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Subject: Re: Fluxgate compass
From: Herbert Prinz (hprinz@XXX.XXX)
Date: Tue Jan 29 2002 - 14:18:24 EST
An important functional difference between a magnetic needle compass and a fluxgate
compass is that in the former the sensor and the indicator are one and the same
part of the instrument, whereas in the latter they are different. If one wants to
get a reading free of deviation, it is therefore a logical necessity on a needle
compass to bias the sensor (i.e. to undo the magnetic bias it receives from its
surroundings), whereas in the fluxgate it makes more sense to merely establish the
bias and then apply the respective correction to the indicator. Of course, this
could be done through analog cuircuitry emulating compensation magnets, but the
times of analog circuitry are gone and so it's done by discrete logic (a.k.a. s/w
calibration).
The strive for eliminating deviation from a conventional compass is merely a matter
of convenience in order to keep deviation tables small. Presently, I see no other
advantage in it. But it was essential when needle compasses provided the control
signal for autopilots.
Herbert Prinz (from 1368950/-4603950/4182550 ECEF)
> Before even starting to
> develop a deviation table for a compass, a good compass adjustor first
> tries to minimize the compass's deviation though use of the compensating
> magnets within the compass and even external devices such as quadrential
> balls and Flinders sticks (the latter two are used mostly on steel vessels
> since, as one might imagine, it's quite difficult to get a compass to read
> correctly when surrounded by a big lump of iron). Is this sort of
> compensation also done for fluxgate compasses? Is it the same as or
> different from the way it is done for traditional compasses?
>
> As a practicing electrical engineer, my first guess would be that they
> would be done the same. But I also realize that fluxgate compasses work
> quite differently than conventional compasses -- internally, they have two
> sensors and compare the fore-and-aft component of the earth's magnetic
> field to the athwartships component to determine magnetic direction rather
> than actually swinging a magnetized needle. This might require different
> compensating practices (especially on a steel vessel).
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