DC motor control principle and drive circuit often used in ROS learning platform

When using the ROS robot to build a map, it is necessary to run autonomously in the room and collect map information. In this process, the forward and reverse rotation of the motor and the speed of the motor need to be controlled to adapt to the robot's straight-moving, turning and other actions.

The forward and reverse control principle of the brushed motor is very simple. You only need to swap the positive and negative poles of the motor power supply line to achieve the forward and reverse control of the motor. In the automatic control system, we cannot manually swap the positive and negative pole power supply sequence of the motor. We need to use the program with the hardware circuit to achieve it.

As shown in the figure below, four power tubes (which can be MOS tubes or IGBTs) are used to build a bridge circuit. Two wires are led out from the center of the bridge arm and connected to the power supply pins of the motor.

When the microcontroller is used to control Q2, Q3 to turn on, and Q1, Q4 to turn off, the current flows through the positive pole of the power supply, through Q3, the motor coil, Q2 and flows to GND. It is assumed that the motor rotates forward in this state.

When the microcontroller is used to control Q1, Q4 to turn on and Q2, Q3 to turn off, the current flows through the positive pole of the power supply, Q1, the motor coil, Q4 and flows to GND. It is assumed that the motor reverses in this state.

The forward and reverse control of the motor can be easily achieved by switching between the two states of the bridge circuit. The forward and reverse control of the brushed motor can also be simply achieved using two relays, but this method is not convenient for speed control, so it will not be introduced here.

DC brush motor speed regulation principle

According to the motor voltage balance equation

From the formula, we can see that the motor speed n is proportional to the supply voltage. Therefore, the purpose of speed regulation can be achieved by changing the motor supply voltage.

In power electronics, the DC voltage can be modulated by controlling the on and off of the switch tube through PWM wave. And the modulated voltage satisfies the relationship Vout=D*Vin, where D is the duty cycle of the PWM wave, which is equal to the ratio of the duration of the high level in one PWM cycle to the PWM cycle.

Speed ​​regulation method: When performing transistor control, you can choose three different chopping modes: HPWM-LON, HON-LPWM, and PWM-ON-PWM. I usually use the HPWM-LON mode, which means the upper tube is PWM and the lower tube is turned on.

Brushed DC Motor Drive Circuit

The H-bridge circuit uses 4 high-current NMOS tubes. The gate 100 ohm resistor plays a role in suppressing surge current. The 10K resistor forms a gate-source parasitic capacitance discharge loop. The gate diode provides a low-impedance MOS tube shutdown path to speed up the MOS tube shutdown. (The parameters of the components in the circuit should be adjusted according to the actual PCB)

In the half-bridge driving circuit, when the gate-source voltage of the MOS tube is higher than the threshold voltage, the MOS tube starts to conduct. The threshold voltage of IRF3710 is 4V. However, when only 4V voltage is used to drive the MOS tube, the Rds of the MOS tube is relatively large, and the MOS tube cannot flow through excessive current, as shown in the following figure:

As can be seen from the figure, as the gate-source voltage increases, the current capacity of the MOS tube also increases. Therefore, in the driver design process, I used a 12V power supply as the driver of the MOS tube. When the MOS tube is turned on, the MOS can have a very small Rds, which makes the MOS tube have a greater current capacity.

In the circuit, C7 is used as a bootstrap capacitor. When driving the upper arm of the H-bridge circuit, since the source of the upper MOS tube is at a higher voltage (24V), the voltage of the MOS tube G pole should be 12V higher than the source pole to be able to conduct (Vgs=36V). Here, the internal circuit of the half-bridge driver chip raises the gate of the MOS tube to 36V by taking advantage of the fact that the voltage across the capacitor cannot change suddenly. At this time, the gate-source voltage of the MOS meets the conduction condition. Since the bootstrap capacitor C7 needs to be charged at intervals continuously, the PWM duty cycle of this circuit cannot reach 100%, so special attention should be paid when programming.

The optocoupler isolation circuit uses optocoupler devices to electrically isolate the driver from the main controller to prevent the motor driver from interfering with the main controller.