Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2000-03-02
2002-09-24
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S202000, C324S243000
Reexamination Certificate
active
06456069
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 a magnetic field at the surface of a ferromagnetic material.
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 maldng 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 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).
A ship or submarine with a ferromagnetic hull produces a magnetic field signature which is dependent on the hull material magnetic characteristics, it's geometry in the earth's magnetic field, and stresses which are applied to the hull. Present degaussing systems sense the magnetic fields relatively close to the hull, and adjust the degaussing coil currents to minimize the fields at a distance below the vessel which can be sensed by magnetic influence sea mines. The ferromagnetic hull material's characteristics are used in complex ship models which are able to predict a vessels magnetic field signature below the ship. However, the characteristics may change significantly with respect to stress, heading, and time.
Ferromagnetic material sample characteristics are presently measured using ASTM “Standard Methods of Testing Magnetic Materials, which include DC fluxmeter, and alternating current techniques; see
ASM Standard Methods of Testing Magnetic Materials,
A34-70, American National Standards Institute, Part 8, incorporated herein by reference. Other techniques include balance permeameters for feebly magnetic materials, and portable permeameters, sometimes used as “magnaflux probe” for non-destructive testing of structural materials; see Sery, R. S.,
Permeameter Development and Use for Measuring Magnetic Permeability of SSN and High Strength Steels,
NSWC TR 80-347, Oct. 1, 1980, Naval Surface Weapons Center, White Oak, Md., incorporated herein by reference.
Other 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 379-401; 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 characteristics of a ferromagnetic material such as that of a ship's hull.
It is 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.
All magnetic materials can be characterized by a hysteresis curve, which is a two dimensional plot of Induction (B in weber/m
2
=10
4
gauss) versus Magnetic Field intensity (H in ampere/meter=0.01256 Oersted). The ratio of B/H is defined as the magnetic permeability of the material (weber/m-amp=henry/meter=newton/amp
2
or 1 hy/m=79.6×10
3
gauss/oersted), and varies non-linearly with respect to field amplitude, and mechanical stress.
At the surface of a ship or submarine hull or any ferromagnetic material, the normal component of the Induction (B) and the transverse component of Field Intensity (H) are each continuous across the surface boundary. The fields at the surface are dependent on the bulk material magnetic properties (permeability), which are dependent on the ambient magnetic field, the stress on the material which changes the characteristics of the magnetic material, and other local effects.
The “Ferromagnetic Surface Magnetic Field Sensor” (“FSMFS”) in accordance with the present invention features measurement of magnetic field at the surface of a ferromagnetic material (e.g., at the surface of a ship's hull) by measuring either or both the transverse H field and the normal B (Induction), using the ferromagnetic properties of the material as part of the sensor transducer. This invention advantageously senses magnetic characteristics of ferromagnetic material while obviating the need to alter such material.
The present invention provides a fluxgate device for sensing the transverse component of the magnetic field intensity H at a surface area of a ferromagnetic entity. The inventive device comprises a magnetic core, a drive winding and two sens
Holmes John J.
O'Keefe Edward C.
Scarzello John F.
Aurora Reena
Kaiser Howard
Lefkowitz Edward
The United States of America as represented by the Secretary of
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