Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02232474 1998-OS-19
ELECTROMAGNETIC ANTI-SHOPLIFTING SYSTEM
Field Of Invention
This invention relates to a method and apparatus for surveillance of articles
and
in particular to an improvement in a method and apparatus for detecting or
preventing the theft of articles of value and more particularly it concerns
the
method and apparatus capable of distinguishing labels from other objects
within
an oscillatory electromagnetic field.
Background To Invention
There are in existence several systems for detecting or preventing the theft
of
articles of value. One of these corresponds to U.S. Pat. No. 3,292,080 granted
to E.M. Trikilis on December 13, 1966 which makes use of a magnetometer and
utilizes a magnetized object which identifies the article unless check-out
procedure has removed the magnetism from the object.
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. However, there
are
severe health problems with this system involving danger to people from the
nuclear radiation.
A further system involves the use of radio frequency generating device
imbedded
in a rubber pad. The radio frequency emitting device is fastened to articles
and
if not removed will energize radio frequency detecting antenna 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 article at the
time it is sold, permitting the buyer to pass through the doorway without
attracting
the attention of the store detectives.
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CA 02232474 1998-OS-19
The following United States patents illustrate a number of alternative
proposals
which will serve as background to the invention described and illustrated
herein
namely:
U.S. Pat. No. 3,631,442; U.S. Pat. No. 3,747,086; U.S. Pat. No. 3,754,226;
U.S. Pat. No. 3,790,945; U.S. Pat. No. 3,820,103; U.S. Pat. No. 4,527,152;
U.S. Pat. No. 4,888,579; U.S. Pat. No. 5,459,452; U.S. Pat. No. 5,602,556;
U.S. Pat. No. 5,661,470;
Objects Of The Invention
The principal object of this invention relates to providing an improved method
and
apparatus for detecting or preventing the theft of articles of value.
Another very important object of this invention is to provide electrical
circuitry
which has inherently high sensitivity and simultaneously small probability of
false
alarms.
It is a further object of this invention to provide a method for generating a
constant oscillating magnetic field for both in phase and out of phase modes.
The present invention provides a new method to increase sensitivity and
simultaneously decrease probability of false alarms.
Features Of Invention
It is a feature of this invention to provide an apparatus for detecting the
passage
of an object through a surveillance zone comprising a transmitting coil for
generating an oscillatory electromagnetic interrogation field within the
surveillance
zone, a label secured to an object, whereby said label is adapted to cut or
link
up with the electromagnetic field during its traversal through the
electromagnetic
interrogation field regardless of the label's spacial orientation, and thereby
generating signals captured by a receiving coil including electronic circuitry
adapted to set off an alarm.
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More particularly, the electronic circuitry is adapted to minimize distortion
of the
generated signal by the label. The electronic circuitry is also adapted so as
to
differentiate label signals from other signals such as pop cans generating
lower
harmonics.
It is a further feature of this invention to provide for a field generating
system
which shall generate sufficient lines of flux to switch a label in any one of
the
three vectors.
It is a further feature of this invention to provide for a broad band passive
fundamental filter and signal amplifier system receiver antenna which while
having a signal gain of ten substantially nulls the fundamental frequency of
the
generated oscillating magnetic field and is made sufficiently lossy such that
it
does very little or no wave shaping to the signal generated by the label.
More particularly, it is a feature of this invention to provide for electronic
circuitry
capable of processing the signal such that it is not distorted or wave shaped
and
the signal retain its inherent characteristics.
It is a further feature of this invention to provide coherent filtering of
said signal
generated by the label from other signals.
Yet another feature of this invention resides in providing transmitting coils
capable
of being driven in and out of phase with respect to one another so as to
generate
oscillating magnetic lines of flux having one vector in the in phase mode and
two
vectors in the out of phase mode. The label is adapted to cut or link up with
one
or more of the vectors.
It is another important feature of this invention that the phase of the
receiving
coils match that of the transmitting coil so as to maximize the capture of
signals
generated by a label in response to the in and out phase generation of an
oscillating magnetic field produce by the transmitting coils.
