WO2010018546A1

WO2010018546A1 - A dual frequency rfid tag

Info

Publication number: WO2010018546A1
Application number: PCT/IB2009/053542
Authority: WO
Inventors: Brian Michael O'neill
Original assignee: Ipico Innovation Inc; Van Der Walt, Louis, Stephanus
Priority date: 2008-08-13
Filing date: 2009-08-11
Publication date: 2010-02-18

Abstract

The invention relates to a dual frequency RFID tag (10). It has a first antenna coil (14) resonant at a first frequency and a second antenna coil (16) resonant at a second frequency. The coils (14, 16) are arranged relative to each other such that coupling between the coils (14, 16) is minimised, resulting in little or no current being induced in the second coil (16) by the first coil (14). In one embodiment, the coils (14, 16) are arranged relative to each other such that a magnetic field created by the first,coil (14) couples with the second coil (16) both positively and negatively, thereby substantially cancelling out in the second coil (16) the magnetic field generated by the first coil (14).

Description

Tl TLE: A dual frequency RFI D tag

Fl ELD OF I NVENTI ON

This invention relates to an RFI D tag, and particularly to a dual frequency RFI D tag.

BACKGROUND OF I NVENTI ON

The I nventor is aware of a dual frequency RFI D (Radio Frequency Identification) system including an RFI D tag powered by a low frequency (LF) energising field generated by an RFI D reader. The RFI D tag transmits data back to the RFI D reader at a different frequency, typically in the high frequency (HF) band. The LF field is typically at 125 kHz to 135 kHz, while the HF field is typically at 6.8 MHz or 13.56 MHz.

Such a dual frequency RFI D tag as described above contains two antenna coils, one resonant at LF and the other resonant at HF. These coils are aligned in that, for a planar configuration, they are arranged concentrically (refer to Figure 1 ) and that, for a cylindrical configuration, they are arranged co-axially (refer to Figure 2). I n both of these conventional configuration methodologies, the magnetic field generated by the HF coil extends largely through the LF coil as well. There is thus a great deal of coupling between these two coils. As illustrated in Figure 1 , when the HF and LF coils are arranged concentrically, most of the magnetic field goes through both HF and LF coils in the same direction. The HF coil does not load the LF coil significantly, as at 125 kHz the HF coil behaves like an open circuit. However, the LF coil can load the HF coil significantly, since the LF coil behaves like a short circuit at 6.8 MHz. The effect of the loading of the LF coil on the HF coil is that some power is dissipated in the LF coil, instead of being transmitted to the RFI D reader. The

Q-factor of the HF coil is effectively reduced and the resonance frequency can be shifted.

For repeatable performance, it is also important that the relative positions and spacing between the two coils be tightly controlled during manufacturing.

Figure 3 and Figure 4 show diagrammatic representations of the magnetic field created by cylindrical and planar configurations respectively. Since the current in the coils is actually oscillating at LF or HF frequencies, the actual magnetic field is not stationary but correspondingly oscillates and changes direction at the LF or HF frequency. Figure 5 shows an alternative representation of the magnetic field in the planar configuration of Figure 1 . I n this two-dimensional representation, the '+ ' and '-' indicate magnetic field lines of opposite direction

The Applicant accordingly desires a dual frequency RFI D tag which overcomes or at least alleviates some of the aforementioned drawbacks.

SUMMARY OF I NVENTI ON

The invention provides a dual frequency RFI D tag having a first antenna coil resonant at a first frequency and a second antenna coil resonant at a second frequency, characterised in that the coils are arranged relative to each other such that coupling between the coils is minimised, resulting in little or no current being induced in the second coil by the first coil.

Therefore, the dual frequency RFI D tag in accordance with the invention differs from prior art dual frequency RFI D tags in which the magnetic field created by the first coil goes through the second coil in substantially a single direction only, resulting in the first coil inducing a current in the second coil thereby loading the first coil.

The first coil (of the dual frequency RFI D tag in accordance with the invention) may be an HF coil, resonant at HF. The HF may be 6.8 MHz to 13.56 MHz. The second coil may be a LF coil, resonant at LF. The LF may be 125 kHz to 135 kHz.

AS the HF coil induces no or little current in the LF coil, the LF coil therefore does not load the HF coil significantly or at all, resulting in more energy being transmitted back to the RFI D reader, in use.

