Re: Source Sealed Potentiometers?

Frnak McKenney wrote:

Paul,

Thanks for joining in.

I recall that the OP wanted to be certain of the turntable
position without the need of any prior rotation. ...

[%X------------Robot Car Description----------%X]


So I think I can understand why the OP might not like to use that
_particular_ approach in solving his problem. <grin!>

A situation he was most careful to explain he wished to avoid.

... A serial
encoding track would not fit this bill.

I'm puzzled as to exactly what you're saying here. A simple
counter-example to what I _think_ you're saying here would be an
all-white track with one black bar -- all ones but for one zero, or,
if you prefer, all zeroes but for one one. If this fits your
definition of a "serial encoding track", then imagine 180 optical
sensors around the disc that this track is drawn upon -- won't
scanning those sensors report the exact-within-2deg position of the
disc? And I won't have to rotate the disc to figure out what its
position is.

I had been reading some other links thrown up by this thread which had
sector encodings on a single track. This would require an, admittedly,
small rotation to read the entire code with the minimal number of sensors
(two adjacent patterns required to be certain). Of course, sensor rich
solutions might be of interest to the OP if the individual sensors were
cheap enough and met his environmental criteria.

One sensor that might fit some of his criteria would be this one:-

http://news.thomasnet.com/fullstory/474495/2562

However, as it is only IP67, it may just fall a bit short of his intended
usage environment. It may also be out of his budget.

The documents mentioned in the other thread:

http://www.taosinc.com/downloads/pdf/encoderdesign4b.pdf
http://www.taosinc.com/downloads/pdf/IOSDN1.pdf

They were mentioned here also.

describe a somewhat<grin!> less sensor-hungry approach to using a
single-track encoder pattern to determine absolute position, in
their case to 1/4096 of a full turn (0.088 degrees). They use a
single sensor (really, 128 photodiodes in a single package) with a
single-track 256-element pattern to accomplish this; I'm on my
third reading, and it's starting to make sense.

I'm also having trouble translating the "tracked" approach out of
the optical domain and into the magnetic... um, "field" (oooog!)
to satisfy the OP's concerns regarding outdoor use. It's not that I
think that this translation _couldn't_ be done, it's that I get
stuck at the part where I picture _my_ rather clumsy fingers trying
(for the third time) to glue 256 magnets of two different widths
around the circumference of the OP's platform in the correct
pattern.

Hm. What if I wrapped _one_ flexible magnetic strip around the
platform's rim and then covered it with a carefully punched strip of
metal? Will enough of the strip's magnetic field get through so
that a linear Hall Effect sensor can tell the difference between a
narrow and a wide slit? I suspect I can print-and-paste-and-punch a
pattern more reliably than I can glue individual itsy-bitsy
magnets... I need to think about this.

The problem with magnetic fields is that they easily stray into zones where
they are not needed. It doesn't take much to divert them. So, I do not see
the magnetic encodings being able to be used in as fine a pattern as
optical ones. Either the encoding of a distinct number of segments and
doing finer position detection by other (absolute) means or coming up with
a full magnetic encoding scheme that gives the desired 2 degree resolution,
would seem to be a sensible direction for the OP to take.

Anyway, if I misunderstood your comment, I apologize. Please give it
another "whack".

It is probably the nature of our differences in experience that led to the
differing take. Lately, I have only dipped into the newsgroups on a more
eratic basis than usual due to being quite occupied with other things.
However, now that the OP has explained a bit more about his application it
seems that there are plenty of potentially good ideas coming forward and
some lively discussion about the merits of each one.

I can imagine that there would be a few interesting applications for such a
scheme with the environmental requirements as the OP stated. I hope he
finds the most suitable solution for his needs and his wallet.

Frank McKenney, McKenney Associates

--
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Paul E. Bennett ....................<email://peb@xxxxxxxxxxxxxxxxxx>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
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