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Still another important feature of this invention resides in providing
fortransmitting
coils having a parallelogram configuration.
More particularly, it is a feature of this invention to provide electronic
circuitry
capable of time domain blanking and signal recognition. The signal may be
recognized by utilizing pulse width detection or correlation.
It is a further feature of this invention to provide for a method for
detecting the
presence of an object when the object is in an interrogation zone having an
oscillatory electromagnetic field. More particularly, the method comprises of
securing to the object a label capable of generating signals when placed in
the
surveillance zone, whereby the signals are captured by receiving coils
including
electronic circuitry adapted to set off an alarm.
Brief Description Of Drawings
These and other objects and features will become apparent in the following
description to be read in conjunction with the sheets of drawings in which:
FIG. 1 is a perspective view of the anti-shoplifting system illustrating coil
housing
units, electronic circuitry device, and alarm.
FIG. 1 A is a schematic view of the coil units.
FIG. 2 is a perspective view of the label illustrating its internal magnetic
materials.
FIG. 3 is a perspective view of the deactivating system.
FIG. 4 is a side elevational view of one of the coil housing units which
includes
a partially broken view to illustrate its internal components.
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FIG. 5 is a cross-sectional view of the coil housing unit taken along the
lines 5-5
in Fig. 3 revealing the transmitting coil and receiving coil.
FIG. 6 is a diagram to assist in the explanation of the operation of the
aiding and
opposition mode.
FIG. 7A is a schematic illustration of the interconnection of the transmitting
coil
in the constant mode.
FIG. 7B is a schematic illustration of the interconnection of the transmitting
coil
in the alternating mode.
FIG. 8A is a graphic illustration of the alternating current along
corresponding
points of the transmitting coil when driven in phase.
FIG. 8B is a graphic illustration of the alternating current along
corresponding
points of the transmitting coil when driven out of mode.
FIG. 9 and 10 are diagrammatic views illustrating the generated magnetic
field.
FIG. 11 is a diagrammatic view of the receiving coils mounted within the coil
housing units.
FIG. 12 is a graphic illustration of the fundamental frequency.
FIG. 13 is a graphic illustration of the fundamental frequency including the
fundamental frequency of the signal generated by the label.
FIG. 14 is a block diagram illustrating the various components in the
electronic
circuitry device for two gates system.
FIG. 15 is a block diagram illustrating the various components for three gates
system.
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CA 02232474 1998-OS-19
Detailed Description Of Invention
In the preferred embodiment of this invention, the improved system for
detection
of marked or tagged objects within a magnetic field has been adapted to
comprise an improved anti-shoplifting device generally depicted in FIGS. 1, 2
and 3.
FIG. 1 includes two coil housing units 2 and 4 which have a surveillance zone
6
intermediate said spaced coil housing units 2 and 4. The two coil housing
units
2 and 4 are adapted to generate an oscillatory electromagnetic interrogation
field
within said surveillance zone 6 in a manner to be described herein.
A marker element, tag or label generally illustrated as number 8 in FIG. 2 is
attached to each object or article (not shown) to be surveyed by the system
described herein. When there has been an unauthorized passage of the label
8 through the surveillance zone 6 (as in the case of shoplifting) the label 8
will
cut or link a sufficient number of generated lines of flux thereby generating
a
signal to be received by the coil housing units 2 and 4. The signal is
communicated to an electrical detection circuitry 10 by means of electrical
conductors 12 and 14, which will activate the alarm 44.
When the shopper has paid for the article or object the label 8 is inserted
into the
deactivating device 46 illustrated in FIG. 3. The deactivating device 46 will
deactivate the label 8 so that when the label 8 is passed through the
surveillance
zone 6 there are no signals generated by the label 8; this avoids any false
alarm
of shoplifting through alarm 44.
