Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE
HIGH RESPONSE ELECTRONIC ARTICLE SURVEILLANCE
SYSTEM RESPONDERS AN~ METHODS FOR PRODUCING SAME
BACKGROUND OF THE lNv~:NllON
Field of the Invention
This invention relates to electronic article
surveillance systems and more particularly it concerns
novel responders, also known as targets, used in such
systems, as well as novel methods for manufacturing
such responders or targets.
Description of the Related Art
Electronic article surveillance systems are used to
detect the unauthorized taking, or theft, of books or
merch~n~ise from libraries, stores, etc. In general,
these systems include a wave generator which generates
electromagnetic or magnetic waves near the exit from a
protected area. The books or merch~n~ise to be
protected are provided with targets or responders
which, when taken through the exit, produce a
characteristic disturbance of the electromagnetic or
magnetic waves. A monitor is also positioned at the
exit to detect these disturbances; and when a
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characteri~tic disturbance is detected, an alarm is
energized.
There are different types of targets or responders,
which operate in different ways and generally at
different frequencies. This invention is concerned
with targets or responders which comprise a strip of
low magnetic coercivity material which is easily
magnetically saturable. These targets or responders
generally operate at lower frequencies, for example in
the range of about 50 Hz to about 2500 Hz.
It has been found that targets in the form of strips of
an iron and or cobalt alloy in the amorphous state,
which have been heat treated under controlled
conditions, produce especially distinctive and easily
detectable signals when placed in an alternating
magnetic field.
U.S. Patent No. 4,660,025 and U.S. Patent No. 4,980,670
describe amorphous markers (also known as "targets" or
"responders") which are said to produce distinctive
responses by virtue of a large Barkhausen
discontinuity. That is, when the responder is placed
in an alternating magnetic interrogation field, where
it is driven alternately in opposite directions into
and out of magnetic saturation, it disturbs the field
in such a manner that sharp signal peaks are produced.
These signal peaks are described as being quite unlike
the signals produced by ordinary metal objects in the
same field; and they can therefore be distinguished
from such ordinary objects.
U.S. Patent No. 5,029,291 describes a target or sensor
element which produces even more unique responses which
are characterized by an asymmetrical wave form.
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All of the above described targets or markers are
formed by first cutting a continuous strip of amorphous
metal into predet~rm;ned f;n;sh~ target lengths and
then subjecting those lengths to a special heat treat
process. In U.S. Patent No. 5,029,291 the heat treat
process is carried out in the presence of a small
magnetic field, e.g. 0.3 oersteds directed along the
length of the strip. In all these cases the heat treat
must be carried out after the strip has first been cut
into the lengths of the individual finished targets.
If heat treatment were carried out on the continuous
strip before it is cut into individual target lengths,
the cutting of the heat treated strip will adversely
affects its magnetic characteristics. Thus, it is not
possible with the prior art to produce a spool of heat
treated material from which finished targets or
responders can be cut; and targets or responders with
special magnetic characteristics cannot be produced
except by a process which requires first severing and
then heat treating. This is expensive and time
consuming; and the targets or responders cannot be
handled conveniently as they could if they could be
simply cut from a supply spool as they are needed.
SUMMARY OF THE INVENTION
The present invention overcomes the above described
problem of the prior art and provides novel targets or
responders which are characterized by an especially
distinctive response characteristic and which can be
made in the form of a continuous strip on a roll from
which individual lengths may be cut without adversely
affecting their magnetic characteristics.
According to one aspect of the invention, there is
provided a novel high response marker for use in
electronic surveillance systems. This novel marker
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comprises an elongated strip of material consisting
essentially of the formula:
Co,~ Fe~2T~3B,~4
where:
T is a transition metal chosen from at least one
of the elements of the group consisting of vanadium,
titanium, niobium, zirconium, chrome and manganese; and
where:
~ = 0-85~;
~ = 0-85~;
~ = 0.5-5~; and
14 = 15-30%'.
(All percents are expressed as atomic percentage).
According to another aspect of the invention, there is
provided a novel method for manufacturing a high
response marker. According to this method a continuous
strip is formed of an amorphous alloy consisting
essentially of the formula
Co~,Fe~T~B~4
where T is a transition metal chosen from at least one
of the elements of the group consisting of vanadium,
titanium, niobium, zirconium, chromium and manganese
and where ~, = 0-85~; ~ = 0-85~; ~ = 0.5-5~; and
~4 = 15-30~ (all percents being atomic percentages).
The strip is formed preferably by melt spinning the
molten composition and then as it cools through its
annealing temperature range, the strip is subjected to
a magnetic field along its length which is in the range
of 180-300 millioersteds.
~RIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagrammatic side elevational view of a
spin casting apparatus used in manufacturing novel
material according to the present invention;
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Fig. 2 is a top view of the apparatus of Fig. 1;
Fig. 3 i8 a magnetic hysteresis diagram of the novel
material of Fig. 1;
Fig. 4 is a time derivative diagram of the magnetic
flux in the novel material of Fig. 1;
Fig 5 is a magnetic hysteresis diagram of prior art
amorphous material;
Fig. 6 is a time derivative diagram of the magnetic
flux in the novel material of Fig. 1; and
Fig. 7 is a perspective view showing a spool of novel
material produced according to the present invention
and the manner in which novel individual targets are
separated from such spool.
DETAILED DESCRIPTION OF THE PREFERRED EM~ODIMENT
Conventional spin casting apparatus, such as shown in
Figs. 1 and 2, may be used to produce novel strip
material from which targets or responders may be cut
according to the present invention. Basically, this
apparatus comprises a melting crucible 10 in the form
of a vertical ceramic tube with an extrusion nozzle 12
at the bottom and a removable pressure sealed cover 14
at the top. A pressurized inert gas, such as argon, is
applied to the crucible 10 via the cover 14 to force
the liquid contents of the crucible out through the
nozzle 12. A high frequency induction coil 16
surrounds the crucible 10 along its length; and high
frequency electrical current is applied to the coil
from an external source to produce induction heating
and melting and mixing of the crucible contents.
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The crucible 10 is first charged by ~no~ing the cover
14 and charging the crucible with material to form a
desired alloy. In accordance with the present
invention, the alloy would have a composition according
to the formula: Co~l Fe~ T~ B~ where T is a transition
metal chosen from one or more of the group consisting
of vanadium, titanium, niobium, zirconium, chromium and
manganese and where xl is 0-85~; ~ is 0-85~; X3 iS 0.5-
5.0~; and X4 iS 15-30~ (all atomic percentage).
In a preferred alloy: xl=73~, x2=5~, x3=2% and x4=20~ and
T is vanadium. The alloys of this nvention are
especially characterized by the absence of silicon,
which will be explained hereinafter. The purity of the
composition should be at least that which is ordinarily
used to make amorphous strip usable for responders or
targets for electronic article surveillance systems.
A spin cooling wheel 18, preferably of copper, is
positioned to rotate about a horizontal axis 20 with
its periphery 22 just below the nozzle 12. In the
preferred embodiment the wheel diameter is 30
centimeters and it is spun about its axis at
approximately 2500 revolutions per minute so that the
wheel surface moves at a linear speed of about 40
meters per second. The speed and diameter of the wheel
should be such as to cause the molten alloy from the
nozzle 12 to become rapidly quenched on the wheel
periphery 22 and solidified into an amorphous state and
then thrown off as a continuous solid strip 24 where it
cools further and then becomes collected in a
receptacle (not shown). The linear speed of the
periphery 22 of the cooling wheel 18 affects the
thickness of the finished strip. Preferably the strip
24 should have a thickness of about 20-30 micrometers;
and for this thickness to be achieved, the wheel
periphery should move at 35-40 meters per second.
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As can be seen in Fig. 1, after the continuous strip 24
separates from the spin cooling wheel 18, it follows a
ballistic trajectory along which it falls in a curved
path toward the earth. During this movement, the strip
air cools and passes through several temperature zones,
as indicated by the letters A, B and C in Fig. 1. In
the region between the nozzle 12 and point A, the strip
24 cools from its initial solidification temperature to
its Curie temperature, which in the case of the
preferred alloy is about 500 C. At this point, the
material of the strip 24 begins to become
ferromagnetic. Also at this point, the strip should be
out of the range of magnetic fields produced by the
induction coil 16; so that it will not be influenced by
that field. The strip 24 then moves to point B, where
it enters a zone of best annealing temperature, which
is approximately 400 C. The strip 24 then passes to
point C where it has cooled to about 250 C, and below
which no further annealing takes place. Between points
B and C the strip undergoes an annealing process in
which it acquires its desired magnetic properties.
As can be seen in Fig. 1, the earth's magnetic field
extends upwardly from the horizontal at an angle of
about 45; and the spin cooling wheel 18 is positioned
such that the portion B-C of the path of the strip 24
is at a controlled angle to the direction of the
earth's magnetic field, namely the component of the
earth's field along the length of the strip will be in
the range of 180-300 millioersteds (mOe). Also, as
shown in Fig. 2, the spin cooling wheel is positioned
such that its plane of rotation is in a controlled
angle to the direction of the earth's magnetic field.
