1. ELECTRICAL
SERVO MOTOR

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

3. 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.

4. 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.

5. 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.

6. 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.

7. Physical Properties of a
Servo 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.

8. Physical Properties of a
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.

9. Physical Properties of a
Servo Motor
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.

10. 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 .

11. 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.

12. 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.

13. 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 .

14. Industrial Servo Motor

15. 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 .

16. How Does a Servo Motor
Work?
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.

17. How Does a Servo Motor
Work?
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 .

18. 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.

19. 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.

20. 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 .

21. 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.

22. Servo Motor Types
1 . Rotary Servo Motor

23. Servo Motor Types
2 . Linear Servo Motor
A 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.

24. Servo Motor Types
2 . Linear Servo Motor

25. 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.

26. Servo Motor Feedback
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 .

27. Servo Motor Feedback
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.

28. Servo Motor Feedback
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.

29. 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

30. 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

31. END