U.S. patent application number 13/012620 was filed with the patent office on 2011-11-24 for method and apparatus for sensing a magnetic field.
This patent application is currently assigned to Assurance Technology Corporation. Invention is credited to Thomas W. Jewitt, Kai W. Li, Louis S. Palecki, William C. Place, Antonio G. Rizzo.
Application Number | 20110285389 13/012620 |
Document ID | / |
Family ID | 36943539 |
Filed Date | 2011-11-24 |
United States Patent
Application |
20110285389 |
Kind Code |
A1 |
Jewitt; Thomas W. ; et
al. |
November 24, 2011 |
METHOD AND APPARATUS FOR SENSING A MAGNETIC FIELD
Abstract
A magnetic screening system uses directional gradiometers with
high resolution and accuracy to measure magnetic field signatures
of target objects (e.g., gun, knife, cell phone, keys) in a volume
of interest. The measured signatures can be compared to signatures
of known objects stored in a local database. Various mathematical
processes may be used to identify or classify target object
signatures. In a network of magnetic screening systems, the
magnetic screening systems can transmit signatures to a central
signature database, and a management computer can share the central
signature database with all of the magnetic screening systems on
the network. The magnetic screening system can operate in multiple
modes, such as a tracking mode, measurement mode, and self-test
mode. Through use of unique processes and designs, the magnetic
screening system can achieve a high rate of processing persons for
target objects.
Inventors: |
Jewitt; Thomas W.; (Ayer,
MA) ; Li; Kai W.; (Chelmsford, MA) ; Palecki;
Louis S.; (Southborough, MA) ; Place; William C.;
(Acton, MA) ; Rizzo; Antonio G.; (Nashua,
NH) |
Assignee: |
Assurance Technology
Corporation
Carlisle
MA
|
Family ID: |
36943539 |
Appl. No.: |
13/012620 |
Filed: |
January 24, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12459774 |
Jul 7, 2009 |
7898248 |
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13012620 |
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11981717 |
Oct 31, 2007 |
7573257 |
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12459774 |
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11169147 |
Jun 28, 2005 |
7319321 |
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11981717 |
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11073424 |
Mar 4, 2005 |
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11169147 |
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Current U.S.
Class: |
324/244 |
Current CPC
Class: |
G01V 3/081 20130101 |
Class at
Publication: |
324/244 |
International
Class: |
G01R 33/02 20060101
G01R033/02 |
Claims
1. A gradiometer comprising: at least three magnetometers; and a
processor unequally weighting outputs of the at least three
magnetometers and combining the unequally weighted outputs to
orient a direction of sensitivity toward at least one direction of
interest.
2. The gradiometer according to claim 1 wherein the processor
weights the outputs with non-integer weights.
3. The gradiometer according to claim 1 wherein the processor uses
output weights calculated using a deterministic mathematical
technique.
4. The gradiometer according to claim 1 wherein the processor
adjusts the output weights digitally.
5. The gradiometer according to claim 1 wherein spacing of the
magnetometers is determined before the weights are determined.
6. The gradiometer according to claim 1 wherein the number of
magnetometers and associated weights are selectable to develop
arbitrary response patterns.
7. A method of sensing a magnetic field, the method comprising:
applying unequal weights to outputs of at least three magnetometers
in a gradiometer; and combining the unequally weighted outputs to
orient a direction of sensitivity toward at least one direction of
interest.
8. The method according to claim 7 wherein applying the outputs
includes applying the outputs with non-integer weights.
9. The method according to claim 7 further including calculating
the output weights in a deterministic mathematical manner.
10. The method according to claim 7 further including adjusting the
output weights digitally.
11. The method according to claim 7 further including determining
spacing of the magnetometers before determining the weights.
12. The method according to claim 7 further including selecting a
number of magnetometers and associated weights to develop arbitrary
response patterns.
13. A gradiometer comprising: means for applying unequal weights to
outputs of at least three magnetometers in a gradiometer; and means
for combining the unequally weighted outputs to orient a direction
of sensitivity toward at least one direction of interest.
Description
RELATED APPLICATION(S)
[0001] This application is a continuation application of U.S.
application Ser. No. 12/459,774, filed Jul. 7, 2009, which is a
continuation of U.S. application Ser. No. 11/981,717, filed Oct.
31, 2007, now U.S. Pat. No. 7,573,257, which is a continuation of
U.S. application Ser. No. 11/169,147, filed on Jun. 28, 2005, now
U.S. Pat. No. 7,319,321, which is a continuation of U.S.
application Ser. No. 11/073,424, filed on Mar. 4, 2005, now
abandoned. The entire teachings of the above applications are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] In recent years, screening for weapons at entrances of
public places, such as airports, government buildings, public
schools, and amusement parks, has increased to ensure safety for
the public at those places. Screening for weapons can include
requiring people entering such public places to pass through a
magnetic screening system such as a portal metal detector. Although
people have become accustomed to passing through portal metal
detectors, the process remains relatively slow for a number of
reasons.
[0003] One reason for the slow process is that people must empty
their pockets of all metallic objects, remove their coats, and
sometimes remove their shoes. The objects and clothing are either
physically inspected by hand or passed through an x-ray machine for
visual screening. Another reason for the slow process is due to
false detection or detection of non-weapon metals, such as wrist
watches, belt buckles, metallic wires found in ordinary garments,
personal adornments such as broaches or hair clips, and loose coins
in pockets.
[0004] Yet another reason for the slow processing is due to machine
settling times, which refers to the amount of time that must be
allowed for the sensors in metal detectors to resettle after a
person passes through it. Allowing a portal metal detector
sufficient time to settle ensures accurate readings of the next
person. A person who passes through the portal metal detector then
stops on the other side very close to the portal metal detector
(i.e., within an `influence` zone) can also influence the metal
detector to such an extent that the metal detector makes a false
detection or misses detecting an object as the next person passes
through it. Therefore, portal metal detector operators must stop
the next person from entering the portal metal detector until the
previous person has passed beyond the influence zone.
[0005] Typically, all of the delays result in a passthrough rate of
between one and two hundred persons per hour. To accommodate large
crowds, many portal metal detectors are operated in parallel, which
leads to staffing, training, and machine calibration issues. If the
passthrough rate were higher, many venues that are currently
equipped with large numbers of portal metal detectors could reduce
the number in use, and venues such as sports stadiums not currently
equipped with portal metal detectors would be so equipped.
[0006] Moreover, today's portal metal detectors are sensitive to
large ferromagnetic objects, such as wheelchairs. When a person in
a wheelchair passes through the portal metal detector, the portal
metal detector is overwhelmed by the metal content of the
wheelchair and unable to detect relatively small metal objects on
the person. In addition, even if the wheelchair is not passing
through the portal metal detector, it can influence the detector to
such an extent that the detector makes erroneous readings.
[0007] In addition to the slow process associated with today's
portal metal detectors and their sensitivity to large ferromagnetic
objects, many portal metal detectors are `active,` meaning they
emit an electro-magnetic field in a volume of interest (i.e., the
area in the portal metal detector). Active detectors can be
dangerous for people using medical devices, such as pacemakers,
that are sensitive to electro-magnetic fields. Passive metal
detectors, which sense a local disturbance in the earth's magnetic
field, do not affect medical devices, but they are sensitive to
local magnetic fields, large ferromagnetic devices, calibration
errors, background offsets, and other measurement disturbances
known in the art.
SUMMARY OF THE INVENTION
[0008] The principles of the present invention apply to multiple
levels of a magnetic screening system and a network managing
multiple magnetic screening systems. In a network embodiment,
signature data ("signatures" or data) of target objects (e.g.,
guns, knives, cell phones, Personal Digital Assistants (PDA's), and
other ferromagnetic materials) and information associated therewith
can be added to and maintained in a central signature database.
From this central signature database, a management computer can
distribute the information and data in the central signature
database to magnetic screening systems on the network for updating
their local databases. As a result, the local databases can be
continually updated, thereby allowing the magnetic screening
systems to have knowledge of more target objects than if operating
independently off the network for increased automation of
detection, identification, or classification processes, among
others, of target objects. Increased automation increases a rate of
processing of people through the magnetic screening systems because
operators of the magnetic screening systems have fewer incidents of
having to manually inspect persons carrying target objects.
[0009] The magnetic screening systems may include arrangements of
gradiometers, each including at least three magnetometers and, in
some embodiments, a gradiometer processor in communication with the
magnetometers. The gradiometer processor scales outputs from the
magnetometers with unequal weights and combines the scaled outputs
to orient a direction of sensitivity of the respective gradiometer
toward a volume of interest (e.g., a pathway through a portal metal
detector). The magnetic screening systems may also include an
arrangement in communication with the gradiometer that uses the
gradiometers in a collective manner to detect a target object. In
addition to detecting a target object, the arrangement processor
can localize the position of the target object, identify the target
object, and optionally classify the target object. The gradiometers
may use passive magnetometers, which do not themselves generate a
magnetic field, thereby allowing people with medical devices, such
as heart pacemakers, to pass through the magnetic screening system.
The arrangement processor may also be used to compare a signature
of the target object measured by the gradiometers against known
signatures stored in a local or central database.
[0010] The gradiometers may be operated in multiple modes. Examples
of modes in which the gradiometers may be caused to operate include
measurement mode, background offset reduction mode, calibration
mode, self-test mode, automatic alignment mode, and diagnostic
mode. Self-test mode can be used to determine operational
readiness. Automatic alignment mode can be used to calculate the
alignment of the gradiometers relative to the earth's magnetic
field, which, in turn, can be used to determine orientations of
each gradiometer to at least one other gradiometer in the magnetic
screening system. Knowing alignment of gradiometers allows for
system operation in a tracking mode, in which multiple gradiometers
can be configured to generate real-time tracks of target objects in
three dimensions. In diagnostic mode, the gradiometers can output
measured field strengths in an unaltered state from the component
magnetometers. During measurement mode, the gradiometers can be
switched from measurement mode to calibration mode or background
offset reduction mode in various sequences and at selectable rates.
