Re: Source Sealed Potentiometers?


Frnak McKenney wrote:
On 2 Oct 2006 06:59:48 -0700, John Mianowski <spamfree@xxxxxxxxxx> wrote:

CBFalconer wrote:
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".
--snip--
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?
--snip--
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.
--snip--
- 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.
--snip--
I'd still need a method to tell when I'm in some exact position, & some
way to tell whether that position is CW or CCW if I'm not exactly there
already, so that I can have a reference angle from which to start
counting teeth. I'm interested in absolute position, not relative.

You can purchase "absolute encoder" discs for optical encoders. These
have black/white or opaque/clear segments that provide a binary value
to a row of sensors so you can read the disc's current position
"directly" (possibly through, say, a Gray Code lookup table).

The same principle can be applied to a magnetic or mechanical encoder.
Put bumps on a disc, or lay down axial rows of tiny magnets to be read by
a row of Hall Effect sensors.

Is there any way you can apply this approach? You'er still faced with
the problem of choosing sensors (optical, Hall Effect, microswitches) that
can operate reliably underwater in fresh water.

I know about absolute encoders, but I still have the problem of needing
to track multiple turns, since I don't have a shaft to directly couple
to or the space to add a full-sized 1:1 gear.

Magnetic strips might be a possibility, to tell me which segment of the
circle I happen to be in, with the resolver providing
more-than-sufficient resolution within that range. If I could
determine magnetic polarity, I should be able to reduce the number of
sensors to 2, with 4 strips staggered around the perimeter of my
turntable:

High/North, Low/North, none, Low/South, High/South.

Then, with 5:1 gearing on the resolver, that I know fits within the
confines of my space, the combination of segment sensor/resolver shold
tell me what I need to know.

So, there's another question for somebody: How can I detect not only
the presence of a magnetic field, but its polarity? Off the top of my
head, I don't know if that's something most Hall-effect sensors are
capable of doing.

Thanks.

JM

.

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 ... What sort of positional resolution (in degrees, ... A prior prototype used a pair of reflective optical sensors, ...
    (comp.arch.embedded)
  • Re: Source Sealed Potentiometers?
    ... A prior prototype used a pair of reflective optical sensors, ... I don't have 100% confidence in being able to count teeth reliably. ... 45 years ago we controlled a gamma ray spectrometer by counting ... The same principle can be applied to a magnetic or mechanical encoder. ...
    (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)