Abstract:
A chipless RFID tag comprises a plurality of disjoint parallel conducting bands formed on a dielectric support, in which conducting bridges interlink neighboring conducting bands, the conducting bridges delimiting, between the conducting bands, portions of dielectric bands of distinct lengths, each portion of dielectric brand determining a resonant frequency of the tag, the set of resonant frequencies of the tag defining an identification code.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a tag capable of storing data remotely readable by an adapted read terminal. The invention more specifically aims at a technology of remote identification by electromagnetic waves, currently called RFID, for “Radio Frequency Identification”, in the art. Devices operating at frequencies ranging between 10 MHz and 10 THz are more specifically considered herein. 
       DISCUSSION OF PRIOR ART 
       [0002]    Data exchange systems in RFID technology are currently used to recognize and/or to identify, at small or medium distance, all types of objects bearing an adapted tag. 
         [0003]      FIG. 1  very schematically illustrates a remote identification system in RFID technology, comprising a read terminal  1  and an identification tag (TAG)  3 . Read terminal  1  especially comprises an antenna, coupled to a radio wave transceiver device. Tag  3  contains identification data and is capable, when placed close to the read terminal, of receiving the signal transmitted by the read terminal and of specifically interfering with this signal according to its identification data. This interaction is detected by read terminal  1 , which can deduce the tag identification data therefrom. 
         [0004]    There mainly exist two types of RFID tags, that is, tags comprising an integrated electronic circuit, called chip tags, and tags comprising no integrated electronic circuit, generally called chipless tags in the art. 
         [0005]    RFID chip tags generally comprise an antenna, an electronic circuit, a memory for storing an identification code, and a transponder for receiving the signal transmitted by the read terminal and for transmitting as a response, in a determined frequency band, a modulated signal containing the identification code stored in the memory. Some RFID chip tags, called active tags, comprise a battery for powering the chip. In other RFID tags, called passive tags, part of the power carried by the radio waves transmitted by the read terminal is used to power the chip. Passive tags have the advantage of requiring no internal power supply. 
         [0006]    Due to the presence of electronic circuits in RFID chip tags, such tags have a non-negligible cost. The forming of chipless tags has been provided to decrease this cost. RFID chipless tags are considered herein. 
         [0007]      FIG. 2  is a perspective view schematically showing an example of chipless RFID tag  21 . Tag  21  is formed from a dielectric substrate  23 , for example, having the shape of a rectangular wafer of 18×35 mm, with a thickness of approximately 1 mm. The rear surface of substrate  23  is covered with a metal ground plane  25 . On the upper surface side of substrate  23  are formed separate parallel conductive strips, five strips  27   a  to  27   e  in the present example. Strips  27   a  to  27   e  differ from one another by their dimensions (length and/or width) and by their surface areas. 
         [0008]    Tag  21  forms a structure with resonant elements capable of interfering with a radio signal transmitted by an RFID read terminal (not shown). Each conductive strip  27   a  to  27   e  behaves as a resonant LC-type circuit, capable of retransmitting a specific electromagnetic wave that can then be detected by the read terminal. Inductance L especially depends on the length of the conductive strip. Capacitance C corresponds to the capacitance formed between the conductive strip and ground plane  25 , and especially depends on the conductive strip surface area and on the thickness of the substrate as well as on its dielectric properties. Thus, each conductive strip  27   a  to  27   e  determines, by its geometry, a resonance frequency of tag  21 . In this example, each strip  27   a  to  27   e  defines a specific resonance frequency ranging between 5 and 6 GHz. 
         [0009]    In operation, the read terminal transmits a radio signal having a spectrum comprising all the resonance frequencies of the tags that it is likely to read. If tag  21  is close to the read terminal, the read terminal detects a peak (and/or a trough) of the signal at the resonance frequencies determined by strips  27   a  to  27   e,  which translates as the appearing of five different lines in the power spectrum of the radio signal. The positions of these five strips in the spectrum enable the read terminal to uniquely identify tag  21 . 
         [0010]    Chipless RFID tags are passive by nature since they require no electric power supply. 
         [0011]    Although the tags described in relation with  FIG. 2  are less expensive to manufacture than chip tags, their cost however remains non negligible. This is especially due to the fact that the support substrate used to form the tag should comprise a ground plane and have a specific thickness and well-defined dielectric properties. 
         [0012]    It would be desirable to have tags of very low cost, which can especially be used as disposable identification devices, for example, in food packaging. 
         [0013]    Further, the data storage capacity per surface area unit of chipless tags of the type described in relation with  FIG. 2  is relatively low. In the example of  FIG. 2 , a tag of 18×35 mm, operating at frequencies approximately ranging from 5 to 6 GHz, only enables to store a five-bit code. It should be noted that by increasing the operating frequency range, the tag size can be decreased. It would however be desirable to have chipless RFID cards having a greater storage capacity per surface area unit, for a given operating frequency range. 
