On Mon, 03 Oct 2005 09:39:54 -0700, Eric R Snow
wrote:

SNIP

The driving method so far is a
belt. I can detect no slipping at all. But I keep getting this .0009
error. I don't know if this is a display error because it's reading
the encoder disc in quadrature. I thought about looking at the disc
itself. It's 2.000 inches in diameter with 2500 marks on it. which
makes the spacing about .0025". This is readable with a magnifier. But
I thought that amplifying the error would be easier because I can see
the movement at the same time as the display changes. I can very
easily see 9 inches movement at 132 feet. And it turns out I have a
target just about that far away. A nice clean cement wall. Thanks for
reading and responding.
Eric

It is almost impossible to get a belt or friction drive to work
with ZERO backlash. Reversal of rotation generates an an error of
twice the compressibility of the drive system and this may well
be enough to account for your error.

Try bolting a rigid arm to your encoder a few inches long
and spring loading this against a solidly anchored stop.
Insertion and removal of feeler gauges between the arm and the
stop will soon show whether your .0009 error is inherent in the
encoder or your friction drive.

Jim