NavList:

Red asked-

| So are you saying, George, that the flux compass creates a table of
readings for
| each of the two axis, and then determines deviation correction by
matching the
| patterns to see the offset and/or repeat in them?

Yes, that's how I understand it.

| We seem to agree with it is storing readings from two sensors (two
axis).

Yes.

| We disagree about whether time is inportant in how that storage or
comparison is
| made,

Yes, that's where we differ, Red and I. And I do not think it would be
a practical proposition, to achieve a sufficiently constant rate of
turn, to do the job by timing, in the way that Red has suggested. I
don't know if any electronic compasses try to do the job in the way
that Red has suggested, but if so, the result would be a very inferior
product, and one very demanding of the calibration procedure. What I
think more likely is that salesmen try to explain it away without
understanding, as Lu seems to have found.

| are you suggesting that rather than taking discrete data points, it
is
| perhaps using analog voltage comparison or simply storing/comparing
minimums
| versus maximums for each axis? Would that be sufficient?

As I read it, the output of each sensor is digitised, by means of an
analogue-to-digital converter (adc). Maybe by a single adc, with
sequential switching, maybe by a dedicated adc for each transducer.
Then a sequence of many such observations from both transducers,
having been stored in memory when taken round a complete turn, is
analysed by a simple program in a microprocessor. I see no technical
problems in any of that. I doubt if it would be done by simply looking
at maxima or minima, but by combining all the points in an overall
analysis, to determine how elliptical the shape turned out to be, and
how offset its centre was. Not a difficult task.

But to understand it better, Red needs to read the two papers that I
gave references to, from which my own information has been derived.

| How would the two sets of readings/values be correlated to determine
the
| deviation then? That's unclear to me.

Read those papers, then ask further if it's still unclear, and I will
try to help if I can.

I should add that there's quite a lot of information in those papers
about how to design a three-axis magnetometer, with tilt sensors about
two of those axes. In which case, it may be possible to make a
"strap-down" compass which doesn't need to be gimballed. But it leaves
doubts, in my own mind, about whether the accelerations that occur to
a craft in a seaway, can be properly filtered out. If anyone has
experience or knowledge of such a non-gimballed device, it would be
interesting to discover more.

George.

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.


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---