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At 09:29 AM 1/30/2002 -0500, Chuck Griffiths wrote:
 >   But, how does making constant rate turns help the fluxgate find
 > magnetic north if it sensed it incorrectly [didn't read 0 degrees when
 > boat was heading 000M] to begin with?  

If one plots deviation as a function of compass direction it should look
like a sine wave (for fiberglass and wood boats at least, not sure about
steel).  If the sine wave is "off center" (ie, the plot shows more westerly
deviation that easterly or vice versa) it is an indication that the lubber
line of the compass is not aligned with the ships keel.

After I learned this fact I plotted the deviation table for my 36' sailboat
and -surprise- it was uneven.  Even though it's a high quality boat, the
compass had been fastened to the steering pedestal with the lubber line of
the compass pointing about 2 degrees to starboard.  Hard to discover, easy
to fix once I had done so.

What I suspect that any decent deviation-determining software does is
simply to even out the deviation readings so the sine-wave is "balanced."

Lu

>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. 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 
>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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