Antenna for use in an RFID tag

Abstract

An antenna for use in an RFID tag comprises an antenna yarn. The antenna yarn comprises metal fibers. The metal fibers are stainless steel fibers. The antenna yarn is wrapped by at least one wrapping yarn thereby covering the full surface of the antenna yarn or of the metal wire or of the bundle of metal wires. The at least one wrapping yarn comprises non-electrically conductive fibers.

Claims

1. Antenna for use in an RFID tag, wherein the antenna comprises an antenna yarn, wherein the antenna yarn comprises metal fibers, wherein the metal fibers are stainless steel fibers, wherein the antenna yarn is wrapped by at least one wrapping yarn thereby covering the full surface of the antenna yarn, and wherein the at least one wrapping yarn comprises non-electrically conductive fibers.

2. Antenna as in claim 1, wherein the stainless steel fibers have an equivalent diameter less than 20 m.

3. Antenna as in claim 1, wherein the antenna yarn comprises at least 80 stainless steel fibers in its cross section.

4. Antenna as in claim 1, wherein the at least one wrapping yarn is at least one tape.

5. Antenna as in claim 1, wherein each of the at least one wrapping yarns is wrapped around the antenna yarn with more than 1000 turns per meter length of the antenna yarn.

6. Antenna as in claim 1, wherein the antenna yarn is wrapped by a wrapping yarn in S-direction; and wherein the antenna yarn is wrapped by a wrapping yarn in Z-direction.

7. Antenna as in claim 6, wherein the number of wrapping turns of the wrapping yarn wrapping the antenna yarn in Z-direction are the same as the number of wrapping turns of the wrapping yarn wrapping the antenna yarn in S-direction.

8. Antenna as in claim 1, wherein the stainless steel fibers are present as filaments; or wherein the stainless steel fibers are present as fibers of discrete length.

9. Antenna as in claim 1, wherein the stainless steel fibers are present as filaments in parallel untwisted arrangement.

10. Antenna as in claim 1, wherein the antenna yarn comprises twisted or cabled multifilament stainless steel fibers, or wherein the stainless steel fibers are present in the antenna yarn as filaments in parallel untwisted arrangement, and wherein the antenna yarn is wrapped by a wrapping yarn in S-direction, and wherein the antenna yarn is wrapped by a wrapping yarn in Z-direction.

11. Antenna as in claim 1, wherein the stainless steel fibers have a martensite percentage by weight less than 5%.

12. RFID tag comprising a transponder chip and an antenna as in claim 1, wherein the antenna is coupled to the transponder chip.

13. Assembly of a textile fabric and an RFID tag as in claim 12, wherein the RFID tag is fixed onto the textile fabric.

14. Assembly as in claim 13, wherein the antenna is fixed onto the textile fabric by means of one or by means of more than one stitching yarn.

15. Apparel product or bed sheet or pillow cover or towel comprising an assembly as claim 13.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

(1) FIG. 1 shows the cross section of an antenna for an RFID tag according to the first aspect of the invention.

(2) FIG. 2 shows the section in a plane through and along the axis of an antenna for an RFID tag according to the first aspect of the invention.

(3) FIG. 3 shows a textile fabric and an RFID tag fixed onto the textile fabric.

MODE(S) FOR CARRYING OUT THE INVENTION

(4) FIG. 1 shows the cross section 100 of an exemplary antenna for an RFID tag according to the first aspect of the invention. FIG. 2 shows the section 200 in a plane through and along the axis of an exemplary antenna for an RFID tag according to the first aspect of the invention. The exemplary RFID-antenna has been made using bundle drawn stainless steel filaments of 12 m equivalent diameter out of 316L stainless steel (according to ASTM A240). A parallel bundle of 275 stainless steel filaments 110 has been twisted with 100 turns per meter in order to obtain a twisted yarn; the antenna yarn. A first wrapping yarn 120 is wrapped in Z-direction around the antenna yarn. A second wrapping yarn 125 is wrapped in S-direction around the antenna yarn. The wrapping yarns 120 and 125 create a full coverage of the surface of the antenna yarn.

(5) The example of FIGS. 1 and 2 shows an antenna yarn wrapped in Z- and in S-direction, however, it is also possible to perform the invention using only wrapping in Z-direction or in S-direction; however wrapping in Z- and in S-direction is preferred.

(6) FIG. 3 shows a textile fabric 330 and an RFID tag 340 fixed onto the textile fabric. The RFID tag 340 comprises a transponder chip 350 and an antenna 360 as in the first aspect of the invention. The antenna 360 is positioned undulating on the textile fabric and forms in the middle of its length a loop 365 with overlapping ends. The antenna 360 is inductively coupled to the transponder chip 350. The RFID tag 340 is fixed onto the textile fabric. The antenna 360 is fixed onto the textile fabric 330 by means of one or more than one stitching yarns 370. The transponder chip 360 is fixed onto the textile fabric by means of an, e.g. transparent, laminating foil 355. Alternatively, the transponder chip can e.g. be fixed onto the textile fabric by means of epoxy blob or glue.

(7) A first example of an antenna for use in an RFID tag comprises an antenna yarn, provided out of 90 stainless steel filaments of 14 m equivalent diameter, twisted together with a twist of 100 turns per meter. The stainless steel filaments of the antenna yarn have been made using bundle drawn stainless steel filaments out of 316L stainless steel (according to ASTM A240). The antenna yarn is wrapped in S-direction and in Z-direction by 76 dTex (=7.6 Tex) non-entangled texturized polyester multifilament yarns. The wrappings are done with 2250 turns per meter length of the antenna yarn. The wrapping non-entangled texturized polyester multifilament yarns cover the full surface of the antenna yarn. The antenna has a diameter of 0.23 mm.

