Nanocrystalline core antenna for EAS and RFID applications

FIELD OF THE INVENTION

The present invention relates to core antennas, and, in particular, to core antennas for electronic article surveillance (EAS) and radio frequency identification (RFID) systems.

BACKGROUND

EAS and RFID systems are typically utilized to protect and/or track assets. In an EAS system, an interrogation zone may be established at the perimeter, e.g. at an exit area, of a protected area such as a retail store. The interrogation zone is established by an antenna or antennas positioned adjacent to the interrogation zone.

EAS markers are attached to each asset to be protected. When an article is properly purchased or otherwise authorized for removal from the protected area, the EAS marker is either removed or deactivated. If the marker is not removed or deactivated and moved into the interrogation zone, the electromagnetic field established by the antenna(s) causes a response from the EAS marker. An antenna acting as a receiver detects the EAS marker's response indicating an active marker is in the interrogation zone. An associated controller provides an indication of this condition, e.g., an audio alarm, such that appropriate action can be taken to prevent unauthorized removal of the item to which the marker is affixed from the protected area.

An RFID system utilizes an RFID marker to track articles for various purposes such as inventory. The RFID marker stores data associated with the article. An RFID reader may scan for RFID markers by transmitting an interrogation signal at a known frequency. RFID markers may respond to the interrogation signal with a response signal containing, for example, data associated with the article or an RFID marker ID. The RFID reader detects the response signal and decodes the data or the RFID tag ID: The RFID reader may be a handheld reader, or a fixed reader by which items carrying an RFID marker pass. A fixed reader may be configured as an antenna located in a pedestal similar to an EAS system.

Historically, transmitting, receiving, or transceiver antennas in EAS and RFID systems have been configured as loop-type antennas. Recently, however, magnetic core antenna configurations have been explored for use in such systems. Materials utilized as the core material in core antennas have included ferrite and amorphous magnetic material.

Ferrite material may be provided as a powder, which is blended and compressed into a particular shape and then sintered in a very high temperature oven. The compound becomes a fully crystalline structure after sintering. Ferrite materials have a higher magnetic permeability than air, and have a relatively low saturation flux density compared, for example, to most amorphous materials. Also, ferrite materials that operate at higher RF (e.g. 15 MHz) frequencies have relatively low permeability and/or saturation flux density.

In contrast to ferrite materials, amorphous magnetic materials lack a distinct crystalline structure. Amorphous magnetic materials e.g., VC6025F available from Vacuumschmelze GmBH Co. (D- 6450 Hanua, Germany), have been successfully utilized in lower frequency EAS applications, e.g., 58 kHz. However, such amorphous magnetic materials do not perform well in the RF frequency range as core loss and permeability decrease performance for frequencies higher than a few 100 kHz.

Accordingly, there is a need for a core antenna for EAS and RFID applications capable of suitable operation frequencies up to the RF range. In addition, there is a need for improved performance of a core antenna in the lower frequency range for EAS as an alternative to ferrite or amorphous materials.

SUMMARY OF THE INVENTION

An antenna consistent with the invention for use in an EAS or RFID includes: a core including a nanocrystalline magnetic material, and a coil winding disposed around at least a portion of the core. The antenna may be implemented in an EAS or RFID system for generating an electromagnetic field to interrogate a marker by providing a controller configured to provide an excitation signal to excite the antenna for operation at a given frequency.

