| In order to alleviate the problems of environmental pollution,climate warming and energy shortage,new energy vehicles with the advantages of energy saving and emission reduction have become the research hotspots of the automotive industry.As a high-performance vehicle,in-wheel moter vehicle is one of the development directions of new energy vehicles,considering that the stability control of the vehicle is a prerequisite to ensure the normal operation of in-wheel moter vehicle.The Active Front-wheel Steer(AFS)adds a set of variable ratio device and electronic control unit on the basis of the traditional steering system,by applying a variable ratio device to the front wheels independently of the steering wheel angle.The AFS system adds a variable ratio device and an electronic control unit to the conventional steering system,and can improve the vehicle’s handling stability by applying a superimposed steering angle independent of the steering wheel angle.At the same time,the AFS system can realize the variable ratio control of steering system through the superimposed angle,which can improve the "low-speed sensitivity" and "high-speed stability" of the vehicle steering compared with the traditional steering system.Therefore,it is necessary to study the variable ratio control and stability control of AFS system.As the Direct Yaw Moment Control(DYC)system is a common means to achieve vehicle stability control,it is necessary to study the DYC system because the wheel hub motor-based electric vehicle can independently adjust the drive wheel torque and easily play the advantages of the DYC system.Considering that the control limits of AFS and DYC on vehicle stability are different,it is necessary to coordinate the control of AFS and DYC in order to take advantage of the respective advantages of AFS and DYC in vehicle stability control and to avoid the interference of DYC with AFS,and the research contents of this paper are as follows:(1)Analyze the structural composition,basic principle,kinematic characteristics,operating mode and performance characteristics of the AFS system.Taking in-wheel moter vehicle steering knuckle as the research object,for the problems of steering knuckle breakage due to insufficient strength and resonance between steering knuckle and wheel motor,topology optimization is carried out with the objectives of steering knuckle stiffness,dynamic inherent frequency and mass,to realize lightweight design of steering knuckle.(2)A reasonable dynamics simulation model is a prerequisite for the control study and simulation verification of the active front-wheel steering system of a hub motor electric vehicle.The AFS system dynamics model and the eight-degree-of-freedom model of in-wheel moter vehicles are built based on MATLAB/Simulink,and a joint simulation platform is built based on Car Sim and MATLAB/Simulink.(3)Based on the built AFS system dynamics model and in-wheel moter vehicle eight-degree-of-freedom complete vehicle model.Firstly,Considering that the AFS system can realize variable transmission ratio control and vehicle stability control,the ideal transmission ratio that varies with vehicle speed and steering wheel angle is determined.In order to eliminate the sudden change of steering wheel torque caused by the sudden change of transmission ratio,the ideal transmission ratio is optimized by S-shaped function,and the ideal transmission ratio is verified under the angular step condition.Based on MATLAB/Simulink,a "human-vehicle-road" closed-loop simulation platform is built,and based on the angular velocity feedback of the transverse pendulum,the stability control strategy of the AFS system is built using the fuzzy PID control algorithm.Finally,the stability control strategy of the AFS system is verified under sinusoidal conditions.(4)The DYC controller is designed with a hierarchical control structure to address the problem,the AFS system is difficult to achieve vehicle stability control when the tires are in the nonlinear region,in which the DYC upper controller uses the sliding mode variable structure control principle to calculate the additional transverse moment required to maintain the vehicle stability state,the DYC lower controller distributes the additional transverse moment to each wheel by the dynamic load distribution method based on the combined differential braking/driving method,and finally the DYC upper and lower controllers are validated based on the step and sinusoidal conditions.(5)In order to take advantage of the advantages of AFS and DYC in vehicle stability control and to avoid the interference of DYC with AFS,a three-degree-of-freedom model of the vehicle is built based on MATLAB/Simulink.At different vehicle speeds and different road adhesion coefficients,the variation of the phase plane stability domain is analyzed.Then AFS and DYC coordinated control strategy is developed,and the phase plane is divided into AFS system working area alone,DYC system working area alone and AFS and DYC system coordinated working area by using the combination of bilinear and elliptic methods,and the weight coefficients of AFS and DYC system are determined. |
