Abstract
A method of using shielded straps with RFID tag designs is disclosed. Specifically, the RFID device, in one embodiment, comprises a bridge conductor which couples the antenna and pair of strap pads together. Thus, the coupling between the bridge conductor and the strap conductor, the coupling between the bridge conductor and the antenna conductor, and the coupling between the antenna conductor and the strap conductor increases the total capacitance of the RFID strap device. Further, the presence of the bridge conductor also reduces the area occupied for a given inductance, and provides a higher effective capacitance when the bridge strap is connected to the antenna.
Claims
1. A radio-frequency identification (RFID) device comprising: a first conductor comprised of at least one pair of strap pads and an RFID chip connected to the at least one pair of strap pads; a second conductor; and a dielectric positioned between the first conductor and the second conductor, wherein the first strap conductor couples to an antenna conductor, wherein the second conductor is a bridge and the second conductor, the first strap conductor, and the antenna conductor overlap each other to provide a mutual area with separating dielectrics and a plurality of capacitors having a value.
2. The RFID device of claim 1, wherein the at least one pair of strap pads is coupled to the antenna conductor via a conductive adhesive.
3. The RFID device of claim 1, wherein the at least one pair of strap pads is coupled to the antenna conductor via capacitance.
4. The RFID device of claim 1, wherein the antenna conductor is attached to a base substrate.
5. The RFID device of claim 1, wherein the value of the plurality of capacitors is determined by (i) the mutual area, (ii) a dielectric constant of a material between the antenna conductor, the first strap conductor and the second conductor, and (iii) an amount of distance separating the antenna conductor, the first strap conductor and the second conductor.
6. The RFID device of claim 1, wherein an area of the second conductor is larger than an area of the pair of strap pads.
7. The RFID device of claim 1, wherein an area of the second conductor is smaller than an area of the at least one pair of strap pads.
8. The RFID device of claim 1, wherein the second conductor is modified via a cutting process.
9. The RFID device of claim 8, wherein the cutting process is a laser cut line.
10. The RFID device of claim 1 wherein a shape and an area of the second conductor is modified via a cutting process.
11. A radio-frequency identification (RFID) strap device comprising: a first strap conductor comprised of a pair of strap pads and an RFID chip connected between the pair of strap pads; a second bridge conductor; and a dielectric positioned between the first strap conductor and the second bridge conductor, wherein the first strap conductor couples to an antenna conductor, and further wherein the second bridge conductor, the first strap conductor, and the antenna conductor overlap each other to provide a mutual area with separating dielectrics and a plurality of capacitors having a value.
12. The RFID strap device of claim 11, wherein the value of the plurality of capacitors is determined by the (i) mutual area, (ii) a dielectric constant of a material between the antenna conductor, the first strap conductor, and the second bridge conductor, and (iii) an amount of distance separating the antenna conductor, the first strap conductor and the second bridge conductor.
13. The RFID strap device of claim 11, wherein an area of the bridge conductor is larger than an area of the pair of strap pads.
14. The RFID strap device of claim 11, wherein an area of the bridge conductor is smaller than an area of the pair of strap pads.
15. The RFID strap device of claim 11, wherein the second bridge conductor is modified via a cutting process.
16. A method of making shielded straps having an increased capacitance across a RFID device comprising: providing a bridge conductor, a strap conductor comprising at least one pair of strap pads, and an antenna conductor; attaching the antenna conductor to the strap conductor; and attaching the bridge conductor to the strap conductor, wherein the bridge conductor, the strap conductor, and the antenna conductor overlap each other to provide a mutual area with separating dielectrics and a plurality of capacitors having a value.
17. The method of claim 16, further comprising modifying one or more of a shape and an area of the bridge conductor via a cutting process.
18. The method of claim 16, wherein the cutting process is performed before attaching the antenna conductor to the pair of strap pads.
19. The method of claim 16, wherein the cutting process is performed after attaching the antenna conductor to the pair of strap pads.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1A illustrates a side perspective view of a RFID strap device with a second conductor added to the opposite side of the strap dielectric in accordance with the disclosed architecture.
(2) FIG. 1B illustrates a top perspective view of the RFID strap device with the second conductor added to the opposite side of the strap dielectric in accordance with the disclosed architecture.
(3) FIG. 2 illustrates a side perspective view of the RFID strap device with added bridge conductor connected to an RFID antenna in accordance with the disclosed architecture.
