U.S. patent number 3,838,409 [Application Number 05/351,018] was granted by the patent office on 1974-09-24 for field strength uniformity control system for article theft detection system.
This patent grant is currently assigned to Knogo Corporation. Invention is credited to Arthur J. Minasy, Ronald Pruzick.
United States Patent |
3,838,409 |
Minasy , et al. |
September 24, 1974 |
FIELD STRENGTH UNIFORMITY CONTROL SYSTEM FOR ARTICLE THEFT
DETECTION SYSTEM
Abstract
An electromagnetic interrogation field in a theft detection
system is made effectively more uniform by energizing different
antenna windings lying in different planes at the same frequency
but at different phases so that the resulting field pattern rotates
in the vicinity of the antenna windings.
Inventors: |
Minasy; Arthur J. (Woodbury,
NY), Pruzick; Ronald (Commack, NY) |
Assignee: |
Knogo Corporation (Westbury,
NY)
|
Family
ID: |
23379238 |
Appl.
No.: |
05/351,018 |
Filed: |
April 13, 1973 |
Current U.S.
Class: |
340/572.7 |
Current CPC
Class: |
G08B
13/24 (20130101); G08B 13/2471 (20130101) |
Current International
Class: |
G08B
13/24 (20060101); G08b 013/24 () |
Field of
Search: |
;340/258C,280 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Swann, III; Glen R.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
What is claimed is:
1. In an article theft detection system the combination of at least
two antenna windings mounted in the vicinity of a checkpoint along
an article egress path, said antenna windings lying in different
planes, an oscillator for generating high frequency electrical
signals, signal transmission means connected between said
oscillator and said antenna windings to energize said windings from
said oscillator, said signal transmission means including means
forming separate signal paths to the antenna windings lying in the
different planes and phase shift means in at least one of said
signal paths for producing a relative phase difference between the
signals applied to the different antenna windings.
2. An article theft detection system according to claim 1 wherein
the planes of said two antenna windings are arranged traversely to
each other and wherein said phase shift means is operative to
produce a net relative phase difference of 90.degree. between the
signals applied to the different antenna windings.
3. An article theft detection system according to claim 2 wherein
at least one antenna winding encircles said egress path and lies in
a plane perpendicular to said egress path.
4. An article theft detection system according to claim 3 wherein
two additional antenna windings lie in planes parallel to said
egress path.
5. An article theft detection system according to claim 4 wherein
said one antenna winding is connected to be energized from one of
said signal paths and wherein both of said two additional antenna
windings are connected to be energized from another of said signal
paths.
6. An article theft detection system according to claim 2 wherein
separate phase shifters are provided in said one and another signal
paths.
7. An article theft detection system according to claim 6 wherein
one of said separate phase shifters is operative to produce a
leading phase shift of approximately 45.degree. and the other phase
shifter is operative to produce a lagging phase shift of
approximately 45.degree..
8. An article theft detection system according to claim 6 wherein
said phase shifters have complementary frequency sensitivity
characteristics whereby the same net phase difference is maintained
at different signal frequencies.
9. An article theft detection system according to claim 6 wherein
one phase shifter comprises a resistor and a capacitor connected in
parallel with each other along one of said signal paths and said
other phase shifter comprises a resistor and an inductor connected
in parallel with each other along said another signal path.
10. Apparatus for generating an electromagnetic interrogation field
at a given checkpoint through which protected articles must pass,
said apparatus comprising at least two antenna windings lying in
different planes in the vicinity of said checkpoint and means for
applying electromagnetic signals of the same frequency but in phase
shifted relationship simultaneously to said antennas.
11. Apparatus according to claim 10 wherein said antenna windings
lie in planes which are substantially perpendicular to each other
and wherein said means for applying electromagnetic signals
includes phase shifter means operative to maintain a substantially
90.degree. relative phase shift between the signals applied to the
antennas in different planes.
12. An article theft detection system comprising a variable
frequency oscillator, a tuning oscillator connected to said
variable frequency oscillator for causing same to produce an output
signal whose frequency varies cyclically at a rate corresponding to
the frequency of said tuning oscillator, means defining an output
signal path including a first junction connected to the output of
said variable frequency oscillator and means forming a pair of
branch signal paths leading from said first junction to associated
antenna junctions, separate antenna windings connected respectively
to said antenna junctions, said antenna windings being positioned
at a given checkpoint and lying in different planes, detector means
connected to said antenna junctions, said detector means being
responsive to the changes in energy level at the junctions which
occur at a rate related to the frequency of said tuning oscillator
to produce an output signal, alarm means arranged to be actuated by
said output signal from said detection means and phase shifter
means arranged in at least one of said branch signal paths between
said first junction and one of said antenna junctions.
