Research On The Control Strategy Of Distributed Four-Wheel Drive Electric Vehicles With Composite Braking Stability

With the rapid development of vehicle electrification and intelligence,distributed four-wheel drive technology based on wheel-side or hub motors implemented in electric vehicles has received widespread attention.As the motor can flexibly adjust the braking or driving torque,it makes the composite braking control strategy where the motor and the conventional braking system work together has become one of the research hotspots for distributed four-wheel drive electric vehicles.Distributed four-wheel drive electric vehicles are different from centralised drive electric vehicles in that all four-wheel motor torques can be quickly responded to and independently controlled.The output torque of the wheel drive motor controls the braking of the vehicle in conjunction with the hydraulic braking system,which will reduce braking stability due to the disturbing yaw moment generated during the distribution of torque among the four wheels of the distributed four-wheel drive electric vehicle.At the same time,the general composite braking control strategy based on braking energy recovery mainly considers longitudinal control,in steering conditions if the composite braking control strategy can be reasonably coordinated,it will also improve braking stability.In response to the above problems,this study relies on the national key research and development program "Multi-system Efficient Integration of In-wheel Motor Action Module and Vehicle Torque Vector Distribution Technology"(Project Number:2021YFB2500703)and Jilin Province Science and Technology Development Plan Projects "Research on decision-making and control of automatic lane change of electric vehicle with X-by-Wire chassis"(Project Number:20230101121JC)to investigate the status of domestic and international research on distributed drive electric vehicles,sideslip angle of vehicle estimation and distributed drive electric vehicle stability control,focusing on a composite braking control strategy for coordinated transverse and longitudinal stability control,including: a distributed four-wheel drive electric vehicle model,a distributed four-wheel drive electric vehicle sideslip angle of vehicle estimator,a composite braking stability control upper layer control strategy and the lower control strategy of the composite braking stability control.Details of the research are as follows:(1)Build a distributed four-wheel drive electric vehicle model.Firstly,the structural characteristics of the distributed four-wheel drive electric vehicle are analysed.Secondly,the seven-degree-of-freedom vehicle dynamics model,the motor external characteristics model,the hydraulic braking torque model,the tyre model,the wheel model and the driver model are established.Finally,the joint simulation platform of Car Sim and MATLAB/Simulink is established and the accuracy of the model is verified by simulation,so as to lay the foundation for designing the composite braking stability(2)Design a distributed four-wheel drive electric vehicle sideslip angle of vehicle estimator.Firstly,the prediction and observation equations are designed to estimate the sideslip angle of vehicle based on the unscented kalman filter and the seven-degree-offreedom vehicle dynamics model to obtain the steady-state value of the sideslip angle of vehicle.Secondly,the sideslip angle of vehicle is estimated based on the radial basis function neural network algorithm to determine the input and output quantities,and the data are collected to form a data set to train the algorithm to obtain the dynamic value of the sideslip angle of vehicle.Finally,the fusion of the unscented kalman filter estimator and the radial basis function neural network estimator is used to estimate the sideslip angle of vehicle,and simulations are carried out to verify the accuracy of the fusion estimator,thus laying the foundation for the composite braking stability control strategy.(3)A composite braking stability control upper layer control strategy is proposed.Firstly,an improved stability determination method based on the energy phase plane is proposed,and the effects of vehicle speed,road adhesion coefficient and steering wheel angle on stability determination are analysed.Secondly,the stability determination result is taken as the condition for the execution of the lateral control strategy,and the sliding mode control is used to design the lateral control strategy and the longitudinal control strategy,and the additional torque required for braking stability control is calculated.Finally,a simulation test is conducted to verify the effectiveness of the upper level control strategy based on the layered idea of designing the composite brake stability control.(4)Design the lower control strategy of the composite braking stability control.Firstly,the longitudinal additional torque is allocated to the four wheels,and the allowable action interval of the braking torque allocation is obtained based on the I curve and ECE regulations,and the longitudinal additional torque allocation control strategy is designed in conjunction with the demanded braking intensity.Secondly,the lateral additional torque is allocated to the driving or braking torque of the four wheels,and the steering wheel angle and the additional transverse moment are used to judge the vehicle state,to make full use of the advantages of four-wheel independent controllability,and to design the lateral additional torque allocation control strategy.Finally,the four-wheel composite braking actuator distribution control strategy is designed based on the weighted least squares method with constraints to coordinate the wheel motor and hydraulic system output wheel torque composite braking,and simulation tests are carried out to verify the composite braking stability control strategy.