Background offset, caused by disturbances within or outside a
volume of interest that affect measurements by the gradiometers,
can be reduced in a real-time manner or in a post-processing
manner, and the magnetometers can be calibrated before every
measurement sample. Use of the above-described modes can yield
accuracies that result in minimized rates of false detection of
known or unknown target objects, including high accuracy in
automatically determining whether a target object is a dangerous
object or a non-dangerous object. Thus, processing rates of persons
passing through the magnetic screening system(s) is increased.
[0011] To increase calibration accuracy, the gradiometers may have
individual calibration circuits available for applying localized
magnetic fields to transducers in the magnetometers that cause a
measurable response by the magnetometer. During a calibration
cycle, the calibration circuit may be used to generate magnetic
fields at least two different levels. The calibration circuit may
be specially designed to limit externally-induced offsets for
improved calibration accuracy. Using the calibration circuit, the
gradiometer processor or other processor can calculate a
calibration curve using various techniques or metrics. The
calibration curve can be used to calibrate every magnetic field
vector sample measured by the respective magnetometer. This
calibration circuit, thus, improves measurement accuracy, which, in
turn, reduces rates of false alarm for increased rate of processing
persons through the magnetic screening system(s).
[0012] As described above, the gradiometers may each include at
least three magnetometers whose outputs are scaled with unequal
weights. The unequal weights may be combined to orient the
direction of sensitivity of the gradiometer toward a volume of
interest. In one embodiment, the weights are non-integer weights
that may be calculated using a deterministic mathematical
technique. In some embodiments, a processor may adjust the output
weights digitally and optionally in real-time. Spacing of the
magnetometers can improve accuracy of the gradiometers. For
example, in the case where the magnetometers are aligned along a
single axis, the outer two magnetometers are preferably spaced
apart as far as possible for enhanced sensitivity for measuring a
magnetic gradient. The magnetometer(s) between the outer two
magnetometers may be arbitrarily positioned relative to the outer
two magnetometers to orient the direction of sensitivity toward the
volume of interest. Optionally, positioning of the magnetometers is
determined before the output weights are determined. In some
embodiments, the direction of sensitivity is entirely toward the
volume of interest, and the sensitivity away from the volume on
interest is substantially zero. In such an embodiment, gradiometers
positioned on a first boundary of the volume of interest do not
detect disturbances behind themselves outside the volume of
interest below a selectable threshold, such as a wheelchair or
other magnetic screening system. The disturbance outside the volume
of interest can be detected by gradiometers on the other side of
the volume of interest and, thus, be treated as a background
disturbance and eliminated from measurements. Again, such
processing techniques and design implementations are used to reduce
rates of false detection, improve accuracy, and ultimately lead to
increased rate of processing of persons passing through the
magnetic screening system(s).
[0013] As a result of the multiple aspects of the present invention
as described above, a magnetic screening system can process 600-700
persons per hour or more compared to previous systems capable of
processing 100-200 persons per hour. Moreover, the magnetic
screening systems can make the screening process much more
acceptable for people since (i) jackets and other typical outerwear
can be worn while passing through the magnetic screening system and
(ii) ferromagnetic objects, such as keys, cell phones, personal
digital assistants, and other ferromagnetic objects do not have to
be removed from pockets as a result of the techniques described
herein.
[0014] Moreover, in some embodiments, arrangements of gradiometers
can be deployed in fixtures other than portals, such as
wastebaskets or other common fixtures, so as to be imperceptible to
persons passing through a volume of interest defined by placement
of the arrangement(s) of gradiometers. Thus, passive metal
detection can be done by the magnetic screening systems that do not
disrupt traffic flow while yielding high rates of detection of
dangerous target objects (e.g., guns, knives, and so forth) and
discriminating non-dangerous target objects (e.g., cell phones and
the like).
[0015] As the magnetic screening system(s) are used, the database
of signatures corresponding to known ferromagnetic objects
increases and, therefore, improves the overall operation of the
system even more over time.
[0016] As a result of the improvements described herein, the
magnetic screening systems may be employed at venues such as sports
stadiums, amusement parks, and other public places in which such
systems were previously thought to be too restrictive on a
flow-through basis or public relations basis. At venues where
magnetic screening systems are currently used in high numbers, such
as airports, the number of magnetic screening systems can be
reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0018] FIG. 1 is a pictorial diagram of an entrance to a public
place having magnetic screening systems (e.g., portal metal
detectors) employing the principles of multiple aspects of the
present invention;
[0019] FIG. 2 is a network diagram of a magnetic screening system
network including the magnetic screening systems of FIG. 1;
[0020] FIG. 3 is a network block diagram corresponding to a subset
of the network of FIG. 2;
[0021] FIG. 4A is a block diagram of a gradiometer used in the
magnetic screening systems of FIG. 1.
[0022] FIG. 4B is an electrical schematic diagram of the
gradiometer of FIG. 4A;
[0023] FIG. 5 is an electrical schematic diagram of a magnetometer
with transducer used in the gradiometer of FIG. 4B;
[0024] FIG. 6A is an electrical schematic diagram of the transducer
of FIG. 5 and an associated calibration circuit;
[0025] FIG. 6B is a plot of data points captured by a calibration
process using the calibration circuit of FIG. 6A;
[0026] FIG. 7A is a magnetic pattern diagram for the gradiometer of
FIG. 4A.
[0027] FIG. 7B is a detailed lobe diagram for the gradiometer of
FIG. 7A;
[0028] FIG. 7C is a lobe diagram for a prior art three-magnetometer
gradiometer;
[0029] FIG. 7D is a detailed lobe diagram for the prior art,
three-magnetometer gradiometer of FIG. 7C;
[0030] FIG. 8 is a vector diagram for the gradiometer of FIG. 7A
measuring a target object;
[0031] FIG. 9 is a timing diagram for multiple modes of operation
for the gradiometer of FIG. 8;
[0032] FIG. 10 is timing diagram for the system operation diagram
of FIG. 9.
[0033] FIG. 11A is a graphical diagram illustrating a real-world
example for the magnetic screening system of FIG. 9;
[0034] FIG. 11B is a detailed timing diagram corresponding to the
real-world illustration of FIG. 11A;
[0035] FIG. 12A is graphical illustration of target object having
magnetic fields detectable by the magnetic screening system of FIG.
11A;
[0036] FIG. 12B is an alternative embodiment of the magnetic
screening system of FIG. 12A using gradiometers in a tracking mode
to track target objects in three dimensions;
[0037] FIG. 12C is an alternative embodiment of the magnetic
screening system of FIG. 12A in which an arrangement of
gradiometers is deployed in non-portal fixtures and can operate in
a tracking mode to track target objects in three dimensions;
[0038] FIG. 13 is an example signature for a target object (e.g., a
gun) captured by the gradiometer of FIG. 4A;
[0039] FIG. 14A is a signal diagram illustrating the signature of
FIG. 13 in a measurement signal affected by a background
disturbance and captured by the gradiometer of FIG. 4A;
[0040] FIG. 14B is the signal diagram of FIG. 14A with a background
offset removed either in real-time or during post processing;
[0041] FIG. 15 is a block diagram of identification and
classification processing used to identify or classify the target
object signature of FIG. 13; and
[0042] FIG. 16 is a graphical diagram illustrating the magnetic
screening system of FIG. 1 that is presenting an indicator produced
by the processing of FIG. 15 to a portal metal detector
operator.
DETAILED DESCRIPTION OF THE INVENTION
[0043] A description of preferred embodiments of the invention
follows.
[0044] The principles of the present invention may be employed at
multiple levels in the magnetic screening system 100 and a network
of magnetic screening systems 100 each including multiple
gradiometers. The multiple levels include at least a network level,
system level, mode of operation level, and gradiometer design
level. A brief overview of each is presented, and details of each,
in turn, follows the brief overview.
[0045] FIG. 1 is a pictorial diagram of a cluster 50 of magnetic
screening systems 100, each using gradiometers configured in a
portal 105. Magnetic screening systems 100 are typically used at an
entrance to a public place, such as an airport, government
building, public school, or amusement park. The magnetic screening
systems 100 are used to ensure safety for the public. The magnetic
screening systems can identify magnetic objects, referred to herein
as "target objects" on a person, as understood in the art. In this
example embodiment, each magnetic screening system 100 includes the
portal 105 (more generally referred to herein as an arrangement of
gradiometers), an operator station 110, and a screening computer
115.
[0046] A traditional magnetic screening system configured with a
portal is capable of processing approximately 100-200 persons per
hour. Through use of the principles of one or more aspects of the
present invention, the number of persons the portal metal detectors
can process increases to between 600 and 700 persons per hour or
more. There are many reasons for the increase in processing rate.
One reason is that the people passing through the portals 105 do
not need to empty their pockets of metal objects. In some
embodiments, the portals 105, screening computers 115, operator
stations 110 can detect, identify, and classify metal objects being
carried by people. For example, processing in the magnetic
screening systems 100 can determine that target objects are a cell
phone, loose change, brassier underwire, shoe support, wristwatch,
hair clip, or other non-dangerous weapon in an automated manner and
exclude these objects from causing an alert signal to an operator.
Additionally, dangerous weapons, such as knives or guns, can be
detected, identified, or classified through use of the processing
according to the principles of the present invention.
[0047] Another reason for the higher processing rate of people
passing through the magnetic screening systems 100 is due to a low
rate of false detection. The reason for a low rate of false
detection is because the magnetic screening systems 100 can remove
background offset before, during, or post measurement acquisition.
