Fluxgate magnetometers evolved during the second World War as a means of detecting submarines. Military applications continue to be important market, but geologists and archeologists have also become big fans of these robust and extremely sensitive instruments. Metrolab uses fluxgate sensors produced by a respected specialist to complement its Hall instruments, extending the range to very low fields.
A fluxgate magnetometer consists of a soft-iron core with two coils wrapped around it: a drive coil and a sense coil. An alternating voltage drives the core continuously through a complete hysteresis cycle, from saturation in one direction to saturation in the other. The sense coil measures the flux changes in the core.
The magnetic permeability of the core – the slope of the B vs. H curve – is modulated as the core goes into and out of saturation : unsaturated, the core has the high permeability of soft iron; saturated, it suddenly drops to the low permeability of free space. This means that the flux density B in the core due to an external field H is also modulated; one can think of the flux due to the external field being switched off as the core saturates, and back on as the core desaturates – hence the name "fluxgate". Clever demodulation schemes allow the magnitude of this gated field to be measured by the sense coil.
Although the basic principle is relatively straightforward, the actual performance depends on many factors:
- Core material: To maximise the signal, the core material should be as magnetically "soft" as possible, with a very narrow hysteresis loop and sharp saturation point. Other characteristics of the core material, such as temperature coefficient, isotropy, and long-term stability, determine other key sensor characteristics.
- Core geometry: Clever core geometries dramatically improve the sensitivity by cancelling the direct transmission of drive to sense signal. One such geometry uses two parallel cores, with the drive coil wound in opposite directions. The sense coil is wound around the pair, so that the flux generated by the two drive coils cancel out.
- Multi-axis: Another core geometry is a ring, with the drive coil wound toroidally and the sense coil around the entire ring – a straightforward extension of the parallel-rod design. The direction of the sense coil determines the sensitive direction of the sensor; two orthogonal sense coils on the same ring make an XY sensor; and with two rings one can make an XYZ sensor. Note that in this design, the axes are centred on a single point.
- Synchronous detection: A single cycle of the drive voltage causes two saturation/desaturation signals from the sense coil: once in the forward and once in the reverse direction. Synchronous 2nd harmonic detection can therefore be used to improve the signal-to-noise ratio.
- Feedback: The dynamic range and linearity can be greatly improved by using the fluxgate signal as a null detector. It controls a current that is fed back into either the sense coil or a separate feedback coil, nulling out the external field.
- Frequency response: A higher drive frequency allows a higher sensor bandwidth, but needs to be traded off against the sensitivity.
- Demodulation electronics: The demodulator must be carefully designed to optimise noise, offset, bandwidth and the temperature coefficient.
-Calibration: To achieve good accuracy, good calibration technique is paramount. Given the great sensitivity of fluxgate sensors, it is especially difficult to create a good zero reference field.