Simple sine wave control of brushless DC motor based on XC866

Preface

With the development of control technology and the increasing demand for energy saving in society, brushless DC motors have been widely used as a new type of high-efficiency motor. Traditional brushless DC motors use square wave control, which is simple to control and easy to implement. However, they have problems such as torque pulsation and commutation noise, and are limited in some applications that require noise. For these applications, sine wave control can solve this problem well.

Introduction to Sine Wave Control of Brushless DC Motors

The sine wave control of the brushless DC motor is to generate a sinusoidal current in the motor winding by applying a certain voltage to the motor winding, and to control the motor torque by controlling the amplitude and phase of the sinusoidal current. Compared with the traditional square wave control, the motor phase current is sinusoidal and changes continuously without sudden changes in the commutation current, so the motor operation noise is low.

According to the complexity of control, the sinusoidal wave control of brushless DC motor can be divided into: simple sinusoidal wave control and complex sinusoidal wave control.

(1) Simple sine wave control:

A certain voltage is applied to the motor winding to make the motor phase voltage a sine wave. Since the motor winding is an inductive load, the motor phase current is also a sine wave. By controlling the amplitude and phase of the motor phase voltage to control the phase and amplitude of the current, it is called voltage loop control, which is relatively simple to implement.

(2) Complex sine wave control:

Different from simple sinusoidal control, complex sinusoidal control targets the motor phase current, establishes a current loop, and directly controls the phase and amplitude of the phase current to achieve the purpose of controlling the motor. Since the motor phase current is a sinusoidal signal, it is necessary to perform current decoupling operations, which is relatively complex. Common ones are field oriented control (FOC) and direct torque control (DTC).

This article will mainly introduce the principle and implementation of simple sinusoidal wave control.

Simple sine wave control principle

Simple sinusoidal wave control is to control the motor current by controlling the amplitude and phase of the motor's sinusoidal phase voltage. Usually, a certain form of voltage is applied to the motor terminal line to generate a sinusoidal phase voltage at both ends of the winding. Common generation methods are: sinusoidal PWM and space vector PWM. Since the sinusoidal PWM principle is simple and easy to implement, it is usually used as a PWM generation method in simple sinusoidal wave control. Figure 1 is a BLDC control structure diagram, where Ux, Uy, and Uz are bridge arm voltages, and Ua, Ub, and Uc are phase voltages of the motor winding. The following introduction to different types of PWM modulation methods will be based on this structure diagram.

Figure 1 Brushless DC motor control block diagram

(1) Three-phase sinusoidal modulation PWM

Three-phase SPWM is the most common sinusoidal PWM generation method, that is, a sinusoidal voltage signal with a phase difference of 120 degrees is applied to the three end lines of the motor. Since the neutral point is 0, the motor phase voltage is also sinusoidal, and the phase is the same as the applied sinusoidal voltage, as shown in Figure 2.

Figure 2 Three-phase modulation SPWM terminal line voltage

(2) Sinusoidal PWM with minimum switching loss

Different from the common SPWM, when the switching loss is minimized by using the sine PWM, the voltages Ua, Ub, and Uc applied to the motor terminals are not sine wave voltages. At this time, the motor center voltage is not 0, but the motor phase voltage is still sine. Therefore, this type of control method is line voltage control. See Figure 3:

Figure 3. Sinusoidal PWM terminal line voltage with minimum switching loss

Among them, Ux, Uy, Uz are the motor terminal line voltages, and Ua, Ub, Uc are the motor phase voltages. It can be seen that the phase difference of the phase voltage is 120 degrees. The relationship between Ux, Uy, Uz and Ua, Ub, Uc is as follows:

After merging, Ux, Uy, and Uz are as follows:

It can be seen that when the switching loss is minimized by using the sine PWM, the phase difference between Ux, Uy, and Uz is 120 degrees, and they are in the form of piecewise functions, not sinusoidal voltages, while the motor phase voltages Ua, Ub, and Uc are still sinusoidal voltages. And within the 120-degree area, the terminal line voltage is 0, that is, the corresponding switch tube is normally open or normally closed. Therefore, compared with three-phase sine PWM, the switching loss is reduced by 1/3.

By controlling the phase and amplitude of Ux, Uy, Uz, Ux, Uy, Uz can be controlled to achieve the purpose of controlling current.

Implementation of Simple Sine Wave Control for Brushless DC Motor

System Structure

The system structure is shown in Figure 4. The working principle is as follows: the Hall input signal is automatically filtered and sampled to obtain a reliable commutation signal, which can be used to estimate the rotor angle and speed. The speed PI regulator calculates the Modulation of the sinusoidal PWM according to the given speed value and the feedback speed value. The position estimation unit estimates the rotor position angle Angle using the speed and commutation information. The lead angle Δ is compensated by the lead angle adjustment unit to obtain Angle. The SPWM unit uses the Modulation and Angle information to generate the SPWM with the minimum switching loss and output it to the inverter unit. The following content introduces the principles and implementations of each unit.

Figure 4 System block diagram

Generation of sinusoidal PWM with minimal switching losses

Since the phase difference among Ux, Uy and Uz is 120 degrees, Ux is taken as an example for analysis.

Ux is a piecewise function, and is a sine function and is symmetrical with . It is only necessary to implement one segment and process the other segment symmetrically.

Implementation:

Therefore, it can be realized by using only a sine table of 0-120 degrees, that is , where M is the amplitude. The realization of Uy and Uz is similar to that of Ux, with a phase difference of 120°.