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CA 02232474 1998-OS-19
Coil Housing Units
The coil housing units 2 and 4 are each more particularly described in FIGS. 4
and 5. Each coil housing unit 2 or 4 is so constructed and driven repetitively
in
alternate in phase (or aiding mode) and out of phase (or opposition mode) such
that a label 8 will cut or link a sufficient number of lines of magnetic flux
generated by the two magnetic field producing coil housing units 2 and 4 at
some
point doing its traversal through the interrogation zone 6 regardless of its
angle
with respect to the magnetic field producing coil units 2 and 4.
Geometric Construction of Coil Housiny Units
In the preferred embodiment the coil housing units 2 and 4 respectively
include
a transmitting coil 48 having four turns as illustrated in FIGS. 4 and 5. Each
of
the turns of transmitting coil 48 are insulated from each other by insulating
material 40. The transmitting coil 48 is wound in a parallelogram
configuration
as illustrated in FIG. 4. The slopes of the two longest inclined members 22
and
24 are respectively from 0° to 60° from the horizontal place.
The other two
shorter members 26 and 28 respectively are in the vertical position.
The transmitting coil 48 is disposed in such a manner that the vertical
members
26, 28 and inclined members 22, 24.
As disclosed in FIG. 6, one notable exception to the similarity of conductors
A,
B and C, D with respect to the above mentioned equivalent longer conductors is
that the electrical current travelling in A and B and also in C and D will be
in
opposite directions, since the reference members are in all cases on opposite
sides of the transmitting coil 48 and the electrical current which always
flows in
the same direction at the same point in the time domain of a continuous
conductor, will necessary be flowing in the opposite direction due to the
geometry
of the transmitting coil 48 and the fact that the sides are opposite.
Therefore, if the current were flowing in an upward direction in element A, it
would have a downward direction in element B and likewise for elements C
and D.
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Driving Coils in Phase and Out Of Phase
The transmitting coil 48 in each of the coil housing units 2 and 4 are driven
by
an alternating current source; however, the alternating current source applied
to
one of the transmitting coils 48 in coil housing unit 2 is fixed while the
alternating
current applied to the other transmitting coil 48 in the coil housing unit 4
is
operating so that the alternating current within transmitting coil 48 of coil
housing
unit 4 is in phase with the alternating current within transmitting coil 48 of
coil
housing unit 2 for a portion of time, and is then out of phase for a portion
time.
FIG. 7a is a schematic illustration of the transmitting coil 48 in coil
housing unit
2 and FIG. 7b is a schematic illustration of the transmitting coil 48 in the
coil
housing unit 4.
FIG. 8a is a graphic illustration of the alternating current along
corresponding
points of the transmitting coil 48 in coil housing units 2 and 4 when the
transmitting coils 48 are operated in phase or in the aiding mode. FIG. 8b is
a
graphic illustration of the alternating current along corresponding points of
transmitting coil 48 in coil housing units 2 and 4 when the transmitting coils
48
are driven out of phase or in the opposition mode.
Consideration must now be given to the case of the generated magnetic field or
lines of flux generated in the aiding and opposition modes of operation.
Vectors Produced in the Aiding and Opposition Modes of Operation
In the case of the aiding configuration, it is noted that conductors A and A~
(which represents the portion of transmitting coil 48 in the vertical members
26
in the coil housing units 2 and 4 respectively) have electrical current
travelling in
the same direction, but conductors A and A~, are displaced in space by their
separation distance of 30"-40" centre to centre. By applying the right-hand
rule
with respect to the flux generated by an electrical current travelling in a
conductor, it is observed that the lines of flux at a point equidistant from
the two
conductors A and A~ shall have an opposite direction and shall in fact cancel
if
the current in the two conductors were the same.
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The same discussion applies to the flux producing elements B and B~.
Generally, the same applies to D and D~ and C and C~, with the exception that
these flux producing elements.
In the case of the opposition configuration, it is noted that conductors A and
A~
have electrical current travelling in opposite directions, but conductors A
and A~
are displaced in space by their separation distance of about 38" centre to
centre.
