WO2004088606A1

WO2004088606A1 - Method of manufactureing deactivating elements for magnetic markers

Info

Publication number: WO2004088606A1
Application number: PCT/IL2004/000257
Authority: WO
Inventors: Evgeni Sorkine
Original assignee: A.C.S. Advanced Coding Systems Ltd.
Priority date: 2003-03-31
Filing date: 2004-03-23
Publication date: 2004-10-14

Abstract

A method and system are presented for segmenting a continuous element of a metal-containing material of a kind capable of changing its magnetic properties under supply of heating energy, to form in this element first and second alternating segments of different magnetic properties. The element is subjected to a thermal treatment process during a relative displacement of the element with respect to a heating zone defined by a heating source operating with a constant heating power. The thermal process results in different heating conditions for alternating segments of the element thereby creating the first magnetic segments spaced by the second non-magnetic segments in the element, and sequential fast cooling of the second segments when leaving the heating zone thereby stabilizing the non-magnetic state of the second segments.

Description

METHOD OF MANUFACTURING DEACTIVATING ELEMENTS FOR MAGNETIC MARKERS

FIELD OF THE INVENTION

This invention is generally in the field of article surveillance techniques utilizing magnetic markers, and relates to a method of manufacturing deactivating elements used in magnetic markers.

BACKGROUND OF THE INVENTION

Article surveillance systems of electromagnetic type are widely used for theft prevention in stores, libraries or the like. Such systems use magnetic markers attached to objects to be protected from unauthorized removal from the protected site. The marker, when subjected to an interrogating magnetic field, provides a response to the field, and thus enables for detecting the presence or absence of the object.

A magnetic marker typically comprises a soft magnetic unit (one or more elongated pieces of soft-magnetic material) characterized by relatively high permeability and low coercivity, and may also include a semi-hard magnetic unit serving as a deactivating element for the soft-magnetic material. In the active state of the marker (i.e., allowing the marker detection), the semi-hard magnetic material (deactivating element) is in its non-magnetized (non-active) state, and thus allows detection of the response of the soft-magnetic material. When magnetized (active), the deactivating element creates a permanent magnetic field that significantly alters the magnetic permeability of the soft magnetic element of the marker and thus makes the soft magnetic element undetectable while located in the interrogation zone of the article surveillance system. In order to improve the marker operation, it is known to make a deactivating element in the form of multiple pieces of semi-hard magnetic material distributed in a space-apart relationship along the length of the active soft magnetic element of the marker. An example of a marker utilizing this concept is described in U.S. Patent No. 4,484,184. According to this technique, the marker is adapted to generate magnetic fields at frequencies that are harmonically related to an incident magnetic field applied within an interrogation zone and that have selected tones providing the marker with signal identity. The marker is an elongated, ductile strip of amorphous ferromagnetic material having a composition defined by the formula M_aN_bO_cX Y_eZ_f, where M is at least one of iron and cobalt, N is nickel, O is at least one of chromium and molybdenum, X is at least one of boron and phosphorous, Y is silicon, Z is carbon, "a"-"f ' are in atom percent, "a" ranges from about 35-85, "b" ranges from about 0-45, "c" ranges from about 0-7, "d" ranges from about 5-22, "e" ranges from about 0-15 and "f ranges from about 0-2, and the sum of (d+e+f) ranges from about 15-25.

However, the manufacture of such markers is complicated because of the problem associated with precise positioning and securing of the pieces of a deactivating element along the marker.

Some types of steel alloys are known to be not magnetic in their annealed state (austenite steels). Cold rolling or die drawing of this steel produces certain amount of martensite phase in the steel, and the material becomes a semi-hard magnetic. Further heating of this material to high temperatures can destroy the martensite phase, so that the steel becomes again austenite and non-magnetic. These phase transitions are described, for example, in the book of L. Colombier and J. Hofrnan, "Stainless and Heat Resistant Steels", Edward Arnold Publishers Ltd, 1967.

The above property can be used for producing an alternating pattern of magnetic and non-magnetic segments in a continuous strip or wire made from such steel alloys, by local annealing in spatially separated regions followed by cooling of the heated spots. In such a way, a segmented deactivating element can be made operating similar to the multi-piece deactivator described above.

