Development of a Tri-Axis Concentric Fluxgate Magnetometer With High Orthogonality and High Linearity

This study fabricates and characterizes a concentric tri-axis fluxgate magnetometer. The vector magnetometer features a tubular soft flux chopper with a toroidal coil and three concentric pick-up coils. The Z-axis pick-up coil is positioned within the cavity of the flux chopper to minimize the sensor’s size. With the X- and Y-axis pick-up coils encircling the flux chopper and the Z-axis coil situated at the center, the three sensing axes are concentric, resulting in an overall size for the vector sensing coils of no more than $10 \times 15 \times 15$ mm. The concentric coils ensure a single field point for the X, Y, and Z sensing axes while achieving high orthogonality. The achieved non-orthogonality errors are less than ±1°. A three-axis fluxgate driving circuit based on second harmonic detection and magnetic field feedback has also been developed to enhance linearity. The built circuit can generate an ac excitation signal and efficiently demodulate the three-axis outputs of the concentric fluxgate sensor, producing three separate dc voltage signals that are proportional to the external quasi-static magnetic field. With feedback, the average non-linearity is 0.1% within the $\pm 60 \; \mu $ T range. The proposed design concept is valuable for developing a compact and sensitive vector magnetometer suitable for precise geomagnetic measurements.