Electricity: measuring and testing – Of geophysical surface or subsurface in situ – For small object detection or location
Reexamination Certificate
2001-05-09
2004-04-20
Barlow, John (Department: 2863)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
For small object detection or location
C324S326000
Reexamination Certificate
active
06724191
ABSTRACT:
SUMMARY OF THE INVENTION
The present invention provides systems and methods which can be employed to locate or detect presence of various materials, including nonferrous metals. These systems include new and useful sensors, circuits, systems and devices which power and/or interoperate with the sensors, and methods of making, operating and using such systems. Any or all of the systems, devices or processes can be combined with other systems, devices or processes disclosed.
1. Devices for Detecting and Identifying Conductive or Magnetic Objects Introduction
Devices according to the present invention are capable of performing sophisticated target location, classification, and recognition of conductive or magnetic objects independently without the use of other devices or systems and can be deployed by themselves in an area survey. This is accomplished by the simultaneous and accurate phase and magnitude measurement of the response of a target object to time varying magnetic fields of several frequencies.
While several types of magnetic/electromagnetic methods have been employed by others, the ability to accurately measure phase and amplitude as a function of frequency has been lacking. This measurement is necessary for the classification of materials. Much of the most sophisticated magnetic target location research to date has been conducted by the United States Navy for the purpose of location and detection of mines and unexploded ordnance. This work has been carried out primarily by the Coastal Systems Station (CSS) of the Navy's Naval Surface Warfare Center. They have developed some of the most sophisticated passive magnetic field sensing instruments. These include both total field and vector magnetometers as well as gradient magnetometers. However, their work does not include active time varying magnetic field generation.
BACKGROUND
Classical electromagnetic theory provides the underlying relationships for the description of operation of devices according to the present invention. An electrical current produces a magnetic field throughout space. The magnetic induction at any point in space, dB, due to a current flowing in an infinitely small (differential) length element is proportional to the magnitude of the current, I, times the vector cross product of the element length, dl, and the distance vector from the element to the point in space, x, divided by the cube of the magnitude of the distance vector:
dB=k
c
I
(
dlGx
)/|
x|
3
. (1)
where k
C
is the proportionality constant. In Gaussian units k
C
=1/c, where c is the speed of light. In MKSA units k
C
=1/(4&pgr;&egr;
0
)
1/2
. Therefore, the total magnetic induction at any point in space can be calculated by integrating over the total current path for all currents of concern.
B=fk
C
I
(
dlGx
)/|
x|
3
. (2)
Furthermore, if the current is varying with time, then both I and dB become functions of time, I(t) and dB(t), and the relation becomes:
B
(
t
)=
fk
C
I
(
t
)(
dlGx
)/|
x|
3
. (3)
In free space the magnetic field, H, is proportional to the magnetic induction, B. The proportionality constant, &mgr;
0
, is the permeability of free space and its value depends on the choice of the system of units. Thus:
H
(
t
)=&mgr;
0
B
(
t
). (4)
The second relationship states that a time varying magnetic induction will produce an electric field, E, over any closed path as follows:
gE
(
t
)·
dl=−k
E
(
d/dt
)
ffB
(
t
)·
nda.
(5)
Equation 5 states that the line integral of E over the closed path whose elements are dl is proportional, k
E
, to the negative of the time derivative of the surface integral, over any continuous surface bounded by the closed path, of the vector dot product of the magnetic induction and the unit surface normal, n, to the surface enclosed by the path. da is a surface element of that surface.
Any material has an associated conductivity, &sgr;
M
, and magnetic permeability, &mgr;
M
. Both the conductivity and permeability are material specific as signified by the subscript, M. These properties react with the local electric and magnetic fields, E and H. This interaction produces both currents and magnetic fields within the material. The magnetic induction in a material produced by the magnetic field is given by:
B
(
t
)=&mgr;
M
&mgr;
0
H
(
t
), (6)
where &mgr;
0
is the permeability of free space and &mgr;
M
is the material's relative permeability. There are three major classes of magnetic materials. They are differentiated by the size of &mgr;
M
. The ferromagnetic materials have large, positive permeabilities. These materials include iron, nickel, cobalt and most of their alloys. The paramagnetic materials, which include most other metals, have permeabilities that are greater than one by only parts per million; their permeabilities are small when compared to those of ferromagnetic materials. The third class of magnetic materials is the diamagnetic materials. These materials have permeabilities that are slightly less than one by amounts equivalent in size to that of the paramagnetic materials. A few of the metals are diamagnetic. Ferromagnetic materials are referred to as ferrous materials and paramagnetic and diamagnetic materials are referred to as nonferrous materials.
The current by produced the imposed electric field, commonly called an eddy current, is proportional to the product of the impedance, Z
M
, which includes the conductivity and the shape functions, and the magnitude of the electric field, E; thus:
I
(
t
)=
k
1
E
(
t
)/
Z
M
, (7)
where k
1
is the proportionality constant. The impedance is in general a complex number and is a function of the shape of the material along with its conductivity. As with magnetic materials there are three major classes of materials based on their conductivities. The conductors, which include metals, have relatively large conductivities. The insulators have conductivities that are a million to a trillion times smaller. In between lie the semiconductors. Materials can also be classified by their permitivities. In general the permitivities are relatively unimportant at the frequencies of interest for the conductors; therefore the effects due to the displacement currents are extremely small and can be neglected in the following arguments. Furthermore the displacement current will add a small −jZ
C
term which will be overwhelmed by the much larger inductive term in the case of conductors. The aspect ratios of the targets are generally close to one. In this case for the materials of interest the impedance is strongly inductive and can be described by the relation:
Z
M
=Z
R
+jZ
L
, (8)
where, Z
R
is the real impedance, which is a function of the conductivity and the shape, and Z
L
is the imaginary part of the impedance, which is a function of the shape.
The materials of interest have high permeabilities, high conductivities, or both. Equations 6 and 7 show that these materials will produce a secondary magnetic field either directly from the applied magnetic field due to &mgr;
M
or indirectly through an induced current due to &sgr;
M
. It is these secondary magnetic fields that are sensed in all types of magnetic/electromagnetic sensing systems. Equation 5 shows that an electric field is only produced if the magnetic field is time varying; therefore, nonferrous materials cannot be detected by a constant magnetic field. The passive magnetic field sensing methods used by the Navy and all others for large area surveys are thus only capable of sensing the ferrous materials. In these methods the residual magnetic field from the earth, approximately 0.00005 T (Tesla) provides the local magnetic field at the target material. A ferrous target then creates a secondary field that can be sensed as a variation in the constant background field of the earth. This field is generally dipole in nature and falls off as the cube of the distance from the target. The field fa
Admiralty Corporation
Barlow John
Kilpatrick & Stockton LLP
Le Toan M
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