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CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to and claims priority to U.S. Provisional Patent Application No. 61/203,060, entitled “OPTIMIZATION OF THE FIELD PROFILE ON A HIGH FIELD STRENGTH MAGNETIC DETACHER,” filed Dec. 17, 2008, the entire contents of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     n/a 
     FIELD OF THE INVENTION 
     The present invention relates generally to a detachment method and magnetic detacher for electronic article surveillance (“EAS”) tags and more specifically to a method and system for optimizing the field profile of a high strength magnetic detacher. 
     BACKGROUND OF THE INVENTION 
     Electronic Article Surveillance (“EAS”) systems are designed to prevent unauthorized removal of an item from a controlled area. A typical EAS system may include a monitoring system and one or more security tags. The monitoring system may create an interrogation zone at an access point for the controlled area. A security tag may be fastened to an item, such as an article of clothing. If the tagged item enters the interrogation zone, an alarm may be triggered indicating unauthorized removal of the tagged item from the controlled area. A security tag is deactivated before a tagged item can leave the controlled area without triggering the alarm. 
     As is known in the art, security tags (also referred to as labels) for EAS systems can be constructed in any number of configurations. The desired configuration of the tag or label is often dictated by the nature of the article to be protected. For example, an EAS label may be enclosed in a rigid housing which can be secured to the monitored item, such as hard tags containing EAS labels which are commonly attached to clothing in retail stores. Some EAS hard tags typically include a plastic tag body which houses an EAS sensor and a locking mechanism including a pin or tack which passes through the item and is clamped to the tag body to secure the item and tag together. Generally, theses tags require a detacher unit to remove the tack from the tag body and allow the item to be separated from the tag. In some applications, a detacher unit may include a magnet assembly which applies a magnetic field to the tag body for releasing the tack. 
       FIG. 1  illustrates a prior art EAS tag  10  having a rigid, e.g., plastic, tag body  12  with a hollow internal chamber  14 . The tag body  12  houses an EAS sensor  16  for triggering an alarm. The EAS tag  10  includes a tack  18  with an enlarged head  20 . As shown, the tack  18  is securely held within the tag body  12  by a magnetic clamping mechanism  22 . In order to remove the tack  18 , the magnetic clamping mechanism  22  must be disengaged using a magnetic detacher. The plastic tag body  12  includes a substantially circular protrusion  24  of sufficient size to completely encase the tack  18  and magnetic clamping mechanism  22 . 
       FIG. 2  illustrates one conventional magnetic detacher unit  26 . The magnetic detacher unit  26  includes a base unit  28  having an indented detaching zone  30  designed to receive protrusion  24  of EAS tag  10  or another magnetic securing device. A high field strength magnet assembly  32 , as shown in  FIGS. 3 and 4 , resides within the base unit  28  and is positioned proximate to the indented detaching zone  28  to present a magnetic field within the detaching zone  30  in order to disengage a magnetic clamping mechanism  22  from a tack  18  of the EAS tag  10 , thereby allowing removal of EAS tag  10  or other magnetic securing device from the previously secured item. 
     As is shown in  FIG. 4 , a magnet assembly  32  for a magnetic EAS tag detacher is shown. The magnet assembly  32  includes a cylindrical core magnet  34  and an oppositely magnetized ring magnet  36  stacked on top of the cylindrical core magnet  34  in order to maximize the axial magnetic field in proximity of a cavity  38  of the ring magnet  36 . In other words, the magnetization of the cylindrical core magnet  34 , indicated by field lines  39   a , is opposite the magnetization of the ring magnet  36 , indicated by field lines  39   b  in the body of the ring magnet  36 . However, as the magnetic field of the ring magnet  36  radiates from body of the ring, the orientation of the magnetic field is actually rotated 180° when the field passes through the cavity  38 . Therefore, within the cavity  38 , the effects of the magnetic fields produced by the ring magnet  36  and the core magnet  34  are additive, thereby increasing the resulting field strength inside the cavity  38 . As discussed below, using this arrangement, the maximum field strength is not provided at particular or optimal location. 