13. An Article Theft Detection System according to claim 12 wherein
said antenna windings include a first winding lying in a plane
transverse to an egress path and a second winding lying in a plane
extending laterally of said egress path.
14. An article theft detection system according to claim 13 wherein
said phase shifter means is operative to produce a net phase
difference in signals applied to said first and second windings of
substantially 90.degree..
15. An article theft detection system according to claim 12 wherein
said phase shifter means includes a pair of phase shifters each
connected along a different one of said branch signal paths, one of
said phase shifters being operative to produce a leading phase
shift and the other being operative to produce a lagging phase
shift.
16. An article theft detection system according to claim 15 wherein
one of said phase shifters comprises a resistor and a capacitor
connected in parallel with each other along one of said branch
signal paths and the other phase shifter comprises a resistor and
an capacitor connected in parallel with each other along the other
of said branch signal paths.
17. An Article Theft Detection System according to claim 12 where
said detector means comprises separate detector devices connected,
respectively, to each of said antenna junctions.
18. A method for generating an electromagnetic interrogation field
at a given checkpoint through which protected articles provided
with electronic responder circuits must pass, said method
comprising the step of electrically energizing antenna coils,
positioned in different planes at said checkpoint, at the same
frequency but at different phases thereby to generate an
electromagnetic field which rotates in the vicinity of the
checkpoint.
19. A method according to claim 18 wherein said coils lie in planes
which are mutually perpendicular and wherein said coils are
energized 90.degree. out of phase with respect to each other.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electronic theft detection systems and
more particularly it concerns novel arrangements for maintaining
the vicinity of a theft detection checkpoint substantially
uniformly filled with an electromagnetic interrogation field.
2. Description of the Prior Art
The present invention is especially suitable for use in conjunction
with electronic theft detection systems of the type described in
U.S. Pat. Nos. 3,493,955 and 3,500,373. In both systems, each of
the articles to be protected from theft has a electronic responder
circuit attached to it. This circuit may be concealed in a wafer
like element which may also serve as a price label or the like for
the protected article. The articles are maintained in an enclosure
having limited egress and checkpoints are set up at each egress. A
transmitter is provided at the checkpoint to transmit an
interrogation signal and receiver means are provided to note any
response produced by the interaction of a wafer responder circuit
with the transmitted signal field in the vicinity of the
checkpoint. In the case of the systems described in U.S. Pat. No.
3,493,955, the wafer responder circuits respond to the transmitted
interrogation signal, which is at first frequency, to produce a
response signal at a second frequency. The receiver means are tuned
to detect this second frequency.
In the case of the system described in U.S. Pat. No. 3,500,373, the
wafer responder circuits are resonant circuits tuned to resonate at
the transmitted interrogation frequency. When these wafer responder
circuits are brought into the transmitted interrogation signal
field they absorb some of the transmitted energy. The receiver
means monitors the transmitted signal, which changes in amplitude
due to this absorption. In order to maximize sensitivity the
transmitter of this system produces an output frequency which
sweeps cyclically over a given range which includes the resonant
frequency of the wafer responder circuits. This causes a series of
responses in the form of impulses which occur at a repetition rate
corresponding to the frequency sweep rate.
The ability of a responder circuit to function effectively in any
electronic theft detection system depends upon the degree to which
the interrogation field is incident upon the responder circuit.
Since these responder circuits are generally in flat wafer-like
form, they exhibit different degrees of sensitivity depending upon
their orientation with respect to the interrogation antenna. In
order to accomodate the different attitudes which responder devices
may assume when carried through a checkpoint there has been
developed a plural antenna system comprising at least two antennas
positioned at right angles to each other at the checkpoint. The two
antennas are energized simultaneously so that as a responder
circuit is turned away from one interrogation antenna it turns
toward the other interrogation antenna so that sensitivity is
maintained. This plural interrogation antenna system is shown and
described in U.S. Pat. No. 3,493,955.
The present invention is directed to a somewhat related but
different problem than the orientation problem described above. It
has been found that even plural interrogation antenna systems
produce an uneven electromagnetic field energy distribution
throughout the vicinity of a checkpoint. This occurs as a result of
the additive and subtractive effects of the fields produced by the
different antennas. As a result of these effects there are
developed "dead zones" or regions of minimal electromagnetic field
intensity. If a responder circuit is caused to follow a path
through the dead zone regions of a checkpoint only minimal
interaction will occur between the interrogation signal and the
responder circuit and it is possible that the passage of the
responder circuit through the checkpoint will not be detected.
