| In recent years,global environmental pollution and energy shortages have become increasingly serious,forcing governments around the world to formulate stricter vehicle emission standards and vigorously develop research and application of new technologies for vehicle energy saving and emission reduction.Pure electric vehicles with characteristics of zero emission and high energy utilization have become the current research hotspots of major automobile manufacturers and scientific research institutes around the world.Regenerative braking system(RBS)can improve vehicle economy and extend vehicle mileage by recovering the energy in the vehicle’s braking process,which is of great significance to the realization of energy saving and emission reduction for pure electric vehicles.However,only using the RBS for braking may not be able to meet the braking performance requirements of vehicles,because the use of RBS is restricted by many factors.Therefore,the braking system of a pure electric vehicle is an electrohydraulic compound braking system that is a combination of the RBS and the hydraulic braking system.How to coordinate control of motor brake and hydraulic brake during braking to ensure the safety and stability of vehicle braking while efficiently recovering braking energy is still an urgent problem to be solved.In order to achieve maximum recovery of braking energy under the premise of ensuring good braking performance of the vehicle,taking the four-wheel-drive pure electric SUV driven by the front and rear dual motors as the research object,a set of electro-hydraulic compound braking system is designed,and a coordinated control strategy of the supporting electro-hydraulic compound braking system is formulated in this paper.The main research work is as follows:(1)Combining the characteristics of the power transmission structure of the fourwheel-drive pure electric SUV,a new electro-hydraulic compound braking system is designed on the basis of the traditional hydraulic anti-lock braking system(ABS),and the corresponding mathematical model of the braking system and the seven-degree-offreedom vehicle dynamics model are established for the proposed electro-hydraulic compound braking system structure scheme.(2)A braking intention recognition algorithm based on fuzzy control,a longitudinal vehicle speed estimation algorithm based on extended kalman filter,and a road recognition algorithm based on the magic tire formula μ-s relationship are designed,which are respectively used to identify the driver’s braking intention,estimate the longitudinal vehicle speed and estimate the peak adhesion coefficient and optimal slip rate of the road surface,and the designed algorithms are simulated in Matlab/Simulink to verify its effectiveness.(3)The optimization of the charging efficiency when the motor and the battery work together is studied,and the optimal distribution law of the front and rear motor braking torque under different braking strengths and vehicle operating conditions is obtained.Based on the above-mentioned law and taking braking regulations as constraints,a braking force distribution control strategy that comprehensively considers braking safety and charging efficiency optimization is proposed.(4)In order to prevent wheel lock and recover braking energy during ABS braking,with braking safety and braking energy recovery efficiency as the control goal,a model predictive control strategy based on particle swarm optimization is proposed for coordination RBS and ABS.This strategy calculates in real time the required braking torque of the wheel to maintain the wheel slip rate at the optimal slip rate,and gives priority to the use of motor to provide the braking torque required by the wheel and controls the hydraulic braking system to provide the remaining required braking torque,thereby completing the wheel anti-lock braking control while recovering the braking energy.(5)The forward simulation model of four-wheel-drive pure electric SUV is established in Matlab/Simulink,and the proposed coordinated control strategy and the traditional coordinated control strategy based on logic threshold control are used to simulate vehicle braking on roads with different adhesion coefficients.The results show that,compared with the traditional coordinated control strategy,the proposed coordinated control strategy can not only ensure braking stability and effectively shorten the braking distance and braking time,but also increase the braking energy recovery efficiency by more than23.29%,verifying the feasibility and effectiveness of the designed electro-hydraulic compound braking system and coordinated control strategy. |
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