     The high field strength magnet assembly  32  includes a cylindrical core magnet  34  and an oppositely magnetized ring magnet  36  stacked on top of the cylindrical core magnet  34  in order to maximize the axial magnetic field in proximity of a cavity  38  of the ring magnet  36 . To permit the removal of the tack  18 , the protrusion  24  of the EAS tag  10  or other magnetic securing device is inserted into the cavity  38  to take advantage of the strong field inside the ring magnet  36 . The magnet assembly  32  provides a substantially vertical magnetic field in the cavity  38  sufficient to force the clamping mechanism  22  to disengage and allow removal of the tack  18  from the tag body  12 . 
     Many different types of magnetic clamping mechanisms  22  are used in a variety of EAS tags and other magnetic securing devices. For example, one such clamping mechanism  22  is shown in  FIGS. 5 and 6 . In this example, the clamping mechanism  22  consists of a spring  40  used in combination with a clutch  42 . The shaft  44  of the tack  18  is inserted into a hollow tube  46  which extends through the protrusion  24  of the plastic tag body  12 . The shaft  44  is inscribed with one or more notches  48   a ,  48   b ,  48   c  (referenced collectively as notch  48 ) which receive the clutch  42  in a locked configuration, thereby preventing the tack  18  from being removed from the plastic tag body  12 . When the EAS tag  10  is secured (See  FIG. 5 ), the spring  40  is in an engaged position supporting the clutch  42  and preventing the clutch  42  from moving in a downward direction and disengaging from the notch  48 . When the EAS tag  10  or other magnetic securing device is presented with the magnetic field of the magnetic detacher unit  26  (See  FIG. 6 ), the clutch  42 , is pulled down and away from the notch  48  and releases the tack  18 . 
     Other magnetic clamping mechanisms  22  may use different locking devices, but the principle operation of the magnetic detacher unit  26  remains the same as described above. To disengage a particular EAS tag  10  or other magnetic securing device, the high field strength magnet assembly  32  must present the needed magnetic field strength at the exact location of the implemented clutch  40 . Because the field strength of the magnet assembly  32  decreases quite rapidly as the distance away from the magnet assembly increases, much stronger magnets than needed are often used in constructing the magnetic detacher unit  26 . Stronger magnets introduce additional cost into manufacturing the magnetic detacher unit  26 . 
     Additionally, the security tags used in an EAS system are replaced over time due to theft, loss, or normal wear and tear. For example, a sales clerk may forget to remove the EAS tag  10  from a purchased item. The security tags designed to be used in conjunction with a specific EAS system having a particular magnetic detacher unit  26  may be replaced with cheaper, “knock-off” EAS tags often provided by sub-standard manufacturers. These “knock-off” tags may not meet the requirements of the EAS system, provide a risk of unauthorized removal and do not, necessarily, have the magnetic clamping mechanism  22  at the same position of the original manufacturer&#39;s EAS tag  10 . Often these “knock-off” tags may be easily detached using a single magnet, essentially rendering the protection offered by the EAS system practically worthless. 
     Therefore, what is needed is a system and method for optimizing the field profile of a high strength magnetic detacher in order to achieve maximum field strength at particular location. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides a method and system for optimizing the field profile of a high strength magnetic detacher in order to achieve maximum field strength at particular location. 
     In accordance with one aspect, the present invention provides a magnetic detacher in which a housing defines an inner volume in which is positioned a core magnet and a ring magnet. The core magnet has a body with a top surface and a bottom surface opposite the top surface. The core magnet produces a first magnetic field. The ring magnet defines a cavity having a first diameter. The ring magnet has a top surface, a bottom surface opposite the top surface. The ring magnet produces a second magnetic field and is axially aligned with the core magnet such that the first magnetic field opposes the second magnetic field along the bodies of the respective magnets and enhances the second magnetic field within the cavity. The top surface of the core magnet is separated from the bottom surface of the ring magnet by a predetermined distance to produce a resultant magnetic field having a first resultant field strength at a specific position that is greater than a second resultant field strength produced at the same position when the top surface of the core magnet abuts the bottom surface of the ring magnet. 
