A new, fine-magnetic-particle detection system is proposed that uses a fundamental-mode orthogonal fluxgate (FM-OFG) gradiometer positioned on a Permalloy shielding disk. The Permalloy disk reduces the gradiometer sensitivity to magnetic field interference, thus enhancing the sensor resolution and measurement signal-to-noise ratio. The proposed detection system configuration offers an inexpensive and compact alternative to commercially available SQUID sensor magnetic contaminant detection systems. Numerical analysis was conducted to investigate the effective shielding area of the Permalloy disk. Experiments with a 10 cm diameter Permalloy disk showed that the FM-OFG gradiometer uniform magnetic field sensitivity was reduced by a factor of 0.25 at a 10 mm sensor baseline. Detection of a steel ball of 120 μm diameter was achievable under 10 Hz magnetic field interference, generating 150 nT/m signal peak-to-peak in the gradiometer output, where the noise peak-to-peak was reduced to 46.8 nT/m. The signal-to-noise ratio of the 120 μm steel ball detection signal was increased by a factor of 4.6 when using the Permalloy disk.
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