(8) A second example of an antenna for use in an RFID tag comprises an antenna yarn provided out of 275 stainless steel filaments of 12 m equivalent diameter, twisted together with a twist of 100 turns per meter. The stainless steel filaments of the antenna yarn have been made using bundle drawn stainless steel filaments out of 316L stainless steel (according to ASTM A240). The antenna yarn is wrapped in S-direction and in Z-direction by 167 dTex (=16.7 Tex) non-entangled texturized polyester multifilament yarns. The wrappings are done with 1250 turns per meter length of the antenna yarn. The wrapping non-entangled texturized polyester multifilament yarns cover the full surface of the antenna yarn. The antenna has a diameter of 0.33 mm. Because the antenna yarn deforms easily and compresses when being touched before wrapping, it is difficult to measure its diameter prior to wrapping.

(9) This antenna was compared with a prior art antenna with the same antenna yarn but having a polymer coating sheath instead of the wrapping yarns. The prior art antenna with the polymer coating sheath has a diameter of 0.55 mm. Analysing the cross section of the polymer sheath showed a coating layer thickness of 0.12 mm and a diameter of the antenna yarn in the antenna 0.31 mm. The coating layer thickness 0.12 mm was the lowest coating layer thickness that could be applied for full coverage of the surface of the antenna yarn with polymer. The wrapped antenna also showed a lower bending stiffness than the coated antenna.

(10) A third example of an antenna for use in an RFID tag comprises an antenna yarn provided out of 275 stainless steel filaments of 12 m equivalent diameter, twisted together with a twist of 100 turns per meter. The stainless steel filaments of the antenna yarn have been made using bundle drawn stainless steel filaments out of 316L stainless steel (according to ASTM A240). The antenna yarn is wrapped in S-direction and in Z-direction by 76 dTex (=7.6 Tex) non-entangled texturized polyester multifilament yarns. The wrappings are done with 2250 turns per meter length of the antenna yarn. The wrapping non-entangled texturized polyester multifilament yarns cover the full surface of the antenna yarn. The antenna has a diameter of 0.30 mm, indicating a larger compaction of the antenna yarn in the wrapping operation than in the second example, and the use of the thinner wrapping yarn than in the second example.

(11) In the three examples, 316L stainless steel (according to ASTM A240) fibers have been used; however, other stainless steel grades can be used in the invention.

(12) Instead of non-entangled texturized multifilament wrapping yarns, other yarns or tapes can be used as non-electrical conductive wrapping fiber material.

(13) Although wrapping in S- and in Z-direction is preferred, wrapping in only one direction (S or Z) can be used in the invention.

(14) A fourth example of an antenna for use is an RFID tag comprises an antenna yarn provided out of 275 stainless steel filaments of 12 m equivalent diameter, twisted together with a twist of 100 turns per meter. The stainless steel filaments of the antenna yarn have been made using bundle drawn stainless steel filaments out of 316L stainless steel (according to ASTM A240). The stainless steel filaments have been annealed at 1000 C. to create an annealed microstructure of the stainless steel filaments. The antenna yarn is wrapped in S-direction and in Z-direction by 167 dTex (=16.7 Tex) non-entangled texturized polyester multifilament yarns. The wrappings are done with 1250 turns per meter length of the antenna yarn. The wrapping non-entangled texturized polyester multifilament yarns cover the full surface of the antenna yarn.

(15) The antenna was virtually free of martensite, as determined by measuring the saturation field via magnetic measurement, and comparison with the measurement value of stainless steel samples with known martensite weight percentage to calculate the martensite weight percentage of the tested sample. An RFID-tag was made comprising a transponder chip and such antenna. The textile fabric with the exemplary RFID tag with the antennas as described in the example has been tested on the effect on MRI-scanner images. The effect was sufficiently low such that it did not affect the MRI images negatively.

(16) The antenna has a diameter of 0.33 mm. This antenna was compared with a prior art antenna that has a polymer coating sheath and an antenna yarn similar in construction as the fourth example but comprising stainless steel filaments with an end-drawn microstructure. The prior art antenna with the polymer sheet has a diameter of 0.55 mm. Analysing the cross section of the polymer sheath showed a coating layer thickness of 0.12 mm and a diameter of the antenna yarn in the antenna 0.31 mm. The antenna of the fourth example showed improved reading distance compared to the prior art antenna yarn with a polymer coating. This improved reading distance is obtained by the difference in conductivity: the antenna of the fourth example has an electrical conductivity of 30 Ohm per meter length of the antenna, whereas the electrical conductivity of the prior art antenna (with antenna yarn with stainless steel filaments with end-drawn microstructure and with a polymer sheath) is 25 Ohm per meter length of the antenna.

(17) A fifth example of an antenna for use in an RFID tag comprises an antenna yarn provided out of 275 stainless steel filaments of 12 m equivalent diameter. The stainless steel filaments are present in parallel untwisted arrangement; this means as a bundle of untwisted, parallel stainless steel filaments. The stainless steel filaments of the antenna yarn have been made using bundle drawn stainless steel filaments out of 316L stainless steel (according to ASTM A240). The antenna yarn is wrapped in S-direction and in Z-direction by 76 dTex (=7.6 Tex) non-entangled texturized polyester multifilament yarns. The wrappings are done with 2250 turns per meter length of the antenna yarn. The wrapping non-entangled texturized polyester multifilament yarns cover the full surface of the antenna yarn.

(18) Other wrapping yarns can be used than the ones listed in the first, second, third, fourth and fifth example. Advantageously, tapes can be used as wrapping yarns. Specific examples of cross sections of tapes that can be used in the invention are e.g. 250 micrometer by 12 micrometer, 350 micrometer by 12 micrometer, 370 micrometer by 12 micrometer and 250 micrometer by 23 micrometer, e.g. in polyester.