Off topic but interesting?? Magnetics

From: Dwight Elvey <elvey_at_hal.com>
Date: Tue Nov 7 20:29:04 2000

Eric Smith <eric_at_brouhaha.com> wrote:
> John Lawson wrote:
> > ALSO: Aircraft compasses are generally of the flux-gate or flux-ring
> > variety, and use a counter-revolving pattern of light to measure the
> > fields, and are much less susceptible to stray fields than the ancient
> > magnetic compasses,
>
> You've got me very curious. I'm familiar with flux-gate compasses which
> do not use light, and with laser ring gyros which do (but, of course,
> are not compasses). What's a flux-ring compass, and how does it use
> light?

Hi
 Flux-gate compasses are completely different than
laser rotational accelerometers.
 The laser devices only detect rotation by integrating
the amount of Doppler shift caused by rotating the ring.
This is subject to drift because it is integrating.
Still, because of the short wavelength of light and
the fact that they pass the light several times in
a loop, these can run for days with almost no drift.
 Flux-gate compasses depend on magnetic fields. They can
be fooled by magnets just like a regular compass.
The way that they work is to have two perpendicular
windings over ( but not through ) a toroidal core.
There is also a winding in the normal manner through
the core. The way it works it that one builds a field
in the inner winding until the core saturates. Since
the field is even around the core, there is no coupling
to the two coils that are perpendicular over the core.
When the coil saturates, it is as though the core is
no longer magnetic. The earths field lines that were
distorted and pulled into the core are now released.
The problem is that this happened slowly so the two
coils didn't see much of this change in field. The current
in the inner coil is cut off quickly ( usually the spike
is dumped in to a zener diode of a high voltage ).
The earths magnetic field suddenly see a magnetic material.
It snaps into the core but in order to do so, the field
lines must cross the windings of the two perpendicular
sense coils. The orientation of north is related to
the vector sum of the pulses generated in these two sense
windings.
 Still, when space craft fly, they keep track of direction
using mechanical gyroscopes. So, what do you know?
Dwight
Received on Tue Nov 07 2000 - 20:29:04 GMT