U.S. patent number 3,790,945 [Application Number 05/201,686] was granted by the patent office on 1974-02-05 for open-strip ferromagnetic marker and method and system for using same.
This patent grant is currently assigned to Stoplifter International, Inc.. Invention is credited to Edward R. Fearon.
United States Patent |
3,790,945 |
Fearon |
February 5, 1974 |
OPEN-STRIP FERROMAGNETIC MARKER AND METHOD AND SYSTEM FOR USING
SAME
Abstract
The specification describes an open-strip marker for use in
detecting the presence of an object in an interrogation zone. Each
object to be detected is provided with a marker comprising an
elongated ferromagnetic element having a high permeability and a
high ratio of length to cross-sectional area. A periodically
varying magnetic field is provided in an area through which the
object is to pass. Upon passage of the object through the area,
reversal of the magnetization of the marker results in a uniquely
characteristic signal for each alternation of the electromagnetic
field. This characteristic signal is sensed to distinguish the
presence of the object.
Inventors: |
Fearon; Edward R. (Tulsa,
OK) |
Assignee: |
Stoplifter International, Inc.
(N/A)
|
Family
ID: |
26897007 |
Appl.
No.: |
05/201,686 |
Filed: |
November 24, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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747050 |
Mar 22, 1968 |
3631442 |
|
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|
680666 |
Nov 6, 1967 |
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Current U.S.
Class: |
340/572.2;
343/787; 340/572.6 |
Current CPC
Class: |
G08B
13/2474 (20130101); G01S 13/753 (20130101); G08B
13/2431 (20130101); G08B 13/2442 (20130101); G08B
13/2437 (20130101); G06K 7/10009 (20130101); G08B
13/2408 (20130101) |
Current International
Class: |
G08B
13/24 (20060101); G06K 7/10 (20060101); G01S
13/00 (20060101); G01S 13/75 (20060101); G08b
013/24 () |
Field of
Search: |
;340/258R,258C,280,224
;325/8,105 ;343/6.5SS,6.8,787,788 ;179/82 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trafton; David L.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional application of U.S. Patent
application Ser. No. 747,050, filed Mar. 22, 1968, now U.S. Pat.
No. 3,631,442, which was filed as a continuation in part of now
abandoned patent application Ser. No. 680,666 filed Nov. 6, 1967.
Claims
1. A system for sensing the passage of an object through a
surveillance area comprising:
means for generating an oscillating magnetic field in said
surveillance area,
an elongated thin open-stripe of ferromagnetic material for being
secured to an object and having an aggregate saturation magnetic
moment of about one pole-centimeter and producing at least one
pulse of magnetic field having frequencies of very high order,
extending up to and including the thousandth harmonic of the
fundamental frequency of the oscillating
2. The marker of claim 1 wherein said generating means generates an
oscillating magnetic field having a peak intensity of at least 3
oersteds.
3. A marker for use in a system for detecting the presence of an
object within an interrogation zone wherein magnetic coils generate
a magnetic field periodically varying at a predetermined
fundamental frequency in the interrogation zone and a sensor
operates a security readout upon detection of high harmonic
frequencies of the fundamental frequency comprising:
a marker for being secured to an object to enable detection of the
presence of the object within the interrogation zone,
said marker comprising an elongated thin ferromagnetic strip having
a maximum permeability of in the range of 400,000,
said strip being about 4 centimeters in length and having a
cross-sectional area of approximately 0.0004 square centimeters and
a thickness not greater than about 0.00125 centimeters,
said strip when disposed with its longitudinal axis generally
parallel to the varying magnetic field generating very high
harmonic frequencies of
4. The marker of claim 3 wherein said ferromagnetic strip is
comprised of
5. The marker of claim 3 wherein said fundamental frequency equals
60 cycles per second and wherein said strip returns harmonic
frequencies
6. A system for sensing passage of objects through a surveillance
area comprising:
means proximate the area for producing oscillating electromagnetic
energy components in the area,
a thin electromagnetically nonlinear marker having a magnetic
permeability of at least 400,000 associated with each object and
dimensioned to have a high ratio of length to cross-sectional area
for re-radiating electromagnetic energy at very high harmonic
frequencies when said components are coupled to the marker, and
means for sensing electromagnetic energy in said area and for
producing a
7. A system for detecting the presence of an object when said
object is in an interrogation zone having a magnetic field
periodically varying at a predetermined fundamental frequency, said
system comprising:
marker means to be secured to an object, said marker means
comprising an elongated ferromagnetic element having a high
permeability capable of generating signals containing harmonics of
said fundamental frequency in excess of the 20th order when placed
in said zone,
radiating means for producing within said interrogation zone said
magnetic field having an intensity sufficient to reverse the
magnetization of said ferromagnetic element while in said zone,
receiving means for detecting said signals containing said
harmonics of said fundamental frequency produced by said
ferromagnetic element, and
security readout and communications means coupled to said receiving
means responsive to said signal to indicate the presence of an
object in
8. A system according to claim 7 wherein said marker element
comprises a ribbon having a maximum permeability of at least
400,000 and a ratio of length to square root of cross-sectional
area in excess of 200, wherein said marker element is positioned in
the zone with its axis approximately parallel with the magnetic
field, and wherein said radiating means for generating said varying
magnetic field radiates into said interrogation zone a 60 Hz
oscillating field having a peak intensity of at least 3
9. A system according to claim 7 wherein said radiating means for
generating said oscillating magnetic field includes:
an oscillatory power source, and
a doorway coil comprising a flat wound coil to said power source to
radiate an oscillating magnetic field into said interrogation zone,
and wherein said receiving means comprises a loop antenna coupled
to an amplifier, the output of which is coupled to said security
readout and communication
10. A method for detecting the presence of an object when said
object is in an interrogation zone having a magnetc field
periodically varying at a predetermined frquency, comprising the
steps of:
securing to an object a marker comprising an elongated
ferromagnetic element having a high permeability capable of
generating signals containing harmonics of said fundamental
frequency in excess of the 20th order when placed in said zone;
radiating within said interrogation zone said magnetic field having
an intensity sufficient to reverse the magnetization of said
ferromagnetic element while in said zone;
receiving said signals containing said harmonics of said
fundamental frequency produced upon reversal of the magnetization
of said marker element when in said zone; and
energizing a security readout and communications system in response
to said
11. A method according to claim 10 wherein said ferromagnetic
element is characterized by having a permeability in excess of
50,000, a coercivity of less than 0.1 oerstead, and dimensions such
that the ratio of length to square root of cross-section area of
said element is in excess of 150, and wherein said marker element
is positioned in the zone with its axis
12. A method according to claim 10 further characterized by
radiating within said interrogation zone said magnetic field having
an oscillation frequency of 60 Hz, and a minimum peak intensity of
at least 3 oersteds and further comprising receiving from said
element said signals containing harmonics of said oscillating
frequency extending up to and including
13. A system for detecting the presence of an object within an
interrogation zone comprising:
coil means extending around said interrogation zone,
an oscillator having a fundamental frequency connected to said coil
means,
marker means for being attached to an object to reflect harmonics
of said fundamental frequency when said object is placed in said
interrogation zone,
circuit means connected to said coil means for detecting even and
odd harmonics present on said coil means and for generating a
voltage representative of the ratio of selected even and odd
harmonics, and
means responsive to said voltage for energizing object indication
means.
