Servomechanism in Control System | Theory and Working principal

Servomechanism, Working Principle, Uses of servomechanism, Advantages, Disadvantages, Future trades, Summary, Servomechanism is an automatic closed


Servomechanism is an automatic closed loop control system. A servomechanism, sometimes shortened as servo,is used for error-sensing of negative feedback to correct the performance of a mechanism.A servo system primarily consists of three basic components -

  1. A controlled device.
  2. A output sensor.
  3.  A feedback system.
Block diagram of Servomechanism
Block diagram of Servomechanism

Here instead of controlling a device by applying the variable input signal, the device is controlled by a feedback signal generated by comparing output signal and reference input signal. When reference input signal or command signal is applied to the system, it is compared with output reference signal of the system produced by output sensor and third signal produced by a feedback system. This third signal acts as an input signal of controlled device. The primary task of a servomechanism to maintain the output of a system at the desired value in the preference of the disturbance.

Working Principle

A servo motor is basically a DC motor along with some other special purpose components that are a DC motor, a potentiometer, gear arrangement and a intelligent circuitry. As we know, a small DC motor will rotate with high speed but the torque generated by its rotation will not be enough to move even a light load. At the initial position of servo motor shaft, the position of the potentiometer knob is such that there is no electrical signal generated at the output port of the potentiometer. This output port of the potentiometer is connected with one of the input terminals of the error detected amplifier.

Now an electrical signal is given to another input terminal of the error detector amplifier. Now difference between these two signals, one comes from potentiometer and another comes from external source will be amplified in the error detector amplifier and feeds the DC motor.

This amplified error signal acts as the input power of the DC motor and the  motor starts rotating in desired direction. As the motor shaft progress the potentiometer knob also rotates as it is coupled with motor shaft with help of gear arrangement.

As the position of the potentiometer knob changes there will be an electrical signal produced at the potentiometer port. As the angular position of the potentiometer knob progresses the output or feedback signal increases. After 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 given to amplifier.

At this condition, there will be no output signal from the amplifier to the motor input as there is no difference between external applied signal and the signal generated at potentiometer. As the input signal to the motor is nil at that position, the motor stops rotating. This is how a simple conceptual servo motor works.

Servomechanism control system
Servomechanism control system

Uses of servomechanism

  1. Positioning servomechanisms were first used in military fire and marine navigation equipment. 
  2. It is also used in automatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation system on boats and planes.
  3. Many autofocus camera also use a servomechanism to accurately move the lens, and thus adjust the focus.

Advantages of Servo control system

  1. Highest torque
  2. Higher speed
  3. Control speed
  4. Speed is proportional to the applied voltage

Disadvantages of Servo control system

  1. More complex
  2. High cost for feedback use
  3. Maintenance difficult
  4. Tuning is required

Future trades

  1. Due to increase of Automation and development of new application of Servo system it has a high scope in future.
  2. Due to price reductions there is a beneficial effect on growth of Servo system in future.
  3. Due to continuous technology improvement in this electronic device system it has a high scope in future.
  4. The serve drives sector is benefiting from the popularity of intelligent drives that combine drive and controller capabilities, which leading it towards high growth.


  • Servo control system reduces the error of the system and improves the output mechanism.
  • Due to blocking of error, the efficiency of the device increases.
  • It is easier to design and implement. It can operate on both AC and DC.

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