Servo Motors and Servo Drives

Servo Motor Selection Flow Chart

Explanation

References

Determine the size, mass, coefficient of friction, and external forces of all the moving part of the Servo Motor the rotation of which affects.

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Determine the operating pattern (relationship between time and speed) of each part that must be controlled.
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.

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Calculate the Acceleration/Deceleration Torque from the Load Inertia or Operating Pattern.

Acceleration/Deceleration 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 motor.

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 product for 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 Items	Check Items
Load Inertia	Load Inertia ≤ Motor Rotor Inertia x Applicable Inertia Ratio
Effective Torque	Effective Torque < Motor Rated Torque Please allow a margin of about 20%. *
Maximum Momentary Torque	Maximum 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 Speed	Maximum 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 Energy	Regenerative 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 Resolution	Ensure that the Encoder Resolution meets the system specifications.
Characteristics of a Positioner	Check if the Pulse Frequency does not exceed the Maximum Response Frequency or Maximum Command Frequency of a Positioner.
Operating Conditions	Ensure that values of the ambient operating temperature/humidity, operating atmosphere, shock and vibrations meet the product specifications.

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

Formulas

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

Load Mass, Ball Screw Pitch, Ball Screw Diameter, Ball Screw Mass and Ball Screw Friction Coefficient

(2) Determining Operating Pattern

One Speed Change, Velocity for a Load Travel, Strokes, Stroke Travel Time, Acceleration / Deceleration Time and Positioning Accuracy

(3) Calculation of Motor Shaft Conversion Load Inertia

Ball Screw Inertia, Load Inertia, Motor Shaft Conversion Load Inertia Calculation

(4) Load Torque Calculation

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

Calculation of Maximum Momentary Torque, Effective Torque

(9) Result of Examination

Result of Examination

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

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