System and method for characterizing ferromagnetic material

Patent metadata

Field	Value
Publication number	US-10067090-B2
Application number	US-201715653036-A
Country	US
Kind code	B2
Filing date	Jul 18, 2017
Priority date	Jun 29, 2015
Publication date	Sep 4, 2018
Grant date	Sep 4, 2018

Abstract

A system and method using magnetic sensing to non-intrusively and non-destructively characterize ferromagnetic material within infrastructure. The system includes sensors for measuring magnetic field gradients from a standoff distance adjacent to ferromagnetic material. The method includes using the system to measure magnetic fields, determining magnetic field gradients measured by a sensor array, and comparing measured and modeled or historical magnetic field gradients at the same or similar positions to identify differences caused by a phenomenon in the ferromagnetic material, and, in a particular embodiment, to recognize defects and developing defects.

First claim

What is claimed is:

1. A method for characterizing a ferromagnetic material, comprising:
providing a plurality of magnetometers arranged in an at least one-dimensional array, the array being positionable at a standoff distance along a z axis from the ferromagnetic material and translatable along an x axis, perpendicular to the z axis, to a plurality of scan positions along the ferromagnetic material, wherein at least three magnetometers of the plurality of magnetometers are mutually spaced apart and disposed within the array along a first axis;
storing, in a memory, data representing a deterministic physics-based model of magnetic fields for missing metal;
for each scan position of the plurality of scan positions, calculating, by a processor, respective differences between a plurality of respective select pairs of the magnetometers, wherein at least one select pair of the plurality of respective select pairs of the magnetometers comprises two non-adjacent magnetometers of the at least three magnetometers disposed along the first axis, and each select pair of at least two other select pairs of the plurality of respective select pairs of the magnetometers comprises two adjacent magnetometers of the at least three magnetometers disposed along the first axis, and wherein each difference of the respective differences is calculated by dividing by a distance separating the respective select pair of magnetometers; and
comparing, by the processor, respective differences between the plurality of respective select pairs of the magnetometers with the data representing the deterministic physics-based model of magnetic fields for missing metal to identify metal missing from the ferromagnetic material.

2. The method of claim 1 , the differences comprising at least one numeric that is derived from magnetic field data measured by at least one magnetometer of the plurality of magnetometers, the numeric being chosen from the group of numerics consisting of: Fourier, Wavelet, magnetic field gradients, gradient Fourier transform, wavelet transform, 2 nd derivative matrices, Hessians and fractal dimension.

3. The method of claim 1 , further comprising characterizing, by the processor, the metal missing from the ferromagnetic material to distinguish between a defect and a non-defect.

4. The method of claim 3 , the step of characterizing including applying, by the processor, a pairwise comparison between measured magnetic field data and modeled magnetic field data to characterize the metal missing from the ferromagnetic material.

5. The method of claim 4 , the step of applying utilizing a pairwise statistical comparison plot.

6. The method of claim 5 , wherein the pairwise statistical plot comprises a plurality of differences between measured magnetic field data and modeled magnetic field data, each difference of the plurality of differences representing a range of values according to a legend.

7. The method of claim 3 , the step of characterizing including determining a signature from measured magnetic field data associated with a non-defect of the ferromagnetic material.

8. The method of claim 1 , wherein the first axis is the y axis.

9. The method of claim 1 , wherein the first axis is the z axis.

10. The method of claim 1 , wherein the deterministic physics-based model of magnetic fields for missing metal comprises a magnetic dipole model.

11. The method of claim 10 , wherein the magnetic dipole model is calculated according to the equation:

(

B
x

B
y

B
z

)

=

1

r
5

⁢

(

C
x

⁡

(

3
⁢

x
2

-

r
2

)

+

3
⁢

C
y

⁢
xy

+

3
⁢

C
z

⁢
xz

3
⁢

C
x

⁢
xy

+

C
y

⁡

(

3
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y
2

-

r
2

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+

Assignees

• Charles Stark Draper Laboratory Inc

Inventors

• Timmons Brian P
• Mangoubi Rami S
• Hoffman Zachary R
• Webb Franklyn R

Classifications

• G01N33/2025

  Gaseous constituents · CPC title

• G01N33/20

  Metals · CPC title

• G01N27/83 Primary

  by investigating stray magnetic fields · CPC title

• G01N17/006

  of metals · CPC title