Re: Source Sealed Potentiometers?

John Mianowski wrote:
CBFalconer wrote:
John Mianowski wrote:

... snip ...

What I need to do is rotate what amounts to a
lazy-susan/turntable/camera mount (pan only). I need to rotate up to
360 degrees, but not continuous. The device sits on a bearing so there
is no center shaft that I might couple to. OD is 3". My plan is to
attach a gear (I've got that part worked out OK) to be driven by an
electric motor. What I need is some sort of position feedback. The
gear I've chosen for the turntable has 156 teeth with a pitch diamter
of 3.25". I figure that with a mating 32-tooth gear, I can attach a
5-turn pot. (ratio = 4.875:1, leaving a little travel at either end).
What I DON'T have, besides a shaft to couple onto directly, is space
for another 3.25"-dia. gear to keep the ratio of turntable to position
sensor at 1:1 or below, & I'd like to avoid using a multi-stage
reduction gear if I don't absolutely have to.

I also have to be able to survive immersion in fresh water for up to 30
minutes.

What sort of positional resolution (in degrees, or even in gear
teeth count) do you require? Something like a pair of leds for
tooth resolution, and a reset mechanism via a microswitch could
provide the position. The pair of leds can detect rotation
direction, and the whole schmeer should be immune to over-rotation,
unlike a pot. Counting the 156 teeth should give you roughly 2
degrees of positional resolution.

Your only worry would be the effect on the light path of the
immersion, which should not be excessive in fresh water.

2 degrees of resolution is enough.

A prior prototype used a pair of reflective optical sensors, with black
& white strips on the rotating assembly. One sensor read a narrow
white "index" stripe that indicated the unit was in its "home"
position, while the other read either black or white, depending on
whether "home" was CW or CCW. That worked well, when I needed to do a
realignment such as on power-up. I had a couple of issues that have
led me away from this solution:

- I don't have 100% confidence in being able to count teeth reliably.
Once I believe there has been a missed step, it's easy enough to do a
re-alignment, but I'd rather not have to do that. The prior prototype
used a stepper motor, & kept a running count of steps after it was
aligned to its "home" orientation. I didn't feel like I could gurantee
that the unit couldn't get physically forced out of position, to where
its actual orientaton would be different from where I thought it is,
based on the step count.

- I don't have a lot of confidence in the use of optical sensors in
what I expect to be a rather dirty environment.

I need to know, with a high degree of certainty, the orientation of the
platform at all times (within the limit of about 2 degrees). Counting
teeth, I believe, amounts to a guess & once the certainty of position
is gone, it's gone.

45 years ago we controlled a gamma ray spectrometer by counting
teeth. The sensors were microswitches that were operated by a cam
on an idler gear. Quite reliable. You would need two such, with
appropriate phasing to tell direction. We used the idler gear
teeth proper as the cams. The whole system was a collection of
relays and an ordinary motor to do the actual moving. Modern
systems would need positive switch debouncing, which can easily be
done with a spdt break before make switch and a set/reset
flip-flop. You would want the sensors out of phase by 1/2 tooth,
but that would not be a critical adjustment. The idlers would have
6 to 8 teeth.

--
Some informative links:
<news:news.announce.newusers
<http://www.geocities.com/nnqweb/>
<http://www.catb.org/~esr/faqs/smart-questions.html>
<http://www.caliburn.nl/topposting.html>
<http://www.netmeister.org/news/learn2quote.html>
<http://cfaj.freeshell.org/google/>


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Relevant Pages

  • Re: Source Sealed Potentiometers?
    ... I need to rotate up to ... gear I've chosen for the turntable has 156 teeth with a pitch diamter ... A prior prototype used a pair of reflective optical sensors, ...
    (comp.arch.embedded)
  • Re: Source Sealed Potentiometers?
    ... I need to rotate up to ... gear I've chosen for the turntable has 156 teeth with a pitch diamter ... A prior prototype used a pair of reflective optical sensors, ...
    (comp.arch.embedded)
  • Re: Source Sealed Potentiometers?
    ... gear I've chosen for the turntable has 156 teeth with a pitch diamter ... Counting the 156 teeth should give you roughly 2 ... A prior prototype used a pair of reflective optical sensors, ...
    (comp.arch.embedded)
  • Re: Source Sealed Potentiometers?
    ... gear I've chosen for the turntable has 156 teeth with a pitch diamter ... What sort of positional resolution (in degrees, ... A prior prototype used a pair of reflective optical sensors, ...
    (comp.arch.embedded)
  • Re: Source Sealed Potentiometers?
    ... I need to rotate up to ... gear I've chosen for the turntable has 156 teeth with a pitch diamter ... What sort of positional resolution (in degrees, ...
    (comp.arch.embedded)