| The shortage of energy and the pollution of environment have been urging traditional vehicles industry to realize transformation.In this case,the rapid development of pure electric vehicles has been ushered in.The characteristics of distributed four-wheel hub motor-driven electric vehicles are the simple construction and the drive torque can be controlled flexibly and independently,which has beaten a new path for studying automotive dynamics.However,nowadays,there are still lots of problems in driving and controlling whole vehicle,differing from mature traditional chassis control system.Based on this point,this thesis focuses on the research of four-wheel-independent electric vehicles and its driving control strategy.The main contents are as follows.Modeling and analyzing the research object were realized in this thesis.Besides,the degree of freedom was simplified on the basis of ensuring the accuracy of the model.Modular modeling the whole vehicle includes seven-degree-of-freedom vehicle body model,wheel model,tire model,wheel hub motor model.Introducing the transmission parameters of each subsystem will lay the foundation for the experiment of designing and simulating the rear driving control strategy.Aimed at the issue of neglecting the coordination effect of the internal force of the frame on each wheel in differential speed control,this thesis analyzed the driving process of traditional vehicles with differential speeds,confirmed the constraint condition of the hub motor electric vehicle with differential driving speeds and analyzed the force of wheels containing the internal force of the frame.Thus,the principle of adaptive differential speeds for whole vehicle's torque control.In addition,considering the dimension of vehicle dynamics in the process of torque control,a hierarchical control method was utilized to design strategies of driving torque control of the whole vehicle under different conditions.Under the steering conditions,aimed at the problem that vehicles are more prone to understeer,oversteer or even lose stability,a control strategy of stability driving torque was proposed to improve the steering stability of vehicles.The control strategy includes the torque calculation layer and the torque distribution layer.In the torque distribution layer,firstly,the expected value of the control target was determined by considering the change in the concerning stiffness of the two-degree-of-freedom model.Center-of-mass roll angle is applied to restraint the yaw rate in order to avoid the existence of coupling and conflict between the control targets.At last,calculating the additional yaw moment through the slip mode control introduced RBF neural network to improve the traditional sliding mode control on the purposes of reducing the system's “bounce”.In the torque distribution layer,considering the transfer of vertical load,a dynamic load distribution algorithm was proposed to distribute the yaw moment and demanded torque of the driver obtained by the upper layer.By designing the experiments of joint simulation between CarSim and Simulink,it was proved that the stability driving torque control can greatly improve steering characteristics and steering stability of vehicles.In the straight-line stable driving condition,aiming at the problem of the short distance of electric vehicles,a control strategy of energy-saving driving torque was raised.In order to obtain the MAP picture of driving efficiency of the hub motor,a motor test bench was established.Besides,considering the impact of motor torque changes on the mobility and comfort of vehicles,multi-objective energy-saving optimal distribution algorithm was designed and fuzzy control was utilized to adjust the target weight coefficient.Meanwhile,the chassis dynamometer experiment under NEDC city road cycling conditions was conducted.The research results indicated that the energy-saving driving torque control can effectively improve the driving efficiency of the hub motor and reduce the energy loss of the entire vehicle system. |
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