14. A marker for being used in a system for sensing passage of
objects through an interrogation zone having a magnetic field
periodically varying at a predetermined fundamental frequency
comprising:
a marker for being secured to an object to be detected by said
system,
said marker comprising an elongated thin ferromagnetic element of
high permeability and characterized by dimensions including a ratio
of length to square root of cross-sectional area of at least 200
such that said marker generates detectable signals containing
harmonics of said fundamental frequency in excess of the 20th order
when said object bearing said marker is placed in said zone with
the longitudinal axis of said ferromagnetic element positioned
generally parallel to the varying
15. A marker for being used in a system for sensing passage of
objects through an interrogation zone having a magnetic field
periodically varying at a predetermined fundamental frequency
comprising:
a marker for being secured to an object to be detected by said
system,
said marker comprising an elongated thin ferromagnetic element of
high permeability and characterized by a saturation magnetization
in excess of 0.1 pole-centimeter such that said marker generates
detectable signals containing harmonics of said fundamental
frequency in excess of the 20th order when said object bearing said
marker is placed in said zone with the longitudinal axis of said
ferromagnetic element positioned generally
16. A marker for being used in a system for sensing passage of
objects through an interrogation zone having a magnetic field
periodically varying at a predetermined fundamental frequency
comprising:
a marker for being secured to an object to be detected by said
system,
said marker comprising an elongated thin ferromagnetic element of
high permeability and having a cross-sectional area of less than
10.sup.-.sup.3 CM.sup.2 and a length of at least 4 centimeters such
that said marker generates detectable signals containing harmonics
of said fundamental frequency in excess of the 20th order when said
object bearing said marker is placed in said zone with the
longitudinal axis of said ferromagnetic
17. A marker for being used in a system for sensing passage of
objects through an interrogation zone having a magnetic field
periodically varying at a predetermined fundamental frequency
comprising:
a marker for being secured to an object to be detected by said
system,
said marker comprising an elongated thin ferromagnetic element of
high permeability and having a cross-sectional area of in the range
of 0.0004 CM.sup.2 such that said marker generates detectable
signals containing harmonics of said fundamental frequency in
excess of the 20th order when said object bearing said marker is
placed in said zone with the longitudinal axis of said
ferromagnetic element positioned generally
18. A marker for being used in a system for sensing passage of
objects through an interrogation zone having a magnetic field
periodically varying at a predetermined fundamental frequency
comprising:
a marker for being secured to an object to be detected by said
system,
said marker comprising an elongated thin ferromagnetic element of
high permeability and low coercive force and having a slender cross
section compared to length, said marker further having a magnetic
moment of a magnitude such that said marker generates detectable
signals containing harmonics of said fundamental frequency in
excess of the 20th order when said object bearing said marker is
placed in said zone with the longitudinal axis of said
ferromagnetic element positioned generally parallel to the varying
magnetic field.
Description
FIELD OF THE INVENTION
This invention relates to object detection, and more particularly
relates to object detection in such applications as anti-pilfering
and sortation systems.
THE PRIOR ART
There are in existence several systems for detecting or preventing
the theft of articles of value. One of these corresponding with
U.S. Pat. No. 3,292,080, granted to E. M. Trikilis, Dec. 13, 1966,
makes use of a magnetometer and utilizes a magnetized object which
identifies the article unless checkout procedure has removed the
magnetism from the object. The magnetized object is attached to or
becomes a part of the merchandise or article of value, and by
energizing the magnetometer system as it passes through the
doorway, is detected. If the magnetized object has been
demangetized it causes no magnetic signal as it passes through the
doorway and is not detected. Demagnetizing is done in the process
of checking out the merchandise. Thus by the checkout procedure an
individual has free passage with the merchandise that has been paid
for or recorded by the clerk. Any additional merchandise not paid
for and however concealed radiates a magnetic influence, and
energizes the magnetometer at the doorway, creating an awareness of
security department personnel that something is being stolen.
Another system involves radioactive material which emits nuclear
radiation. When the label containing the magnetic material is
removed from the merchandise, the radiation is no longer emitted,
and therefore radiation detectors situated in the doorway are not
energized. On the other hand, if the radiation emitters remain on
the merchandise, doorway sensors of nuclear radiation react, and
security personnel are in a position to prevent the theft.
In another system currently being employed in a men's wear
department in Macy's in New York City, the operator uses a radio
frequency generating device embedded in a rubber pad. The radio
frequency emitting device is fastened to the men's clothing, and if
not removed, will energize radio frequency detecting antennae at
the doorway. In the normal course of events, when the merchandise
is sold, a special fastener is unlocked and the radio frequency
emitter is removed from the clothing at the time it is sold,
permitting the buyer to pass through the doorway without attracting
the attention of the store detective.
Another system specifically intended for use in anti-pilferage
application is described in a 1934 French Pat. No. 763,681, issued
to Pierre Arthur Picard, which discloses a remote detection system
employing dynamic magnetic phenomena to detect the presence of an
object, e.g., a book being carried through a doorway. The system of
Picard is based upon the discovery that a piece of metal subjected
to a sinusoidally varying magnetic field induces in a pair of
balanced pickup coils in the vicinity of the applied field a
voltage characteristic of the metal. The patent indicates that high
permeability metals produce an induced voltage including higher
order harmonics of the sinusoidal field than the harmonics of
metals such as iron. Permalloy is listed as a high permeability
material from which the characteristic voltage contains ninth and
eleventh harmonic components, unlike such common metals as copper,
iron, or aluminum, which produce practically no harmonics of such a
high order.
According to the Picard patent, only the composition of a metal
determines the order of the harmonics present in its characteristic
voltage. It is disclosed that marker size and geometry affect the
amplitudes of all frequency components proportionately.
