Residence Time Difference Fluxgate Magnetometer in “Horseshoe-Coupled” Configuration

Abstract

Fluxgate magnetometers are, possibly, the simplest and most convenient magnetic flux sensors and yet capable, in controlled environments and with some well-conceived technical embellishments, to detect magnetic fields in the order of 50 nT or less. In practice, they are limited by the presence of unwanted or contaminating signals, these include the sensor noise floor as well as noise-contamination of the target signal of interest. The residence time difference (RTD) readout technique was conceived as a readout protocol that is remarkable in its simplicity and its ability to outperform the conventional “second harmonic” readout. In this work, we address the issue of the fluxgate sensitivity in the presence of spurious dc magnetic field signals, while ensuring that the sensor delivers a high responsivity to the target signal. This is achieved through a “Horseshoe Configuration”: three rod-core fluxgates are connected in series forming a U (horseshoe) and the output is drawn only from one secondary (or pick-up) coil using the RTD readout mechanism. The two branches of this configuration yield a differential mechanism vis-a-vis the (unwanted) external magnetic field, while the magnetic target is “localized” by being placed in or in the proximity of the air gap of the horseshoe. A theoretical analysis of the efficacy of this configuration considering the geometry, the demagnetizing effect, and the sensing mechanism is carried out. The results confirm the theoretical assumptions: the sensitivity to the (spurious) external magnetic fields can be reduced and the sensitivity to the target signal enhanced, with a concomitant enhanced tolerance to noise. The results appear promising for the detection of very small magnetic fields, e.g., the magnetic fields encountered in the project “IRMA Parkinson Cyclone in Life” for the noninvasive diagnosis of neuroferritinopathies through the detection of a few milligrams of iron inside the brain.