Actively Shielded, Adaptively Balanced SQUID Gradiometer System for Operation Aboard Moving Platforms

Abstract

Extremely high dynamic range is required if magnetometer SQUIDs are to be operated while in motion in the earth's field. We have developed an HTS SQUID gradiometer system that uses active shielding and an adaptive signal processing algorithm to achieve the necessary dynamic range. An array of four thin-film SQUIDs with flux transformers is configured to form two orthogonal magnetometers and a single-axis gradiometer with a baseline of 10 cm. External field changes are cancelled by a three-axis Helmholtz coil set, driven by integral feedback from the SQUIDs, which surrounds the array. A real time adaptive balancing algorithm corrects for gradient offset, electronics scaling errors, sensor misalignments and uniformity errors in the Helmholtz coils. For outside use, light RF shielding is provided by thin layers of Al foil.In laboratory tests, intrinsic noise levels of 20 pT m Hz (1 Hz) and 0.3 pT m Hz (white) were measured. In field trials, the Helmholtz coils provided a shielding factor, for uniform fields, of 50-60 dB (0.1-10 Hz) during roll, pitch and yaw rotations of up to 5 . Adaptive balancing reduced gradient noise to the ambient background level of 80 pT m Hz (1 Hz) and 1 pT m Hz (white), whilst in motion.

The Compensation Method of Vehicle Magnetic Interference for the Magnetic Gradiometer

“…The first magnetometer serves as the reference in (13). Substituting (13) into (9), B 1 can be expressed as…”

“…Bono et al studied on magnetic interference of the magnetic gradiometer onboard the UUV and proposed a scheme for the compensation of vehicle magnetic interference [8]. Keene et al developed a Superconductivity Quantum Interference Device (SQUID) gradiometer system that used active shielding and an adaptive signal processing algorithm to eliminate magnetic interference [9], but he did not expound the compensation algorithm of vehicle magnetic interference. Pei and Yeo developed a prototype magnetic gradiometer onboard UUV for magnetic detection of underwater objects, which was housed in a plastic nose cone consisting of four fluxgate magnetometers, and one of the magnetometers served as the reference reading to the earth magnetic fields and was used to compensate the platform magnetic interference of other magnetometers [10,11].…”

Superconducting sensors and methods in geophysical applications

“…Koch et al [83] use the signal difference of two sensitive magnetometers which are both inductively coupled to a reference magnetometer with low enough sensitivity to reduce the dynamic range requirements. The other variant uses a global feedback scheme [84,85] with a (moderate sensitive) reference sensor triplet which couples back the measured magnetic field using a tri-axial Helmholtz coil system. In both cases, the sensitive magnetometers should be coupled with the same strength to the reference field to reduce the noise introduced by the reference sensors by differencing their signals.…”

Superconducting Magnetic Tensor Gradiometer System for Detection of Underwater Military Munitions

“…All of these prior systems use multiple sensors and calculate the gradients, either in software or electronically, which can be prone to error. Keene et al (2005) described a high temperature SQUID tensor gradiometer aimed at magnetic anomaly detection (MAD) from a moving platform. This system calculates the gradients by subtracting the output of two magnetometers.…”