Integrating fluxgate magnetometer

Details Integrating fluxgate magnetometer Integrating fluxgate magnetometer

2000-03-02

2001-08-21

Patidar, Jay (Department: 2862)

Electricity: measuring and testing

Magnetic

Magnetometers

Reexamination Certificate

active

06278272

ABSTRACT:

BACKGROUND OF THE INVENTION
The present inventions relate to methods, systems and apparatuses for performing measurement pertaining to magnetic field, more particularly to such methods, systems and apparatuses for measuring one or more components of a magnetic field over a linear region.
Ships and submarines are constructed of ferromagnetic materials which produce magnetic field signatures, making them detectable and vulnerable to magnetic influence sea mines and detectable by airborne magnetic anomaly detection (MAD) and underwater electromagnetic surveillance systems.
To reduce the magnetic field signature of ships and submarines, coils are wrapped around the ferromagnetic hull, and fields produced which reduce the vessel's signature. In order to control the coil currents, a degaussing (DG) system must have sensors which accurately measure the signature related magnetic fields, and control algorithms to extrapolate the spatially measured field values to regions under the ship, and adjust the coil currents to minimize the signature amplitude.
It is useful to measure magnetic fields near the hull of naval ships and submarines, so that such measured magnetic fields can be used to control advanced degaussing systems. A large number of “point” sensors are presently employed, but they are expensive and not capable of satisfying the need for measuring fields at all points along the circumference of a ship or submarine hull. It is important to measure these fields produced by local hull anomalies (welds, stresses, bulkheads, etc.) and material inhomogeneities at many locations, for more effective control of the ship's degaussing system. Ideally, by measuring the surface magnetic fields all over the hull (and thereby continuously monitoring the magnetic state of a ship or submarine hull), the magnetic field signature of the ship can be adjusted and maintained at a low level using an advanced degaussing system such as the U.S. Navy's Advanced Closed Loop Degaussing System, thereby making a ship less vulnerable to sea mine magnetic influence fuzes.
Advanced degaussing systems require accurate and spatially distributed magnetic field measurements around the ship, so that ship mathematical model algorithms can precisely control magnetic field signatures below the ship. Some of the problems associated with measuring these fields include: large spatial gradient magnetic fields; local magnetic anomalies; induced magnetic fields caused by heading changes; and, permanent magnetization changes due to pressure-induced hull stresses. Such measurements have been made using traditional fluxgate magnetometers, short baseline gradiometers, etc.
In some cases, there are large spatial magnetic field gradients, close to the hull, which are produced by local hull anomalies (e.g., welds, stresses, bulkheads, etc.) and material inhomogeneities. “Point” triaxial fluxgate magnetometers and gradiometers are presently used to measure these spatial gradients; however, because of these local effects, field measurements at many locations may be not be useful for controlling the shipboard degaussing system.
Fluxgate magnetometers measure the magnetic field intensity using a variety of transducer cores which, normally, are considered to be small “point” field sensors (typically, about one to two inches in length). More generally, fluxgate, fiber-optic and other magnetic field sensitive transducer phenomena measure the magnetic field intensity using a variety of transducer cores which are normally considered point field measurements (wherein the transducers are typically about one to two inches in length).
Pertinent background information is provided by the following papers, each of which is hereby incorporated herein by reference: Lenz, J. E., “A Review of Magnetic Sensors,”
IEEE Proceedings,
Vol. 78, No. 6, June 1990; Gordon, D. I., R. E. Brown and J. F. Haben, “Methods for Measuring the Magnetic Field,”
IEEE Trans. Mag.,
Vol. Mag-8, No. 1, March 1972; Gordon, D. I. and R. E. Brown, “Recent Advances in Fluxgate Magnetometry,”
IEEE Trans. Mag.,
Vol. Mag-8, No. 1, March 1972; Gordon, D. I., R. H. Lundsten, R. A. Chiarodo, H. H. Helms, “A Fluxgate Sensor of High Stability for Low Field Magnetometry,”
IEEE Transactions on Magnetics,
vol. MAG-4, 1968, pp 379-041; Acuna, M. H., “Fluxgate Magnetometers for Outer Planets Exploration,”
IEEE Transactions on Magnetics,
vol. MAG-10, 1974, pp. 519-23.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide method, apparatus and system for measuring magnetic field distribution in a sector of interest—e.g., a distance, an area or a volume—which includes a plurality of different spatial points (i.e., discrete locations in space).
It is another object of the present invention to provide such method, apparatus and system which are more efficient than conventional methods, apparatuses and systems.
It is a further object of this invention to provide such method, apparatus and system which are more economical than conventional methods, apparatuses and systems.
It is another object of this invention to provide such method, apparatus and system which are more reliable than conventional methods, apparatuses and systems.
Another object of the present invention to provide method, apparatus and system for continuously measuring same, for use in association with a magnetic control system such as a ship degaussing system.
According to many inventive IFM embodiments, an “integrating” fluxgate transducer magnetometer measures the magnetic field amplitude component over a linear region, and “integrates” the measured values to obtain the sum of the field component amplitudes over the length of the fluxgate transducer magnetometer's sensor element. A typical inventive Integrating Fluxgate Magnetometer (IFM) is a fluxgate magnetometer having a rigid transducer core which is configured as a long “racetrack” in order to integrate large component gradient magnetic fields near a ferromagnetic entity, e.g., a ship hull or a large piece of machinery. A typical inventive IFM: (i) measures magnetic fields over the length of its elongated transducer element (e.g., the 30 cm length of an inventive prototype tested by the U.S. Navy), and (ii) spatially integrates the component field amplitudes.
In accordance with many embodiments of the present invention, a fluxgate magnetometer comprises a magnetic core, two drive windings and a sense winding. The magnetic core is characterized by a closed magnetic flux path, a core length, a core width and a ratio of the core length to the core width of at least ten. The magnetic core has two approximately straight approximately equal lengthwise core portions and two arcuate end portions. The lengthwise core portions are approximately oriented in rectangular parallel relation. Each lengthwise core portion is characterized by approximately the same lengthwise core portion length, which is substantially the core length. Each drive winding is wound over one lengthwise core portion. The sense winding is wound encompassingly with respect to the combination of the two lengthwise core portions and the two drive windings. Typically, the fluxgate magnetometer is adaptable to transmitting, via the sense winding, an electrical signal which is integratively indicative of the sensed magnetic field components over the lengthwise core portion length.
Certain conventional fluxgate magnetometers implement a magnetic core having the “closed” (or “closed loop”) configuration; that is, the shape of the magnetic core describes a geometrically closed figure. In other words, a closed magnetic core has a closed magnetic flux path. Two closed core configurations which are known in the art are the “ring core” and the “racetrack core” configurations. See, e.g., the following references, each of which is incorporated herein by reference: Pavel Ripka, “Review of Fluxgate Sensors,”
Sensors and Actuators A,
33 (1992), pp 129-141; Pavel Ripka “Race-Track Fluxgate Sensors,”
Sensors and Actuators;
A, 37-38 (1993), pp 417-421;

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