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Brushless motor control principle

August 29, 2018

In order to rotate the motor, the control unit must first determine the current position of the rotor of the motor detected by HALL-SENSOR and then turn on (or off) the power transistors in the converter (inverter) according to the sequence of the stator windings. As shown in the figure below, AH, BH, CH (these are called upper arm power transistors) and AL, BL, CL (these are called lower arm power transistors) in the inverter, so that the current flows through the motor coils to generate the positive direction ( Or reverse) rotating the magnetic field and interacting with the rotor magnet to rotate the motor clockwise/reverse. When the rotor of the motor rotates to the position where the HALL-SENSOR detects another set of signals, the control unit turns on the next set of power transistors, so that the cycle motor can continue to rotate in the same direction until the control unit decides to stop the motor rotor or turn off the power transistor ( Or just turn off the lower arm power transistor) and reverse the rotor of the motor. The power transistors are opened in reverse order.

Basically, the opening of the power transistor can be explained as follows:

AH, BL group AH, CL group BH, CL group BH, AL group CH, AL group CH, BL group, but can not open into AH, AL or BH, BL or CH, CL. In addition, since the electronic components always have the response time of the switches, the power transistors should consider the response time of the components in the interleaving time between the switches. Otherwise, when the upper arm (or lower arm) is not fully closed, the lower arm (or upper arm) has opened, causing a short circuit between the upper arm and the lower arm, and the power transistor is burned.

When the motor rotates, the control unit compares the speed of the HALL-SENSOR signal with a command (COMMAND) consisting of the speed and acceleration/deceleration rate set by the driver (or by software) and then determines the next set (AH). , BL or AH, CL or BH, CL or ...) turn-on and turn-on time. If the speed is not enough, the speed will be shortened, and this part will be completed by PWM. PWM is a method to determine whether the motor speed is fast or slow. How to generate such PWM is the core to achieve more precise speed control. High-speed speed control must consider whether the system's CLOCK resolution is sufficient to master the time to process software instructions. In addition, the data access mode used to change the HALL-SENSOR signal also affects processor performance and the correctness of the judgment and real-time performance. For speed control at low speeds, especially at low start speeds, the HALL-SENSOR signal changes slowly. It is important to retrieve the signal pattern based on the characteristics of the motor, the processing time and the correct configuration of the control parameters. Or the speed echo becomes an ENCODER change as a reference, so the signal resolution is increased for better control. The motor runs smoothly and responds well. It is suitable for P.I.D. control can not be ignored. As mentioned earlier, the brushless DC motor is closed-loop controlled, so the feedback signal is equal to telling the control unit how much the motor speed is now different from the target speed. This is the error (ERROR). Compensation errors are natural, including traditional engineering controls such as P.I.D. controls. But the state and environment of control is complex and variable. These factors should be considered if the control is strong and durable. Therefore, fuzzy control, expert systems and neural networks will be incorporated into the important theoretical control of intelligent sports.

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