Re: Minimal encoder patterns (was: Source Sealed Potentiometers?)

On 6 Oct 2006 11:44:30 -0700, Robert Adsett <sub2@xxxxxxxxxxxxxxxxxxxxx> wrote:

Frnak McKenney wrote:
After my last posting I took a walk and thought a bit about encoders
and encoder patterns. "Absolute" optical encoder wheel patterns
have been around for years; the ones I've seen are basically XxY
binary arrays drawn as concentric circular segments, with the bit
patterns generally based on Grey Codes or something similar.

If I take such an XxY array and string it out as a X*Y-length binary
pattern "track", and if I use a second track as an "index marker"
every Y "bits", I've transformed the M-track absolute position
pattern into a two-track pattern.

Could I do it all with one track? What would I need?

- A "magic" bit pattern P of length N.
- A set of sensors -- M of them.
- P chosen so that every M-length "wrapped" substring of P was
unique.

Something like this?

http://www.taosinc.com/downloads/pdf/encoderdesign4b.pdf

Yes, although they seem to have done a much better job on the
problem than I did. <grin!>

For those who haven't already downloaded this file, it's an App Note
published by TAOS (of color sensor fame) entitled "Design
Considerations for an Absolute Steering Angle Encoder Demo". The AN
describes a method for using a maximal-length sequence code to
generate an optical encoder pattern which can be used with a linear
array sensor to determine the encoder wheel's position.

The authors have started with the same problem I described a few
postings back, but they've clearly thought through not only the
mathematics, but the real-world considerations of such an approach as
well... such as the problem of ensuring continuous readout values
during transitions, and the "bar" width vs. the sensor size and
separation between sensors.

I'm still working my way through some of the paper's calculations,
and I think I'm going to need to dig into the data*** on TAOS's
TSL1401R sensor before I can get much further.

Anyway, it's nice to see their work, and I found some other
interesting App Notes on the TAOS site.

Thanks for the pointer, Robert.


Frank

Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
--
"I've wrestled with reality for thirty-five years, and I'm happy
to state that I've finally won out over it."
-- Elwood P. Dowd in "Harvey"
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