Internally, the mechanism of a servo motor uses a potentiometer attached
to the rotating shaft to sense the position. It measures the width of the
incoming pulse, and applies current to the motor to turn the shaft
correspondingly.
Here are the insides of a servo that’s been dissected. You can see the DC
motor, position potentiometer, and a small PCB. The PCB has a chip on
one side, possibly a small microcontroller.
Inside an RC servo
The other side of the PCB has some discrete transistors, probably in an
H-bridge configuration, which allow the controller to steer current through
the motor in either direction, for both clockwise and counterclockwise
rotation.
Back of the PCB
One Other Useful Servo
Ordinary RC servos turn over a 90° range – it’s useful for turning a steering
linkage, or adjusting the control surfaces on an airplane, but not so useful
as a drive mechanism. That’s where full or continuous rotation servos
come in.
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