|1.||An apparatus for lengthwisely magnetizing randomlyoriented magnetizable strips, said apparatus comprising: at least one set of electrical coils, every said set of electrical coils having an axis associated therewith, every said axis extending in a different direction; means for passing direct electrical current through each said coil to generate a magnetic field of a strength sufficient to magnetize those of said deactivation elements which are generally aligned with said axis of said coil, each said coil being capable of producing a different magnetic field strength than any of said other coils; and means for successively subjecting said magnetizable strips to each said magnetic field produced by each said set of coils, wherein each succeeding said magnetic field is weaker than a preceding said magnetic field.|
|2.||An apparatus according to claim 1, wherein three said sets of electrical coils are provided.|
|3.||An apparatus according to claim 1, wherein two said sets of electrical coils are provided.|
|4.||An apparatus according to claim 1, wherein one said set of electrical coils is provided.|
|5.||An apparatus according to claim 1, further comprising: means for passing alternating electrical current through at least one coil, thereby generating and alternating magnetic field.|
|6.||An apparatus according to claim 1, further comprising a conveyor for carrying said magnetizable strips past said electrical coils.|
|7.||An apparatus according to claim 6, wherein three said sets of electrical coils are provided, each said set of electrical coils being dimensioned and disposed so that said magnetizable strips are successively conveyed past one said set of electrical coils at a time.|
|8.||An apparatus according to claim 1, wherein each said magnetic field is between approximately 100 Oe and 250 Oe.|
|9.||A method for deactivating a randomly oriented magnetizable strip, comprising the steps of: providing a set of electrical coils; positioning said magnetizable strip between said set of electrical coils; generating a magnetic field which surrounds said magnetizable strip, said magnetic field having a first direction relative to said magnetizable strip, by applying electrical current to said set of electrical coils; repositioning said magnetizable strip; applying a succeeding and weaker magnetic field which surrounds said magnetizable strip so that said succeeding magnetic field lies along a second direction relative to said magnetizable strip; and repeating said steps of repositioning and applying until said security strip has been deactivated.|
|10.||A method of deactivating a randomly oriented security strip, comprising the steps of: providing a first set of electrical coils, a second set of electrical coils, and a third set of electrical coils, said first, said second, and said third sets being approximately orthogonally disposed; positioning a magnetizable strip between said sets of coils; and generating successively a first, a second, and a third magnetic field by applying successively electrical current to said sets of coils, each said field being weaker than a preceding said field, wherein after generating said fields, said strip has been lengthwisely magnetized.|
|11.||A method according to claim 10, wherein each said magnetic fields is between approximately 100 Oe and 250 Oe.|
DEVICE AND METHOD FOR DEACTIVATING MAGNETIC SECURITY STRIPS
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to magnetic sensor strips, and more particularly, to devices and methods suitable for deactivating such strips, especially security strips, regardless of the orientation of those strips.
Description of the Related Art
In order to combat the ever-present threat of shoplifting, many retailers employ theft detection systems which can sense the passage of a security element attached to or embedded in the goods to be protected. The security elements found in the displayed merchandise are constructed such that if they are removed from the store, the theft detection system, which is preferably located at all of the store exits, senses their passage out of the store and triggers a suitable response. However, when the goods are purchased, the security elements are removed or deactivated, enabling the customer to leave the store without triggering the theft detection system. Thus,
if a shoplifter attempts to steal goods bearing an activated security element, an alarm can be triggered and the shoplifter can be apprehended.
The security strips are affixed to, or otherwise incorporated into, the goods to be secured. In some detection systems, such as those described in U.S. Patent Nos. 3,493,955, 4,187,509 and 4,299,040, security elements take the form of radio circuits mounted in tags. The tags are attached to the goods in a manner which requires the use of a removal tool to detach the tags. Without that tool, it is difficult, or even impossible, to remove the tags without damaging the goods. Thus, a would-be shoplifter will either set off an alarm, if the goods are removed with the tags attached, or will damage the goods, reducing their value, if the tags are removed. When a customer purchases the goods, the security tags are easily removed at the point of sale using the appropriate removal tool, without damaging the goods.
