Re: Source Sealed Potentiometers?

On Mon, 09 Oct 2006 18:25:58 +0100, Paul E. Bennett <peb@xxxxxxxxxxxxxxxxxx> wrote:
Frnak McKenney wrote:

On 7 Oct 2006 07:20:15 -0700, John Mianowski <spamfree@xxxxxxxxxx> wrote:

Frnak McKenney wrote:
...
You mentioned the possibility of the platform getting "dunked", ...
--snip--
Rotation speed will be very low, & the rotating assembly is supported
on a bearing, so loading on the gears is pretty low. Most plastic
gears are self-lubricating & maintenance is expected to be little more
than cleaning. Gears in need of maintenance typically start getting a
little noisy but still function for a good while. Optical sensors,
OTOH, tend to fail completely & suddenly - such as from dirt on a
sensor lens or a white reflective surface. Magnetic, I would expect
little trouble from.

Less trouble in terms of getting silt deposits, but it feels like
adding lots of tiny magnets (or the equivalent of gear teeth) could be
more work than simply printing an optical-encoder pattern.

I dimly recall that the platform is, what, three inches in diameter?
Yes.

Putting an optical pattern on its uderside would mean... 180 divisions,
so eight binary sensor "bits" to track -- an 180x8 grid, with fairly
closely-spaced "pickups".

Or around the edge, where I have about 0.5" to work with. This is
where I painted my black/white stripes on the 2nd prototype (1/2 black,
1/2 white to tell direction to "home"; narrow white on black to
indicate "home"; stepper to rotate, count/accumulate steps to determine
position) - which worked just fine, except that I'm not 100% satisfied
that it can handle the dirty environment or continue to keep track of
position when/if an external force rotates the turntable.

Well, optical or magnetic, you might check the parallel thread in this
newsgroup titled "Minimal encoder patterns" regarding doing absolute
position sensing with a one-track pattern -- sort of like a linear or
one-dimensional barcode vs. a complex X-Y pattern like a 2-D barcode.

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. ...

My third robot was a modified R/C car which steered with a geared DC
motor and no limit switches, so I used a "bang-bang" approach to
centering the steering. This involved stopping, running the motor
into a full left turn (indicated via a comparator "stall detector"
looking for a spike on the motor current), reversing the motor for a
full left turn... and then assuming that running the motor for half
of that elapsed time would aim the front wheels "straight".

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

... 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.

The documents mentioned in the other thread:

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

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.

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


Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
--
"We don't just borrow words; on occasion, English has pursued other
languages down alleyways to beat them unconscious and riffle their
pockets for new vocabulary." -- James D. Nicoll
--
.

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