The magnetic screening systems 100 can also perform calibration
during measurement, optionally at a high rate, which improves
accuracy of the measurements. Further, because of the removal of
background offset from the measurements, persons who dwell in an
"influence zone" after passing through the portal metal detector
105 does not adversely affect measurements of the next person to
pass through the portal 105.
[0048] Another reason for the portal metal detector system 100 is
able to perform processing at a higher rate over traditional
processing is because it can force resettling of gradiometers in an
arrangement of gradiometers that are inside the portal 105. Typical
portal metal detectors rely on "drift to zero" for resettling
before taking the next measurement without an offset. In the
magnetic screening system 100 according to the principles of the
present invention, the gradiometers can be driven to their
"resettling" state in a forced manner.
[0049] Because of the technical advantages provided by the
principles of the present invention, the magnetic screening systems
100 can process a single person or multiple persons passing through
at the same time, which can save a tremendous amount of time. It
should be noted that the 600-700 person per hour processing rate
does not include the additional gain of having multiple persons
pass through the portal 105, for example, at the same time.
[0050] Also, because of the ability to reduce offset, large
ferromagnetic sources of disturbances are allowed to pass through
the portal 105 that are typically unable to pass through portal
metal detectors. For example, wheelchairs can pass through without
deleterious effect on the performance of the magnetic screening
system 100. This ability to process wheelchairs includes an ability
to allow the wheelchair to pass through the portal 105 or be nearby
or within a given area inside a volume of interest 75 defined by
the portal 105.
[0051] In addition, the gradiometers used by the magnetic screening
system 100 inside the portal 105 are electromagnetically passive.
This means that they do not project magnetic fields which are known
to cause problems for persons using medical equipment that is
sensitive to magnetic fields, such as a cardiac pacemaker. Other
sensitive electronic equipment is also be able to pass through
without any adverse effect since the gradiometers can be passive
gradiometers.
[0052] At the network level, a management computer stores
information and data common to all magnetic screening systems. The
management computer may have or receive common information and data
from clusters or individual magnetic screening systems and share
the stored information or data with other clusters or individual
magnetic screening systems via a communications network, such as
the Internet. An example of data stored in the common database is
signature data for target objects, which can be any ferromagnetic
objects (e.g., gun, knife, cell phone, keys). An example of
information stored in the database includes an indicator (e.g.,
graphic or photograph of a gun) or description associated with the
signature.
[0053] At the system level, the principles of the present invention
may be used to detect, identify, or classify ferromagnetic objects
through use of an arrangement of gradiometers (i.e., magnetic
sensors). The architecture of the magnetic screening systems allows
their response to a particular set of target objects to be tailored
in accordance with sets of target objects that operators choose to
include or exclude. These target objects may be objects that are
threats to safety, such as guns and knives, or any other
ferromagnetic object that may pose a risk of injury. The magnetic
screening system may use a variety of signal processing techniques
in combination to detect, localize, identify, or classify the
ferromagnetic objects. The technique for detecting the
ferromagnetic objects may be completely electromagnetically
passive, in contrast to similar devices that generate an active
magnetic field.
[0054] At the mode of operation level, each gradiometer utilizes at
least three vector magnetometers of any underlying technology to
measure a magnetic field gradient. Typically, the magnetic field
being measured is the earth's magnetic field, but may be a
different, man-made or biologic magnetic field, such as found in
medical applications. The multi-modal gradiometer includes signals
and processing methods to provide high rate, high dynamic range
signals. The multi-modal gradiometers include modes, such as
measurement mode, automatic offset reduction mode, calibration
mode, tracking mode, self-test mode, automatic alignment mode, and
diagnostic mode, and may also include a method to eliminate
hysteresis in the measurements.
[0055] At the gradiometer design level, the principles of the
present invention optimize a response of the gradiometers. In
particular, the principles of the present invention relate to a
method and corresponding apparatus for optimizing near and far
field responses of a plurality of vector magnetometers in the
gradiometers so as create a field response pattern optimized for a
particular sensing application. The method utilizes the individual
magnetometers applied in a certain arrangement to shape the field
response. Additionally, data from magnetometers in the preferred
arrangement is processed so as to enhance shaping of the field
response pattern. In particular, the field response pattern can be
used to reduce effects of extraneous and background signals that
may be undesirable or overwhelming to the signal of interest.
[0056] The description that follows provides details of the
multiple aspects of the principles of the present invention.
[0057] The magnetic screening system 100 detects, selectively
identifies, localizes, or excludes an item from a volume of
interest. This is accomplished by utilizing an arrangement of
high-order magnetic gradiometers (i.e., at least three
magnetometers 400) used in a collective manner combined with global
processing of all the arrangement elements to detect, localize, or
identify the target objects. In other embodiments, a gradiometer
may include one or two magnetometers, and processing may allow an
arrangement of gradiometers to be used as described in reference to
gradiometers with three magnetometers or a reduced set of
processing may be employed for more limited detection of target
objects. The detection method may be entirely electromagnetically
passive.
[0058] One advantage resulting from the use of high order
gradiometers is that the effect of nearby `hard iron` and moving
ferromagnetic objects can be suppressed. The use of a totally
passive sensor system allows for discreet screening of a volume of
interest without the knowledge of the person or persons passing
through the volume of interest. The passive embodiment of the
magnetic screening system also eliminates a possibility of
interference with life sustaining devices, such as cardiac
pacemakers, and does not interfere with the operation of personal
electronic devices, such as cell phones. Another advantage of this
aspect of the present invention is that signal processing elements
tasked with processing measurements by the gradiometers can be
remotely located anywhere there is a suitable communications
connection, preferably digital, to the arrangements of
gradiometers.
[0059] An apparatus embodiment comprises arrays of high order
magnetic gradiometers located near to or remote from a
signal-processing element. The signal-processing element may
control and process a single gradiometer or arrangement or a number
of gradiometer arrangements.
[0060] In use, a preferred embodiment of the system level aspect of
the present invention is operated by a central workstation, where
an operator can respond to automatic announcements the system
generates. This workstation may be physically attached to the
array(s), co-located with the array(s), or located remotely from
the array(s).
[0061] In broad terms, a preferred embodiment of the system may be
implemented in the form of a plurality of arrangements of
gradiometers located at a controlled location (e.g., airport)
having at least one volume of interest connected to an operator
workstation or workstations. A signal-processing element is
co-located with each of the arrangements of gradiometers to provide
detection, location, or classification of target objects.
[0062] In broad terms, an embodiment of a signal processing method
according to a system level aspect of the invention includes the
following steps:
[0063] 1. The arrangement of gradiometers, when operating, collects
data from each of the gradiometers by converting analog field
gradient measurements into a digital measurement utilizing an
Analog-to-Digital (A/D) converter. The digital field gradient
measurements are sampled at a rate greater than 100 Hz or more
preferably greater than 1 kilohertz to maintain the frequency
content of the sampled signals. The measurement sampling occurs
when the gradiometer array has been turned on to begin a sampling
event.
[0064] 2. The data from each magnetic gradiometer is processed to
improve Signal-to-Noise Ratio (SNR) by successively applying a
frequency-enhancing filter and then an optional matched signal
filter. The matched signal filter may include a filtering window of
a magnetic dipole moment. The data may also be normalized to
provide a uniform number of samples for succeeding stages of signal
processing.
[0065] 3. After the SNR enhancement and filtering is performed, the
data set from a gradiometer is then processed for determination of
magnetic features. The magnetic features may include magnitude,
phase, and timing relationships, along with frequency spectra or
other fundamental signal characteristics well known in the art. The
collected data represents a time phased scan of an object as it
passes by the arrangement of gradiometers. Conversely, a fixed test
volume may be scanned by moving the arrangement of gradiometers
past the volume of interest without any loss of generality of the
technical features disclosed herein.
[0066] 4. Once each magnetic field reading has been processed at
the gradiometer level, each gradiometer in the arrangement presents
its results and/or raw data to a collection processor. The
collection processor may first perform a quality check on the data.
At this point, the collection processor utilizes the data from all
the gradiometers in the arrangement to develop an estimate of the
locations, magnitudes, and dipole moments of any objects passing a
threshold for magnitude. The combined data from the arrangement of
gradiometers are averaged to provide a measure of the integrated
background. The measured background is subtracted from each
gradiometer's raw data so as to leave only disturbance data for
further processing. The collection processor also coordinates
timing of the data collection across multiple arrangements of
gradiometers (e.g., multiple portal metal detectors) and provides a
communications node for the arrangements of gradiometers to
communicate with each other or share data or other information
(e.g., photographs corresponding to data).
[0067] 5. The collection processor may utilize all of the
gradiometer data collected to extract further features related to
position, magnitudes, or phases of target objects detected.
[0068] 6. This data set for the collection event is packaged and
forwarded to a computing element dedicated to detection, analysis,
or classification of the target object.
[0069] 7. The signature data collected may be processed through a
plurality of independent processing techniques. A preferred
embodiment utilizes matched filtering, wavelet decomposition, and
soft polynomial decision space boundaries. The results of the
individual classifications may be forwarded to a meta-classifier,
where, in one embodiment, a polynomial Bayesian or other suitable
classifier combines the results to determine a "best" estimate of
the target object's position and classification. Other
meta-classifiers, such as artificial neural networks, may be used
for the final classification step.
[0070] 8. Once the target object is classified, the results are
returned to the operator and optionally transmitted via wire or
wireless network for notifying other systems or personnel. The
results may be graphically displayed on an operator interface, or
otherwise announced to the operator for initiating action.
Additionally, the results may be used to control the state of the
array of gradiometers to prevent further screening from taking
place unless the event is deemed benign.
[0071] FIG. 2 is a network diagram of a magnetic screening system
network 200 that includes a magnetic screening system subnetwork
255 and a management system subnetwork 260. The magnetic screening
system subnetwork 255 includes multiple clusters 50 of magnetic
screening systems 100. In one embodiment, the management system
subnetwork 260 includes a management station 215, signature
database server 220, and network time server 225.
