Servo Motor


Servo Motor

A servo motor is a dc, ac, or brushless dc motor combined with a position sensing device. Servo motors are self-contained electric devices that rotate or push parts of a machine with great precision. A servo system mainly consists of three basic components – a controlled device, a output sensor, a feedback system. The standard servo on the left can range in power or speed to move something quickly, or it can accommodate a heavier load, such as steering a big radio-controlled monster truck or lifting the blade on a radio-controlled earth mover toy.

 The servo circuitry is built right inside the motor unit and has a position-able shaft, which usually is fitted with a gear.  The motor is controlled with an electric signal which determines the amount of movement of the shaft.

Servo motor

Working principle of Servo Motors

 A servo consists of a Motor (DC or AC), a potentiometer, gear assembly. The servo motor has some control circuits and a potentiometer (a variable resistor) that is connected to the output shaft. Gear assembly used because of to reduce RPM and to increase torque of motor. Say at initial position of servo motor shaft, the position of the potentiometer terminal is such that there is no electrical signal generated at the output terminal of the potentiometer.

The potentiometer allows the control circuitry to monitor the current angle of the servo motor. If the shaft is at the correct angle, then the motor shuts off. Electrical signal is given to another input terminal of the error detector amplifier. Now difference between these two signals, one signal comes from potentiometer and another signal comes from other source, will be amplified in the error detector amplifier and tha is feeds the DC motor.

This amplified error signal acts as the input power of the DC motor and the motor starts rotating in desired or forward direction. The angular position of the potentiometer knob progresses the output or feedback signal increases. After some desired angular position of motor shaft the potentiometer knob is reaches at such position.  the electrical signal generated in the potentiometer becomes same as of external electrical signal is given to amplifier.

There will be no output signal from the amplifier to the motor input as there is no difference between external signal and the signal generated at potentiometer. As the input signal to the motor is zero at that position, the motor stops rotating. This is a simple conceptual servo motor works. The output shaft of the servo is capable of travelling somewhere around 180 degrees. Usually, its somewhere in the 210 degree range.


  1. performance is high.
  2. Smaller in size.
  3. Wide variety of components.
  4. High speeds available with specialized controls


  1. Cost is lightly higher.
  2. High performance limited by controls.
  3. High speed torque limited by commutator or electronics.


  • Servo motors are used in radio-controlled airplanes to position control surfaces like elevators, rudders, walking a robot.
  • It is used in industrial applications, robotics for smaller movement, in-line manufacturing.


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