Invasens - Fluxgate explained

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 twin 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 these 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 Fluxgate waveforms 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.

Ring core fluxgate 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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