Research on brushless DC motor drive technology for quadcopter

The ATMEGA16 single-chip microcomputer is used as the control core, and the six MOSFETs of the drive circuit are turned on in turn to achieve commutation by using the back-electromotive force zero-crossing detection; the DC brushless motor control program completes the MOSFET power-on self-test, motor startup software control, PWM motor speed control and circuit protection functions. Practice has proved that the design circuit structure is simple, the cost is low, the motor operation is stable and reliable, and the motor can be operated continuously.

0 Introduction

In recent years, the research and application scope of quadcopters has gradually expanded. It uses four brushless DC motors as its power source. The brushless DC motor is an outer rotor structure that directly drives the propeller to rotate at high speed.

The driving control methods of mainstream brushless motors are mainly divided into two types: with position sensor and without position sensor. Since the brushless DC motor controller in the quadcopter requires small size, light weight, high efficiency and reliability, a brushless DC motor without position sensor is used. This article uses the Langyu X2212 kv980 brushless DC motor.

The brushless DC motor drive control system consists of two parts: the drive circuit and the system program control. The switching characteristics of the power tube are used to form a three-phase full-bridge drive circuit, and then DSP is used as the main control chip. With its powerful computing and processing capabilities, the motor is started and controlled, but the circuit structure is complex and costly, and lacks economy.

The commutation of the brushless DC motor adopts the back-EMF zero-crossing detection method. Once the back-EMF zero-crossing point of the third phase is detected, preparation for commutation is made. The back-EMF zero-crossing detection adopts the virtual neutral point method to determine the rotor position by detecting the back-EMF zero-crossing point of each phase of the motor. The motor current commutation theory based on the voltage variation law of the three-phase winding terminal of the motor can greatly improve the system control accuracy.

The drive circuit of the brushless DC motor in this paper adopts a three-phase six-arm full-bridge circuit, and the management control chip of the control circuit is implemented by ATmega 16 single-chip microcomputer to give full play to its high performance and rich resources, so the peripheral circuit structure is simple. The brushless DC motor adopts software start and PWM speed control to achieve the start and stable operation of the motor, greatly improving the speed regulation and control performance of the brushless DC motor of the quadcopter.

1 Three-phase six-arm full-bridge drive circuit

The brushless DC motor drive control circuit is shown in Figure 1. The circuit adopts a three-phase six-arm full-bridge drive mode, which can reduce current fluctuations and torque pulsations, so that the motor outputs a larger torque. Six power field effect tubes are used in the motor drive part to control the output voltage. The power supply voltage of the DC brushless motor drive circuit in the quadcopter is 12 V. In the drive circuit, Q1~Q3 uses IRFR5305 (P channel) from IR Company, and Q4~Q6 is IRFR1205 (N channel). The field effect tube has a built-in freewheeling diode to provide a current path when the field effect tube is turned off to avoid reverse breakdown of the tube. Its typical characteristic parameters are shown in Table 1. T1~T3 uses PDTC143ET to provide drive signals for the field effect tube.

As shown in Figure 1, A1~A3 provide the gate drive signal of the upper arm of the three-phase full-bridge, and are connected to the hardware PWM drive signal of the ATMEGA16 microcontroller, and the motor speed control is achieved by changing the duty cycle of the PWM signal; B1~B3 provide the gate drive signal of the lower arm, which is directly provided by the I/O port of the microcontroller and has two states: on and off.

The brushless DC motor drive control adopts a three-phase six-state control strategy. The power tube has six trigger states. Only two tubes are turned on at a time. The commutation is performed every 60° electrical angle. If the AB phase is turned on at a certain moment, the C phase is cut off and there is no current output. The single-chip microcomputer uses the switching characteristics of the MOSFET according to the detected motor rotor position to realize the power-on control of the motor. For example, when Q1 and Q5 are turned on, the AB phase is turned on. At this time, the current flows from the positive pole of the power supply → Q1 → winding A → winding B → Q5 → the negative pole of the power supply. Similarly, when the MOSFET is turned on in the order of Q1Q5, Q1Q6, Q2Q6, Q2Q4, Q3Q4, Q3Q5, as long as accurate commutation is performed at the right time, the continuous operation of the brushless DC motor can be achieved.

