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    Re: Fluxgate compass /benefits of 3 axis
    From: George Huxtable
    Date: 2002 Jan 31, 2:39 AM

    Mike Wescott said-
    >george@HUXTABLE.U-NET.COM said:
    >> But on a surface craft there are all sorts of random accelerations
    >> taking place, in unpredictable directions, of a magnitude comparable
    >> with that of gravity, depending on the sea state. These get hopelessly
    >> jumbled up with the acceleration due to gravity. Can a three-axis
    >> accelerometer somehow unscramble these accelerations?
    >I suspect that a suitable low-pass filter could be used on the outputs
    >to get a reasonably accurate indication of the direction of the constant
    >1 G accelation.
    >        Mike Wescott
    >        Wescott_Mike@EMC.COM
    George Huxtable replies-
    No, I think that would not work. Trouble is, you need a FAST
     response to the tilting of the deck to correct the fluxgate compass
    quickly for changes of tilt as the vessel gets chucked about. Otherwise,
    the indicated course would vary wildly in rough weather, even if the vessel
    continued to travel in a straight line. That would upset the use of this
    compass in a self-steering system, which is presumably its intended use.
    Mike Westcott's suggestion would provide a good AVERAGE course, but that
    would not be sufficient for a feedback steering application.
    The problem is that the frequency-spectrum of the changes of tilt is very
    similar to the frequency-spectrum of the accelerations as the vessel is
    pushed about, both being caused by the same waves. That is why it would be
    difficult, if not impossible, to separate them by any sort of filtering.
    All is not lost, though. As I pointed out in an earlier posting, "gyro"
    devices exist, (that don't necessarily contain gyroscopes), which can
    measure changes of deck-tilt without using gravity, and these would be
    insensitive to the accelerations that the waves cause. The simpler devices
    of this type are able only to measure short-term CHANGES in the tilt, not
    the steady longterm value of the tilt itself. The output of each such
    device could be passed through a high-pass filter, and combined with the
    output of a gravity-sensor in the same plane. Just as Mike Wescott
    suggests, the output of the gravity sensor would first be passed through a
    low-pass filter to remove its fast, wave-caused, fluctuations, and just
    retain a steady averaged value. That gravity sensor could be a pendulum, or
    eustacean tubes (as in the inner ear), even an accelerometer.
    A combination of the two devices , as described above, would allow the
    system to maintain its own model of the tilt of the vessel, short-term and
    long-term, which is the requirement for correcting the output of a
    strapped-down fluxmeter.
    Brian Whatcott has referred earlier to a military instrument with a
    strapdown fluxgate, and it may perhaps work in the manner described above.
    I am not claiming that such a device is impossible- far from it! What I
    suggest is that it requires more instrumentation than the three
    accelerometers that are referred to in the blurb. Perhaps it is in its
    maker's interest to conceal some of the internal goings-on.
    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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