SUMMARY OF THE INVENTION
The present invention overcomes the above described field
distribution problems and provides an interrogation field in which
dead zones are effectively minimized.
According to the present invention there are provided at least two
interrogation antennas at a checkpoint. These antennas are
positioned in different planes, preferably at right angle to each
other. The antennas are energized simultaneously at the same
interrogation frequency. However this energization is controlled so
that a phase difference exists in the energization of the different
antennas. Preferably this phase difference is ninety degrees. As a
result of the energization of antennas lying in different planes
with signals of different phase, there is produced in the vicinity
of the checkpoint a rotating electromagnetic field. Thus, while a
dead zone may exist in the field at any given orientation thereof,
the rotation of the field causes the dead zone to move so that the
entire region of the checkpoint is effectively filled with the
electromagnetic field. In effect the dead zones are eliminated and
the responder circuits are more likely to be detected.
There has thus been outlined rather broadly the more important
features of the invention in order that the detailed description
thereof that follows may be better understood, and in order that
the present contribution to the art may be better appreciated.
There are, of course, additional features of the invention that
will be described hereinafter and which will form the subject of
the claims appended hereto. Those skilled in the art will
appreciate that the conception upon which this disclosure is based
may readily be utilized as a basis for the designing of other
structures and other methods for carrying out the several purposes
of the invention. It is important, therefore, that the claims be
regarded as including such equivalent structures and methods as do
not depart from the spirit and scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
A specific embodiment of the invention has been chosen for purposes
of illustration and description, and is shown in the accompanying
drawings forming a part of the specification, wherein:
FIG. 1 is a block diagram of a swept frequency electronic theft
detection system in which the present invention is embodied;
FIG. 2 is a perspective view illustrating the arrangement of
antenna coils according to the present invention;
FIG. 3 is a fragmentary circuit diagram showing phase shifting
circuits used in the system of FIG. 1;
FIGS. 4 A-D are diagramatic representations showing magnetic field
relationships around portions of antenna coils during antenna
energization at different intervals in a cycle of energization
according to the prior art; and
FIGS. 5 A-D are views similar to FIGS. 4 A-D but showing magnetic
field relationships at different intervals in a cycle of
energization according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The swept frequency theft detection system of FIG. 1 is, in part,
like that shown and described in U.S. Pat. No. 3,500,373. As shown,
there is provided a tuning oscillator 10 which produces a voltage
whose amplitude varries at a given rate, e.g., 300 cycles per
second. This varying voltage is applied to a voltage tuneable swept
radio frequency oscillator 12 which is designed to produce an
output voltage which is nominally 2 megahertz (MHZ). When the
oscillator 12 is controlled by the tuning oscillator 10 however,
its output frequency is swept between 1.95 MHZ and 2.05 MHZ at a
300 cycle per second rate.
The swept frequency output of the oscillator 12 is supplied to a
junction 14 (see FIG. 3) from which it branches to "lead" and "lag"
phase shifters 16 and 18. These phase shifters, which will be
described in greater detail hereinafter in connection with FIG. 3
act, respectively, to advance and retard the phase of the signals
applied to them. In the case of the lead phase shifter 16, the
signal phase is advanced by approximately 45.degree., while in the
case of the lag phase shifter, the signal phase is retarded by
approximately 45 degrees. There is thus produced a net phase
differential of 90.degree. at the outputs of the two phase shifters
16 and 18.
The output of the lead phase shifter 16 is applied to an amplifier
20; and the output of this amplifier is applied to a transverse
antenna junction 22. A transverse antenna winding 24 is connected
to the junction 22. The junction 22 is also connected to a first
detector 26; and this is in turn connected to an alarm 28.
The output of the lag phase shifter 18 is applied to an amplifier
30; and the output of this amplifier is applied to a pair of
lateral antenna junctions 32 and 34.
Each of the detectors 26 and 40; and the alarm 28 may be
constructed and arranged as described in connection with FIG. 2. As
is there shown, the transverse antenna winding 24 comprises a
multiturn coil lying in a plane which is substantially
perpendicular to a path of egress (indicated by an arrow A) through
a checkpoint. Various physical means (not shown) are provided to
confine the movement of protected articles, which are equipped with
electronic responder circuits, so that they can exit from an
enclosure only via the path shown by the arrow A. The protected
article thus must pass either through or very near the transverse
antenna winding 24 during egress from the enclosure.
The two lateral antenna windings 36 and 38 also comprise multiturn
coils. However these antennas are arranged on opposite sides of the
egress path with their planes oriented parallel to the path and
perpendicular to the plane of the transverse antenna winding
24.
