Research On BLDC Motor Sensorless Direct Torque Control

The brushless DC motor is widely used in industry due to its advantages suchas low price and high torque density. Traditional BLDC position detection isachieved by position sensor. In order to reduce the cost, and due to in theapplications such as high temperature and high voltage, position sensor mightbecome invalid, BLDC motor sensorless control have become current researchfocus. Traditional position detecting methods might fail in high power applications,and is limited by conduction modes. The torque ripple is usually high when theBLDC operates in traditional conduction modes, so it is necessary to study thetorque ripple suppression algorithm.Firstly, three conduction modes of BLDC motor are discussed in this paper,and torque ripple in different conduction modes is compared. Simulation models areestablished, and the relationship between conduction modes and the torque ripple isdiscussed. The method of improving the torque when the DC voltage is constant isanalyzed. The limits of the phase voltage zero-crossing detecting method arestudied by means of analyzing the relationship between the phase voltage and theback EMF.Secondly, the sliding mode observer is proposed in order to solve the problemthat the BLDC motor sensorless control has its shortcomings, such as traditionalposition detection methods are restricted by conduction mode and fail because ofthe phase current freewheeling. Improved switching function is proposed to inhibitthe high frequency noise. The extended state sliding mode observer is analyzed toavoid amplitude attenuation and phase shift. Simulation models are established inMatlab/Simulink and the results show that the proposed sliding mode observer canestimate the rotor position and is not influenced by conduction modes and phasecurrent.Thirdly, direct torque control method is analyzed to inhibit the torque ripple.Based upon sliding mode observer, the improved torque control method is proposed,and it avoids calculating the stator flux linkage, which is hard to be accurate. Threeways of section division are compared and simulation models are established to findout which section division method would cause the lowest motor loss.In the end, a DSP control platform based upon TMS320F2812is designed. Theexperimental results prove that the proposed sliding mode observer can estimate therotor position accurately, and the sources of error are discussed, and on the basis ofthe analysis, the error suppression methods are proposed.