High Sensitivity Magnetometers

Volume 19 of the series Smart Sensors, Measurement and Instrumentation pp 63-102


Orthogonal Fluxgate Magnetometers

  • Mattia ButtaAffiliated withCzech Technical University in Prague Email author 

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Orthogonal fluxgate is a particular type of fluxgate, which recently gained popularity. As all fluxgate sensors it is based on the gating of magnetic flux in a ferromagnetic core; however, in orthogonal fluxgates the excitation field and the measured field are orthogonal. This leads to different sensor structure, most notably to the absence of an excitation coil, making the construction of an orthogonal fluxgate very simple. In this chapter we will first analyse the principle of operation of orthogonal fluxgates in order to explain the mechanism which generates the output signal. Then, we will examine how the sensor is build, especially the structure of the core and the techniques typically used in order to minimize the amplitude of excitation current. Next, a particular type of orthogonal fluxgate—the so-called coil-less fluxgate—is presented: its name comes from the lack of the pick-up coil, for the output voltage is derived directly from the core’s termination thanks to helical anisotropy of the core. The most important part of the chapter is however focused on fundamental mode orthogonal fluxgate; in this type of sensor a large dc bias is added to the excitation current in order to suppress the Barkhausen noise, that is the main source of noise in fluxgates. The resulting output has very low noise: we show how, properly designing the core geometry and modifying the anisotropy by annealing we can achieve noise as low as 1 pT/√Hz at 1 Hz. Another part of the chapter is focused on magnetic gradiometers based on orthogonal fluxgates, typically used when the sensor has to be used in noisy environment and the magnetic field to be measured has large gradient and small amplitude. Finally a comparison with similar sensors, such as wire-based GMI, is presented: we show similarities and differences, especially regarding the methods for signal extractions and we explain why orthogonal fluxgates perform better.