Another type of security system detects the passage of magnetic security strips. Such magnetic strips and detection systems are described in U.S. Patent Nos. 4,074,249, 4,384,281, 4,568,921, and 5,029,291, all assigned to the assignee of the present invention. Because the magnetic security strips can be made flat and fairly small, they are quite inconspicuous, and can be placed in goods in such a way that neither purchasers nor shoplifters are aware of them. Thus, these systems are less likely to be defeated, since a would-be shoplifter may not even realize a security system is being used.
Since the magnetic strips need not be removed to allow them to pass through the detector, the strips can be permanently attached to or incorporated in the goods to
be protected. Alternatively, the magnetic strips can be mounted on tags which are attached to the merchandise in the manner previously described.
A particular benefit of this type of system is that it is not necessary to remove the magnetic security strips when the goods are purchased, but only to alter the strips' magnetic states. Thus, the strips can be left in place in the goods. This is particularly advantageous when the protected items are being borrowed, rather than bought, since the magnetic security strips can be left on or in the items and reused. For example, lending libraries frequently make use of such strips to safeguard their collections.
As explained in the assignee's aforementioned U.S. patents, security systems which employ magnetic strips only detect those strips having a particular magnetic state. Typically, a magnetic security strip is detectable when it is unmagnetized state, and is not detectable when it has been magnetized along its length.
To activate a magnetic security strip so that it can be detected, the strip must be demagnetized so that it has little or no residual magnetism. Typically, this is done by applying a sufficiently strong alternating magnetic field to the strip, and gradually decreasing the strength of that field. As the magnetic field weakens, the magnetic security strip will be demagnetized; the magnetization of the strip follows a gradually decreasing "Steresis curve, in accordance with known principle; f demagnetization. The activated (demagnetized/ security strip is attached to the merchandise to be protected (this can be done either before or after demagnetization) .
The activated (demagnetized) strip can be detected in the following manner. The security strip is subjected to an alternating magnetic interrogation field, which causes the demagnetized strip to be driven into and out of magnetic saturation. As the strip passes into and out of saturation, it produces harmonic disturbances in response to the interrogating magnetic field. These disturbances are detected using a system which triggers the desired reaction, i.e., activating an alarm.
To deactivate the magnetic security strip, all that need be done is to magnetize the strip along its length. This can be done by applying a sufficiently strong magnetic field along the length of the strip. Now, when the strip is interrogated by the alternating magnetic interrogation field, the interrogation field is not sufficiently strong to drive the strip out of and back into saturation, and so the strip will not produce a detectable response to that applied field.
One way to deactivate (magnetize) a strip is to magnetize the strip along its length by individually determining the orientation of the strip and then applying a suitable magnetic field along the length of the strip, thus magnetizing the magnetizable elements on each security strip in the direction of the applied field. However, it is important that the direction of magnetization be along the length of the strips. Therefore, .in the past it has been necessary to individually orient the strips with the direction of the magnetizing field. However, when a large number of randomly oriented strips are to be deactivated by the process of reorienting and magnetizing each individual strip which is attached to a protected article, the procedure is laborious and slow.
One device for the bulk deactivation of magnetic strips is set forth in U.S. Patent No. 5,126,720 (the "'720 patent"). The '720 patent teaches a device that can deactivate bulk quantities of goods having randomly oriented security strips. The example shown in this patent has several sets of permanent magnets, each set being held in a frame in a specific manner. The magnets are arranged so that as an item bearing magnetic security strips passes through the frame it encounters several orthogonal magnetic fields, each succeeding field being weaker than the preceding field. The patent explains that each field is able to demagnetize security strips which lie approximately parallel to that field. By providing three fields arranged at right angles, it is possible to demagnetize all security strips, regardless of their orientations.
