Magnetic sensor operation onboard an AUV: magnetic noise issues and a linear systems approach to mitigation

Abstract

The U. S. Navy has demonstrated that total-field magnetometers and gradiometers as well as tensor fluxgate and superconducting magnetic gradiometers can be used in towed underwater platform environments. In these applications, the principal platform noise issue has been that caused by the rotation of onboard magnetic materials in the large background earth's magnetic field. The associated induced magnetic polarization changes and eddy currents cause secondary magnetic fields to be generated at the onboard sensor, and these overwhelm the magnetic signature of objects to be detected and localized.In these applications it has been established that knowledge of the earth's field vector time history in the platform reference frame can be used in a relatively simple fixed-parameter filter model to estimate the "motion noise" contribution to the measured signal, and to effectively remove it. The associated model has been modified to contain up to three magnetic dipole sources, and measured, time-windowed data has been used to simultaneously remove platform motion noise and localize magnetic targets.To operate a magnetic sensor onboard an AUV, it is necessary to deal with the problems encountered on a towed platform, that is, motion noise in the earth's field, but now there are numerous additional magnetic sources that are independent of the external field. These include onboard current loops and magnetic materials that move relative to the AUV platform. Examples are control surface mechanisms, motors and controllers, onhoard processors, and switch magnets.To model the noise from all magnetic sources, it is still necessary to monitor the earth's magnetic field onhoard the platform. It also is necessary to add measurements of onboard currents and magnetic field measurements made very near any sources that move relative to the platform. The basic issue here is that all onboard magnetic sensors will respond to the large earth's magnetic field changes due to rotation of the AUV in the earth's field. The additional magnetic field sensors should be positioned such that the additional field changes due to the local sources are measurable relative to the rotational changes.We describe both frequency-domain and time-domain filter models that incorporate onhoard AUV reference sensor measurements to cancel platform noise, and apply the models to data collected as described in companion papers. We evaluate the relative performance of the time-domain and frequencydomain models, and show that it depends on the measurement sets used to perform the modeling. We conclude that windowed time-domain filters hold much promise for future AUV-resident magnetic sensor systems

Progress in the development of buried minehunting systems

“…Since the magnetic noise generated by electrical subsystems and propulsion units may effectively limit the sensitivity of higher performance sensors, the RTG can be considered as a good compact and affordable candidate for UUV operation. The existing version of the RTG has been used in several land-based tests to establish the effectiveness of magnetic sensors onboard UUVs [14]- [16]. Sensitivities obtained from these tests are essentially at the noise floor of the sensor under ideal "magnetically quiet" conditions.…”

Magnetic sensor operation onboard a UUV: magnetic noise investigation using a total-field gradiometer

“…The filter model is the same as that used previously [4], and is repeated here for convenience. th reference, respectively, and the sum is over all N reference sensor channels (N=7 in our case).…”

“…In a previous OCEANS paper [4], we investigated the operation of a variety of sensors on a variety of prototype UUVs. For the purposes of this paper, we focus on the particular measurements involving the Polatomic P2000 totalfield gradiometer and the BLUEFIN BPAUV UUV, and we compare these measurements to measurements made with the same sensor mounted on a completely redesigned vehicle, the BLUEFIN RELIANT UUV.…”

Attitude Filter for Intervention-AUVs working in Tandem with Autonomous Surface Craft

“…When operating in open water scenarios, the attitude of the vehicle is typically estimated from the fusion of information from several sensors such as accelerometers, rate gyros, Doppler Velocity Loggers (DVL), acoustic positioning systems, and from flux-gate magnetometers or digital compasses. Typical targets, such as large metal structures, ship wrecks, metallic drills, and other Man-made objects, are known for having strong magnetic signatures which cause space anomalies and magnetic distortions (Wynn and Bono [2002]), thus rendering the onboard magnetometers unsuitable for heading determination, but largely adopted for the detection of buried objects (Kumar et al [2005]) and underwater structures investigation, see Pei et al [2010], Ioannidis [1977]. This paper addresses the design of an AUV attitude determination system that relies upon a team of two vehicles, comprising of an Autonomous Surface Craft (ASC) and an AUV in a tandem configuration, which cooperatively navigate the surroundings of a subject of interest.…”