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Subject: Re: Fluxgate compass/Calibrate
From: Brian Whatcott (inet@XXX.XXX)
Date: Wed Jan 30 2002 - 10:20:57 EST
At 08:29 AM 1/30/02, Chuck Griffiths wrote:
>...
>When one first installs a fluxgate, I would imagine that it has two errors.
>First, when the ship is on a heading of exactly "0", relative to local
>variation, the fluxgate reads something other than north, due to deviation
>(the
>disturbance of the sensor by magnetic fields on the ship). Second, all other
>headings will also have deviation induced errors, e.g. the display
>indicates 80
>degrees when it should read east....
> how does making constant rate turns help the fluxgate find
>magnetic north if it sensed it incorrectly to begin with?
For a definitive answer, you need to talk to the software engineer who rigged
the early auto correction firmware. Companies try to hold these
algorithms as
trade secrets, but it is obvious from examining the various specs, that
there is
a lively business in reverse engineering code of this kind.
So I can only tell you what I would do: capture a table of headings at equal
time intervals beginning when triggered, and ending when the initial
heading returned.
Compute the least squares straight line through these co ordinates.
Compute a table of differences between the straight line and the actual
heading sensed at equal heading intervals.
This procedure has the helpful property of disposing deviations equitably on
either side of the "real" mag heading, and provides the best chance of
minimizing deviations at any heading.
Did someone wonder how to generate a table of minimum deviations of a
three axis flux gate? No? - Oh good!
Brian Whatcott
Altus OK Eureka!
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