     In accordance with another aspect, the present invention provides a magnet assembly for use in a magnetic detacher in which the magnet assembly has a core magnet has a body with a top surface and a bottom surface opposite the top surface. The core magnet produces a first magnetic field. A ring magnet defines a cavity having a first diameter. The ring magnet has a body with a top surface and a bottom surface opposite the top surface. The ring magnet produces a second magnetic field and is axially aligned with the core magnet such that the first magnetic field opposes the second magnetic field along the bodies of the respective magnets and enhances the second magnetic field within the cavity. The top surface of the core magnet is separated from the bottom surface of the ring magnet by a predetermined distance to produce a resultant magnetic field having a first field strength at a specific position that is greater than a second field strength produced at the same position when the top surface of the core magnet abuts the bottom surface of the ring magnet. 
     In accordance with yet another aspect, the present invention provides a method for detaching a magnetic securing device from an item. The magnetic securing device is secured by a clutch mechanism engaging a magnetic locking mechanism. The magnetic securing device is received in a magnetic electronic article surveillance tag detacher in which the magnetic electronic article surveillance tag detacher includes a core magnet and a ring magnet. The core magnet has a body with a top surface and a bottom surface opposite the top surface. The core magnet produces a first magnetic field. The ring magnet defines a cavity having a first diameter. The ring magnet has a body with a top surface and a bottom surface opposite the top surface. The ring magnet produces a second magnetic field and is axially aligned with the core magnet such that the first magnetic field opposes the second magnetic field along the bodies of the respective magnets and enhances the second magnetic field within the cavity. The top surface of the core magnet is separated from the bottom surface of the ring magnet by a predetermined distance to produce a resultant magnetic field having a first resultant field strength at a specific position that is greater than a second resultant field strength produced at the same position when the top surface of the core magnet abuts the bottom surface of the ring magnet. The field strength at the specific position disengages the clutch mechanism to release the magnetic locking mechanism. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a side view of a prior art electronic article surveillance (“EAS”) tag with a magnetic locking mechanism; 
         FIG. 2  is a perspective view of a prior art magnetic EAS detacher unit; 
         FIG. 3  is a perspective view of a prior art magnet assembly for an EAS detacher unit; 
         FIG. 4  is a side view of a prior art magnet assembly for an EAS detacher unit illustrating magnetic field orientation of each magnetic component; 
         FIG. 5  is a cross-sectional view of a prior art magnetic locking mechanism of an EAS tag in a locked position; 
         FIG. 6  is a cross-sectional view of a prior art magnetic locking mechanism of an EAS tag in an open position; 
         FIG. 7  is a side view of a magnet assembly for an EAS detacher unit constructed in accordance with the principles of the present invention; 
         FIG. 8  is a side view of a magnet assembly for an EAS detacher unit having an optional shield and booster unit, constructed in accordance with the principles of the present invention; 
         FIG. 9  is a graph illustrating magnetic field strength versus distance for a core magnetic component; 
         FIG. 10  is a graph illustrating magnetic field strength versus distance for a ring magnetic component in accordance with the principles of the present invention; 
         FIG. 11  is a graph illustrating the resulting composite effects of the magnetic field strength versus distance for a magnetic assembly having the ring component abutting the core component; 
         FIG. 12  is a graph illustrating a shifted magnetic field strength versus distance curve for a ring magnetic component displaced by a 4 mm gap in accordance with the principles of the present invention; and 
         FIG. 13  is a graph illustrating the resulting composite effects of the magnetic field strength versus distance for a magnetic assembly having the ring component displaced by a 2 mm gap in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before describing in detail exemplary embodiments that are in accordance with the present invention, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to implementing a system and method for optimizing the field profile of a high strength magnetic detacher. Accordingly, the system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     One embodiment of the present invention advantageously provides a method and system for fine-tuning the magnetic field profile of a magnetic assembly in a magnetic detacher unit in order to use the magnetic detacher with a specific mechanical tag design. The use of a spacer element enhances the magnetic field produced within the zone of interest (detaching zone). Additionally, a booster element constructed from, for example, soft ferromagnetic material, aids in enhancing the magnetic field further out into the detaching zone. 
