Research On Torque Coordination Control Of Six-Wheel Skid Steering Vehicle Driven By In-Wheel Motor

The skid steering vehicle is different from the Ackerman steering vehicle,which is mainly realized by the rotational speed difference formed by the different torques of the wheels on both sides.The rapid development of in-wheel motor drive technology is more conducive to the function improvement of multi-wheel drive skid steering vehicles.The vehicle with slip steering will have both longitudinal slip and lateral slip when steering,and the nonlinear and multi-dimensional coupling characteristics of the vehicle are enhanced significantly.How to improve the steering sensitivity and handling stability of the slip-steering vehicle under different road adhesion coefficient and different steering demand is the difficulty of the current wheel motor-driven slip-steering vehicle torque coordinated control.Based on the wheel skid steering vehicle developed by the research group,this paper studies the key issues such as vehicle steering characteristics analysis,dynamic variable speed ratio design,key state parameter observation,and six-wheel torque coordination distribution.Firstly,this paper analyzes the steering characteristics of the skid steering vehicle,and deduces the ideal yaw rate formula based on the driving condition.The steady-state slip-steering mechanism of the wheeled vehicle is analyzed,the vehicle model is simplified to a single-axis skid steering model,and the steady-state steering forming process of the wheeled skid steering vehicle is demonstrated.The steering performance of the skidding vehicle is compared with that of the Wilhelm Ackermann steering vehicle,and a conclusion is drawn that the skidding vehicle has a smaller yaw rate gain.Combined with the two-degree-of-freedom dynamic model of the skid steering vehicle,a dynamic variable steering speed differential ratio algorithm for six-wheel slippage steering vehicle is proposed,a three-variable nonlinear function of road adhesion coefficient,vehicle speed and the ratio of dynamic steering speed difference is designed.The corresponding evaluation methods and analytical expressions for the steering characteristics and motion variables of skid steering vehicles under complex driving conditions,different steering intentions and speeds are obtained,which are used as the input of the subsequent vehicle state parameter estimation and control strategies.Secondly,the vehicle dynamic model of skid steering is constructed and the vehicle state parameter observer is developed.Based on the analysis conclusion of the steering characteristics of the skid steering vehicle,a nine-degree-of-freedom vehicle dynamics model is constructed.In order to accurately obtain the key vehicle state parameters required by the torque coordinated control system,the lateral velocity and road adhesion coefficient of the vehicle under different driving conditions are estimated based on the reduced-order nonlinear observer,and the convergence of the observer is analyzed.The verification results based on Truck Sim/Simulink co-simulation platform show that the reduced-order nonlinear road adhesion coefficient estimator does not need to calculate the inverse matrix in real time,which has fast convergence speed and small calculation amount.Thirdly,this paper develops a coordinated torque control strategy for skid steering vehicles.The control variable is the yaw rate of the skid steering vehicle.The upper layer adopts the sliding mode control method to make decisions on the yaw moment required to maintain the motion of the skid steering vehicle,so as to improve the robustness of the system.The adaptive fuzzy approximation algorithm is used to construct a continuous discrete symbolic function to reduce the system chattering.The lower layer is a six-wheel torque coordination distributor.Based on the driving force and braking force applied to different wheels under different driving conditions,an optimal allocation method of wheel torque based on yaw moment contour is proposed.which divides the slip steering vehicle into 12 operating modes,and the yaw moment determined by the upper controller is coordinated to the six wheels of the vehicle,so as to realize the torque coordinated control of the skid steer vehicle.Finally,the typical working condition simulation and real vehicle test of the skid steering vehicle are completed.The test results show that the torque coordinated control strategy developed in this paper can improve the high steering sensitivity of the skid steering vehicle,the low road surface in handling stability,the torque coordinated control response is rapid and the robustness is good.