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Due to its advantages such as small torque fluctuation, high efficiency, low noise and fast dynamic response, brushless DC motors (BLDC motors) have been widely used in various applications in recent years, including air-conditioning compressors, variable-frequency refrigerators, washing machines, high-speed vacuum cleaners, sweepers, drones and even electric vehicle controllers.
According to Allied Market Research, the global brushless DC motor market will increase from US$33.2 billion in 2020 to US$72.2 billion by 2030. Many chip manufacturers are needed to provide support for this, and Qorvo is one of the leading suppliers. Its highly advantageous integrated motor control solutions and FOC algorithms can achieve more complex control forms and help address various unique challenges.
Recently, Zhang Shao, senior system/application engineer for motor control applications at Qorvo, gave a speech related to BLDC and FOC, popularizing the relevant knowledge about FOC and explaining the methods Qorvo uses to simplify the FOC implementation plan.
What is FOC?
FOC is the abbreviation of Field-Oriented Control, but in most cases, it is called vector control. To understand what FOC is, we must first start with motor drive.
Zhang Shao introduced that, as shown in the figure below, this is a typical drive system, whose DC bus voltage can be powered by a battery or obtained from three-phase rectification. The main function of the power and three-phase power circuits is to convert the DC bus voltage into three-phase AC voltage and current, thereby achieving the purpose of controlling the three-phase AC motor.
The blue box on the left of the picture contains the digital controller and analog circuits related to motor control; the right of the picture contains an MPOS, which can display the overall position information of the motor with the help of devices such as Hall sensors or software.
The figure below shows a typical dual-loop motor control system, where the inner loop is the "current loop" and the outer loop is the "speed loop". In the inner "current loop", some important modules include mathematical modules such as "Clark transformation" and "Park transformation", motor current signal detection modules, and modules that can provide motor speed and angular position information.
According to Zhang Shao, these modules are very important, so in the following speech, he introduced the functions of each module in detail.
First, let's look at one of the math modules, the "Clark Transformation", which converts the three-phase motor currents Iu, Iv and Iw into two-phase Ialpha and Ibeta. The figure on the right shows the process and results of the "Clark Transformation".
After obtaining a new Ialpha and Ibeta through the "Clark Transformation", we use the "Park Transformation" to obtain Iq and Id - the current signal in the rotating coordinate. More specifically, through the "Park Transformation", we can convert the three-phase AC current signal into a two-phase DC current signal, so that the control of the three-phase AC motor can become as simple as the control of the two-phase DC motor.
"This is the essence of vector control," Zhang Shao emphasized. "Through this transformation, we can control a three-phase AC motor as if it were a two-axis DC motor," Zhang Shao continued.
Next, let's take a look at the current sampling circuit as shown in the figure below. The leftmost one is a three-phase converter based on three resistors, the middle one is the differential sampling circuit of the PAC controller, and the rightmost one is the corresponding PWM control waveform. "In order to improve the performance of vector control, we usually recommend that the three-phase circuit be detected at the same time. The PAC controller has ADC sampling and maintenance functions. We also support three-phase differential circuits to better detect current." Zhang Shao said. He also pointed out that in order to improve the sampling quality of current, Qorvo recommends the intermediate sampling circuit with the switch tube turned on under the converter, which can avoid the influence of the controller's switching action on the current signal.
In some applications, customers tend to use a single resistor mode to detect the three-phase current of the motor to reduce system costs. The left figure below shows such an application. According to Zhang Shao, another advantage of this method is that we only need to use one differential circuit to detect the current. At the same time, when detecting the circuit, it is also hoped that the circuit is detected in the middle of the PWM effective vector to reduce the influence of the switching signal on the current sampling.
"In single resistance mode, in order to detect the current, we must do it during the effective vector period. In the case of zero vector, we cannot detect the current of the motor. In order to improve the current detection accuracy, we recommend using an asymmetric current method." Zhang Shao emphasized.
Regarding the current detection part, we can see a concise summary from the figure below.
Zhang Shao went on to say that after detecting the motor's current signal, it is hoped that the feedback current can track its given value. To achieve this goal, they usually use the D-axis and Q-axis current controllers, among which the PI-based current controller is widely used. In order to improve the performance and response speed of current control, the Kp and Ki parameters of the PI controller need to be carefully adjusted.
In Zhang Shao's opinion, the adjustment of these two parameters is very complicated, but Qorvo provides the function of "automatic diagnosis of motor parameters" to detect the parameters of the motor. Thus, the Kp and Ki parameters of the "current loop" and "speed loop" are calculated according to the motor parameters, which greatly simplifies the process of motor adjustment.
According to Zhang Shao, the output of the circuit controller is the given value of the voltage (reference value of the D axis and Q axis), and then through the reverse "Clark transformation", Valpha and Vbeta are obtained. From the left figure below, we can also get the voltage given value of the three-phase current from Valpha and Vbeta, and then use it as the input of the modulator to get the output of the "duty cycle", and then control the three-phase inverter to achieve the purpose of motor control.