By applying the right-hand rule with respect to the flux generated by an
electrical
current travelling in a conductor, it is observed that the lines of flux at a
point
equidistant from the two conductors shall have the same direction and shall
add
and produce twice the flux if the current in the two conductors were the same.
This addition to produce twice the flux if the electrical currents in the two
conductors were the same holds true for flux producing elements B and B~, and
also to C and C~, and D and D~, with the exception that the latter four flux
producing elements have a slope.
It is further understood that the predominant field or lines of flux generated
by the
transmitting coil 48 are in a vector perpendicular to the place in which the
coil
exits and that the strongest field is within, or through the transmitting coil
48,
since all four sides or current producing members add to one another. The
magnetic field in the centre of the coil 48 would be about four times as much
as
the fringing field as if a measurement was made at a distance equal to 1~ of
the
sum of the distance from the centre to each edge of the conductor and from any
one of the conductors on the outside of the coil 48.
FIG. 10 illustrates that three vectors of magnetic flux are generated in the
surveillance zone 6, and that two of the vectors are perpendicular with
respect
to one another while the third vector is displaced from the vertical.
The aiding configuration is used primarily to produce the magnetic field which
is
generally at a point in the centre of one of the coils perpendicular to the
plane in
which the coil lies and is in the same direction as the other transmitting
coil 48
at the same point in time domain producing the strongest field of the three
vectors produced, as illustrated in FIGS. 9 and 10.
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The opposition configuration as illustrated in FIG. 9 and 10 is used to
generate
the fringing fields that produce the other two vectors, and that the exact
vector
produced will be determined by the plane in which the two conductors lie. The
vector produced in the opposition configuration will be at right angles with
respect
to said conductors. The fringing fields add at points equidistant from the two
conductors. All other points between the conductors produce a strong magnetic
field across the entire 38" spread in all vectors.
Fundamental Freauency
In the preferred embodiment, the transmitting coils 48 in the coil housing
units 2
and 4 are driven in phase for 13 to 15 milliseconds. During this time interval
an
oscillating magnetic field is generated; the vector of said generated magnetic
field
is perpendicular to the face of the transmitting coils 48 as illustrated in
FIGS. 9
and 10. The application of alternating current to transmitting coil 48 in coil
housing 4 is then stopped for 8 milliseconds so as to allow switching said
alternating current to drive the transmitting coil 48 in coil housing unit 4
in the out
of phase mode as previously described. During the opposing configuration, an
oscillating magnetic field is generated having two vectors, one of which is
perpendicular to the plane formed by the two conductors A and A~ and that the
other of which is perpendicular to the plane formed by the conductors B and
B~.
The transmitting coils 48 in coil housing units 2 and 4 are driven in the out
of
phase mode for 13 milliseconds.
The cycle of generating one vector in the aiding configuration for 13 to 15
milliseconds, stopping for 8 milliseconds, and then generating the vectors in
the
opposing configuration for 13 to 15 milliseconds is repeated during the entire
operation of the anti-shoplifting system.
In this manner, the transmitting coils 48 of coil housing units 2 and 4
generate
a prescribed fundamental frequency suitable to resonate the coils 48 in coil
housing units 2 and 4. In the preferred embodiment the capacitance and
inductance of the transmitting coils 48 in coil housing units 2 and 4 are
selected
so that they operate in resonance to generate an oscillating magnetic field
having
a fundamental frequency of 6 KHz, which is graphically illustrated in FIG. 12.
page 11 /26
CA 02232474 1998-OS-19
Marker Element
As previously described the transmitting coils 48, in coil housing units 2 and
4)
operate in resonance to generate an alternating magnetic field having a
fundamental frequency of 6 KHz. During this in phase operation, magnetic field
will be generated in the surveillance zone 6, whose vector is orientated as
described in FIGS. 9 and 10. During the out of phase operation, a magnetic
field
will be generated in the surveillance zone 6, having two vectors as described
in
FIGS. 9 and 10.
