Research On Sensorless Control Of Brushless DC Motor Based On Improved Sliding Mode Observer

In recent years,unmanned aerial vehicles(UAVs)have been widely used in military,agriculture,forestry and mining industries.Brushless DC motor has the characteristics of high power density and simple structure,which is widely used in the power of agricultural plant protection UAV.Sensorless control can not only reduce the cost of the whole system but also meet the requirements of the complex working environment of plant protection UAV.Therefore,this thesis takes brushless DC motor as the research object,and studies the hardware circuit design and software program of low speed I/F(Current frequency ratio control)and medium and high speed sliding mode observer algorithm respectively.The main research contents are as follows :(1)In this thesis,the mathematical models of brushless DC motor in three-phase stationary coordinate system,two-phase stationary coordinate system and two-phase dq rotating coordinate system are deduced,and the load model of UAV blade is established.The principle of vector control of brushless DC motor is expounded,and the simulation model of double closed-loop vector control is built and verifed.(2)This thesis analyzes the principle of the I/F control method used in the low speed stage and the sliding mode observer used in the medium and high speed stage.Based on the double closed-loop vector control speed regulation system,the traditional sliding mode observer is constructed,and the reasons for the chattering and the large speed estimation error in the traditional sliding mode observer are analyzed.Aiming at these problems,this thesis improves the traditional sliding mode observer by using the variable reaching law and saturation function instead of the fixed gain and symbol function of the traditional sliding mode observer.Aiming at the error in the process of back electromotive force extraction,a back electromotive force observer based on model reference adaptive is designed.Finally,the motor speed and rotor position are extracted by phase-locked loop.The simulation results show the maximum estimation position error of rotor position decreases by 50 %.(3)In this thesis,the hardware controller with STM32F407IGT6 as the main control chip is designed and manufactured for the UAV power system,including power supply circuit,half bridge pre drive circuit and drive circuit,analog sampling circuit,over current protection circuit and communication and program download circuit.The software adopts the programming idea of high cohesion and low coupling,and the full speed domain sensorless control scheme is designed by module.Finally,according to the simulation experiment and application background,the experimental platform is built,the hardware and software of the controller are debugged,the experimental data waveform is analyzed,and the feasibility of the control scheme proposed in this paper is verified.The estimated rotor position of the motor is compared with the Hall signal,and the estimated rotor position lags the Hall position signal by 0.1600 ms / cycle,which meets the requirements of the control system.