Accordingly, the ratio of two individual harmonic components for a
particular material would be the same regardless of the material's
size or geometry. The Picard patent further discloses that the
ratio between at least certain selected components is
characteristically different for different materials. Picard
emphasizes that the size of the metal piece to be used as a marker
is important; not to control the order of the harmonics present,
but rather to provide a signal large enough to be detected.
All of the foregoing systems have severe difficulties of one kind
or another. The Trikilis system requires a rather large piece of
ferromagnetic material for the marking of the merchandise. If too
small a piece of ferromagnetic material is used, ambient variations
in the magnetic field are greater than the changes caused by the
Trikilis merchandise marker. In the case of the radioactive dot,
there is a severe health problem involving danger to people from
the nuclear radiation, and involving danger to those who remove the
markers and store them. The system in use in Macy's Store
unfortunately is limited by the extreme costliness of the radio
frequency transmitter, and the limited period of time during which
its emission can be maintained by the little batteries with which
it is provided. True, larger radio frequency emitting pads could be
made, but these tear or injure the clothing and are impractically
bulky.
The Picard patent fails to appreciate the dependence of the
harmonic signal on the shape of the high permeability element. The
resultant detection of signals containing only nine to 11 order
harmonics, e.g., frequencies on the order of 650 Hz when excited by
a 50 Hz field, allows the occurrence of false alarms resulting from
the presence of other magnetic materials as well as relatively
common electrical noise. The Picard system purports to avoid such
limitations by measuring the ratio of two harmonics.
SUMMARY OF THE INVENTION
I have discovered a practical solution to the problems presented
but not solved by the workers in the prior art as described above.
As a matter of convenience, I choose to employ electromagnetic
radiation. However, because of the inconvenience of supplying
energy in a contraband marking, the energy to be radiated from the
contraband marked device is delivered, instead, from structural
members of my sensing doorway.
I have found it extremely difficult to re-radiate or reflect energy
in a distinctive manner from any merchandise marker for the reason
that all solid bodies and all electrically conductive masses
(including the human body which is largely composed of salt water)
also reflect or disperse electromagnetic radiation and therefore
must be considered in the recognition of any merchanidse marking. A
human being reflects more electromagnetic energy than any practical
size of merchandise marker.
I have solved the problems just described by my discovery of an
extremely simple device which can receive energy and re-emit it,
receiving the energy in a frequency spectrum entirely distinct from
the frequency spectrum which is re-emitted. I do this by making use
of the properties of electrically and electromagnetically nonlinear
systems. In general, it is the property of a nonlinear system that
if a frequency F is imposed at an energy level at which the
nonlinearity of the system becomes important, the system will
generate frequencies 2F, 3F, 4F, etc. Similarly, when signal
sources are imposed on a nonlinear system, the sources delivering
approximately equal energy in each of two frequencies, the
nonlinear system will generate other frequencies, not originally
present. If the frequencies imposed are F.sub.1 and F.sub.2, the
nonlinear system will generate signals having frequencies F.sub.1 +
F.sub.2, F.sub.1 - F.sub.2, F.sub.1 + 2F.sub.2, 2F.sub.1 +
2F.sub.2, and various other combinations of sums and differences of
multiples of the frequencies imposed. It is an essential part of my
invention that I have discovered a merchandise marker which
constitutes a nonlinear system and which therefore can generate
phenomena such as those which have just been discussed.
I have discovered a theft detection and prevention system which
comprises a surveillance doorway containing among other things
emitters of electromagnetic energy adapted to emit electromagnetic
energy into a region of space. My system also includes an
electromagnetic signal detection means situated at or near the
surveillance doorway designed and adapted to receive energy
re-radiated from a merchandise marker device.
In accordance with a more specific aspect of the invention, a
marker is provided which, when secured to an object, enables the
detection of that object in an interrogation zone such as a
doorway. Such a zone has a magnetic field periodically varying at a
predetermined fundamental frequency. The marker comprises an
elongated ferromagnetic element of high permeability which is
capable of responding to the magnetic field to generate cusp-like
signals containing harmonics of the fundamental frequency of very
high order. Such a marker preferably has dimensions to provide a
very high ratio of length to cross-sectional area, and further has
a permeability in the range of 400,000 or greater, and a coercivity
of about 0.02 oersteds. In this invention, not only is a larger
detectable signal produced by the marker than is produced by pieces
of the same material not having the necessary dimensions, but by
specially selecting the dimensions of the marker, the signal
resulting from the presence of the very high harmonics is far
greater than that resulting from greater amounts of the same
materials having non-preferred dimensions. Such harmonics are many
times higher order than those observed by Picard or obtainable in
any materials except the specially selected shapes of uncommon
metals, thereby providing a system not susceptible to false alarms.
Furthermore, the marker of my invention readily lends itself to
concealment in various articles of merchandise, making compromise
of my system difficult.
The present invention is further directed toward a system for
detecting the presence of an object when the object is in an
interrogation zone such as a doorway, in which the zone has a
magnetic field periodically varying at a predetermined fundamental
frequency. The system comprises the marker as described above, a
radiating means for producing the magnetic field within the
interrogation zone having an intensity sufficient to reverse the
magnetization of the marker element when in the zone, a receiving
means for detecting cusp-like signals containing the high order
harmonics of the fundamental frequency which are produced by the
ferromagnetic element, and a security readout and communications
means coupled to the receiving means responsive to the signal to
indicate the presence of an object in accordance with the harmonic
content. In the preferred embodiment, the marker element is
positioned in the zone with its axis approximately parallel with
the magnetic field.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a somewhat diagrammatic view of a typical installation of
the system of the invention;
FIG. 2 illustrates one form of a contraband marking element for use
in the present system;
FIG. 3 illustrates another embodiment of a marker for use with the
invention;
FIG. 4 is a schematic of an embodiment of a system for radiating
electromagnetic energy to detect the presence of a marker according
to the invention;
FIG. 5 is a diagram to assist in explaining the operation of the
present energizing and detecting system;
FIG. 6 is a diagram illustrating the filter and coil system of the
invention;
FIG. 7 illustrates the preferred claimed marker of the present
invention;
FIG. 8 illustrates a typical waveform generated by the preferred
marker shown in FIG. 7; and
FIG. 9 illustrates the harmonic detection system of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
I now turn to FIG. 1 which is a general view of the manner in which
the present system operates in a store to prevent theft of
merchandise. Merchandise 1 is provided with contraband marker
elements 2. The checkout stand area 3 contains a deactivating
device 4 which is capable of changing the electromagnetic
properties of the contraband marker elements 2. An energizing and
detecting system 5A situated in the 5B and 5C vicinity of the
outgoing doorway 6 detects the contraband marker elements 2, and
identifies those which have not been subjected to change at the
checkout stand area 3 by the deactivating device 4. In the use of
my system, one way traffic, enforced by perhaps a turnstile 7,
takes care of persons entering the store, prohibiting the carrying
of merchandise from the store to areas outside the store except
through my outgoing doorway 6. The turnstile 7 is provided at the
entry portal 8.
