Abstract:
A single turn loop electronic article surveillance (EAS) gate. The gate includes a closed magnetic core, a multi-turn primary winding wound around the core, and a secondary loop passing through the core once. The secondary loop is a self-supporting single turn loop, and the secondary loop is a transmit loop that generates a magnetic drive field for the article detection gate system.

Description:
TECHNICAL FIELD 
     The present invention relates to electronic article surveillance (EAS) systems, and more specifically, to EAS systems with a self-supporting single turn transmit loop. 
     BACKGROUND 
     EAS systems can be used to prevent unauthorized removal of merchandise from stores or retail settings, and prevent removal of books or other lending media from libraries without the item first being checked out. EAS systems typically include a detection system, often made of two side gates between which an individual must pass to exit a store or library. The gates generate an electromagnetic field that is used to excite a response from a security tag attached to the merchandise, book or media. A variety of tags are used in EAS systems and include magnetomechanical (also known as acousto-magnetic), radio frequency identification (RFID) and magnetic (also known as magneto-harmonic) tags. 
     EAS systems present a variety of challenges, including providing gates that generate sufficient magnetic drive fields to fully cover the area between two gates; designing or placing tags that are not easily shielded from a magnetic drive field generated by the EAS gates; and providing cost-effective EAS gates and tags that are proportional in value to the merchandise or media they are being used to track. 
     SUMMARY 
     The present invention provides an elegant solution for an EAS gate where the transmit loop is made of a single turn loop. This construction provides several advantages over existing EAS gates. For example, the single turn loop construction provides a light-weight solution, allowing the gate to be more easily supported. The single turn loop construction allows for manufacturing options that are lower-cost than current EAS gate constructions due to both reduced material and labor costs. A simpler construction can also result in reduced likelihood of gate failure. The present invention also allows for construction of an EAS gate without the need for a molded exterior, providing for a narrower gate with improved aesthetics. 
     In one embodiment, the present invention provides a single turn loop electronic article surveillance gate. The gate includes a closed magnetic core; a multi-turn primary winding wound around the core; and a secondary loop passing through the core once. The secondary loop is a self-supporting single turn loop, and the secondary loop is a transmit loop that generates a magnetic drive field for the gate. 
     In another embodiment, the present invention provides an electronic article surveillance system. The electronic article surveillance system includes a gate including a receive coil and a transmit loop. The transmit loop is a rigid loop forming a secondary loop of a transformer. The transformer further includes a closed magnetic core and a primary winding wound around the core. The receive coil is comprised of two symmetrical loops. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description in connection with the accompanying drawings, in which: 
         FIG. 1  shows a single loop turn EAS gate. 
         FIG. 2  shows an exemplary closed magnetic core. 
         FIG. 3  shows an illustration of the magnetic drive field generated by a transmit loop of an EAS gate. 
         FIG. 4  shows an EAS gate system including a transmit loop and a receive coil. 
         FIGS. 5A-5K  show exemplary cross section shapes for a transmit loop. 
         FIG. 6  is a chart illustrating resistance associated with various cross sections for a transmit loop. 
         FIG. 7  shows a circuit diagram illustrating a single loop turn EAS gate. 
     
    
    
