Control Principle Of Brushless DC Motor

The control principle of the brushless DC motor, to make the motor rotate, the control part must first determine the position of the rotor of the motor sensed by the hall-sensor, and then decide to open (or close) the power in the inverter according to the winding of the stator. The order of the transistors, AH, BH, CH in the inverter (these are called upper arm power transistors) and AL, BL, CL (these are called lower arm power transistors), make the current flow through the motor coil in sequence to produce forward (or reverse) ) rotates the magnetic field and interacts with the rotor's magnets so that the motor turns clockwise/counterclockwise. When the motor rotor rotates to the position where the hall-sensor senses another group of signals, the control unit turns on the next group of power transistors, so that the circulating motor can continue to rotate in the same direction until the control unit decides to turn off the power if the motor rotor stops. transistor (or only turn on the lower arm power transistor); if the motor rotor is to be reversed, the power transistor turn-on sequence is reversed.

Basically, the opening method of power transistors can be as follows: AH, BL group → AH, CL group → BH, CL group → BH, AL group → CH, AL group → CH, BL group, but must not Open as AH, AL or BH, BL or CH, CL. In addition, because electronic parts always have the response time of the switch, the response time of the power transistor should be taken into account when the power transistor is switched off and on. Otherwise, when the upper arm (or lower arm) has not been completely closed, the lower arm (or upper arm) has already Turn on, as a result, the upper and lower arms are short-circuited and the power transistor is burned out.

When the motor rotates, the control unit will compare the command (Command) composed of the speed set by the driver and the acceleration/deceleration rate with the speed of the hall-sensor signal change (or calculated by software), and then decide the next group ( AH, BL or AH, CL or BH, CL or ...) switches are turned on, and how long they are on. If the speed is not enough, it will be long, and if the speed is too high, it will be shortened. This part of the work is done by PWM. PWM is the way to determine whether the motor speed is fast or slow. How to generate such PWM is the core of achieving more precise speed control.

The speed control of high rotation speed must consider whether the CLOCK resolution of the system is sufficient to grasp the time for processing software instructions. In addition, the data access method for the change of the hall-sensor signal also affects the performance of the processor and the correctness and real-time performance of the judgment. As for low-speed speed control, especially low-speed starting, the change of the returned hall-sensor signal becomes slower. It is very important to acquire the signal method, process the timing, and configure the control parameter values appropriately according to the motor characteristics. Or the speed return change is based on the encoder change, so that the signal resolution is increased for better control. The motor runs smoothly and responds well, and the appropriateness of the P.I.D. control cannot be ignored. As mentioned earlier, the brushless DC motor is a closed-loop control, so the feedback signal is equivalent to telling the control unit how far the motor speed is from the target speed, which is the error (Error). Knowing the error, it is necessary to compensate, and the method is traditional engineering control such as P.I.D. control. However, the state and environment of control are actually complex and changeable. If the control is to be sturdy and durable, the factors to be considered may not be fully grasped by traditional engineering control, so fuzzy control, expert system and neural network will also be included as intelligent Important theory of P.I.D. control.