Noise sources in miniature flugate sensors. Part II: the case of sensors with Fe77.5Si7.5B15 amorphous wire cores

Abstract

As a continuation of Part I this paper is devoted to fluxgate sensors that employ Fe77.5Si7.5B15 amorphous wires as magnetic cores, with attention being paid to fluxgate miniaturization. Two types of fluxgate sensors have been constructed. The first type is based on the high axial anisotropy exhibited by amorphous wires and employs two coils, an excitation and a receiving one. The second type employs the large Matteucci effect, owed to helical anisotropy induced into such wires either by mounting under torsion or by torsion annealing. The latter fluxgate type employs one single excitation coil. The exploitation of the Matteucci effect indicates that it may lead to the construction of miniature fluxgate sensors. The voltage-output of a series of such sensors has been sampled and processed. The repeatability of the flux-reversal process in the presence of helical or axial anisotropy is quantitative and qualitative investigated to allow for understanding the nature of noise. The existence of noise-sources that have been modeled in Part I is investigated and their power spectral density is estimated, from the experimental results. The influence of several manufacturing parameters in the noise at the output of fluxgates sensors is experimentally verified. Such parameters are the dimensions of the core, the presence and thickness of the glass-cover in glass-covered wires, the level of applied stress and torsion, working temperature level, and the characteristics of the excitation waveform. An irregular behavior at the hysteresis loop of fluxgates that employ the Matteucci effect is observed, which could be understood as a kind of perming effect. Measurements of the repeatability and hysteresis errors, of the sensors transfer function (output-voltage vs input-field), are presented, along with measurements of the cross-field sensitivity. The latter errors do not count as noise, but may limit the performance of fluxgates significantly, especially when miniaturization is attempted. Finally some design rules and manufacturing hints are proposed for fluxgate-magnetometer noise reduction.