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Subject: Re: Fluxgate compass
From: Chuck Griffiths (griffiths_chuck@XXX.XXX)
Date: Wed Jan 30 2002 - 09:29:34 EST
OK, I'll throw in my fluxgate question that has been eating at me since I
installed a fluxgate for my autopilot a couple years ago.
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.
Now, I can imagine that after the compensating program is run, the fluxgate
could compare the rate of turn indicated during the compensating turns and
adjust indicated headings such that they match headings as they should have
appeared during the turns. To continue my example, the fluxgate could sense the
rate of turn and know that 90 degrees of turn have been completed so the display
should indicate east, the previous error between 80 and 90 degrees could be
corrected. But, how does making constant rate turns help the fluxgate find
magnetic north if it sensed it incorrectly to begin with? Of course, I checked
it after I corrected it by comparing headings to sun bearings so I know it
works, I'm just wondering if someone can explain how it works.
____________________Reply Separator____________________
Subject: Re: Fluxgate compass
Author: Brian Whatcott <inet@XXX.XXX>
Date: 1/29/02 6:33 PM
At 05:16 PM 1/29/02, you wrote:
>... What exactly
>is a bolt-down sensor? And what errors does it "abolish"?
>
>As I understand it, a fluxgate compass, like any other compass, needs to
>establish the direction of the horizontal component of the Earth's field,
>with respect to the direction of the vessel. For this to happen, somehow
>the fluxgate compass, just like any other compass, has to be aware of the
>direction of the horizontal. ...
>George Huxtable.
George, the comments I posted were on the basis of examining the
constructional details of a fluxgate. In view of your praiseworthy
efforts in describing the Lunar, I'll try to return the favor by taking
a look at what the commercial fluxgate offerings provide.
It may be that for a 2-axis fluxgate, a pendulous mount might be adopted.
This would retain the pendulous errors associated with a compass card
of the conventional kind. I'll get back to the list, if I find useful
engineering input.
A three axis fluxgate can be mounted rigidly. This is sometimes called a
'bolt-down' sensor.
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