The '720 patent notes that it is possible to mathematically formulate the positions of the permanent magnets used to deactivate randomly-oriented magnetic security strips, but then goes on and states that it is more convenient to obtain the requisite field pattern by just shifting the magnets.
To generate the deactivation field, the '720 uses permanent magnets. As shown in Figs. 11-14 of that patent, permanent magnets produce magnetic fields which "bulge". Accordingly, the strength of those magnetic fields will vary depending upon where in the field an object is.
In addition to being non-homogenous, the magnetic fields produced by permanent magnets have limited strength. Even with improvements in permanent magnet technology, it is difficult to provide permanent magnets having large, intense, homogenous magnetic fields.
SUMMARY OF THE INVENTION
In order to facilitate the use of magnetic security strips, it is desirable to be able to deactivate these strips either before or after they have been attached to the objects to be protected. It is also desirable to be able to deactivate a large quantity of randomly- oriented strips at a given time.
The present invention allows the rapid, sure deactivation of magnetic security strips, no matter what their orientation is with regard to one another and the deactivation apparatus. A further benefit of this invention is that it employs a structure which facilitates conveyance of the objects to be processed. Moreover, this invention is capable of deactivating a great number of strips at one time.
In one embodiment of the present invention, an apparatus is provided for lengthwisely magnetizing randomly-oriented magnetizable strips, and this apparatus includes at least one set of electrical coils, every set of such electrical coils having an axis associated therewith, each axis extending in a different direction, and means for passing direct electrical current through each coil to generate a magnetic field of a strength sufficient to magnetize those deactivation elements which are generally aligned with the axis of the coil, each coil being capable of producing a different magnetic field strength than any of the other coils. Means are also provided for successively subjecting the magnetizable strips to each magnetic field produced by each set of coils, wherein each succeeding magnetic field is weaker than a preceding magnetic field.
Another aspect of this invention is a method for deactivating a randomly-oriented magnetizable strip,
and this method involves the steps of υroviding a set of electrical coils, positioning a ma letizable strip between thf. set of electrical coils, and generating a magnetic field which surrounds the magnetizable strip by applying electrical current to said set of electrical coils (the magnetic field has a first direction relative to the magnetizable strip) . This method also involves repositioning the magnetizable strip and applying a succeeding and weaker magnetic field which surrounds the magnetizable strip so that this succeeding magnetic field lies along a second direction relative to the magnetizable strip, and repeating the steps of repositioning and applying until the security strip has been deactivated.
A further aspect of this invention is a method of deactivating a randomly-oriented security strip by the steps of providing a first, second and third set of electrical coils, these sets being approximately orthogonally disposed, and positioning a magnetizable strip between the sets of coils. The method also involves generating successively first, second, and third magnetic fields by applying successively electrical current to the sets of coils, each field being weaker than a preceding field, so that after generating the fields, the strip has been lengthwisely magnetized.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an embodiment of this invention using three sets of Helmholtz coils;
FIGS. 2A, 2B and 2C are side cross-sectional views of the embodiment shown in FIG. 1 as taken along 2-2' depicting the successive application of magnetic fields to deactivate randomly-oriented security strips;
FIG. 3 is a perspective view of another embodiment of this invention using two sets of Helmholtz coils;
FIG. 4 is a perspective view of a further embodiment of the invention using one set of Helmholtz coils;
FIG. 5 is a side cross-sectional view of an embodiment of the invention in which a conveyor belt is provided to carry large quantities of goods through the frame past Helmholtz coils;
FIG. 6 is a perspective view of an embodiment of the invention in which goods can be deactivated without interruption or pause as they are carried along a conveyor; and
FIG. 7 is a cross-sectional view along 7-7' of the embodiment of the invention in FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
To activate a magnetic security strip it is necessary to eliminate substantially any overall magnetization of the strip. This can be done by surrounding the strip in an alternating magnetic field, and gradually decreasing the strength of that field. The magnetization of the strip will describe a hysteresis loop as it follows the alternating magnetic field. As the strength of that applied demagnetizing field decreases, the size of the hysteresis loop representing the magnetism of the security strip will decrease, until eventually the strip has no (or little) residual magnetism. The field strength can be reduced, or, alternatively, the object being demagnetized can be moved further from the field generator. In accordance with known principles, the increased separation will reduce the field strength.