The coils may be arranged relative to each other such that a magnetic field created by the first coil couples with the second coil both positively and negatively, thereby substantially cancelling out in the second coil the magnetic field generated by the first coil.

I n a first embodiment, the first (e.g. HF) and second (e.g. LF) coils may be offset, or misaligned, relative to each other. If the dual frequency RFI D tag is of planar configuration, the first and second coils may be eccentric (but still arranged in the same plane or in parallel planes) . More particularly, the first and second coils may overlap. Approximately half of the second coil may overlap the first coil, thereby causing an opposing magnetic field in two opposite directions to have approximately the same magnitude, resulting in little or no current being induced in the second coil. - A -

I n a second embodiment, at least one of the coils may overlap itself, or be twisted, thereby inverting the polarity of the generated magnetic field. More particularly, the first (e.g. HF) coil may overlap itself, for example having a figure-of-8 shape. The first coil may then be arranged concentrically within the second coil and the magnetic field generated by opposite ends of the first coil may have opposite polarities, thereby cancelling itself out from the perspective of the second coil and inducing little or no current in the second coil.

Alternatively, if the dual frequency RFI D tag is of cylindrical configuration, the coils may be orthogonal relative to each other. Thus, the magnetic field from the first coil may not couple at all, or only negligibly, with the second coil. I n this configuration, the second (e.g. LF) coil may be wound around a ferrite core. The first (e.g. HF) coil may be a cylindrical air coil.

The invention extends to a dual frequency RFI D system, which includes: at least one dual frequency RFI D tag as defined above; and at least one RFI D reader.

The invention extends further to a method of assembling a dual frequency RFI D tag having a first antenna coil resonant at a first frequency and a second antenna coil resonant at a second frequency, the method being characterised by arranging the coils relative to each other such that coupling between the coils is minimised, resulting in little or no current being induced in the second coil by the first coil.

Arranging the coils relative to each other may include arranging the coils such that a magnetic field created by the first coil couples with the second coil both positively and negatively (i.e. in two opposite directions) , thereby substantially cancelling out in the second coil the magnetic field generated by the first coil. The method may include offsetting the coils relative to each other such that the coils are eccentric, yet substantially coplanar and overlapping (e.g. if the RFI D tag has a planar configuration).

The method may include twisting or inverting one of the coils or otherwise causing it to overlap itself (e.g. having a figure-of-8 shape), thereby allowing generation of magnetic fields in different parts of the coil of opposite polarity. I n such case, the method may include: twisting one of the coils such that it overlaps itself and defines two opposite halves; and bending (e.g. around a cylindrical support) the coil such that the opposite halves are generally orthogonal relative to each other.

The method may include includes arranging the coils such that they are orthogonal (e.g. if the RFI D tag has a cylindrical configuration).

BRI EF DESCRI PTI ON OF DRAWI NGS

The invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings.

I n the drawings: Figure 1 shows a schematic view of a planar configuration of a dual frequency RFI D tag in accordance with the prior art;

Figure 2 shows a schematic view of a cylindrical configuration of a dual frequency RFI D tag in accordance with the prior art;

Figure 3 shows a schematic view of a magnetic field pattern generated by a coil of the RFI D tag of Figure 2;

Figure 4 shows a schematic view of a magnetic field pattern generated by a coil of the RFI D tag of Figure 1 ; Figure 5 shows a schematic view of an alternative magnetic field pattern generated by a coil of the RFI D tag of Figure 1 ;

Figure 6 shows a schematic view of a planar configuration of a dual frequency RFI D tag in accordance with the invention; Figure 7 shows a schematic view of an alternative planar configuration of a dual frequency RFI D tag in accordance with the invention; and

Figure 8 shows a schematic view of a cylindrical configuration of a dual frequency RFI D tag in accordance with the invention.

DETAI LED DESCRI PTI ON OF PREFERRED EM BODI M ENT

Referring to Figure 6, reference numeral 10 generally indicates a dual frequency RFI D tag, in accordance with the invention. The RFI D tag 10 has a planar configuration and is manufactured in the form of a credit card tag.

The RFI D tag 10 has a first coil 14 and a second coil 16. The first coil 14 is an HF coil resonant, for example, at 6.8 MHz. The second coil 16 is a LF coil resonant, for example, at 125 kHz. The LF coil 16 consists of a large number of windings, in the range of 50 to 500 windings. The HF coil 14 consists of just a few windings, in the range of 1 to 3 windings. The two coils 14, 16 are in the same plane, laminated between two sheets of plastics material 18.