As shown in FIG. 2, all of the antenna windings have one end
connected to ground. The opposite end of the transverse antenna
winding 24 is connected to the junction 22 while the opposite end
of the lateral antenna windings are connected to the junctions 32
and 34.
The system of FIGS. 1 and 2 operates to detect the presence of
resonant responder circuits (not shown) carried on protected
articles which pass through and by the antenna windings 24, 36 and
38 along the egress path. The responder circuits are tuned to
resonate at a frequency within the sweep range of the oscillator
12. Thus the responder circuits may be tuned to resonate at 2
MHZ.
When no responder circuit is present in the egress path, a
relatively high impedance is presented to the antenna windings and
most of the energy from the amplifiers 20 and 30 passes by the
junctions 22, 32 and 34 and becomes incident upon the detectors 26
and 40. As long as a relatively constant energy level is applied to
the detectors they do not produce any alarm actuating signal.
When a responder circuit passes by or through the antenna windings,
it resonates and absorbs energy each time the frequency of the
oscillator 12 sweeps by the resonant frequency of the responder
circuit. When the responder circuit is tuned to resonate at 2 MHZ
and the oscillator frequency sweep between 1.95 MHZ and 2.05 MHZ,
this resonant response occurs twice during each sweep cycle or at a
600 response per second rate. The resonant responses cause a
decrease in impedance in the vicinity of the antenna windings so
that more of the transmitted energy passes out from the windings
during the resonant response. This results in a decrease in the
energy level applied to the detectors 26 and 40. The detectors, as
described in U.S. Pat. No. 3,500,373, are provided with special
arrangements for detecting these energy decreases and for actuating
the alarm 28 when they occur.
The phase shifters 16 and 18 may be of any suitable construction
which will produce a difference in phase between the lateral and
transverse antenna windings. The particular phase shifter
construction shown in FIG. 3 has been found to be quite suitable
for the present application. As shown in FIG. 3, the lead phase
shifter 16 comprises a resistor 42 connected in series between the
junction 14 and the amplifier 20. A capacitor 44 is connected
across the resistor 42. The lag phase shifter 18 comprises a
resistor 46 connected in services between the junction 14 and the
amplifier 30. An inductor 48 and a capacitor 50 are together
connected across the resistor 46. It has been found that when the
resistor 42 is about 680 ohms and the capacitor 44 is about 150
picofarads the lead phase shifter 16 will produce a phase shift of
close to +45.degree. for frequencies in the range of 1.95 - 2.05
MHZ. Also when the resistor 46 is about 680 ohms and the inductor
48 is about 47 microhenries, the lag phase shifter will produce a
phase shift of close to -45.degree. for frequencies in the range of
1.95 - 2.05 MHZ. The capacitor 50 is used to prevent short
circuiting of the resistor 46 by the inductor 48; and it has been
found that this capacitor will serve this function without
adversely affecting phase shift for the frequencies mentioned when
its capacitance is approximately 0.1 microfarad.
The phase shifters 16 and 18 may be adjusted to produce any
differential in the net phase shift between the lateral and
transverse antenna windings. However, as will be seen, a 90.degree.
net phase shift should produce a more uniform field distribution
with the antenna arrangement of FIG. 2. Also, it is not necessary
that two separate phase shifters be used. A single phase shifter
capable of producing a 90.degree. phase shift in the signal to one
of the amplifiers 20 or 30 would produce a similar result. The use
of two phase shifters, each of which produces only a 45.degree.
phase shift, however, permits a more accurate phase shift over the
frequency sweep range with less expensive construction. Also it has
been found that the frequency sensitivity of the two phase shifters
16 and 18 is complimentary during the 1.95 to 2.05 MHZ frequency
sweep. Thus when the frequency at any instant is such that the lead
phase shifter 16 produces less lead, that same frequency causes the
lag phase shifter 18 to produce greater lag so that the net phase
difference remains essentially the same as frequency shifts.
The manner in which the above described antenna orientation and
phase shifted antenna excitation serves to produce a revolving
field which eliminates dead zones can be seen in a comparison of
FIGS. 4 and 5. FIGS. 4 and 5 each comprise a group of section views
taken along line 4--4 of FIG. 2. These section views sever the
vertical portions of both the transverse and horizontal antenna
windings 24, 36 and 38. Thus there are shown severed ends of
vertical portions 24a and 24b of the transverse antenna winding 24,
severed ends of the vertical portions 36a and 36b of the lateral
antenna winding 36 and severed ends the vertical portions 38a and
38b of the lateral antenna winding 38. As can be seen in FIG. 2,
when current flows upwardly in the vertical portion 24a, it flows
downwardly in the opposite vertical portion 24b. This also applies
to vertical portions 36a and 36b and 38a and 38b. Reverse current
flows occur, of course, during one half of each cycle of antenna
energization.