A method of establishing extended detection range in an EAS or RFID system consistent with the invention includes: providing a nanocrystalline core antenna including a core and at least one coil winding disposed around at least a portion of the core, the core including nanocrystalline magnetic material; and exciting the antenna for operation up to and including RF frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, together with other objects, features and advantages, reference should be made to the following detailed description which should be read in conjunction with the following figures wherein like numerals represent like parts: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is block diagram of an exemplary EAS system consistent with the invention;</li><li>FIG. 2 is a block diagram of one embodiment of a nanocrystalline magnetic core antenna consistent with the invention;</li><li>FIG. 3 is one exemplary circuit diagram of a controller for use with the system of FIG. 1;</li><li>FIG. 4 is a perspective view of an exemplary nanocrystalline core antenna consistent with the invention;</li><li>FIG. 5 is a partial cross-sectional view of the nanocrystalline core of FIG. 4 showing the insulated laminations and taken along the line 5-5 of FIG. 4.</li><li>FIG. 6A is a perspective view of another exemplary nanocrystalline core antenna consistent with the invention illustrating a resonant primary coil winding and a non-resonant secondary coil winding for transmitter, receiver, or transceiver mode operation.</li><li>FIG. 6B is an perspective view of a portion of the antenna of FIG. 6A showing the primary and secondary windings in greater detail</li><li>FIG. 7 is a plot of magnetic flux density versus magnetic field intensity for an exemplary nanocrystalline core antenna consistent with the invention.</li><li>FIG. 8 is a plot of relative permeability versus magnetic field intensity for an exemplary nanocrystalline core antenna consistent with the invention.</li><li>FIGS. 9A-9C are detection performance plots illustrating detection range for an EAS tag in lateral, horizontal, and vertical orientations, respectively, in an exemplary system consistent with the invention.</li></ul>

DETAILED DESCRIPTION

For simplicity and ease of explanation, the present invention will be described herein in connection with various exemplary embodiments thereof associated with EAS systems. A core antenna consistent with the present invention may, however, be used in connection with an RFID system. It is to be understood, therefore, that the embodiments described herein are presented by way of illustration, not of limitation.

Turning to FIG. 1, there is illustrated an EAS system 100 including a nanocrystalline core antenna 109 consistent with the invention. The EAS system 100 generally includes a controller 110 and a pedestal 106 for housing the core antenna 109. The controller 110 is shown separate from the pedestal 106 for clarity but may be included in the pedestal housing. In the exemplary embodiment of FIG. 1, the antenna 109 is configured as a transceiver and the associated controller 110 includes proper control and switching to switch from transmitting to receiving functions at predetermined time intervals. Those skilled in the art will recognize that there may be a separate transmitting antenna and receiving antenna located on separate sides of the interrogation zone 104.

An EAS marker 102 is placed, e.g. at a manufacturing facility or retail establishment, on each item or asset to be protected. If the marker is not removed or deactivated prior to entering an interrogation zone 104, the electromagnetic field established by the antenna will cause a response from the EAS marker 102. The core antenna 109 acting as a receiver will receive this response, and the controller 110 will detect the EAS marker response indicating that the marker is in the interrogation zone 104.

Turning to FIG. 2, a block diagram 200 of one embodiment of a nanocrystalline magnetic core antenna consistent with the invention configured as a transceiver antenna is illustrated. In the illustrated embodiment 200, a winding is placed around the nanocrystalline magnetic core and coupled to a series resonating capacitor C2. The core antenna with this winding is represented by the inductor L2, the resistor R2, and the series resonating capacitor C2 in the series RLC circuit 218. As is known to those skilled in the art, the value of the series resonating capacitor C2 is selected to resonate or tune the antenna circuit at the desired operating frequency. Another winding, represented by L1, may be placed around the core antenna and then coupled to the transmission line or cable (depending on the operating frequency) 212, which is in turn coupled to a controller 210 having appropriate excitation and detection circuitry to support both transmit and receive functions. The winding L1 is inductively coupled to the series resonating RLC circuit 218.

The controller 210 may be adapted to operate using pulsed or continuous waveform detection schemes, including swept frequency, frequency hopping, frequency shift keying, amplitude modulation, frequency modulation, and the like depending on the specific design of the system. For instance, the controller 210 may provide a limited duration pulse at a given operating frequency, e.g., 8.2 MHz, to the transmission line cable 212 during transmission. The pulse is transmitted via the transmission line cable 212 to the core antenna load. The transmission line cable may have an impedance, e.g., 50 ohms, that matches the signal generator impedance to prevent reflections. At lower frequencies, e.g. 58 kHz, the transmission line or cable is not important in impedance matching. In addition, the impedance transformer L1 may match the resonant core load impedance of the series RLC circuit 218 to the transmission cable 212.