[0072] The magnetic screening system 100 includes respective
arrangements of gradiometers 230 and an arrangement processor, also
referred to herein as a portal Central Processing Unit (CPU) 240.
In the embodiment of FIG. 2, the arrangements of gradiometers 230
are configured in frames defining the portals 105. The gradiometers
230 and arrangement processor 240 communicate with each other via
an intra-portal communications bus 235. Any standard or
application-specific communications protocol may be used for
communications over the intra-portal communications bus 235.
[0073] The portal CPUs 240 can communicate with one another and the
operator station(s) 110 via an inter-portal communications bus 245.
The screening computers 115 communicate with the portal CPUs 240
and operator station(s) 110 via the inter-portal communications bus
245. It should be understood that one or more inter-portal
communications buses 245 may be used with one or more
communications protocols in any combination. Further, the
inter-portal communications buses 245 may be wire, wireless, or
fiber optic with associated network interface support hardware and
software.
[0074] A network device 205, such as a router, may also be
connected to the inter-portal communications bus 245 within the
cluster 50 of magnetic screening systems 100. The network device
205 provides communications services for the operator station(s)
110 or other processor in the cluster 50 to communicate with other
clusters 50 in the magnetic screening system subsystem 255 via an
inter-cluster bus 247 using an appropriate protocol. The
inter-cluster communications bus 247 may be wired, wireless, or
optical.
[0075] The magnetic screening system subnetwork 255 may communicate
with the management system subnetwork 260 via a wide area network
(WAN) 210, such as the Internet. On each side of the WAN 210 is a
network device 205 that communicates over a WAN communications bus
250. The WAN communications bus 250 may use an Internet Protocol
(IP) communications protocol, Voice-Over-IP (VoIP) communications
protocol, or any other suitable protocol for communicating
information or measurement data between the magnetic screening
system clusters 50 and the computing devices 215, 220, or 225 in
the management system subnetwork 260 over a management system bus
249. It should be understood that the WAN 210 may include packet
switched or circuit switched networks using associated
communications protocols.
[0076] Within the management system subnetwork 260, the management
station 215 provides many services. First, the management station
215 provides high-level communications with the operator stations
110. Second, the management station 215 may provide high level
processing, such as analysis on information or data captured by one
or more magnetic screening systems 100. Third, the management
station 215 maintains a central database (not shown) of signatures
of target objects measured in a controlled environment or measured
by a magnetic screening system 100 in the field. The signatures may
be maintained in the signature database server 220 or other
server(s). Fourth, the management station 215 may assist in
uploading and downloading database of target object signatures from
and to the operator stations 110. The uploading and downloading
processes allows the magnetic screening systems 100 to share
signature measurements of target objects stored in their local
databases (not shown) and use all the signatures in the central
database stored in the signature database server 220 for field
measurements.
[0077] The network time server 225 provides a means of
synchronizing and tagging screening events in a uniform fashion
across all the elements of the system. The preferred embodiment of
the system provides for the common time base across all elements to
produce more value in the records of incidents and signatures
gathered during operation. The network time server is not required
for the system to operate in other embodiments.
[0078] FIG. 3 is a block diagram of the magnetic screening system
network 200 of FIG. 2 with indications of signals on the data
buses. Starting at the gradiometers 230, the intra-portal
communications bus 235 carries sensor data 315 from the
gradiometers 230 to the portal CPU 240. A portal camera 305 may be
employed at a magnetic screening system 100 and transmit camera
images 310 to the portal CPU 240. The camera images 310 may be
images of people passing through the respective portal metal
detector 105. The portal CPU 240 may associate the camera images
310 with respective target object signatures measured for use in
later identification of persons carrying a suspected dangerous
target object.
[0079] The portal CPU 240 sends the sensor data 315, possibly in a
processed form 315', via the inter-portal communications bus 245 to
the screening computer 115 optionally with the camera images 310.
The screening computer 115 forwards screening results 320 to the
operator workstation 110 for providing an indication of whether or
not the person passing through the portal 105 of the magnetic
screening system 100 is carrying a target object. The screening
computer 115 may provide the screening results 320 in the form of
signature data, identity of the target object (e.g., gun, knife,
cell phone, keys), or classification of the target object (e.g.,
dangerous object, non-dangerous object, unknown object).
[0080] In this embodiment, the screening computer 115 also
communicates with the management station 215 via the network paths
245, 247, 249, 250 described above. On the network paths, the
screening computer 115 and management station 215 communicate
queries and responses 325 and optionally other information or data.
The screening computer 115 may communicate alarm files 330 and
related information or data with the signature data server 220. The
screening computer also communicates time service 335 with the
network time server 225.
[0081] It should be understood that the embodiments of FIGS. 2 and
3 are merely examples of possible configurations of processing
associated with arrangements of gradiometers 230. For example, the
portal CPU 240, screening computer 115, and operator workstation
110 may be combined into a single computer system. Similarly, the
management station 215, signature data server 220, and network time
server 225 may also be combined into a single computer system. In
addition, the portal 105 in FIGS. 1 and 2 include gradiometers 230
arranged in two vertical columns or arrays.
[0082] It should be understood that the gradiometers 230 may be
configured in any other arrangement(s) that defines at least one
boundary of a volume of interest, e.g., a pathway through which a
person walks to be screened for target objects. For example, an
arrangement of gradiometers 230 may define one boundary of a volume
of interest and a wall or fixture can define a second boundary of
the volume of interest, where the first and second boundaries
define a pathway. For example, in one embodiment, one vertical
column of the portal 105 is populated with gradiometers 230. In
another embodiment, a single gradiometer may define a boundary and
in a manner similar to other arrangements of gradiometers 230 may
be used to take measurements.
[0083] FIG. 4A is a block diagram of an individual gradiometer 230.
In one embodiment, the gradiometer 230 includes a processor 410,
such as a digital signal processor (DSP), to communicate with at
least three magnetometers 400a, 400b, and 400c (collectively 400)
using an intra-gradiometer bus 230. The gradiometer DSP 410 also
communicates with the arrangement processor 240 over the
intra-portal bus 235.
[0084] The processor 410 may be analog, substantially digital, or
completely digital depending on various factors for design
implementation. It should be understood that supporting circuitry
(not shown) which allows the gradiometer processor 410 to
communicate with the magnetometers 400, may also be employed.
Examples of other circuitry include memory, registers, analog
circuits, or supporting processors. Further, the gradiometer
processor 410 may include multiple gradiometer processors 410 for
parallel processing purposes.
[0085] In addition to a uniaxial layout of the magnetometers 400,
the magnetometers may also be positioned offset in one or more axes
from each other for purposes of achieving particular orientations
of sensitivity for the gradiometer 230.
[0086] In other embodiments, the gradiometers 230 in an arrangement
may not have an "on-board" processor 410. In such a case, the
arrangement processor 240 performs functions described herein in
reference to the gradiometer processor 410.
[0087] FIG. 4B is a electrical schematic diagram of an example
embodiment of the gradiometer 230. The gradiometer processor 410
communicates with a field programmable gate array (FPGA) 412,
which, in turn, communicates with the magnetometers 400.
[0088] Each magnetometer 400 in this embodiment includes the same
circuitry, so the following description applies to each of the
magnetometers 400. At the front end of the magnetometers 400 is a
digital-to-analog converter (DAC) 415. In one embodiment, the DAC
415 includes a left (L) output channel 417L and a right (R) output
channel 417R (collectively 417). Both of the output channels 417
connect to an input of a magnetic sensor 420, which senses a
gradient in a magnetic field. The output of the magnetic sensor 420
is connected to an amplifier 425, such as a sense amplifier capable
of amplifying very low level voltages without adding significant
noise in the amplifying process. The output of the amplifier 425 is
an analog-to-digital converter (ADC) 430. The ADC 430 provides a
digital output to the FPGA 412, which, in turn, provides a digital
signal to the gradiometer processor 410.
[0089] In operation, the gradiometer processor 410 issues a command
signal to the FPGA 412 for commanding one or more of the
magnetometers 400. The command signals output by the processor 410
may correspond to a mode of operation of the gradiometer 230,
including measurement mode, background offset reduction mode,
calibration mode, self-test mode, automatic alignment mode,
diagnostic mode, or tracking mode. These modes are described in
detail below beginning in reference to Table I and FIG. 9.
[0090] The FPGA 412 transmits the command received from the
processor 410 to the corresponding magnetometer(s) 400. As
illustrated, an offset control signal 435 output by the left
channel 417L of the DAC 415 indicates that the processor 410 is
commanding the magnetometers 400 to operate at least part time in
background offset reduction mode. As also illustrated, a bridge
drive signal 440 output by the right channel 417R of the DAC 415
indicates that the processor 410 is commanding the magnetometers
400 to operate at least part time in measurement mode or another
mode that causes the magnetic sensor 420 to take measurements. For
example, both signals 435 and 440 are used during background offset
reduction mode since one portion of the mode is a measurement
period (bridge drive signal 440) of a background disturbance
causing a magnetic offset of the magnetometer 400, and another
portion of the background offset reduction mode is an offset
reduction period (offset control signal 435). Further discussion of
the background offset reduction mode and other modes is presented
below in reference to FIGS. 9 through 12C.
[0091] FIG. 5 is a detailed electrical schematic diagram of the
magnetometers 400 of FIG. 4. In this embodiment, the magnetic
sensor 420 includes a magnetic transducer 500, which is in the form
of a traditional Wheatstone bridge. The transducer 500 includes two
legs of static elements 505a and two legs of variable elements
505b. In the case of a magnetic transducer, the static elements
505a and variable elements 505b are the same elements, but a
magnetic shielding is placed over the static elements so that
external magnetic disturbances influences only affect the
unshielded variable elements 505b. The type and arrangement of the
variable elements of the magnetometer is not material to the
operation of the gradiometer. In other embodiments, the
magnetometer bridge may have one or multiple active sensing
elements. Such other arrangements are well known to practitioners
of the art in magnetometers.
