Feedback: Matching moments of inertia

Staff -- Control Engineering, 1/1/2007

Dear editor:

I read your article (CE, 12/06, p. 18, “Technology Update: Matching moments of inertia”) with interest. Having spent 38 years in R&D on machine tools at G&L I am quite familiar with "inertia match" indexes of performance for industrial machine servo drives. Control Engineering published an article on inertia match a number of years ago. I still refer to that article in my lectures.

As you point out the best inertia match is 1:1. As an index of performance we could live with a 3:1 mismatch as you stated.

There is another variable that enters into the "match" thinking. Most servo motors used in general industrial applications used ceramic magnets. With a 3:1 mismatch we could still get satisfactory performance. The reason for this is that we can get good "load isolation" with a ratio since the load reflected inertia to the motor is reduced by the square of the ratio. This fact is very important when we have machine axes that are 20,000 lbs of more. If a lead screw is used that will also reduce the inertia by the square of the lead.

Now we also have high performance servo motors (low inertia) that use high coercive force magnets like neodymium-iron-boron. The best match for these motors is 1:1, but now the reflected inertia must be much lower to get the increased performance. Unfortunately, many users think they can use these motors on a machine axis that worked well with higher reflected inertia with a ceramic type motor, and still get the desired performance. I always try to point this out to seminar attendees.

The manufacturers of low inertia motors claim they can use a 10:1 inertia mismatch. This can lead to instability if you don't know what you are doing. That article by Control Engineering shows that the accel time will have to be increased to avoid instability.

Another problem that is often overlooked is that the low inertia motors have a much shorter thermal time constant which can be a heating problem for drive forcing such as repeated accelerations.

Read December 2006 article online at www.controleng.com/archive.

George Younkin, P.E., MSEE, Life FELLOW-IEEE, Industrial Controls Consulting, A Division of Bull's Eye Research Inc., Fond du Lac, WI.

For information on George Younkin's book, Industrial Servo Control Mechanisms: Fundamentals and Applications, visit www.crcpress.com.