What is a Fluxgate ?
The fluxgate magnetometer is a magnetic field sensor for vector
magnetic field. Its normal range is suitable for measuring earth’s
field and it is capable of resolving well below one 10,000th of that.
It has traditionally been used for navigation and compass work as
well as metal detection and prospecting. Not difficult to construct
it is often forgotten in today’s world of silicon and MEMS devices.
Fluxgate magnetometer designs fall into
broadly two styles, those employing rod cores
and those using ring cores. Whilst there are
many alternative designs mostly based on rod
cores none have reached the state of development
and performance attributed to two styles.
For this reason this page is intended to apply
only to the twin rod and ring core fluxgate variants.All fluxgates use a highly permeable core which
serves to concentrate the magnetic field to be
measured. The core is magnetically saturated
alternatively in opposing directions along any
suitable axis, normally by means of an excitation
coil driven by a sine or square waveform.
Prior to saturation the ambient field
is channelled through the core producing
a high flux due to its high permeability.
At the point of saturation the core
permeability falls away to that of vacuum
causing the flux to collapse. During the
next half cycle of the excitation waveform
the core recovers from saturation and the
flux due to the ambient field is once again
at a high level until the core saturates in
the opposite direction; the cycle then
repeats. Despite the magnetisation reversals
due to the excitation, the flux from the
ambient field operates in the same direction
throughout. A sense coil placed around the
core will pick up these flux changes the
sign of the induced voltage indicating flux collapse or recovery. The name fluxgate
clearly derives from the action of the core gating flux in and out of the sense coil.
This process is shown in the figure on the left as idealised waveforms, and it can
clearly be seen that the sense voltage is twice the frequency of the excitation.
Demodulation schemes often employ 2nd
harmonic detection for this reason. In practice
for a single rod shaped core the sense coil
will pick up the excitation drive as well as
the signal voltage which due to its high level
can prove troublesome to remove electronically.
A common solution for this is to use two parallel
cores with the excitation phase reversed from
one to the other. The sense coil picks up the
signal but the induced excitation voltage is
cancelled by the phase reversal, producing
waveforms similar to those shown here.As described, the voltage of the flux change peaks is from Faraday’s
law proportional to the magnetic field; a simple sensor can be used in
this way. However a superior design will employ a coil (the sense coil
often doubles up for this task) to feedback a magnetic field in opposition
to the sensed field such that the two fields cancel one another. In this
mode of operation, where the fluxgate is used as a null detector, the
current in the feedback coil is proportional to the sensed field.. The
technique improves linearity of measurement, allows a much greater
dynamic range to be achieved and is used by the majority of modern
devices.
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