Methods of manufacturing likewise deactivating elements are described for example in the following patent publications: EP 0756255; US 6,166,636; and WO 01/63577. The manufacturing method of EP 0756255 utilizes electrical heating of a 301 type stainless steel strip. The heating is affected in several treatment stations, where each station includes two contact heads spaced at relative distance from each other and connected to a source of current. The processed strip is fed intermittently between the contact heads, and the heating current is switched on and off to produce the desired periodic structure of the deactivation strip. Since the process is intermittent, high production speeds are difficult to achieve. The production method described in WO 01/63577 utilizes electrical heating of a 304-type stainless steel wire in a continuous process. The wire is spooled at constant high speed over two fixed electrodes, while the heating current is switched on and off. Also proposed in these three publications, is a laser heating method for thermal treatment of a semi-hard magnetic strip. Laser heating, though being effective, needs a powerful (hundreds of Watts in the beam power) and expensive laser station to be installed in the production line. In an alternative embodiment proposed in US 6,166,636, the strip is conducted over a heated gearwheel in a continuous ran, the spacing of the individual teeth of the gearwheel corresponding to the expanses of the magnetic regions. The gearwheel is brought to a temperature higher than the phase conversion temperature.

SUMMARY OF THE INVENTION

There is a need in the art to facilitate patterning of a strip or wire of a magnetic material to produce in this material spaced-apart semi-hard magnetic regions spaced by non-magnetic regions. Such a patterned strip or wire can be used as a segmented deactivating element in a magnetic surveillance marker. The present invention provides a novel effective technique of patterning a magnetic material, utilizing the above-described principles of phase transitions in a metal-containing material (e.g., steel alloy material). In distinction to the known methods of the kind specified, the present invention takes into account a need for controlling the magnetic structure of the strip/wire during the patterning process, as well as such property of this material as the need for fast cooling of the heated phase in order to stabilize the austenite non-magnetic structure of the treated regions of the strip/wire.

It is known in the art that if the cooling process is slow, the steel structure turns to ferrite characterized by small but noticeable magnetization. For example, with the temperature condition used in the technique of US 6,166,636 (1000°C or more), the most part of heat exchange between the treated strip and the heated gearwheel goes by radiation, and therefore all parts of the treated strip will be heated to high temperatures, and thus the treatment will be ineffective. The present invention solves the above problem by applying a process of thermal treatment to a thin (a few tens of microns thickness) continuous strip or wire of a metal-containing material, of a kind capable of changing its magnetic properties under supply of heating energy, with a constant thermal power and non- uniform heat exchange conditions of said process, thus forming in this strip/wire a pattern of segments of stable different magnetic properties.

There is thus provided, according to one aspect of the invention, a method of segmenting a continuous element of a metal-containing material of a kind capable of changing its magnetic properties under supply of heating energy to form in said element first and second alternating segments of different magnetic properties, the method comprising applying a thermal treatment process to said element during a relative displacement of said element with respect to a heating zone defined by a heating source operating with a constant heating power, while providing different heating conditions for alternating segments of said element thereby creating the first magnetic segments spaced by the second non-magnetic segments in said element, and sequentially providing fast cooling of the second segments thereby stabilizing the non-magnetic state of the second segments.

The term "element" used herein signifies a strip of wire of the metal- containing material. The metal-containing material to be segmented is in its semi-hard magnetic state. The segmentation results in the formation of spaced-apart non-magnetic segments (regions). The thermal treatment of the strip/wire material utilizes fast cooling of the material region immediately after achieving a magnetic state change of this region, thus stabilizing the non-magnetic austenite structure in said region of the strip or wire.

The different heating conditions are achieved due to the provision of non- uniform heat exchange conditions by passing the strip or wire through a heated zone, while the strip or wire lies on a patterned cooled surface formed by an arrays of spaced-apart cooled surface regions spaced-apart from each other a predetermined distance, which is equal to that of a required length of non-magnetic segments to be obtained in the strip/wire. The heating power is thus immediately transferred from the first strip/wire segments, contacting with the cooled surface regions, to these cooled surface regions, the temperature of the strip/wire in these first segments thus rising only slightly, while the second segments of the strip/wire that are aligned with the spaces between the cooled surface regions are quickly heated above the phase conversion temperature of the strip/wire material. The second segments thus become non-magnetic. By this, the material segments of the different magnetic properties are defined by the heated segments lying on, respectively, the cooled surface regions and the spaces between these regions. When the second segments of the strip/wire (those lying above the spaces between the cooled surface regions) go out of the heated zone, they are rapidly cooled by radiation (since the initial temperatures of the heated segments are above the phase transition temperature, typically around 1000°C, and the ambient temperatures outside the heating zone are around the room temperature) and by conductive heat exchange between the strip/wire material and the cooled surface. In such a way, the non-magnetic state of the second segments is stabilized. The patterned surface is constituted by a surface formed with an array of spaced-apart grooves or slots. Preferably, the cooled surface is an outer surface of a rotating wheel having the patterned area.

