From: pad@galaxy.nsc.com (Paul Denny x8349)
Subject: fluxgate compasses
Keywords: how it works
Organization: National Semiconductor, Santa Clara
Date: Fri, 4 Sep 1992 16:51:43 GMT

I recently asked here how fluxgate compasses work and received the
following explanation by email from Franklin Antonio. I felt that
it was an elegant explanation that would be of interest to sci.physics
and as such I have reproduced his explanation. All credit for this
posting are due to him and any errors remain mine - flame me not him.

enjoy! (I did)
 
From: Franklin Antonio <ANTONIO@qualcomm.com>

In a recent sci.physics posting, you ask how fluxgate compasses work.

The trick that requires explanation is how you can get a coil to produce
a signal which is proportional to the strength of one component of a
STATIC magnetic field.  (ie the Earth's magnetic field)  Obviously, if you
can pull that off, you can do it twice, with two coils oriented 90 degrees
apart, then take the 4-quadrant arctangent of the two signal levels, and get 
the direction of the static field.  (or if you prefer doing it in 3d, you
can do the same with three coils.)

To make a signal which is a measurement of a static magnetic field, the
fluxgate compass gates the flux of the earth's magnetic field.  

In the absence of local magnetic materials, the earth's magnetic field
looks locally uniform.  That is to say the lines of force are curved
but gently so, so that locally you can think of them as uniform and
straight, like the lines on a piece of writing paper.  Now, consider
a piece of magnetic material of high permeability.  Perhaps a piece of
iron or ferrite.  Put this into the previously uniform field, and now
the lines of force bend, because the lines would rather go thru the
high permeability material than thru free space.  If we had a way to
gate the permeability of that hunk of iron, that is to vary it in a
regular patern high/low/high/low/... then we would make the lines of
force of the earth's magnetic field move in and out of the iron.  Now
we would have a varying (rather than static) magnetic field which
is proportional to the strength of the static (Earth's) magnetic field
which we were trying to measure.  Of course, now that it's varying, we
can measure it easily.

How to make the permeability of that magnetic material gate on and off?
Since permeability is in general a nonlinear function of total field,
you can do this with bias.  Consider a toroidal magnetic core (iron or
ferrite) with two coils wound on it.  One coil is for bias, the other
for sensing.  Put a large gated signal into the bias winding.  Here
large means large enough so that the nonlinear properties of the magnetic 
material come into play.  When the bias signal is off, the core has a 
high permeability.  When the bias signal is on, the core has a lower
permeability.  Now observe the sense winding.  On it, we will see two
signals.  One is the bias signal obviously, and we need to filter that
out.  The bias frequency is chosen to be high enough that we can easily
lowpass filter the sense output to eliminate the bias.  The second 
component in the sense winding will be caused by some of the earth's
magnetic field being alternately pulled into the core (by the permeability
of the core material), and displaced (when the permeability drops).
This signal is then rectified, and measured.

This is usually done using two (or three) cores, oriented at right angles,
so that the resulting signals are the components of the field in the
corresponding directions.

In a practical implementation, the bias might be something like a 100 kHz
sine wave, which is gated on/off at a rate of a few per second.  Bias level
would be many ampere-turns, so to put the core well into saturation when
bias is on.

Accuracy is limited by your ability to wind a very uniform (symmetrical)
sense coil, and other such non-ideal characteristics of the implementation.

There is a NASA patent.  I had a copy once, but no longer have it handy.
Check your favorite patent database, and search on "flux-gate".


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