I turn now to FIG. 2 which illustrates one form of contraband
marking element suitable for use in my system. The element includes
an easily saturable high permeability filament or narrow ribbon 9
of specialized magnetic material such as the one known by the
tradename Superpermalloy, which has a maximum permeability of over
800,000 and a coercive force of about 0.002 oersteds. The filament
extends parallel to, and is so situated as to collect the magnetic
flux from two pole piece coupons 10 which are also composed of high
quality magnetic material of the type having a very low coercive
force and a very high maximum magnetic permeability. In this
embodiment, the coupons 10 are preferably composed of materials
having a maximum permeability in the vicinity of 50,000 or
thereabouts. Attached to the coupons 10 I provide masses of rigid
plastic substance such as polymerized methyl methacrylate 11. In
use, the device is assembled between layers of paper 12 (or
plastic) illustrated in exploded view (removed from the vicinity of
the filament 9 and coupons 10). The filament 9 and coupons 10 are
not shown in exploded form, but are illustrated realistically. In
use, the filament 9 is spaced from the coupons 10 by a few
thousandths of an inch, the space being occupied by a lubricating
particle suspension such as silicone oil with magnesium oxide
particles in it. Any other suitable lubricant may be employed,
together with particles of suitable size. For example, petroleum
lubricant and carbon particles are satisfactory. Fluorocarbon oil
with bentonite clay suspended in it is suitable. In use of the FIG.
2 device, the space between the filament 9 and the adjacent layer
of paper 12 is also filled with a suitable particle suspension
lubricant, to space the filament 9 apart from the paper 12 by an
appropriate distance.
In use, the contraband assembly described in FIG. 2 encounters, at
the outgoing doorway 6 of FIG. 1, a combination of electromagnetic
fields producing an oscillating component of magnetic field
parallel with the axis of the filament 9. In one embodiment, the
oscillating component of magnetic field, as provided in the
outgoing doorway 6, includes contributions of two separate
frequencies. The magnetic fields thus provided have a component
parallel to the axis of the filament 9 as shown in the FIG. 2
device, and are of sufficient magnitude to bring about a
substantial degree of magnetic saturation of the filament 9
parallel to its axis. Because of the nonlinearity of the magnetic
phenomena occurring in the filament 9, summation and difference
tones are produced and radiated in the form of electromagnetic
radiation from the device shown in FIG. 2. The above described
phenomena occur when merchandise 1 (FIG. 1) carrying a contraband
marker element 2 shown in detail in FIG. 2 is taken through the
outgoing doorway 6 (FIG. 1) without paying for it.
When, on the other hand, the customer pays for the merchandise 1
(FIG. 1) the merchandise 1 (FIG. 1) is presented in the vicinity of
the deactivating device 4 (FIG. 1) in the checkout stand area 3
(FIG. 1). The deactivating device 4 (FIG. 1) delivers an extremely
strong magnetic field, a field so strong that it is sufficient to
induce a very large magnetic flux not only through the filament 9,
but also in the coupons 10 of the device of FIG. 2. The large
magnetic flux induced in the coupons 10 (FIG. 2) by the
deactivating device 4 (FIG. 1) would normally bring about an
elongation (or in some instances possibly a shortening) of the
material composing the coupons 10 (FIG. 2) in the direction in
which the magnetic flux is induced.
However, the plastic substance 11 attached to each coupon 10 is
rigid and non-magnetic. The plastic substance 11 being firmly
attached to the coupons 10 resists the dimensional change which
would otherwise occur due to a strong magnetic flux in the coupons.
The clamping effect which the plastic substance 11 thus exerts
corresponds with a mechanical strain improsed on the magnetic
material of the coupons 10. The mechanical strain being beyond the
elastic limit of the said magnetic material, it undergoes cold-work
which destroys its superior magnetic properties, degrading its
maximum permeability from the vicinity of 50,000 to the general
vicinity of one or two thousand.
A FIG. 2 device assembled as described in the foregoing paragraph
and deactivated as described, still has demonstrable nonlinear
magnetic properties. However, the doorway field intensity required
to induce nonlinear behavior of the filament 9 is substantially
altered, for the reason that the maximum permeability of the
coupons 10 being lowered, they do not collect magnetic flux from
the outgoing doorway 6 environement and feed it into the filament 9
as efficiently as they did before their magnetic properties were
degraded. Thus it is possible for an energizing and detecting
system 5 existing in the vicinity of the outgoing doorway 6 to
determine the presence of unsold merchandise 1, and at the same
time be sensitive to the fact that the same individual, or one
nearby, is also carrying merchandise which has been properly paid
for and carries deactivated contraband marker elements 2 (FIG. 1).
(I note that the contraband marker elements 2, as actually
illustrated in FIG. 1, are not deactivated, being shown inside the
store area.)
FIG. 3 illustrates a modified form for the coupons 10 (FIG. 2). In
this modified form the coupons 10 are not attached to any plastic
clamping substance 11 over their entire surface, but material of a
different nature, either more or less magnetic than the material of
the coupons, or not magnetic at all, is deposited in a periodically
spaced pattern in a plurality of closely spaced stripes 13
equi-distant from each other on the specially arranged coupon 14,
which has properties generally similar to the properties of the
coupons 10 of FIG. 2. Because of the mass of such periodically
spaced stripes 13, and because of their other properties by which
they are differentiated from the magnetic material composing the
specially arranged coupon 14, the specially arranged coupon 14 in
combination with the stripes 13, exhibits a mechanical resonance
tending to vibrate in such a manner that the material situated at
the stripes 13 undergoes a minimum of movement and/or dilatation.
If mass is the predominant characteristic of the material at the
stripes 13, the stripes 13 will correspond with a minimum of
movement. If mechanical stiffness predominates, the stripes 13 will
correspond with very little change of dimension at the frequency of
the resonance, and with the specially arranged coupon 14 vibrating
in the resonant mode. Because of the influence of the periodically
spaced stripes 13, the specially arranged coupon 14 will always
exhibit a sharply determined mechanical resonance in the manner
just described.
