Radio frequency identification tags with compensating elements

• US 7,268,687 B2
• Filed: 03/23/2004
• Issued: 09/11/2007
• Est. Priority Date: 03/23/2004
• Status: Expired due to Fees

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Egbert, William C., Herdtle, Thomas
• Primary Examiner(s): TWEEL JR, JOHN ALEXANDER
• Application Number:
  US10/807,072
• Publication Number:
  US 20050212707A1
• Time in Patent Office: 1,267 Days
• Field of Search: 340/572.7, 340/572.1, 340/572.4, 340/572.5
• US Class Current: 340/572.7
• CPC Class Codes: G06K 19/07749 the record carrier being capable of non-contact communication, e.g. constructional details of the antenna of a non-contact smart card
• CPC Class Codes: G06K 19/07771 the record carrier comprising means for minimising adverse effects on the data communication capability of the record carrier, e.g. minimising Eddy currents induced in a proximate metal or otherwise electromagnetically interfering object
• CPC Class Codes: H01Q 1/22 by structural association with other equipment or articles
• CPC Class Codes: H01Q 1/2225 used in active tags, i.e. provided with its own power source or in passive tags, i.e. deriving power from RF signal
• CPC Class Codes: H01Q 7/00 Loop antennas with a substantially uniform current distribution around the loop and having a directional radiation pattern in a plane perpendicular to the plane of the loop

Abstract

Radio frequency identification (RFID) tags include compensating elements. The compensating elements enhance the operation of a compensated RFID tag, even when in close proximity to other RFID tags, whether the other tags are compensated or uncompensated. The compensating elements can include a closed loop of conductive material added to a RFID tag antenna. The conductive loop compensates the RFID tag performance when multiple RFID tags are in close proximity, keeping the frequency response of the assembled group of tags substantially centered near the operating frequency of the RFID system.

Claims

1. A compensating element comprising a closed loop of conductive material having a size and proximity to an inductive loop antenna of an RFID tag for electromagnetic coupling to the inductive loop antenna to substantially maintain an operating frequency of the inductive loop antenna at or near an operating frequency of an RFID system in the presence of other RFID tags.

2. The compensating element of claim 1, wherein a parasitic current is induced in the compensating element in response to the electromagnetic coupling to the inductive loop antenna.

3. The compensating element of claim 1 wherein the conductive material comprises one of a die cut metal foil, a patterned metal foil, an electroplated conductive metal, a printed conductive ink, and a printed precursor material reduced to a conductive state.

4. The compensating element of claim 1 having a substantially rectilinear shape.

5. The compensating element of claim 1 having a substantially circular shape.

6. The compensating element of claim 1 further comprising a substrate upon which the closed loop is disposed.

7. The compensating element of claim 6 further comprising an adhesive layer disposed on one side of the substrate.

8. The compensating element of claim 7 wherein the compensating element and the adhesive layer are disposed on the same side of the substrate.

9. The compensating element of claim 7 wherein the compensating element and the adhesive layer are disposed on opposite sides of the substrate.

10. A radio frequency identification (RFID) tag, comprising:

an inductive loop antenna; and

a comnpensatixig element sized and positioned on the RFID tag proximate the inductive loop antenna for electromagnetic coupling to the inductive loop antenna to substantially maintain an operating frequency of the inductive loop antenna at or near an operating frequency of an RFID system in the presence of other RFID tags.

11. The RFID tag of claim 10, wherein a parasitic current is induced in the compensating element by a primary current in the inductive loop antenna.

12. The RFID tag of claim 10 wherein the compensating element is positioned for electromagnetic coupling to the inductive loop antenna such that an RFID system interrogating antenna is able to detect the compensated RFID tag when in close proximity to other RFID tags.

13. The RFID tag of claim 10, further including a RFID die having identification information stored therein.

14. The RFID tag of claim 10 wherein the compensating element comprises a closed loop of conductive material.

15. The RFID tag of claim 14 wherein the closed loop has a substantially rectilinear shape.

16. The RFID tag of claim 14 wherein the closed loop has a substantially circular shape.

17. The RFID tag of claim 14 wherein the closed loop is electrically isolated from the inductive loop antenna.

18. The RFID tag of claim 14 wherein the closed loop is electrically connected to the inductive loop antenna.

19. The RFID tag of claim 14 wherein the closed loop is disposed within an innermost loop of the inductive loop antenna.

20. The RFID tag of claim 14 wherein the closed loop is disposed between loops of the inductive loop antenna.

21. The RFID tag of claim 14 wherein the closed loop is disposed outside an outermost loop of the inductive loop antenna.

22. The RFID tag of claim 14 wherein the compensating element has an angular displacement of between 0 and 45 degrees with respect to an axis of the inductive loop antenna.

23. The RFID tag of claim 14 wherein the conductive material comprises one of a die cut metal foil, a patterned metal foil, an electroplated conductive metal, a printed conductive ink, and a printed precursor material reduced to a conductive state.

