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Lead Contents

Servomotors / Servo Drivers

Servomotors and Servo Drivers are provided for the purpose from replacing stepping motors to high-speed, high-precision control. Models with MECHATROLINK II communications are also provided. Concerning Servomotors/Servo Drivers, OMRON offers different products in different areas. So please click on the banner "Products links" in the top right corner to reach the country website nearest you.

Overview Features
Principles Classifications
Engineering Data Further Information

Related Contents

Primary Contents

Servo Motor Selection Flow Chart

Determine the size, mass, coefficient of friction, and
external forces of all the moving part of the Servo
Motor the rotation of which affects.
Determine the operating pattern (relationship
between time and speed) of each part that must be
Convert the operating pattern of each controlled
element into the motor shaft operating pattern.
Operation Pattern Formula
The elements of the machine can be separated so
that inertia can be calculated for each part that
moves as the Servo Motor rotates.
Calculate the inertia applied to each element to
calculate the total load inertia of the motor shaft
conversion value.
Inertia Formulas
Calculation of Friction Torque
Calculates the frictional force for each element,
where necessary, and converts it to friction torque
for a motor shaft.
Calculation of External Torque
Calculates the external force for each element,
where necessary, and converts it to external
torque of a motor shaft.
Calculates the total load torque for the motor shaft
conversion value.
Load Torque Formulas
Select a motor temporarily based upon the motor
shaft converted load inertia, friction torque, external
torque and r.p.m of a motor.
Calculate the Acceleration/Deceleration Torque
from the Load Inertia or Operating Pattern.
Torque Formulas
Calculate the necessary torque for each part of the
Operating Pattern from the Friction Torque, External
Torque and Acceleration/Deceleration Torque.
Confirm that the maximum value for the Torque for
each operating part (Maximum Momentary Torque)
is less than the Maximum Momentary Torque of the
Calculate the Effective Torque from the Torque for
each Operating part, and confirm that it is less than
the Rated Torque for the motor.
Calculation of Maximum
Momentary Torque,
Effective Torque
Calculate Regenerative Energy from the Torque of
all the moving parts.
Please see the user
manual of each
productfor the details
on calculation of the
regenerative energy.
Check if the the number of encoder pulses meets
the system specified resolution.
Accuracy of Positioning
Check if the calculation meets the specifications of
the temporarily selected motor.
If not, change the temporarily selected motor and
re-calculate it.
The following table
Specialized Check ItemsCheck Items
Load InertiaLoad Inertia ≤ Motor Rotor Inertia x Applicable
Inertia Ratio
Effective TorqueEffective Torque < Motor Rated Torque
Please allow a margin of about 20%. *
Maximum Momentary TorqueMaximum Momentary Torque < Motor
Maximum Momentary Torque
Please allow a margin of about 20%. *
For the motor Maximum Momentary Torque,
use the value that is combined with a driver
and the one of the motor itself.
Maximum Rotation SpeedMaximum Rotation Speed ≤ Rated Rotation
Speed of a motor
Try to get as close to the motor's rated
rotations as possible. It will increase the
operating efficiency of a motor.
For the formula, please see "Straight-line
Speed and Motor Rotation Speed".
Regenerative EnergyRegenerative Energy ≤ Regenerative Energy
Absorption of a motor
When the Regenerative Energy is large,
connect a Regenerative Energy Absorption
Resistance to increase the Absorption
capacity of the driver.
Encoder ResolutionEnsure that the Encoder Resolution meets
the system specifications.
Characteristics of a PositionerCheck if the Pulse Frequency does not
exceed the Maximum Response Frequency
or Maximum Command Frequency of a
Operating ConditionsEnsure that values of the ambient operating
temperature/humidity, operating atmosphere,
shock and vibrations meet the product

*When handling vertical loads and a load affected by the external torque, allow for about 30% of capacity.


Formulas for Operating Patterns

Inertia Formulas

Load Torque Formulas

Acceleration/Deceleration Torque Formula

Calculation of Maximum Momentary Torque, Effective Torque

Positioning Accuracy

Straight Line Speed and Motor Rotation Speed

Sample Calculations

(1) Machinery Selection

(2) Determining Operating Pattern

(3) Calculation of Motor Shaft Conversion Load Inertia

(4) Load Torque Calculation

(5) Calculation of Rotation Speed

(6) Motor Temporary Selection [In case OMNUC U Series Servo Motor is temporarily selected]

*Note that this value changes according to the Series.

(7) Calculation of Acceleration/Deceleration Torque

(8) Calculation of Maximum Momentary Torque, Effective Torque

(9) Result of Examination

Note:This example omits calculations for the regenerative energy, operating conditions, or positioner characteristics.