US 20130006552A1
(19) United States
(12) Patent Application Publication (10) Pub. No.: US 2013/0006552 A1
Peyton et al.
(54)
(43) Pub. Date:
WALK THROUGH METAL DETECTION
(52)
Jan. 3, 2013
US. Cl. ....................................................... .. 702/57
SYSTEM
(75) Inventors: Anthony J. Peyton, Bolton (GB); David
W. Armitage, Malpas (GB); Liam A.
(57)
ABSTRACT
Marsh, Stockport (GB); Christos
Ktistis> WarringtOn_(GB); William
Robert BrefkwPPnllearts Whaley
Bndge (GB)’ An Jarvl’ ESPOO (FD
-
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of magnetic ?eld generators and magnetic ?eld detectors
(73)
Asslgnee'
(21)
Appl_ No. 13/175 785
’
Filed;
JUL 1, 2011
(22)
The present application is a detection system for locating and
characterizing an object placed in a detection area in a three
dimensional space. The detection system includes a plurality
scan systems’ Inc" Torrance’ CA
Publication Classi?cation
arranged on opposite sides of the detection area and a control
system for enabling generation of a magnetic ?eld in the
detection area by the magnetic ?eld generators and for mea
suring of the magnetic ?eld modi?ed by the object at each of
the magnetic ?eld detectors. The detection system also
includes a processor for processing the measured magnetic
?eld to obtain a data set characterizing the object and a loca
tion of the object. The processor applies a reconstruction
(51)
Int. Cl.
process on a prede?ned number of measurements of the
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US 2013/0006552 A1
Jan. 3, 2013
US 2013/0006552 A1
WALK THROUGH METAL DETECTION
SYSTEM
FIELD OF THE INVENTION
[0001] The present application relates to electromagnetic
(EM) inspection/detection systems. More particularly, the
[0008]
In another embodiment, the present application dis
closes a method for classifying a metal object detected by the
detection system described above into de?ned classes using
the electromagnetic characteristics.
[0009] In yet another embodiment, an electromagnetic
characteristic of a metal object, such as the magnetic polar
present application relates to a system for locating and char
isability dyadic, is estimated using a novel coil con?guration.
acterizing a metal object located on a person.
[0010] In still yet another embodiment, a reconstruction
algorithm is employed for calculating at least one electro
magnetic characteristic of the metal object, such as the mag
BACKGROUND
[0002] Walk-through metal detectors (WTMDs) compris
ing an array of transmitter coils and an array of detector coils
are Well knoWn and Widely used for screening of personnel at
secure locations such as airports, prisons, government build
ings and the like. WTMDs typically operate using coupling
betWeen pairs of coils, providing a multi-zone system With a
coil pair for each zone, each pair operating as an independent
metal detector. In conventional systems, the transmitter coils
are typically located in one panel Whereas the receiver coils
are located in an opposite panel, Wherein the tWo panels
comprise a Walk-through portal or “hallWay”.
[0003] While such systems are capable of detecting the
presence of metal objects, they are limited in their ability to
discriminate betWeen different types of metal objects or accu
rately locate metal objects on the subject. Although there have
been attempts to produce imaging metal detectors and even
tomographic metal detectors that can address these concerns,
these systems are limited due to the poor quality of the images
that they produce. In general, such imaging systems do not
produce images With su?icient quality to reliably discern the
shape of the object. In addition, these conventional metal
detection systems may not produce good results because they
attempt to represent the metal object With a tWo-dimensional
response. It should be appreciated by those of ordinary skill in
the art that a metal object has an inherent three dimensional
response that is not taken account by employing a simple
tWo-dimensional approach.
[0004]
The limitations of currently available metal detec
tors are Well knoWn, such as the loW sensitivity to loW con
ductivity, non-magnetic metals, e.g. titanium, and false posi
tives caused by innocuous objects, Which, in turn, result in
longer queues at checkpoints and borders. In recent times,
X-ray backscatter imaging techniques and systems and some
millimeter-Wave scanning methods and systems are becom
ing more Widely deployed. HoWever, these are high perfor
mance, high cost systems Which are only suited to speci?c
screening applications and there are issues, such as negative
public perceptions regarding radiation exposure and/ or an
invasion of privacy, relating to their use.
[0005] Therefore, What is needed is a metal detection sys
tem that is capable of characterizing and locating the position
of hidden objects by combining spectroscopic, tomographic
and ultra-Wide band techniques.
[0006]
There is also a need for a neW generation of electro
magnetic screening equipment for detecting metallic objects
Which addresses the limitations of prior art, and also has
minimal impact on the environment.
netic polarisability dyadic.
[0011]
In one embodiment, the present application dis
closes a detection system for locating and characterizing an
object Within a detection area in a three-dimensional space
comprising a plurality of magnetic ?eld generators arranged
on at least a ?rst side of the detection area; a plurality of
magnetic ?eld detectors, arranged on at least a second side of
the detection area, Wherein the second side is opposite to the
?rst side; a control system for generating a magnetic ?eld in
the detection area by the magnetic ?eld generators and for
measuring a modi?ed magnetic ?eld at each of the magnetic
?eld detectors, Wherein the generated magnetic ?eld is modi
?ed by the object; and at least one processor con?gured to
process the measured modi?ed magnetic ?eld to obtain a data
set characterizing the obj ect and a location of the object,
Wherein said at least one processor is con?gured to execute a
plurality of instructions de?ning a reconstruction process on
a prede?ned number of measurements of the modi?ed mag
netic ?eld.
[0012] Optionally, the detection system further comprises
an alarm generation unit for generating an alarm correspond
ing to at least one prede?ned type of object based on one or
more parameters in the characteristic data of the object,
Wherein the alarm generation module is adapted to execute a
classi?cation process for determining the type of the object
based on one or more prede?ned categories associated With
the object. The detection system further comprises a move
ment sensor arranged to measure a position of at least a ?rst
part of the object relative to at least one of the magnetic ?eld
generators, the magnetic ?eld detectors, or another part of the
object, Wherein the measured position is combined With the
data set characterizing the object. The movement sensor com
prises a motion sensor or a video camera. The movement
sensor comprises a light sensor. The movement sensor pro
duces a visual output displaying one or more of a measured
position and a category of the object passing through the
detection area, Wherein the visual output is obtained by com
bining a photographic image of the object With at least a
portion of the data set characterizing the object and the mea
sured position of at least a part of the object.