FIG. 3 is a more detailed block diagram of an exemplary controller 310 configured for operation using a pulse detection scheme. The controller 310 may include a transmitter drive circuit 318, which includes signal generator 311 and transmitter amplifier 312. The signal generator 311 supplies an input signal to the transmitter 312 at a desired frequency such as RF frequency levels. The term "RF" as used herein refers to a range of frequencies between 9KHz and 300MHz.

The transmitter 312 drives the nanocrystalline magnetic core antenna represented by inductor LA, resistor RC, and resonating capacitor CR. The transmitter drive circuit 318 thus provides a burst to the core antenna at a given frequency for a short period of time to produce a sufficient electromagnetic field at a sufficient distance from the core antenna in an associated interrogation zone. A marker in the interrogation zone excited by this electromagnetic field produces a sufficient response signal for detection when the core antenna is connected to the receiver circuit portion of the controller 310.

After a short delay following the transmission burst, the nanocrystalline magnetic core antenna is coupled to the receiver circuit 322 when the switch controller 324 instructs the switch S1 to open. The switch controller 324 effectively switches the core antenna into and out of transmit and receive modes. During the transmitter pulse, the receiver circuit 322 is isolated from the antenna load at node 330 through the decoupling network formed by capacitor CDEC and resistor RDEC and the input protection circuit 334. After the transmission pulse, there is sufficient delay to allow the energy from the transmitter circuit 318 to fully dissipate. The switch controller 324 then disconnects the transmitter amplifier 312 from the antenna by opening switch S1. The alternating transmit and receive modes continue in such a pulse mode.

A perspective view of a nanocrystalline magnetic core antenna 400 consistent with the invention is illustrated in FIG. 4. The core antenna 400 may be utilized as the transceiver antenna of the system of FIGS. 1 and 2, a transmitter antenna, or a receiver antenna. The nanocrystalline magnetic core antenna 400 includes a core assembly 404 with a coil winding 406 thereon. The coil winding 406 may be coupled to a transmission line and controller as previously detailed. Those skilled in the art will recognize that the dimension of a core antenna consistent with the invention may vary depending on application and performance requirements. In exemplary embodiments, the core may have a length in a range from 20 to 80 cm, and may have a cross-sectional area from 0.02 to 1 cm².

FIG. 5 is a partial cross sectional view of the core assembly 404 taken along the line 5-5 of FIG. 4. In the illustrated exemplary embodiment, the core assembly 404 generally includes stacked ribbons 508 of nanocrystalline material laminated together with a suitable insulation coating 510. The insulation coating 510 electrically isolates each ribbon 508 from adjacent ribbons to reduce eddy current losses.

As will be recognized by those skilled in the art, nanocrystalline material begins in an amorphous state achieved through rapid solidification techniques. After casting, while the material is still very ductile, a suitable coating such as SiO₂ may be applied to the material. This coating remains effective after annealing and prevents eddy currents in the laminate core. The material may be cut to a desired shape and bulk annealed to form the nanocrystalline state. The resulting nanocrystalline material exhibits excellent high frequency behavior, and is characterized by constituent grain sizes in the nanometer range. The term "nanocrystalline material" as used herein refers to material including grains having a maximum dimension less than or equal to 40nm. Some materials have a maximum dimension in a range from about 10nm to 40nm.

Exemplary nanocrystalline materials useful in a nanocrystalline core antenna consistent with the invention include alloys such as FeCuNbSiB, FeZrNbCu, and FeCoZrBCu. These alloys are commercially available under the names FINEMET, NANOPERM, and HITPERM, respectively. The insulation material 510 may be any suitable material that can withstand the annealing conditions, since it is preferable to coat the material before annealing. Epoxy may be used for bonding the lamination stack after the material is annealed. This also provides mechanical rigidity to the core assembly, thus preventing mechanical deformation or fracture. Alternatively, the nanocrystalline stack may be placed in a rigid plastic housing.

FIGS. 6A and 6B are perspective views of another exemplary nanocrystalline magnetic core antenna 600 consistent with the invention. As shown, the core antenna 600 includes a nanocrystalline core assembly 602 with a primary resonant coil winding 604 and a secondary non-resonant coil winding 606. A capacitor 608, shown particularly in FIG. 6B, is coupled to the primary winding 604 for tuning the resonant frequency of the primary winding.

