Eric R Snow wrote:

On Tue, 04 Oct 2005 19:15:41 GMT, "Mike Young"
wrote:

"R. O'Brian" wrote in message
news:JBy0f.407$784.353@lakeread08...
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

You probably need a metal belt, a thin flat endless ribbon of stainless
steel or some such alloy. Dimensional stability is excellent at constant
temp. See
http://www.machinedesign.com/BDE/mec...emech1_20.html

Or phosphor bronze... I can see that working for partial turns, but not
multiple turns as Eric is looking for. Two opposing straps wound not quite
the full circumference, fixed firmly at the ends, will track like a worm and
rack.

It would be instructive to disassemble a dial indicator and look inside.
Mebbe I would just find a way to mount the encoder to the de-faced
indicator, rather than mess with mirrors, lasers, belts, oscilloscopes,
quadrature waveforms, ... unless the idea is to mess, to reinvent, rather
than measure.
I know how indicators work. And they involve anti-backlash gears. But
that misses the point. The encoder should have NO mechanical lash. The
disc with the lines is mounted directly to the shaft. A metal ribbon
if the friction was high enough. I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS


The turn-on and turn-off positions do not happen at exactly at the same
position when the shaft is reversed. This is where your "backlash" is
coming from. If you want better accuracy, use a higher count per rev.
encoder.

John