Electricity: measuring and testing – Magnetic – Magnetic field detection devices
Reexamination Certificate
2002-07-29
2004-03-30
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetic field detection devices
C324S260000, C324S247000
Reexamination Certificate
active
06714008
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatuses for determining a magnetic field, a magnetic field vector or a value related thereto, more particularly for determining a static magnetic field signature associated with an object whereby the applied uniform magnetic field (such as associated with a test facility) and/or the earth's background magnetic field are accounted for in such determination.
Laboratory measurement of the static magnetic fields surrounding large objects can be a difficult process. These experiments are typically conducted within the uniform volume of a large magnetic calibration facility whose coil system can apply fields in three orthogonal directions. In the past, such laboratory tests have included spacecraft, large-scale magnetic models of naval vessels, and full-scale ship equipment such as engines, electric motors, and generators. See the following papers, each of which is incorporated herein by reference: T. N. Roy, “Spacecraft magnetic field modeling,”
IEEE Trans. Magn.,
vol. 13, pp 914-919, January 1977; A. V. Kildishev, S. A. Volokhov and J. D. Saltykov, “Measurement of the spacecraft main magnetic parameters,” in
Proc.
1997
IEEE Autotestcon Systems Readiness Technology Conf.,
pp 669-675, 1997; R. A. Wingo, J. J. Holmes and M. Lackey, “Test of closed-loop degaussing algorithm on a minesweeper engine,” in
Proc.
1992
Amer. Soc. Naval Eng.,
May 1992. In naval applications, the field pattern around and under a ship is called its signature.
The purpose of laboratory experiments is usually to measure an object's field at discrete locations around it, and then to mathematically predict the flux distribution or signature in all space. See F. M. Duthoit, L. Krahenbuhl and A. Nicolas, “The boundary integral equation method for the extrapolation of field measurement,”
IEEE Trans. Magn.,
vol. 21, pp 2439-2442, 1985, incorporated herein by reference.
In addition, onboard magnetic field compensation systems called “degaussing” systems are adjusted (calibrated) to minimize the amplitude of the surrounding field. See the following papers, each of which is incorporated herein by reference: K. R. Davy, “Degaussing with BEM and MFS,”
IEEE Trans. Magn.,
vol. 30, pp 3451-3454, September 1994; X. Xu and L. Zeng, “Degaussing of cylinders magnetized in Earth's magnetic field—a 2-D model of degaussing of submarine,”
J. Electromag. Waves Appl.,
vol. 12, pp1039-1051, 1998; F. Le Dorze, J. P. Bongiraud, J. L. Coulomb, P. Labie and X. Brunotte, “Modeling of degaussing coil effects in ships by the method of reduced scalar potential jump,”
IEEE Trans. Magn.,
vol. 34, pp 2477-2480, September 1998; M. Norgren and S. He, “Exact and explicit solution to a class of degaussing problems,”
IEEE Trans. Magn.,
vol. 36, pp 308-312, January 2000.
Minimization of a naval vessel's static magnetic field signature is important in reducing its vulnerability to magnetically actuated mines. Also, spacecraft signature reduction is necessary to mitigate attitude control problems, and interference to onboard magnetic instruments and low-energy electron experiments. See aforementioned paper T. N. Roy, “Spacecraft magnetic field modeling,”
IEEE Trans. Magn.,
vol. 13, pp 914-919, January 1977.
Large, triaxial, calibration coil facilities presently exist at several U.S. Naval facilities, including the Naval Surface Warfare Center detachment at Panama City, Fla. In addition, a new coil facility is under construction at the Center's Carderock Division, West Bethesda, Md. that will accommodate test items up to 44 tons in weight. The new facility's coil system will generate a uniform magnetic field within a cube 3 meters on a side, with a peak-to-peak spatial variation less than 0.05% of the field along the primary axis. An inducing field can be generated over the uniform volume with a dynamic range of ±50,000 nT, and can be held stable to within ±1 nT, according to D. Whelan of the Naval Surface Warfare Center, West Bethesda, Md. as stated in a private communication (e-mail) to joint inventor John J. Holmes on Jan. 18, 2000. However, laboratory testing of large magnetic objects can present special problems.
An important requirement for a magnetic testing facility is the ability to remove the applied uniform field (facility plus earth's field) from the measurement of an item's signature. Typically, the applied field is eliminated by physically removing the test object from the facility, then recording and storing the sensor readings while energizing each calibration coil, one at a time, with a known current. This “background” data can be scaled; based on real-time measured coil currents or other facility control sensors, and subtracted from all subsequent measurements of the test object. However, removing a test item from inside a coil system is not always viable, especially if the object is physically large, heavy, or has an irregular shape. In some cases, sensors and sections of the coils themselves must be moved to accommodate removal of the test item from the facility. Exact realignment of the sensors and coils is always difficult and time consuming, reducing accuracy and repeatability of the experiments.
A magnetic gradiometer measures the difference between two magnetometers separated by a short distance and mounted on a rigid and thermally stable bar that is usually made of titanium. Special care is taken in the design and construction of a gradiometer to insure that the gain and orientation of its two magnetometers do not change over time with environmental conditions. A commercial product which exemplifies this is Gradiometer Type GS2, manufactured by the ULTRA Electronics Magnetics Division, Fallow Park, Rugely Road, Hednesford, Cannock, Staffordshire, England WS12 5QZ.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide method and apparatus for measuring the magnetic signature of an entity so as to remove the influence of any external field (e.g., an applied magnetic field and/or the earth's magnetic field) from such measurement.
It is a further object of the present invention to provide such method and apparatus for measuring the magnetic signature of an entity which is large or cumbersome or otherwise does not readily admit of portability or transportability relative to a magnetic test facility.
In accordance with many embodiments of the present invention, a method for determining at least one magnetic field signature of an entity comprises the steps of performing gradiometric measurements and processing the gradiometric measurements. The performing of the gradiometric measurements includes using plural gradiometers which are arranged so as to generally describe a closed three-dimensional geometric shape which surrounds the entity. The processing of the gradiometric measurements includes determining the multipole moments of the entity based on the gradiometric measurements, wherein the multipole moments correspond to the closed three-dimensional geometric shape. According to typical inventive embodiments, the closed three-dimensional geometric shape is a prolate spheroid which surrounds the entity, and the multipole moments are the prolate spheroidal multipole moments of the entity (i.e., the multipole moments pertaining to the same prolate spheroidal shape which is geometrically conceived to surround the entity as described by the arrangement of the gradiometers).
Further provided according to the present invention is a computer program product. The present invention's computer program product comprises a computer useable medium having computer program logic recorded thereon for enabling a computer to determine at least one magnetic field signature of an entity. The computer program logic comprises means for enabling the computer to determine the prolate spheroidal multipole moments of the entity. The prolate spheroidal multipole moments (i.e., the multipole moments pertaining to the same prolate spheroidal shape which is
Holmes John J.
Hood Bruce R.
Scarzello John F.
Kaiser Howard
Patidar Jay
The United States of America as represented by the Secretary of
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