[0013] Optionally, the detection further comprises a dis
placement sensor con?gured to detect a displacement of at
least a part of the object aWay from a reference position, and
to correct for the displacement When processing sets of mea
surements of the modi?ed magnetic ?eld, Wherein the refer
ence position is relative to at least a part of the object and the
displacement is relative to at least a part of the object. The
closes a detection system for characterizing and locating one
or more metal objects in three-dimensional space by using
object is a conductor of electricity. The object is a ferromag
netic object. The magnetic ?eld generators are electrical con
ductor coils through Which electric current is passed to gen
erate magnetic ?eld. The magnetic ?eld detectors are
electrical conductor coils in Which electric current is gener
ated due to changes in a magnetic ?eld. The magnetic ?eld
electromagnetic characteristics of the objects.
detectors are solid state magnetometers. The at least one
SUMMARY OF THE INVENTION
[0007]
In one embodiment, the present application dis
Jan. 3, 2013
US 2013/0006552 A1
processor reconstructs a path followed by the object in three
dimensional space. The control system generates a magnetic
?eld by generating electrical current in the magnetic ?eld
generators and measures the magnetic ?eld at each of the
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and other features and advantages of the
present application Will be further appreciated, as they
magnetic ?eld detectors by detecting induced signals in the
become better understood by reference to the detailed
description When considered in connection With the accom
detectors. The control system comprises one or more data
panying draWings, Wherein:
acquisition and conditioning electronics for collecting con
ditioned signals from the magnetic ?eld detectors and at least
one processor for possessing the collected conditioned sig
nals to generate images.
[0019] FIG. 1 is a schematic diagram of a detection system,
in accordance With an embodiment;
[0020] FIG. 2 illustrates a more detailed, magni?ed vieW of
[0014] Optionally, the detection system further comprises
at least one processor adapted to process at least a video signal
obtained from one or more video cameras placed around the
detection area. The data set for characterizing the object com
prises one or more electromagnetic properties of the object
and a sequence of coordinate points that describe a path the
object has traveled inside the detection area. The data set for
characterizing the object comprises a complex magnetic
polarisability dyadic of the object, Wherein the magnetic
polarisability dyadic describes a three-dimensional scattering
effect of the object With respect to the generated magnetic
?eld. The magnetic ?eld generators and detectors are each
con?gured in an array for obtaining near zero background
coupling and loW susceptibility to mechanical movement.
[0015]
In another embodiment, the present application dis
closes a method for locating and characterizing an object
Within a detection area in a three-dimensional space compris
a portion of the detection system depicted in FIG. 1;
[0021] FIG. 3A is a graph shoWing transmitter coil geom
etry in tWo-dimensional space, in accordance With one
embodiment;
[0022]
FIG. 3B is a graph shoWing receiver coil geometry
in tWo-dimensional space, in accordance With one embodi
ment;
[0023] FIG. 3C is a graph shoWing both transmitter and
receiver coil geometry in three-dimensional space, including
appropriate portal dimensions, in accordance With one
embodiment;
[0024] FIG. 4 is a graphical illustration of an exemplary
output of a reconstruction algorithm used in conjunction With
a detection system of the present application;
[0025] FIG. 5 illustrates an exemplary visual output of the
detection system, superimposed upon a subject under inspec
tion, in accordance With one embodiment;
[0026] FIG. 6 illustrates a complex magnetic polarisability
dyadic calculated from the reconstruction algorithm, in
ing: providing a Walk through the detection area, Which com
prises a plurality of magnetic ?eld generators arranged on at
accordance With one embodiment; and
[0027] FIG. 7 is a graph shoWing a typical spectrum of an
least a ?rst side of the detection area and a plurality of mag
netic ?eld detectors, arranged on at least a second side of the
illustrated in FIG. 6.
detection area, the second side being opposite and parallel to
eigenvalue of the complex magnetic polarisability dyadic
DETAILED DESCRIPTION OF THE INVENTION
the ?rst side; generating a magnetic ?eld in the detection area
by the magnetic ?eld generators; measuring a modi?ed mag
netic ?eld at each of the magnetic ?eld detectors, Wherein the
generated magnetic ?eld is modi?ed by the object; and pro
cessing the measured magnetic ?eld to obtain a data set char
acterizing the object and a location of the object, Wherein a
reconstruction process is applied to a prede?ned number of
measurements of the modi?ed magnetic ?eld.
[0016]
Optionally, the method further comprises measur
ing a position of at least a ?rst part of the object relative to at
least one of the magnetic ?eld generators, the magnetic ?eld
detectors, or another part of the object, Wherein the measured
position is combined With the data set characterizing the
object. The method further comprises producing a visual
output using a movement sensor that shoWs one or more of a
measured position and a category of the object passing
through the detection area, Wherein the visual output is
obtained by combining a photographic image of the object
With characteristics of the data set of the object and With the
measured position of at least a part of the object. The method
further comprises a) detecting a displacement of at least a part
of the object aWay from a reference position and b) correcting
for the displacement When combining sets of measurements
of the modi?ed magnetic ?eld, Wherein the reference position
is relative to at least a part of the object and the displacement
is relative to at least a part of the object.
[0028] The present application discloses multiple embodi
ments. The folloWing disclosure is provided in order to enable
a person having ordinary skill in the art to practice the claimed
inventions. Language used in this speci?cation should not be
interpreted as a general disavoWal of any one speci?c embodi
ment or used to limit the claims beyond the meaning of the
terms used therein. The general principles de?ned herein may
be applied to other embodiments and applications Without
departing from the spirit and scope of the invention. Also, the
terminology and phraseology used is for the purpose of
describing exemplary embodiments and should not be con
sidered limiting. Thus, the present application is to be
accorded the Widest scope encompassing numerous altema
tives, modi?cations and equivalents consistent With the prin
ciples and features disclosed. For purpose of clarity, details
relating to technical material that is knoWn in the technical
?elds related to the claimed inventions have not been
described in detail so as not to unnecessarily obscure the
disclosure.
[0029] In one embodiment, the present application dis
closes a personnel screening and inspection system that is
capable of detecting, characterizing and locating a metallic
object in three-dimensional space.
[0030] In one embodiment, the present application dis
closes a detection system for characterizing and locating one
or more metal objects in three-dimensional space by using
present shall be described in greater depth in the draWings and
electromagnetic characteristics of the objects.
