HIGH RESPONSE ELECTRONIC ARTICLE SURVEILLANCE SYSTEM RESPONDERS AND METHODS FOR PRODUCING SAME

BACKGROUND OF THE INVENTION 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 merchandise 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 merchandise 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

characteristic 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.

All of the above described targets or markers are formed by first cutting a continuous strip of amorphous metal into predetermined finished 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

comprises an elongated strip of material consisting essentially of the formula:

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: _xl = 0-85%; _Λ = 0-85%; _ri = 0.5-5%; and _x4 = 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 COr _f Fer _f Tr _f Br _f 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 _xl = 0-85%; _ώ = 0-85%; _ύ = 0.5-5%; and _x4 = 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.

BRIEF 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;

Fig. 2 is a top view of the apparatus of Fig. 1;

Fig. 3 is 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 EMBODIMENT

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.

The crucible 10 is first charged by removing 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 _xl Fe^ T^ B _x4> 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 _X1 is 0-85%; _X2 is 0-85%; _X3 is 0.5- 5.0%; and _x4 is 15-30% (all atomic percentage) .

In a preferred alloy: _X1 =73%, _X2 =5%, _X3 =2% and _x4 =20% and T is vanadium. The alloys of this invention 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.

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.

It has been found that when the strip 24 is maintained parallel to a continuous magnetic field of 180-300 mOe along its length while it cools through its annealing range, e.g. 400°-250° C, the material of the strip 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 material 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

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 believed 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.

The quenching temperature will affect the cooling rate and the width of the strip 24 for a given pressure of inert gas applied to the cover 14 of the crucible 10. This temperature is 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

Co ₇₄ Fe ₆ B ₂₀ No sharp jump was observed

Co ₇₃ Fe ₅ Cr ₁ V,B ₂₀ Weak sharp jump was observed Co ₇₃ Fe ₅ Cr _{1 5} B ₂₀ Very weak sharp jump was observed

Co ₇₃ Fe ₅ Cr ₂ B ₂₀ Weak sharp jump was observed

Co ₇₃ Fe ₅ . ₅ Vι _j B ₂₀ Fairly strong sharp jump was observed

Co ₇₃ Fe ₅ V ₂ B ₂₀ Strong sharp jump was observed

Co ₇₂ Fe ₄ V ₄ B ₂₀ Weak sharp jump was observed Co _{70 5} Fe _{4 5} Si ₁₅ B ₂₀ No sharp jump was observed .

It has also been found that when the direction of the strip movement during cooling in relation to the direction of the earth's magnetic field has an effect on the magnetic properties. Several strips with the composition Co ₇₃ Fe ₅ V ₂ B ₂₀ were cooled while moving 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 During Cooling Hysteresis Characteristic

Greater than 2 Oe No sharp jump 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.

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 revolutions 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 ₇₃ Fe ₅ V ₂ B ₂₀ 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 400°C and 250°C.

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.)

Target 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 containing 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 exhibit 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 400°C to about 250°C. 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

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

frequency magnetic fields, for example fields which alternate at 60 Hz. This makes possible more complex signal processing.