Design On Brake System Of Four-wheel Drive Electric Vehicle

With the rapid development of pure electric vehicle,four-wheel drive electric vehicle characterized by multi-motor and electronic control has become one of the vital developing aspects of electric vehicle.On four-wheel drive electric vehicle,every wheel is driven independently and there is no mechanical transmission,which asks for stability control under braking condition.On four-wheel drive electric vehicle,there is more than one motor for energy recovery.Therefore,distribution of electric and mechanical braking on four wheels needs to be studied.This thesis proposes a distributed electric and mechanical braking control strategy,reforms a four-wheel drive electric vehicle,and analyses the strategy through simulation and rapid control prototype experiment.Parallel braking strategy and series braking control strategy for four-wheel drive electric vehicle are proposed,which apply hierarchical control and have function of both braking stability and energy recovery.Parallel braking control strategy for four-wheel drive electric vehicle consists of three layers: determining expected yaw moment,determining the electric braking torque added on two wheels on one side,and energy recovery;series braking control strategy for four-wheel drive electric vehicle consists of four layers: determining total braking torque and expected yaw moment,determining braking torque of four wheels,divide braking torque into electric and mechanical braking torque,and energy recovery.These two strategies optimize stability and safety of four-wheel drive electric vehicle under braking condition,and have function of energy recovery.This thesis builds driver model,braking torque control model and energy recovery model with MATLAB/Simulink and vehicle dynamic model with CarSim.This thesis simulates braking stability and energy recovery performance of parallel and series braking strategy under a highspeed corner braking condition.Simulation results show that parallel and series braking strategy are useful in stability control,and series braking strategy is better than parallel braking strategy in energy recovery.With the assistance of cooperative enterprise,a mass-production electric vehicle is reformed to a four-wheel drive electric vehicle,including powertrain system and chassis system reform.Firstly,by comparing schemes of hub motor and wheel-side motor,power system driven by wheelside motor is determined;secondly,the selection of motor,battery and reducer,which are main parts of power system,is determined on a multi-objective optimization method;finally,according to the actual vehicle situation,a power system structure of wheel-side motor is designed.Power system and chassis system of the vehicle are reformed by the cooperative enterprise.A four-wheel drive electric vehicle parallel braking platform is built by LabVIEW software and cRIO real-time controller hardware.A rapid control prototype experiment on parallel braking strategy is carried out using the four-wheel drive electric vehicle on corner braking and single lane braking condition.Results show that main dynamic parameters and motor response are as expecteed,and the vehicle keeps stable,which proves the effectiveness of the parallel braking strategy.Experiment results match simulation results,which proves accuracy of simulation model and reliability of simulation.The strategy processing time of computer platform is less than 20 ms per circle,which proves that the rapid control prototype experiment is real-time and experiment results are reliable.