     In the following description of illustrated embodiments, reference is made to the accompanying drawings, in which are shown by way of illustration various embodiments in which the invention may be practiced. It is understood that the embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number. 
     DETAILED DESCRIPTION 
     The following description provides more detailed information about the present invention, including illustrations of various embodiments of the present invention. Magnetic EAS systems are typically formed with pairs of gates. Each gate may include both a winding or loop for transmitting a signal to generate a magnetic field and a second winding or loop to receive or detect signals created by tags responding to the generated magnetic field. 
     The transmit loop of an EAS gate typically produces a magnetic field of magnitude greater than 50 microTesla (μT) over much of an area large enough to create a portal so that a human can easily walk between a pair of such EAS gates. Such a portal often has a cross section sized in the range of 1 square meter. The magnetic field is generated by driving current through a loop or coil of wire. The magnitude of the magnetic field is directly proportional to the product of the current times the number of coil turns (units of Amp-turns). To produce a 50 μT or greater magnetic field over the desired volume, something like 200 Amp-turns must be generated in the drive winding. Such a magnetic field can be created, for example, by driving 10 Amps through a coil of 20 turns. 
     Multi-turn transmit coils can have several costs associated with them. For example, such coils typically require some type of mechanical support, which increases manufacturing costs. Multi-turn transmit coils can also generate acoustic noise via the Lorentz force which tends to cause mutual attraction of adjacent turns of a coil when current is driven through the turns of the coil, often resulting in an audible vibration that can be objectionable. 
     Multi-turn transmit coils can require higher voltages than those required in the current invention. For example, some multi-turn transmit coils may have voltages as high as 100 V on their transmit coils. Such voltage levels may require that the multi-turn loop be electrically insulated to prevent human exposure to high voltages. Such insulation may be in the form of a large molded structure, further increasing manufacturing cost. 
     In contrast to a transmit coil made with several (or even many) turns, a single turn loop as described in the present disclosure can generate a sufficient magnetic field by driving about 200 Amp-turns in a single turn loop. Such a construction can provide the appropriate Amp-turns with lower voltages, such as voltages in the range of 1 V to 5 V. Such a loop can be self supporting and have several other advantages as discussed throughout the present disclosure. 
       FIG. 1  shows a diagram of a single turn loop EAS gate  10 . Gate  10  includes transmit loop  12 , magnetic core  14  and multi-turn winding  16 , which together function as a transformer to produce a magnetic field surrounding the gate. Gate  10  functions to drive sufficient Amp-turns (often in the range of about 200 Amp-turns) through transmit loop  12  to generate a magnetic field for detecting security tags attached to items that may be carried between a pair of gates. 
     The transformer can have a range of configurations. For example, multi-turn winding  16  may have a variety of turns for example, between 50 and 100 turns. The current driven through multi-turn winding may vary inversely with the number of turns in the multi-turn winding  16 . For example, in an embodiment with 84 turns of multi-turn winding  16 , the current level may be in the range of 2.25 to 2.75 amps. The Amp-turns driven by transmit loop  12  can be calculated by multiplying the current driven through multi-turn winding  16  by the number of turns in multi-turn winding  16 . For example, to drive 200 Amp-turns through transmit loop  12 , 2.38 amps could be driven through an 84-turn winding. Alternatively, 4 Amps could be driven through a 50-turn winding. 
     Physically, transmit loop  12 , closed magnetic core  14  and multi-turn winding  16  can be constructed from a variety of materials. In one embodiment, transmit loop  12  can be made, for example, in one embodiment, of round, solid copper with a 0.375 inch diameter or copper pipe with an outer diameter of 0.625 inches and a wall thickness of 0.62 inches. While a variety of sizes and configurations can be used for transmit loop  12 , a structure with a smaller diameter can often result in lower loss when transmitting a signal and can be more aesthetically pleasing. Transmit loop  12  may also be made from aluminum. Transmit loop  12  may also be made from a mixed composition comprising primarily aluminum or copper. Transmit loop  12  may have a variety of shapes, covering a range of area. In one embodiment, transmit loop  12  encloses an area in the range of 0.8 m 2  to 1.0 m 2 . In some embodiments, transmit loop  12  may be self-supporting such that it holds its vertical structure when mounted at the base. 
     Closed magnetic core  14 , as discussed in further detail with respect to  FIG. 2 , may be formed from a molded, wrapped or otherwise fashioned magnetic material, such as silicon iron, ferrite or any other appropriate materials as may be apparent to one of skill in the art. A closed core is one that is continuous or substantially continuous. An example of a closed form is a toroid. 
     Multi-turn winding  16  is wrapped around closed magnetic core  14  and may be made of any appropriate conductive material, such as stranded copper, aluminum or another type of wire. The wire could additionally be insulated. The number of turns for multi-turn winding  16  may vary based on the specific design. 
       FIG. 2  shows an exemplary closed magnetic core  24 . In this embodiment, magnetic core  24  is formed of a thin magnetic strap  22  wrapped or coiled about a center form  26 . In some embodiments, center form  26  may be temporary and used only for forming closed magnetic core  24 . Magnetic strap  22  may be made of a variety of materials, for example, silicon iron, ferrite or any other appropriate materials as may be apparent to one of skill in the art. 
     Magnetic core  24  may also be a variety of shapes. In one embodiment, magnetic core  24  may be cylindrical. Magnetic core  24  may also be toroidal, have a square circumference, or be any other appropriate shape. 
     The size of magnetic core  24  may vary based on a variety of factors. Saturation of the core can limit how small the magnetic core  24  may be and other factors such as cost and cosmetics can serve as upper size limiting factors for magnetic core  24 . In some embodiments, the cross section of magnetic core  24  may be in the range of 500 to 1,500 mm 2 . Additionally, the diameter of magnetic core  24  should be large enough to allow the turns of winding  16  and loop  12  ( FIG. 1 ) to pass through the center hole of magnetic core  24 . 
