Re: Source Sealed Potentiometers?


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.

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.

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.

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.

So: is there any way of putting limit switches (or some sort of sensor)
on the platform?

Yeah, but I expect that more often than not, when it powers up, it's
not going to be triggering any switch, so I won't know anything about
the initial position.

JM

.

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