By controlling M and x, the amplitude and phase of the motor phase voltage can be controlled.

Relationship between Sinusoidal PWM Control with Minimum Switching Losses and Hall Position Sensor

Usually, brushless DC motors use Hall sensors to locate the rotor position. Since the traditional control method is square wave control, three Hall sensors can meet the requirements. The relationship between the position of the Hall sensor and the rotor back EMF is shown in Figure 5, that is, the Hall sensors are installed at positions where the back EMF is 30°, 90°, 150°, 210°, 270°, and 330°. The specific Hall output value is related to the specific installation method of the Hall.

Figure 5 Relationship between BLDC Hall sensor output and back EMF

When using sinusoidal PWM with minimum switching loss to control BLDC, the relationship between the motor terminal line voltage and the Hall sensor output is shown in Figure 6.

Figure 6 Relationship between terminal line voltage and Hall state when using sinusoidal PWM with minimum switching loss

As shown in Figure 2, when the switching loss is minimized by using the sinusoidal PWM, the motor terminal line voltage leads the phase voltage by 30°. Therefore, it can be concluded that the motor phase voltage is synchronized with the back EMF when using sinusoidal wave control.

Since the phase voltage leads the phase current, the phase current lags the back EMF.

Speed ​​calculation

The speed calculation depends on the Hall sensor. Ideally, the interval between two adjacent Hall states is 60°. In practical applications, due to installation errors, the actual interval is not 60°, which will introduce calculation errors. This document uses the output of a Hall sensor as a reference for speed calculation, as shown in Figure 7. The high and low levels are 180 degrees respectively, which will not introduce installation errors. This information can be used to calculate the motor speed.

Figure 7 Speed ​​calculation

The calculation formula is as follows: Where: f is the electrical frequency, P is the number of motor poles

Angle estimation

Unlike square wave control, the angle in sine wave control changes continuously, and the three common Hall sensors in BLDC can only provide six angle information, namely 0°, 60°, 120°, 180°, 240°, and 300°. Other angle information cannot be directly obtained. The average speed method is usually used. Assuming that the motor speed is stable within a certain period of time, the angle and speed information at the previous Hall commutation are interpolated to obtain other angle information, as shown in Figure 8.

Figure 8 Angle estimation

It can be seen that the speed fluctuation of the motor will directly affect the error of angle calculation. In the scheme, the average value of three adjacent 180° commutation times is used to calculate the speed information, as shown in Figure 9.

Figure 9 Calculation of speed using multiple averaging method

That is , the angle error caused by rotation speed fluctuation is reduced.

Speed ​​PI

The speed control uses a PI regulator, the input is the speed setting and speed feedback, and the output is the amplitude Modulation of the sine PWM with the minimum switching loss. The formula is as follows:

Where: y is the output of the PI regulator. In the specific implementation, the integral link adds an anti-integral saturation function to limit the maximum and minimum values ​​of the integrator output, and at the same time adds a saturation limit to the output value of the entire PI regulator. The implementation block diagram is as follows.

Figure 10 PI regulator block diagram

start up

Before the brushless DC motor is started, the rotor is stationary, and only the absolute position information of the motor can be obtained using the Hall sensor. Since there is no commutation, the motor speed information cannot be obtained, so the angle information required for sinusoidal control cannot be calculated using the average speed method. Therefore, during the motor startup phase, the sinusoidal control method cannot be directly switched to, and the square wave control method is used to start. After the motor is started and reliable commutation information is obtained, the sinusoidal wave control can be switched to. In order to prevent large speed fluctuations, it is necessary to pay attention to the smooth transition of the phase and amplitude of the current before and after the switching.

The current waveforms before and after the ideal switching are shown in FIG11 as follows.

Lead angle adjustment

As can be seen from the previous content, the output of the Hall sensor reflects the back EMF information of the rotor, and the sinusoidal phase voltage generated according to the Hall state is in phase with the rotor back EMF. However, since the motor is an inductive load, the motor phase current lags behind the phase voltage. That is, the motor phase current lags behind the back EMF. When the Hall maximum torque is output, the motor phase current is synchronized with the back EMF, so it is necessary to adjust the voltage phase so that the generated phase voltage leads the back EMF, that is, the lead angle Δ. Appropriate adjustment of Δ can make the phase current and the back EMF in phase, increase the output torque, and improve the system efficiency. The adjustment of the lead angle can be manually adjusted in the form of experiments, or automatically adjusted using a certain algorithm.

Figure 11 Ideal switching from square wave control to sine wave control

Experimental Results

The control method proposed in this paper is implemented using Infineon's high-performance 8-bit microcontroller XC866. XC866 integrates a dedicated motor control unit CCU6E (providing a dedicated BLDC control mode) and a high-performance ADC module, making it an ideal choice for controlling brushless DC motors. The motor is a brushless DC fan with a rated power of 35W and a pole pair number of 4. Square wave control is used at startup, and sine wave control is switched when the speed stabilizes. Figure 12 shows the motor phase current under sinusoidal PWM control with minimal switching loss.

Figure 12 BLDC phase current using sine wave control with minimum switching loss

summary

This paper introduces a sine wave control scheme for brushless DC motors based on sine PWM with minimum switching loss, and implements and verifies the system based on InfinEON high-performance 8-bit microcontroller XC866. Compared with traditional square wave control, the motor has low running noise due to the use of sine wave drive technology, and the switching loss is reduced by 1/3 compared with SPWM, which can well meet the requirements of noise and efficiency in brushless DC fan applications. Therefore, this type of control scheme will have great application prospects.