Research On Sensorless Control System Of Brushless DC Motor

Brushless DC motors(BLDCM)has been widely used due to its excellent performance,However,the drawbacks of traditional rotor position sensors in the cost and reliability limit the application of BLDCM.The research on sensorless BLDCM drives has important practical significance.In this paper,depth theoretical analysis,simulation and experimental study on several key technologies such as rotor position detection method,commutation error correction strategy and motor start-up control algorithms are performed for sensorless BLDCM.The sensorless control of BLDCM by detecting the third harmonic back-EMF is a relatively simple and potentially low-cost technique.By the Fourier series analysis of BLDCM back EMF waveform,and the theoretical derivation on the characteristics of the full-bridge PWM modulation,the principle of the traditional back-EMF third-harmonic method is analyzed in detail.Then,an improved back-EMF third harmonic rotor position identification theory is presented.This method does not depend on motor parameters,without the need to reconstruct the motor mid-point,and provides a triple gain for the third harmonic back-EMF amplitude,theoretically with better low-speed performance.The waveforms of the third harmonic back-EMF were analyzed under the condition of accurate,advanced and lagging commutation.In order to correct commutation errors,a novel closed-loop correction algorithm based on the integration of the third harmonic back-EMF is proposed.The integration difference between the adjacent 60 electrical degrees interval before and after the commutation point is utilized as the feedback of the PI regulator to compensate the errors automatically.The paper analyzes the relations between the stator windings equivalent inductance and the initial rotor position,then designs a method to determine the initial rotor position by comparing DC bus current when voltage vectors injected into the stator windings.In the process of motor acceleration,the motor is driven by two-phase voltage vector.By applying two three-phase voltage vectors which have a 120 electrical degrees phase angle difference,and the relative size of the DC bus current response is compared to track the rotor position.The difference value of current response is proportional to the difference value of the back-EMF between stator phases.The simulation files of the model program based on Simulink and DSP Builder are designed,and a system experimental test bed based on FPGA is established for proposed sensorless control schemes of BLDCM.The signal processing techniques such as Bessel filter,FIR digital filter,median average filter and pulse anti-shake filter were used to test the control performance of sensorless BLDCM.Experimental results reveal the effectiveness of the proposed schemes.