Double Closed-Loop Control for Brushless DC Motor

First and foremost, it should be noted that the torque is changing when the BLDC motor is driven by the 120° square wave. If the current is constant, the instant torque is changing within every Hall phase shift section. (It is regarded as a part of the sinusoidal wave.) Therefore, the relationship between the current and torque mentioned here should be interpreted as the relationship between the average current and the average torque.

In the closed-loop, the current loop can control the average current. In the general double closed-loop design, the detected current is an instant value rather than the average value. Besides, the current increases from zero to certain value after phase shift of the brushless DC motor, when the duty ratio is constant. Even if the electric current loop is added, the current also increases from 0. Because of the electric motor's inductance, there is a minimum for the rise of the current. In some electric motor control, the current is also fluctuating.

The double closed-loop itself can influence the electric motor control. It joins the current loop as an ordinary PID algorithm. Due to influence of this loop, the dynamic process will slow down. Here, dynamic refers to the regulation process after stabilization of the brushless DC motor and under the disturbance. This conclusion suggests that there is a time constant for the electric motor speed loop and current loop regulation. These two time constants, if put together, will slow down the system regulation. However, we often think that the double loop accelerates the regulation process. This is wrong, because the double closed-loop is proposed for the electric motor's launch process. When the closed-loop is ideal for the BLDC motor, it can launch the electric current. When it is stable, there is only the speed loop. However, the double closed-loop design can hardly ensure that it has just one loop. Therefore, when there are two loops, the influence degree is different under two conditions. When it is started, the current loop plays the decisive role. When the speed is stable, the speed loop plays the dominant role.

Role of the current ring: At the core of the current ring is the current ring itself, which can operate at the maximum permissible torque. Proceeding from this point, the current loop mainly has three functions:
1. Acceleration of the launch process
2. Anti-pull-in current protection
3. Anti-interference of the voltage fluctuation

PID compilation of the double closed-loop is carried out in three steps: 1. speed loop compilation and debugging, 2. current loop compilation and debugging, 3. compilation of the double closed-loop. The compilation of the double closed-loop consists of two aspects, namely output of the speed ring and demarcation of the current ring. PID parameter regulation is to regulate with the PIK of the speed loop remaining unchanged or to regulate I, or in accordance with the principle of regulating P of the current loop and decreasing I.

Besides, if the current loop is added and there are requirements of the current detection precision and scope, the digital PID should basically has one deviation. However, the digital controller itself has one requirement of the deviation, that is, it should not be too small. If it is too small, the cutoff error can be easily caused. If it is too large, the calculation results and P value might be improper, for the PID regulation should be consistent with the AD scope.

The current loop and the speed loop are debugged separately, because the PID value is different under the high-speed and the low-speed or when the current is small and large. In terms of different loops, PID debugging is necessary under different values. The sectional PID can be increased to ensure a good applicability for the brushless DC motor.