electric servo motor

ELECTRICAL SERVO MOTOR

A Servo Motor is a motor which is partof a servomechanism. It is typically paired with some type of encoder to provide positioning and speed feedback.

Servo Motor

Servo Motor Basics

A Servo Motor is defined as an automatic device that uses an error-correction routine to correct its motion. The term servo can be applied to systems other than a Servo Motor; systems that use a feedback mechanism such as an encoder or other feedback device to control the motion parameters.

Servo Motor Basics

Typically when the term servo is used it applies to a 'Servo Motor' but is also used as a general control term, meaning that a feedback loop is used to position an item.

Servo Motor Basics

A servomechanism may or may not use a servo motor. For example, a household furnace is a servomechanism that is controlled by a thermostat. Once a set temperature is reached, there is feedback signaling it to shut off; making it a “servo” in nature. The term “servo” describes more of a function or task, than it does a specific product line. For this guide, we will discuss servo motors specifically.

Servo Motor Basics

A servo motor can be a DC, AC, or brushless DC motor, combined with a position sensor; in most cases, a digital encoder. A servo motor is typically the motor selected when it is essential that there is a high degree of confidence that the servo motor and drive system will closely track what is asked of it.

Physical Properties of aServo Motor

A Servo Motor consists of three major parts: a motor, control board, and potentiometer (variable resistor) connected to the output shaft. The motor utilizes a set of gears to rotate the potentiometer and the output shaft at the same time. The potentiometer, which controls the angle of the servo motor, allows the control circuitry to monitor the current angle of the servo motor.


The motor, through a series of gears, turns the output shaft and the potentiometer simultaneously. The potentiometer is fed into the servo control circuit and when the control circuit detects that the position is correct, it stops the servo motor. If the control circuit detects that the angle is not correct, it will turn the servo motor the correct direction until the angle is correct.


Normally a servo motor is used to control an angular motion of between 0 and 180 degrees. It is not mechanically capable (unless modified) of turning any farther due to the mechanical stop build on to the main output gear.

Where are Servo Motors used?

Servos are extremely useful in robotics and automation. Servo motors are used across various automation fields specifically where the motor must be able to operate at a range of speeds without overheating, operate at zero speed while being able to retain its load in a set position, as well as operate at low speeds .

Where are Servo Motors used?

The aerospace industry makes use of servo motors in their hydraulic systems to contain system hydraulic fluid. The servo motor is relatively small in size, yet very powerful. A servo motor also draws power proportional to the mechanical load.

What Industries are Servo Motors used in?

Servo motors are seen in applications such as factory automation, robotics, CNC machinery, and packaging. The feedback lets the drive know its position, speed, and torque to detect unwanted motion. Pharmaceutical industries are driven be the need to create smaller devices; ones that are easier to operate and function more efficiently.

Industrial Servo Motor

The grey/green cylinder is the brush-type DC motor. The black section at the bottom contains the planetary reduction gear, and the black object on top of the motor is the optical rotary encoder for position feedback. This is the steering actuator of a large robot vehicle .

Industrial Servo Motor

How Does a Servo Motor Work?

Typical servo motor mechanism is not complex. The servo motor has control circuits and a potentiometer that is connected to the output shaft. The shaft, which is the output device, links to a potentiometer and control circuits that are located inside the servo .


The potentiometer, coupled with signals from the control circuits, control the angle of the shaft – anywhere from 0 to 180 degrees, sometimes further. The potentiometer allows the control circuitry to monitor the current angle of the servo motor.


If the shaft is at the correct angle, the servo motor idles until next positioning signal is received. The servo motor will rotate the correct direction until the angle is correct. Each servo motor works off of modulation known as Pulse Coded Modulation, or PCM. The motor has a control wire that is given a pulse application for a certain length of time .

How are Servo Motors controlled?

Servo motors operate on negative feedback, meaning that the control input is closely compared to the actual position via a transducer. If there is any variance between physical and wanted values, an error signal is amplified, converted, and used to drive the system in the direction necessary to reduce or eliminate error.

How are Servo Motors controlled?

Servo motors are controlled by a pulse of variable width that is sent from a micro-controller output pin to the servo motor’s control wire. The shaft angle is determined by the duration of the pulse, also known as pulse width modulation (pwm ). This pulse has to have specific parameters such as; minimum pulse, a maximum pulse, and a repetition rate.

Servo Motor Types 1 . Rotary Servo Motor

A rotary Servo Motor is what most people think of when they think of a Servo Motor. The three types of Rotary Servo Motors are: AC Servo Motor, Brush DC Servo Motor, and Brushless DC Servo Motor. The motion of a rotary Servo Motor is often converted into linear motion by the use of a screw thread (ball screw or lead screw), or with the use of belts and pulleys .

Servo Motor Types

1 . Rotary Servo Motor

A Rotary AC Servo Motor is an AC type motor that is used with a feedback device. These are typically used in smaller applications, because a large AC Servo Motor is typically inefficient when compared to its DC or Brushless counterparts.

Servo Motor Types

1 . Rotary Servo Motor

Servo Motor Types

2 . Linear Servo MotorA linear Servo Motor is a flattened

out Servo Motor where the rotor is on the inside, and the coils are on the outside of a moveable u-channel. Both Servo Motor types are becoming more popular as Servo Motor prices continue to come down.

Servo Motor Types

2 . Linear Servo Motor

Servo Motor Feedback

There are two options for Servo Motor feedback controls, either a servo encoder or a servo resolver. A servo encoder and a servo resolver provide the same solution in many applications, but are vastly different. They are both used to sense speed, direction, and position of the Servo Motor output shaft.


The resolver on the Servo Motor uses a second set of rotor and stator coils called the transformer to induce rotor voltages across an air gap. The resolver does not use any electronic components, therefore it is very robust with a high temperature range, and is inherently shock-resistant due to its design. A resolver is mostly used in harsh environments .


The optical encoder on the Servo Motor uses a rotating shutter to interrupt a beam of light across an air gap between a light source and a photo detector, over time the wear associated with the rotating shutter reduces the longevity and reliability of the encoder. The application will determine whether a resolver or an encoder is needed.


Encoders are more accurate and are easier to implement so they should be the first choice for any application. The only reason to choose a resolver is environmental concerns and longevity requirements.

Stepper Motor Versus Servo Motor

Stepper Motor Advantages Servo Motor Advantages

Stability: A stepper motor can drive a wide range of frictional and inertial loads

High output: Power in relation to the servo motor size and weight

Does Not Require Feedback: The stepper motor also acts as the position transducer

Encoder: Determines the accuracy and resolution of the servo motor

Price: Relatively inexpensive High-efficiency: The servo motor can approach 90% efficiency at light loads

Standardized: NEMA standard frame size and performance

High Torque to Inertia Ratio: The servo motor can rapidly accelerate loads

Plug and Play features: Easy to set up and use

A servo motor has 2-3 times continuous power for short periods

Stepper Motor Versus Servo Motor

Stepper Motor Advantages Servo Motor Advantages

Safety: The stepper motor stops if there is a malfunction or interference

A servo motor has 5-10 times rated torque for short periods

Excellent Low Speed Torque: The stepper motor has the ability to drive several loads without gearing

A servo motor stays cool because the current draw is proportional to load

Repeatability: Accurately returns to the same location - open loop system

A servo motor maintains usable high speed torque of 90% of NL RPM

Overload safe: A mechanical overload cannot damage the stepper motor

A servo motor performs quietly at high speeds silently

Longevity: If the specifications of the motor are not exceeded, the stepper is good for 10,000 hours of operation

A servo motor has a resonance-free and vibration-free operation

electric servo motor

Technology