Comparison between stepper Motor and Servo Motor

[kk1234]

How can I size a Servo Motor based on an existing Stepper Motor ?
I mean, I want to replace a Stepper Motor by a Servo motor.
Based on Stepper motor data (Amps, RPM), how can I select a Servo Motor ?

 

[SERVOCAM]

You can make the swap if your not looking to change the application cycle times, mechanics, loading..etc. You could choose a servo with the same or higher rotor inertia and same or higher torque & speed.

The best way to do it is to resize the application. Inertia is the most important factor in every servo/stepper controlled system for sizing. Reason is because Acceleration Torque is defined as Inertia x Rotational acceleration and you need to watch your load-to-rotor inertia matching.

What is the reason for wanting to switch to servo? Stepper old and let the smoke out?
Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. -

http://www.aerotech.com

"Dedicated to the Science of Motion"

 

[jcpitts]

http://handyboard.com/faq/display.php?key=dcservo

 

[RadarRay]

Servocam, did you mean "Same or LOWER motor inertia". If you are adding the statement "same or higher torque", I do not see how you can say "higher inertia".

 

[SERVOCAM]

Duty Cycle is:


On Time
-------------------- x 100
On Time + Off Time


>Servocam, did you mean "Same or LOWER motor inertia".
- No, higher inertia. It comes to Load-to-rotor inertia ratio. If you go with a motor that has less inertia, then you could have too large of an inertia mismatch and have stalling problems w/ stepper or tuning issues w/ servo. Inertia-ratio is more of a servo thing than stepper. If the stepper has the torque, it doesn't not really matter what the mismatch is, it's going to do the job.

Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. -

http://www.aerotech.com

"Dedicated to the Science of Motion"

 

[RadarRay]

Servocam,
I understand that we all respond to applications here looking through a keyhole, not being enough information to adequetely analyse the problem, but I have to disagree with your statement.

For example, say the reflected inertia matched the motor inertia in the original system, so servo wise it was perfect. So by going higher on the motor the system was out of match 2:1. Then net system, if it had the same torque as the original, would respond slower.

Taking a motor of equal torque and half the original motor inertia, the system would respond faster.

And with todays modern controls, it should handle a larger mismatch in either direction and still perform. A 5:1 mismatch doesn't seem to bother any of the servo people I have worked with lately.

Given this data, I would still go with the same torque, lower inertia, motor option, and feel more confident it could handle the application.

 

[SERVOCAM]

I was stating to go with same or higher inertia for the fact if your at a 8:1 currently, and you put in a lower inertia motor, you may now be at 15:1. Typicaly we try to stay at 10:1 or less. That is why I stated it that way. If you don't know what the current mismatch is, you will be safe going to a higher (or slightly higher) inertia rotor.

I have had servo's in 121:1 mismatch and been able to control them. I'm sure that a small change in the inertia will affect his application if he is using stepper.

If your at 2:1...8:1, then yes, you have room to go with a lighter inertia servo, just try to say under 10:1.

Sorry for not explaining more. Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. -

http://www.aerotech.com

"Dedicated to the Science of Motion"

 

[skills]

The tolerable intertial mismatch is also highly dependant upon the rigidness of the system and particularly the coupling mechanism and its lash.

 

[SERVOCAM]

Yes, good point to add skills!

Nothing like a resonance ring to help break the couplers too!

But still, the 10:1 ratio for general (non-super-high-performance) applicaions is a good rule to follow.

Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. -

http://www.aerotech.com

"Dedicated to the Science of Motion"