Providing multiple windings 604, 606 on a single core 602 allows use of the core to transmit at one frequency and receive at another frequency as long as sufficient frequency separation is provided. Using two windings operating at separate frequencies, such as 58 kHz and 13.56 MHz, also allows use of a single antenna as a transmitter and/or receiver at either frequency so that the antenna assembly can be plugged into a system operating at either frequency without special tuning. Additionally, multiple windings may be used such that the transmitter winding is tuned to 13.56 MHz and the receiver winding is tuned to 6.78 MHz (half-frequency) to facilitate operation using a frequency division scheme.

Turning to FIG. 7, there is provided a BH plot 700 for an exemplary nanocrystalline magnetic core antenna consistent with the invention constructed as shown in FIG. 4 using a FINEMET core. The exemplary nanocrystalline magnetic core antenna was 60 cm long by 0.5 cm wide, by 0.5 cm high and operated at 1 KHz. In general, the plot includes a linear region at fields below saturation (H fields between about +/- 170 A/m) and a flat region at saturation (H fields above and below about +/- 250 A/m). The slope of the linear region determines the permeability. In general, a higher permeability results in a more sensitive antenna when configured to act as a receiver antenna.

FIG. 8 is a plot 800 of relative permeability versus H-field in A/m at a frequency of 1 kHz for the same exemplary 60 cm x 0.5 cm x 0.5 cm nanocrystalline magnetic core antenna exhibiting the BH plot of FIG. 7. As indicated, the relative permeability is about 5000 or higher at H fields between 0 and about 100 A/m. The permeability decreases relatively linearly until saturation at about 250 A/m where it begins to drop off even further. Of course, as the antenna operating frequency increases, permeability decreases. Nonetheless, high permeability is maintained compared to conventional core antenna configurations. For example, the same exemplary 60 cm x 0.5 cm x 0.5 cm nanocrystalline magnetic core antenna exhibiting the BH plot of FIG. 7 and permeability characteristic of FIG. 8 and operated at frequencies from 8.2 to 13.56MHz exhibits a minimum relative permeability of 300. Due to the relatively high permeability and saturation level of nanocrystalline material, as indicated, for example, in FIGS. 7 and 8, a nanocrystalline core antenna used as a receiver antenna exhibits increased detection performance compared to conventional core antenna configurations.

FIGS. 9A-9C are detection performance plots 900, 902, 904 illustrating detection range for an EAS tag in lateral, horizontal, and vertical orientations, respectively, associated with an axially arranged pair of nanocrystalline magnetic core antennas consistent with the invention. The two nanocrystalline magnetic core antennas were 60 cm long x 0.5 cm wide x 0.5 cm thick and provided in a 58 kHz detection configuration. The dimensions of the plots in each of FIGS. 9A-9C correspond to the height and width dimensions of the tested area. The shaded area of each figure shows detection of an EAS tag. Non-shaded areas are areas in which an EAS tag is not detected. As shown, the exemplary antenna configuration exhibits a detection range between about 0 cm and 90 cm over a large height range from about 0 cm to 150 cm. In addition, the detection rate, also referred to as the pick rate, for the lateral orientation was 93.1%. The pick rate for the horizontal orientation was 79.3%, and the pick rate for the vertical orientation was 95.6%. The exhibited detection range and pick rates compare favorably with those of amorphous core antennas.

There is thus provided a nanocrystalline core antenna for use in EAS and RFID systems. The nanocrystalline antenna is constructed from nanocrystalline material and exhibits excellent performance characteristics at RF frequencies. The performance of the antenna results in improved detection range in EAS and RFID systems compared to conventional antenna configurations. The embodiments that have been described herein, however, are but some of the several which utilize this invention and are set forth here by way of illustration but not of limitation. It is obvious that many other embodiments, which will be readily apparent to those skilled in the art, may be made without departing materially from the spirit and scope of the invention as defined in the appended claims.