[0031] In one embodiment, the present application dis
detailed description provided beloW.
closes a detection system comprising a plurality of magnetic
[0017]
The aforementioned and other embodiments of the
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US 2013/0006552 A1
?eld generators Which, in one embodiment, are transmitter
coils and a plurality of magnetic ?eld detectors, Which, in one
embodiment, are receiver coils, located around a detection
area de?ning a portal through Which a subject stands or Walks,
coils 16. As illustrated, the arrays 10, 14 are arranged on
opposite sides of a support frame 18 Which de?nes an arch,
portal or gate 20, Within Which is an imaging area 22 through
and a control system that is arranged to generate magnetic
?eld and measure the generated magnetic ?eld at each of the
detectors. In one embodiment, the magnetic ?eld is generated
by generating electrical current in the transmitter coils. In one
magnetic ?eld generators 12 is arranged on at least a ?rst side
of the detection area. In one embodiment, a plurality of mag
netic ?eld detectors 14 is arranged on at least a second side of
the detection area, the second side being opposite to and/or
embodiment, the magnetic ?eld is measured by detecting
parallel to the ?rst side. The detection system thus comprises
induced signals in the receiver coils.
an array 10 of transmitter coils 12 and an array 14 of receiver
[0032]
coils 16.
[0040] In the embodiment illustrated in FIG. 1, there are
In an embodiment, the object to be detected is a
conductor of electricity. In another embodiment, the object is
a ferromagnetic object. Further, in yet another embodiment,
the magnetic ?eld generators are electrical conductor coils
Which a person can Walk. In one embodiment, a plurality of
equal numbers of transmitter coils 12 and receiver coils 16,
With each transmitter coil 12 being level With, opposite to,
through Which electric current is passed to generate magnetic
and/or parallel to a respective receiver coil 16. In one embodi
?eld; and the magnetic ?eld detectors are electrical conductor
coils in Which electric voltage is generated due to changes in
ment, there are eight transmitter coils 12 and eight receiver
coils 16. It should be understood by those of ordinary skill in
the art that any number of receiver coils may be employed
Without departing from the scope of the present application.
In the embodiment described herein, but not limited to such
embodiment, the transmitter and receiver coils are also
a magnetic ?eld, thereby enabling the magnetic ?eld to be
measured. In another embodiment, the magnetic ?eld detec
tors are solid state magnetometers.
[0033]
In another embodiment, the present application dis
closes a method for classifying a metal object detected by the
detection system described above into de?ned classes using
the electromagnetic characteristics.
[0034] In yet another embodiment, an electromagnetic
characteristic of a metal object, such as the magnetic polar
isability dyadic, is estimated using a novel coil con?guration.
[0035] In still yet another embodiment, a reconstruction
algorithm is employed for calculating at least one electro
magnetic characteristic of the metal object, such as the mag
equally spaced Within the panels.
[0041] In alternate embodiments, various other coil con
?gurations may be employed, such as different numbers of
transmitters and receiver coils; using transmitter and/or
receiver coils of different sizes; using transmitter and/or
receiver coils of different orientations; using transmitter and
receiver coils of different geometries, Which Will be described
in greater detail beloW; and positioning the coils With unequal
spacing, for example, positioning a greater number of coils
netic polarisability dyadic.
near the ?oor level to enhance the characterization of a metal
[0036]
lic objects carried in a subject’s shoes.
[0042] A control system 30 is arranged to generate and
control a varying electrical current independently in each of
the transmitter coils 12, thus generating a magnetic ?eld. In an
embodiment, the control system 30 measures signals gener
ated in each of the receiver coils 16 due to the generated
magnetic ?eld in the form of electrical currents. The control
In one embodiment, the present application dis
closes a detection system for characterizing and locating one
or more metal objects in three-dimensional space by using
electromagnetic characteristics of the objects further com
prising at least one processor con?gured to process measure
ments of a generated ?eld by, for example, measuring the
induced signals to generate a data set characterizing one or
more of the detected objects, together With their location, as
they pass through or across the sensitive region of the inspec
tion system.
[0037]
The at least one processor or processing means
refers to a processing device, such as a chip, semiconductor,
or any other processor, that is con?gured to access a memory
storing a plurality of instructions de?ning a reconstruction
method and to execute the plurality of instructions in order to
reconstruct the characteristics of each object, and, if required,
the path each object has taken through space.
[0038] In one embodiment, the data set for characterizing
the object comprises one or more electromagnetic properties
of the object, and a sequence of coordinate points that
describe the path the object has traveled inside the detection
area. The data set for characterizing the object comprises a
complex magnetic polarisability dyadic of the object,
Whereby the magnetic polarisability dyadic describes a three
dimensional scattering effect of the object With respect to the
generated magnetic ?eld. Further, in an embodiment, the
magnetic ?eld generators and detectors are each con?gured in
an array for obtaining near zero background coupling and loW
susceptibility to mechanical movement.
[0039] FIG. 1 is a schematic diagram of a detection system,
in accordance With one embodiment of the present invention.
The detection system comprises an array 10 of magnetic ?eld
generator coils 12 and an array 14 of magnetic ?eld detector
system 30 comprises data acquisition and conditioning elec
tronics 40 and a processing system 42, Which, in one embodi
ment, is a host computer. The data acquisition and condition
ing electronics 40 collects data from the receiver coils 16, and
the processing system 42 processes the collected conditioned
signals to generate images and executes instructions to per
form detection methods. In one embodiment, detection sys
tem also comprises camera 44 and camera 46 to generate
images of the imaging area 22, described in greater detail
beloW.
[0043] FIG. 2 illustrates a magni?ed vieW ofa portion ofthe
detection system depicted in FIG. 1. Referring to both FIGS.
1 and 2 interchangeably, the detection system further com
prises four video cameras, tWo cameras 44 and 46 positioned
on each side of the gate 20 and tWo (not shoWn in FIG. 1 or 2)
positioned overhead on each side of the gate 20 for providing
a bird’s eye vieW.
[0044]
The tWo cameras 44, 46 are orientated so that they
both image the imaging area 22 from different angles, as
illustrated in FIG. 2, and thus provide front and rear vieWs of
a subject Walking through the imaging area 22 to determine
the position of the subject passing through the gate 20.
