Abstract: This paper first introduces the development of three-phase brushless DC motors at home and abroad and the research status of their control systems. It then discusses the structure, operating principle, characteristic analysis, and rotor position signal detection method of three-phase permanent magnet brushless DC motors. Finally, the hardware circuit and corresponding software of the control system are designed.
Keywords: Three-phase brushless DC motor; Atmega8L microcontroller; PID closed-loop control
Abstract: The paper first introduced the study status of permanent magnet brushless DCmotor at home and abroad, and then discussed its structure, operation principles, characteristics as well as its rotor position signal detection methods; secondly, designed the control system's hardware and software accordingly.
Keywords: Three phase BLDC motor; Atmega8L micro-c; PID closed-loop control
1 Introduction
Brushless DC motors are one of the fastest-growing varieties in the small motor industry in recent years. With the trend of smaller, lighter, and thinner audio-visual products, quieter and more energy-efficient home appliances, and increased demand for luxury cars, the demand for brushless DC motors has increased rapidly. Brushless DC motors use electronic commutation to replace brushes and commutators, and have the advantages of high reliability, high efficiency, long life, convenient speed adjustment, and low noise. In recent years, there has been a lot of research on the design and control of brushless DC motors in China, but there are still many areas that need improvement compared with mature foreign products. Moreover, many brushless DC motor manufacturers have not provided specific control schemes. Therefore, research on the control of brushless DC motors is very necessary [1].
Because brushless DC motors have advantages such as small size, light weight, high efficiency, good speed regulation performance, small moment of inertia, and no excitation loss, they have a wide range of applications in various fields. On the one hand, brushless DC motors have obvious advantages over other asynchronous motors, such as simpler feedback devices, higher power density, larger output torque, and full utilization of the potential of both the motor and the inverter. Therefore, the application and research of brushless DC motors have received unprecedented attention [2]. According to statistics, the use of brushless DC motors is growing at a high rate every year. On the other hand, brushless DC motors have more advantages over brushed DC motors, such as simple motor body structure, no sparks during operation, low electromagnetic interference, and no noise. Therefore, they have a wide range of applications.
2. System Hardware Design
2.1 Overall Hardware Architecture of Three-Phase Brushless DC Motor
The hardware of this control system mainly consists of a control circuit, a drive circuit, a display circuit, and an RS485 interface circuit, as shown in Figure 2-.
As can be seen from the operating principle of permanent magnet brushless DC motor, the average working current of permanent magnet brushless DC motor is inversely proportional to the speed, and the fan blade load of air conditioner motor is constant, that is, the torque is linearly related to the speed. Therefore, the electromagnetic torque of brushless DC motor can also be controlled by controlling the speed [3]. This system is a speed closed-loop system. The position signal of Hall position sensor is processed and sent to a dedicated driver chip to generate a speed pulse signal. After processing by the microcontroller, it is converted into speed. Then, the incremental PI algorithm is used to obtain the PWM control signal. The dedicated integrated driver chip is driven by the optocoupler isolation circuit to control the speed in a closed loop. At the same time, the microcontroller also monitors the operating status of the control system. When the system has faults such as short circuit, overcurrent, and overvoltage, the microcontroller will block the PWM output signal, stop the motor, and display the fault through the LED circuit. Since the customer has different requirements for the control system, the various parts of this system are designed to maintain their independence while leaving corresponding interfaces to form a complete system.
2.2 Control Circuit
The main hardware control circuit of this system consists of an AtmegaBL microcontroller, a PWM signal generation and processing circuit, a current detection circuit, a speed detection circuit, an isolation circuit, and an interface circuit, as shown in Figure 2-2.
After the PWM signal is generated, it needs to be processed to obtain the desired output signal. The PWM signal generated by the ATmegaBL is optocoupled to generate a PWM signal of the same period at pin 3 of P521. After voltage division and filtering, a speed regulation voltage of 0 to 6V is output for the drive circuit. The Zener diode in the figure stabilizes the voltage at pin 4 of P521 to 9V. The PWM signal at pin 3 of P521 is smoothed after two stages of RC filtering. P521 plays a role in electrical isolation between the main control circuit and the drive circuit.
3. System Software Design
This control system utilizes the C language and employs modular and structured programming. Modular programming means dividing a large program into several smaller modules, each maintaining relative independence and connected only by a few output parameters. This allows for separate design of each program module, making program debugging and modification easier. Structured programming involves the use of efficient transfer and calling between subroutines, enabling modules to be effectively combined into a whole, clearly transferring the flow from one program module to the next.
This control system software design adopts a foreground/background system, which consists of an infinite loop program plus multiple interrupt service routines. Before the main program initializes, all interrupts and the watchdog timer should be disabled to ensure that initialization is not interrupted by system resets. When there are many tasks, a real-time operating system (RTOS) is required to improve the utilization of the microcontroller's MPU and ensure that each task runs as expected in real time. This control system software includes a main program and interrupt service routines. The main program mainly consists of system initialization, rotor speed calculation, and speed PID closed-loop control, completing most of the functional tasks. The interrupt routines mainly detect interrupt events and notify the main program to handle them accordingly, completing necessary real-time functions. This is done to minimize CPU time consumption by interrupts, ensuring reliable operation of all program functions. The interrupt routines in this control system are mainly used for serial interrupt reception of speed setpoints from the host computer, timer interrupt detection of current, display of speed values, and related faults. The program design is shown in Figure 3-1.
I/O port initialization mainly sets whether the port is an input or output, the initial value of the output, and whether pull-up resistors are needed; A/D initialization mainly sets the channel of the AD converter, the analog reference voltage, and the clock frequency; Max7219 initialization mainly sets the internal brightness, decoding mode, scan bit number, and other register settings; serial port initialization sets it to multi-machine communication mode, baud rate of 9600 bit/s, 1 start bit, 9 data bits, and 1 stop bit; AtmegaBL has three timers, T0 and T2 are 8-bit, and T1 is 16-bit [4-5]. In this control system, T0 is used to generate a 2-second timer interrupt signal, T2 is used to generate a PWM wave signal corresponding to the user-set speed, and T1 is used to capture the FG pulse signal to calculate the motor speed. After setting the above values, interrupts and watchdog timers are enabled to respond to interrupt service routines and to prevent program crashes and subsequent reset and restart.
References
[1] Zhang Chen. Principles and Applications of Brushless DC Motors (Second Edition). Beijing: Machinery Industry Press, 2004.
[2] Deng Xingzhong. Electromechanical Transmission Control. Wuhan: Huazhong University of Science and Technology Press, 2001.
[3] Wen Zhaofang. Electrical Machines and Control. Beijing: Beijing Institute of Technology Press, 2004.
[4] Hu Hancai. Microcontroller Principles and Interface Technology. Beijing: Tsinghua University Press, 1996.
[5] Zhou Tengqin. Computer Control Systems. Xi'an: Northwestern Polytechnical University Press, 1998.
