Research On Braking Control Strategy Of Electric Vehicle

With little pollution emission, low noise, high thermal efficiency, and important role in improving the energy structure, Electric Vehicle (EV) has become the development trend of vehicle, drawing more and more attention. As a key point of EV technology, braking technology has become the hot issue. Braking system of EV consists of mechanical braking and regenerative braking.EV has the function of regenerative braking, which is one of the most significant differences from conventional vehicle with Internal Combustion Engine(ICE).During regenerative braking, the motor works as a generator, storing a portion of the Mechanical energy into energy storage devices, thereby the energy efficiency improved. The key issue of EV braking technology that how to control mechanical braking torque and regenerative braking torque to improve the energy efficiency on the premise of ensuring safety is of great significance and research value.The EV studied in this thesis was a pure electric sedan transformed from a conventional vehicle with ICE. The thesis mainly makes a study of the control strategy of Electric Brake-force Distribution (EBD) in mechanical braking and the control strategy of coasting and braking in regenerative braking.In studying of the control strategy of EBD, through analyzing forces of vehicles during braking, ideal brake-force distribution curve wherewith to analyze characteristics and control requirements of EBD is derived. The thesis presents three working conditions of EBD, and by analyzing control requirements of every working condition, a control strategy of EBD based on wheel slip ratio and acceleration threshold is proposed. The control strategy identifies working conditions by vehicle information, and then adopts different strategies for every different working condition. Simulation based on ve-DYNA and Hardware-In-the-Loop test based on dSPACE are performed, and results show that the EBD control strategy meets the functional requirements of controlling rear wheel pressures in every working conditions, preventing rear wheels locking before front wheels, improving braking comfort and ensuring high braking efficiency.Regenerative braking system should be analyzed before developing the control strategy of regenerative braking. The thesis modeled regenerative braking system of EV that consists of vehicle dynamic model, driveline model, model of motor system and battery model. According to actual physical process, constraints of regenerative braking, including braking regulations, characteristics of motor, characteristics of battery and braking comfort are analyzed.The EV that studied here adopts the scheme of parallel compound braking that mechanical braking torque is incontrollable and only regenerative braking torque can be controlled.In this thesis, regenerative braking is separated into coasting energy recovery and braking energy recovery, and then control strategy is developed for each condition. For coasting energy recovery, according to rolling optimization concept, an off-line optimization control strategy of the regenerative braking torque is proposed. During coasting, the control strategy aims at maximum of the energy recovered in fixed time to plan the regenerative braking torque on the premise of satisfying constraints. The executing time of the optimization control strategy is so long that it's impossible for on-line computing with Electronic Control Units (ECU) on the vehicle, therefore table looking-up and interpolating is adapted here to implement the control strategy, besides, influencing factors of the velocity, SOC of battery and voltage of battery are introduced to limit output torque of the look-up table and smooth the regenerative braking torque. For braking energy recovery, the thesis adopts parallel compound braking as a result that the mechanical braking torque is incontrollable, and a control strategy of braking energy recovering is proposes based on drive's braking intention. In the control strategy, drive's braking intention is interpreted from brake pedal stroke that used to plan the regenerative braking torque, besides, influencing factors of the velocity, SOC of battery and voltage of battery are introduced to limit planned torque and smooth the regenerative braking torque.In developing of the automotive electronic software, V-cycle development based on model is widely used and is adopted in this thesis. The model of control strategy is constructed and simulated based on Matlab/Simulink, and then C code is generated form model via Targetlink, the automatic code generator, after the code downloaded to ECU, Hardware-In-the-Loop Simulation (HILS) is carried out to validate the control strategy.The thesis at last carries out HILS, Vehicle Chassis Dynamometer (VCD) test, and road test to calibrate key parameters during regenerative braking, providing data for further developing and applying of the control strategy. During test, data of Controller Area Network (CAN) bus is recorded and displayed on-line, and control parameters are calibrated by applying the CAN Calibration Protocol (CCP).Test results show that motor switches smoothly from drive mode to brake mode and contrarily, the control strategy realizes function of energy recovering and ensuring the braking comfort.