[0045] In various embodiments, any number of cameras
may be used to improve the positional accuracy or extract
three-dimensional information of the location of the subject
passing through the gate 20. All of the cameras are connected
Jan. 3, 2013
US 2013/0006552 A1
to the computer 42 (shown in FIG. 1) and are used to deter
interference picked up by the receiver coils and external elec
mine the entrance and exit of the subject through the system.
[0046] With reference to FIG. 1, in an embodiment, the
computer 42 is arranged to process the video image signals
tromagnetic interference generated by the transmitter coils.
[0052] Third, it is preferred that the background coupling
(i.e. the mutual magnetic coupling When no object is present)
received from at least the video cameras 44, 46 and the elec
betWeen any transmitter and any receiver coil pair is virtually
tromagnetic signals received from the receiver coils 16.
Zero.
Exemplary processing algorithms executed by the computer
[0053] Fourth, it is preferred that the coil array is insensitive
to mechanical movement of the array, and, in particular, dis
42 are described in detail in later portions of this application.
[0047] Further, as shoWn in FIG. 2, in one embodiment,
gate 20 has the folloWing dimensions: a depth 50 of approxi
mately 320 mm, a height 52 of approximately 2026 mm, and
a Width 54 of approximately 860 mm, Which coincide With the
x, y, and Z-axes, respectively.
[0048] In a typical operational scenario at least one of the
transmitter coils 12 is energiZed With a sinusoidally varying
AC signal, thereby creating a magnetic ?eld. This magnetic
?eld is modi?ed by electrically conductive or ferromagnetic
objects Within the ?eld and the resultant ?eld changes are
measured With the array of receiver coils 16. In one embodi
ment, the array of transmitter coils 12 is energiZed serially
and/ or sequentially around the object, and for each transmit
ter coil, the ?eld is measured With each of the receiver coils
16, for determining the electromagnetic properties of the
object. Properties such as the polarisability may be measured
by using a reconstruction algorithm after combining the
resultant detected signals.
[0049] In various embodiments the accuracy of the recon
structions are based upon the number and positions of trans
mitter coils 12 and receiver coils 16. Therefore, in one
placement in the Y-Z plane (the plane perpendicular to the
Walkthrough direction, as shoWn in FIG. 2) as the gate has the
loWest shear stiffness in this particular plane. This is an
important consideration because an array With greater stiff
ness is less sensitive to mechanical vibrations that are trans
mitted through, for example, the ?oor. In an extreme case,
mechanical vibrations may also be transmitted by a person
that accidentally bumps into the gate as he/ she Walks through
it.
[0054]
Fifth, it is preferred that neighboring transmitter and
neighboring receiver coils have near Zero net mutual mag
netic coupling as this helps to ensure good isolation betWeen
adjacent channels and minimize crosstalk. This property is
more important if the coils are resonated because resonant
currents Would otherWise be able to induce a voltage in their
neighboring coils.
[0055] Sixth, it is preferred that the transmitter coils are
collectively able to excite the object With magnetic ?eld com
ponents in all three directions (x, y, and Z as shoWn in FIG. 2).
As the object folloWs any path through the gate in the x
direction, the receiver coils are simultaneously and collec
embodiment, the present application is directed toWards sys
tively able to detect the magnetic ?eld components produced
by the object in all three directions.
tems and methods for assessing the response of a candidate
coil con?guration. In one embodiment, a processor is con?g
con?guration having the characteristics outlined above,
[0056]
FIGS. 3A and 3B are graphical illustrations of a coil
ured to quantify both the error in the predicted object position
and the error in the predicted magnetic polarisability dyadic
of the object as a function of the signal-to-noise ratio (SNR)
tively.
of the signals from the receiver coils. The error in the pre
tWo-dimensional space, in accordance With an embodiment
dicted position and the predicted magnetic polarisability
dyadic can then be assessed for different candidate coil con
?gurations in order to determine the optimum con?guration,
such as the number of transmitter coils 12 and receiver coils
16; the position of transmitter coils 12 and receiver coils 16;
and the geometries of transmitter coils 12 and receiver coils
16. In a preferred embodiment, and the embodiment
described in this speci?cation, the coil con?guration includes
at least eight transmitter coils 12 and at least eight receiver
coils 16. It should be understood by those of ordinary skill in
the art that any number of transmitter and receiver coils can be
used, depending upon the candidate coil con?guration char
acteristics described above and described in greater detail in
the paragraphs that folloW.
[0050] Furthermore, a preferred coil con?guration should
also have the folloWing characteristics and/or properties.
First, it is preferred that all transmitter coils are contained
Within a panel on one side of the gate While all the receiver
coils are contained Within a panel on the opposite side of the
gate. This alloWs the transmitter and receiver circuitry to be
separated and alloWs for the minimiZation of unWanted
capacitive crosstalk.
[0051]
Second, it is preferred that all transmitter and
receiver coils are gradiometer Wound, With equal areas of
clockWise and counterclockwise Windings. This coil geom
etry is Well-knoWn by those skilled in the art, because the coil
geometry helps to minimiZe both external electromagnetic
shoWing both transmitter coils and receiver coils, respec
[0057]
FIG. 3A illustrates a transmitter coil geometry in
of the present invention, shoWing the position of the Wires of
one transmitter coil in both the x-axis (in meters) and the
y-axis (in meters) on a grid 300. As shoWn, the coil 302 is
constructed in the shape of a squared-off ?gure eight having
tWo symmetrical half-sections 302a and 302b, Where one
half-section is Wound in a clockWise direction and the other
half-section is Wound in a counterclockWise direction. As
shoWn in FIG. 2, the x-direction is horiZontal and in the
Walk-through direction of the portal, Whereas the y-direction
is vertical.
[0058] FIG. 3B illustrates a receiver coil geometry in tWo
dimensional space, in accordance With one embodiment.
FIG. 3B shoWs the position of the Wires of one receiver coil in
both the x-axis and the y-axis on a grid 305.As shoWn, the coil
307 is Wound in three sections, Where tWo outer sections 307a
and 30719 are Wound in a direction opposite to a central section
3070.
[0059] FIG. 3C is a graphical illustration shoWing eight
individual transmitter coils (331T, 332T, 333T, 334T, 335T,
336T, 337T, 338T) and eight individual receiver coils (331R,
332R, 333R, 334R, 335R, 336R, 337R, 338R) mounted in the
side panels 318 of the portal 320 in three-dimensional space.
