| The brushless DC motor(BLDCM) is a kind of electromechanical integration machinery, and it has been rapidly developing in recent years with the help of the development of power electronic technique and the appearance of new type of permanent magnet material. With it's widely used in many fields, the sensorless control especially back-EMF method of the motors is more useful than sensor control in many applications.In this thesis, the common methods of sensorless control of BLDCM are presented, and the principle of back-EMF method is introduced in detail. Two kinds of methods of the zero-crossing of back-EMF sensing are analyzed,and the mothod of direct back-EMF is adopted to design the zero-crossing of back-EMF sensing circuit in which the motor neutral point voltage information isn't needed. The zero crossing signal of the true phase back EMF can be directly obtained from the motor terminal voltage by properly choosing the PWM and switch control strategy. As the result, the method proposed is not sensitive to the switching noise, no filtering of the motor terminal voltage is required. The circuit model is established by PSPICE software and the simulation results verify the theoretical analysis. In this system, the BLDC motor applied in the massage chair is used as a sample and the hardware of the BLDCM control system on direct back-EMF method are designed , each part of this system is introduced in detail. In additional, the methods to reduce electromagnetic interference(EMI) are also analyzed.The common start methods of sensorless BLDCM control system are introduced, and the three-step start technique, including the rotor pre-setting, accelerating and state-changing step, is discussed carefully. The control software flow of direct back-EMF is introduced around three-step start technique in detail.Finally, The experimental results show the method useful and correct.
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