The RFI D tag 10 also includes a PCB module 12 containing the dual frequency RFI D chip (e.g. EM4322), a power storage capacitor and tuning capacitors for each of the coils 14, 16. Further, each of the coils 14, 16 is connected electrically to the FCB module 12.

The RFI D chip of the FCB module 12 is powered from an energizing 125 kHz field produced by an RFI D reader (not illustrated). The 125 kHz field is picked up by the LF coil 16 and rectified inside the RFI D chip to provide a DC supply to the RFI D chip. A power supply capacitor of the order of 500 nF to 1 uF stores enough power to allow the chip to transmit data (typically an I D number) back to the RFI D reader. The data is encoded using Pulse Position Encoding (PPE) and transmitted by pulsing the HF coil 14 so that it rings/ resonates at 6.8 MHz.

At the LF (i.e. at a frequency of 125 kHz) , the HF coil 14 behaves like an open circuit and the LF coil 16 induces little or no current in the HF coil 14. Therefore, the HF coil 14 does not load the LF coil 16 significantly.

However, the LF coil 16 can load the HF coil 14 significantly, since the LF coil 16 behaves like a short circuit at 6.8 MHz. I n order to minimise this loading, the coils 14, 16 are arranged relative to each other such that coupling between the coils 14, 16 is minimised. More specifically, the coils 14, 16 are arranged relative to each other such that a magnetic field generated by the HF coil 14 couples with the LF coil 16 both positively and negatively (i.e. in two opposite directions), thereby substantially cancelling out in the LF coil 16 the magnetic field generated by the HF coil 14, resulting in little or no current being induced in the LF coil 16 by the HF coil 14.

I n this embodiment, the HF coil 14 is arranged to be offset (i.e. eccentric or misaligned) relative to the LF coil 16, so that the magnetic field generated by the HF coil 14 couples both positively (indicated by the '+ ' signs in region 14.1 ) and negatively (indicated by the '-' signs in region 14.2) to the LF coil 16. The opposite polarities in the different regions 14.1 , 14.2 effectively cancel each other out (from the perspective of the LF coil 16) and thereby minimise the coupling and resultant loading by the LF coil 16 on the HF coil 14. Conveniently, the strength of the magnetic field in the respective regions 14.1 , 14.2 is substantially equal, thereby cancelling each other out completely or nearly completely, resulting in no or very little current being induced in the LF coil 16 resulting in no or very little loading on the HF coil 14.

Referring now to Figure 7, reference numeral 100 indicates an alternative embodiment of a dual frequency RFI D tag, in accordance with the invention. The RFI D tag 100 has a planar configuration and is manufactured in the form of a credit card tag (much like the RFI D tag 10).

Also like the RFI D tag 10, the coils 102, 16 of the RFI D tag 100 are arranged relative to each other such that a magnetic field created by a first coil, i.e. the HF coil 102, couples with a second coil, i.e. the LF coil 16, both positively and negatively (i.e. in two opposite directions) . However, in contrast with the RFI D tag 10, the coils 102, 16 of the RFI D tag 100 do not overlap each other. I nstead, the HF coil 102 overlaps itself and has a figure- of-8 shape. Therefore, regions 102 .1 , 102.2 at opposite ends of the HF coil 102 generate magnetic fields having opposite polarities. The HF coil 102 is then arranged concentrically within the LF coil 16 and the magnetic field generated by the HF coil 102 effectively cancels itself out, from the perspective of the LF coil 16, and induces little or no current in the surrounding LF coil 16.

Normally, a figure-of-8 arrangement is not preferred over a single or untwisted loop arrangement (like the HF coil 14 of the RFI D tag 10), since the transmission from the two halves of the figure-of-8 will largely cancel each other out at long range from the RFI D tag 100, resulting in a reduced reading range for such a RFI D tag 100. If, however, the RFI D tag 100 is bent or wrapped around a cylindrical object (e.g. about or parallel to axis 104) so that the two halves or portions 102.1 , 102.2 of the RFI D tag 100 are generally orthogonal to each other, the two magnetic fields generated by each halve will not cancel each other out at long range.