When using contraband marker elements 2 manufactured generally in
accord with FIG. 1, but provided with specially arranged coupons
14, the deactivating device 4 (FIG. 1) should be energized at the
frequency corresponding with the resonance, or the deactivating
device 4 should be made resonance seeking with respect to the
desired mode of mechanical motion. Inducing the resonant motion,
the deactivating device 4 (FIG. 1) causes mechanical energy to
build up in the specially arranged coupon 14 until the amplitude of
movement and the amplitude of stress and strain involved in the
resonant oscillations approaches the elastic limit of the magnetic
material composing the specially arranged coupon 14. As has been
described before in conjunction with the deactivation of coupons
such as the coupons 10 (FIG. 2), the coldwork result from the
movement causes the magnetic properties of the specially arranged
coupons 14 to be degraded from the general vicinity of a maximum
permeability of 50,000 to a maximum permeability in the vicinity of
one to two thousand. As before, the contraband marker elements 2
(FIG. 1) in which coupons of whatever type have been degraded, are
recognizable, and may be differentiated from other contraband
marker elements 2 (FIG. 1) which have not passed through the
deactivating process, and not had their coupons degraded.
Although the provision of periodically deposited stripes 13 on the
specially arranged coupon 14 helps to define and select a
particular resonance at which the specially arranged coupon 14 will
oscillate, as will be later described in detail, the coupon without
stripes 13 and without the plastic substance 11 (FIG. 2) can also
be induced to oscillate in a resonance mode. In fact resonant
oscillations can be induced at a wide variety of modes comprising
an extensive plurality of possible choices of resonant frequencies.
This, in fact, is the chief difference between an ordinary
unclamped coupon such as the coupon 10 of FIG. 2 (but without
plastic substnce 11 and without stripes 13 as provided in FIG. 3)
and the specially arranged striped coupon 14 of FIG. 3. Because of
the stripes, the specially arranged coupon 14 of FIG. 3 prefers a
particular mode of resonant oscillation and the striped structure
13 tends to suppress the other modes which are a feature of an
unclamped and unstriped coupon. In fact the convenience of the
stripes lies in this, that the otherwise extremely large diversity
of possible oscillatory frequencies is reduced by the stripes 13 to
one chosen and preferred mode and frequency. Through the provision
of this feature, a specially arranged coupon 14, because of its
thickness, mechanical characteristics, and because of the
periodicity of the stripes 13, is distinctly recognizable and can
be differentiated at the outgoing doorway 6 (FIG. 1).
Thus it is possible, using the present system, and using the
provisions of my FIG. 3 to distinctly characterize the contraband
marker elements 2 (FIG. 1) being employed by Woolworth's, or for
example by Sears Roebuck. In fact, using the recognition
capabilities intrinsic in contraband markers thus manufactured, the
outgoing doorway 6 energizing and detecting system 5 (FIG. 1) can
report at the Sears Roebuck store when it detects merchandise 1
that was stolen at Woolworth's and determine that it is Woolworth
merchandise that is being observed.
Attention is now directed to the energizing and detecting system 5
(FIG. 1) situated in the outgoing doorway 6 (FIG. 1). Because there
are three perpendicular coordinates available in space of three
dimensions, two energizing systems and detecting devices can be
arranged to work in a non-interacting manner. In fact, it is a
characteristic of one embodiment of the invention that within the
limits of accuracy of adjustment of the position and orientation of
the electromagnetic radiating and receiving components, two
radiating components radiate independently, neither one being
capable of transmitting energy into the other one, and further, the
detecting or receiving pickup does not receive energy directly from
either of the radiating devices. These arrangements of course are
valid only when the space in the doorway is empty, there being no
contraband marker elements 2 (FIG. 1) in it. This type of
arrangment which has been generally recited above is depicted in
more detail in FIG. 4.
In FIG. 4 I have pictured two pedestals 31, each containing near
its center a pair of sending coils 32. All the sending coils 32 are
connected in parallel (or they could have been connected in
series). For illustration only, I will suppose that the frequency
by which these sending coils 32 are energized is 21 kilohertz. Each
such sending coil 32 is separately tuned to exhibit the highest
possible impedence at 21 kilohertz. For illustration only, the coil
may be composed of 99 turns of No. 20 copper wire wound on a 1 inch
diameter coil form in a single layer to produce 99 turns in a total
length of 3-1/2 inches. Such a coil may be resonated to 21
kilohertz by the use of an electrical capacity of not less than 1
microfarad and not more than 1.1 microfarad. The combination of one
of these coils 33 with its resonating capacitor 34 (as shown in the
inset), when energized at the resonant frequency, represents an
entirely resistive impedence and in the illustrative case exhibits
a resistance between 100 and 150 ohms. A parallel combination of
four such resistive loads has a combined effect adapted to
efficiently load the voice coil outputs of some available audio
amplifiers.
Similarly, there are situated at the bottom and at the top of each
of the pedestals 31, coils 35 intended for transmitting another
chosen frequency such as (for illustration only) 24.5 kilohertz.
The four coils 35 which are intended for 24.5 kilohertz radiation
may be constructed similarly and resonated similarly, but, of
course, resonate with a correspondingly smaller electrical capacity
for each coil. The combination of the first group of four coils 32
is connected to a source of electrical energy 36 at 21 kilohertz.
The combination of the second group of four coils 35 is connected
to a separate, entirely independent, source of electrical energy 37
at 24.5 kilohertz. Because of the arrangement which I have chosen
for the first group of coils 32 and for the second group of coils
35, there is no appreciable mutual inductance acting to deliver 21
kilohertz energy into the 24.5 kilohertz, or vice versa.
At four other locations I present four more coils 38 with their
axes perpendicular to the plane of the paper. Because all the
contributions of the first group of four coils 32 and the second
group of four coils 35 lie in the plane of the paper, the four
coils 38 with their axes perpendicular to the plane of the paper do
not receive energy neither at 24.5 kilohertz, nor at 21 kilohertz.
The four coils 38 with their axes perpendicular to the paper are
resonated at 3.5 kilohertz by choosing an appropriate electrical
capacitance. In order to achieve good sensitivity in these coils,
and in order that they may be resonated efficiently at the
frequency of 3.5 kilohertz, more copper is required in the winding,
preferably four layers of No. 20 wire, each layer containing 99
turns more or less. The capacity required to resonate such a coil
is in the general vicinity of two microfarads for 3.5
kilohertz.