As illustrated in FIG. 3C, the receiver coils overlap one
another to ensure that neighboring receivers have near Zero
net mutual coupling. Also, adjacent transmitter coils are over
lapped in a similar manner to ensure that neighboring trans
mitters have near Zero net mutual coupling. This feature of the
Jan. 3, 2013
US 2013/0006552 A1
transmitter and receiver coil array helps to avoid interference
betWeen neighboring coils, especially When the coils are reso
nated With tuning capacitors.
other object categories could be de?ned and used by operators
for particular classi?cation purposes.
[0065]
In one embodiment, a plurality of measurements
[0060] In an embodiment, the transmitter and receiver coils
are gradiometer Wound as described above With respect to the
captured simultaneously from the transmitter and receiver
preferred properties of the coil con?guration. Thus, the trans
mitter and receiver coils have equal areas of clockWise and
nals’. For the gate shoWn in FIG. 3C, the eight transmitter
coils and eight receiver coils result in 64 possible independent
coil combinations are de?ned as a ‘set of measurement sig
counterclockwise Windings. This coil geometry is Well
measurement signals (8x8). HoWever, signals from transmit
knoWn to those of ordinary skill in the art, as it helps to
ter coils to receiver coils that are located far from the trans
minimize both external electromagnetic interference picked
up by the receiver coils and external electromagnetic inter
ference generated by the transmitter coils. The gradiometer
Winding con?guration ensures that the magnetic ?eld pro
mitter coils have little sensitivity to metal objects and conse
quently are ignored. So in this particular embodiment, the ‘ set
of measurements signals’ is limited to 34 measurements as
summariZed in the table beloW:
duced by a transmitter coil drops off very quickly to Zero
outside the gate as the magnetic ?eld produced by the coun
terclockWise section of Winding cancels the ?eld produced by
the clockWise section of Winding. Hence, the transmitter coil
produces little interference to other Walk-through metal
detectors that may be positioned in the vicinity. Similarly, the
gradiometer Winding con?guration ensures that the receiver
TABLE 1
Measurements Derived From Coil Pairings
Transmitter
Coil
Couples to
Receiver Coils
Number of
Measured
Coil Pairs
object to x, y, Z magnetic ?eld components as the object
331T (bottom ofgate)
332T
333T
334T
335T
336 T
passes through the gate of the detection system of the present
invention. Also, in an embodiment, the transmitter and
338T (top ofgate)
331R, 332R, 333R
331R, 332R, 333R, 334R
331R, 332R, 333R, 334R, 335R
332R, 333R, 334R, 335R, 336R
333R, 334R, 335R, 336R, 337R
334R, 335R, 336R, 337R, 338R
335R, 336R, 337R, 338R
336R, 337R, 338R
3
4
5
5
5
5
4
3
coils are relatively immune to pick-up from the distant
sources of electromagnetic interference by reciprocity.
[0061] In various embodiments, the transmitter and
receiver coils are designed in a manner so as to expose an
337 T
receiver coils are con?gured in an array for near Zero back
TOTAL
ground coupling and for loW susceptibility to mechanical
movement, especially shear strain on the gate.
[0062] With reference back to FIG. 1, the video image
signals received from at least the video cameras 44, 46 and the
electromagnetic signals received from the receiver coils 16,
Which are input to the computer 42, are fed to a processor
executing reconstruction instructions. In an embodiment, the
reconstruction instructions are Written in MatLab. In other
embodiments, the algorithm may be coded in any suitable
programming language. The reconstruction instructions are
executed by the computer 42 to estimate the position and the
magnetic properties (eg. magnetic polarisability dyadic) of a
[0066]
34
The siZe of the set of measurement signals affected
the error in the predicted positions and the predicted magnetic
polarisability dyadic of the object. The siZe of the set of
measurement signals also affects the number of objects that
can be simultaneously reconstructed, Which, in this particular
embodiment, is typically 4 or less. It should be noted herein
that the 34 measurement signal sets represent a loWer limit of
the number of signals sets that can be measured. In other
embodiments, a greater number of coils and thus, a greater
number of measurement signal sets are preferred. Therefore,
metallic object by using the data collected from the object as
it travels through the gate 20 of the detection system 100.
[0063] A characteristic data set for each detected object
in preferred embodiments, at least 34 measurement signal
may be a characteristic of each detected object together With
change as one or more metal objects pass through the gate.
Each measurement signal is sampled at a rate of 100 samples
a sequence of coordinate points or other suitable parameters
that describe the path that each object has traveled either
through or across the sensing region of the detection system.
In an embodiment, a complex magnetic polarisability dyadic
is used to suitably characterize the object. A magnetic polar
isability dyadic describes the three-dimensional scattering
sets should be obtained.
[0067]
The set of measurement signals are time varying and
per second, Which gives adequate temporal resolution for
objects passing through the gate at Walking speed or less.
Consequently there are 100 sample instants per second for
each measurement signal in this exemplary embodiment of
the present invention.
effect of the object to the applied magnetic ?eld. In various
other embodiments, other similar characteristics of detected
objects may be used, and thus, the present application is not
limited to the representation described herein.
[0064] In one embodiment, the detection system is con?g
that indicate the location and the magnetic polarisability
dyadic of the object being scanned, are calculated using a
ured to generate an alarm for a prede?ned type of obj ect based
example), then a sequence of x, y, and Z coordinates are
on one or more parameters in the characteristic data for the
calculated by the reconstruction process together With an
object. A classi?cation method may be applied to the charac
teristic data for this purpose. The classi?cation method is
used to determine Which category the object belongs to, for
estimation of the magnetic polarisability dyadic of the metal
example, either a threat object or an innocuous object. If any
detected object falls in the threat category, an alarm is acti
vated. As is evident to those of ordinary skill in the art, various
[0068]
For every sample instant, the x, y, and Z coordinates
reconstruction process. Since the measurement signals con
sist of a sequence of samples (100 per second in this
lic object. If there is more than one metallic object then the
reconstruction algorithm can be extended to calculate mul
tiple x, y, and Z coordinate sequences and multiple magnetic
polarisability dyadic, With one x, y, and Z sequence and one
dyadic per object.
