US 4,906,929 AGranted

Continuous real time nulling gradiometer for single and multicomponent field gradients and field measurements

AssigneeThe Boeing Company

InventorRaymond D. Rempt

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Abstract

A fiber-optic magnetic field gradiometer uses a plurality of magnetic transducers to simultaneously determine multicomponents of the gradient and field strength of an external magnetic field so as to permit accurate determination of the location of a ferromagnetic object located in an array of objects. The magnetic transducers are made of several layers of magnetostrictive glass which are wrapped with an optical fiber and immersed in an applied magnetic field to null out material differences and the earth's magnetic field. The null conditions for each adjacent pair are accomplished without disturbing the null conditions of the other adjacent pair. The nulling technique may be accomplished in real time and does not require cutting off the drive signals to adjacent coils. Thus the nulling may be accomplished simultaneously for all coils as the balancing of each coil is independent of its neighbors. A magnetic field to be detected along the axes of the magnetic transducers causes an optical path length change in the fibers. By employing eight magnetic transducers, all gradients and fields may be determined at the same time. Further by employing additional transducers, for a total of thirteen, the second derivatives of the fields may also be determined.

Claims33

Dependent claims

A multicomponent nullable magnetic field gradiometer comprising:

(a) first through fourth magnetic transducers, T(k), where k=1,2,3 and 4, each having a magnetically sensitive axis oriented along a first, x direction, wherein said first and third magnetic transducers are colinear and said second and fourth magnetic transducers are displaced along a second, y direction and a third, z direction respectively relative to said first and third magnetic transducers, said x, y and z directions being mutually perpendicular;
(b) first through fourth optical fibers, one fiber wrapped around each of said magnetic transducers wherein said fibers define first through fourth optical paths with said first magnetic transducer arranged in one arm of an interferometer and said second through fourth magnetic transducers arranged in another arm of said interferometer;
(c) laser means coupled to said fibers for providing laser light within said fibers, wherein the laser light travels along the optical paths defined by said fibers;
(d) an applied magnetic field generating means for immersing each of said magnetic transducers in an applied magnetic field having an AC and a DC component, said generating means including:
(1) means for applying a plurality of AC bias fields having frequencies w(k) to respective magnetic transducers T(k) where k=1,2,3 and 4 and common frequency w(1,2) to transducers T(1) and T(2), common frequency w(1,3) to transducers T(1) and T(3) and common frequency w(1,4) to transducers T(1) and T(4),
(2) means for applying a variably adjustible DC magnetic field to each of said first through fourth magnetic transducers, and
(3) means for separately adjusting the magnitudes of aid DC magnetic field and AC fields corresponding to said common frequencies so as to null said interferometer separately and independently for each magnetic transducer;
and
(e) first means operative when said fibers are subjected to an external magnetic field to be measured for detecting a change in the length .DELTA.L.sub.1,j , of the optical paths defined by said fibers between said integral values 2, 3 and 4, said changes in length proportional to the gradient of Bx in the x, y and z directions.

6 more elements

A method of detecting multicomponent magnetic field gradients of an external magnetic field comprising the steps of:

(a) orienting a magnetically sensitive axis of a first through fourth magnetic transducer along a first given direction, defined as the x direction, along which the gradient of the external magnetic field is desired to be measured, said magnetic transducers wrapped with a first through fourth optical fiber, respectively;
(b) transmitting a coherent radiation beam through each of said first through fourth optical fibers, said first through fourth optical fibers defining, respectively, first through fourth optical paths, wherein said first magnetic transducer is positioned in one arm of an interferometer and said second through magnetic transducers are positioned in another arm of said interferometer;
(c) applying a bias magnetic field having AC and DC components to immerse each of said magnetic transducers in said bias magnetic field;
(d) in the absence of said external magnetic field, detecting an optical path length difference between adjacent pairs of said first through fourth optical paths and adjusting said AC and DC components of said bias magnetic field to null said interferometer by the steps of:
(1) applying a plurality of AC bias fields having frequencies w(k) to respective magnetic transducers T(k) where k=1,2,3 and 4 and common frequency w(1,2) to transducers T(1) and T(2), common frequency w(1,3) to transducers T(1) and T(3) and common frequency w(1,4) to transducers T(1) and T(4),
(2) applying a variably adjustible DC magnetic field to each of said first through fourth magnetic transducers, and
(3) separately adjusting the magnitudes of said DC magnetic field and AC fields corresponding to said common frequencies so as to null said interferometer separately and independently for each magnetic transducer;
and
(e) then, in the presence of said external magnetic field, detecting optical path length differences between adjacent pairs of said first through fourth magnetic transducers to thereby detect said field gradients of said external magnetic field.

6 more elements

A method of nulling and balancing a gradiometer comprising the steps of:

(a) positioning a first and second magnetic transducer associated with first and second coils respectively in separate arms of an interferometer,
(b) applying a common AC field of frequency w to said first and second coils of said first and second magnetic transducers,
(c) applying a first AC field of frequency w1 to said first magnetic transducer via said first coil,
(d) applying a second AC field of frequency w2 to said second magnetic transducer via said second coil,
(e) applying a first DC bias field to said first transducer via said first coil;
(f) applying a second DC bias field to said second transducer via said second coil, and
(g) adjusting said first, second and common AC fields and said first and second DC bias fields to null said first and second magnetic transducers and balance said interferometer.

4 more elements

Dependencies are inferred from the claim text.

Facts

Application: 07/325,830Filed 20 Mar 1989 · continuation in part
Priority: 6 Mar 1987
Granted: 6 Mar 1990
Term ends: Not estimatedThe source records don’t support an estimate.

Classification3 codes

G01R 33/0327
Using magneto-optic devices, e.g. Faraday or Cotton-Mouton effect: with application of magnetostriction
G01R 33/022
Measuring direction or magnitude of magnetic fields or magnetic flux: measuring gradient
G01R
Measuring electric variables