Gyros and Compass Modules

[protek@shreve.net]

    Electric fields do not have an effect on compass's.  Its the magnetic
field that the compass senses.  Inside the hull you usually have 50 to 200
amp lines that generate magnetic fields that cause the compass to swing
based on whether the motor is pulling current or not.  This effect can be
minimized but not totally.  Compensation will not work in this case.  The
hull of a sub is to a degree a magnetic shield IF the hull is steel.
Compass's mounted outside can take advantage of this shielding effect to
some degree.  The problem is that the hull itself is magnetic and can be
fairly strong compared to the flux density of the earth's magnetic field.  
    I am in the process of trying to get a workable compass mounted outside
the hull.  I have the compass constructed with necessary electronics
already.  But when I use a hand-held compass in the location that I desire
to place the external compass, the indication is effected greatly by the
hull, or perhaps other metal in the area such as lifting frames and beams
etc.  I need to take my sub outside of my hangar where I keep it and see
how bad the distortion actually is in this case, by pointing it in several
positions in the full 360 degree range.  I may try compensation, or I may,
if the problem is not to bad, use correction in the microprocessor dealing
with the data gathering.  This can be done with a look-up table.  In other
words, each reported heading is a pointer into an array where the corrected
heading can be read by the microprocessor.
   Small magnet compensation can be tricky because if you use a magnet to
allign the field at a certain heading, and make the strength too great, you
will distort the readings for the other directions.

Gary Boucher



>flux gate compass's really operate very well in the presence of STRONG
electric fields.  the one on my dad's boat is 2 feet in front of 2 VERY
large engines, in fact 2 454's :)  In steel boats they use conventional
compasses, but they use compensation weights...  Well not really weights,
they are large balls of iorn positioned around the compass to even out the
magnetic fields.  As for the motor screwing up the compass's reading,
either put the motor in a properly insulated cage/ gauss shield.  OR you
can put the motor inside the boat and the compass outside :)  (I think the
boat would make a GREAT cage to stop the motor's magnetic field)
>