A Position Sensorless Control Strategy For BLDCM Based On Back-EMF Function

The BLDCM has been widely used in electric vehicles,numerical control machine tools,aviation and national defense due to its high power density,high output torque,high reliability and simple structure.Normally,rotor position sensor is usually required in order to realize precise commutations.However,the installation of the position sensor will increase the motor volume,and the installation precision will directly affect the motor performance.Therefore,the position sensorless control strategies for BLDCM have been attracted extensive attention from domestic and foreign scholars.Under high speed and heavy load conditions,the floating phase of BLDCM has longer commutation freewheeling time.Sometimes the freewheeling interval even exceeds 30 electrical degrees,which result in conventional methods fail to detect the back-EMF zero-crossing signal,causing commutation failure.Base on the back EMF and back-EMF slope of the floating phase,a novel sensorless control strategy for the brushless DC motor is proposed in this paper.By dividing the back EMF and back EMF slopes obtained online with the offline stored reference values,a back-EMF function independent of the motor speed is constructed.The threshold value directly corresponds to the motor commutation signal,and no back-EMF zero-crossing signal is needed.Detecting the voltage of floating phase by synchronous sampling technology and obtaining the back EMF and back-EMF slope of the floating phase,the calculation of the back EMF function can be performed.When the value of the back EMF function is equal to the set threshold,the motor commutation signal is output.The calculation of the back EMF function does not need the low-pass filter,so the strategy also avoids the phase shift problem caused by filtering.At the same time,the threshold of the back EMF function directly corresponds to the motor commutation signal and the output of the commutation signal has nothing to do with the calculation interval of the back EMF function.Therefore,the motor can be controlled under the condition that the commutation time is more than 30 electrical degrees.Finally,an experimental platform with DSP+FPGA as the core is built.The correctness of theoretical analysis and the effectiveness of the proposed control strategy are verified under low speed and light load conditions,high speed and heavy load conditions and rated conditions.The experimental results show that the position sensorless control method proposed in this paper can accurately obtain the motor commutation signal under the condition that the back-EMF zero-crossing signal cannot be detected.