RFID UHF antenna and matching network embedded in disposable conducting covers

A UHF RFID antenna is integrated into the disposable metal cover of foam, plastic, metal or cardboard containers.

BACKGROUND OF THE INVENTION

Radio Frequency Identification (RFID) tag and reader systems operate over a wide range of radio frequencies, including low frequency (LF) applications, high frequency (HF) applications and ultra-high frequency applications (UHF). LF applications typically reside in the range from about 125 to about 148.5 kHz, HF applications typically operate at about 13.56 MHz while UHF applications typically reside in the range from about 300 MHz to about 3 GHz. The “read range” of an RFID tag is typically defined as the distance from which the RFID reader can communicate with the RFID tag. Passive LF and HF applications typically provide only very short read ranges and typically require the RFID reader to be separated from the tag by no more than about 2 centimeters to about 30 centimeters to achieve successful communication. Passive UHF applications typically allow for longer read ranges, enabling RFID tags to be located from about 2 meters to about 12 meters or more for successful communication with an RFID reader. Typically, various environmental factors can detune an RFID tag and modify the operating frequency to potentially affect the power received by the RFID tag. This affects the read range for the RFID tag. For example, RFID tags in the presence of conducting media such as metals and liquids may experience detuning due to absorption or parasitic capacitance. Detuning may also arise from the capacitance spread in the assembly process. For example, if the direct attaching process of the RFID tag to the antenna has a misalignment or an imperfect contact, parasitic capacitance may be introduced.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, containers having a disposable metal cover, such as, for example, plastic, foam, metal or cardboard cups with a metal foil cover can be RFID tagged by integrating the UHF RFID antenna into the packaging, in particular, disposable metal cover120(seeFIGS. 1aand1b). Typical consumer examples of these container types are yogurt cups or dehydrated soup cups. If cup110is metal, there may exist resonances between cup110and disposable metal cover120which effect the resonance frequency of disposable metal cover120which operates as a UHF antenna. Having cup110made of metal introduces an extra capacitance from disposable metal cover120to ground and introduces inductive coupling between cup110and UHF RFID matching network310(e.g. seeFIGS. 3band3c). Additionally, if cup110is metal, the volume of cup110will typically effect the design. Note that the effects of having cup110made of metal can typically be mitigated by appropriate design of UHF RFID matching network310(e.g. seeFIG. 3b) and because the resonance of disposable metal cover120which functions as the UHF antenna is typically more than about 50 to about 100 MHz higher than the operating frequency in accordance with the invention. The UHF RFID antenna integrated into disposable metal cover120receives power from the RFID reader and that power is used to activate UHF RFID Integrated Circuit (IC)330(seeFIG. 3b). Typical power output for the RFID reader is on the order of 4 watts and UHF RFID IC330(seeFIG. 3b) typically needs a power level of −18 dBm for activation in reading mode and typically needs a power level of −15 dBm for activation in write mode. Embodiments in accordance with the invention have typical Q (quality factor) values less than about 20 and the embodiments in accordance with the invention function in both the radiative near field zone and in the radiation zone.