[0092] In operation, the DAC 415 presents the bridge drive signal
440 to a bridge driver amplifier 515, which may produce a bridge
drive current 520 whose level is set, in part, by a compensation
resistor 510 providing a voltage V.sub.Rc at a junction with the
bridge driver 515 negative input. The bridge driver 515 can then
correct error in the bridge drive current 520. Thus, the transducer
500 produces a differential voltage output Vb 525 that is amplified
by the amplifier 425 whose output is sampled by the ADC 430.
[0093] The offset control signal 435 is presented to an offset
control circuit 535, which includes a drive amplifier 540 and
magnetic field generator 540 for producing a magnetic field that
drives offset of the magnetic transducer 500 to a "reduced" state.
The reduced state is a state in which background offset caused by
large ferromagnetic elements in the volume of interest or within a
zone of influence of the gradiometer is cancelled from the magnetic
transducer 500 (i.e., background offset reduction mode).
[0094] It should be understood that the magnetic transducer 500 may
be other forms of magnetic transducers known in the art adapted to
detect magnetic fields as described herein. For example, the
magnetic field may be the earth's magnetic field (45,000 nTeslas)
and fields of target objects (100 nTeslas or less). The magnetic
fields may also be much larger, as in the case of medical sensing
applications. Therefore, the offset control circuit 535 is
preferably capable of producing a magnetic field over a wide range
or the offset control circuit 535 is specially designed for
particular sets of applications.
[0095] FIG. 6A is an electrical schematic diagram of a calibration
circuit 600 that may be used to calibrate the magnetic sensors 420
in the magnetometers 400. The calibration circuit 600 includes a
precision current source 605 with feedback circuit 607 to produce a
precision calibration drive current 608. The precision calibration
drive current 608 travels on a pair of crossing traces 610 to a
magnetic field generator 615, which may be a simple wire loop or
more complex magnetic field generator 615. The magnetic field
generator 615 produces a precise, known, calibration magnetic field
617 in magnetic relationship with the magnetic sensor 420 of the
magnetometer 400 and causes a measurable response by the
magnetometer 400. The output Vb 525 of the magnetic sensor 420 may
be fed to a controller 625, which drives an offset correction drive
circuit 635 and magnetic field generator 640 for producing an
offset correction magnetic field 645 for correcting the offset
caused by the calibration circuit 600.
[0096] The offset correction circuitry 635, 640 may be the offset
control circuit 535 used for calibration in this case, or may be a
separate circuit. In either case, any number of signals used to
correct for the measured response of the magnetometer 400 can be
used as a calibration metric, which may be scaled or offset in
real-time or post-processing.
[0097] The controller 625 may be an analog controller or a digital
controller. In the case of a digital controller, it may be
implemented in the gradiometer processor 410 or in a separate
digital processor. In either of the digital processor cases, the
magnetic sensor output Vb 525 is a sampled form provided by the ADC
430 that is sampled according to techniques well known in the art.
Further, the controller 625 may use any applicable control law,
such as a proportional, integral, differential (PID) control law.
Since digital controllers can be updated in software, the
controller 625 is preferably a digital controller.
[0098] Continuing to refer to FIG. 6A, a calibration cycle includes
measurements of at least three calibration points in one
embodiment. A timing sequence is listed at the input to the
calibration precision current source 605. A first calibration point
(Cal A) does not use the current source 605 to produce a drive
current 608. Instead, the first calibration point is a measure of
the magnetometer 400 immediately after background offset reduction
mode has removed offset from the magnetometers 400 and calibration
measurements of Vb 525 are taken and averaged over 100 msec, for
example. A second calibration point (Cal B) applies a low level
voltage to the current source 605 to produce a low level drive
current 608 for 100 msec, for example, during which measurements
are taken and averaged. A third calibration point (Cal C) applies a
high level voltage to the current source 605 to produce a high
level drive current 608 for 100 msec, for example, during which
measurements are taken and averaged.
[0099] FIG. 6B is a plot 650 of results of the calibration
measurements based on the example calibration cycle just described.
The plot includes three points, 655a, 655b, and 655c corresponding
to calibration points, Cal A, Cal B, and Cal C. A linear fit,
polynomial fit, or other suitable technique may be employed to
determine a calibration curve 660. The gradiometer processor 410
uses the calibration curve 660 to improve accuracy of measurements
of gradient magnetic fields by the magnetometers 400 in a manner
well known in the art.
[0100] At the gradiometer design level, the principles of the
present invention provide a method or corresponding apparatus to
arrange and process signals from an array of multiple magnetic
sensing devices (e.g., gradiometers) to control near and far field
responses of the arrangement. The arrangement may then be used to
sense signals (e.g., local disturbances in the earth's magnetic
field), which are near the magnetic sensing devices in a volume of
interest, while excluding signals that are in a particular
direction away from the volume of interest.
[0101] One advantage of this aspect of the present invention is
that the user of the magnetic sensing device does not need to
shield or otherwise prevent extraneous signals from interfering in
the measurement of the desired signals or target objects. By
improving the directionality of the magnetic sensing device,
expense and complexity of otherwise eliminating external signals or
sources of interference is saved.
[0102] Another advantage of this aspect of the present invention is
that the improved directionality of the magnetic sensing device
allows the usage of the magnetic sensing device in an area
otherwise unsuitable for magnetic measurements. This is
accomplished by automatically excluding background signals normally
encountered during measurement, such as signals from light
fixtures, power lines, and ferromagnetic objects in close proximity
to the magnetic sensing device.
[0103] The principles of this aspect of the present invention may
be implemented in the form of a plurality of magnetic sensors, such
as vector magnetometers, to form a gradiometer. A preferred
embodiment of the gradiometer does not require a specific number of
magnetometers or a specific technology of magnetometers. Using the
technique described herein can optimize the gradiometers.
[0104] The magnetometers may be Superconductivity Quantum
Interfering Devices (SQUID), Anisotropic Magnetoresistive (AMR) or
Giant Magnetoresistive (GMR) sensors, spin tunneling devices, or
simply a wire loop or solenoid for the magnetic field detector.
[0105] Once a gradiometer has been devised, the magnetometers can
be optimized by application of a technique for determining distance
and weighting of the magnetometers. The distance between the
outermost magnetometer pair is selected based on the application in
which the gradiometer is to be used. For example, compact baselines
may be preferred for "close-in" medical magnetic field
measurements, while large baselines may be preferred for
large-scale field measurements, such as the location of large
ferromagnetic objects or deposits on the ocean floor. The technique
of optimization does not require a specific baseline for successful
application.
[0106] The development of a unidirectional gradiometer entails
positioning at least one magnetometer between the outermost
magnetometers on a selected baseline. One common configuration in
the art is called a second order gradiometer, discussed below in
reference to FIGS. 7C and 7D. The second order gradiometer rejects
only the signals along the perpendicular from the baseline between
the two magnetometers.
[0107] A digital signal processor or other digital computing device
may be employed to operate a preferred embodiment of the
gradiometer. Analog processors are also possible once the
magnetometers' positions and weights have been set; however, an
analog processor has a drawback in that it is not easily adjustable
once fabricated. The proposed digital gradiometer has the advantage
that the pattern of sensitivity can be adjusted as required after
the fabrication through digitally updating the weights associated
with the respective magnetometers that are stored in the digital
processor or memory associated therewith.
[0108] In broad terms, a preferred embodiment of the gradiometer
includes at least three magnetic sensors in linear alignment on a
common axis. A sensing axis and polarity of each magnetometer is
aligned with that of the other magnetometers along the common axis.
The positions of the outermost magnetometers are selected based on
criteria for the application of use. The position of the
magnetometer(s) between the outermost magnetometers is/are
calculated utilizing an embodiment of a magnetometer
positioning/weighting optimization method.
[0109] In broad terms, an embodiment of the magnetometer
positioning/weighting method includes the following steps:
[0110] 1. Once a baseline has been selected, the number of "inner"
magnetometers is chosen depending upon the user's desire for
directivity or other technical criteria.
[0111] 2. The position of the magnetometers is established using
any applicable numerical method.
[0112] 3. The position and vector magnitude of the interfering and
desired magnetic signals are modeled, optionally utilizing the same
numerical method as in Step 2 immediately above.
[0113] 4. An equation for received magnetic signal combinations is
input to a numerical optimizer, such as least squares or successive
approximation optimizer, as is common in the art.
[0114] 5. Constraints of the method are entered into the
optimization equation. The constraints include:
[0115] (a) The sum of the signal weightings must be zero.
[0116] (b) The received interference signal must be zero or reduced
to a desired value.
[0117] (c) The received signal of interest must be maximized.
[0118] 6. The optimizer then changes the signal weighting while
moving the inner magnetometers until a satisfactory solution is
found. The values of the weightings are not constrained to
positive-only or negative-only values. The values can be continuous
or discrete, positive or negative, so long as the conditions of
Step 5 above are met.
[0119] 7. Once the numerical optimizer has reached a level of being
sufficiently close to the signal goals, the optimization process
can be stopped, and the calculated values can be used for
construction of the magnetic field gradiometer(s).
[0120] FIG. 7A is a diagram of a gradiometer 230 with magnetometers
400a, 400b, and 400c positioned using the process just described.
In one embodiment, the two outer magnetometers 400a and 400c are
arbitrarily positioned, and at least one other gradiometer 400b is
positioned relative to the outer two magnetometers 400a and 400c.