     After the multi-turn winding is wrapped about magnetic core  24 , the construction, including both magnetic core  24  and multi-turn winding  16  may be potted to prevent mechanical motion generated by current driven through the multi-turn winding  16 . Potting often includes filling the voids with a material that hardens. The hardening material is often non-conductive, can prevent moisture or motion and therefore acoustic noise, and can increase durability and improve insulation resistance. 
       FIG. 3  shows an illustration of the magnetic field flux  38  generated by a transmit loop  32  of an EAS gate  30 . Arrows  33  indicate the direction of current in transmit loop  32 . Resulting magnetic field flux  38  follows the Right Hand Rule with respect to transmit loop  32 . The magnitude of magnetic drive field is dependent on the number of Amp-turns being driven via transmit loop  32 . 
     The frequency of magnetic drive field can range significantly, but typically is a lower frequency. For example, the frequency of magnetic drive field may be about 400 Hz, about 500 Hz, or in the range of 200 Hz to 1,000 Hz. There are a number of factors to consider in determining the frequency of magnetic drive field. As the frequency of the magnetic drive field increases, the transmit loop  32  experiences greater loss due to increased transmit loop AC resistance. However, a higher frequency magnetic drive field increases the amplitude of the output of the receive coil for a security tag passing through the field. 
     One important challenge in designing an EAS gate is to ensure there are no “dead zones” between a pair of EAS gates  30  such that a tag passing through that zone would not be detected. One way to shift the coverage of the magnetic drive field to change the shape of transmit loop  32 . For example,  FIG. 4  shows an EAS gate with a varied transmit loop shape to provide improved magnetic field coverage. EAS gate  30  may be configured such that the transmit loop  32  is capable of generating a magnetic drive field extending at least 0.4 meters, 0.5 meters or more from EAS gate  30 . 
       FIG. 4  shows an EAS gate pair  40  including a transmit loop  42  and a receive coil  48 . In this configuration, individual EAS gates  41  are spaced about 1 meter from each other. The distance between EAS gates  41  can be determined by accessibility requirements of the respective jurisdiction. Each EAS gate  41  includes a potted magnetic core  44  with a multi-turn winding through which current is driven to transmit loops  42 . Transmit loop  42  generates a magnetic drive field, with each magnetic drive field extending approximately half-way between the EAS gate pair  40 . 
     Each EAS gate  41  further includes a receive coil  48 . Receive coil  48  may have a variety of configurations. In this configuration, receive coil  48  wound on area filler  49  and is co-planar with transmit loop. Receive coil  48  may be contained in the same housing, lattice or gate as transmit loop  42 , and may or may not be co-planar with transmit loop  42 . 
     In one configuration, receive coil  48  can be formed of two symmetrical loops  48 A and  48 B. The respective loops  48 A and  48 B are wound oppositely, i.e., one clockwise and the other counterclockwise. One benefit of using two symmetrical loops is that the two loops  48 A and  48 B cancel out the induced voltage from the drive circuit flux from the transmit loop  42  so that loops  48 A and  48 B can adequately detect disturbances in the field by tags. Receive coil  48  may enclose a relatively large area. For example, in some embodiments, receive coil  48  or receive coils  48 A and  48 B cumulatively may enclose 75% or more of the area enclosed by transmit loop  42 . 
     In the configuration illustrated in  FIG. 4 , transmit loops  42  each include four horizontal segments  42 A- 42 D. Horizontal segments  42 A- 42 D can prove advantageous as they provide variation in the direction of the magnetic drive field generated by transmit loops  42 , to allow for more complete field coverage of an area between the EAS gate pair  40 . 
     In some configurations, transmit loop  42  may be made of several pieces of material joined together or may be constructed of a single piece of material joined in one location. Because of the low voltage possible as described in the present disclosure, transmit loop  42  may form the outer structure for EAS gate  41 , and the surface of transmit loop  42  may be exposed so that an individual walking through or near EAS gate pair  40  may touch transmit loop  42 . 
     EAS gate pair  40  is configured to work collectively so that the cumulative magnetic drive field emitted by transmit loops  42  covers the majority of the area between EAS gate pair  40 . When a soft magnetic material, RF tag, or other tag or security item passes between EAS gate pair  40 , the item disturbs the magnetic field, and receive coils A and/or B may then detect the presence of such item or soft magnetic material and trigger an alarm. 
     While  FIG. 4  illustrates one embodiment for an EAS gate pair  40 , an EAS gate pair consistent with the present disclosure may take a variety of configurations, with changes in distance between EAS gates  41 , configuration of transmit loops  42  and associated receive coils  48 , difference in mounting, area filler  49  and other items. 
       FIGS. 5A-5K  show exemplary cross section shapes for a transmit loop. While transmit loop  42  in  FIG. 4  is illustrated as a round copper pipe, various shapes for the cross section of the transmit loop will impact the resistance of the transmit loop  42  acting as a conductor. The shape of a cross section of transmit loop  42  also affects the aesthetic appeal of an EAS gate incorporating transmit loop  42 . 
       FIGS. 5A-5K  show exemplary cross-sectional shapes for a transmit loop conductor including round cross-section  51 , rectangular cross-section  52 , square cross-section  53 , flat cross-section  54 , U-shaped cross-section  55 , V-shaped cross-section  56 , T-shaped cross-section  57 , D-shaped cross-section  58 , solid round cross-section  51 A, solid rectangular cross-section  52 A and solid square cross-section  53 A. In these various cross-sectional shapes, corners may be rounded consistent with the cross-sectional shapes. Additionally, some cross-sections are closed, such as round cross-section  51 , rectangular cross-section  52 , square cross-section  53 , and D-shaped cross-section  58 . Others are open, such as flat cross-section  54 , U-shaped cross-section  55 , V-shaped cross-section  56 , and T-shaped cross-section  57 . Cross sections may be solid or hollow. A transmit loop consistent with the present invention may have either a close or an open cross-section. 
     Example 1 
     Cross-Sectional Shape Variation 
     The AC resistance of a variety of cross-sections that could be used in copper transmit loops consistent with the present disclosure was modeled with respect to changing frequency. The results of such modeling are shown in  FIG. 6 . 
     The various types of cross-sections modeled and the corresponding result identifier are shown in Table 1 below. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Cross Sections Modeled in FIG. 6 
               
