CHIPLESS RFID TAG, A CHIPLESS RFID SYSTEM, AND A METHOD FOR ENCODING DATA ON A CHIPLESS RFID TAG

Abstract

The present invention relates to a chipless RFID tag (T), comprising: —a dielectric substrate (1); and —electromagnetic resonators (3) excitable by an external electromagnetic field and respectively arranged on separate spatial locations of the dielectric substrate (1) forming a row, and configured to resonate at a common resonant frequency. The dielectric substrate (1) defines several predetermined encoding areas that include the separate spatial locations, so that data is encoded by the presence/absence of operative electromagnetic resonators (3) thereon. The present invention also relates to a system comprising the chipless RFID tag of the invention and a RFID reader (R) reading an encoded code by detecting the presence/absence of attenuation peaks on an electromagnetic wave providing the external electromagnetic field to the electromagnetic resonators (3). The present invention also relates to a method for encoding data on a chipless RFID tag defined according to the present invention.

Claims

1. A chipless RFID tag, comprising: a dielectric substrate; electromagnetic resonators excitable by an external electromagnetic field and respectively arranged on separate spatial locations of said dielectric substrate; wherein said electromagnetic resonators are arranged on said dielectric substrate forming at least one row, and are configured to resonate at a common resonant frequency, and in that said dielectric substrate defines several predetermined encoding areas that include at least said separate spatial locations, so that data is encoded by the presence/absence of operative electromagnetic resonators on each of said predetermined encoding areas.

2. The chipless RFID tag according to claim 1, wherein said common resonant frequency is the fundamental frequency of each of said electromagnetic resonators.

3. The chipless RFID tag according to claim 1, wherein all of said electromagnetic resonators have substantially the same dimensions, geometry, and composition.

4. The chipless RFID tag according to claim 1, wherein said predetermined encoding areas are equidistant to each other, and said at least one row is a linear or circular row running along at least one surface of said dielectric substrate.

5. The chipless RFID tag according to claim 1, wherein said electromagnetic resonators are planar electromagnetic resonators that can be excited by means of said external electromagnetic field.

6. The chipless RFID according to claim 5, wherein said planar electromagnetic resonators are split ring resonators having at least one slit, or S-shaped split ring resonators, or spiral resonators, or open-loop resonators, or any planar resonator that can be excited by an external electromagnetic field.

7. The chipless RFID tag according to claim 1, wherein said dielectric substrate is a flexible substrate, including plastic substrates and paper substrates.

8. A chipless RFID system, comprising: a chipless RFID tag defined according to; and a RFID reader comprising: a dielectric support relatively movable with respect to the dielectric substrate of the RFID tag; at least one element for at least propagating an electromagnetic wave providing said external electromagnetic field to the electromagnetic resonators, said at least one element being arranged on said dielectric support to move therewith during said relative movement to adjacent locations to said predetermined encoding areas, such that said at least one element is loaded with the electromagnetic resonators of the predetermined encoding areas; and detection means configured and arranged for detecting the presence/absence of attenuation peaks on said electromagnetic wave, or on an electrical signal associated thereto, induced by the electromagnetic resonators, and also configured for providing, based on said detections, the data encoded in the RFID tag, in the form of a code having at least one bit per predetermined encoding area, and at least two possible alternate logic states per bit determined by the presence/absence of a respective of said attenuation peaks.

9. The system according to claim 8, wherein the at least one element is arranged on said dielectric support to sequentially move along said adjacent locations during said relative movement, such that the at least one element is sequentially loaded along time with the electromagnetic resonators of the predetermined encoding area adjacent thereto at at least some of the adjacent locations.

10. The system according to claim 9, wherein said RFID reader comprises a guide for guiding the dielectric substrate of the chipless RFID tag with respect to the dielectric support during said sequential relative movement along said adjacent locations, so that for each adjacent position the corresponding encoding area is distanced from the at least one element below a certain distance that guarantees near-field electromagnetic coupling.

11. The system according to claim 8, wherein said at least one element is a transmission line electrically fed through an input port and generating said electrical signal at an output port, and wherein said detection means are connected to said output port of said transmission line for detecting said generated electrical signal and the presence/absence of attenuation peaks thereon.

12. The system according to claim 11, wherein said transmission line is a coplanar waveguide having a central conductor strip and two return conductors, one to either side of the central conductor strip, and separated therefrom by respective slots, wherein said electromagnetic resonators are arranged such that during said relative movement they pass transversally to the transmission line, in a parallel plane, causing attenuation peaks in said electrical signal when any of the electromagnetic resonators or a portion thereof, is aligned with any of said slots.