2 Back EMF zero-crossing detection

In order for a brushless DC motor to operate normally and continuously, the rotor position must be detected to achieve accurate commutation. There are three main ways to detect the motor rotor position: photoelectric encoder, Hall sensor, and sensorless measurement [10]. Since the brushless DC motor of a quadcopter requires a simple system structure and light weight, a position sensorless method is used, using the induced electromotive force generated by the third phase to delay commutation by 30° when it passes through the zero point. Although this method is more troublesome when starting the motor and has poor controllability, it is suitable for quadcopter motors that do not need to be started frequently during normal flight due to its simple circuit and low cost.

Due to the two-phase conduction mode of the brushless DC motor, the non-conducting third phase can be used to detect the magnitude of the back electromotive force. As shown in Figure 2, the back electromotive force detection circuit, the neutral point N is connected to the AIN0 of the microcontroller, and Ain, Bin, and Cin are connected to the ADC0, ADC1, and ADC2 of the microcontroller respectively. The voltage of the neutral point N is constantly compared with the voltage of the three terminals of the three phases A, B, and C to detect the zero-crossing point of the induced electromotive force of each phase. The positive input terminal of the analog comparator of the ATMEGA16 microcontroller is AIN0, and the negative input terminal selects ADC0, ADC1, and ADC2 according to the configuration of the ADMUX register, thereby utilizing the multiplexing function of the analog comparator of the microcontroller. When the A and B phases are powered on, the back electromotive force of C is compared with the neutral point N. Similarly, the zero-crossing event of each phase can be successfully detected.

After the back EMF of the motor is detected, the zero crossing point of the back EMF can be found, and the commutation operation is performed with a delay of 30° electrical angle after the back EMF crosses zero.

3 Control Program Design

3.1 Power-on self-test of drive control circuit

The brushless DC motor drive control section includes three parts: MOSFET self-test, motor start control, and voltage and current monitoring functions. The power-on self-test process of the drive control circuit is shown in Figure 3, including MOSFET short-circuit characteristic and conduction characteristic tests to prevent overcurrent from damaging the circuit.

3.2 Software startup control

The back EMF detection method can only be performed after the motor is running normally. When the motor does not rotate or the speed is very low, its back EMF cannot be detected, so the software startup method is used. For the control of the brushless DC motor without position sensor, this paper adopts a three-step startup method. First, the A and B phases are energized for a period of time to fix the motor rotor position; the six states are commutated in turn, and the power-on time is gradually reduced; the back EMF of the third phase is detected. If it is normal, the startup is successful, otherwise it is restarted. The specific startup process is shown in Figure 4.

3.3 System protection function design

The system protection functions of the quadcopter include voltage and current monitoring functions. Battery voltage monitoring circuit: The battery voltage is reduced to the input range allowed by the microcontroller A/D conversion (0~5 V) through a simple voltage divider circuit, and the voltage monitoring is used to prevent the motor from stopping when the voltage is insufficient; current detection circuit: The current is sampled through a 0.01 Ω resistor, converted into voltage, and sent to the A/D conversion port of the microcontroller to prevent high current from damaging the circuit when a fault occurs. When monitoring the current, a simple numerical average filter method is used to reduce the impact of the instantaneous peak current on the measurement results.

4 Conclusion

This paper implements the drive circuit and system control software program design of the DC brushless motor of the quadcopter. The drive circuit adopts a three-phase six-arm full-bridge circuit, MOSFET as the switching element, and uses the ATmega 16 microcontroller as the control chip. The back electromotive force zero-crossing detection and software startup control method are used, and the commutation is delayed by 30°. After normal startup, the microcontroller outputs PWM to achieve brushless DC motor speed regulation. At the same time, the voltage and current monitoring circuit is designed to ensure the safety of the system. Therefore, the system can normally drive the brushless DC motor without position sensor and can be applied to quadcopters.