       SUMMARY 
       [0014]    Thus, an object of an embodiment of the present invention is to provide a chipless RFID tag at least partly overcoming some of the disadvantages of conventional chipless RFID tags. 
         [0015]    An object of an embodiment of the present invention is to provide such a tag which is less expensive and easier to manufacture than conventional chipless RFID tags. 
         [0016]    An object of an embodiment of the present invention is to provide such a tag that can be easily formed on any type of support, for example, by simple printing or screen printing of conductive tracks on a single surface of any type of package (for example, made of cardboard or paper). 
         [0017]    An object of an embodiment of the present invention is to provide such a tag enabling to store more data per surface area unit than conventional chipless RFID tags. 
         [0018]    Thus, an embodiment of the present invention provides a chipless RFID tag comprising a plurality of separate parallel conductive strips formed on a dielectric support, wherein conductive bridges interconnect neighboring conductive strips, the conductive bridges delimiting, between the conductive strips, portions of dielectric strips of different lengths, each dielectric strip portion determining a resonance frequency of the tag, the resonance frequencies of the tag altogether defining an identification code. 
         [0019]    According to an embodiment of the present invention, a dielectric strip, not shorted by a conductive bridge, is arranged between each pair of neighboring conductive strips interconnected by a conductive bridge. 
         [0020]    According to an embodiment of the present invention, all neighboring conductive strips are interconnected by conductive bridges. 
         [0021]    According to an embodiment of the present invention, the width of a conductive strip comprised between two neighboring dielectric strips is at least equal to three times the width of the adjacent conductive strips. 
         [0022]    According to an embodiment of the present invention, the conductive strips are U-shaped in top view. 
         [0023]    According to an embodiment of the present invention, the conductive strips have the shape of portions of circles in top view. 
         [0024]    According to an embodiment of the present invention, the conductive strips are rectilinear, pairs of neighboring strips having the same length and pairs of neighboring strips having different lengths. 
         [0025]    According to an embodiment of the present invention, the portions of dielectric strips all have the same width. 
         [0026]    Another embodiment of the present invention provides a method for coding data readable by an electromagnetic wave transceiver, comprising the steps of: forming a plurality of separate parallel conductive strips on a dielectric support; forming conductive bridges interconnecting neighboring conductive strips, so that the conductive bridges delimit, between the conductive strips, portions of dielectric strips of different lengths, each dielectric strip portion determining a resonance frequency; and associating with each resonance frequency a portion of the data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The foregoing and other objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0028]      FIG. 1 , previously described, very schematically illustrates a remote identification system in RFID technology; 
           [0029]      FIG. 2 , previously described, is a perspective view schematically showing a chipless RFID tag; 
           [0030]      FIG. 3  is a top view schematically showing an embodiment of a chipless RFID tag; 
           [0031]      FIG. 4  is a top view schematically showing another embodiment of a chipless RFID tag; 
           [0032]      FIG. 5  is a top view schematically showing another embodiment of a chipless RFID tag; 
           [0033]      FIG. 6  schematically shows the power spectrum of the electromagnetic signal seen by a read terminal in the presence of the tag of  FIG. 5 ; 
           [0034]      FIG. 7  is a top view schematically showing three alternative embodiments of the tag of  FIG. 5 ; 
           [0035]      FIG. 8  schematically shows the superposition of the power spectrums of the electromagnetic signal seen by a read terminal in the presence of each of the tags of  FIG. 7 ; 
           [0036]      FIG. 9  schematically shows another embodiment of a chipless RFID tag; and 
           [0037]      FIG. 10  schematically shows another embodiment of a chipless RFID tag. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. 
         [0039]      FIG. 3  is a top view schematically showing an embodiment of a chipless RFID tag  31 . Tag  31  is formed on a dielectric support  33  and supports conductive patterns on a single one of its two surfaces. Four separate parallel rectilinear conductive strips  35   a  to  35   d  are formed on this surface. Strips  35   a  to  35   d  are identical, aligned along a direction perpendicular to the strips, and spaced apart from one another by a same step. Thus, conductive strips  35   a  to  35   d  delimit three identical rectilinear dielectric strips  37   a  to  37   c.  Conductive bridges interconnect neighboring conductive strips to delimit, between the conductive strips, portions of dielectric strips of different lengths. In this example, two conductive bridges  38   a  and  38   b  respectively interconnect neighboring left-hand conductive strips  35   a  and  35   b  and neighboring right-hand conductive strips  35   c  and  35   d.  Thus, each of the left-hand and right-hand dielectric strips, respectively  37   a  and  37   c,  is divided into two portions of dielectric strips. The tag thus comprises four portions of dielectric strips of different lengths  39   a  to  39   d.  Central dielectric strip  37   b  thus is not shorted by a conductive bridge. 
