Re: EMC handling backlash with linear encoders and lookup tables

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Doug Fortune wrote:

> Maybe once we find a reasonably priced linear encoder (which
> of course directly senses table movement) then it will be
> feasible to have EMC directly characterize the backlash
> along the length of each axis (say every 1/10th, 1/100th or 1/1000")
> and rather than a simple fixed value, the backlash is looked
> up near that location on every reverse of an axis.

Theoretically doable.

> In fact, a slight extension of this characterization (say every
> 1/1000")  would even compensate automatically for the
> non-linearity of the acme or ball screw.

That is fairly standard, and is called machine error compensation.
It usually requires high accuracy encoders (either linear or rotary)
which resolve down to 25 uInches or better.  (That's .000025"
per encoder count.)  I'm hoping EMC will have this, someday.
I managed to snag some high-precision ground ballscrews that
are good to about .0015" over their 36" travel!  These had
compensator cams on the resolvers in the original unit, but I
left out that mechanism when I retrofitted them with shaft encoders.
Obviously, I didn't lose much if I'm still getting .0015" error
over that length.  So, waiting for machine error compensation
is not a big deal.

Jon