Since an oscillating magnetic field having three separate and distinct vector
components is generated, any label 8 which traverses through the interrogation
zone 6, will be cut or will link a sufficient number of lines of flux at some
point
during its passage through the field, regardless of the angle of the
orientation of
the label 8.
The prior art also discloses that the use of grain or domain orientated
material
was necessary in the use of marker element 8. However, a label 8 having a
unipole orientation may be utilized where the anisotrophy is such that the He
is
the same regardless of whether the applied magnetic field is parallel to the
longest dimension or the shortest one.
In the preferred embodiment, the label 8 comprises of ferromagnetic material
30,
which is magnetically soft or easily magnetized. When the label 8 passes
through the magnetic field oscillating at the fundamental frequency of 6 KHz,
the
ferromagnetic material 30 becomes magnetized by the oscillating magnetic
field.
As the oscillating magnetic field alternates, the ferromagnetic material
switches
poles at a fundamental frequency and induces perturbations or anomalies on the
oscillating lines of flux of the generated magnetic field. This induced signal
has
a fundamental frequency and harmonics thereof which combine with the
fundamental frequency of the generated magnetic field, as illustrated in FIG.
13.
The signal generated by the label 8 is depicted as number 32 in FIG. 13.
The harmonic signal 36 is received by a receiving coil 32, located within coil
housing units 2 and 4.
page 12/26
CA 02232474 1998-OS-19
As previously stated the marker element 8 may be deactivated in the
deactivating
device 46 so that no signals 32 will be generated in the surveillance zone 6
during the passage through. This is accomplished by including magnetically
hard
material 34 within the label 8 which becomes magnetized in deactivating system
46 to such an extent that the magnetically hard material 34 will prevent the
switching of the ferromagnetic material 30 in surveillance zone 6.
Receivincl Coil
The receiving coil 36 is more particularly disclosed in FIGS. 4 and 5. The
particular configuration of the receiving coil 36 is that of a figure eight.
The
reasoning behind the particular choice is that the receiving coil 36 acts as a
passive filter element; that is if the area of the two halves of figure eight
are the
same, the fundamental frequency of 6 KHz is nulled or substantially
eliminated;
yet, the signal 32 induced by the marker element 8 is not nulled, since the
marker element 8 cannot be in both regions of the figure eight at the same
time.
In the preferred embodiment, the receiving coil 36 comprises of ten turns of
wire
located in a wire ribbon cable, the ends being so interconnected such that a
ten
turn coil is formed, as illustrated in FIG. 5. Since the receiving coil 36 is
comprised of ten turns of wire, the receiving coil 36 also acts as a passive
gain
stage, that is, by utilizing ten turns a voltage gain of 10 is accomplished.
Electrostatic shielding 38 is placed over the receiving coil 36 so as to
shield the
receiving coil 36 against receiving electrostatic signals from the ambient
atmosphere. However, it is obvious that the electrostatic shielding 38 does
not
extend over the entire extent of the figure eight of the receiving coil 36,
otherwise, the electrostatic shielding 38 would change the characteristics of
the
receiving coil 36.
Other receiving coils used in the trade have a resonant frequency of
approximately 130 KHz, which is where most of the energy from the signal 32 of
the label marker element 8 lies.
page 13/26
CA 02232474 1998-OS-19
The receiving coil 36 herein, is designed to have a much higher resonant
frequency than used in the trade. In the preferred embodiment the resonant
frequency of the receiving coil 36 is 400 KHz. The reason why the receiving
coil
was designed to have higher resonant frequency than the signal 32 generated
by the label 8 is that a coil when excited at its resonant frequency will ring
or
resonate; once a receiving coil 36 rings, one loses the characteristic of the
exciting signal and obtains the characteristic of the receiving coil 36, and
accordingly, the signal 32 generated by the label 8 loses its distinctiveness.