13. The system according to claim 8, further comprising at least an additional electromagnetic resonator arranged on a second face of the dielectric support, opposite to a first face on which the at least one element is arranged, configured and arranged for avoiding inter-resonator coupling between the electromagnetic resonators of the dielectric substrate, wherein said additional electromagnetic resonator has substantially the same dimensions, geometry, and composition as each of the electromagnetic resonators arranged on the dielectric substrate but oriented at 180° with respect thereto, and is arranged to be alternatively aligned with each of the electromagnetic resonators of the encoding areas when the at least one element is at the corresponding adjacent location, to provide a broadside-coupled resonators structure formed by the resulting pair of the so aligned electromagnetic resonator and additional electromagnetic resonator, said broadside-coupled resonators structure being associated to a target resonance frequency, and wherein the system comprises a power source configured to generate and feed the at least one element with a harmonic signal tuned at said target resonance frequency.

14. The system according to claim 11, wherein said transmission line is a microstrip line loaded with a resonant element identical to the electromagnetic resonators of the chipless RFID tag, and configured as a bandpass structure, so that each time the electromagnetic resonators of the chipless RFID tag cross the microstrip line, detuning in the response is achieved, resulting in attenuation peaks in an electrical harmonic signal used to feed the microstrip line.

15. The method for encoding data on a chipless RFID tag according to claim 1, wherein in the chipless RFID tag the electromagnetic resonators are present in all the encoding areas at equidistant spatial locations, and the method comprises encoding data by physically altering some of said electromagnetic resonators to make them inoperative, said physical alteration being performed by short-circuiting or cutting the electromagnetic resonators, thus providing the logic state ‘0’ to the corresponding so made inoperative electromagnetic resonator .

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The previous and other advantages and features will be better understood from the following detailed description of embodiments, with reference to the attached drawings, which must be considered in an illustrative and non-limiting manner, in which:

[0052] FIG. 1 schematically shows the chipless RFID system of the second aspect of the invention, for an embodiment, and also the working principle thereof. The cross sectional view is depicted in the inset. The black arrows indicate tag motion in a reading operation.

[0053] FIG. 2 shows a square S-shaped SRR coupled to a CPW transmission line (a) and a 3D view of the broadside coupled S-SRR (BC-S-SRR) (b).

[0054] FIG. 3 shows a working prototype of the chipless RFID reader of the system of the second aspect of the invention, for an embodiment, particularly a transmission line reader, including the CPW (a) and a S-SRR (b), and a photograph of the fabricated encoder (c), i.e. of the chipless RFID tag formed by a plurality of S-SRRs. CPW dimensions, in ref. to FIG. 2(a), are (in mm) W=1.2 and G=0.48; S-SRR dimensions are (in mm) l.sub.1=3.8, l.sub.2=2.96, C.sub.0=0.4 and s=0.2.

[0055] FIG. 4 is a plot which shows the measured frequency response of the tag-CPW of FIG. 3, for different relative positions of the tag in the vicinity of the reference position.

[0056] FIG. 5 is a plot showing the attenuation of the tag-CPW at the reference frequency as the tag moves completely (transversally) above the CPW, for the prototypes of FIG. 3.

[0057] FIG. 6 includes three plots which show measured normalized envelope for 3 fabricated encoders, i.e. chipless RFID tags, with the indicated codes, for the prototype of FIG. 3, i.e. for a “1111111111” code, and for other two prototypes with other codes encoded in the tag, specifically for the following codes: “1010101010” and “1101101101”.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

[0058] FIG. 1 shows an embodiment of the system of the second aspect of the invention, which comprises: [0059] a chipless RFID tag T, comprising:

[0060] a dielectric substrate 1 (in this case, a secure paper); and

[0061] electromagnetic resonators 3 (also identified in the Fig. as R.sub.0 to R.sub.N−1) excitable by an external electromagnetic field, respectively arranged on separate spatial locations of the dielectric substrate 1 each corresponding (for the illustrated embodiment) to a respective predetermined encoding area (of a plurality of equidistant predetermined encoding areas) of the tag T, and configured to resonate at a common resonant frequency; [0062] a RFID reader R comprising:

[0063] a dielectric support 2 relatively movable with respect to the dielectric substrate 1 of the chipless RFID tag T, according to a sequential relative movement;

