Before understanding the working principle of
servo motor we should understand first the basic of servomechanism.
Servomechanism
A servo system mainly consists of three basic
components - a controlled device, a output sensor, a feedback
system.
This is an automatic closed loop control system. 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 a third signal produced by a feedback system. This third signal acts as an
input signal of controlled device.
This input signal to the device presents as long as there is a logical
difference between reference input signal and the output signal of the system.
After the device achieves its desired output, there will be no longer the
logical difference between reference input signal and reference output signal
of the system. Then, the third signal produced by comparing theses above said
signals will not remain enough to operate the device further and to produce a
further output of the system until the next reference input signal or command
signal is applied to the system. Hence, the primary task of a servomechanism is
to maintain the output of a system at the 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 AC motor) along with some other special purpose components that make a DC motor a servo. In a servo unit, you will find a small DC motor, a potentiometer, gear arrangement and an intelligent circuitry. The intelligent circuitry along with the potentiometer makes the servo to rotate according to our wishes. 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. This is where the gear system inside a servomechanism comes into the picture. The gear mechanism will take high input speed of the motor (fast) and at the output, we will get an output speed which is slower than original input speed but more practical and widely applicable.
Say at 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 detector 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 progresses 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.