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Subject: Re: Fluxgate compass /benefits of 3 axis
From: Michael Wescott (wescott_mike@XXX.XXX)
Date: Fri Feb 01 2002 - 19:44:50 EST
George Huxtable:
> 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.
> 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.
We'll have to agree to disagree. I believe that in most situations the
accelerations are small enough or of short enough durations to make it
possible to get a reasonable indication of vertical. And consequently to
develop an electronic compass at least as accurate as a standard magnetic
compass. In any case, they're too pricey for me and my boat.
--
Mike Wescott
Wescott_Mike@XXX.XXX
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