I assume this is due to mechanical error external to the encoder and/or PID resolution, not actual reading error of the encoder?
Max.

Yes, that's the vented mechanical error from the indicated position. Repeatability of positioning in a system like this more important than the absolute position as there are software offsets to correct for just about everything including wall flex from the vacuum chamber the disk is in.

But when things go bad, they go really bad on a large disk spinning at 1200 rpm. (didn't happen to us)

https://www.flickr.com/photos/12772565@N05/sets/72157604136768179

 

Someone let the desk geeks operate the real man machinery!