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    Re: gyroscopic compasses
    From: George Huxtable
    Date: 2007 Jun 22, 21:51 +0100

    Geoffrey Kolbe asked-
    | Does anyone know of any "hand held" or "portable" gyroscopic compasses
    | available? I would assume that laser gyroscopic compasses could be made
    | quite small and - as there are no moving parts - be reasonably cheap if
    | made in sufficient quantity.
    That's a topic that I am interested in but don't know much about, and what I
    do know may be badly out of date.
    Ring laser "gyros" are not really gyros at all; there's nothing gyroscopic
    in their action, though they tackle the same task as a gyroscope. They are
    used in inertial navigation of aircraft and space vehicles to determine
    orientation. They commonly come in sets of three, one for each axis, and
    have very high performance. They work by sending laser light around a closed
    path, both clockwise and anticlockwise, using tubes and mirrors, and look
    for small differences in transit time. They are enormously expensive,
    although versions using fibre-optic coils have brought the price down.
    There's a problem with coupling of the two beams causing some sort of
    locking, which is alleviated by "dithering" of the assembly mechanically; in
    which case you couldn't say that there are no moving parts. I don't know
    whether that problem, and solution, applies also to the fibre-optics type.
    Unless there have been radical developments in ring-laser gyros in recent
    years, I doubt if they provide a cost-effective solution to Geoffrey's
    There's another, simpler, much cheaper, type of "gyro", the solid-state
    gyro. These have been applied to short-term inertial or dead-reckoning
    navigation of land vehicles, to bridge the reception gaps in GPS signals,
    which occur particularly in city conditions, between tall buildings. These
    gyros have a tiny, resonant, vibrating element on a chip, and again, can be
    used in sets of three. Again, they are not actually gyroscopic in any way.
    They work a bit like a tiny version of a Foucault pendulum, which does its
    best to keep swinging in the same plane. They give out a steady voltage that
    is proportional to the rate of rotation about a certain axis. But there's a
    sensitivity to changing temperature, which also offsets that output voltage,
    and gives rise to drift in the output signal even when there is no rotation.
    It's that temperature drift that bedevils these solid state gyros, implying
    that they are useful only for looking for short-term changes in orientation,
    not at long-term rotation. That fits in with the land vehicle GPS
    requirement. Holding them at a thermostatically-controlled temperature
    helps, but is of course an extra complication, using power. I can imagine
    such gyros might be useful at sea in short-term correction of the outputs of
    a 3-axis fluxgate, which is strapped to the vessel rather than gimballed, to
    allow for the vessel's motions in a seaway.
    I'm hoping that this might trigger a conribution from someone else with more
    up-to-date knowledge of these devices.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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