PSN-L Email List Message

Subject: Re: fluxgates vs Speake and Precision Navigational magnetometers
From: "Charles R. Patton" crpatton@......
Date: Sun, 06 Feb 2000 20:09:18 -0800

I=92ll answer the questions as best I understand them.  You wrote:
=93How much material are you using in the core? I was thinking that for
efficiency the strip should be cut into some narrower strips to make the
diameter of the coil smaller and thus having more core and less air.=94

A bit of background to answer this question.  As I understand it, there
are two major types of magnetic anti-theft systems (as opposed to RF
tags and the like.)  They both impose a low level AC magnetic field on
the area you pass through (the gate like structures at the doorway are
the drive and sense coils.  In the case of the tags on CD=92s, the strip
is magnetostrictive so it actually physically resonates at a multiple of
the drive frequency.  The sense coil looks for this resonance.  The
other strip, a mild-steel strip, adjacent to the very shiny, but
somewhat pock-marked Metglas, is used to deactivate the tag.  On
check-out, the sales clerk magnetizes the mild steel, which thereby
holds the Metglas in saturation, thereby blocking the resonance effect.
The library strip is Permalloy about 4 =93 x 0.1=94 x 0.001=94 and usuall=
visible down in the spline of the book.  When it goes through the AC
field, it saturates causing a second harmonic to appear, just like the
flux gate.  This is picked up by the sense coil indicating that there is
a strip (and thereby the book it is attached to) in the field.  For my
experiments, the local librarian was kind enough to give me about 10 of
the strips.  I took a standard plastic coffee stirrer (the straw style)
and wound one and two layer versions with #36 magnet wire.  I squeeze
the whole coil in a vise to give it an oval shape that the Permalloy
strip will slip down inside easily.  A single layer arrangement will
saturate with about 12 mA.  The earth=92s field is equivalent to about 3
mA in this coil structure.  With 10 V drive it takes about 15 us to go
from plus to minus saturation.  I don=92t advise trying to cut narrower
strips.  It=92s hard to control the cut with homemade equipment and the
effect will be to broaden the saturation time as different parts of
strip saturate, making it harder to determine saturation.  Additionally
it is very touchy to cut Metglas.  It has many of the characteristics of
thin glass.  I was only able to cut it two ways.  One was a pair
stainless steel scissors that I would very carefully hone between cuts
to make sure there were no nicks in the blade.  A nick, and a great big
dimple or sometimes a tear would appear.  The other method was to scribe
the Metglass with a diamond scribe very carefully, then folding and
cracking along the scribe.  In either case, I did this with a jig I
built so I could reasonably control the strip width and hold it
constant.  But this was from 2=94 wide, several foot long Metglas designe=
for EM shielding so I could get a handle on it and hold it evenly.

You then wrote:
  =93Any thoughts on using the Metglass strips in a parallel core design
flux gate device? One would thing the high perm and easy saturation
would make for a much lower power device.

This question stumped me.  Can you elaborate on what you meant by a
parallel core design?  I have seen so many designs, some with multiple
cores, I=92m confused just which one you mean.

Charles R. Patton


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Larry Cochrane <cochrane@..............>