"Selecting different neutral voltages can form different modulation control methods. At present, space vector modulation is widely used in the field of motor control. This is due to its ability to provide a sinusoidal current waveform. At the same time, this modulation method can also provide a higher voltage transfer ratio, thereby achieving a higher motor control speed." Zhang Shao said.
In the previous sections, Zhang Shao introduced the coordinate transformation principle, current controller, current sampling circuit and modulator respectively. Next, he will introduce the motor rotor position module. Zhang Shao pointed out that as shown in the figure, we can get the rotor position and speed of the motor through three methods: HALL, Encoder (QEP) and Sensorless (Estimator), which is the core part of vector control. Qorvo's software solution supports HALL, QEP and Estimator modes at the same time. "As shown in the figure, we also support two hybrid modes such as Hall-Estimator and QEP-Estimator." Zhang Shao emphasized.
Now that we have a basic understanding of FOC, let's take a closer look at its application in three-phase brushless DC motors.
Advantages of FOC in three-phase BLDC applications
To understand this advantage, we must first understand the relevant principles.
"According to Ampere's law, injecting current into the winding will generate a magnetic field, the strength of which is related to the value of the current. Applied to three-phase motors, after we inject three-phase alternating current into the three-phase winding, a rotating magnetic field will be generated on the stator side, which will interact with the permanent magnet magnetic field of the stator to generate torque. This is the basic principle of motor torque generation," explained Zhang Shao.
We can also see from the figure below that when the winding magnetic field and the stator magnetic field are at different angles, the torque generated is different. Our goal is to maximize the torque while reducing the motor stator current.
After having a basic understanding of the principle of torque generation, we will introduce the control method of three-phase brushless DC motor, including FOC. In addition, as shown in the figure, six-step trapezoidal wave is another method to achieve control.
From the figure below, we can see the specific process of the six-step trapezoidal wave generating a rotating magnetic field on the stator side. "We usually divide 360 degrees into six sectors, and each sector has a fixed switch tube turned on, so that a rotating magnetic field can be formed in the 'stator winding' to obtain the direction of rotation. As shown in the figure, the formation of different vectors is determined by different switch tubes." Zhang Shao said.
From the figure below, we can see that we can obtain more optimized torque by selecting the appropriate vector. In summary, this six-step trapezoidal wave method cannot always maintain the torque angle at 90 degrees. It can only reach this angle when it is in the middle of the sector, and only at that moment can it generate the maximum torque.
Through the following figure, we can understand the principle of rotor torque generation under the "vector control" mode. As shown in the figure, we can relatively simply and constantly maintain the stator magnetic field and the winding magnetic field at 90 degrees, thereby generating greater torque, which is fundamentally different from the "six-step trapezoidal wave" control method.
In summary, the "vector control" method has the following advantages. Qorvo provides customers with a combination of software and hardware solutions to lower the threshold for customers to develop related products and shorten the development cycle.
Zhang Shao said that the "six-step trapezoidal wave" control method produces greater noise than the "vector control". This is mainly caused by the torque fluctuation caused by the current harmonics of the motor winding.
"In application scenarios that require reducing vibration, lowering the harmonic content of the motor, and reducing noise, FOC will be a better choice," said Zhang Shao.
Qorvo lowers the implementation threshold of FOC
According to Zhang Shao, in order to simplify the implementation of FOC applications, Qorvo provides a combination of software and hardware solutions:
In terms of hardware, in addition to providing a series of PAC controllers based on Arm cortex-M0 and Arm cortex-M4, Qorvo also provides test boards developed based on these controllers, on which customers can run different control methods; in terms of software, Qorvo provides a variety of algorithms for vector control. In order to simplify the parameters of the controller for customers, Qorvo has developed a motor parameter setting function, which can obtain the parameters of the current controller and the speed controller by setting the resistance and inductance of the motor.
"We also have more advanced algorithms that can detect motor faults," said Zhang Shao. He pointed out that, as shown in the figure, Qorvo's products also support different development environments to facilitate developers' development. Back to FOC, Qorvo provides a variety of functions to meet the needs of different occasions. "We also provide a user interface to facilitate customers to debug their motors." Zhang Shao emphasized in his speech.
In order to run some vector control software, as shown in the figure below, we need to build a foundation based on Qorvo hardware. Customers can choose the appropriate PAC debug board for configuration according to their needs.
Zhang Shao concluded that, as shown in the figure below, Qorvo's FOC Firmware supports a variety of functions.
At the same time, Qorvo also provides a user-friendly interface that integrates multiple modules to simplify customers' product development.
In order to facilitate customers to debug motors and optimize motor parameters, Qorvo also provides customers with different debugging functions.
"The vector control functions mentioned above can be enabled or disabled before the compilation process. For specific usage, please refer to the manual provided by Qorvo," said Zhang Shao.
Zhang Shao said that on the company's website, Qorvo provides customers with detailed information and related support, and welcomes customers to visit and consult.
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