| Distributed drive electric vehicle(DDEV)provides more potential and opportunity for vehicle active safety technology because of applying wheel-side /in-wheel motor.At present,in order to ensure the reliability of braking,most of the distributed drive vehicles have installed a parallel hydraulic braking system even though the anti-drag torque can be provided by the distributed drive motor.However,the existing electro-hydraulic composite braking strategies often have the problems of small motor participation and over-dependence on hydraulic braking under emergency braking conditions.Due to the neglect of the advantages of fast response of the motor,this also restricts the performance of distributed drive electric vehicles in emergency braking conditions.Therefore,this paper aims to improve the performance of distributed drive vehicles under emergency braking conditions,the main contents are as follows:Firstly,the structure and working principle of the electro-hydraulic composite braking system is introduced,the nonlinear characteristics of the hydraulic brake and the motor brake are studied,and the mathematical models are built.A new adaptive frequency division electro-hydraulic hybrid braking strategy is designed through the idea of frequency reconstruction,which distribute the high frequency part of the desired braking torque to the motor brake so as to give full play to the advantages of the fast response of the motor,while distribute the low frequency part to hydraulic brake in order to reduce the working power of the motor.According to the frequency-domain characteristics of the desired braking torque signals,a cost function is designed to determine the cut-off frequency of the frequency reconstruction.Secondly,the method of lateral stability control under braking conditions is studied.The direct yaw-moment control(DYC)and the antilock braking system(ABS)of the DDEV is combined by considering the trade-off between braking performance and lateral stability of the vehicle under specific working conditions.The ABS / DYC is jointly controlled by model predictive control(MPC)to obtain the expected braking torque of each wheel during emergency braking.Then,the influence of road conditions on the vehicle braking stability is considered.Based on the mixture of Gaussians model,a tire-road friction coefficient(TRFC)observer is proposed.After that,the feedforward braking torque is calculated by the observed TRFC,so as to further improve the braking performance and lateral stability of the vehicle in emergency braking conditions.Finally,the ABS / DYC cooperative MPC control subsystem,the TRFC based feed-forward control subsystem,and the adaptive frequency division electro-hydraulic hybrid braking system are integrated.Through the high-precision vehicle simulation software Truck Sim,the adaptive frequency division electro-hydraulic composite braking system,ABS/DYC cooperative MPC controller and integrated system are simulated and verified.The simulation results show that the adaptive frequency division electro-hydraulic composite braking strategy can significantly improve the braking performance compared with the traditional hydraulic braking system;and can reduce the power of the motor compared with the pure motor braking.It is verified that the ABS / DYC cooperative MPC controller can simultaneously ensure the braking performance and lateral stability of the vehicle by changing the MPC weights under different working conditions.And verify the effectiveness of the adaptive frequency division based electro-hydraulic hybrid braking system system by real car experiments. |
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