A flat ribbon cable is used to form the receiving coil 36 since it has a lower
distributed capacitance and gives a resonant frequency of approximately
400 KHz.
The receiving coil 36 is made more lossy by the placement of one K ohm
resistor
across its terminals as illustrated in FIG.14. The K ohm damping resistor is
added to prevent the receiving coil 36 from ringing with anything but a large
signal at its resonant frequency.
Therefore, a receiving coil 36 is disclosed which has a filter gain system
with a
broad band pass of about 400 KHz with a gain of ten that does not distort the
signal at all and yet, is a passive element.
It is important that the phase of the receiving coil 36 matches that of the
transmitting coil 48 so as to maximize the capture of signal 32 generated by
the
label 8 in response to the in and out of phase generation of oscillating
magnetic
field produced by the transmitting coils 48.
Accordingly, the phase of the receiving coil 36 mounted adjacent the
transmitting
coil 48 within coil housing unit 2 is wired so as to be in phase with the
transmitting coil 48 in coil housing unit 2. Since the phase of the
transmitting coil
48 in the coil housing unit 2 is held constant and the phase of the receiving
coil
36 in coil housing unit 2 is also held constant.
page 14/26
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The phase of the receiving coil 36 mounted adjacent the transmitting coil 48
within coil housing unit 4 is wired so as to be in phase with the transmitting
coil
48 in coil housing unit 4. Since the phase of the transmitting coil 48 in coil
housing unit 4 is alternated to be in and out of phase with respect to the
transmitting coil 48 in coil housing unit 2, receiving coil 36 mounted within
coil
housing unit 4 is wired in phase with respect to the transmitting coil 48 in
coil
housing unit 4 so as to be alternating in and out of phase with respect to the
receiving coil 36 in coil housing unit 2, but remain in phase with
transmitting coil
48 in coil housing unit 4. Therefore, the phase of the receiving coil 48 in
coil
housing unit 4 remains in phase with the transmitting coil 48 in coil housing
unit
4 as the phase of the transmitting coil 48 in coil housing unit 4 is switched
to be
in and out of phase with respect to the transmitting coil 48 in coil housing
unit 2.
Electronic Circuitry
Once the signal is recovered from the receiving coil 36 without any wave
shaping
the signal 32 is extracted from the signal without substantial alteration by
the
electronic circuitry generally depicted as number 10 in FIG. 1 and more
specifically itemized in FIG. 14 and FIG. 15.
FIG. 14 is a block diagram of the circuitry fortwo gates system which extracts
the
generated signal 32 and which is capable of differentiating between object
signals. The block diagram includes two receiving coils 36A and 36B, impedance
matching and gain stages 15A and 158) summing stations 16A and 16B,
high-pass filter systems 17A and 17B, low-pass filter systems 18A and 18B,
automatic gain control stages 19A and 19B, switches 20A and 20B, signal
recognition stages 21 A, 21 B, and alarm circuitry 14.
When the transmitting antennas are driven out of phase, switches 20A and 20B
are in a position when the voltage gain is 1. When the antennas are driven in
phase switches 20A and 20B are in a position when the voltage gain is 0. So
the
values of the analog signals after summing stations 15A and 15B in the middle
of the gate are about the same for both direction of the field because of
doubling
signals from both posts only for the out of phase direction of the field, when
the
field is two times weaker than the field for the in phase direction.
page 15/26
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Doubling of signal is very useful to increase the sensitivity and decrease a
common value of amplifier, and accordingly to decrease the probability of
false-alarms.
The same values of analog signals for both directions of the field are very
useful
for signal recognition stages 21 A, 21 B and accordingly for sensitivity and
probability of right alarms.
This invention is very useful for reliability of the system because if the
troubles
happen with any parts of one post (except transmitting coils 48, receiving
coils
36A and 36B, impedance matching and gain stages 15A and 15B, summing
stations 16A and 16B), the system would continue to work with the same
sensitivity in every places of the gate but only slower. Moreover, if troubles
happen with some parts of both posts, the system would continue to work
properly.
