Servo Motors

Servo Motor Basics

A servo motor is a linear actuator that accurately controls velocity, acceleration, angular and linear position. It is made up of a motor with a sensor for position feedback, and a sophisticated controller specifically designed for the servo motor. The term “servo motor” refers to the control mechanism – a motor that is suitable in a closed-loop control system. Servo motors are often used in applications related to CNC machinery, robotics, or automated manufacturing. What contributes to this diversity in the application is the servo motor’s low cogging and compact nature. Let’s read on for a closer look at the servo motors.

Figure 1. Servo Motor, Solpower

Types of Servo Motors

There are two types of servo motors: DC and AC.

DC Servo Motors
This type of motor is driven by a brushed direct current (DC) motor. Although DC motors were widely adopted back in the day because of their small size and low cost, their use is significantly lower in today’s industries. DC motors are not suitable for large projects because they are not designed for high current surges.

AC Servo Motors
These servo motors are driven by an alternating current (AC) motor. Despite the more complicated controls, advanced control technology has enabled them to be compatible with most applications today. What also makes AC motors more versatile than DC motors today is the ability to handle high current surges, making them the best candidate for a wide range of industrial machinery.

Construction of Servo Motors

The design of servo motors shares many similarities with other motors, except that it features an encoder as one of its key components. 

  • Stator Winding: This type of winding is also known as the field winding on the motor, which is located on the stationary part of the motor.
  • Rotor Winding: This type of winding is located on the rotating part of the motor. It is also referred to as the armature winding of the motor.
  • Bearing: Bearings are used for the movement of the shaft, and are typically classified into the front bearing and back bearing.
  • Shaft: The shaft of the motor is the iron rod on which the armature winding is wound.
  • Encoder: An encoder is a sensor that determines the speed as well as the revolution per minute (RPM) of the motor, usually mounted on the rear of the motor.

Figure 2. Structural Design of Servo Motors

Working Principles

The mechanism behind a servo motor is the automatic closed-loop system, and the controlling device plays a crucial role here. The controller is typically made up of a comparator and a feedback path. When an output signal is detected, the comparator compares the required reference signal captured by the encoder. And when the input signal (i.e. feedback signal) is received, the motor starts to work. Basically, the comparator decides whether the motor is ON or OFF. The quality of the motor operation often has much to do with the quality of the controlling device. Let’s check out a video to see how a servo motor operates:

(Copyright@learnchannel, YouTube)

Applications of Servo Motors

Servo motors are very versatile and are used in a wide range of industries. In specific applications, servo motors serve as a replacement for conventional AC motors, stepper motors, hydraulic and pneumatic systems. See below for an outline of servo motors’ key applications: 

Robotics is one of the key industries that utilize server motors. The compact and lightweight nature makes them ideal for use in robotic arms and other highly automated equipment.

Equipment in the defense industry is largely exposed to harsh environments. Like a motor that is resistant to high temperature and shock, it's a perfect candidate for defense-related equipment.

Machine Tools
Machine tool applications often make use of these highly accurate servo motors, especially in precision machining where productions are carried out at maximum precision.

Oil & Gas
The servo motors used in this industry should be able to withstand high pressure and temperature. They should also comply with the explosion-proof requirements as per the specific application.

Other applications of servo motors include:

  • Radio control
  • Control of amusement equipment
  • Control of automatic doors on trains
  • Control of a variety of manufacturing machinery, such as presses and molding machines.
  • Control of conveyor systems
  • Washing equipment in the food and beverage industry


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