Study On Stability Integrated Control Algorithm Of Driving/Steering/Braking Systems For Four-Wheel-Independent Electric Vehicle

With the increasing scarcity of traditional energy sources and the growing pollution,government and car manufacturers are committed to the electric cars to alleviate thetransport pressure on energy and the environment. On the other hand, consumers requirevehicle performance to increase. However, conventional vehicles have developed for morethan a century and the performance is relatively difficult to increase furthermore because ofthe limitation of mechanical structures. Full-wire four wheel-motor independent drive,independent steering, independent braking electric vehicle complies with the trend of vehicleelectrification. And it has more controllable degrees of freedom. Therefore, it is suitable toadopt vehicle chassis integrated control technology to allocate driving, steering and brakingsystem actuator actions to achieve the global optimum of vehicle performance. Therefore,four wheel independent electric vehicles equipped with integrated chassis control technologyhave good development prospects. In this thesis which is supported by National NaturalScience Foundation of China(50775096) and National Science Foundation for DistinguishedYoung Scholars of China（51105165）, based on full-wire four-wheel-independent electricvehicles, integrated control algorithm structure is designed, the stability control objective isproposed to maximize tire attachment margins, and stability integrated control algorithm isdeveloped.To explore the dynamic characteristics of four-wheel-independent electric vehicle, andprovide the validation platform for stability integrated control algorithm, dynamic simulationmodel of four-wheel-independent electric vehicle is established. The model can reflect thecoupling among vehicle longitudinal, lateral, and yaw movement, and the dynamic response characteristics of driving motors and steering motors. Moreover, it can simulatemulti-moving modes which are unique to four-wheel-independent vehicles. Besides, for thesimulation of stability integrated control, the model is validated in harsh working conditionssuch as high speed and low adhesion conditions.Integrated control algorithm designed in this thesis uses a hierarchical centralized controlstructure, which has high integration degree so that it can achieve the theoretical optimalcontrol performance, and also simplify vehicle decoupling control problem. The algorithmstructure is divided into two layers. The upper layer calculates the required total verticalforce, total lateral force and total yaw moment according to vehicle movement targets, byusing vehicle inverse dynamics. Therefore, the driving intention is realized. The lower layertakes four wheel steering angles and four wheel diving torques as eight independent controlvariables, and adopts optimal control methods to assign tire adhesions so that to enhancevehicle stability. At the same time, the total virtual control outputs are transformed intospecific control output values which are the control targets of vehicle actuators.To design the algorithm, in the upper layer, three degrees of freedom vehicle model isadopted, system feedback stabilization matrix is constructed, and the multi-inputmulti-output system multi-target tracking algorithm is designed based on model predictivecontrol theory. In the lower layer of the algorithm, with consideration of the effect of tirevertical load and road adhesion condition, tire adhesion margins are taken as allocation targetto calculate tire longitudinal force and tire lateral force optimally. Tire longitudinal forces arerealized directly by driving system and braking system. However, to control tire lateralforces, tire inverse model is built in this thesis. Therefore, the steering system can control tirelateral forces by controlling steering angles to obtain corresponding tire slip angles.Finally, the four-wheel-independent electric vehicle stability integrated control structure andthe algorithm is validated by simulation of three harsh working conditions. The results showthat the integrated control algorithm can achieve driver’s driving intentions, keep vehicle slipangle to be zero, and control the utilization of tire adhesions to be equal so that the maximumtire adhesion utilization is reduced. Vehicle with and without stability integrated control are compared, and it show that stability integrated control algorithm can improve vehiclestability.