Preferably, the magnetic properties of the treated metal-containing element are continuously monitored with the aim to provide the necessary quality of the manufactured material. The monitoring process consists of passing the segmented element through a DC magnetic field region, thus magnetizing the semi-hard segments of the element, and then passing this element through a vicinity of a receiving coil, thus causing an AC voltage induced in the coil and enabling detection of a profile of this voltage, and consequently the shape and value of magnetization of a periodic magnetic structure of the strip/wire element.

The method of the present invention can advantageously be used for manufacturing a deactivating element for a magnetic marker.

There us thus provided according to another aspect of the present invention a method for manufacturing a deactivating element for use in a magnetic marker, the method comprising: segmenting a continuous element of a metal-containing material of a kind capable of changing its magnetic properties under supply of heating energy to form in said element first and second alternating segments of different magnetic properties, the method comprising applying a thermal treatment process to said element during a relative displacement of said element with respect to a heating zone defined by a heating source operating with a constant heating power, while providing different heating conditions for alternating segments of said element thereby creating the first magnetic segments spaced by the second nonmagnetic segments in said element; and providing substantially fast cooling conditions for the second segments outside the heating zone thereby stabilizing the non-magnetic state of the second segments. According to another aspect of the present invention, there is provided a system for manufacturing a segmented continuous element of a metal-containing material of a kind capable of changing its magnetic properties under supply of heating energy, the system comprising: (a) an array of spaced-apart cooled surface regions spaced from each other a predetermined distance defining a length of segments to be formed in the element;

(b) a heating source accommodated so as to define a heating zone in the vicinity of the cooled surface regions and operable with substantially constant heating power to apply thermal treatment to a material located within the heating zone;

(c) a driving assembly operable to provide a relative displacement between the element to be segmented and the heating zone, while said element lies on said cooled surface regions; the system being thereby operable to apply the thermal treatment process to said element with different heating conditions for alternating segments of said element at a constant power of the thermal process, and thus create in said elements segments of different stable magnetic properties.

According to yet another aspect of the present invention, there is provided an element of a metal-containing material having segments of different magnetic properties, said element being manufactured by applying to said material a thermal treatment process at substantially constant power and different heating conditions for alternating segments of the material under treatment.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic illustration of the thermal treatment device and process of the present invention;

Fig. 2 is a block diagram of a device for monitoring the magnetic properties of a magnetic strip/wire under treatment; and

Fig. 3 exemplifies an output voltage waveform measured with the monitoring device of Fig. 2.

DETAILED DESCRD?TION OF THE INVENTION

The treatment technique of the present invention provides for sufficiently fast cooling of previously heated (treated) regions (segments) of a deactivating element material (constituted by a strip or wire of metal-containing material such as stainless steel or the like alloy) to ensure production of a stable non-magnetic austenite structure of said material within the segment previously heated above the phase conversion temperature. Such sufficiently fast cooling of the spaced-apart heated segments in the treated strip or wire can be achieved, if the lengths of these heated segments are small, and if the non-heated material segments in the spaces between the heated segments are efficiently cooled. However, it should be noted that the smaller the heated segments, the more intensive should be the heating source.

For the convenience of description, the deactivating material or deactivating element will be addressed hereinafter as a strip. It should however be understood that the same considerations are valid for the deactivating element of a wire form.

Referring to Fig. 1, there are schematically illustrated the principles of the thermal treatment method and system in accordance with the invention for manufacturing a segmented continuous element 1 of a metal-containing material of a kind capable of changing its magnetic properties under supply of heating energy.

A treatment system 100 includes an array 2B of spaced-apart cooled surface regions 2A; a heating source 4 accommodated so as to define a heating zone HZ; and a driving assembly 6 associated with the element 1 and/or the surface 2B and operable to provide a relative displacement between the element 1 and the heating zone, while the element 1 lies on the surface regions 2A. The heating source 4 is operable with substantially constant heating power to apply thermal treatment to a material located within the heating zone HZ. The cooled surface regions 2A are made of metal with good thermal conductivity (e.g., copper with a thermal conductivity value of 400W/m.K, or aluminum with the corresponding value of 200W/m.K). This cooled surface regions are formed as a patterned area of the cooled surface 2B, the pattern being in the form of an array of grooves 3 spaced- apart from each other by the surface regions 2A of a predetermined distance / defining a length of segments to be formed in the element.