FIGS. 1aand1bshow an exemplary embodiment in accordance with the invention.FIG. 1ashows a cross section of cup110affixed with disposable metal cover120and metal tab130. In accordance with an embodiment of the invention, the diameter of disposable metal cover120is typically 10 cm or less. Note that in accordance with the invention, disposable metal cover120need not be round but may be square, rectangular, octagonal or other suitable geometric shape. Metal tab130is typically structurally integrated into disposable metal cover120.FIG. 1bshows a top view of cup110which is typically hidden below disposable metal cover120(seeFIG. 1a). The material for disposable metal cover120typically includes aluminum. In accordance with the invention, disposable metal cover120and tab130have an RFID tag antenna integrated into them.FIG. 2shows a cross-section of multilayer structure200that is used for disposable metal cover120and tab130. Conducting layer210of disposable metal cover120and tab130is typically made of aluminum, typically thicker than about 5 μm, and is attached using glue layer220to insulating substrate230. In accordance with the invention, insulating substrate230is typically made of plastic such as polyethylene terephthalate (PET) but may be made of another suitable insulator and is typically thicker than the skin depth at the operating UHF frequency. Glue layer220may be formed by the partial melting of insulating substrate230during the lamination process used to create disposable metal cover120and tab130when a plastic such as PET is used. In accordance with the invention, the laminating process typically combines a sheet of metal or metal foil (e.g. aluminum) with one or more other materials such as paper or plastic (e.g. PET) using a glue, pressure and typically heat for controlling glue viscosity and drying or thermosetting the bonding agent. Four typical methods for laminating aluminum foil are wet bonding, dry pressure or thermoplastic-bonding, extrusion bonding and hot melt bonding.

Insulating substrate230is attached to the rim of cup110using glue layer240. Typical thicknesses for insulating substrate230are on the order of from about 20 μm to about 100 μm. Glue layer240is typically a glue that is suitable for attachment to the rim of cup110.

Due to the mechanical design of disposable metal cover130having insulating substrate230coated with conducting layer210, UHF RFID matching network310(seeFIG. 3b) may be integrated directly into disposable metal cover120and tab130by cutting or etching a matching loop formed by arms128and129into conducting layer210on tab130and form part of UHF RFID matching network310. A typical three-dimensional electromagnetic simulation software program used for design of UHF RFID matching network310is CST MICROWAVE STUDIO available from COMPUTER SIMULATION TECHNOLOGY OF AMERICA, Inc. 429 Old Connecticut Path, Suite 505, Framingham, Mass. 01701. UHF RFID integrated circuit (IC)330(seeFIG. 3a) may be directly attached to matching network310or to UHF RFID STRAP carrier520(seeFIG. 5) which is attached to matching network310.

FIG. 3ashows arm128and arm129of UHF RFID matching network310(seeFIGS. 3band3c) on tab130. In accordance with an embodiment of the invention, part of metal layer210on tab130is precisely removed to create arm128and arm129using an appropriate etchant and mask or other suitable process for the metal used for metal layer210and exposing an area of insulating substrate230on tab130. The size of the area enclosed by arm128and arm129is typically adjusted to provide the desired UHF RFID matching network310. Note notch127inFIG. 3athat is etched or otherwise introduced into metal layer210to provide an attachment location for UHF RFID IC330.

UHF RFID matching network310functions to match the impedance of disposable metal cover120to UHF RFID IC330as shown inFIG. 3b. Maximum power is delivered from the RFID reader to disposable metal cover120when the input impedance of disposable metal cover120is the complex conjugate of the impedance of UHF RFID IC330(conjugate impedance matching). A typical complex impedance for UHF RFID IC330is, for example, (10-150j)Ω. UHF RFID IC330may be directly attached to UHF matching network310(seeFIG. 4) or to UHF RFID STRAP carrier520which is attached to UHF matching network310(seeFIG. 5).

FIG. 3cshows the relationship of an embodiment in accordance with the invention to an equivalent circuit model. Equivalent circuit model360corresponds to disposable metal cover120which functions as a UHF RFID antenna having inductance362, capacitance364and resistance366. In accordance with the invention, typical values for inductance362are in the range from about 30 nH to about 50 nH, typical values for capacitance364are in the range from about 0.2 pF to about 2 pF and typical values for resistance366are in the range from about 5Ω to about 100Ω at the operating frequency that is typically about 915 MHz.

