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Re: [PSUBS-MAILIST] Re: Compass instruments
At work we use an ordnance grade laser ring gyro INS (IMU). We
specially select to get the highest accuracy ones from the lot that we are
allowed. We use them because they can tolerate much higher G forces.
What about using one of the gyros you have (Carsten) and then a GPS
antennae on a float to update the position as needed. (I guess anytime you
feel that you need an updated accurate position (direction and velocity).)
I believe a flux gate measures magnetic flux electronically (with hall
effect transistors) to make an electronic compass. It probably is the same
princeple as the one availible in cars these days, but I'm not sure.
Has anyone tested how deep the GPS signals can be recieved under water?
Perhaps water proofing an antennae is the cheapest, simplest, and most
accurate. I doubt it but perhaps someone could test it to rule it out and
way?
Jay.
Jay,
wrt Electric directional gyros. electric DG's as we
call them they're good for the short term, but
precess (due to internal friction) about 1-5 degrees
every 15 minutes (most of them about 2-3 deg in 15
min). So they need to be constantly reset to the
compass. They're good because they don't get big
errors from turns, climbs and descents. So in
practice we steer from the DG and keep resetting it.
- or if you're in a fancy plane there is a thingy
called a flux valve I think(a remote flux sensor
usually in a wingtip or the top of the tail), which
automatically updates it, relieving the pilot of this
work and potential for an error. Virtually all
commercially operated a/c will have one of these
self-updating DGs.
If you want the correct terminology I'll look it up.
>From a pilot perspective a DG is not a compass.
Compasses use magnetism. Not sure if thats the way
the words are used in other fields.
For a sub I'd suggest a Inertail Ref. Sys which uses
laser rings because its small, low power consumption,
sturdy and reliable. They're pricey, but much less
than a true INS with gyros and accelerometers. A
compass might be used but maybe only at depths and
after figuring the errors due to the subs mag field
under certain operating conditions. I guess it'd have
to be located someplace without a stong magnetic field
like a nose probe or on a conning tower aerial??
ps attitude gyros obviously self correct to down.
Curious problem.
Gnight,
Kent.
--- jbarlow@bjservices.ca wrote:
>
> Kent,
>
> Do electric compasses such as they are talking
> about, precess. That is
> drift over a period of days. I thought that maybe
> they (airplane
> compasses) would be good for 8-10 hours not 8-10
> days., Is that true you
> think?
>
> Jay.
>
David Buchner
<buchner@wcta.net> To: personal_submersibles@psubs.org
Sent by: cc:
owner-personal_submersible Subject: Re: [PSUBS-MAILIST] Re: Compass instruments
s@psubs.org
22/02/2003 03:52 PM
Please respond to
personal_submersibles
On Saturday, Feb 22, 2003, at 11:55 US/Central, Alec Smyth wrote:
> I just re-read your message. My first response was in a bit of a rush,
> as my wife was calling from downstairs that the house was flooding due
> to the melting snow. But priorities are priorities, and I had to
> finish my PSUBS message!
Besides -- what're you gonna do: Make the snow stop melting? You folks
must be buying out all the shop-vacs and bilge pumps at the local
hardware stores... No snow melting here today -- high of about 10.
> Anyway, between you and Cliff I'm really indebted for the research.
> Imagine if I went out and spent a couple of thousand dollars on an
> electric gyro only to find out what you just said about the 15 minute
> duration! But my question is this... if you put a fluxgate in the
> sail, it should work whether you're surfaced or submerged. What's the
> need for having a gyro as well?
I was reading a couple of earlier messages in this thread, and was
wondering whether this limitation on aircraft gyros is due to an
expense issue, or a national security one -- as in, the technology for
really solid fancy IMU's is controlled due to its usefulness in stuff
like missile guidance. A relative of a relative of a friend works for a
company that builds and tests stuff like that (laser gyros, is what I
heard) and apparently part of the job of his lab is testing
export-grade equipment to make sure it's _not too accurate_. Weird.
Apparently, I've missed part of this discussion... and am lazy. What is
a fluxgate? Is that the name for the thingamagig the electronic compass
in my car uses?
- - -
David
buchner@wcta.net
Osage, MN, USA