I call attention to the fact that the cores of these windings have
not been specified thus far. It is a preferred choice to wind them
on non-magnetic, electrically non-conducting material, for the
reason that ferromagnetic material (because of its nonlinear
properties) imparts to my system undesirable interactions between
the energy sources. Electrically conducting material, on the other
hand, destroys the quality of the inductive performance of all the
coils. As a matter of fact, an air core coil of 99 turns, made in
the manner that I have described, has a Q in the vicinity of 500 at
21 kilohertz when wound on a wooden core. The resonance cannot be
found, nor the inductance measured well enough to determine the Q
if it is wound on an electrical conductor as a core.
The combination of the four coils, as described, with their axes
perpendicular to the paper (each coil resonated at 3.5 kilohertz by
appropriate electrical capacitance) delivers its output to the
ingoing end of a high gain tuned amplifier 39 adapted to
selectively receive and amplify electrical signals at 3.5
kilohertz. The amplifier 39 delivers its output to an alarm
mechanism 40, or to a carrier frequency module, which is discussed
further on. To achieve a closer impedence match with respect to the
commonly prevailing input resistance of the amplifiers that are the
most convenient, I may choose to vary from the connections shown in
FIG. 8, and connect the four receiving coils 38 (the ones with
their axes perpendicular to the paper) in series. The resistive
component of these coils (with their resonators connected) comes
out for each such resonated system in the vicinity of 100 ohms,
with the result that the seris of four of them are a close match to
the communications impedance figure of 500 ohms, a common choice
for amplifiers, filters, etc.
I turn now to FIG. 5 presented for the purpose of diagrammatically
assisting in the explanation of the manner of functioning of the
energizing and detecting system 5 (FIG. 1) which I have
particularly detailed and described in connection with FIG. 4. In
FIG. 5 the axis X may be taken to represent the action of the 21
kilohertz radiator, the perpendicular axis Y illustrates the action
of the 24.5 kilohertz radiator, and the axis Z represents the
receiving sensitivity or direction of the 3.5 kilohertz receiving
coils 38 of FIG. 4. The vector .theta. is illustrated in a
direction not parallel to nor perpendicular to any of the three
axes. The vector .theta. represents the direction in which a
contraband marker element 2 (FIG. 1) is capable of receiving and
re-radiating energy. Because the vector .theta. has an appreciable
component in all three axes, the contraband marker element 2 (FIG.
1) oriented in accord with this vector is able to receive energy
concurrently at 21 kilohertz, and likewise at 24.5 kilohertz. For
similar reasons, if the contraband marker element 2 (FIG. 1)
re-radiates at 3.5 kilohertz (not being deactivated) then detection
axis Z is so directed with respect to the vector .theta. that the
said detection system is not insensitive to radiation emitted by
the contraband marker element 2 (FIG. 1).
The user, considering the information presented in connection with
FIG. 4, and the information just presented in connection with FIG.
5, will realize that the reception of a 3.5 kilohertz in the system
is a distinctive and an exclusive evidence of the presence of
contraband marker elements 2 (FIG. 1). One or more such elements
must be in the domain of energy radiation and sensitivity provided
by the arrangements shown in FIG. 4 to deliver a 3.5 kilohertz
signal. Other entities than contraband marker elements are not
entirely without effect, but they do not present the same
effects.
To aid the understanding of another modification of my sistem which
I have described, I turn again to FIG. 5. In FIG. 5 I have
represented the directions of action of the energy source
frequencies X and Y (21 and 24.5 kilohertz sources) and the
direction of sensitiviy of the system that detects the difference
tone Z in the form of three perpendicular axes. To the worker
skilled in the art, it is evident that if contraband vector .theta.
is exactly perpendicular to either of the signal source axes X or
Y, energy is eliminated which corresponds with the vector to which
the vector .theta. is perpendicular. Furthermore, if the vector
.theta. lies in the X - Y plane, it is perpendicular at all times
to the axes Z, which therefore prohibits the reception of any
energy in the signal receiving system 38, (FIG. 4). It is, in fact,
true that the vector .theta. must have appreciable and comparable
components or direction cosines aligned with all three of the
vectors X, Y, and Z. For those directions .theta. which do not
fulfill these conditions, either the difference tone signals are
not produced or they are not observed (if produced) by the
contraband marker element 2 (FIG. 1). The fact that there are so
many blind spots and so many requirements on the direction of
contraband, causes the system, conceived as in the foregoing, to
sometimes fail to recognize contraband markers passing through the
outgoing doorway 6 (FIG. 1). It still remains a fact that nothing
other than a contraband marker will ring the alarm. However, a way
has been discovered to reduce the inconvenience resulting from the
above noted limitations (which now and then permit a contraband
marked piece of stolen merchandise to get through).
The user will note in FIG. 4 that in the foregoing I have excluded
the energy from the 21 kilohertz source from getting into the 24.5
kilohertz source by arranging for separate radiators, and arranging
that these be non-interacting because of their perpendicularity
arrangement. Another approach to excluding wrong pathways of signal
energy is quite applicable in the frequency range which I have
chosen, an approach not dependent on geometry. My modification
permits advantages in the simplification of the doorway
structure.
The system which is contemplated for the reduction of the number of
blind spots in respect to the direction of the vector .theta. (FIG.
5) substitutes rigorously designed wave filters, containing passive
elements only. These perform the function performed by the
geometric isolation in the system of FIG. 4. Such wave filters can
be designed for the range of frequency in the vicinity of 20 to 50
kilohertz without the use of ferromagnetic material or anything
else which would impose a nonlinearity. The wave filters thus used,
if provided in a sufficient number of sections, propagate the
desired energy substantially without loss and are able to reject
the unwanted signal frequencies to whatever extent is desired,
through the use of a sufficient number of networks. A properly
designed M or .pi. derived filter network will exclude unwanted
frequencies by over one hundred decibels in just a few
networks.
Lattice type filters may be employed for single frequency rejection
and are extremely effective. In fact, the only serious limitation
on the rejection brought about by a lattice type filter is imposed
by variation in frequency of the signal which it is desired to
reject. A lattice type filter, for example, may comprise two
electrical capacitances and two inductive elements as the four
components of a bridge. The input to the bridge and the output to
the bridge have a ratio which theoretically is infinite at the
frequency at which it balances. Thus it is theoretically possible
to exclude a single frequency to any extent, by a single network of
such a filter. At the same time a single network lattice filter can
transmit very efficiently energy corresponding with signal
frequencies that are substantially different from the signal
frequency at which the bridge balances.