(4) FIG. 3 illustrates a top perspective view of the RFID strap device with multiple conductive layers, the bridge layer, the strap pads, and the antenna conductor in accordance with the disclosed architecture.
(5) FIG. 4 illustrates a side perspective view of the RFID strap device showing coupling between the strap, antenna, and bridge conductors in accordance with the disclosed architecture.
(6) FIG. 5A illustrates a top perspective view of the RFID strap device showing loop inductor in accordance with the disclosed architecture.
(7) FIG. 5B illustrates a top perspective view of the RFID strap device with bridge strap which reduces the size of the loop inductor in accordance with the disclosed architecture.
(8) FIG. 6A illustrates a top perspective view of the RFID strap device showing the folded dipole length with standard strap in accordance with the disclosed architecture.
(9) FIG. 6B illustrates a top perspective view of the RFID strap device with bridge strap which increases the folded dipole length in accordance with the disclosed architecture.
(10) FIG. 7A illustrates a top perspective view of the RFID strap device wherein the shield element is larger than the strap conductors in accordance with the disclosed architecture.
(11) FIG. 7B illustrates a top perspective view of the RFID strap device wherein the shield element is smaller than the strap conductors in accordance with the disclosed architecture.
(12) FIG. 8 illustrates a top perspective view of the RFID strap device wherein the bridge conductor is altered via a laser cut line in accordance with the disclosed architecture.
DETAILED DESCRIPTION
(13) The innovation is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding thereof. It may be evident, however, that the innovation can be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate a description thereof.
(14) The present invention discloses a method of using shielded straps with RFID tag designs. Specifically, the RFID strap device comprises a bridge conductor which couples the antenna and at least one pair of pads (also referred to herein as strap pads or strap conductors) together. Thus, the coupling between the bridge conductor and the strap conductor, the coupling between the bridge conductor and the antenna conductor, and the coupling between the antenna conductor and the strap conductor increases the total capacitance of the RFID strap device.
(15) The amount the capacitance increases depends on one or more of (i) the overlap area between (a) the bridge conductor and the strap conductor, (b) the bridge conductor and the antenna conductor, and (c) the antenna conductor and the strap conductor; the dielectric constant and thickness of intervening materials between each of the strap conductor, the bridge conductor, and the antenna conductor. In most applications an increase of up to 4 times the capacitance of the chip attached to a strap without a shield would be desirable, as higher values would make design of a broad-band antenna for an RFID tag coupled to the strap difficult. For example, a typical strap used for a UHF RFID tag may have a capacitance in the region of 1 pF, therefore a shielded strap would have a range of capacitance between 1 pF and 4 pF.
(16) The increased capacitance provided by the presence of the bridge conductor can have a number of beneficial effects on the design of the RFID tag it is used with. For example, having an increased strap capacitance reduces the required inductance to achieve resonance, as discussed further herein.
(17) It is common for a UHF RFID tag to include an inductive element as part of the antenna connected across the strap, the inductive element intended to resonate at a given frequency, for example, the intended operating frequency of the RFID tag. This inductor is generally made as a planar loop of a given width of conductor and area. Because having an increased strap capacitance, which can be achieved by use of a bridge conductor, reduces the required inductance to achieve resonance, a designer can make a loop having a smaller area, and hence occupy less of the total area available for the rest of the antenna structure, thus allowing increased performance to be achieved. Alternatively, use of a loop having a smaller area can allow the use of a wider conductor. A wider conductor can have a number of benefits; for example: the resistance is lower, and hence less energy is lost when a current flows through it at the operating frequency; a different fabrication method can be used, for example, an etching process is required to define a 0.2 mm line, whereas a cutting process may be used for a 1 mm line, a cutting process advantageously being lower cost than an etching process.
(18) Referring initially to the drawings, FIGS. 1A-B illustrate an RFID device 100 that incorporates at least a second conductor 102. The second conductor may be, for instance, but is not limited to, a bridge or shield conductor. While the present disclosure discusses the utilization of a second conductor 102, the present disclosure further contemplates the utilization of any number of additional conductors and is not limited to a specific amount. Specifically, the RFID device 100 comprises a first conductor, which is at least one pair of conductor pads 106 and a second conductor 102 (also referred to as a bridge or shield conductor) with a dielectric 104 positioned between the second conductor 102 and the at least one pair of conductor pads 106. In some embodiments, the first conductor can be a strap.