Equivalent circuit model370shows the “T-match” configuration that is created by T-match arms128and129together with disposable metal cover120. The “T-match” configuration acts as an impedance transformer to match the impedance of disposable metal cover120to UHF RFID IC330. The impedance transformer in equivalent circuit model370is typically adjusted by controlling the area of insulating substrate230enclosed by arms128and129(e.g. making the arms128and129wider or narrower as well as longer or shorter). For example, the input impedance may be increased if the width of arms128and129is decreased. The impedance step up ratio provided by inductors372and374depends on the current division factor between metal cover120and arms128and129. Details regarding “T-match” configurations may be found in “The art of UHF RFID antenna design: impedance matching and size reduction techniques” by Gaetano Marrocco, IEEE Antennas and Propagation Magazine, Vol. 50, No. 1, pp. 66-79, 2008 which is hereby incorporated by reference in its entirety.

Equivalent circuit model380corresponds to inductances382and384of metal arms128and129, respectively, on tab130that connect UHF RFID IC330to disposable metal cover120. A typical value for inductances382and384is on the order of about 10 nH. Finally, equivalent circuit model390corresponds to resistance392and capacitance394of UHF RFID IC330. A typical value for resistance392is on the order of about 2 KΩ and a typical value for capacitance394is on the order of about 1 pF.

In accordance with an embodiment of the invention,FIG. 4shows UHF RFID IC330directly attached to arms128and129in cross-sectional view350(seeFIG. 3b) to provide a connection for UHF RFID IC330to matching network310. Matching network310is attached to insulating layer230by glue layer220.

In accordance with an embodiment of the invention,FIG. 5shows the use of UHF RFID STRAP carrier520to connect UHF RFID IC330to matching network310in cross-sectional view350(seeFIG. 3b). UHF RFID STRAP carrier520is attached to arms128and129which form a matching loop structure. Note that in accordance with the invention, the shape does not need to be a loop and could be, for example, semi-rectangular. Arms128and129are attached to insulating layer230by glue layer220. UHF RFID IC330is typically directly attached to UHF RFID STRAP carrier520.

In accordance with the invention,FIG. 6ashows the imaginary value of the complex impedance as a function of frequency at notch127for six different distances of the center of notch127on tab130from the perimeter of disposable metal cover120having a diameter on the order of about 50 mm. (e.g seeFIG. 3a). All other values are fixed so that arms128and129enclose less area as the location of notch127is moved inward a total of about 9.5 mm and the imaginary impedance decreases as the inductance is reduced. Notch127is successively moved in by about 1.9 mm. Curve610shows the imaginary impedance as a function of frequency at notch127for the reference position where notch127is about 15.5 mm away from the perimeter of disposable metal cover120. Curve620shows the imaginary impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve610. Curve630shows the imaginary impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve620. Curve640shows the imaginary impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve630. Curve650shows the imaginary impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve640. Curve660shows the imaginary impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve650and notch127is a distance of about 6 mm away from the perimeter of disposable metal cover120for curve660.

In accordance with the invention,FIG. 6bshows the real value of the complex impedance as a function of frequency at notch127for six different distances of the center of notch127on tab130from the perimeter of disposable metal cover120having a diameter on the order of about 50 mm (e.g seeFIG. 3a). All other values are fixed so that arms128and129enclose less area as the location of notch127is moved inward a total of about 9.5 mm and the real impedance decreases as the inductance is reduced. Notch127is successively moved in by about 1.9 mm. Curve615shows the real impedance as a function of frequency at notch127for the reference position where notch127is about 15.5 mm away from the perimeter of disposable metal cover120. Curve625shows the real impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve615. Curve635shows the real impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve625. Curve645shows the real impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve635. Curve655shows the real impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve645. Curve665shows the real impedance as a function of frequency at notch127where notch127is about 1.9 mm closer to the perimeter of disposable metal cover120than for curve655and notch127is a distance of about 6 mm away from the perimeter of disposable metal cover120for curve665.

As will be apparent to those skilled in the art, in accordance with the invention other impedance matching techniques may be used, such as nested shape slot techniques which typically require a different location for the attachment of the UHF RFID IC to the disposable metal cover.

While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.