Key Control Technology Of Sensorless Brushless DC Motor

Sensorless control strategy for brushless DC motor (BLDCM) can not only solves the problems, such as complicated craftwork, restricting of running environment, which generated by installing position sensor, but also improves the reliability and anti-interference ability of the system. This paper aims at studying rotor position detection, sensorless start-up strategies, and torque pulsation in commutation and non-commutation period of brushless DC motor without position sensor. The specific research contents include the following aspects:1. The status and level of the sensorless control strategy research are summarized. The mathematical model of BLDCM is set up. And based on the mathematical model, the simulation of the BLDCM with position sensor is established. Simulation module is verified according to experimental results and theoretical analysis. Sensorless BLDCM control system platform is built up, and design scheme is verified after experiment, which lays foundations of the theoretical basis and provides contrasting basis of sensorless technology.2. Aiming at the back electromotive force (BEMF) zero-crossing detection problem in extreme speed region (Namely the BEMF signal is too larger at high speed period, thus causing detecting circuit to abnormally work and even to damage the circuit; However at the low speed, the signal is so low that can hardly be detected.), a novel BEMF zero-crossing detecting method and the correspondingly detecting circuit is proposed. Sampling method is switched between high speed region and low speed region, according to the duty ratio of speed feedback signal. Zero-crossing detection is effectively improved in low speed region by using low speed sampling method in the pulse width modulation_OFF(PWM_OFF) interval. While in high speed region, high speed sampling method is used in pulse width modulation_ON(PWM_ON) interval and corresponding hardware circuit (voltage division circuit being controlled by transistor paralleling with each phase circuit) is switched. In the meantime, duty ratio of control signal is adjusted linearly according to the rotor speed, by which the on-off of transistor is controlled and then mediates the resistance of resistance divider of switching circuit. Therefore, the dangerous state that the BEMF amplitude is greater than the power supply of the detecting circuit can be avoided. Experiment results indicate that the new proposed BEMF zero-crossing detecting method can ensure brushless DC motor operate steadily in a wider speed region.3. Aiming at the starting problem of sensorless BLDCM, a novel quasi-closed-loop start-up strategy is proposed, which integrates the precise pre-location of initial rotor-position and the location optimization in accelerating process. When short-width-time pulse voltage vectors are injected into3-phase stator windings, the initial rotor-position are locked in a60°electrical angle range through detecting the DC bus current. On this basis, the precise pre-location is achieved by detecting the current when constant voltage is applied to the assigned stator windings and comparing the threshold of the current. After pre-location, location optimization and acceleration process proceeds. In this process, the current comparing threshold is used as condition to ensure the correct commutation in accelerating period. The system can be switched to BEMF detection mode when the rotor is accelerated from standstill to certain speed (when BEMF zero-crossing signal produces). Experimental results show that, using current threshold as feedback insure that location, acceleration and switching steps are all under closed-loop control. Pre-location accuracy and commutation sensitivity can be adjusted by changing the threshold value, thus ensuring the system to cater to different needs and applications.4. Aiming at the adverse effect of torque ripple which restricts its application in senior field, a direct torque control method which bases on voltage space vector is applied in non-commutation period. In the commutation period, torque ripple is viewed as controlled variable and using PWM method to restrain the produce of torque ripple in the switching of inverter. Direct torque control has the difficulty to get BEMF, so nowadays BEMF reconfiguration with sliding mode variable is used to solve such problem. However, its discontinuous switching characteristics cause system shake. Therefore, sectional sliding mode variable structure is proposed to reconfigure BEMF and restrain system shake. Sliding mode variable structure observer module, flux linkage, torque calculate module, flux linkage section selection module and voltage space vectors selection module were firstly built and simulated in Matlab/Simulink. Then, direct torque control method system with sectional sliding mode variable structure for BLDCM was built and simulated. Simulation result verifies the effectiveness. In the end, direct torque control system platform is built up. Torque ripple is restrained effectively for both commutation period and non-commutation period. Timeliness and robustness are improved compared to traditional control systems.