     In another embodiment, a magnetic shield element with a similar foot print as the ring magnet may also help condense the field into the cavity of the detacher unit. A shield element with a thickness of only a fraction of millimeter also effectively reduces the stray field to the outside environment. This shielding minimizes the possibility of destroying magnetic cards (such as credit card, gift card, etc.) or attracting other ferrous objects, such as tools, cook wares, etc. 
     Referring now to the drawing figures in which like reference designators refer to like elements, there is shown in  FIG. 7 , an exemplary magnetic assembly of a magnetic detacher unit provided in accordance with the principles of the present invention and designated generally as  50 . Although discussed below in relation to one embodiment for use with a magnetic EAS tag  10  having a magnetic clutch and pin, the principles of the present invention may be used with any magnetic securing device, including but not limited to, keepers, savers, EAS tags, pinless EAS tags, bottle EAS tags, etc. Magnet assembly  50  includes a cylindrical core magnet  52  which is separated from an oppositely magnetized ring magnet  54  by a spacer  56  which aids in projecting the resultant magnetic field further out into the detaching zone. The ring magnet  54  includes a central cavity  58  and is axially aligned with the core magnet  52  and the spacer  56 . Although shown as a cylindrical magnet, the geometric shape of the core magnet and the ring magnet are not essential to the spirit of the present invention. In other words, the core magnet and the ring magnet may be any shape, e.g., elliptical, rectangular, cuboidal, cylindrical, etc., as long as the ring magnet includes a central cavity portion which resides atop the core magnet. 
     The spacer  56  may be constructed preferably from non-ferrous materials, for example, plastic, cloth, etc. Alternatively, the ring magnet  54  and the core magnet  52  may be secured in the magnetic detacher unit such that they are separated from each other by an air gap. The spacer  56  may include a cavity (not shown) having a diameter equal to the diameter of the cavity  58  in the ring magnet  54  in order to accommodate insertion of the protrusion  24  on EAS tag  10  (see  FIGS. 1 ,  5  and  6 ) or other magnetic securing device. The resulting magnetic field strength of the magnet assembly is dependent upon the separation distance between the ring magnet  54  and the core magnet  52 , e.g., the height of the spacer. 
     In accordance with the present invention, for any specific magnetic EAS tag  10  or other magnetic securing device, a spring  40  ( FIGS. 5 and 6 ) may be designed in such a way that the clutch  42  is responsive to a minimum magnetic field strength at a specific height. This feature allows for the design of more robust EAS tags  10  which cannot be removed from a protected article except by using its corresponding magnetic detacher unit  50 . As a result, the ring magnet  54  is chosen such that its coercivity is strong enough to sustain its magnetization in the presence of the opposing magnetic field from the core magnet  52 . It is possible to have a design such that the diameter of the core magnet  52  equals to the inner diameter of the ring magnet  54 . In such a case, the high coercivity of the ring magnet  54  is not as critical. 
     Referring now to  FIG. 8 , an alternative embodiment of the present invention may further include a booster element  60  and/or a shield element  62 . The booster element  60  may be constructed of soft ferromagnetic material to further enhance the magnetic field strength of the core magnet  52  and aid in projecting magnetic field further out into the detaching zone. The shield element  62  may have a similar foot print as the ring magnet  54  and may also help condense the magnetic field into the cavity  58  of the magnet assembly  50 . A shield element  62  with a thickness of only a fraction of millimeter effectively reduces the stray magnetic field to the outside environment, thereby minimizing the possibility of destroying magnetic cards (such as credit card, gift card, etc.) or attracting other ferrous objects, such as tools, cook wares, etc., be constructed of, for example, steel or other soft ferromagnetic materials. 
