Patent Application
20060038683
The present invention relates to an apparatus and method for providing an RFID tag on a metal closure for a container such as a metal bottle cap.
Mounting an RFID tag within a plastic cap for a container, e.g., a beverage bottle, has presented no difficulty since the plastic material does not significantly affect the transmission of the electromagnetic signal transmitted to the RFID tag.
However, the use of an RFID tag with a metal container closure or cap present certain design difficulties. As used herein, metal cap is understood to mean any metal closure for any type of container. Furthermore, references herein to bottles and metal caps for bottles is not to be understood as limiting the scope of the invention but merely illustrative of a particular application for the invention. At the high RF frequencies used for communication with an RFID tag, some transmitted signal energy will diffract and reflect into a metal cap from the open end of the metal cap so long as the fluid contents within the container remain below the bottom of the cap. However, a full container will likely prevent the RF signal from reaching an RFID tag mounted within a metal cap. Furthermore, since an RFID tag normally does not include an integral battery and is powered by the received RF energy, sufficient RF energy has to reach the RFID tag to power the integrated circuit chip on the RFID tag. It is unlikely that this would occur for an RFID tag mounted within a metal cap absent special circumstances, such as positioning the interrogator antenna at a very close range and at a specific orientation to the metal cap. Consequently, a conventional RFID tag mounted completely inside a metal cap does not appear to be practical.
Microstrip antenna technology originated in microwave transmission lines etched into radio frequency integrated circuits and into copper-clad printed circuit boards. A microstrip transmission line is a metal conductor path (usually etched copper) separated from an expansive conducting surface (ground plane) by an insulating dielectric layer. The width of the transmission line and the thickness of the dielectric medium determine the characteristic impedance of the transmission line, and thereby the efficiency of RF power transmission from one device to another. If the length of the microstrip transmission line is adjusted to be one-half the wavelength of RF waves in the dielectric layer, and if one or both ends of the transmission line are not connected to a device, then that transmission line radiates energy (or receives it) as an antenna. Consequently, the same technology and the same process steps can be used to produce an antenna and the necessary impedance matching components, resulting in lower manufacturing costs.
For these reasons, microstrip antennas are commonly used in connection with the interrogator of a RFID system. These antennas have the desirable characteristic of laying flat on a surface with minimum protrusion from that surface. However, they are not commonly used on RFID tags, primarily for the following three reasons: 1) The characteristic length of a simple microstrip antenna is one-half of the wavelength, whereas it is one-quarter of the wavelength for an electric dipole antenna. Consequently, for a given frequency of operation, the microstrip antenna must be twice the length the electric dipole antenna. 2) The simplest microstrip antennas have a narrower bandwidth than the electric dipole antenna, resulting in tighter manufacturing tolerances for the microstrip antenna. 3) Since the patch of the microstrip antenna is more massive than the wire antenna, the RFID tag IC chip must have more substantial power conversion and switching devices than is necessary for the wire antenna in order to modulate the backscattered RF energy return to the interrogator.
The use of a microstrip antenna for an RFID tag has been disclosed in U.S. Pat. No. 6,215,402, which includes several designs for patch antennas and impedance matching components for an RFID tag, and U.S. Pat. No. 6,329,915, which describes the use of an additional insulating material with high electric permittivity that is applied to the surface on top of the microstrip antenna in order to further reduce the size of the antenna. However, neither of these patents discloses the use of an RFID tag having a microstrip antenna on a metal closure for a container.
The use of specially designed slots etched into the interior of a patch antenna to broaden the bandwidth of a microstrip antenna without changing the overall form factor of the antenna is disclosed in an article by Ali, Sittironnarit, Hwang, Sadler, and Hayes, entitled “Wideband/Dual-Band Packaged Antenna for 5-6 GHz WLAN Application,” that appeared in the February, 2004 issue of the journal IEEE Transactions on Antennas and Propagation. However, this article does not disclose the use of an RFID tag having a microstrip antenna on a metal bottle cap.
Accordingly, it is an object of the present invention to provide an RFID tag employing an antenna that can be mounted on the exterior of a metal closure for a container and that provides the same functionality as a conventional RFID tag mounted on a plastic closure for a container.
It is a further object of the present invention to provide an RFID tag for mounting on a metal cap that is not subject to close tolerances in manufacturing.
The present invention is directed to an RFID tag system which communicates with a base station at a predetermined frequency for use with a container having a metal closure. The RFID tag system includes an antenna and insulator adapted to be mounted to an exterior surface of the metal closure and an RFID chip coupled to said antenna and adapted to be coupled to the metal closure. In a first embodiment, the RFID chip is mounted outside the metal closure. In a second embodiment, the RFID chip is mounted within the metal closure and connected to the antenna outside the metal closure through an electrical feedthrough connection in the metal closure.