(19) The second conductor 102 can be any suitable conductor as is known in the art, such as, but not limited to, an aluminum foil, a copper foil or a printed conductive ink. Further, the second conductor 102 can be any suitable size, shape, and configuration as is known in the art without affecting the overall concept of the invention. One of ordinary skill in the art will appreciate that the shape and size of the second conductor 102 as shown in FIGS. 1A-B is for illustrative purposes only and many other shapes and sizes of the second conductor 102 are well within the scope of the present disclosure. Although dimensions of the second conductor 102 (i.e., length, width, and height) are important design parameters for good performance, the second conductor 102 may be any shape or size that ensures optimal performance during use.
(20) The RFID device 100 further comprises an RFID chip 108 that is preferably positioned between the conductor pads 106, and is mounted on a suitable carrier (shown in FIG. 2), such as a plastic, paper, fabric, corrugated cardstock, foam or any other suitable material. The second conductor 102 is then added to the other or opposite side of the strap dielectric 104 from conductive pads 106 and RFID chip 108, coupling to the pair of conductor pads 106 and potentially to a separate antenna conductor.
(21) As shown in FIG. 2, the RFID device 200 comprises a bridge conductor 202 (also referred to as a second or shield conductor) connected to, or in communication with, an antenna conductor 210. Specifically, the bridge conductor 202 is coupled to a conductor such as a strap conductor 206 and comprises a dielectric 204 positioned between the bridge conductor 202 and the strap conductor 206. The strap conductor 206 further comprises at least one pair of strap pads 208 with an RFID chip 214 positioned between the strap pads 208. The strap pads 208 may then be attached to the antenna conductor 210 via any suitable method as is known in the art, such as the application of a conductive adhesive or non-conductive adhesive (not shown). When attached to the antenna conductor 210 by a conductive adhesive, the strap pads 208 may be coupled to the antenna conductor via the conductive adhesive. In other embodiments, the coupling between the antenna conductor 210 and the strap pads 208 is via capacitance or any other suitable method of coupling as is known in the art, such as magnetic coupling. For instance, a magnetic loop can couple to an antenna, such as antenna conductor 210, when it is adjacent to it. Additionally, the antenna conductor 210 can be made of any suitable material that is known in the art, such as, but not limited to, aluminum foil, copper foil, or printed conductive ink. The antenna conductor 210 can then be attached to an antenna base layer 212 to complete the RFID strap device 200. In another embodiment shown in FIG. 3, an RFID device 300 utilizing a strap comprises at least three conductive layers: a bridge layer 302; a pair of strap pads 304, to which RFID chip 306 is attached; and an antenna conductor 310. While FIG. 3 illustrates the utilization of three conductive layers, the present invention is not limited to any number of conductive layers. These three conductive layers (302, 304, and 310) overlap each other to provide a given mutual area 308 with separating dielectrics, thus creating capacitors. The value of each capacitor is determined, at least in part, by the mutual area 308, separating distance, and dielectric constant of the material between each conductive layer (302, 304, and 310). Additionally, there are small fringing capacitors created on the RFID device 300, but generally these are smaller than the overlap capacitances created by the conductive layers (302, 304, and 310).
(22) Additionally, FIG. 4 discloses a RFID device 400 and the coupling between a strap conductor 406, an antenna conductor 408, and a bridge conductor 402. As disclosed, the coupling is not via the conducting dielectric 404, but rather via capacitance. C.sub.SA represents the coupling 410 between the strap conductor 406 (i.e., strap pads) and the antenna conductor 408. In one embodiment, a thin dielectric adhesive is used for this joint, making this capacitance large. C.sub.BS represents the coupling 412 between the bridge conductor 402 and the strap conductor 406, and C.sub.BA represents the coupling 414 between the bridge conductor 402 and the antenna conductor 408. C.sub.C is the capacitance of the RFID chip 416. Generally, and considering that C.sub.SA is relatively large in comparison to the other capacitances, C.sub.BS and C.sub.BA are effectively in parallel; therefore, the capacitance added in parallel across C.sub.C can be expressed by the following formula: (1/C.sub.B)=2((C.sub.BA+C.sub.BS)), where C.sub.B is the total capacitance added by the presence of the bridge conductor 402, and the capacitance presented to the antenna conductor 408 and an inductor structure as part of the resonating element is increased to C.sub.C+C.sub.B. Alternatively, the joint may be conductive by using an isotropic conductive paste, an anisotropic conductive paste, thermal, laser or ultrasonic welding, crimping or other methods.
