On 05 Oct 2005 00:19:27 -0400, Greg Menke
wrote:

Ned Simmons writes:

In article ,
says...
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

What concerns me about using a wrapped cable is that the turns of cable
want to walk across the drum as a result of the helical wrap. This will
cause the cable to be pulled out of line with it's attachment points,
which changes number of pulses per inch of carriage travel. If the
stroke is long relative to the drum's circumference the cable may either
slip back into line, cross over itself, or walk off the end of the drum.

I've done this, and even with a fairly short stroke, large drum, and low
accuracy requirement, it was a real pain to get working reliably. I
don't mean to be discouraging, but I haven't heard, or thought of, any
idea yet that gives me the warm fuzzies.

Ned Simmons

Sorry didn't catch the previous post. One way to avoid the cable
walking is to set up a parallel shaft nearby and turn a couple
"windlasses"- bigger collars on each shaft so the cable can bear more on
each. Wouldn't hurt to put notches on the collars to help guide the
cable. So the coil goes around the first, then around the second, then
back around the first. Not wrapping all the way around each shaft, but
looping around the pair of shafts. The aux shaft should be free to
spin. Then as the cable drives the shaft (or vice versa), the cable
won't walk or slip. Maybe you only need a few loops around the shafts &
can keep things fairly compact.

Gregm
Greg,
I can't quite see waht you are describing. Maybe I'm too tired. The
encoder shaft cannot get any bigger. This shaft is 1.000 in
circumference. This is so the encoder outputs 10,000 pulses per
revolution. Could you explain your idea more clearly so I can
understand it? I'm sure everyone else here has already visualized the
setup but my brain is fogged this morning.
ERS