In the present example, the cooled surface 2B is constituted by the circumference of a wheel 2. The driving assembly 6 is constituted by first and second drive mechanisms A and €B associated with, respectively, the wheel 2 and the element 1 supply means (e.g., a conveyor, which is not specifically shown). These drive mechanisms are operable by a control unit (not shown) so as to provide rotation of the wheel 2 with a linear speed equal to that of the movement of the strip 1. In this specific example, the heating source 4 is a gas torch operable to supply a narrow intensive flame 5 to the heating zone. A conventional torch working with propane and oxygen may be used as a heating source. However, it is advantageous for the purposes of the present invention to use a hydrogen/oxygen torch, since such torch has the ability of producing a very intensive and narrow flame, with temperatures reaching 2500°C. Hydrogen/oxygen torches are commercially available being equipped with simple and non-expensive electrolyze units that generate the mixture of hydrogen and oxygen from water, so that only distilled water and electricity are the process expenses. The only emission of such torch is water vapor. The system 100 operates in the following manner: The thin metal-containing strip 1 (deactivating element), in its semi-hard magnetic state, is passed around a cooled wheel 2_t while the wheel 2 rotates with a linear speed equal to that of the movement of the strip l.The treated strip 1 is heated by the narrow intensive flame 5. Due to a small thickness of the strip 1, the heating power of the flame 5 is immediately transferred from the strip segments 1A that contact the wheel surface regions 2A to the cooled wheel surface, so that the temperature of the metal- containing strip in these segments 1 A rises only slightly. As for those regions IB of the strip 1 that are aligned with the slots 3 in the wheel surface, these strip segments IB are quickly heated by the flame 5 above the phase conversion temperature (e.g., 900C for 301-type stainless steel). As a result, the strip segments IB become nonmagnetic, and the strip 1 is formed with semi-hard magnetic regions 1A spaced by non-magnetic regions IB. During the rotation of the wheel 2, when the heated parts of the strip 1 go out of the flame 5, they are rapidly cooled, due to their small lengths, by a conductive heat exchange between these strip segments and the cooled wheel 2. Due to the provision of the array of slots and narrow flame, the heat exchange is non-uniform at the constant power supply, and therefore the segments of the strip IB that were quickly heated, are cooled faster. Hence, the non-magnetic state of the material within the segments IB is stabilized immediately after the formation of this state.

Fig. 2 presents a schematic diagram of a device 200 for monitoring the magnetic properties of the deactivator material 1 resulting from the above- described thermal treatment.. The device 200 includes a source of DC magnetic field 11 (permanent magnet, e.g., a rare earth magnet or a strontium ferrite magnet); a receiving coil 12; preferably, an amplifier 13; and a detector (oscilloscope) 14.. After passing the thermal treatment station 100 described above, the strip 1 passes through the vicinity of the permanent magnet 11, and the semi-hard magnetic segments 1A of the strip 1 are magnetized by the permanent magnet 11. When the magnetized segments of the strip 1 sequentially, segment-by-segment, pass through - l i ¬

the vicinity of the receiving coil 12, an AC voltage is induced in the coil. This voltage is amplified and then received at the oscilloscope 14, where the AC waveform is displayed, enabling detection of the shape and the amplitude of the waveform indicative of the periodic magnetic structure of the treated strip 1. The device 200 also preferably includes a demagnetizing assembly formed by an AC generator 15 and a coil 16 wound on a C-core. Having passed through the vicinity of the receiving coil 12, the strip 1 runs over the demagnetizing coil 16, which is constantly supplied with power from the AC generator 15, and the strip segments 1A are thus demagnetized. This may be necessary for providing the deactivator strip material in its demagnetized state before spooling.

The following is a specific, but non-limiting example, of the method of the present invention.

The wheel (2 in Fig. 1) is made from aluminum and has a 260mm diameter. Ninety slots 3 with a 2mm width and a 6mm depth are cut uniformly on the wheel surface, so that the distance between each two adjacent slots (and consequently, a length of the semi-hard strip segments) is 8mm. The heating apparatus was the LIGA 12 apparatus commercially available from Liga Ltd, Saint Petersburg, Russia. The electrical power was set to 0.5kW, and the flame had a 1.5mm diameter and about 8mm length of its hottest conic part. Strips of the 301 -type stainless steel with 25 and 54 microns thickness and with 0.63 to 1.5mm width were used in test runs.

In their initial state, the strips were semi-hard magnetic, with a coercive force of 2800A/m. In the beginning of the treatment process, the torch position was adjusted so that a stable picture of AC pulses was seen on the oscilloscope. The strip movement speed was varied from 10 to 30 m/min. Fig. 3 shows the AC wavefoπn of the treated strip. In the normal process conditions, the amplitudes and the periods of the observed AC pulses are uniform and stable. Material defects and insufficient power of heating result in decreased amplitudes or absence of pulses. The ready deactivator strip was tested by introducing it in magnetic markers together with appropriate soft magnetic elements (made of amorphous alloy strips or glass-coated microwires). The tested markers showed the deactivation performances similar to those of commercially available markers with pieces of semi-hard magnetic material.