As a result, as the strip 24 passes through its
annealing temperature, from about 400 C to about 250
C, it is subjected to a continuous magnetic field of
180-300 mOe along its length.
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It has been found that when the strip 24 is maint~; nP~
parallel to a continuous ~agnetic field of 180-300 mOe
along its length while it cools through its ~nne~ling
range, e.g. 400-250 C, the material of the strip
S acquires very unique magnetic properties which are
illustrated in Figs. 3 and 4. Fig. 3 shows the
magnetic hysteresis characteristic of a target or
responder which has been cut from the strip 24 after it
has completely cooled. The length of the target or
responder in this case is about 3.81 centimeters and
the hysteresis is measured at a frequency of 60 Hertz.
As can be seen, when the s terial is subjected to a
magnetic field of about 0.15-0.20 oersteds in each
direction, magnetic flux in the strip undergoes a sharp
change (a). The time derivative of this change, which
is shown in Fig. 5, is seen as very sharp pulses (b)
which are unlike any magnetic response produced by
other materials.
This can be appreciated by comparing the magnetic
hysteresis and time derivative curves of Figs. 3 and 4
with those of Figs. 5 and 6. Figs. 5 and 6 are the
magnetic hysteresis and time derivative curves,
respectively, for a 3.81 centimeter long target or
responder of a prior art amorphous alloy used for
electronic article surveillance responders or markers.
This material, which is supplied by Allied-Signal
Company of Morristown, New Jersey, under the
designation Metglas~ 2714AZ amorphous strip, has a
distinctive magnetic hysteresis characteristic but does
not exhibit the sharp changes in flux in the low field
(less than 1 oersted) as shown in Fig. 3.
Consequently, the time derivative shown in Fig. 6
includes only pulses of small height and large width.
A particularly significant feature of the present
invention is that the unique magnetic properties
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described above are unaffected by the action of
severing the continuous strip into individual lengths.
Although the exact reason for the unique magnetic
characteristics of the amorphous strip of the present
invention has not been fully determined, it is believed
that the absence of silicon in the alloy is a
significant factor. When silicon is present, it
combines quickly with oxygen in the air to form a
silicon oxide coating around the surface of the strip
which impedes oxidation of the metallic components.
Thus, silicon containing alloys require several hours
for proper annealing. On the other hand, when silicon
is not present, the iron and/or cobalt will form a very
thin oxide coating in the short time during which the
strip air cools through its annealing temperature. It
is believed that this oxide coating becomes
magnetically coupled to the strip 24 and cooperates
with the strip to produce the unique magnetic
characteristic illustrated in Figs. 3 and 4. The oxide
coating is belleved to be only one or at most a few
atoms thick and, therefore, when the strip is cut into
individual targets or responders its overall magnetic
properties are only insignificantly affected.
In the preferred embodiment the strip 24 should have a
width in the range of 0.4-1.0 millimeters. When
targets or responders are cut from wider or narrower
strips the unique magnetic characteristics are not
reliably produced.
It is also preferred that rotational speed of the spin
cooling wheel 18 be such that its peripheral surface
moves at a speed in the range of 35-40 meters per
second. This speed affects the cooling rate and the
thickness of the strip, as well as the percentage of
its amorphous content.
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The quenching temperature will affect the cooling rate
and the width of the strip 2~ for a given pressure of
inert gas applied to the cover 14 of the crucible 10.
This temperature i8 controlled by adjustment of the
power to its induction coil 16. This adjustment
meanwhile affects the alternating magnetic field in the
vicinity of the surface of the wheel 18 where the strip
is formed.
Experiments have been conducted to ascertain the
operative and preferred limits of the present
invention. In each of these experiments targets or
responders of 3.8 cm length were cut randomly from
strips made under different conditions; and the
magnetic characteristics of these targets or responders
were examined in a 60-100 Hz AC field of 0.4-1.0
oersteds.
The following compositions were tested (the subscripts
are atomic percentage) with the following results being
observed.
Composition Effect on Shape of Hysteresis
Characteristic
Co74Fe6B2o No sharp jump was observed
Co73FescrlvlB2o Weak sharp jump was observed
25 Co~3Fe5crlsB2o Very weak sharp jump was observed
co73Fe5cr2B2o Weak sharp jump was observed
co73Fe5.5vl.sB2o Fairly strong sharp jump was
observed
Co~3Fe5v2B~o Strong sharp jump was observed
co72Fe4v4B2o Weak sharp jump was ob~erved
Co7o5Fe45silsB2o No sharp jump was observed.
