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Mike Wescott says-

 Mounting the 3-axis acceleration sensor
>near the axes of roll, pitch and yaw (especially roll) would minimize the
>acceleration affects of those motions leaving relatively short impulse
>accelerations, or longer accelerations that are much smaller than 1G. Both
>of which can be filtered.
>--
>        Mike Wescott
>        Wescott_Mike@EMC.COM

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George Huxtable replies-

Well, yes, that would the best compromise position for the sensor, and
would minimise any linear accelerations that were caused by the tiltings of
the vessel. But that would fix only a small part of the problem.

It's the linear accelerations of the whole vessel, caused by the wave
motions, that upset any sensor of orientation that relies on gravity. The
powerful sideways shove of a wave in a cross-sea. The heave of the craft on
to the top of a wave, the sinking pit-of-the stomach feeling as it drops
into a trough. The brakes-hard-on  effect as the bow digs into a wave on
the way up, and the exhilarating (and accelerating) long swoop down into
the valley of a following sea. We are all familiar with these forces, and
the accelerations that go with them, which make it so hard to keep one's
feet in rough weather. It's just part of going to sea. We accept it, and
occasionally enjoy it. There's no positioning of a sensor that will give it
any immunity at all to those forces.

Another accelation needs to be considered, also, even in smooth water: the
effect, on a fast power vessel, when you put the helm over at speed. The
vessel tilts on the turn, and we need to by know how much. But more than
that, it introduces a sideways acceleration too, during the turn, which
perturbs any sensor of the vertical, such as gimbals or a pendulum or a
u-tube or a 3-axis accelerometer, and all by just the same amount.

The poor old gimbal system, on a magnetic compass or a fluxmeter, does its
best with these accelerations. So does a more complex gravity sensor, such
as a 3-axis accelerometer.  The disturbances occur on just the same sort of
time-scale as the tiltings of the vessel that the gravity-sensor is there
to detect, so in spite of what Mike says, I think it is quite impossible to
filter out one and retain the other.

All that a gimbal system, or any other such gravity sensor, can do is to
ASSUME that any disturbance due to acceleration is negligibly small
compared with that due to tilt. And that is a poor assumption, as we know
by the limited performance of our compasses (needle or fluxgate) in rough
weather. This isn't too serious if the compass is simply being used to
measure the average course, as long as there are landmarks around us which
we can watch to maintain a short-term heading. The problems occur when we
want to feed the compass information immediately back to the steering to
try to maintain a steady course.

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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