Inexpensive linear encoders

Hello, CNC folks,

I've been following these discussions in a digest form for about a year
now, and I've learned quite a few things from you all. Fantastic. Now I
think I finally have time to write instead of just reading, and perhaps
to give something to the group.

So,

If you are looking for inexpensive linear encoders and strips, visit
your local computer surplus store and look for an old (and big) HP
PaintJet XL. I happened to find two of them, and paid very little. Few
people want to buy old printers that they can't find new ink cartridges
for, so the prices can nowadays be very reasonable.

I opened my new PaintJet's cover and found a two feet long (59
centimeters to be precise) encoder strip inside. A closer look to the
strip under my microscope revealed the pitch to be 180 lines per inch.
The encoder module attached to the print head carrier is an old HP model
QEDS-9558, and it has a separate strip guide that will protect the
strip's center part from being in any contact with the reader and thus
being scratched. The guide touches the strip's edge areas, but those
areas are free from pattern.

The attainable in-quadrature resolution of 720 cpi (35.28 microns) isn't
very high, but it might be fine for an inexpensive DRO. Besides, I think
it's possible to double the resolution by using two suitably spaced
reader heads on one strip, combined with some logic gates. I have to try
this to be sure, but I recall someone else has also proposed something
like that.

The PaintJet XL also has a small DC servo motor and a smallish dual
shaft 400-step Vexta stepper. The stepper isn't very strong, but it
still might be useful for light applications. The motors are driven with
two L298 driver ICs that are quite usable for experimenting with small
steppers. Quite a lot of stuff for ten bucks (or euros, if you prefer).

Do you know any other good sources for cheap encoder strips? Many
inkjets do have a strip, but I have no idea about the resolutions. At
least the newer HP printers seem to have a 150 lpi strip, so they might
be a bit less useful for us hobbyists. Perhaps an old Epson capable of
1440 dpi printouts might have a 360 lpi strip?

BTW, I never let a discarded printer pass by without having a quick look
inside... ;)




Cheers,

Vesa

--
Vesa T. Korhonen - corhoin@...
from Helsinki, Finland