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Electric Servo Motor Basics

Servo motors push or rotate objects with high precision, facilitating industrial equipment and robotics systems that manipulate the environment. Servo motors come in AC and DC varieties – both capable of achieving high torque in a compact, lightweight package. This makes them suitable for many applications.

An Overview of the Servo Mechanism

There are three important parts to any servo:

  • The controlled device
  • The output sensor
  • The feedback system

A servo is a closed loop system – if any of the components are missing, it cannot perform. To control the motion and ultimate position of the shaft, positive feedback is used. That is, the feedback signal continuously compares the output signal and a reference input.

The system continues to generate the feedback signal as long as there is a difference between the input and output. Thus, the system must interpret the feedback signal and respond accordingly as it changes over time. If feedback fails, the system will malfunction.

How Does a Servo Motor Work?

Although servos are powerful and versatile, they are relatively simple. Aside from the motor, the key components include a potentiometer, a gear assembly, and the circuit providing controlling functions – the most sophisticated element.

The gear assembly is used to reduce total revolutions per minute and increase torque. At the initial position of the servo motor shaft, no signal is produced – however, a corresponding error detector signal is generated.

The difference between the potentiometer signal and the signal from the other input results in the error signal. That signal, in turn, is the input for the motor, which begins to rotate. As the angular position of the potentiometer changes, so does the feedback signal.

The motor stops rotating when the difference between the potentiometer and external signal is zero.

Controlling the Servo Motor

PWM – pulse with modulation – is generated by control wires to operate the servo motor. The servo motor anticipates a pulse every 20ms, and interprets it to determine how far to turn. The maximum turning radius is typically 90 degrees in either direction from the neutral state.

Although there are only a few components to an electric servo motor, performance can vary greatly from one motor to another. For example, some motors exceed standards and turn up to 210 degrees.

To develop an efficient, powerful motor, experience and knowledge are required. MCMA offers an in-depth electric motor and motion control course that is offered for presentation at your company location.


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