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A servo motor is a rotary actuator or motor that allows for a precise control in terms of angular position, acceleration and velocity, capabilities that a regular motor does not have. It makes use of a regular motor and pairs it with a sensor for position feedback. The controller is the most sophisticated part of the servo motor, as it is specifically designed for the purpose.
Servo motors are not actually a specific class of motor but are a combination of specific parts, which happen to include a DC or AC motor, and are suitable for use in a closed-loop control system. They are used in robotics, automated manufacturing and computer numerical control (CNC) machining applications.
The servo motor is a closed-loop servomechanism that uses position feedback in order to control its rotational speed and position. The control signal is the input, either analog or digital, which represents the final position command for the shaft. A type of encoder serves as a sensor, providing speed and position feedback. In most cases, only the position is reported. The final position is reported to the controller and this is compared to the initial position input, and then if there is a discrepancy, the motor is moved in order to get to the correct position.
The simplest servo motors use DC motors and position sensing through a potentiometer and also use big-bang control, which means that the motor moves at maximum speed until it stops at the designated position or is stopped. This is not widely used in industrial motion control as it can be quite inaccurate, but these kinds of servo motors are popular in radio-controlled devices such as model aircraft and toy cars. Sophisticated servo motors for industrial use have both position and speed sensing as well as implement proportional-integral-derivative control algorithms, allowing the motor to be brought to its position quickly and precisely without overshooting, as the speed of the shaft can also be controlled.