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Flux Gate
Magnetometers or
Magnetic Induction
Compass.
Here we see a
beautiful device. We
see many variations
and the best thing
is that they work by
simply moving
ElectroMagnetic
Flux. They are also
very simple devices.
Some pictures to
give you an idea on
the construction:
Parallel core
magnetometers,
Vacquier
configuration shown
on the left and
Foster configuration
on the right:
Ring Core sensor
developed jointly by
the Naval Ordnance
Laboratory and NASA
Ames Research Center:
How a Fluxgate Works
Fluxgate sensors are
typically ring cores
of a highly
magnetically
permeable alloy
around which are
wrapped two coil
windings: the drive
winding and the
sense winding (as
shown in the
figure). Some
sensors will also
have a third
feedback winding, if
the sensor is to
operate in closed
loop.
It is helpful to
think of the ring
core as two separate
half cores shown in
blue and green in
the figure. This
ring core is set up
to measure the field
in the direction of
Hext. As the
current flows
through the drive
winding, one half
core will generate a
field with a
component in the
same direction as
Hext and the other
will generate a
field with a
component in the
opposite direction
as Hext.
DRIVE WAVEFORM
An example drive
waveform is shown in
Figure 2a. The
transitions are
infact more 'square'
than shown in the
figure, here they
are exaggerated to
emphasize what is
happening in the 2
half cores.
No external field
In the absence of an
external field (Hext=
0) the two half
cores go into and
come out of
saturation at the
same time. The
fields generated exactly
cancel out as shown
in Figure 2b and
there is no net
change of flux in
the sense winding,
and hence no voltage
induced.
With external
magnetic field
When there is an
external field, the
half core generating
a field in
the opposite
direction of the
external field (for
first transition in
Figure 2c, shown in
green) comes out of
saturation sooner
and the half core in
same sense as the
external field comes
out of saturation
later. During this
time the fields do
not cancel out and
there is a net
change in flux in
the sense winding
(shown in
black). According
to Faraday's law,
this net change in
flux induces a
voltage, shown in
black in figure 2d.
Similarly towards
the end of the
transition, the half
core now generating
a field in the same
direction as Hext
goes into saturation
sooner. Consequently,
there are two spikes
in voltage for each
transition in the
drive and the
induced voltage is
at twice the drive
frequency.
Measuring the field
The size and phase
of the induced
spikes tells us
about the magnitude
and direction of the
external field. To
help amplify this
signal to make it
easier to detect,
the fluxgate
magnetometers
produced by Imperial
College use a
capacitor to tune
the sense winding.
The tuned sensor
waveform is shown in
red in Figure 2d.
Ref:
Imperial College
Now lets have a look
at our geometries
and see if we can
see any
similarities:
Certainly we can see
some similarity's
here, and maybe the
actual operational
characteristics are
similar to?
_____________________
To reach new
horizons...
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