The initial applied demagnetizing field must be strong enough to erase any preexisting magnetization which the strip might have. Thus, the field strength which must be provided in a strip activator will depend, among other things, on the coercivity and other properties of the strip being activated.
The activated strips are detected by providing an interrogating magnetic field. This field regularly fluctuates, and these fluctuations affect the activated (demagnetized) security strips. The demagnetized strips become magnetized by the fluctuating interrogation field, and because of the hysteresis of those security strips, harmonic disturbances caused in response to the interrogating field can be detected and used to trigger a suitable alarm.
In order to deactivate a magnetic security strip, it is necessary to magnetize the strip along its length. As previously explained, this requires the application of a sufficiently strong magnetic field parallel to the strip. When processing bulk quantities of goods having randomly-oriented strips, it is impractical to apply magnetic fields precisely along each strip, since the many strip orientations would require the application of the same number of fields. Furthermore, it is possible that subsequently-applied fields might "undo" the effects of preceding fields.
Applicants' invention makes feasible the deactivation of bulk quantities of randomly-oriented magnetic security strips through the application of only a minimal number of magnetic fields. This is possible because each magnetic security strip can be magnetized by a magnetic field which is generally aligned along but which is nevertheless not parallel to the length of the strip. It has been determined that each magnetic
strip can be magnetized along its length by applying a sufficiently strong magnetic field at an angle which is within 45° of the longitudinal axis of the strip.
Accordingly, it is possible deactivate a magnetic security strip, the orientation of which is unknown, by applying successively three magnetic fields to the strip. These fields should be orthogonal to one another so that each strip is exposed to at least one field which comes within 45° of the longitudinal axis of the strip. In order to prevent a strip which has been properly magnetized from being affected by a subsequent field, the intensity of the succeeding fields should be such that every field is weaker than the immediately preceding field, as taught in U.S. Patent No. 5,126,720.
The weakest magnetic field must still be strong enough to magnetize a suitably oriented security strip. The minimum field strength will depend upon the particular properties of the magnetic strips being used.
The present invention is intended to be used with "slug" type strips in which several magnetic bodies are disposed on the surface of a single substrate, as depicted in Fig. 11 of U.S. Patent No. 5,126,720. However, the present invention is not limited to such strips, and will also cover any other types of strips which can be deactivated by this invention.
Several embodiments of the present invention will now be proposed, each embodiment being capable of deactivating randomly oriented magnetic security strips.
One embodiment of the present invention employs three sets of Helmholtz coils to generate the requisite
magnetic fields. Helrr^oltz coils are formed by providing two prefers ' / circular conductive coils, the coils being oriented so that they lie in parallel planes, and being separated by a distance which is not more than approximately the radius of the coils. A homogeneous (uniform) magnetic field can be created between the coils by applying electrical current thereto. In accordance with well-known electromagnetic principles, the magnetic field will be perpendicular to the planes of the coils, and the field strength will depend upon the electrical current flowing through the coils and the number of loops of conductor in each circular coil.
Another reason why Helmholtz coils, a species of electromagnets, are preferred, is because as electromagnets, their field strength is related to the applied current. Thus, field strength can be increased merely by increasing the energizing current. Electromagnets are capable of producing much larger and stronger magnetic fields than permanent magnets, both important considerations when trying to deactivate a large quantity of goods.
