In recent years, the research and application range of quadcopters has been gradually expanded, and 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 a high speed.
The drive control mode of the brushless mainstream motor is mainly divided into two types of control methods: a position sensor and a position sensor. Since the brushless DC motor controller requires a small size, light weight, high efficiency and reliability in a quadcopter, a brushless DC motor without a position sensor is used.
The brushless DC motor drive control system includes two parts: a drive circuit and a system program control. The three-phase full-bridge drive circuit is formed by the switching characteristics of the power tube, and then the DSP is used as the main control chip. With its powerful arithmetic processing capability, the start and control of the motor are realized, but the circuit structure is complicated and costly, and lacks economy.
The commutation of the brushless DC motor uses the back-EMF zero-crossing detection method. Once the zero-crossing of the back-EMF of the third phase is detected, it prepares for the commutation. The back-EMF zero-crossing detection uses a virtual neutral point method to determine the rotor position by detecting the zero-crossing point of the back EMF of each phase of the motor. The motor current commutation theory based on the voltage variation law of the three-phase winding end of the motor can greatly improve the system control accuracy.
The driving circuit of brushless DC motor adopts three-phase six-arm full-bridge circuit. The control circuit of control circuit is realized by atmega16 single-chip microcomputer, in order to give full play to its high performance and rich resources, so the peripheral circuit structure is simple. The brushless DC motor adopts software startup and PWM speed control to realize the starting and stable operation of the motor, and greatly improve the speed regulation and control performance of the four-axis aircraft brushless DC motor.
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 driving mode, which can reduce current fluctuations and torque ripples, so that the motor outputs a large torque. In the motor drive part, six power FETs are used to control the output voltage. The DC brushless motor drive circuit power supply voltage in the four-axis aircraft is 12 V. In the drive circuit, Q1~Q3 adopts IRFR 5305 (P-channel), Q4~ Q6 is IRFR 1205 (N-channel). The FET has a freewheeling diode to provide a current path for the FET to turn off to avoid reverse breakdown of the tube. Typical characteristic parameters are shown in Table 1. T1~T3 PDTC 143ET is used to provide driving signals for the FET. .
Table 1 MOSFET tube parameters
It can be seen from Fig. 1 that A1~A3 provide the three-phase full-bridge upper arm gate drive signal, and is connected with the hardware PWM drive signal of the atmega16 microcontroller, and realize the motor speed control by changing the duty ratio of the PWM signal; B1~B3 The lower gate arm drive signal is provided, which is directly provided by the I/O port of the single chip microcomputer, and has two states of conduction and cutoff.
Figure 1 Three-phase six-arm full-bridge drive circuit for brushless DC motor
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, and each 60° electrical angle is reversed. If the AB phase is turned on at a certain time, Phase C is off and there is no current output. According to the detected rotor position of the motor, the MCU uses the switching characteristics of the MOSFET to realize the energization 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 to 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, and Q3Q5, the continuous operation of the brushless DC motor can be realized as long as the accurate commutation is performed at an appropriate timing.
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