Those skilled in the art will readily appreciate that modifications and changes can be applied to the embodiment of the invention as hereinbefore exemplified without departing from its scope defined in and by the appended claims.

Claims

CLAIMS:

1. A method of segmenting a continuous element of a metal-containing material of a kind capable of changing its magnetic properties under supply of heating energy to form in said element first and second alternating segments of different magnetic properties, the method comprising applying a thermal treatment process to said element during a relative displacement of said element with respect to a heating zone defined by a heating source operating with a constant heating power, while providing different heating conditions for alternating segments of said element thereby creating the first magnetic segments spaced by the second non- magnetic segments in said element, and sequentially providing fast cooling of the second segments thereby stabilizing the non-magnetic state of the second segments.

2. The method of Claim 1, wherein the metal-containing material to be segmented is in its semi-hard magnetic state, the segmentation resulting in the formation of the second non-magnetic segments spaced by the first semi-hard magnetic segments of said material.

3. The method of Claim 1 or 2, wherein the different heating conditions and said fast cooling are achieved by passing said element through and out of the heating zone while said element is supported on a cooled patterned surface in the form of an array of spaced-apart surface regions, thereby resulting in that the first segments contacting the surface regions and the second segments aligned with the spaces between these regions are differently heated when in the heating zone and differently cooled when going out of the heating zone due to non-uniform heat exchange between the heated element and the support thereof .

4. The method of Claim 3, wherein the second segments are heated to a higher temperature than the first segments, when passing through the heating zone, and are cooled faster than the first segments when going out of the heating zone, while on the cooled surface.

5. The method of Claim 4, wherein the second segments, while passing through the heating zone, are heated to a temperature higher than a phase conversion temperature of said material.

6. The method of any one of Claims 3 to 5, wherein lengths of the first and second segments are defined by dimensions of the pattern in the patterned surface.

7. The method of any one of Claims 3 to 6, wherein the cooled surface is an outer surface of a rotating wheel having the pattern in the form of an array of spaced-apart grooves.

8. The method of any one of preceding Claims, comprising monitoring the magnetic properties of the segmented element.

9. The method of Claim 8, wherein the monitoring comprises passing the segmented element through a DC magnetic field region, to thereby magnetize the first segments of the element, and then passing this element through a vicinity of a receiving coil, thus causing an AC voltage induced in the coil and enabling detection of a profile of this voltage, and consequently shape and amplitude of a waveform of aperiodic magnetic structure of said element.

10. The method of any one of preceding Claims used for manufacturing the deactivating element for use in a magnetic marker.

11. An element of a metal-containing material having segments of different magnetic properties, said element being manufactured by any one of Claims 1 to 8.

12. A system for manufacturing a segmented continuous element of a metal- containing material of a kind capable of changing its magnetic properties under supply of heating energy, the system comprising:

(a) a patterned surface formed by an array of spaced-apart cooled surface regions spaced from each other a predetermined distance defining a length of segments to be formed in the element;

(b) a heating source accommodated so as to define a heating zone in the vicinity of the cooled surface regions and operable with substantially constant heating power to apply thermal treatment to a material located within the heating zone; (c) a driving assembly operable to provide a relative displacement between the element to be segmented and the heating zone, while said element lies on said patterned surface; the system being thereby operable to apply the thermal treatment process to said element with different heating conditions for alternating segments of said element at a constant power of the thermal process, and thus create in said elements segments of different stable magnetic properties.

13. The system of Claim 12, wherein said patterned cooled surface is an outer surface of a wheel having the pattern formed by an array of spaced-apart grooves.

14. The system of Claim 13, wherein the driving assembly has a first drive mechanism operable to rotate the wheel with a predetermined speed.

15. The system of Claim 14, wherein the driving assembly has a second drive mechanism for moving said element with respect to the wheel.

16. The system of Claim 15, wherein the wheel is rotated with a linear speed substantially equal to that of the movement of said element.

17. The system of any one of Claims 12 to 16, wherein the heating source is a gas torch operable to supply a narrow intensive flame to the heating zone.

IS. The system of Claim 17, wherein the torch operates with propane and oxygen.

19. The system of Claim 17, wherein the torch operates with hydrogen and oxygen.

20. The system of Claim 17, wherein the flame has a temperature of up to about 2500°C.