×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Servo motor inertia ratio

Servo motor inertia ratio

Servo motor inertia ratio

(OP)
Hey all,

I understand that inertia ratio between the load and the rotor can be a good guideline for how to size a servo motor and gearbox.

From what I have seen as a general rule of thumb, people suggest a load:rotor ratio of less than 10:1 or as close to 1:1 as possible.
I am wondering if there would be any harm in gearing down to be less than 1:1, like 1:10.

I imagine there would be no issues, as it would reduce the load on the motor or improve the acceleration of the load up to operating speed.

I should say, this servo is just being used to index a rotary table.

What thoughts do you have?

RE: Servo motor inertia ratio

Although it seems sensible that a 1:1 inertia ratio will be the target, it is not always feasible or cost-effective. Many servo motor manufacturers recommend keeping the inertia ratio at or below 10:1, although there are many applications that work well at much higher ratios.

The best inertia ratio for an application is the dynamics of the step and the accuracy needed.

RE: Servo motor inertia ratio

There's probably been more nonsense offered about "inertial matching" issues than any other in all the years I have been in servo control. There is really nothing "optimal" about a 1:1 ratio.

In the real world, what you typically need to worry about most is how your resonant and anti-resonant frequencies of the load-coupling-load system vary with changing ratios. Increasing the load inertia too much can lower these sqrt(k/J) frequencies into the zones you want actual operation, which is problematic.

The 10:1 ratio limit is simply a common rule of thumb to keep you out of trouble in common systems, mainly rotary servo motors with a small-diameter helical coupling to the load. On the other hand, with direct-drive motors where the "coupling" has the full diameter of the motor, resulting in a very high stiffness "k", you can successfully use a ratio of hundreds to one.

There are some secondary issues that get worse with high inertia ratios. If you have the system tuned well with a large load attached, you may get bad behavior with the load unattached, even just for test purposes. If you have backlash, the load is effectively unattached during the transition, and you can get problematic behavior then as well.

There is absolutely no problem here in going below a 1:1 ratio if it helps other system goals.

Curt Wilson
Omron Delta Tau

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close