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    Re: Fluxgate compass /benefits of 3 axis
    From: George Huxtable
    Date: 2002 Feb 1, 09:54 +0000

    Jared Sherman said-
    > < Trouble is, you need a FAST
    > response to the tilting of the deck >
    >There are now solid-state accelerometers, literally you can buy a single
    >IC chip off the shelf that is a solid-state accelerometer capable of
    >generating data "instantly" for all three axes of motion. Among other
    >things they are used to track for inertial navigation systems. I don't
    >think they've replaced anything on the Polaris submarines, but they should
    >be more than adequate for generating correction data on a fluxgate
    George Huxtable responds-
    We are discussing trying to make a fluxgate system immune from the effects
    of sea-state. Doesn't matter how small, cheap, compact, fast, an
    accelerometer is.  Any device that measures acceleration due to gravity
    will find it inextricably confused with the other transient accelerations
    that rough weather creates.
    Some sort of sensor of the vertical is certainly required, to establish the
    averaged attitude of the vessel (e.g. any long-term heeling). A 3-axis
    accelerometer could do that part of the job perfectly well, but a fast
    response in that sensor is not needed. Quite the reverse, in fact: fast
    changes should be filtered out, to remove the contribution from short-term
    accelerations caused by the sea-state. What's needed is a fast sensor for
    ROTATION, to measure changes of deck-tilt about the two relevant axes, roll
    and pitch, independent of gravity and acceleration, to correct the output
    of the fluxmeter.
    A strap-down inertial navigator for an aircraft (or, presumably, a sub.)
    requires such rotation-sensors, to determine the orientation of its 3-axis
    accelerometer. The ultimate instrument for doing that job on an aircraft is
    the ring-laser "gyro".
    George Huxtable.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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