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It has also been found that when the direction of the
strip ll~OV~ t during cooling in relation to the
direction of the earth's magnetic field has an effect
on the magnetic properties. Se~eral strips with the
composition Co73Fe~V2B20 were cooled while mo~ing at
different angles relative to the earth's magnetic field
so that they would each be subjected to a different
magnetic field along its length. The following effects
on the magnetic hysteresis characteristics of these
strips were observed:
Strength of Magnetic Field
Component Along Length of Effect on Shape of
Strip Durinq Cooling Hysteresis Characteristic
Greater than 2 Oe No sharp jump
, 15 390-890 mOe 65~ of strips exhibited a
jump in two magnetization
directions; 20~ exhibited a
jump in one magnetization
direction; and 25~ exhibited
no jump.
180-300 mOe 100~ of strips exhibited a
sharp jump in two
magnetization directions
50-100 mOe 15~ of strips exhibited a
sharp jump in a single
magnetization direction and
85~ exhibited no jump.
The width of the strip 24, (and the width of the
targets or responders which are cut from the strip) is
controlled by the size of the nozzle 12. This width
should be less than 1.8 millimeter and preferably in
the range of 0.8 - 1.0 millimeter.
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The cooling wheel speed should be in the range of 2000
to 2700 revolutions per minute, for a 30 centimeter
diameter wheel; and preferably the wheel speed should
be between 2000 and 2500 re~olutions per minute. Also,
the power supplied to the induction heating coil 16
should be in the range of 5-10 kilowatts.
A preferred strip is made from a 150 gram ingot of the
alloy Co~Fe~V2B20 which is melted by the application of
10 kilowatts of induction power from the coil 16 and
extruded at a width of 0.4-1.0 millimeter onto the
periphery of a 30 centimeter diameter cooling wheel
which is rotating at about 2500 revolutions per minute
with the periphery moving at a certain angle to the
direction of the earth's magnetic field such that a
component of the earth's field which extends along the
length of the strip is in the range of 180-300
millioersteds. The strip should be protected from all
other magnetic fields as it cools through its annealing
range, which is between 400C and 250C.
The field at which a target or responder made according
to the present invention will exhibit a sharp signal
depends on the length of the target. The following
table illustrates the amplitude of the applied
alternating magnetic field, of 60 Hz, at which a sharp
change will occur for different length targets or
responders. (The width of the targets is 1 mm.)
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Tar~et Length Applied Field
10.16 cm. 0.10 Oe
7.62 cm. 0.15 Oe
6.35 cm. 0.20 Oe
5.08 cm. 0.25 Oe
3.81 cm. 0.30 Oe
Fig. 7 shows a spool 30 cont~ining the strip 24 rewound
thereon after being cast and cooled as above described.
The strip 24 may be shipped while on the spool 30 for
use at a desired location; and there, individual
targets or responders 24a can be cut from the strip 24
to any desired length, depending on the magnetic field
at which one wishes the responder to exnibit a sharp
signal. As mentioned above, cutting of the continuous
strip 24 into individual targets or responders 24a does'
not adversely affect the magnetic characteristics of
the finished target or responder.
The strip 24 as above described was formed in the
presence of the earth's magnetic field during the
cooling of the strip through its annealing range from
about 400C to about 250C. The strip must be isolated
during this time from the effects of other magnetic
fields. It is not necessary that the earth's field be
precisely aligned with the length of the strip 24
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during this time, so long as the component of the
earth's field along the length of the strip is in the
range of 180-300 millioersteds. It is also not
necessary that the bias be supplied by the earth's
magnetic field. An artificially generated field may be
substituted, provided that some means are taken to
isolate the strip from other fields including the
earth's magnetic field. It has been found convenient
to use the earth's field because in most latitudes the
earth's field extends at an angle from the horizontal
such that a component thereof, in alignment with the
ballistic trajectory of a strip being thrown from a
spin cooling wheel, will be in the required range.
An especially notable feature of the present invention
is that the novel targets or responders 24a exhibit
their most distinctive effects when interrogated at low
frequencies. Prior art targets or responders are best
distinguished from ordinary metal objects when they are
subjected to high frequency alternatively magnetic
interrogation fields which alternate at frequencies
higher than 200 Hz. However, such high frequencies
make it difficult to measure the time between responses
produced in successive interrogation cycles. The novel
targets of this invention can best be distinguished
from other metal objects when interrogated with lower
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frequency magnetic fields, for example fields which
alternate at 60 Hz. This makes possible more complex
signal processing.