The flow of current through the various antenna windings is
accompanied by a circular magnetic field surrounding the winding
wires as illustrated by arrows B in FIGS. 4 and 5. The direction of
the various circular magnetic fields B corresponds to the direction
of current flow through the winding which each field surrounds.
Thus, the direction of the circular magnetic field reverses itself
upon each half cycle of antenna energization.
The distribution of electromagnetic energy in the vicinity of the
egress path A will now be described with regard to the magnetic
fields B produced by currents in the antenna windings 24, 36 and
38.
In the successive 90.degree. intervals of antenna energization
represented by FIGS. 4A-D, each of the three antenna windings 24,
36 and 38 is energized in phase, according to the prior art. Thus
all antenna windings conduct maximum current at the same time and
all antenna windings conduct zero current at the next following
90.degree. interval.
The major direction of magnetic field strength, which is
represented by an arrow C in FIG. 4 is determined according to the
mutual additive and subtractive effects of the circular magnetic
fields B in the various antenna windings. As can be seen in FIG.
4A, the major magnetic field energization C is at an angle .alpha.
with respect to the path A. After the next 90.degree. interval the
magnetic field energization diminishes to zero as shown in FIG. 4B.
Thereafter as shown in FIG. 4C, the major magnetic field
energization C is again at the angle .alpha. with respect to the
egress path A, but is reversed in direction. Finally, as shown in
FIG. 4D, after the last 90.degree. interval the magnetic field
strength again diminishes to zero.
It will be appreciated from FIG. 4 that the major magnetic field
strength C always lies along a path which crosses the egress path A
at an angle .alpha.. Also by considering the additive and
subtractive effects of the circular magnetic fields B within the
various quadrants (a), (b), (c) and (d), it will be seen that a
minimum magnetic field strength is always present in quadrants (b)
and (d) while maximum magnetic field strength is present only in
quadrants (a) and (c). Thus, by taking care to traverse the
checkpoint along a path indicated by the dashed arrow D, one can
cross the path of maximum field strength at substantially a right
angle to it and thereby minimize the duration required to pass
through the high intensity region of the field. Also, by following
the path of the arrow D, one will traverse the checkpoint via the
quadrants (b) and (d) of minimum field energization. These
quadrants make up dead zones within which only minimal response can
be obtained from a rebroadcaster circuit. As a result a possibility
exists in this prior art arrangement, for a protected article to
pass through a checkpoint undetected if it follows a particular
path. Turning now to FIG. 5, wherein operation according to the
present invention is shown, it will be seen that the transverse
antenna winding 24 is energized at a 90.degree. phase relationship
to the energization of the lateral antennas 36 and 38. As can be
seen in FIG. 5A, when maximum current flows through the lateral
antenna windings 36 and 38, no current flows through the transverse
antenna winding 24. The additive and subtractive effects of the
circular magnetic fields B are such that they produce a net maximum
magnetic field energization C which is at a right angle .beta. to
the egress path A. After the next 90.degree. interval, as seen in
FIG. 5B, the current through the lateral antenna windings 36 and 38
diminishes to zero and the current through the transverse antenna
winding 24 becomes maximum. The net effect is to swing the maximum
magnetic field energization C into alignment with the egress path
A. Thereafter, after the next 90.degree. interval of antenna
energization, as shown in FIG. 5C, current in the transverse
antenna winding 24 diminishes to zero while current through the
lateral antenna windings 36 and 38 again reaches maximum in the
opposite direction from that shown in FIG. 5A. This causes the
maximum magnetic field energization C to swing further around until
it again is at a right angle .beta. to the egress path A. Finally
after the last 90.degree. interval of antenna energization, as seen
in FIG. 5D, the circular magnetic fields B cause the maximum
magnetic field energization to swing back along the egress path
direction A.
It will be appreciated that by interpolation it can be shown that
the maximum magnetic field vector C revolves circularly during
antenna energization. Thus any dead zones are swept around and the
entire vicinity of the egress path A is electromagnetically
energized uniformly. Further, since the electromagnetic field
pattern moves continuously there is no path along which one may
pass in which the magnetic field strength is always minimal. Thus
there is provided a means for improving the detection reliability
of responder circuits in electronic theft detection system.
While the invention has been described with reference to the
preferred form thereof, it will be obvious to those skilled in the
art to which the invention pertains, after understanding the
invention, that various changes and modifications may be made
therein without departing from the spirit and scope of the
invention, as defined by the claims appended hereto.
* * * * *