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'[OT] inertial guidance for an underwater PIC as a '
2000\05\18@033503 by

The subject says it all and if it is feasible we could even get rid of the
[OT] ;)

how hard would this be?  Just need and LED direction indicator to find the
way back to the boat, so accuracy would not have to be better than a few
meters over an hour of swimming (boat would probably drift anyway).

Is this reasonable on a PIC or better suited to an HC11.  Are there reject
accelerometer modules that are not good enough for helicopters (a source of
crashed ones?) but would be OK for diving?  I am thinking cost on this one.

Is there another way (pitot (sp?) tubes??, under water GPS type boat
produced ultrasonics?))

cya,    Andrew...

> {Original Message removed}
Really, really, freakin' hard.  On subs, the Navy uses dual \$20 Million
(pretty sure) gyros to do it, and they are the size of a refrigerator.  To
make a first order approximation of how to do this with an accelerometer-
from basic physics (d=.5*a*t^2)  (It has been a while, so please forgive me
if I did not remember correctly :) if you take an offset of 1/10,000 of a g,
over an hour, the position could be off as much as 6.3 kilometers.  You
don't do a long period INS (intertial navigation system) with cut rate
accelerometers.  You use devices like ring laser gyros and do interferometry
on light. To be within 10m after an hour you need a drift of less than
1.6x10^-7 g.  Not something you're going to get with an off the shelf part.
>From what I've seen, an INS that would satisfy your requirements would be in
the \$100,000+ range.

An INS is best over the short term- periodically zeroing out the drift with
some other reference, such as a GPS.  Kalman filters are used a lot to
integrate different sensors to make an accurate INS.  The Ultrasonics, on
the other hand, have a much better chance of success, at least in telling
you range- 3D position is harder unless you have a pretty large array of
transducers to home in on (based on geometric dilution of precision).
Underwater, GPS has its own limitations, unless you are willing to surface
to get a fix on your position.

I believe this topic has been discussed at length before on the PICLIST.  It
is really hard, if you could do it cheaply, many, many people would be

Matt Bennett

{Original Message removed}
"McMeikan, Andrew" wrote:
>
> The subject says it all and if it is feasible we could even get rid of the
> [OT] ;)
>
> how hard would this be?  Just need and LED direction indicator to find the
> way back to the boat, so accuracy would not have to be better than a few
> meters over an hour of swimming (boat would probably drift anyway).
>
> Is this reasonable on a PIC or better suited to an HC11.  Are there reject
> accelerometer modules that are not good enough for helicopters (a source of
> crashed ones?) but would be OK for diving?  I am thinking cost on this one.
>
> Is there another way (pitot (sp?) tubes??, under water GPS type boat
> produced ultrasonics?))

It's really early AM, but...

I wonder if you could detect a 'ping' from the depth finder.

--
Best regards

Tony

http://www.picnpoke.com
salespicnpoke.com

On Fri, 19 May 2000 06:58:39 +1000, Tony Nixon
<Tony.NixonENG.MONASH.EDU.AU> wrote:

> I wonder if you could detect a 'ping' from the depth finder.

I doubt it, they are very directional, and typically point straight
down.

However, you could probably have a little beeper, and send a tone when
you press a button, which would then start a omni-directional pinger
on the boat. It would be relatively simple to have a microphone
mounted on each shoulder, and hook them to a PIC, and use the time
difference from the pinger reaching each mic to tell you which
direction to go. Of course, you'd need to rotate into a couple
different positions to eliminate the 90/270 problem, but that would be
easy enough...

I'm planning on using exactly that technology for a homing beacon for
my autonomous sub -- a pair of mics spread a foot apart gives you
something like a 500 uS (or is it 1000?) difference when the mics are
inline with the sound source. Easily timable on a PIC.

Later,
Jon

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