Jan. 3, 2013
US 2013/0006552 A1
[0069]
In one embodiment, an iterative process is used to
invert data Where the position and properties of an object are
estimated simultaneously by minimizing a residual, as shoWn
beloW in Equation 3 betWeen the measured data and a calcu
lated data produced by a solution to the forWard problem. The
forWard problem refers to the process of calculating the esti
mated values of the measurement signals, if the position and
dyadic of each object are knoWn. The residual represents the
square of the error betWeen the estimated measurement sig
nals and the actual measurement signals. When the residual is
zero, there is no error betWeen the estimated and actual mea
measurements taken by the transmitter and receiver coils to
be matched. The calibration factor is a set of complex num
bers Which adjusts magnitude and phase of each measure
ment value. In an exemplary embodiment, a calibration object
consisting of a 38 mm diameter spherical ball ?lled With
ferrite poWder is passed through a vertical tube in a speci?c
location on the detection system. The forWard problem (i.e. a
routine that calculates the estimated values of the measure
ment signals, When the position and dyadic of the object are
known) is solved for the ball’s location and magnetic polar
isability dyadic. Then, for every coil combination a complex
sured signals and therefore the x, y, and Z positions and dyadic
for each metallic object is calculated exactly.
factor that best ?ts the measurements to the forWard problem
[0070] An exemplary magnetic polarisability dyadic is
bration object is taken to be the unity matrix as a reference.
depicted as M beloW. The dyadic M is a fundamental property
of the metallic object, Which is dependent on its metallic
composition, shape and orientation. The dyadic M has 9
complex numbers, With each complex number representing
the phasor response of the object in a particular direction (x,
y, or Z) to the component (x, y, or Z) of the magnetic ?eld,
Which has been applied to the object. Reciprocity of electro
magnetic induction stipulates that the dyadic, M, should be
diagonally symmetrical as shoWn in Equation 1. Therefore
the magnetic polarisability dyadic, M, has 6 independent
complex values. Furthermore, since each independent com
plex value has 2 scalar components (Real and Imaginary) and
there are 3 positional values (x, y, Z), Which makes the number
of variables to calculate by the reconstruction algorithm as 15
for each sample instant and each metallic object:
M =
mll "112 "113]
"121
"122
"123
"131
"132
"133
[EQUATION l]
Where m1; = "121, mm = "131, "123 = "132
is computed. The magnetic polarisability dyadic of the cali
[0074]
During the operation of the detection system of the
present invention, When an alarm is triggered, the measure
ments from the 34 coil combinations are imported into the
reconstruction algorithm. From each channel combination, a
?xed length of 200 measurements (approximately 2 s) is
recorded. A background or DC offset is subtracted from each
channel end and a trigger level is used to select the region
Where there is at least a prede?ned level of signal to noise
ratio.
[0075] In one embodiment, a ?rst estimation of the position
of the object is obtained using an empirical algorithm. At least
one signal from one or more straight-coupled channels are
used to estimate the height of the object, While the cross
coupled channels are used to estimate the horizontal z-coor
dinate of the object. The straight coupled channels are de?ned
as having the transmitter coils and receiver coils directly
facing and corresponding With each other, such as for
instance, transmitter coil 331T With receiver coil 331R, or
transmitter coil 332T With receiver coil 332R, or transmitter
coil Tn With receiver coil Rn using the coil numbering con
vention shoWn in TABLE 1 earlier. Similarly, the cross
coupled channels are at angles to each other and consist of
transmitter coil n With receiver coil m, Where n is not equal to
[0071] In various embodiments, from the 34 measurements
only a small number, less than the number of variables
required to be inverted, typically have signal levels above the
noise levels. This makes the problem ill-posed. In order to
improve the condition of the problem, it is assumed that the
m
[0076] A Walking speed is taken to be the same for every
Walk as a ?rst estimate. Estimated position points, having a
?xed distance betWeen them, comprise the object’s trajectory
along the X axis and are taken to be central to the gate of the
value of M does not change With time and more sets of
measurements that have been taken in different instances are
added to it.
detection system. Finally, an assumed magnetic polarisability
[0072] In various embodiments, as described above, the
forWard problem calculates the estimated values of the mea
the least squares solution of the non-linear problem:
surement signals, if the position and dyadic of each metallic
object are knoWn. The accuracy of the forWard problem is
important for the convergence of the iterative reconstruction
process. The reconstruction algorithm takes the dot product
of magnetic polarisability dyadic, the magnetic ?eld pro
duced by the transmitting coil and the magnetic ?eld pro
duced by the receiving coil When it is supplied With unit
current, Which can be expressed as:
Flj:( M -Hi(x))-Hj(x)
[EQUATION 2]
Where, F is the forWard problem, H the magnetic ?eld and x is
the coordinate’s vector. The indices i and j indicate the trans
mitter and receiver number respectively. The magnetic ?elds
are pre-computed for all the coils and the volume of the gate
on a grid With 10 mm spacing.
[0073] In one embodiment, before operation, the detection
system is calibrated in order for the forWard problem and the
dyadic, such as the unity matrix, is taken as a starting point.
[0077] A modi?ed Levenberg-Marquardt method produces
arg min([[D-F(x, 191 W)
[0078]
[EQUATION 3]
Where D is a measurement vector. The forWard
problem is solved for the latest coordinates and polarisability
to ?nd a coil signal as Well as a gradient of the received signal
When any of the unknoWn variables are perturbed. Then a
residual R is computed and Jacobean J is populated With
gradients. The solution of the problem is given by:
[x, M ]:(JTJ+ALTL)’1JTR
[EQUATION 4]
[0079] Where R:D—F(x,l\7l ), 7» is a regularization param
eter and L depicts a regularization matrix. If the neW coordi
nates found are to be outside of the volume of the portal, then
the regularization parameter is increased and the problem is
solved again. The process is repeated iteratively until the
residual stops decreasing or the residual becomes loWer than
the tolerance or the regularization reaches a maximum value.
Jan. 3, 2013
US 2013/0006552 A1
[0080]
In one embodiment, the detection system is a secu
rity detection system in Which the transmitter and receiver
coils are mounted on support means arranged to alloW a
person to Walk through the imaging area. The processing
means may be arranged to generate a plurality of sets of data
as an object moves through the imaging area, and to combine
the sets of data to form a resultant data set. The object may, for
example, be a person together With their clothing and any
articles they are carrying With them.
ment, the circle 508 may be used to track the location of the
metal object on a video stream of the person 502 Walking
through the gate 504.
