Technical procedures to select basic parameters of a fluxgate magnetometer

Abstract

In fluxgate magnetometers, the external magnetic field is generally measured from the second harmonic induced voltage in the pick-up coil. Thus, toimprove the performance of a fluxgate magnetometer design, careful considerations should focus on some operational parameters and how to calibrate them to attainan optimal performance with a minimum intrinsic noise at the output. Basically, two main factors are considered: i) the output dependence on variations of the evenharmonics amplitudes with the excitation current; and ii) adjustments for a perfect timely match between the core saturation and the time-width of the sampling. Thebest sensitivity of the magnetometer is obtained in a portion of the hysteresis curve where the 2nd harmonic component predominates. Since this region depends on theamplitude of the excitation, it becomes necessary to calibrate the excitation amplitude in order to get the highest 2nd harmonic amplitude in the pick-up coil. When theamplitude of the excitation current is modified, the amplitude of the even harmonics also changes, and consequently the magnetometer will operate below its optimalsensitivity, depending on how much the current has changed. Similar result is observed when the core is built with a different inductance (different number of wraps)which requires operational tests to be carried out to verify the level of the core saturation by adjusting the amplitude and the frequency to get the best performancefrom the magnetometer. Changes in the amplitudes of the excitation current generally occur with changes in the excitation current waveform, affecting the timing ofthe core saturation. To avoid bad synchronization between the excitation current and the sampling, the phase shift circuit is locked with the excitation current circuitin order to work together and variations on the excitation current phase is automatically corrected in the sampling circuit. In summary, finding the moment whenthe 2nd harmonic has its greatest value, adjusting the time-width of the synchronous detector and avoiding phase’s shifts between excitation and detection circuitscan produce a magnetometer performing at its lowest noise output.