicle stability and control inputs in a centralized and optimized manner.The controlled vehicle driving on the icysnowy road can track its yaw rate reference signal,and effectively suppress the vehicle lateral velocity and tire slip ratio with more reasonable and accurate additional torques.Finally,for the adaptive adjustment requirements of vehicle stability control systems in complex driving environments,a cooperative control strategy for vehicle transverse and longitudinal stability based on stability regions is proposed.Based on the centralized control strategy,the adaptive weight and constraint adjustment scheme is designed to meet the changing control requirements by combining the stability regions.For the stable/critically stable/unstable regions identified based on Lyapunov’s stability theory,different safety control objectives are considered respectively;stability coefficients are designed based on the stability regions to quantitatively describe the stability degree of the vehicle as a judgement indicator for dynamic switching of control objectives,so as to realize adaptive adjustment of target weights and constraints according to the vehicle state and to improve the overall stability control performance of the vehicle driving on icy and snowy road conditions in a more targeted manner.The overall stability control performance is improved. |
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