[0087] FIG. 6 illustrates a complex magnetic polarisability
dyadic calculated from the reconstruction process, in accor
dance With an embodiment of the present invention. The FIG.
illustrates a magnetic polarisability dyadic, Which is a 3x3
matrix 602 of complex numbers that is symmetrical about a
diagonal. The magnetic polarisability dyadic describes an x, y
In one embodiment, the detection system comprises
and Z response to applied ?eld in the x-direction respectively
movement sensing means arranged to measure the position of
at least a part of an object, for example relative to either the
transmitter or detector coils or another part of the object. The
movement sensing means may be arranged to use the changes
of position When combining sets of data. The movement
for the terms in roW 604; the x, y and Z response to the applied
[0081]
sensing means may be a video camera or other imaging sys
tems, or may comprise other forms of sensor such as light
sensors.
[0082] In one embodiment, the detection system uses the
movement sensing means to shoW the location and, if desired,
the category of a metal object passing through the detection
system. For example, the output of the detection system may
be a video or photograph of a person being screened. Super
imposed on this video or photograph may be a graphical
representation of the characteristics of detected object encod
ing the eigenvalues of the magnetic polaris ability dyadic
?eld in the y-direction respectively in roW 606; and the x, y
and Z response to the applied ?eld in the Z-direction respec
tively in roW 608. In various embodiments, reciprocity of
electromagnetic induction ensures that the matrix 602 is
diagonally symmetrical. The values in the matrix 602 are
calibrated against a circular spherical ferrite poWder calibra
tion object. As illustrated the largest value is 0.8579 in the
middle position (y-excite, y-detect) indicating a ferromag
netic object. The values depicted in section 610 in FIG. 6 are
eigenvalues of the magnetic polarisability dyadic. These val
ues are unique characteristics to the object passed through the
gate of the detection system of the present invention, and can
be fed into a classi?cation algorithm to separate knoWn
innocuous metal objects from knoWn threat metal objects.
[0088]
In an embodiment, a range of frequencies are
the movement sensing means to enhance the operation of the
applied to the transmitter and receiver coils of the detection
system and the coupling versus frequency on the full array of
coil pairs is measured. In one embodiment, the range of
frequencies is betWeen 5 kHZ and 50 kHZ. The range of
reconstruction process. For example, the detected object must
frequencies chosen takes into consideration those frequencies
be associated With a ?xed position on the object as the object
for Which typical metallic threat objects, such as knives and
passes through the detection system; for instance a subject
Wearing a metal Wrist Watch Will produce a detected object
Which should be located on the Wrist. The co-location of
guns, exhibit signi?cant changes in their electromagnetic
response as characteriZed by changes in their magnetic polar
isability dyadic. The range of frequencies is applied simulta
detected object in the magnetic data and visual data is used to
enhance the operation of the reconstruction process or verify
the results of the reconstruction process.
neously via a suitable Waveform. The individual signal com
ponents at each frequency are extracted using demodulation
techniques Well knoWn to those skilled in the art of signal
together With the location.
[0083]
Also, in one embodiment, the detection system uses
[0084] Further, in another embodiment, the detection sys
tem comprises displacement sensing means arranged to
detect displacement of a part of the object aWay from a ref
erence position, and to correct for the displacement When
combining sets of data. This sensing means may also com
prise an imaging system, such as a video camera, or other
processing. The three eigenvalues of the magnetic polaris
ability dyadic for a metal object is calculated for each fre
quency by extending the reconstruction algorithm described
earlier to produce a spectrum for each. FIG. 7 illustrates a
typical spectrum of an eigenvalue of the complex magnetic
polarisability dyadic illustrated in FIG. 6. The spectrum fur
forms of sensors. The reference position may be a position
relative to at least a part of the object. The displacement may
be a displacement relative to at least a part of the object.
ther characteriZes the metal object as shoWn in FIG. 7, Which
contains both the real and imaginary components of the
[0085]
[0089] The spectrum for each eigenvalue is used to improve
the estimation of the classi?cation algorithm described above
FIG. 4 illustrates an exemplary output of a recon
struction process used in conjunction With the detection sys
tem, in accordance With an embodiment of the present inven
tion. FIG. 4 illustrates the 3D coordinate sequence 402 of a
metal object as it passes through the gate 404 of the detection
system of the present invention.
[0086] FIG. 5 illustrates an exemplary visual output of the
detection system, in accordance With an embodiment of the
present invention. Here the position of a metal object is super
imposed upon a person 502 Walking through the gate 504 of
the detection system of the present invention. In this case the
person 502 is carrying a ferritic steel penknife in the left
trouser pocket 506 With the blade pointing in a vertical direc
tion. As can be seen in FIG. 5, the reconstruction process has
located the position of the knife as shoWn by the circles 508
superimposed on the image of the person 502. In an embodi
eigenvalue plotted against frequency, shoWn as 702 and 704,
respectively.
because the shape of the curve is a function of the material
properties of the object (magnetic permeability [LR and elec
trical conductivity 0) and the siZe (area, A, length L) and
shape (aspect ratio Asp) of the object. Consequently different
types of object Will have different spectral characteristics.
[0090] At loW frequencies, the imaginary component of the
eigenvalue is approximately Zero as the magnetic ?eld is not
alternating frequently enough to induce su?icient eddy cur
rents into the metallic object. At loW frequencies, the real
component of the eigenvalue is determined by the magnetic
properties of the object, such as the relative permeability [1R of
the material making up the object. Magnetic objects With a
large cross-sectional area, A, or length, L, Will also have a
large real response at loW frequencies. At very high frequen
Jan. 3, 2013
US 2013/0006552 A1
cies, the magnetic ?eld changes suf?ciently fast to ensure that
5. The detection system of claim 3 Wherein the movement
all the induced eddy current ?oWs are virtually on the surface
sensor comprises a light sensor.
of the metal object and no electromagnetic ?eld penetrates
inside. As a result, the imaginary component of the eigenvalue
is also zero, but the real component of the eigenvalue is
negative and abject With a large cross-sectional area and thus,
sensor produces a visual output displaying one or more of a
Will have a more negative response, While objects With a small
aspect ratio, Asp, Will have less effect. At intermediate fre
quencies the real component changes from being positive to
negative for magnetic metallic objects and the imaginary
component passes through a peak in magnitude.