Unequal weights are calculated for the magnetometers that the
processor 410 uses to scale outputs of the magnetometers 400. The
processor 410 combines the scaled outputs to orient a direction of
gradiometer sensitivity, represented by a sensitivity lobe 700a,
toward a volume of interest, which is to the left in the example of
FIG. 7A.
[0121] In one embodiment, the positions of the magnetometers 400
are determined, optionally along a single axis, according to a
number of parameters, such as available size of a structure into
which the gradiometer 230 is to be deployed. To improve sensitivity
of the gradiometer, the outer two magnetometers 400a and 400c are
preferably positioned as far apart as possible within a given
constraint. The middle magnetometer 400b in a portal metal detector
application is positioned closer to the outer magnetometer 400c
farther away from the volume of interest without touching that
outer magnetometer 400c.
[0122] In this embodiment, after the positions of the magnetometers
400 are set, the weights are calculated preferably using
deterministic mathematical techniques, such a through use of a
least squares optimization technique. For flexibility of design,
the weights can be non-integer weights. Use of non-integer weights
allows the sensitivity lobe 700a to be optimized for use in a given
application. In the case of the gradiometer 230 employing a digital
processor 410, the digital processor 410 can adjust the weights
during operation, thereby changing the characteristics of the
sensitivity lobe 700a. Adjusting the weights to change
characteristics of a sensitivity lobe can be compared to changing
phase delays in a phased array radar system to effect the steering
of the sensitive lobe in a desired direction.
[0123] FIG. 7B is a detailed plot of the sensitivity lobe 700a. The
positions of the magnetometers 400a, 400b, and 400c are represented
by lower case `x` in the plot. As expected, sensitivity is much
higher closer to the magnetometers 400 than farther away from the
magnetometers. Of particular note, the sensitivity lobe 700a does
not extend to the right of the magnetometers 400 based on the
spacing and weighting.
[0124] FIG. 7C is a diagram of the gradiometer 230 in which the
magnetometers 400 are positioned equally distributed along a single
axis and having equal weights assigned thereto for scaling their
outputs. A corresponding sensitivity curve 700b, which is
peanut-shaped, indicates that the sensitivity of the gradiometer
230 extends into a volume of interest (to the left) and also into a
volume that may not be of interest (to the right). As a result, a
magnetic disturbance that is outside the volume of interest, such
as a wall, other portal metal detector, or other machine, can
influence measurement results in a detrimental manner.
[0125] FIG. 7D is a detailed plot of the sensitivity lobe 700b
corresponding to the equally-spaced, equally-weighted embodiment of
FIG. 7C.
[0126] FIG. 8 is a vector diagram which, along with the equations
that follow, provides a more detailed analysis of the process used
to make and use the gradiometer 230 of FIG. 7A.
[0127] The magnetic field produced by a single magnetic dipole is
given by:
B ( r i ) = .mu. 0 4 .pi. ( 3 [ M ( r i ) ] r i 4 - M r i 3 ) E1
##EQU00001##
[0128] Where r.sub.i is the vector position of the dipole 800
relative to the sensitive elements 400a, 400b, 400c making up the
preferred second order gradiometer. M is the vector dipolar
magnetic moment of the target object. Complex objects that make up
the targets of interest may be viewed as a collection of dipole
objects with their magnetic fields superimposed upon each other
without any loss of generality. The optimization method holds for
complex objects as well as simple objects in this formulation.
[0129] The sensed magnetic field at each magnetometer is
proportional to the area, A, of the magnetometer and the
orientation of the sensitive axis, V, to the dipole moment's
principal axis. The responsitivity constant .epsilon. of the sensor
material in units of changed characteristics (resistance, voltage,
current) per unit area is also included to properly scale the
expected output of the magnetometer. The response of the individual
magnetometer is then:
Q.sub.i=.epsilon.AB(r.sub.i)V.sub.iE2
[0130] The equation indicates the measured signal is proportional
to the collection area multiplied by the sensitivity of the
materials used and the vector product of the field strength and
direction of the target object relative to the sensitive axis of
the magnetometer.
[0131] The preferred gradiometer configuration 230 includes three
magnetometers, each having a different response to the target
object due to differing geometries and aspects relative to each
other and the target object. The summary response of the k-th
individual gradiometer is made up of the weighted sum of the
individual responses of the n magnetometers:
T k = i = 1 n a i Q i E3 ##EQU00002##
[0132] T.sub.k is the output of the magnetic field gradient sensed
by the gradiometer 230. The same equation is used as the basis for
the optimization of the a.sub.i coefficients allowing the
practitioner to adjust the response lobes of the gradiometer as
described elsewhere in this document.
[0133] Turning now to operation of the gradiometers 230 and
arrangement of gradiometers 230, the many processors associated
with the gradiometers 230 cause them to operate in multiple
modes.
[0134] At the multi-modal operational level, the present invention
includes an implementation of a gradiometer having at least three
vector magnetometers. In a preferred embodiment, the magnetometers
are each independently controlled and measured. Each magnetometer
may include independent biasing, control, and measurement circuits.
Offset of the magnetometers is controlled without use of an
additional magnetometer specifically designed to do so. Common mode
coupling is eliminated by independence of the individual
magnetometers in preferred embodiments. The effect of the
independence allows adjustment and refinement of the overall
gradiometer output without undesirable effects of convolving the
errors of the magnetometers together.
[0135] The magnetometers within the gradiometer can be switched
between an active measurement mode and a non-active mode, where the
non-active mode is also referred to herein as a "background offset
reduction mode." During the non-active mode, correction for a
plurality of errors common to vector magnetometers can be
accomplished by the gradiometer or processor(s) associated
therewith.
[0136] One advantage of the multi-modal aspect of the present
invention is that the gradiometer performance can be continuously
maintained against drift and errors induced by changing
environmental conditions. Another advantage of the multi-modal
aspect of the present invention is that, because of the continuous
adjustments against drift and errors, the gradiometers can produce
data at high rates. Common gradiometers produce data at 10 to 20 Hz
rates. The gradiometer of the present invention can produce rates
up to 50,000 Hz with no loss of dynamic range or accuracy.
[0137] The gradiometer 230 at its core comprises at least three
vector magnetometers 400. Each magnetometer 400 may have a bias
source driven by a controlled Digital-to-Analog (D/A) converter, an
Analog-to-Digital (A/D) converter for the measured magnetic field,
and digitally-controlled support circuitry for offset adjustment,
typically by another, independent, digital-to-analog
conversion.
[0138] In use, a preferred embodiment of the multi-modal
gradiometer is operated by a Digital Signal Processor (DSP) element
that can process the data from the magnetometers, provide offset
estimations, and perform other functions for successful operation
of the circuit.
[0139] Table I provides a listing of the multiple modes and a
definition corresponding to each:
TABLE-US-00001 TABLE I Mode Definition Measurement mode Use of
gradiometers 230 to measure a volume of interest for target
objects. Background offset Use of gradiometers 230 to measure a
background reduction mode offset and processing applied in either
real-time or post-processing to remove the background offset from
measurements. Calibration mode The processor 410 applies a magnetic
field generated locally at the magnetometers 400 and measures a
reaction by the magnetometers 400 to determine a calibration curve
for compensating measurements made by magnetometers 400. Self-test
mode The processor 410 puts components of the gradiometer in states
to compare measured performance of the components in those states
against specified performance in those states to determine
operational readiness. Automatic alignment The processor 410
captures and averages mag- mode netic field strength over a long
duration while compensating for background disturbances to
calculate the alignment of the gradiometers 230 relative to the
earth's magnetic field or other magnetic field providing a common
influence on the gradiometers 230. Relative orientations of
gradiometers 230 can be determined and used during measurements.
Diagnostic mode The process 410 captures and outputs the measured
field strengths by each magnetometer in an unaltered state (i.e.,
raw measurement data).
[0140] FIG. 9 is a timeline 900 of an example of multiple modes of
operation of an arrangement of gradiometers 230. The timeline 900
includes alternating modes of operation: background offset
reduction mode 905 and calibration mode 910/measurement mode 915.
The timeline 900 can be applied in at least two different ways for
using the gradiometers 230 in a portal metal detector 100
application. The first way the timeline 900 can be applied is to
alternate modes of operation while processing a line of people
proceeding through the portal 105 of the magnetic screening systems
100 of FIG. 1, where background offset reduction mode 905 occurs at
times no one is walking through the volume of interest and
calibration mode 910 and measurement mode 915 occur at times
someone is walking through the volume of interest. The timeline 900
can also apply to a time when someone is walking through the volume
of interest, where the modes 905, 910, and 915 alternate a
selectable number of times. The more times the background offset is
reduced and calibration is performed, the better the accuracy of
the measurements.
[0141] FIG. 10 is a timing diagram 1000 showing relative timing of
portions of the timeline 900 and the bridge bias voltage 440'. In
this embodiment, during opposite periods of a 1 msec frame time
1020 or other rate suitable for the application in which the
gradiometers 230 are employed, a process executing the measurement
mode of operation switches between calibration mode 910 and
measurement mode 915. Alternatively, the process may include
background offset reduction mode 905, which includes set/reset
settle 1010a and active background control 1010b followed by
measurement mode 915. In another embodiment, calibration mode 910
and background offset reduction mode 905 are executed in a
selectable manner on opposite phases of the bridge bias voltage
440' from measurement mode 915. For example, a timing sequence may
be as follows: background offset reduction mode 905, measurement
mode 915, calibration mode 910, measurement mode 915, and repeat.
In another example, background offset reduction mode 905 may not
occur during the same phases. In yet another example, background
offset reduction mode 905 occurs every nth time a measurement mode
915 occurs. In still yet another embodiment, calibration mode 910
occurs every nth time a measurement mode 915 occurs. It should be
understood that any number of combinations of mode sequences can be
employed, which may be dictated by a false alarm rate or other
metric associated with the measurements.