[0064] an element 4 for propagating an electromagnetic wave providing said external electromagnetic field to the electromagnetic resonators 3, the element 4 being arranged on the dielectric support 2 to move therewith during said sequential relative movement to adjacent locations to the predetermined encoding areas, such that the element 4 is sequentially loaded along time (t.sub.0 to t.sub.N-1) with the electromagnetic resonators 3 of the predetermined encoding areas adjacent thereto; and

[0065] detection means configured and arranged for detecting the presence/absence of attenuation peaks on said electromagnetic wave, or on an electrical signal associated thereto, induced by the electromagnetic resonators 3, and also configured for providing, based on said detections, the data encoded in the chipless RFID tag T, in the form of a code having at least one bit per predetermined encoding area, and at least two possible alternate logic states per bit determined by the presence/absence of a respective of said attenuation peaks.

[0066] For the embodiment illustrated in FIG. 1, each separate spatial location corresponds to a respective predetermined encoding area, but for other embodiments for which the encoded code includes one or more logic states “0” defined by the absence of a resonator 3, the encoding area(s) defining said logic state “0” includes none of said separate spatial locations.

[0067] For the embodiment illustrated in FIG. 1, element 4 is a transmission line, particularly a CPW, electrically fed through an input port P1 and generating the above mentioned electrical signal at an output port P2, and the detection means comprise an envelope detector connected to the output port P2 of the CPW for detecting the generated electrical signal and the presence/absence of attenuation peaks thereon.

[0068] As shown in FIG. 2(a), the CPW 4 has a central conductor strip 4a and two return conductors 4b, 4c, one to either side of the central conductor strip 4a, and separated therefrom by respective slots, and the electromagnetic resonators 3 are S-SRRs, for the illustrated embodiment, arranged such that during the relative movement they pass transversally to the transmission line 4, in a parallel plane, causing attenuation peaks in the electrical signal when any of the electromagnetic resonators 3 is aligned with said slots, particularly, for the illustrated embodiment, when an upper loop of the S of the S-SRR is aligned with the upper slot and a lower loop of the S is aligned with the lower slot.

[0069] In the following, the working principle for the present invention, and the implementation of a prototype of the system of the second aspect of the invention and measurements made thereon will be described.

Working Principle of the Proposed Chipless RFID System:

[0070] Reading of the proposed chipless RFID tags is based on electromagnetic coupling between the tag T and the reader R, a coplanar waveguide (CPW) transmission line fed by a harmonic signal tuned at the resonance frequency of the set of resonators. Note that with the proposed approach, the reading distance is preferably limited to the sub-millimeter scale in order to guarantee line-to-resonator coupling. Different to previous multi-resonator/transmission-line based chipless tags, where the resonant elements and transmission line are etched or printed on the same substrate and communication with the reader is achieved by means of cross polarized antennas (which are essential part of the tag, as well), according to the present invention tag reading is performed by near-field coupling.

[0071] As depicted in FIG. 1, in a reading operation the tag T must be forced to transversally move above the transmission line 4 in close proximity to it, so that electromagnetic coupling between the line 4 and the resonators 3 is possible. Each time a resonator 3 lies on top of the line 4, the coupling prevents signal to be transmitted through the line 4, effectively modulating the injected harmonic signal. Thus, the ID signature is obtained by reading the amplitude of the envelope (output) signal, inferred from an envelope detector. The required proximity between the reader R (specifically, transmission line 4) and the tag T (specifically, set of resonators 3) is not necessarily an issue in certain applications such as those indicated earlier (authentication and security). The main advantage of the present invention is that associated to the fact that the number of bits is not limited by bandwidth constraints, as occurs in multi-resonator based chipless tags. Indeed, in applications such as secure paper (e.g., to avoid counterfeiting or copying of corporate or official documents), it is potentially possible to print the tag resonators along one of the edges, opening the path to the design of chipless RFID tags with unprecedented data capacity.

Tag and Reader Design, Fabrication and Characterization:

[0072] In order to prove the present invention works and provides the above asserted advantages, the present inventors have developed a proof-of-concept working prototype, which is illustrated in FIG. 3.

[0073] The prototype implements encoders, i.e. chipless RFID tags T (shown in FIG. 3(c)) on a Rogers RO4003C substrate 1 with thickness h=203 μm and dielectric constant ε.sub.r=3.55, through a drilling machine. The resonant elements 3 are square S-shaped split ring resonators (S-SRRs), formerly used in [9] for the implementation of multi-resonator chipless tags. Such resonant elements [see the typical topology in FIG. 2(a)] are electrically small and can be properly excited by the counter magnetic field lines generated in the slots of the CPW 4 of the reader R when such particles are aligned and oriented as depicted in FIG. 2(a).

