What are your "Precision" Requirements?

Submitted by Corey Foster, Senior Automation Application Engineer
"Precision" is a term that needs to be broken down into several components that include accuracy, repeatability, flatness, straightness, etc. Many customers say they "require" certain accuracy, but is the requirement really "accuracy" or is it "repeatability?"
 
Here is a popular graphical description of the difference:
 
 
Using requirements of 5 microns, a 5 kg load, and only 50mm of travel, you can end up with a much different result on your solution depending upon whether you mean 5 micron "accuracy" or 5 micron "repeatability.
 
5 MICRON ACCURACY

For a 5 micron accuracy requirement, you probably will need a small linear motor stage with a 0.5 linear encoder to make sure you have 5 micron accuracy over 50mm of travel. At this stage, the unit may cost several thousand dollars to achieve 5 micron accuracy.
 
 
 
5 MICRON REPEATABILITY

For about HALF the cost, however, you might end up with a different stage that gets 5 micron repeatability. As shown here.
 
These two solutions have vastly different form factors and capabilities...and therefore price tags. The question is then: how do I know if I really only care about "repeatability?" A few questions to ask yourself when deciding: 
 
  • Does your application have pre-defined positions that you'll be using over and over again?
  • Or are you following a path that you need to follow very closely?

If you are moving to pre-defined positions and have a way of calibrating those positions, then once you have moved to the pre-defined positions, calibrated it, and adjusted for any inaccuracy, you know that you can hit that same position within a 5 micron repeatability with a solution that is less expensive than one that you might have otherwise ended up with.