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Subject: Re: Fluxgate compass /benefits of 3 axis
From: George Huxtable (george@XXX.XXX)
Date: Fri Feb 01 2002 - 04:54:07 EST
Jared Sherman said-
>George-
> < Trouble is, you need a FAST
> response to the tilting of the deck >
>
>There are now solid-state accelerometers, literally you can buy a single
>IC chip off the shelf that is a solid-state accelerometer capable of
>generating data "instantly" for all three axes of motion. Among other
>things they are used to track for inertial navigation systems. I don't
>think they've replaced anything on the Polaris submarines, but they should
>be more than adequate for generating correction data on a fluxgate
>compass.
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George Huxtable responds-
We are discussing trying to make a fluxgate system immune from the effects
of sea-state. Doesn't matter how small, cheap, compact, fast, an
accelerometer is. Any device that measures acceleration due to gravity
will find it inextricably confused with the other transient accelerations
that rough weather creates.
Some sort of sensor of the vertical is certainly required, to establish the
averaged attitude of the vessel (e.g. any long-term heeling). A 3-axis
accelerometer could do that part of the job perfectly well, but a fast
response in that sensor is not needed. Quite the reverse, in fact: fast
changes should be filtered out, to remove the contribution from short-term
accelerations caused by the sea-state. What's needed is a fast sensor for
ROTATION, to measure changes of deck-tilt about the two relevant axes, roll
and pitch, independent of gravity and acceleration, to correct the output
of the fluxmeter.
A strap-down inertial navigator for an aircraft (or, presumably, a sub.)
requires such rotation-sensors, to determine the orientation of its 3-axis
accelerometer. The ultimate instrument for doing that job on an aircraft is
the ring-laser "gyro".
George Huxtable.
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george@XXX.XXX
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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