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Re the question of whether a fluxgate sensor may be bolted down or must be
gimballed-

Thanks to Brian Whatcott for providing useful information about available
commercial fluxgate instruments. This confirms my view that a "boltdown
fluxmeter" is unsuitable where the platform it is attached to can be
heeled, as in a  vessel at sea, unless some horizontal reference is
provided. We seem to agree that the majority (perhaps all) of the
instruments on the marine market require some form of internal gimballing,
so are subject to similar disturbance by accelerations as is a gimballed
magnetic compass.

Noe let's look at what what sellers say about "boltdown" instruments. Brian
quotes KVH as follows-

>KVH has some amusing caveats to offer in their FAQs (frequently asked
>questions)
>in answer to the question:
>What is a strap-Down Fluxgate?
>
>They assert that a 3 axis flux gate compass must be held rigidly horizontal (!)
>
>(KVH)
>

My comment: I don't think that they are being amusing, but entirely serious.

Another quote is-

>Towed Sonar Heading Sensor (HS)
>The CHS100 Sensor is a specialized product designed specifically for towed
>sonar array applications. It is small, low power and designed to replace
>traditional compass solutions. Unlike compass solutions, there is no
>floating gimbal in the CHS100. The unit has been tested in latitude as high
>as 85 Degrees.
>Download CHS100, Towed Sonar Heading Sensor, Datasheet (PDF).

My comment: This is an application that's far removed from the world of the
surface vessel. Presumably, a towed sonar sensor has no problem with heel,
and little problem with the roll-and-pitch of a craft that's subject to the
agitation of the surface. My guess would be that the makers presume that
the orientation of a towed sonar scanner will remain steadily horizontal.

The most interesting instrument quoted by Brian seems to be this-

>I expect this offering from
>Crossbow at San Jose will be the wave of the future:
>
>
>[sample text]
>The CXM544 sensor detects the Earth's magnetic field using a 3-axis
>magnetometer. The sensor computes a continuous measure of orientation using
>the 3-axis accelerometer as a gravitational reference field. The product
>uses the SoftSensor� architecture to compensate for temperature drift,
>alignment, and other errors. The CXM544 finds applications in old well
>logging, marine systems, and magnetic compassing. ...
>Download CXM544, Micro Orientation Sensor, Datasheet (PDF)

A 3-axis accelerometer is a fancy sort of pendulum device, under another
name. It measures the components of acceleration in 3 directions with
respect to the 3 axes of the vessel, effectively by precise measurement of
the positions of 3 weights on springs. Those weights are affected by the
buffeting of the vessel by the waves (in rather the same way that the
gimballing of the compass is) by two factors.

1. The direction of gravity, the effect of the waves and wind in tilting of
the deck of the vessel from the horizontal (which is what is needed to
correct readings from the strapped-down fluxmeter).

2. The various accelerations, sideways, up and dowm, fore and aft, that the
vessel is subject to as the waves push it about.

Those two factors together affect the accelerometer, which is quite unable
to distinguish between them. It's exactly the same problem that faces a
gimballed compass.

The trouble is that in a sea, the tiltings (to which the fluxgate must
respond promptly if it is to display the corrected magnetic heading) vary
on the same sort of timescale as do the pushes and shoves (to which,
ideally, it should not respond at all). It seems difficult to find a way of
filtering the signals to separate the two effects. Indeed, I doubt that it
can be done. It would be interesting to learn more about the instrument
described above.

A suggestion.

Devices do exist for determining the amount of ROTATION about an axis,
independent of the acceleration forces, which could be used in place of the
3-axis accelerometer. In the aviation industry, these tend to be known as
"gyros", whether or not they contain a gyroscope. In the air, one can be
used for each axis of a strapdown three-axis accelerometer to determine its
changes of orientation, in an inertial navigation system. Modern "gyros"
include the ring-laser type, very accurate but enormously expensive. At the
other extreme is the vibrating-reed type, cheap and compact, but prone to
drift over a few minutes.

I think a case could be made for using such vibrating-reed "gyros" to
determine the short-term changes of attitude of a vessel in order to
correct a strapdown fluxgate compass. But because of the drifts, a real
reference for the longer-term direction of gravity would be needed as well.
This could be a pendulum, or it could be liquid in u-tubes. Used in this
configuration, that level-device could now be highly damped, mechanically
or electronically, to filter out all its short-term fluctuations, which
would instead be provided by the "gyros". A combination of these devices,
filtered appropriately, would allow the system to maintain a model of where
the horizontal lies at any moment. It would know which way is up.A fluxgate
system along those lines might perhaps provide the best of all worlds.

The examples supplied by Brian Whatcott have not shaken my opinion that
there is no compass, magnetic or fluxgate, appropriate for a surface
vessel, that can do without some sensor to determine the horizontal.

George Huxtable.

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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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