(23) FIG. 5A discloses one aspect of the invention of the present disclosure, specifically, a loop inductor 502 with a standard strap. The loop inductor 502 is shown at the typical inductor size to provide the desired resonant frequency, with C.sub.C being the capacitance of the RFID chip 500. In contrast, FIG. 5B discloses a loop inductor 506 with a bridge conductor, and thus having a higher capacitance. For example, in one embodiment, the capacitance of the loop inductor is in the region of 1 pf to 4 pF at UHF frequencies. Because the bridge conductor increases the capacitance of the loop inductor, it also advantageously reduces the inductor size needed to provide the desired resonant frequency, with C.sub.C being the capacitance of the RFID chip 500 and C.sub.B being the total capacitance 504 added by the presence of the bridge conductor. Thus, the area occupied for a given inductance, assuming that the line width is unchanged, is reduced by the presence of the bridge conductor and the higher effective capacitance given when the bridge conductor is connected to the antenna.
(24) As shown in FIG. 6A, a loop area or central matching resonator 602 without a bridge conductor is shown. Specifically, the folded dipole length 600 is shown within the given area 608 with the standard strap. FIG. 6B discloses a loop area or central matching resonator 606 with a bridge conductor. Specifically, the folded dipole length 604 is shown within the given area 608 with the bridge conductor. Thus, the benefit of using the bridge conductor on an antenna that is required to fit inside a given area 608 is shown with an increase in folded dipole length 604. Specifically, for dipole antennas, the efficiency and performance and ease of matching the RFID chip, the antenna is related to how much dipole length can fit into a given area 608. In FIG. 6A, therefore, without the bridge conductor, the central matching resonator 602 occupies a relatively large area, so the space available for the folded dipole length 600 is reduced, so less length can be used. For example, in an antenna of 70 mm×14.5 mm using an unbridged strap (i.e., no bridge conductor), the central loop (i.e., inductor) may occupy a 32 mm×8 mm, which is 256 mm.sup.2; on the other hand, using a strap with twice the capacitance may require a central loop (i.e., inductor) occupying 16 mm×8 mm, which is 128 mm.sup.2. As a result, an area of 128 mm.sup.2 can be used for other elements of the antenna such as the dipole. In FIG. 6B, the size of the central matching resonator 606 is reduced by using a bridge conductor and hence more folded dipole length 604 can be fit in.
(25) In an alternative embodiment shown in FIGS. 7A-B, the size and shape of the shield element or bridge conductor 700 and 704 over the top of the strap pads 702 may be varied depending on the wants and/or needs of a user. For example, as shown in FIG. 7A, the area of the bridge conductor 700 is much larger than the area of the strap pads 702, giving high bridging capacitance. In FIG. 7B, in contrast, the size of the bridge conductor 704 has been reduced and is much smaller than the area of the strap pads 702, hence giving lower levels of bridging capacitance. Thus, the bridge conductor can be a variable structure regulating the bridging capacitance based on the wants and/or needs of a user.
(26) Additionally, as shown in FIG. 8, the RFID device 800 comprises a bridge conductor 802 that can be modified via a cutting process, such as via a laser or mechanical die cutting, to alter the bridging capacitance. Specifically, the cutting process is typically a cut line 804, such as a laser cut line, but can be any suitable cutting process as is known in the art. The cutting process can be performed before or after the pair of strap pads 806 and RFID chip 808 are attached to an antenna (not shown). The cutting process can modify the shape and/or area of the bridge conductor 802 and, therefore, allow the bridging capacitance to be changed. Changing the bridging capacitance allows for tuning of the total RFID chip 808 and bridge capacitance.
(27) This change in capacitance can be used to accommodate manufacturing tolerances or shift the operation frequency of an antenna between two bands. For example, for ultra-high frequency (UHF) tags Europe uses a frequency between 865 MHz and 868 MHz wherein the United States uses a frequency between 902 MHz and 928 MHz. Thus, by using a cutting process to modify the shape and/or area of the bridge conductor 802 to change the bridging capacitance, the same RFID device 800 can be used in two different bands merely by changing the bridging capacitance. Use of the same RFID device 800 having a bridge conductor with a variable bridging capacitance can advantageously reduce manufacturing and operational costs because it allows for the use of one RFID device design in multiple frequency bands.
(28) What has been described above includes examples of the claimed subject matter. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the claimed subject matter, but one of ordinary skill in the art may recognize that many further combinations and permutations of the claimed subject matter are possible. Accordingly, the claimed subject matter is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is interpreted when employed as a transitional word in a claim.