[0074] In order to optimize the area occupied by the S-SRRs, it is necessary to minimize its separation as much as possible. This results in inter-resonator coupling and simultaneous coupling between the line and several S-SRRs, consequently appearing multiple transmission zeros located at positions difficult to predict a priori. To solve this problem, an identical S-SRR 5 has been etched in the back substrate side of the CPW transmission line, but oppositely oriented [see FIG. 2(b)]. By this means, the resonance frequency of the structure that results when the S-SRR 5 of the line 4 and one of the S-SRRs SRRs 3 of the tag chain are just one on top of the other (giving rise to the broad-side coupled S-SRR, BC-S-SRR), is significantly smaller, thereby preventing coupling with the neighbor S-SRRs 3. This means that the frequency of the harmonic feeding signal must be actually tuned to the frequency of the BC-S-SRR.

[0075] The CPW transmission line 4 and the S-SRR 5 of the reader R have been etched on opposite sides 2a, 2b of a Rogers RO3010 substrate 2 (previously called dielectric support) with thickness h=635 μm and dielectric constant ε.sub.r=10.2. The bottom and top photographs of this line (a 50 Ω line) are depicted in FIGS. 3(a) and (b). Such Fig. includes the photograph of a 10-bit tag T [FIG. 3(c)] with all the resonators 3 present and operative (i.e., all states corresponding to the logic ‘1’).

[0076] To characterize such tag-reader system, a guiding channel G has been made in the top side 2b of the CPW 4, as shown in FIG. 3(b). Moreover, the tag T shown in FIG. 3(c), including substrate 1 and resonators 3, has been attached to a rigid substrate (with thickness h=1.6 mm and dielectric constant ε.sub.r=4.7) for mechanical stability. Said rigid substrate is not seen in FIG. 3(c) because it remains under substrate 1 and is thus hidden by the same.

[0077] FIG. 4 depicts the measured frequency response for different positions of the tag T in the vicinity of the reference position (corresponding to a perfect alignment between the S-SRR 5 of the reader R and one of the S-SRRs 3 of the tag T). It can be seen that when the tag T is not situated in the reference position, the attenuation at the reference frequency (the notch frequency of the BC-S-SRR) severely decreases.

[0078] A complete displacement of the fabricated tag T above the CPW 4 has been carried out, as it is required in a reading operation, and recorded the attenuation at the reference frequency. The result, depicted in FIG. 5, reveals the presence of 10 attenuation peaks, corresponding to the ten S-SRRs 3 of the considered tag T.

Experimental Setup and Tag Reading Operation:

[0079] The proof-of-concept of the chipless RFID system of FIG. 1 is implemented as follows. The harmonic signal is generated by means of the Agilent E44338C signal generator, whereas an oscilloscope (Agilent MSO-X-3104A) is used for tag readout. The envelope of the modulated signal at the output of the CPW 4 is extracted by means of an isolator (ATM ATc4-8), used to prevent reflections, and a Schottky diode (Avago HSMS-2860), acting as envelope detector.

[0080] With this experimental setup, the ID signatures of three fabricated encoders, i.e. chipless RFID tags T, have been obtained. To this end, the time-varying envelope variation within temporal windows of predefined time (t.sub.w) is recorded, providing the ID code, as illustrated in FIG. 6. The minima are not exactly at same positions within the readout windows since the in-house guiding system does not guarantee uniform displacement velocity of the tag T.

[0081] For the codes of the two lower plots of FIG. 6, the prototype of the tags T are as that shown in FIG. 3(c) but with the S-SRRs 3 placed at the encoding areas corresponding to the “0” logic states physically altered to become inoperative or with said encoding areas empty of any S-SRR 3.

[0082] The obtained results point out the potential of the present invention for achieving chipless RFID encoders with unprecedented data capacity, useful in applications such as authentication or security, where the reading distance can be sacrificed in favor of the number of bits. The number of bits can be significantly increased by simply adding further S-SRRs to the codes. Thus, high data capacity can be achieved without penalizing the complexity of the reader.

[0083] A person skilled in the art could introduce changes and modifications in the embodiments described without departing from the scope of the invention as it is defined in the attached claims.