When wire is added to the receiving coils 36A and 36B, the capacitance of the
receiving coils increases; accordingly the impedance matching stage 15A and
15B is necessary so that the coax connecting the receiving coils to the
interrogator will not detune the receiving coils. The impedance matching stage
15A and 15B also includes a gain stage. In practice it was discovered that by
adding a gain at this point, the signal to noise ratio (SlN) was greatly
improved.
The gain is so designed that the fundamental frequency 6 KHz is not amplified
and the lower cut-off frequency is 48 KHz. This stage has a gain of
approximately
200 for frequencies above 50 KHz and below 400 KHz and a gain of
approximately unity at the fundamental frequency of 6 KHz. The upper cut-off
frequency of 400 KHz was inserted to eliminate the radio frequency pick up
from
the receiving coils 36A and 36B. The impedance matching and gain stage
essentially amplifies the fundamental frequency of signal 32 two hundred times
while the fundamental frequency of the oscillating magnetic field is amplified
by
one. In this manner the fundamental frequency generated by the label 8 is
emphasized so as to facilitate its analysis.
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The signals out of summing stages 16A and 16B are then processed through a
filtering system (which will be more fully described herein) with maximum care
being given to do as little wave shaping as possible, since the electronic
circuitry
described herein is adapted to isolate the distortion caused by the label 8,
therefore the electronic circuitry must be designed so as not to cause or
generate
a distortion through our own faulty systems.
Care must be given to the filtering system utilized) otherwise, problems may
result due to system non-linearities which are induced and generated by our
own
system signals which look very similar to the generated marker element
signal 32.
For this reason, one preferred filtering technique is the use of a transversal
filter
in a band-pass configuration such as sampled data filter which is linear in
phase.
These filters typically have transition rates exceeding 150 dB/octave, and
have
more than 40 dB stop band rejection making them ideal for critical filtering
situations.
Where less critical filtering is acceptable, the more common types of design,
such
as Butterworth may be employed with the final result being that some
additional
processing may be required to give close to the accuracy of a system employing
a transversal filter.
High-pass Filter
The cut-off frequency of high-pass filters 17A and 17B is selected to be high
enough with a steep enough slope to efifectively remove the fundamental
frequency of 6 KHz from the signal; but leaving enough lower order harmonics
to be able to discriminate signals which generate larger lower order harmonics
along with higher order harmonics such as pop cans and large ferrous objects.
A Butterworth's filter (flattest response) was utilized so that the signal is
free from
any wave shaping. It was determined that a 50 KHz lower cut-off frequency and
a scope of 24 dB per octave would give the best results.
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The high-pass filter imparts a slight gain of 2.6 or amplification of 8.3 dB
to the
signal a in.
Low pass Filter
Since the high-pass filters 17A and 17B enhances noise, the low-pass filters
18A
and 18B with a flat response was installed to clean up the signal and get rid
of
any radio frequency that was picked up by the circuit.
The upper cut-off frequency of the low-pass filter was determined
experimentally
to operate optimally at 400 KHz or more.
The low-pass filter imparts a gain of 2.6 or amplification of 8.3 dB to the
signal.
Automatic Gain Control StacLe
Once the signal has been filtered, it is passed through an automatic gain
control
stage 19 so that the amplitude of each signal will be substantially equal
before
attempting signal recognitions.
A fairly efficient automatic gain control system is required having a dynamic
range of 60 dB without distortion. The automatic gain control system must be
designed so as to accommodate a very weak signal in the middle of the gates
(2 mv) or a strong signal almost touching the gate (500 mv). The output of the
automatic gain control will be constant, therefore, all signals will be of
equal
amplitude when attempting signal recognition.
The gate input signal is first amplified than part of this signal is sent to
the
feedback network which will control the level of the input to maintain a
constant
generated output.
page 18/26
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Time Domain Blanking
Since the signal 32 generated by the label 8 occurs only at certain points in
time
corresponding to the in phase and out of phase timing any signal generated
during any other interval of time may be blanketed out by a time domain
blanking
circuit so as to further eliminate any false alarms.