The above and related objects, features and advantages of the present invention will be more fully understood by reference to the following detailed description of the presently preferred, albeit illustrative, embodiments of the present invention when taken in conjunction with the accompanying drawing wherein:
Referring now to the drawing, and in particular to
As discussed above, the IC chip of RFID tag 110 may be located either outside the metal cap or inside the cap. Locating the chip outside the cap results in lower manufacturing costs since no feed-through connections are required. However, there may be functional incentives to locate the chip inside the cap, in which case one or more electrical feed-through connections are required to conduct signals from the antennal patch to the IC chip.
The microstrip patch antenna is naturally adapted to metal caps because the metal cap serves as the ground-plane for the antenna. The complementary metal surface (i.e., the patch) of the microstrip antenna is positioned on top of the metal cap with an insulating spacer between the two metal surfaces.
Two radio frequency bands are allocated by the Federal Communications Commission for RFID systems, 2.4 GHz and 5.8 GHz. Both of these frequency bands are used for other applications, including wireless telephones and wireless local area networks.
The characteristic dimension of the antenna that causes it to be tuned to a specific frequency (and the harmonics of that frequency) is larger for the simple patch antenna (one-half wavelength) than it is for a one-quarter wavelength electric dipole antenna, although more complex patch antennas can be fabricated that are the same characteristic length. Consequently, the simplest (and least costly) of 2.45 GHz patch antennas would barely fit on top of the smallest standard metal cap (1⅛ inch diameter). There are other design options that could make it possible, from a technical standpoint, to use 2.45 GHz, although at a higher manufacturing cost. Alternatively, the 5.8 GHz microstrip antenna has a characteristic dimension of less than 1 inch and thus fits more easily on the top of conventional metal bottle caps.
When using a microstrip patch antenna, the RFID IC chip may be located either outside of the metal cap or within the metal cap. Locating the IC chip on the outside surface results in lower manufacturing cost, since feed-throughs are required to connect the antenna to the IC chip when the IC chip is mounted within the metal cap. Although a single feed-through could be used to connect the antenna to the IC chip, thereby reducing manufacturing costs, when two feed-throughs are employed, the length of the antenna patch can be reduced by 50%.
The microstrip antenna is preferred for a metal cap because, when properly designed, (1) it is more efficient receiving and re-radiating the resonant RF energy, (2) it offers a low profile on the bottle cap and (3) there is sufficient space on the top of the bottle cap to place the antenna if the system is operated at 2.45 GHz or at 5.8 GHz. Furthermore, the higher frequency 5.8 GHz microstrip antenna allows more design freedom and could lead to a lower-cost metal cap with integral RFID tag.
The characteristic length of the antenna patch, and the dielectric permittivity of the insulating layer, determine the frequencies at which the antenna may be used. Consequently, the diameter of the metal cap is the main consideration in selecting one of the two frequency bands that have been allocated by the Federal Communications Commission in the U.S. for use in RFID systems. The 2.45 GHz frequency band is widely used for RFID applications, while only a few systems have been developed for RFID at the higher 5.8 GHz frequency band. However, relevant radio technology at 5.8 GHz has been developed extensively for other applications such as cordless telephones and wireless local area networks.
The characteristic length of the antenna patch is plotted as a function of the dielectric permittivity of the insulating layer at frequencies of 2.45 GHz (plot 160) and 5.8 GHz (plot 150) in
A table of the dielectric permittivity for various low-loss insulating materials manufactured by the Rogers Corp. is shown in Table I.
The data from
Since the simplest patch atennas have only a 2% to 5% bandwidth, it may be desirable in terms of manufacturability to increase the bandwidth of a microstrip patch antenna to ensure that RFID tags are not tuned away from the frequency of the associated interrogator due to variations in component tolerances that arise in the manufacturing process. As one of skill in the art will readily recognize, an RFID tag having an increased bandwidth will still be able to communicate with an associated interrogator, even if the center frequency of the RFID tag varies from its intended value because of manufacturing tolerances, the influence of nearby dielectric materials or other factors. One method to increase the bandwidth of a patch antenna is disclosed in U.S. Patent Publication No. 2003/0222763, incorporated herein by reference. In that publication, a method is disclosed that increases the bandwidth of a patch antenna by 14% or more by etching slots in the patch antenna. An example, based on the methods disclosed in this publication is shown in
In particular, the RFID tag system 310 includes the same components as the RFID tag system 210 of
In some applications, it may be necessary to position the RFID IC chip be inside the metal cap. For example, it may be necessary to employ the RFID tags of the present invention in a larger system having interrogators that operate at a 2.8 GHz transmission frequency. In that case, since, as discussed above, the antenna patch could take up most of the area on the top of a metal cap, only the antenna patch could be positioned outside the metal cap and the antenna connected to the RFID chip is mounted inside the cap and connected to the external antenna via a feed-through connection, i.e., a wire connection that passes through the metal cap.
If the bandwidth of the system illustrated in
Now that the preferred embodiments of the present invention have been shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is to be construed broadly and limited only by the appended claims, and not be the foregoing specification.