             
          
           
               
                   
                   
                 Dimensions 
               
               
                 Cross-Section 
                 Shape 
                 (in inches) 
               
               
                   
               
               
                 61 
                 Flat 
                 0.1250 × 1 
               
               
                 62 
                 Tubular 
                 0.072 wall thickness × 
               
               
                   
                   
                 0.625 outer diameter 
               
               
                 63 
                 Solid round 
                 0.375 diameter 
               
               
                 64 
                 Solid round 
                 0.4375 diameter 
               
               
                 65 
                 Square 
                 0.375 × 0.375 
               
               
                   
               
             
          
         
       
     
     The various types of cross-sections modeled and the identifying number are shown in Table 1 above. Providing modeling of AC resistance allows one to intelligently choose a cross section for a transmit loop based on the other design features. 
     For example, in a configuration where the transmit loop may be operated at a variety of frequencies, a cross-section with relatively flat AC resistance may be desirable for the purpose of consistent performance. For example, the resistance of cross-section  62  is relatively flat across the shown frequency range and may be a good option for such a design scenario. 
     In another scenario, for a gate designed to operate at a particular frequency, for example, 400 Hz, it may be desirable to choose a cross section for transmit loop that is the lowest at that particular frequency. Therefore, cross-section  64  would provide a good option for a gate designed to operate at 400 Hz. 
     Example 2 
     Exemplary Drive Circuit for Transmit Loop 
       FIG. 7  shows a circuit diagram for an exemplary gate  70 . Gate  70  includes transmit loop  72  and receive coils  74 A and  74 B. Receive coils  74 A and  74 B are similar receiver sub-coils wound in opposite directions and connected in series. The receive coils  74 A and  74 B are connected to a differential amplifier  75 . The amplified signal from receive coils  74 A and  74 B is passed to the remainder of the receive circuitry  76  where the received signal is further processed. 
     A signal source  77  generates a sinusoidal signal which is passed to a driver circuit  78 . In this example, the RMS value of the signal generated by signal source  77  is 0.59 V. The driver circuit  78  provides the current and voltage required to generate the desired magnetic field via the single turn loop. In this instance, the drive current and voltage generated by driver circuit  78  is 2.5 Amps and 22.1 V. The inductance presented by the primary of the toroidal transformer is resonated with capacitor  79 , with a value of 5.5 μF so that the driver circuit  78  drives a purely resistive load. The toroidal transformer  80  and capacitor  79  provide an impedance match between the driver circuit  78  and the single turn loop  72 . The current driven through the single loop turn 200 Amps and the voltage across it is 1.6 V. The magnetic field is generated by single turn loop&#39;s  72  200 Amp-turns.