BLDC motor control algorithm Basic structure of field-oriented vector control of AC motors

All controlled variables are converted to DC instead of AC through mathematical transformation. The goal is to independently control torque and flux. There are two methods of field oriented control (FOC):

Direct FOC: The direction of the rotor magnetic field (Rotor flux angle) is directly calculated by the flux observer.

Indirect FOC: The direction of the rotor magnetic field (Rotor flux angle) is obtained indirectly by estimating or measuring the rotor speed and slip.

Vector control requires knowledge of the position of the rotor flux and can be calculated through advanced algorithms using knowledge of the terminal currents and voltages (using a dynamic model of the AC induction motor). However, from an implementation perspective, the demand on computing resources is critical.

Vector control algorithms can be implemented in different ways. Feedforward techniques, model estimation and adaptive control techniques can all be used to enhance response and stability. 3. Vector Control of AC Motors: A Deeper Look At the heart of the vector control algorithm are two important transformations: the Clark transform, the Park transform and their inverse operations. Using the Clark and Park transforms, the rotor current can be controlled to the rotor area. Doing so allows a rotor control system to determine the voltage that should be supplied to the rotor to maximize torque under dynamically changing loads. Clark Transformation: The Clark mathematical transformation modifies a three-phase system into two coordinate systems:

Where Ia and Ib are components of the orthogonal datum planes, and Io is the unimportant homoplanar part

Figure 5: Relationship between three-phase rotor current and rotating reference frame

4. Park transformation: Park mathematical transformation converts a bidirectional static system into a rotating system vector

The two-phase α, β frame representation is calculated by the Clarke transformation and then input to the vector rotation module, where it is rotated by an angle θ to conform to the d, q frame attached to the rotor energy. According to the above formula, the conversion of the angle θ is achieved.

Basic structure of field-oriented vector control of AC motor

The Clarke transformation uses three-phase currents IA, IB and IC, where the currents of IA and IB in the fixed coordinate stator phases are transformed into Isd and Isq, becoming elements in the Park transformation d, q. The currents Isd, Isq and the instantaneous flow angle θ calculated by the motor flux model are used to calculate the electric torque of the AC induction motor.

Figure 6: Basic principle of vector control of AC motor

These derived values ​​are compared with reference values ​​and updated by a PI controller.

Table 1: Comparison of scalar and vector control of electric motors An inherent advantage of vector-based motor control is that the same principle can be used to select the appropriate mathematical model to control various types of AC, PM-AC or BLDC motors respectively.

Vector Control of BLDC Motors

BLDC motors are the main choice for field-oriented vector control. Brushless motors using FOC can achieve higher efficiency, with the highest efficiency reaching 95%, and are also very efficient at high speeds. 1. Stepper motor control

Figure 7 Stepper motor control usually uses bidirectional drive current, and its motor steps are achieved by switching windings in sequence. Usually there are three drive sequences for this stepper motor: ① Single-phase full-step drive: In this mode, its windings are energized in the following order, AB/CD/BA/DC (BA means that the energization of winding AB is in reverse direction). This sequence is called single-phase full-step mode, or wave drive mode. At any one time, only one phase is energized. ② Two-phase full-step drive: In this mode, both phases are energized together, so the rotor is always between two poles. This mode is called two-phase full-step, which is the normal drive sequence for two-pole motors and can output the maximum torque. ③ Half-step mode: This mode combines single-phase stepping and two-phase stepping to energize: single-phase energized, then two-phase energized, then single-phase energized..., so the motor runs in half-step increments. This mode is called half-step mode, and the effective step angle of each excitation of the motor is reduced by half, and its output torque is also lower. All three modes can be used for reverse rotation (counterclockwise), but not in the opposite order. Usually, stepper motors have multiple poles to reduce the step angle, but the number of windings and the drive order remain unchanged. 2. General DC motor control algorithm General motor speed control, especially the motor using two circuits:

Phase angle control

PWM chopping control

① Phase angle control Phase angle control is the simplest method for general motor speed control. The speed is controlled by changing the firing angle of the TRIAC. Phase angle control is a very economical solution, but it is not very efficient and is prone to electromagnetic interference (EMI).

Figure 8: Phase angle control of universal motors Figure 8 shows the mechanism of phase angle control, which is a typical application of TRIAC speed control. The phase shift of the TRIAC gate pulse generates an effective voltage, which produces different motor speeds, and a zero-crossing detection circuit is used to establish a timing reference to delay the gate pulse. ②PWM chopping control PWM control is a more advanced solution for universal motor speed control. In this solution, the power MOSFET or IGBT switches on the high-frequency rectified AC line voltage, thereby generating a time-varying voltage for the motor.

Figure 9: PWM chopping control of a general purpose motor

The switching frequency range is usually 10-20KHz to eliminate noise. This universal motor control method can achieve better current control and better EMI performance, therefore, higher efficiency.