When the signal has been retrieved correctly it will appear only at certain
points
in time corresponding to in phase and out of phase timing. If one blanks the
signal out except for the correct moment only those signals generated by the
label 8 will appear.
In previous systems time domain blanking was implemented but since the signal
was ringing, the signal would spill over into the time when the label 8 signal
would appear and thus cause false alarm. The stronger the signal, the longer
the ring and the more likely that it would spill over into the time domain
where the
signal from the label 8 would appear. Therefore, in previous systems a strong
signal from something like a pop can would ring long enough to spill over into
the
time where the signal 32 from the label 8 would appear and thus cause a
false alarm.
Since the precise location of the signal 32 is known, the aperture in the time
domain blanking circuit can be made much narrower so as to eliminate further
the possibility of false alarms.
Signal Recognition
Once the signal has been retrieved, kept at a uniform amplitude, and having
95%
of the false signal discarded by utilizing time domain blanking, the signals
can
then be analyzed to determine whether it is the correct signal.
Algorithm responsible for recognition of the ferromagnetic marker and for
discriminating the marker from similar signals produced by metal objects and
electromagnetic interference is compared a single marker image byte for byte
against a known good marker image which was collected during learning mode.
If the two images match then an alarm is sounded.
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CA 02232474 1998-OS-19
Additional Signal Processing
When working with weaker signals caused by either spreading the transmitting
coils 48 further apart in order to afford a wider passage way and a larger
surveillance zone 6, or by reducing the label 8 length to extend its utility
and
reduce its costs, another method of signal recognition is the use of signal
averaging. In a preferred form, signal averaging consists of two charge-
transfer
devices, each with thirty-two taps equally spaced one sample time apart, but
with
the taps individually connected to a set of capacitors by means of a transfer
gate.
Each set of capacitors also has a reset switch to delete the previously store
information before accepting signals from a new signal interration cycle, thus
allowing flexibility in selecting any number of signals to be averaged with a
signal
processing algorithm based on the first order differential equation of each of
the
individual storage sights or taps. The algorithm is effectively the same as
that
of a single pole recursive filter; however, it is not subject to the
degradation of the
signal to noise ratio inherent in recursive integration passed by the process
recycling a "coherent noise".
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.
Pumping in more than two sates s sy tem
Pumping in three gates system involve non-stable, different pumping of
transmitting antenna in the middle gate and partly in other two gates. For two
different modes in-phase and out-of-phase, fields from two outside gates add
or
subtract from the field of the middle post. Influence of middle post to
outside
posts is in smaller proportions. Electromagnetic field change on transmitting
antenna between in-phase and out-of-phase can be about 120-150V for the
middle gate and 60-80V for each outside gate. Such type of field change leads
to non-stable work, more probability of false alarms and more complicated
calibration process. In the present invention the phase orientation of
receiving
antenna for one of the outside gate is opposite to the another outside and the
middle gates. By this the outside gates work in different phase modes
according
to the middle gate and so eliminate electromagnetic field influence on it.
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CA 02232474 1998-OS-19
For more than three gates system the same method can be used. So for four
gates system the first and second gates have the same phase orientation of
receiving antennas but the third and fourth gates have the opposite. For five
gates system the first, second and fifth gates have the same phase orientation
of receiving antennas but the third and fourth gates have the opposite.
FIG. 15 is a block diagram of the circuitry for three gates system. The block
diagram includes two receiving coils 36A, 36B and 36C, impedance matching and
gain stages 15A, 15B and 15C, summing stations 16A) 16B and 16C, high-pass
filter systems 17A, 17B and 17C, low-pass filter systems 18A, 18B and 18C,
automatic gain control stages 19A) 19B and 19C, switches 20A, 20B and 20C,
signal recognition stages 21 A, 21 B and 21 C and alarm circuitry 14.
Excluding
connection between third and middle gates prevent tripled noise in a middle
gate.
page 21 /26