Valuable benefits can be obtained by using Helmholtz coils to process magnetic security strips. For ex—*.ple, magnetic fields produced by Helmholtz coils ar ~ highly homogenous. This homogeneity means that the magnetic field strength and orientation for the field generated by a given Helmholtz coil will be nearly constant for all positions within the Helmholtz coil. Since the field strength anc -rection do not vary, it is easy to choose the proper _ze and strength Helmholtz coil. This is in contrast to permanent magnets, where the magnetic fields produced by each permanent magnet will interfere with one another,
making calculation of the overall magnetic field more complicated.
Another benefit of using Helmholtz coils is that the magnetic fields can be generated whenever it is desirable to do so, and can be turned off at other times. Thus, it is possible to apply one magnetic field at a time to the security strip. In contrast, permanent magnets, such as those shown in the '720 patent, cannot be turned off. An additional benefit is that since the Helmholtz coils can be turned off, the Helmholtz coil deactivator will be less likely to inadvertently affect magnetic strips or other magnetizable items.
Helmholtz coils are also particularly suitable because they have a fairly small length to diameter ratio. Using Helmholtz coils it is possible to construct compact, strong electromagnets which can nevertheless fully-enclose an object to be magnetized in a homogenous magnetic field. In contrast, solenoids, another species of electromagnets, have large length to diameter ratios, and so may require more space than a comparable Helmholtz coil.
Favorable results can be achieved using three orthogonal, successively-applied magnetic fields to deactivate the randomly oriented strips.
As previously noted, it is possible that a magnetic strip which has been magnetized along its length by one deactivating field can later be demagnetized by a different field. This effect can be avoided, however, by applying successively weaker fields and by suitably selecting the field strengths. Field strengths of from 100-250 Oe are thought to be preferable. By this it is meant that the weakest applied magnetic field is at
least 100 Oe, while the strongest magnetic field is not more than 250 Oe. The 100 Oe lower limit represents the minimum field strength required to magnetize the typical material from which the magnetic security strips are made; if more or less coercive materials are used, this lower limit can be adjusted accordingly. The upper limit on the strength of the applied field is suggested in order to avoid magnetizing the articles to be protected. In particular, it may be that the security strips are used to protect recorded magnetic medium. In that case, care must be take not to deactivate the strips using a field which is so strong that the recordings on the medium itself are affected. It has been determined that by limiting the strength of the magnetic field to less than approximately 280 Oe, the security strips can be deactivated without affecting any recordings on the magnetic recording medium. Again, if more or less coercive magnetic medium are to be protected, this limit on field strength can be adjusted accordingly. Alternatively, if the protected articles are not recorded magnetic media, stronger fields can be used.
A variety of embodiments of the present invention have been proposed, each of which is capable of deactivating randomly oriented magnetic strips. Because such devices use electromagnets, rather than permanent magnets, they are able to generate magnetic fields which are strong enough to deactivate magnetic strips throughout a large volume of space.
As shown in Figs. 1 and 2a-2c, c device for deactivating magnetic securi ε .ps consists of a rectangular frame 1 to which are attached three sets of Helmholtz coils 3a-3c' . Each set of Helmholtz coils
3a-3c' is mounted on opposite sides of the frame 1, and by virtue of the frame geometry, this means that each
set, 3a, 3a', 3b, 3b', 3c, 3c' will be able to generate a magnetic field which lies at 90° to the other generated fields.
For the sake of appearance and wear, one or more of the Helmholtz coils can be surrounded by a shroud or casing. However, it is necessary to be able to convey the materials to be processed into and out of the frame. Thus, unless the frame is to be partially disassembled each time materials are to be processed, access will have to be provided to the interior of the frame. This can be done by enclosing only the rim of one or more of the Helmholtz coils, or by making a shroud removable.
As shown in Fig. 5, it is possible to use a conveyor belt 5 running past two facing Helmholtz coils 3a, 3a' (not shown) . This allows objects to be processed rapidly, since the items can be fed on the conveyor belt 5, which stops when the items are properly positioned within the frame 1.
