What is a RC Servo Motor?

RC servo motor, short for radio control servo motor, can also be regarded as a DC servo motor, but its characteristics and application fields are different from DC servo motors. RC servo motors are mostly used for steering control of remote control models. Potentiometers (variable resistors) are used to detect the position, and the rotation angle can reach 180°.

Figure 1 shows the schematic diagram of the RC servo system. The RC (ratio control) servo motor consists of a controller and a motor. The controller includes a position deviation detection circuit, a motor drive amplifier and a monostable multivibrator. And the motor includes a DC motor and a potentiometer.

The working principle of the RC servo system can be illustrated by the timing diagram shown in Figure 2. The target position is specified by an external PWM signal, the signal period is 20ms, and the pulse width is 1～2ms (specific parameters are subject to the RC servo motor data sheet).

Next is position detection. According to the resistance value of the potentiometer built in the servo motor, the output pulse width of the monostable multivibrator changes. The pulse width of the position detection is consistent with the target position, and the range is 1 ~ 2ms.

The detection position does not reach the target position, and the position deviation detection circuit outputs a forward pulse. If the detected position passes the target position, a reverse pulse is output.

The motor drive amplifier determines the drive voltage polarity and drive time of the DC motor according to the position offset pulse width of the forward and reverse pulses. After that, through position feedback, the RC servo motor is positioned at the target position. Furthermore, the adjustment of the servo gain is completed by the motor drive amplifier, the deviation pulse is measured, and the drive time is adjusted.

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