24. The RFID tag of claim 14 wherein the closed loop is disposed within 10 line widths of at least one loop of the inductive loop antenna.

25. The RFID tag of claim 14 wherein the closed loop is disposed within 2 line widths of at least one loop of the inductive loop antenna.

26. The RFID tag of claim 10 wherein the compensating element has an axis that is substantially aligned with an axis of the inductive loop antenna.

27. The RFID tag of claim 10 wherein the compensating element lies substantially in a plane parallel and proximate to a plane of the inductive loop antenna.

28. The RFID tag of claim 10 wherein the compensating element is substantially coplanar with the inductive loop antenna.

29. The RFID tag of claim 10 wherein the RFID tag resonates at a frequency of approximately 13.56±

1.0 MHz.

30. The RFID tag of claim 10 wherein the compensating element is physically separate from the inductive loop antenna.

31. The RFID tag of claim 10 wherein the compensating element comprises at least one loop of the inductive loop antenna electrically connected to at least one other loop of the inductive loop antenna.

41. The RFID tag of claim 10, wherein the inductive loop antenna is a multi-turn inductive loop antenna.

42. The RFID tag of claim 10, wherein the operating frequency of the inductive loop antenna is inversely proportional to a distance between the compensating element and the inductive loop antenna.

43. The RFID tag of claim 10, wherein the inductive loop antenna is electromagnetically coupled to an interrogating magnetic field generated by an RFID reader, and wherein the compensating element is not electromagnetically coupled to the interrogating magnetic field generated by the RFID reader.

44. The RFID tag of claim 24, wherein the RFID tag resonates at a frequency of approximately 13.56±

1.0 MHz.

32. A radio frequency identification (RFID) tag, comprising:

an inductive loop antenna; and

a compensating element positioned for electromagnetic coupling to the inductive loop antenna,

wherein the compensating element comprises at least one loop of the inductive loop antenna electrically connected to at least one other loop of the inductive loop antenna,

wherein the compensating element comprises at least two loops of the inductive loop antenna, and wherein each of the two loops of the inductive loop antenna is electrically connected to a different one other loop of the inductive loop antenna.

33. The RFID tag of claim 32 wherein the at least two loops of the inductive loop antenna electrically connected to at least one other loop of the inductive loop antenna are adjacent loops.

34. The RFID tag of claim 32 wherein the at least two loops of the inductive loop antenna electrically connected to at least one other loop of the inductive loop antenna are non-adjacent loops.

35. A radio frequency identification (RFID) tag, comprising:

an inductive loop antenna; and

a compensating element positioned for electromagnetic coupling to the inductive loop antenna,

wherein the compensating element comprises at least one loop of the inductive loop antenna electrically connected to at least one other loop of the inductive loop antenna, and

wherein the at least one loop of the inductive loop antenna is electrically shorted to the at least one other loop of the inductive loop antenna.

36. A Radio Frequency Identification (RFID) tag for placement on a conductive surface, comprising:

a substrate;

an inductive loop antenna positioned on the substrate;

a compensating element sized and positioned on the RFID tag proximate the inductive loop antenna for electromagnetic coupling to the inductive loop antenna to substantially maintain an operating frequency of the inductive loop antenna at or near an operating frequency of an RFID system in the presence of other RFID tags; and

a dielectric spacer positioned between the substrate and the conductive surface.

37. The RFID tag of claim 36 wherein the dielectric spacer has a dielectric constant less than 10.

38. The RFID tag of claim 37 wherein the dielectric spacer has a dielectric constant less than 3.

39. The RFID tag of claim 36 wherein the dielectric spacer has a thickness of less than 10 mm.

40. The RFID tag of claim 36 wherein the dielectric spacer has a thickness of less than 5 mm.

45. A method comprising:

selecting a size for a compensating element;

forming the compensating element according to the selected size; and

positioning the compensating element on an RFID tag proximate an inductive loop antenna so as to provide electromagnetic coupling by the compensating element to the inductive loop antenna to substantially maintain an operating frequency of the inductive loop antenna at or near an operating frequency of an RFID system in the presence of other RFID tags.

46. The method of claim 45, wherein the conductive loop antenna is a multi-turn antenna, and wherein positioning the compensating element comprises positioning the compensating element interspersed with loops of the inductive loop antenna.

47. The method of claim 45, further comprising electrically connecting the compensating element to antenna via a conductive jumper connecting an innermost loop of the inductive loop antenna to a point on a perimeter of the compensating element.

48. The method of claim 45,

wherein selecting the size of the compensating element comprises selecting a diameter of the compensating element based on a diameter of the inductive loop antenna, and

wherein a frequency response of the inductive loop antenna is greater when the diameter of the compensating element is sized within a range bounded by a diameter of an innermost loop of the inductive loop antenna and a diameter of an outermost loop of the inductive loop antenna than when the diameter of the compensating element is not sized within the range.