The Composite Power System And Energy Distribution Strategy For Four Wheel Independent Drive Electric Vehicle

Four wheel independent drive electric vehicle with less power transmission components and high maneuverability has attracted more and more attention.However,at present,due to the limitation of the power battery technology,the four wheel independent drive electric vehicle has short range of endurance and short service life of battery,which limits the development of the four wheel independent drive electric vehicle.The composite power is composed of battery,ultracapaitor,and so on.As the power source of electric vehicle,the composite power can not only meet the instantaneous high power demand of electric vehicle,but also improve the power performance of the electric vehicle.Besides,the rational distribution of the energy of the composite power system and the recovery of the braking energy can increase the driving range of the electric vehicle.In this paper,the composite power system(composed of lithium batteries,ultracapacitors,and DC/DC converters)and energy distribution management strategy on the four wheel independent drive electric vehicle were studied.The main research contents include:(1)Characteristics analysis and model establelish of component characteristics of composite power system.The charge and discharge tests of the lithium battery and the ultracapacitor in the composite power system were carried out and the working characteristics were analyzed.The working characteristics of the DC/DC converter and the working characteristics of the permanent magnet brushless DC motor were analyzed.According to the operating characteristics of the components of the composite power system,the basic equations and the simulation models were esTablelished.(2)Topology design of composite power system.On the basis of the comprehensive analysis of the topology of the existing composite power system,the working characteristics of the four wheel independent driving electric vehicle and the energy utilization efficiency of the composite power system,the structure of the four wheel independent driving electric vehicle with two modes of driving and braking working conditions was designed.The parameters of the components of the composite power supply system are calculated,matched and selected with the aims of the power and economy of the whole vehicle and the constraints of the charging and discharging conditions of the ultracapacitor and the battery.Due to the unbalanced voltage among the ultracapacitor groups of the composite power system,the methods using a flying capacitor to banlance the voltage of the ultrcapacitor group was analyzed.(3)The power distribution strategy of the composite power system.According to the existing fuzzy control strategy,the energy distribution strategy based on speed were analyzed and the driving characteristics of the vehicle in urban driving condition,an improved composite power distribution strategy based on speed was designed.The upper and lower limited SOC of the ultracapacitor was determined by this strategy with the current speed of the vehicle.According to the relationship between the actual and limited value of SOC,as well as the driving demand power of the vehicle,the input and output energy of the battery and the ultracapacitor were distributed reasonably to realize a better role of peak load and valley filling of ultracapacitor,and protect battery from the impact of large current,and improve the energy utilization of the composite power system.(4)Simulation analysis of system.The simulation model of the composite power system and its energy distribution strategy were esTablelished in SIMULINK.The simulation analysis of the working process of composite power system of the four wheel independent drive electric vehicle under the different energy distribution strategy and different structure of power system were carried out.The performance of the composite power system was compared with that of the single power system.The performance of the dual-mode composite power system structure and energy distribution strategy on the four wheel independent electric vehicle was verified.Finally,built the composite power system on the existing four wheel independent drive electric test platform in the laboratory.Using NI data acquisition card to test the four wheel independent driving electric vehicle under single working condition and urban cycle condition.And the power performance,economical efficiency,braking energy recovery performance of the vehicle and composite power system were tested.The simulation and test results are as follows.In the condition of ensuring the power of the vehicle,The ultracapacitor in the composite power system bears most of the peak current,which can better play the role of peak shaving and valley filling to protect the battery from large current than the single power system and improve performance of the battery.Compared with the existing fuzzy control strategy and the speed based energy distribution strategy,the improved energy distribution strategy for the four wheel independent drive electric vehicle can make the composite power system better and more stableto coordinate the energy distribution between the battery and the ultracapacitor,thus stableilizing input and output current of battery,giving full play the role of ultracapacitor in peak shaving and valley filling.Besides,Compared with the composite power system structure with single working mode,the simulation result shows that the energy recovery efficiency under the improved dual-mode composite power system structure is increased by 13.53%,the test result shows that the energy recovery efficiency under the improved dual-mode composite power system structure is increased by 10.26%.which prolong the vehicle's endurance mileage effectively.