Referring now to Figure 8, another embodiment of a dual frequency RFI D tag 200 is illustrated. I n this embodiment, the RFI D tag 200 is still configured such that the coils 14, 202 are arranged relative to each other such that coupling between the coils 14, 202 is minimised. I n this embodiment, a LF coil 202 is implemented by winding a large number of windings 202.1 around a ferrite core 202.2. The HF coil 14 consists of just a few windings in a flat plane (as in the RFI D tag 10 of Figure 6) . The axis of the ferrite core 202.2 is arranged in the same plane as the HF coil 14 which results in the windings of the HF coil 14 being orthogonal or perpendicular to the windings 202.1 of the LF coil 202. There will then be minimal or no coupling between the magnetic field generated by the HF coil 14 and the LF coil 202, and therefore minimal or no loading of the HF coil 14 by the LF coil

202.

The I nventor believes that the invention as exemplified is advantageous as it provides a dual frequency RFI D tag 10, 100, 200 in which the LF coil 16, 202 couples minimally or not at all with the HF coil 14, 102 thus reducing or eliminating the load which the LF coil 16, 202 places on the

HF coil 14, 102.

A further advantage is that small relative shifts in position of the coils 14, 102, 16, 202 will not affect the loading significantly, resulting in easier and more repeatable manufacturing of RFI D tags 10, 100, 200.

Claims

CLAI MS

1. A dual frequency RFI D tag having a first antenna coil resonant at a first frequency and a second antenna coil resonant at a second frequency, characterised in that the coils are arranged relative to each other such that coupling between the coils is minimised, resulting in little or no current being induced in the second coil by the first coil.

2. An RFI D tag as claimed in claim 1 , in which the first coil is an HF coil resonant at a frequency between 6.8 MHz to 13.56 MHz.

3. An RFI D tag as claimed in claim 1 or claim 2, in which the second coil is a LF coil resonant at a frequency between 125 kHz to 135 kHz.

4. An RFI D tag as claimed in any of the preceding claims, in which the coils are arranged relative to each other such that a magnetic field created by the first coil couples with the second coil both positively and negatively, thereby substantially cancelling out in the second coil the magnetic field generated by the first coil.

5. An RFI D tag as claimed in claim 4, in which the coils are offset relative to each other.

6. An RFI D tag as claimed in claim 5, which is of planar configuration and in which the coils are eccentric, yet substantially coplanar and overlapping.

7. An RFI D tag as claimed in claim 6, in which half of the second coil overlaps the first coil, thereby causing an opposing magnetic field having approximately the same magnitude, resulting in little or no current being induced in the second coil.

8. An RFI D tag as claimed in claim 4, in which at least one of the coils overlaps itself, thereby inverting a polarity of the generated magnetic field.

9. An RFI D tag as claimed in claim 8, in which the first coil overlaps itself in a figure-of-8 shape and is arranged concentrically within the second coil such that the magnetic field generated by opposite ends of the first coil has opposite polarities, thereby cancelling itself out from the perspective of the second coil and inducing little or no current in the second coil.

10. An RFI D tag as claimed in any of claims 1 to 3 inclusive, which is of cylindrical configuration and in which the coils are orthogonal relative to each other.

1 1 . An RFI D tag as claimed in claim 10, in which the second coil is wound around a ferrite core and in which the first coil is a cylindrical air coil.

12. A dual frequency RFI D system, which includes: at least one dual frequency RFI D tag as claimed in any of the preceding claims; and at least one RFI D reader.

13. A method of assembling a dual frequency RFI D tag having a first antenna coil resonant at a first frequency and a second antenna coil resonant at a second frequency, the method being characterised by arranging the coils relative to each other such that coupling between the coils is minimised, resulting in little or no current being induced in the second coil by the first coil.

14. The method as claimed in claim 13, in which arranging the coils relative to each other includes arranging the coils such that a magnetic field created by the first coil couples with the second coil both positively and negatively, thereby substantially cancelling out in the second coil the magnetic field generated by the first coil.

15. A method as claimed in claim 14, which includes offsetting the coils relative to each other such that the coils are eccentric, yet substantially coplanar and overlapping.

16. A method as claimed in claim 14, which includes twisting one of the coils such that it overlaps itself, thereby allowing generation of magnetic fields of opposite polarity in different parts of the coil.

17. A method as claimed in claim 16, including: twisting or inverting one of the coils such that it overlaps itself and defines two opposite halves; and bending the coil such that the opposite halves are generally orthogonal relative to each other.

18. A method as claimed in claim 13, which includes arranging the coils such that they are orthogonal.