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    Re: Fluxgate compass / more
    From: George Huxtable
    Date: 2002 Jan 30, 10:04 AM

    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.
    
    ------------------------------
    
    george---.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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