Sensorless BLDC Motor Controller Design Based on TB6588FG

Abstract: Sensorless BLDC motor has a series of advantages such as simple DC motor structure, reliable operation, and easy maintenance. It has been widely used in various fields of industrial control. This design adopts Toshiba three-phase DC brushless driver TB6588FG and MSP430F12012 as the core hardware circuit, and introduces in detail the software implementation method of motor speed control circuit, motor protection circuit and signal detection and adjustment of motor speed regulation.
Keywords: TB6588FG; three-phase brushless motor; MSP430F2012

Introduction
Permanent magnet brushless DC motor and its control have always been a hot topic in the development of motors. Brushless DC motors are small in size and high in efficiency. They have the characteristics of simple structure, reliable operation and convenient maintenance of AC motors, and good speed regulation characteristics of DC motors without mechanical commutators. They are now widely used in various fields of industrial control. This control system designs a hardware platform with SP430F201 2 single-chip microcomputer and TB6588FG integrated driver as the core. The microprocessor collects the speed feedback pulse of TB6588FG and uses the digital potentiometer to adjust the voltage for closed-loop speed control.
In the control of sensorless brushless DC motors, the judgment of rotor position, motor start-up and current commutation are all completed by the control system. The functions are very complex and put forward high requirements for the hardware and software design of the control device. The sensorless brushless DC motor control method based on DSP is adopted. With the help of the powerful computing and processing capabilities of DSP, the back electromotive force detection commutation and open-loop start-up implemented by software are completed. The circuit design is relatively complex, which contradicts the requirement of simple circuit design for the valve control system, and there is also a certain degree of difficulty in software compilation. This design uses the TB6588FG dedicated controller, which makes the system control circuit very simple. It has a drive circuit inside, does not require an external MOS tube, and does not require the use of high-end controller chips such as DSP. Only an ordinary MSP430 can complete the system control function. The software is greatly simplified. The microprocessor only needs to process the external switch signal to achieve reliable operation of the motor.

1 System overall design
In valve control design, the biggest challenge for designers is how to design a simple circuit and place electronic components in a limited space. In this design, MSP430F2012 is used as the control core of the system to handle the start and stop of the motor, speed detection, speed regulation, and external switch quantity. All control functions related to the motor are handled by TB6588FG. The hardware block diagram of the system is shown in Figure 1.

TB6588FG is a dedicated control chip designed for three-phase sensorless brushless DC motor drive. It adjusts the speed by changing the analog voltage to change the PWM duty cycle. The typical current of the drive motor is 1.5 A, and the peak value is as high as 2.5 A. It has overcurrent protection function. The supply voltage is 7-42V. It has forward and reverse control function. It has all the functions required to control the star-connected sensorless brushless DC motor. The main functions include: starting circuit, reverse electromotive force commutation control, PWM speed control, undervoltage and current limiting protection, etc. Using TB6588FG reduces the workload of the microcontroller in terms of hardware and software. All the work can be done with the small and cheap MSP430F2012. The microcontroller mainly completes the motor speed pulse detection, and then the control algorithm adjusts the digital potentiometer to change the analog voltage of the speed control to adjust the speed. TB6588FG has undervoltage and current limiting function to protect the motor.

2 Motor Control Circuit Design
The TB6588FG motor control circuit is shown in Figure 2.

3 Motor start-up
The motor starts based on the analog input voltage of VSP. When this voltage is greater than 1 V, the motor starts to start. At this time, the motor is in a stationary state and the position of the motor rotor cannot be determined. First, an excitation voltage is passed to put the motor into DC activation mode. When the motor rotates, the reverse electromotive force of the motor coil is detected. Then the driver enters the forced communication mode, and the time of the forced communication mode is determined by the external capacitor. As shown in Figure 3, stage a is the DC activation time, and the adjustment formula of the DC activation time is TFIX=0.69×C2×R1. Stage b is the forced communication time. Different motors can be used to adjust the motor start by changing the values ​​of C1 and C2 capacitors.

4 Overcurrent protection circuit
In actual applications, valve control often encounters unexpected motor stalling, which may cause the power supply and motor to burn out. The current limiting protection of TB6588FG is achieved through an internal comparator, as shown in Figure 4. The driver has a comparator inside, with a fixed voltage of 0.5 V at the negative input end. The current flowing through the motor coil is sampled by the R1 resistor and sent to the driver, and connected to the positive input end of the comparator through the filter network inside the driver. If the current flowing through R1 is greater than the set value, the voltage will be greater than 0.5 V, and the driver will turn off the internal drive circuit. In this circuit, R2 and C2 act as RC filters to prevent protection malfunctions caused by noise during PWM switching. For example, when R1=0.33 Ω, the maximum current limiting current is: Iout=0.5 V/0.33 Ω≈1.5 A.

5 Speed ​​detection control
5.1 Speed ​​detection
TB6588FG has a speed output pin FG_OUT. When the motor is not started or the motor is abnormal, this pin outputs a low level. When the motor is started normally, this pin outputs a speed signal. The MSP430 F2012 timer has a capture function. The speed is calculated by capturing the upper and lower edges of the speed pulse, and then the resistance of the digital potentiometer is adjusted to adjust the voltage of the control speed.
5.2 Speed ​​adjustment circuit
CAT5119 has a 32-bit node and a digital potentiometer with a resistance of 10 kΩ. It is adjusted through the two I/O ports of MSP430F2012 , and the adjustment method is UP/DOWN. By setting CAT5119, the maximum resistance can be adjusted to 10 kΩ and the minimum to 100 Ω. In this way, the control speed range is 500-2500 rpm. For example, when the speed needs to be adjusted to 1200 rpm, the resistance of the digital potentiometer is dynamically adjusted to adapt to the change of the load and stabilize the speed within the set range. The analog voltage regulation circuit is shown in Figure 5.

5.3 Program software flow
Because a dedicated motor driver chip is used, the motor control software is very simple. The main task is to set the working mode of the internal timer of MSP430F2012. It has two capture timers. The capture trigger mode selects the upper and lower edge capture, and then the numbers of the two captured timers are subtracted to get the speed. If there is an error with the setting, the speed is adjusted by adjusting the analog voltage. The interrupt service program flow of the software speed control is shown in Figure 6.

Conclusion
Aiming at the characteristics of the sensorless three-phase brushless motor control system of the valve, the motor speed control system designed by MSP430 microcontroller combined with TB6588FG is adopted. The hardware circuit of TB6588FG is used to simplify the overall circuit design, and at the same time reduce the difficulty of software programming, making the whole system more stable and reliable.