Research On Mobility Of Hybrid Off-road Vehicle Driven By Wheel Hub Motor

The wheel hub motor drive system has accurate controllability and torque response performance at millisecond level,which is conducive to independent control of tire forces.Consequently,it has been widely studied and applied in high-performance offroad vehicles to improve vehicle dynamic responsiveness,steering flexibility,handling stability,trafficability and other mobility indicators in recent years.In addition,the extended range hybrid system with great reliability mounted on the four-wheel electric driven off-road vehicle can meet the requirements of strong survivability and the convenience of energy replenishment.However,the design difficulty of distributed drive control is improved owing to the complex and variable driving conditions and the electromechanical coupling characteristics between the hybrid power unit and the wheel hub motor drive system,which requires in-depth analysis and research.Hybrid off-road vehicle driven by wheel hub motor is regarded as the research object in this dissertation.Based on the purpose of achieving the mobility design indexes such as ejection start-up accelerating,pivot steering,steering flexibility,handling stability and trafficability,the vehicle driving environment perception technology and the control method of range extension system as well as the torque vector allocation strategy are studied deeply in order to realize the global optimization of driving power utilization of off-road vehicle under various driving conditions.The specific research content can be summarized into the following aspects:(1)In order to make full use of the redundant torque control freedom and dynamic responsiveness of the hub motor drive system to achieve the comprehensive optimization objectives of power performance,trafficability and stability,it is necessary to realize the real-time perception of vehicle motion state and driving environment,and then form the closed-loop dynamic regulation mechanism of multidegree-of-freedom torque control.Firstly,based on the dynamic perception system constructed in this paper,which is composed of multi-information fusion kinematic measurement,vehicle dynamics model and adaptive correction unscented particle filter,the longitudinal and lateral velocity estimation method and the wheel attachment limit observer are designed.And then,the nonlinear characteristics of various driving conditions of off-road vehicles are analyzed,and a driving environment perception strategy with comprehensive optimization of robustness and accuracy is proposed.The test results show that the observation error of attachment limit of each wheel is small when the vehicle motion state changes approximately linearly.Moreover,the instantaneous change of the attachment state of each wheel can be quickly captured under the disturbance of strong nonlinear conditions.(2)In view of the coordination control problem between the power requirements of the off-road vehicle,whose total weight is about six tons,and the safety constraints of its hybrid power system when accelerating rapidly,a nonlinear auto-regressive neural network(NAR)power demand prediction model is established to accurately predict the power demand in transient conditions.Furthermore,the dynamic safety constraint domain of the hybrid system is designed based on the premise of driving reliability,and the dynamic prediction algorithm is developed to realize the coordinated optimization of drive power utilization and safety.The test results show that the power responsiveness and following accuracy of hybrid power system can be significantly improved by the transient tracking control of required power.The acceleration performance indicators are achieved,and the peak discharge rate and pulse discharge time of the battery are effectively reduced.(3)A multi-DOF torque vector distribution and control method is designed to quickly pass off-road conditions.Specifically,taking drive power utilization optimization as the overall control objective,a torque vector feedforward pre-allocation method is proposed to dynamically balance the utilization ratio of each wheel adhesion based on the real-time perception information of wheel dynamic load and adhesion coefficient.In addition,the grounding height and adhesion conditions of each driving wheel would have strong nonlinear changes when driving on uneven terrain or on rain,snow and gravel road,which increases the difficulty of accurately estimating the adhesion limit of each wheel.In order to improve the robustness of driving torque control and optimize the convergence speed of each wheel’s motion state to a stable linear region,a dynamic second-order sliding mode variable structure feedback controller for all-wheel torque is designed according to the attachment limit of each wheel.The test results show that the global optimization of the attachment utilization rate of each wheel and the fast convergence of slip rate are realized under the road surface with different adhesion conditions and variable-adhesion roads.Additionally,the vehicle can stably and efficiently pass steep slope,twisted road,vertical steps and other extreme off-road terrain,the performance index of fast passing off-road conditions is achieved.(4)The additional yaw moment output from the distributed drive system can adjust the steering characteristics and the heading angle of vehicle in real time,and carry out pivot steering in the narrow space such as urban streets and bridge deck,greatly improving the vehicle’s steering mobility.Due to the tire stability margin decreases gradually with the aggravation of steering motion,the excessive yaw moment is easy to cause the vehicle sliding instability and driving efficiency decline.In order to solve the coordination optimization problem between yaw responsiveness and stability,the desired yaw velocity and sideslip angle are tracked accurately based on vehicle dynamics model,phase plane analysis and nonlinear finite time control technology at first.Furthermore,the control priority of responsiveness and desired yaw trajectory for pivot steering under different adhesion conditions are developed.Then the vehicle yaw rate is tracked based on model prediction algorithm,and the adaptive sliding mode controller is introduced to adjust the wheel slip rate to ensure the stability of longitudinal and lateral motion.The test results show that the lateral dynamic control adaptively optimizes steering flexibility and handling stability,and achieves accurate tracking of the desired pivot steering trajectory.