In this embodiment of the invention, it is envisioned that the items to be deactivated, whether individually or in bulk, on pallets or in boxes, are carried by the conveyor belt 5 into the rectangular frame 1. Once the items are properly positioned inside the frame 1, the conveyor belt 5 stops. The three sets of Helmholtz coils are successively activated, as discussed above. When these procedures are finished, the conveyor belt 5 carries the items out of the frame.
As shown in Fig. 7, by providing a rectangular frame 7, the longest axis of which is oriented in the direction of conveyance, and suitably positioning the different sets of Helmholtz coils 9, 9', 11, 11', 13, 13', the items can be demagnetized while the conveyor belt 5 is
moving. Since the conveyor belt 5 need not be stopped while the Helmholtz c s are activated, the rate at which items can be deactivated is increased. In this form of the invention, the moving items leave one magnetic field and enter another, until they finally emerge deactivated from the rectangular frame 7.
It is also possible to deactivate magnetic security strips using only two sets of Helmholtz coils. Such an embodiment of the invention is simpler and less expensive than the aforementioned versions; however, it also requires some manipulation of the goods or apparatus. This embodiment, as shown in Fig. 3, consists of a rectangular frame 1; however, only two sets of Helmholtz coils 3a, 3a' and 3b, 3b' are provided. As shown in Fig. 3, one of these sets is formed by two Helmholtz coils, one 3a lying on the right, the other 3a' on the left side of the frame 1. The other set is formed by Helmholtz coils lying above 3b and below 3b' the frame 1 (other coil configurations could also be used) .
Since this embodiment of the invention only contains two sets of Helmholtz coils, it can only provide two orthogonal magnetic fields, that is, a vertical field and a horizontal field, running across the width of the frame 1. The requisite third orthogonal field can be obtained simply by rotating the pallet, box, or object to be de-activated by 90° about its vertical axis, and then activating the set of Helmholtz coils 3a, 3a' which generates the horizontal magnetic field (these three fields should be successively weaker, as previously explained) . Now, the items have been exposed to three orthogonal fields, and have been deactivated. Alternatively the object could have been rotated by 90° about its horizontal axis, and then the top and bottom coils 3b, 3b' could have been activated.
It will be apparent that the two orthogonal sets of coils can be arranged in other ways. For example, the coils could have been located only on the front, back and sides of (or, alternatively, above and below) the frame. Again, to provide the third field, the goods would be rotated by 90° so that the top and bottom of the goods become the sides, or otherwise.
A particularly simple embodiment of this invention uses only a single set of Helmholtz coils. Although Fig. 4 depicts these coils alongside the frame 1, it will be appreciated that they could just as easily have been mounted above and below the frame. Regardless of how the coils 3a, 3a' are oriented, after the first magnetization, the goods are rotated by 90° relative to the direction of the magnetic field, and are again subjected to the magnetic field. Then, they are rotated by 90° relative to the plane of the previous rotation into an orientation other than the previous orientation, and are subjected to another magnetic field. These fields are varied in the manner already, described. Since the goods have now been exposed to three orthogonal magnetic fields, these fields being successively weaker, all magnetic security strips will have been deactivated, regardless of their orientation.
Although the aforementioned embodiments of the invention can process pallets or boxes of goods, they can also process individual items. The processing rate also can be increased by simply collecting all the goods in one box and processing them simultaneously.
While Helmholtz coils are presently preferred, the principles of the present invention could be achieved using suitable dimensioned and disposed solenoids or other magnetic field generators. If solenoids are used, the solenoid coils could be supported using a
suitable non-magnetic material such as plastic or aluminum.
Having thus described the invention with particular reference to the preferred fonr-p thereof, it will be obvious to those skilled in th,. art to which the invention pertains, in view c. this specification and the accompanying drawings, that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined by the following claims.
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