[0091]
Hence, the present application discloses a detection
system Which characterizes and locates one or more metal
objects in a three-dimensional space by using electromag
netic characteristics of the objects. It Will be appreciated that
various above-disclosed embodiments, other features and
functions, or alternatives thereof, may be desirably combined
into many other different systems or applications.
[0092] The above examples are merely illustrative of the
many applications of the system of present invention.
Although only a feW embodiments of the present invention
have been described herein, it should be understood that the
present invention might be embodied in many other speci?c
forms Without departing from the spirit or scope of the inven
tion. Therefore, the present examples and embodiments are to
6. The detection system of claim 3 Wherein the movement
measured position and a category of the object passing
through the detection area, Wherein the visual output is
obtained by combining a photographic image of the object
With at least a portion of the data set characterizing the object
and the measured position of at least a part of the object.
7. The detection system of claim 1 further comprising a
displacement sensor con?gured to detect a displacement of at
least a part of the object aWay from a reference position, and
to correct for the displacement When processing sets of mea
surements of the modi?ed magnetic ?eld, Wherein the refer
ence position is relative to at least a part of the object and the
displacement is relative to at least a part of the object.
8. The detection system of claim 1 Wherein the object is a
conductor of electricity.
9. The detection system of claim 1 Wherein the object is a
ferromagnetic object.
10. The detection system of claim 1 Wherein the magnetic
?eld generators are electrical conductor coils through Which
electric current is passed to generate magnetic ?eld.
11. The detection system of claim 1 Wherein the magnetic
be considered as illustrative and not restrictive, and the inven
?eld detectors are electrical conductor coils in Which electric
tion may be modi?ed Within the scope of the appended
claims.
We claim:
1. A detection system for locating and characterizing an
current is generated due to changes in a magnetic ?eld.
12. The detection system of claim 1 Wherein the magnetic
object Within a detection area in a three-dimensional space
processor reconstructs a path folloWed by the object in three
comprising:
dimensional space.
14. The detection system of claim 1 Wherein the control
a. a plurality of magnetic ?eld generators arranged on at
least a ?rst side of the detection area;
b. a plurality of magnetic ?eld detectors, arranged on at
least a second side of the detection area, Wherein the
second side is opposite to the ?rst side;
c. a control system for generating a magnetic ?eld in the
detection area by the magnetic ?eld generators and for
measuring a modi?ed magnetic ?eld at each of the mag
netic ?eld detectors, Wherein the generated magnetic
?eld is modi?ed by the object; and
d. at least one processor con?gured to process the mea
sured modi?ed magnetic ?eld to obtain a data set char
acterizing the object and a location of the object,
Wherein said at least one processor is con?gured to
execute a plurality of instructions de?ning a reconstruc
tion process on a prede?ned number of measurements of
the modi?ed magnetic ?eld.
2. The detection system of claim 1 further comprising an
alarm generation unit for generating an alarm corresponding
to at least one prede?ned type of obj ect based on one or more
parameters in the characteristic data of the object, Wherein the
alarm generation module is adapted to execute a classi?cation
process for determining the type of the object based on one or
more prede?ned categories associated With the object.
3. The detection system of claim 1 further comprising a
movement sensor arranged to measure a position of at least a
?rst part of the object relative to at least one of the magnetic
?eld generators, the magnetic ?eld detectors, or another part
of the object, Wherein the measured position is combined With
the data set characterizing the object.
4. The detection system of claim 3 Wherein the movement
sensor comprises a motion sensor or a video camera.
?eld detectors are solid state magnetometers.
13. The detection system of claim 1 Wherein the at least one
system generates a magnetic ?eld by generating electrical
current in the magnetic ?eld generators and measures the
magnetic ?eld at each of the magnetic ?eld detectors by
detecting induced signals in the detectors.
15. The detection system of claim 1 Wherein the control
system comprises:
a. one or more data acquisition and conditioning electron
ics for collecting conditioned signals from the magnetic
?eld detectors; and
b. at least one processor for possessing the collected con
ditioned signals to generate images.
16. The detection system of claim 1 further comprising at
least one processor adapted to process at least a video signal
obtained from one or more video cameras placed around the
detection area.
17. The detection system of claim 1 Wherein the data set for
characterizing the object comprises one or more electromag
netic properties of the object and a sequence of coordinate
points that describe a path the object has traveled inside the
detection area.
18. The detection system of claim 1 Wherein the data set for
characterizing the object comprises a complex magnetic
polarisability dyadic of the object, Wherein the magnetic
polarisability dyadic describes a three-dimensional scattering
effect of the object With respect to the generated magnetic
?eld.
19. The detection system of claim 1 Wherein the magnetic
?eld generators and detectors are each con?gured in an array
for obtaining near zero background coupling and loW suscep
tibility to mechanical movement.
Jan. 3, 2013
US 2013/0006552 A1
20. A method for locating and characterizing an object
21. The method of claim 20 further comprising measuring
Within a detection area in a three-dimensional space compris
a position of at least a ?rst part of the object relative to at least
ing:
one of the magnetic ?eld generators, the magnetic ?eld detec
tors, or another part of the object, Wherein the measured
position is combined With the data set characterizing the
a. providing a Walk through the detection area, Which com
prises a plurality of magnetic ?eld generators arranged
on at least a ?rst side of the detection area and a plurality
object.
22. The method of claim 20 further comprising producing
of magnetic ?eld detectors, arranged on at least a second
side of the detection area, the second side being opposite
more of a measured position and a category of the object
and parallel to the ?rst side;
b. generating a magnetic ?eld in the detection area by the
passing through the detection area, Wherein the visual output
is obtained by combining a photographic image of the object
magnetic ?eld generators;
c. measuring a modi?ed magnetic ?eld at each of the mag
netic ?eld detectors, Wherein the generated magnetic
?eld is modi?ed by the object; and
d. processing the measured magnetic ?eld to obtain a data
set characterizing the object and a location of the object,
Wherein a reconstruction process is applied to a pre
de?ned number of measurements of the modi?ed mag
netic ?eld.
a visual output using a movement sensor that shoWs one or
With characteristics of the data set of the object and With the
measured position of at least a part of the object.
23. The method of claim 20 further comprising a) detecting
a displacement of at least a part of the object aWay from a
reference position and b) correcting for the displacement
When combining sets of measurements of the modi?ed mag
netic ?eld, Wherein the reference position is relative to at least
a part of the object and the displacement is relative to at least
a part of the object.