[0142] Continuing to refer to FIG. 10, calibration mode 910
includes measuring positive samples 1005a and negative measurements
1005b corresponding to the bridge bias voltage 410'. Similarly,
measurement mode 915 includes measuring positive samples 1015a and
negative samples 1015b corresponding to the bridge bias voltage
410'. Multiple samples may be measured and averaged or otherwise
computed to determine a noise resistant measurement. In addition,
the background offset reduction mode 905 includes a set/reset
settle time 1010a, during which the magnetometers 400 are driven to
reset, and an active background control time 1010b, during which a
background magnetic field is measured and its effect on the
magnetometers 400 is reduced.
[0143] FIG. 11A is a graphical diagram of an example application in
which an arrangement of gradiometers 230 is deployed to detect
target objects carried by a person. In this example, the person 125
at time T1, walking from left to right, approaches the portal 105
of the magnetic screening system 100. During this time (T1), the
magnetic screening system 100 operates in the background offset
reduction mode 905. As the person 125 approaches the portal 105,
the person 125 passes a first pair of optical sensors 1105a that
senses the person 125 disrupting an associated optical beam 1110a.
In response, the magnetic screening system 100 in communication
with the first pair of optical sensors 1105a exits background
offset reduction mode and enters measurement and calibration modes
910, 915, during time (T2).
[0144] During time T2, the person 125 passes through the volume of
interest 75 whose boundaries are defined on both sides, in this
embodiment, by the gradiometers 230 deployed in the vertical
columns on either side of the portal 105. The sensitivity lobes
700A, described above in reference to FIGS. 7A and 7B, extend
through the volume of interest 75 and enable the gradiometers 230
to detect any ferromagnetic objects being carried by the person
125. The person 125 continues to a second pair of optical sensors
1105b having its own optical beam 1110b, which is interrupted as
the person 125 passes. Upon notification that the person 125 has
exited the volume of interest 75, the magnetic screening system 100
again returns to background offset reduction mode 905 in time
T3.
[0145] It should be understood that the optical sensors 1105 are an
example of sensors that can be used to detect when the person 125
is approaching or leaving the volume of interest 75. Motion
detectors, active floor mats, or the magnetic screening system 100
itself may also be used to detect positions of the person relative
to the volume of interest 75. In other embodiments, the system 100
may be operated without sensors for detection of entry of a person
in the volume of interest 75. For example, an operator 130 (FIG. 1)
may trigger measurement mode to begin. As another example,
measurement mode may occur continuously with calibration mode and
background offset reduction mode being used on a periodic basis, on
an "as needed" basis such as through automatic triggering based on
a metric associated with a gradiometer 230 or magnetometer 400, or
on any other basis.
[0146] Of particular interest in the example application 1100 of
FIG. 11A is that the person 125 does not need to remove his jacket
1115 to allow the magnetic screening system 100 to detect,
identify, or classify any metal objects being carried therein. In
addition, the person 125 does not need to remove from his clothing
any potential target objects, such as a cell phone, loose change,
keys, or other ferromagnetic items, that can be sensed by the
magnetic screening system 100 incorporating the principles of the
present invention on some or all of the different levels described
herein.
[0147] FIG. 11B is a representation of the timing sequence 1100
illustrated in FIG. 11A. T1, T2 and T3 are shown in relation to
multiple timing diagrams corresponding to the time T2 in which the
person 125 is passing through the volume of interest 75. As
indicated, once the person 125 interrupts the first optical beam
1110a, the magnetic screening system 100 begins to take samples and
produce output samples 1105. As described above in reference to
FIG. 10, the bridge bias voltage 440' occurs with (i) calibration
mode 910 and optionally background offset reduction mode 905
occurring during a first period and (ii) measurement mode 915
occurring during a second period.
[0148] At the start of the measurement process, a set/reset pulse
1130a triggers positive to indicate that the measurement mode 915
has begun. During the measurement mode 915 period of the bridge
bias voltage 440', the ADCs 430 (FIG. 4B) samples at a high rate,
such as 10 kHz or higher, in this embodiment, to produce sixteen
samples 1120a during the positive portion of the bridge bias
voltage 440' and sixteen samples 1120b during the negative portion
of the bridge bias voltage 440'. The samples are processed by the
processor 410 and other processors, such as the screening computer
115 (FIG. 2). ADC samples 1115a during the first period of the
bridge bias voltage 440' are considered invalid since they are
taken during a "dead time," which is the period as described above
during which the calibration mode 910 or background offset
reduction mode 905 may be occurring and, therefore, the
measurements do not relate to any target objects. The ADC samples
1115b during the measurement mode 915 may be decreased through
decimation, for example, an output as output samples 1125 from the
ADC 430 (FIG. 4B) for processing by the processor 410.
[0149] The timing diagram 1100 during the measurement period in
time T2 continues and repeats so long as the person 125 in FIG. 11A
is between the first and second pairs of optical detectors 1105a,
1105b, i.e., in the volume of interest 75. If a continuous line of
people are passing through the volume of interest 75, the
measurement mode may continue without interruption, and, in such a
case, it is preferable that the background offset reduction mode
905 occur on a regular basis to ensure full dynamic range of the
magnetometers 400 is maintained so that the gradiometer is
maintained in a linear operating region.
[0150] It should be understood that the example timing diagram of
FIG. 11B may be different in alternative embodiments of the
arrangement of gradiometers 230. It should also be understood that
the time frame 1020 of 1 msec may be faster or slower in such other
embodiments. For example, the gradiometer processors 410 may cause
the gradiometers 230 to sample at a rate greater than 50 Hz. The
gradiometer processors 410 may also cause the gradiometers to
switch between the measurement mode and the calibration mode at a
rate greater than 0.1 Hz. In such an embodiment, the portal metal
detector system 100 calibrates very slowly, as might be the case
during a slowly passing person 125. In other embodiments, it is
preferable to enter calibration mode 910 at least a few times while
the person 125 is in the volume of interest 75. Thus, although
calibrating at a 1 kHz rate may be excessive in some applications,
it provides for a more accurate measurement. However, it should be
understood that overall system error budgets can be achieved with
slower rates, so the rates can be determined depending on the
application or on a case-by-case basis.
[0151] FIGS. 12A-12C are system-level diagrams in which the
assembly of gradiometers 230 can be used to detect target objects,
such as a gun 1250a. In each of these embodiments, the measurement
mode, background offset reduction mode, calibration mode, self-test
mode, automatic alignment mode, diagnostic mode, or tracking mode
may be employed.
[0152] Beginning with FIG. 12A, the target object 1205a, which
produces a particular magnetic field disturbance 1210a due to the
target object's influence on the earth's magnetic field or other
magnetic field, traverses on a path 1215 on a person 125 into a
volume of interest 75 in the portal 105. The gradiometers 230 in
the portal 105 are operating in a measurement mode, and, as
described above, their sensitivity lobes 700a are directed
horizontally into the volume of interest 75 through proper design,
as described above.
[0153] FIG. 12B illustrates the portal 105 with gradiometers 230
operating in the tracking mode. In the tracking mode, pluralities
of the gradiometers 230 generate real-time tracks of target objects
in three dimensions. Thus, instead of only providing horizontal
look angle data by pairs of gradiometers 230, as illustrated in
FIG. 12A, the gradiometers 230 of FIG. 12B sense and report
three-dimensional positional information of the target object
1250a. The portal CPU 240 and screening computer 115 (FIG. 2) can
provide the processing power for operating the gradiometers 230 in
the tracking mode.
[0154] FIG. 12C is a top view of an alternative embodiment of a
magnetic screening system 100 in which an arrangement of
gradiometers 230 is employed to provide three-dimensional tracking
information. In this embodiment, at least three gradiometers 230
are distributed about a volume of interest 75 for detecting,
identifying, classifying, tracking, or combination thereof, target
objects, such as a gun 1205a or cell phone 1205b. The cell phone
1205b has an induced magnetic field disturbance 1210b that is
different from the magnetic field disturbance 1210a produced by the
gun 1205a in the earth's magnetic field or other magnetic field. In
either case, the arrangements of gradiometers 230 may be
distributed in non-portal like fixtures, such as wastebaskets,
planters, vending machines, or other discreet security fixtures, so
as not to be intrusive or noticeable by patrons of a venue, such as
an amusement park, sports arena, airport, government building, or
other place in which detection of ferromagnetic objects is of
interest.
[0155] In addition to providing a tracking mode, the processors
associated with the arrangements of gradiometers 230 can initiate
an auto-alignment process for each gradiometer. The arrangement
processor 240 (FIG. 2) in communication with its respective
arrangement of gradiometers 230 causes the gradiometers 230 to
capture and average magnetic field strength over a long duration
while compensating for background disturbances to calculate the
alignment of the gradiometers 230 relative to the earth's magnetic
field. In this way, the processors, such as the arrangement
processor 240, screening computer 115, or other processor used for
auto-alignment, can determine relative orientations of each
gradiometer 230 to at least one other gradiometer in the system.
Thus, in the scenario 1200a of FIG. 12A, it can be seen that the
gradiometers 230 directly across from one another in the portal 105
are aligned with respect to one another. In the portal 105 used in
the scenario 1200b of FIG. 12B, it can be seen that all
gradiometers 230 are known relative to all other gradiometers 230
in the portal 105. In the scenario 1200c depicted in FIG. 12C,
multiple arrangements of gradiometers 230 know orientations of
other arrangements of gradiometers 230. In this way, tracking
target objects 1205a, 1205b can be done with little set-up time and
added expense.
[0156] In some embodiments, a diagnostic mode is possible in which
the processor 410 (FIG. 4A) associated with the magnetometers 400
can capture and output the measured field strengths by each
magnetometer 400 in an unaltered state. A higher-order processor
such as the portal processor 240, screening computer 115, operator
workstation 110, management station 215, or other processor tasked
to run diagnostic tests can determine whether a failure, error, or
other impairment to normal operations of any gradiometer 230 in the
magnetic screening system 100 or magnetic screening system network
200 (FIG. 2) is a potential to adversely affect measurement.