For 20 kilohertz or more, substantially perfect inductances
(inductances with a Q in the realm of thousands) can be delivered
in the space of a few cubic inches, and need not contain more than
an ounce or two of copper wire. Again in the frequency spectrum
involving a metal box comprised of iron or copper, and with a coil
spaced from the walls, inside the box, the coil neither radiates
nor absorbs electromagnetic energy appreciably in this kilohertz
range. Capacitances constructed of aluminum foil and wound with
such a dielectric as wax paper (or mylar or polystyrene) give a
substantially perfect electrical performance in my preferred
frequency range. It is, accordingly, entirely feasible to
contemplate the substitution of rigorous filtering in place of the
previously described geometric means of arranging radiator coils so
that energy is not transferred from one system to another.
Moreover, the use of well designed filters has a further advantage,
that the presence of conducting bodies of any description in the
doorway 6 (FIG. 1) does not cause energy to flow from one system to
the other, since the wave filters function independently of
whatever bodies are situated in the doorway 6 (FIG. 1). On the
contrary, the geometric arrangement of coils is sensitive to the
presence of electrically conducting bodies in the doorway 6 (FIG.
1) and the favorable results which is achieved by making these
coils 32, 35, and 38 (FIG. 4) perpendicular are partly destroyed
whenever a large electrically conducting body passes through the
outgoing doorway 6 (FIG. 1).
I turn now to FIG. 6 which illustrates the plan comprised in a
general way in the foregoing discussion. In FIG. 6, for simplicity
I illustrate one common radiating and receiving means 41, and only
one, since this shows the flexibility of my modified plan most
clearly. In the block diagram, the user will note that there are
provided three distinct wave filters, each connected at its input
to a separate electrical entity. The electrical entity to which the
first two wave filters are connected is in each instance an
oscillator. For convenience, the filters 42 and 43 are also
designed by the symbol F.sub.1 and F.sub.2 to indicate the center
of a pass band which each of the said filters 42 and 43 selectively
transmits. The third filter 44 is designated by the symbol F.sub.1
- F.sub.2 to indicate the fact that the center of its pass band is
chosen at the difference frequencies corresponding with the
difference between the two frequencies F.sub.1 and F.sub.2. The
filters in question are deliberately taken from designs which
permit extremely strong selectivity and extremely high exclusion of
the unwanted frequencies.
As an example of a frequency corresponding with a capability of
extremely strong filtering, F.sub.1 may be 31 kilohertz, F.sub.2
may be 21 kilohertz, and F.sub.1 - F.sub.2, 10 kilohertz. These
frequencies can be very stringently filtered against one another
and, in fact, exclusivity can be achieved to whatever extent is
required. I therefore indicate these entities as being each
connected to a single electronic device in the doorway detecting
and energizing system 41. A suitable doorway sensing and detecting
device 41 adapted for the purpose is a flat wound coil 41
diagrammatically shown in FIG. 6. Such a flat wound coil serves
effectively because the two input energy sources 46 and 47 cause a
concurrent influence on the contraband at the frequencies F.sub.1
and F.sub.2 whenever a contraband element has a significant
component of its vector .theta. in a direction not in the plane of
the coil. In a completely reciprocal manner, the illustrated
doorway coil 42 is able to receive energy at the difference tone
F.sub.1 - F.sub.2 with good efficiency, and can do so whenever the
contraband marker element 2 (FIG. 1) exhibits an appreciable
component perpendicular to the plane of the doorway (shown in FIG.
6) (at the time the contraband element 2 (FIG. 1) is passing
through the plane of the said doorway).
I refer again to FIG. 6. In this figure it will be noted that there
is provided two frequency sources F.sub.1 and F.sub.2, and two
filter systems. It is obvious that if the frequency sources which
deliver energy at F.sub.1 and F.sub.2 are adjusted so that the
frequency F.sub.1 = F.sub.2, and furthermore, if I impose the
requirement that these two alternating current energy sources be in
phase, then, in this degenerate case, the entire system comprising
the frequency sources delivering energy at the two frequencies
F.sub.1 and F.sub.2 has the same effect as one oscillator and one
filter. Accordingly therefore I achieve the same result if I simply
omit the filter F.sub.1 and the oscillator 46. In a system
comprised by such an omission, since F.sub.1 = F.sub.2, the
quantity F.sub.1 - F.sub.2 has no significance as alternating
current for the reason that F.sub.1 - F.sub.2 equals zero. However,
in modulation products, as has been stated earlier, one of the
functions that is generated is F.sub.1 + F.sub.2. For the case in
which F.sub.1 = F.sub.2, F.sub.1 + F.sub.2 is of course 2F.
In the modification of the system which I am now describing with
the help of FIG. 6, the oscillator 46 and the filter 42 are
omitted. I provide the substitution of a filter adapted to pass the
frequency 2F.sub.1 instead of a filter 44 (as illustrated) to pass
the frequency F.sub.1 - F.sub.2. The recognition of contraband
marked merchandise by this modified system is identically the same
as has been described in other embodiments of my invention. As will
be later described, this single frequency system is particularly
useful with the marker element shown in and described in
conjunction with FIG. 7. From an engineering standpoint it is
required that the filter 43 of FIG. 6, be adapted to particularly
stringent rejection of the frequency 2F. In a lattice filter
designed for single frequency rejection elimination of the unwanted
frequency 2F.sub.1 from the output of this filter can be
accomplished to more than 100 decibels in two meshes, providing the
stability of the frequency of the oscillator 47 is sufficiently
good. This is easily arranged by employing crystal control to
stabilize the oscillator 47. I envision the use of a temperature
insensitive cut of the quartz crystal and, if necessary, I employ a
temperature controlled environment to further improve the frequency
stability of the oscillator 47. The stability of oscillators has
been controlled within one part per billion over long periods by
the careful use of these techniques. Since I do not need such
extreme frequency control, the adequacy of the methods which I
propose is quite obvious.
In the use of my anti-shoplifting systems there is a problem of
communicating the warning signal indicating that merchandise is
being stolen, and bringing the indication to the attention of
security guards who are not, necessarily, at the same place. To
make this procedure convenient in finished buildings where the
wiring is already in place, I propose the use of ordinary carrier
frequency signaling techniques that are well known in the art, and
propose that the carrier frequency signals be inserted on the
electric power system.
Since my warning devices are electrically powered, it is convenient
to insert the carrier warning signal on the cord through which the
power requirements of the system are served, making communications
connections of a separate nature unnecessary. The electronic
equipment necessary to put the carrier frequency warning message
into the power cord will generally be a part of, or will be
situated close to the other parts of the anti-shoplifting system.
In fact all these things may be on the same panel rack or may be
built up in the same stack of shielded boxes, as proves convenient.