     In  FIG. 9 , a graph is provided which illustrates the magnetic field strength of a core magnet  52  measured as a function of distance (in millimeters), with the reference point at the top surface of the core magnet  52 .  FIG. 10  is a graph illustrating the magnetic field strength along the center of a ring magnet  54 , also measured as a function of distance (in millimeters), with the reference point at the bottom surface of the ring magnet  54 . In the example shown, it should be noted that the magnetic field strength of the ring magnet  54  measured in  FIG. 10  peaks at a distance of approximately 4 mm.  FIG. 11  is a graph illustrating the resulting composite effects of the magnetic field strength versus distance for a typical magnetic assembly  50  which has the ring component  54  abutting the core component  52 , e.g., there is no spacer  56 , no air gap, etc. between the ring magnet  54  and the core magnet  52 . 
     As can be seen from  FIGS. 9-11 , if an EAS tag  10  or other magnetic securing device is designed based on a required magnetic field strength at a distance of less than 4 mm, then no spacing between the core magnet  52  and ring magnet  54  produces the highest magnetic field. However, if an EAS tag  10  or other magnetic securing device using these same magnets needs a magnetic field strength at more than 4 mm height, for example 10 mm, then shifting the magnetic field strength of the ring magnet  36  in relation to the core magnet  34  increases the resultant magnetic field strength inside the cavity  38 . Such may be the case where the clutch  42  ( FIGS. 5 and 6 ) is positioned at the 10 mm point. 
       FIG. 12  is a graph illustrating the magnetic field strength of a ring magnet  54 , offset from the original field strength profile by 4 mm. In other words, a 4 mm spacer  56  is inserted between the ring magnet  54  and the core magnet  52 .  FIG. 13  is a graph illustrating the resultant field strength produced by the offset ring magnet  54  combined with the core magnet  52 . As can be seen from  FIG. 13 , although the resulting magnetic field is reduced at 4 mm (the top surface of the spacer  54 ), the magnetic field strength at 10 mm is increased approximately seven hundred Oersted. 
     Another added benefit for providing a space between the core magnet  52  and the ring magnet  54  is the reduction of the magnetic instability due to the opposing field configuration. A 1 mm spacing reduces the surface magnetic field by about six hundred Oersted, e.g., from 5.5 kOe to about 4.9 kOe seen at the ring magnet  54  surface. 
     The present invention advantageously tunes the resultant magnetic field strength of magnetic assembly having a combination of a ring magnet and a cylindrical core magnet to provide an optimal magnetic field strength at a predetermined distance away from the surface, e.g., at substantially the location of clutch of the EAS tag or other magnetic securing devices. This feature allows a magnetic assembly of a magnetic detacher e.g., the clutch location, to be tuned to operate only with specifically designed EAS tags or other magnetic securing devices. 
     Additionally, because the magnetic field strength of the magnet assembly is increased in comparison to prior art magnets, a weaker core magnet may be used to achieve the same field strength previously requiring stronger magnets, thereby reducing the overall cost of the magnet assembly. 
     Unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. Significantly, this invention can be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be had to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.

Summary:
A magnetic detacher has a core magnet and a ring magnet. The core magnet has a body with a top and bottom surface, and produces a first magnetic field. The ring magnet defines a cavity. The ring magnet has a body with a top and bottom and produces a second magnetic field. The ring magnet is axially aligned with the core magnet such that the first magnetic field opposes the second magnetic field along the bodies and enhances it within the cavity. The top surface of the core magnet is separated from the bottom surface of the ring magnet by a predetermined distance thereby producing a resultant magnetic field having a first resultant field strength at a specific position greater than a second resultant field strength produced at the same position when the top surface of the core magnet abuts the bottom surface of the ring magnet.