Re: Source Sealed Potentiometers?


Frnak McKenney wrote:
On 4 Oct 2006 13:33:49 -0700, John Mianowski <spamfree@xxxxxxxxxx> wrote:

Frnak McKenney wrote:
On 3 Oct 2006 07:18:15 -0700, John Mianowski <spamfree@xxxxxxxxxx> wrote:
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.
--snip--
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.

And you can't add optical or magnetic sensors under the platform or
around its perimeter?

I could mount the sensors fixed, & the "sensed" material on the
rotating platform.

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:

Ah. I misread the "2 degrees" (e.g. 1/180 revolution) resolution
requirement, so I was picturing someone embedding _lots_ of littly teeny
magnets in a clear resin disc. <grin>

No, I think you got it the 1st time. I don't have the room to connect
a resolver so that 1 turn of the platform = 1 turn of the resolver
shaft; I have to gear it down & can make 5:1 work. Trouble is, there
would be 5 points in the turntable's rotation where the resolver
produces the exact same output, & I need a way of knowing which of them
is correct.

I think I lost part of the picture here. If your "resolver wheel" makes
five turns for the one full turn of the platform, doesn't _every_
reported resolver output represent five possibple platform positions?

Exactly, & I need to narrow it down to just 1.

But... why do you need a separate resolver?

That was part of somebody's proposed solution, to figure out which of
the 5 possible positions is actual.

If you were using, for example,
optoreflector sensors, why not just paint a black and white "absolute
position" pattern on the bottom of the _platform_? It would cut down on
the number of moving parts.

I've done that & it works very well in prototype. I'm not confident,
though, that it will work reliably in the messy environment that the
final version will be going into. The need for frequent
maintenance/cleaning is something that I'd like to avoid if I can.

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.

Take a look at these linear Hall Effect sensors. Their output is a
voltage that varies in proportion to the magnetic field, rising for
one pole and dropping for the other:

Linear Hall-Effect Sensors
http://www.allegromicro.com/hall/linear.asp

A1301 and A1302 Continuous-Time Ratiometric Linear Hall Effect Sensors
http://www.allegromicro.com/sf/1301/

For examples of other types, look at:

Hall-Effect Sensors
http://www.allegromicro.com/hall/

Another approach would be to use two sensors next to each other and bias
one with a small magnet to reverse the polarity it responds to.

MIght be something there - I'll have to get some & see what I can make
happen.

Great. The AllegroMicro has a few tutorials as well, and more material
can be found scattered around the 'web.

But, as you point out, if you use a "sensor wheel" that is making
multiple turns while the platform is making its full limit of 360
degrees, none of this _guarantees_ that your monitor didn't go to sleep
and so miss a whole rotation of the sensor wheel.

Not just miss a step, but know exactly where the turntable is pointed
on power-up.

That, too. I hadn't thought about that part.

Let us know what you finally come up with. We'll be more than happy to
steal your solution and an use it in our own projects. <grin!>

Sure. You're all more than welcome to it. Whatever "the solution"
ends up being, probably won't be for awhile (it's hobby-related,
subject to the parameters of funding & time that all hobbies must exist
within - nothing planned to ever be sold).

Thanks!

JM

.

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