Analysis of Sensorless Control Method for Brushless DC Motor
This paper discusses the position sensorless control method of brushless DC motor. First of all, its core is to obtain the rotor position signal. Currently, the following methods are commonly used in the market.
Flux linkage observation
The motor flux linkage signal is directly related to the rotor position, so the rotor position signal can be determined by the value of the rotor flux linkage. However, the motor rotor flux linkage cannot be directly detected. In order to obtain the rotor flux linkage value of the motor, the phase voltage and current of the motor must be measured first, and a function equation directly related to the rotor flux linkage independent of the rotor speed must be established to calculate the flux linkage value. This method is computationally intensive, and the phase voltage and current contain a large amount of interference signals. Accurate measurement requires high hardware and software costs, so it is rarely used.
2. Back EMF zero crossing detection method
In a brushless DC motor, the back EMF of the winding is usually positive and negative. When the back EMF of a phase winding crosses zero, the rotor's straight axis coincides with the phase winding axis. Therefore, as long as the opposite potentials are detected. At zero point, several key positions of the rotor can be known, thereby eliminating the rotor position sensor and implementing the position sensorless brushless DC motor control. This is currently the most widely used sensorless BLDCM control method.
The disadvantage of this method is that the back EMF signal is zero or very small at rest or low speed, it is difficult to accurately detect the back EMF of the winding, so that an effective rotor position signal cannot be obtained, and the system has a low speed performance, which requires an open loop method to start; In addition, in order to eliminate the interference signal caused by PWM modulation, it is necessary to deeply filter the back EMF signal, which causes the phase shift of the signal related to the motor speed, and the phase shift is compensated to ensure correct commutation.
