Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-02-07
2001-03-06
Mullen, Thomas (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C029S602100
Reexamination Certificate
active
06198393
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to the preparation of a continuous loop inductor for a RF-EAS resonant security tag circuit useful in conjunction with an electronic security system.
Presently there are two basic types of RF-EAS security tags commercially available. One type is a reusable tag which can be fastened to the products desired to be protected, while the other is of the disposable type that can be adhered to the packaging in which the products are packaged, or prepared as a part of the packaging material itself. These tags utilize technology based on tuned circuits that operate in the radio frequency range. An example of the disposable type is disclosed in applicant's pending U.S. patent application Ser. No. 09/362,614, assigned to the present assignee herein.
The disposable tags use discrete inductor and capacitor elements that are applied to a dielectric substrate. Such elements may be formed by conventional fabrication methods for forming printed circuits including selected use of laminated substrates having an interior dielectric layer laminated on both surfaces with a conductive composition such as aluminum or copper. The conductive layers may be printed with an etchant resistant material in the form of the desired circuit, and after etching, the remaining conductive material forms the desired circuit. Alternatively, the resonant tag circuits can be formed by an additive process whereby an activatable composition is printed on a dielectric substrate in the form of the desired circuit. Likewise, the circuits can be formed by stamping or die cutting the circuits out of thin metal sheets which are then adhered or laminated to the dielectric substrate. Efforts to print inductor loops using conductive inks are disclosed for example in U.S. Pat. Nos. 5,442,334 and 5,781,110. However, conductive inks generally have a resistivity nearly one order of magnitude greater than solid metals such as aluminum. Likewise, the fabrication of inductor elements using die-stamping techniques (or, alternatively, die-cutting and subsequent transfer), especially of an inductor foil containing many thin loops, is a difficult and exacting process since fracture of the loop at any point results in an open circuit and ineffective inductor. Thus there remains a need in the art to develop a method and or technique to successfully apply inductors to substrates in the fabrication of RF-EAS security tags.
SUMMARY OF INVENTION
RF-EAS (Radio Frequency-Electronic Article Surveillance) circuits are often comprised of a single inductor and capacitor in a loop, forming an LC resonant circuit. The production of LC circuits with sharp (i.e., frequency-specific) resonance requires that L/C (the Q factor) be as great as possible, which generally requires inductors with a large number of loops. Also, the total surface area occupied by the circuit is usually required to be as small as possible so that the EAS tag is not noticeable or obtrusive. These two requirements encourage the use of inductors with thin, tightly spaced loops. Another requirement of the inductor is that its electrical resistance be kept as low as possible. This requirement is best met by using solid metal foils rather than printed or hot-stamp pigmented materials. Moreover, as noted before, die-stamping of inductors containing many thin loops is a difficult and tricky process. Thus, in an effort to overcome these problems, the present invention comprises a composite foil/ink inductor element which incorporates the best features of each process. For this purpose, the inductor element according to the present invention is comprised of a plurality of discrete, unconnected foil loop segments, the ends of which are connected by printing conductive ink patches to form a continuous loop. It is known, for example, as shown in U.S. Pat. No. 4,835,524, to leave a gap in an inductor loop which is later closed with a conductive ink patch to provide a fusible link in the loop for the purpose of deactivating an RF-EAS tag. However, in the present invention, the purpose of the conductive ink patches is to close the gaps in a discontinuous inductor loop made up of segments of a metal foil to produce an active tag.
In accordance with the present invention, the inductor loop combines metal foil and conductive ink in a way that allows easier stamping and transfer of the foil, while minimizing the portion of the total inductor path length occupied by conductive ink rather than solid foil. The stamped (solid foil) portions of the inductor loop comprise essentially discrete, linear segments of foil. When these segments are stamped onto a suitable dielectric substrate, and the waste foil removed, the ends of the waste foil are self-supporting thus reducing the chance for a rip or tear between the excess foil removed and the segment that remains on the substrate. The inductor loop is then made continuous by printing a conductive ink patch between the free ends of each foil segment. The composite foil/ink inductor of the present invention may be used in all situations which would normally employ a solid foil inductor, including, but not limited to, LC circuits employed in EAS security tags or as the inductor/antenna element of transponder units used in “smart cards” (e.g., phone cards, banking cards, security clearance cards, etc.).
REFERENCES:
patent: 4835524 (1989-05-01), Lamond et al.
patent: 5442334 (1995-08-01), Gallo et al.
patent: 5445922 (1995-08-01), Maple
patent: 5532667 (1996-07-01), Haertling et al.
patent: 5781110 (1998-07-01), Habeger, Jr. et al.
Mullen Thomas
Westvaco Corporation
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