Servo Motor Working Principle:
Before understanding the working principle of servo motor we must understand the basics of servomechanism.
 Servomechanism:
A servo system basically consists of three basic components – a controlled device, an output sensor, a feedback system.
It is an automatic closed loop control system. Here instead of controlling the device by applying a variable input signal, the device is controlled by a feedback signal generated by comparing the output signal and the reference input signal.
When a reference input signal or command signal is applied to the system, it is compared to the system’s output reference signal produced by the output sensor, and a third signal produced by the feedback system. This third signal acts as the input signal of the controlled device.
The device presents this input signal as long as there is a logical difference between the reference input signal and the output signal of the system.
After the device achieves the desired output, there will be no logical difference between the reference input signal and the system’s reference output signal. Then, the third signal generated by comparing the above theses will not be sufficient to drive the device further and generate further output of the system until the next the reference iis nput signal or command signal is applied to the system. .
Therefore, the primary function of a servomechanism is to maintain the output of the system at a desired value in the presence of disturbances.
Working Principle of Servo Motor:
A servo motor is basically a DC motor (in some special cases it is an AC motor) along with some other special purpose components that make the DC motor a servo. In a servo unit, you will find a small DC motor, a potentiometer, a gear arrangement and an intelligent circuitry. Intelligent circuitry with potentiometers makes the servo rotate as per our desires. As we know, a small DC motor will rotate at high speed but the torque produced by its rotation will not be enough to move even a light load.
This is where a gear system within a servomechanism comes into the picture. The gear mechanism will take the high speed (higher) of the motor and at the output, we will get the output speed which is lower than the original input speed but more practical and widely applicable.
At say the starting position of the servo motor shaft, the position of the potentiometer knob is such that no electrical signal is generated at the output port of the potentiometer. And This output is port of the potentiometer is connected to one of the input terminals of the error detector amplifier. An electrical signal is now fed to the second input terminal of the error detector amplifier. Now the difference between these two signals, one coming from the potentiometer and the other coming from an external source, will be amplified in the error detector amplifier and feed the DC motor.
This amplified error signal acts as the input power to the DC motor and the motor starts rotating in the desired direction. As the motor shaft moves the potentiometer knob also rotates as it is connected to the motor shaft with the help of a gear arrangement.
As the position of the potentiometer knob changes, an electrical signal is generated at the potentiometer port. As the angular position of the potentiometer knob advances, the output or feedback signal increases. After the desired angular position of the motor shaft, the potentiometer knob reaches such a position that the electrical signal generated in the potentiometer becomes the same as the external electrical signal given to the amplifier.
In this case, there will be no output signal from the amplifier to the motor input because there is no difference between the external applied signal and the signal generated at the potentiometer. Since the input signal to the motor is zero at this position, the motor stops rotating. We know that This is how a simple conceptual servo motor works.