I visualize such carrier frequency systems as a valuable and useful
feature in combination with the other elements of my invention. In
FIG. 6, the carrier frequency module is, as desired, the element
48.
In FIG. 6 the operator will note that there are six electrical
connections, comprising three pairs, going from the systems: (a) 46
and 42, (b) 47 and 43, and (c) 48 and 45. U.S. Pat. No. 2,520,677
(Aug. 29, 1950) makes a similar use of six wires in the form of
three pairs, and provides an especially effective means for
filtering out the noise from the signal frequency F.sub.1 .+-.
F.sub.2, (f.sub.1 = F.sub.2, is used in the discussion in this
patent application). I contemplate the use of all the same means
and methods for improving the signal to noise ratio in this
anti-shoplifting system, and employ the same in combination with
the other features of my anti-shoplifting system to better reject
unwanted noise and electrical disturbances of all kinds.
I refer once more to FIG. 6, and particularly I employ the device
of FIG. 6 with the omission of elements 43, 44, 45, 47, and 48. I
further describe the filter F.sub.1 (element 42) as a
non-significant component comprised in this use of my FIG. 6 device
as simply a pair of wires going straight through from left to
right. In effect I omit the function of this filter. In this use of
the FIG. 6 device I also construe the oscillator 46 as one emitting
relatively very strong electrical oscillations, and one which may
at times be adjusted or at least have its frequency reset to
another value as required. Further the oscillator 46 may be a
"warble" oscillator adapted to cyclically retraverse a small range
of frequency.
In the use which I am now describing for the FIG. 6 device, I
insert the coil identified in FIG. 6 as "doorway" at the point
shown for the device 4 in FIG. 1. The coil 41 is asumed to be taken
to a proper scale so that it will fit in the space provided at
location 4 in FIG. 1. My FIG. 6 device so arranged is, in fact,
suitable to perform the deactivating function. To assure the upward
radiation of a strong electromagnetic effect through the belt 2A of
the checkout stand 3 shown in FIG. 1, I arrange the design of the
checkout stand so that there are no closed metallic loops between
the device 4 and the merchandise 1 with contraband marker 2. I
further designate that the plane of my FIG. 6 coil 41 will be the
same as the plane of the largest side of the box shaped space
deignated at numeral 4 in FIG. 1. For this use, and for all the
other uses of the FIG. 6 device, it is understood that the
mechanical coil support which is illustrated in FIG. 6 is an
electrically non-conducting material, and a non-ferromagnetic
material.
A preferred embodiment of a marker element is shown in FIG. 7. The
marker comprises an extremely favorable high permeability
ferromagnetic material, as for example, a substance having a
maximum permeability of 400,000 or thereabouts and a coercive force
of 0.02 oersteds. The marker is provided with a very slender
cross-section compared with length, as for example a
cross-sectional area of 0.0004 square centimeters, and a length of
4 centimeters or more, the same being comprised in a ribbon not
thicker than 0.00125 centimeters thick. The marker in the preferred
embodiment is thus provided with a ratio of lengths to square root
of cross-sectional area which exceeds 200. If such a contraband
marker element is presented with its axis approximately parallel to
the oscillating magnetic field in a doorway such as is illustrated
in FIG. 1, the oscillating magnetic field having an intensity of
the order or magnitude of three oersteds (such a contraband
element, being generally similar to element 9 in FIG. 2 and
comprised of for example, as previously noted, of superpermalloy)
the magnetic element so chosen returns harmonic frequencies of a
very high order, extending up to and including 1.6 megacycles when
excited by a frequency such as sixty cycles per second. Such
harmonic frequencies reflected by the marker art thus in excess of
the 20th order of the fundamental frequency.
Referring now to FIG. 8, the reflected waveform from the marker
shown in FIG. 7 is illustrated. When the marker 49 is present in
the interrogation zone of the invention, the reflected waveform
output consists entirely of odd harmonics of the power frequency of
sixty cycles. As may be seen from FIG. 8, the odd harmonics are
present in evenly spaced alternating cusps. This conclusion is
particularly rigorous for the case in which the loop antenna which
receives the energy is chosen with a very insufficient number of
turns and produces in an approximately rigorous manner an
electrical voltage proportional to the time derivative of the
surface integral of the magnetic flux threading through the loop
antenna.
For a particular sensing system and a particular frequency of the
cyclically reversing magnetic field, this time derivative will
depend on the change in magnetic moment of the element. If the
magnetization in the element is reversing in each half cycle, the
magnetic moment of the saturated element itself is the upper limit
of this change. The magnetic moment of a piece of ferromagnetic
material can be determined from the well known relationship.
B = .mu. H = H + 4 .pi. m/V
where B is the magnetic induction, .mu. is the permeability, H is
the applied field, m is the magnetic moment of the element, and V
is its volume, i.e., the product of its cross-section and length.
For a square loop material such as permalloy the maximum
permeability occurs near the coercive force. Thus from the previous
marker example, one can substitute values for permeability,
coercive force, length, and cross-section to show that the magnetic
moment change is approximately one electromagnetic unit (e.m.u.) or
pole centimeter of magnetic moment. With the present apparatus,
detection across a usefully wide exitway is possible with only 0.1
e.m.u. of magnetic moment change. The loop antenna may be element
5B of FIG. 1, for example.
In the use of the contraband elements of the particularly
advantageous type which we have described, we employ the
arrangement shown in FIG. 9 for the electronic energizing and
readout at the doorway. In this use of the previously described
FIG. 6 arrangement, we omit elements 42 and 46, energizing the
doorway with but a single frequency. The element 44 which has been
hitherto characterized as a wave filter, we characterize instead in
FIG. 9 as an electronic device 44' for selecting even and odd
harmonics present on the ingoing leads to element 44'. The device
44' in this arrangement delivers a voltage proportional to the
ratio of the selected even and odd harmonics on the wires going out
to amplifier element 45'. In such a manner of use, the FIG. 9
device and the doorway coil 41' illustrated in connection with it,
serve to energize the security readout system and communications
system 48' (which relies on the output of the amplifier 45') for
the purpose of energizing alarms, lighting lights, etc.
In addition to the use of the systems and apparatus disclosed
herein as an anti-shoplifting means, the invention may equally well
be utilized in various arrangements for classification, recognition
on production lines, security, and for identification of objects
such as I.D. cards, cancelled tickets, and other such similar
applications.
Whereas the present invention has been described with respect to
specific embodiments thereof, it will be understood that various
changes and modifications will be suggested to one skilled in the
art, and it is intended to encompass such changes and modifications
as fall within the scope of the appended claims.
* * * * *