Research On Control Strategy Of Sensorless High Speed Brushless DC Motors

In recent years,high speed brushless DC motor has developed rapidly with its advantages of good reliability,simple control,high efficiency,small size and high power density.In order to accurately detect the rotor position,brushless DC motor is usually installed position sensor,but doing like this would increase the cost and make it more complicated for the system design,besides,its reliability and anti-interference ability would be decreased,so the motor without position sensor control technology has become a hotspot in current researches,and this paper will focus on the control strategy for sensorless brushless DC motor,the concrete work is as follows:First of all,the related technologies of high-speed brushless DC motor are overviewed,including the six-step control method,the back EMF detection method and speed modulation mode.Finally deciding to adopt the back EMF zero crossing method to detect back emf,and after considering the operability of the experiment and the influence of freewheeling,the speed modulation mode of upper and lower arm alternating PWM is selected.Then on the basis of a deep understanding of the working principle of brushless DC motor,the mathematical model is constructed,and the commutation logic of the motor is further analyzed: the specific implementation method to detect the back EMF zero crossing point is the terminal voltage method based on the reconstruction of the neutral point;and the response of direct commutation to the problem that the freewheeling time exceeds 30 electric angle in the motor control process is given.As for the problem of commutation delay,the calculation formula of delay Angle is provided and the commutation compensation is carried out.At the same time,for the motor starting problem,considering the success rate of starting under the condition of large load and that the design should be as simple and convenient as possible,a method based on software that needs to increase frequency and voltage is proposed.Then the hardware system needed for the experiment is designed.Firstly,the reason why the main control chip is selected is explained.Secondly,in the power supply part,the power supply chip is used to realize the voltage conversion step by step to meet the power supply of the main control chip and the driver chip respectively.In the part of the back EMF information collection,the designed circuit not only satisfies the terminal voltage method of reconstruction of neutral points mentioned in the control strategy above,but also realizes the signal filtering processing.And at last,some analog signal acquisition circuits are as well described in detail.Finally,in order to verify the proposed control strategy,the model was built on the Matlab/Simulink platform,and an experimental platform was built based on a domestic microchip MM32SPIN27 PF.The measured three-phase terminal voltage,phase current,back EMF signal and speed waveforms are all in line with expectations,which proves the feasibility of the selected control strategy.