[0157] FIG. 13 is a time plot of a signature representing a
measurement of a target object, such as a gun 1205a, passing
through a volume of interest 75 as measured by gradiometers 230 in
a portal 105 or other arrangement of gradiometers 230. The
signature 1300 is therefore a representation of a time measurement
of a target object's magnetic field induced by the earth's magnetic
field as measured by the gradiometers 230 as a person 125 carries
the target object through a volume of interest 75 in which the
gradiometers 230 have oriented their directions of sensitivities.
As well understood in the art, the signature 1300 is different for
every target object. Therefore, the signature 1300 for a gun 1205
is different for the signature for a cell phone 1205b, so
processors 410, 240, or other processors adapted to identify or
classify the target objects can do so if (i) the resolution of
measurement is high enough (i.e., the number of samples taken as a
target object passes through a volume of interest is at a high
enough rate) and (ii) the processing is adapted to discern
differences between or among target objects or classifications of
target objects.
[0158] To determine signatures for target objects, there are
several ways to train a magnetic screening system 100. One way to
train the system 100 is to perform a measurement of a target object
in multiple orientations as it progresses through a volume of
interest 75 being measured by an arrangement of gradiometers 230.
Such a case can be done in a controlled manner with a robot moving
a target object through the volume of interest 75 in different
orientations. The signature 1300 is captured and stored in a
database along with an identifier and optional graphical
representation of the target object for use in field deployed
magnetic screening systems 100.
[0159] Another way to train the magnetic screening systems 100 is
for a target object unknown to the system to cause an alert to an
operator of the magnetic screening system 100 that the target
object is unidentified (i.e., its signature is not found in a local
signature database maintained by the magnetic screening system 100,
cluster 50 of magnetic screening systems 100, or network wide
level. In such case, the operator of the magnetic screening system
100 can visually inspect the target object, which may be, for
example, a newly-released cell phone or other ferromagnetic object
newly introduced in the consumer market, for example. The operator
of the system can then add the signature of the target object to a
database at the arrangement of gradiometers level, cluster of
arrangements of gradiometer level, or network wide level, and
associate a description, identifier, or graphical representation
with the signature in the database. In this way, the next time the
same signature is identified by a magnetic screening system 100
having the signature in its database, the system 100 can inform the
operator of what the target object is. If the same target object is
carried through a volume of interest 75 and again not recognized by
the magnetic screening system 100, it may be because the target
object was carried through the volume of interest in a different
orientation from the previous time in which the identity of the
target object was determined and entered into the signature
database(s). In this case, the operator of the magnetic screening
system 100 can choose to enter the new signature to the database
and associate it with the identifier, classification, or graphical
representation previously entered in the signature database. In
this way, the magnetic screening systems 100 can adaptively learn
of new target objects without having to be learned in a controlled
environment by a manufacturer, distributor, or other company
associated with producing, distributing, or selling magnetic
screening systems 100. In some embodiments, signatures measured or
previously associated with known target objects are displayed on
the operator station 110 to allow the operator 130 to make an
informed choice as to whether to add the newly acquired signature
1300 to the database(s).
[0160] In the magnetic screening system network 200 of FIG. 2, the
screening computers 115 may transmit respective local databases of
signatures to the management station 215 for storage of the newly
identified signatures 1300 corresponding to the target objects
periodically or on an event driven basis. For example, periodically
may mean the local database of target object signatures is uploaded
to the signature database server 220 on an hourly basis, daily
basis, weekly basis, or monthly basis. An event driven basis may be
done as a result of detection of a target object, either known or
unknown, an operator request, or upon initiation of a
self-test.
[0161] The management station 215 may also transmit the central
database of signatures stored on the signature database server 220
to the local databases stored at the magnetic screening systems
100, for example, periodically or on an event driven basis. The
event driven basis in this case may be on receipt on an unknown
target object from one of the screening computers 115, an operator
request, receipt of a new signature, a system reboot, or a system
power-up. It should be understood that other events, foreseen or
unforeseen, may also be used as a trigger to either upload or
download signatures of target objects, either previously known or
unknown, for use by the magnetic screening systems 100 to
continually improve on their ability to detect, identify, classify,
or otherwise recognize a target object so as to continue to reduce
a rate of false alarms, which ultimately result sin higher speed
processing of people passing through the volume(s) of interest
75.
[0162] FIG. 14A is a time plot 1400a of a measurement of the target
signature 1300 as it passes through the volume of interest 75. In
this example, a source of background offset affects measurements by
the gradiometers 230. The effect manifests itself in the form of a
slope of the curve over which the target object's signature 1300 is
superimposed. At repeating intervals, the magnetometers 400 in the
gradiometer 230 are reset, as described above, so as to ensure full
dynamic range for the next measurement. A source of background
offset is a wheelchair, for example, that is nearby the arrangement
of gradiometers 230. The period for reset may be when the person
125 crosses through the optical beam 1110a (FIG. 11A) or may be on
a sample-by-sample basis at a 1 kHz interval (FIG. 11B) or other
measurement rate.
[0163] FIG. 14B is a plot 1400b of the measurement curve 1400a of
FIG. 14A with the slope of the curve removed through use of a
background offset reduction process, either in real-time in a
sample-by-sample basis or through use of post-processing. In the
case of post-processing, instead of resetting the magnetometers 400
before each sample measurement is made, a measurement of the
background offset is captured and associated with each sample point
and provided to the arrangement processor 240 for use in
mathematical removal of the background offset. The result is a
curve 1400b with offset removed so that the target signature 1300
is more easily discernable. In addition, the target signature 1300
can be normalized, optionally in amplitude, time, phase, or
combination thereof, for more easily being matched to a signature
in a local signature database or central signature database. It
should be understood that various processing improvements may also
be applied. For example, after normalization, a data reduction
process, encryption process, time-frequency analysis, or other
processing may be employed so as to make further processing, data
sharing, or other use of the data be done in a more efficient
manner.
[0164] FIG. 15 is a block diagram of an example process performed
on the target signature 1300. During field measurements 1505, the
target signatures 1300 are captured by the magnetometers 400 of the
gradiometers 230. The gradiometer processors 410 perform a number
of processes 1510 on the target signatures 1300. The processes 1510
may include a wavelet transform 1520, matched filter 1525, fuzzy
logic 1530, and joint time-frequency analysis 1535. The target
signature 1300 is provided to each of these processes for use in
analysis.
[0165] The wavelength transform 1520 produces a frequency versus
time table or other data representation with magnitudes determined
as a function of frequency and time. The matched filter 1525
compares the target signature to filters with impulse response
possibly matching the target signature, which, when matched,
results in a dipole and indication of where the target object is
located on the person 125 passing through the volume of interest.
The fuzzy logic 1530 includes empirical rules (e.g., item in sock)
that, when matched, generates an indication, output by the fuzzy
logic 1530, to have the person 125 be stopped for a search, since,
for example, a person carrying a ferromagnetic object tucked in a
sock is likely to be concealing a weapon.
[0166] Another form of processing is the joint time-frequency
analysis 1535, which can be used to generate a contour map of the
target object so that further processing or a magnetic screening
system operator can visually see the target object on a display
(FIG. 1).
[0167] Each of the processes described may be performed by the
gradiometer processor 410 at the gradiometer level. Each of the
processes 410 can also output data or information for use by the
screening computers 115 to further process the target object
signatures 1300. Examples of processing executed by the screening
computers 115 is a neural network or polynomial decision tree 1540
that can classify the target object into one of multiple classes,
such as a dangerous object, non-dangerous object, or unknown
object. This can be done by determining a percentage of match of a
large amount of uncharacterized data to known signatures stored in
a local database. The result from the processing 1515 by the
screening computer 115 is an indicator 1545, such as text (e.g.,
"gun"), icon, color light indicator 120 (FIG. 1) or other means for
alerting another machine or security personnel. In the case of a
physical machine, such as a turnstile or other mechanism that
controls a passageway may be placed into a "locked" position so the
person 125 carrying the dangerous object or unknown object can be
searched. Otherwise, the turnstile or other mechanism can remain in
an unlocked state to allow the person to pass. Any other type of
machine, such as computer, alarm system, paging system, and so
forth may also receive an alert signal.
[0168] FIG. 16 is a graphical diagram of the person 125 passing
through the portal 105 of the magnetic screening system 100 (FIG.
1). The magnetic screening system operator 130 is standing by and
observes an identifier 1545 (e.g., "gun") displayed on the system
display 110. In response, the magnetic screening system operator
130 is able to stop the person 125 for inspection or "pat down" to
locate all ferromagnetic objects being carried. If the
ferromagnetic object turns out to not be a gun, the operator 130
may enter such information into a local signature database for
future reference. This information, as described above, may be
uploaded or sent to a central database for use in updating its
records and disseminating the new signature data or information to
all of the magnetic screening systems 100 to reduce false
alarms.
[0169] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
[0170] While this preferred embodiment of the system is totally
electromagnetically passive, other embodiments may employ active
field generation to stimulate the target objects into
electro-magnetic oscillations that may be detected. The active
driving functions may simply be wire loops stimulated with radio
frequency pulses or antennae designed to produce localized fields.
In any actively driven embodiment, the operating principles of the
apparatus and optimization methods remain the same.
[0171] The arrangement of the magnetometers shown in the invention
has been linear in nature. There are no implied limitations of the
location of the magnetometers for the screening application. The
magnetometers, so long as their location is known and they are
proximal to the area to be monitored, can be in any configuration.
The optimization method and operating modes incorporated in the
invention can be equally applied as effectively to a different,
non-linear distribution of the constituent magnetometers.
* * * * *