         [0040]    Tag  31  forms a structure with resonant elements capable of interfering with an electromagnetic signal transmitted by an RFID read terminal (not shown). Each dielectric strip portion  39   a  to  39   d  is mainly surrounded with a U-shaped conductive path. Thus, each dielectric strip portion  39   a  to  39   d  defines an LC-type resonant circuit capable of retransmitting a specific electromagnetic wave which can then be detected by the read terminal. Inductance L especially depends on the length of the U-shaped conductive path, and thus on the length of the dielectric strip portion. The two parallel branches of the U-shaped conductive path, separated by the dielectric strip portion, form capacitance C. Thus, each dielectric strip portion  39   a  to  39   d  determines, by its length, a resonance frequency of tag  31 . The resonance frequencies of the tag altogether define an identification code. The tag identifier is thus especially determined by the length and/or the position of conductive bridges  38   a  and  38   b.    
         [0041]    According to an example of an RFID tag forming method, tags comprising the basic pattern created by the parallel conductive strips may be formed at a large scale, and the final user may be given the possibility of forming the conductive bridges by himself, for example, by printing with a conductive ink. An advantage of such a method is that it enables the final user to customize the identifiers of its tags. 
         [0042]    Central dielectric strip  37   b,  non-shorted by a conductive bridge, has the function of avoiding stray coupling phenomena between resonant regions of the tag. Thus, a modification of the length of a dielectric strip portion causes a modification of the resonance frequency associated with this strip portion, but has no influence upon the resonance frequencies associated with the other strip portions. 
         [0043]      FIG. 4  is a top view schematically showing another embodiment of a chipless RFID tag  41 . Tag  41  is formed on a dielectric support  43 . On one surface of support  43  are formed three separate parallel rectilinear conductive strips  45   a  to  45   c.  Strips  45   a  to  45   c  are spaced apart from one another by a same step. Thus, conductive strips  45   a  to  45   c  delimit three identical rectilinear dielectric strips  47   a  and  47   b.  Conductive bridges interconnect the neighboring conductive strips to delimit, between the conductive strips, portions of dielectric strips of different lengths. In this example, two conductive bridges  48   a  and  48   b  respectively interconnect neighboring conductive strips  45   a  and  45   b  and neighboring conductive strips  45   b  and  45   c.  Thus, each of dielectric strips  47   a  and  47   b  is divided into two portions of dielectric strips of different lengths. The tag thus comprises four portions of dielectric strips of different lengths  49   a  to  49   d.  Unlike tag  31  of  FIG. 3 , tag  41  does not comprise a central dielectric strip not shorted by a conductive bridge. Central conductive strip  45   b  is provided to have a sufficient length, to avoid stray coupling phenomena between resonant slots of the tag. As an example, central strip  45   b  has a width at least equal to three times the width of lateral strips  45   a,    45   c.    
         [0044]    An advantage of RFID tags of the type described in relation with  FIGS. 3 and 4  is that they are easier to manufacture than tags of the type described in relation with  FIG. 2 . Indeed, unlike tag  21  of  FIG. 2 , tags  31  and  41  of  FIGS. 3 and 4  comprise no ground plane. Tags  31  and  41  may be formed, by deposition or by printing with a conductive ink, on a single surface of any dielectric support. Tags may in particular be formed directly on the objects which are desired to be tagged, for example, on food packagings. 
         [0045]      FIG. 5  is a top view schematically showing a preferred alternative embodiment of a chipless RFID tag  51 . Tag  51  is formed on a dielectric support  53 . One surface of support  53  supports separate parallel conductive strips in the shape of interleaved Us. In this example, the tag comprises three conductive strips  55   a  to  55   c,  strips  55   a  and  55   c  respectively being the outer strip and the inner strip of the pattern. The strips are spaced apart from one another by a same step. The two parallel branches of the U formed by inner strip  55   c  are spaced apart by a distance equal to the step separating strips  55   a  to  55   c  from one another. Thus, conductive strips  55   a  to  55   c  delimit two U-shaped dielectric strips,  57   a  and  57   b,  and a rectilinear dielectric strip  57   c,  between the parallel branches of the U formed by strip  55   c.  Conductive bridges  58   a  and  58   b  are formed on outer and inner dielectric strips, respectively  57   a  and  57   c,  thus delimiting three dielectric strip portions  59   a  to  59   c  of different lengths. To avoid stray coupling phenomena between resonant regions of the tag, central dielectric strip  57   b  is not shorted by a conductive bridge. 
         [0046]    Tag  51  forms a structure with resonant elements capable of interfering with an electromagnetic signal transmitted by an RFID read terminal (not shown). As in the case of the RFID tags described in relation with  FIGS. 3 and 4 , each dielectric strip portion  59   a  to  59   c  determines, by its length, a resonance frequency of the tag. The tag resonance frequencies altogether define an identification code. 
         [0047]      FIG. 6  schematically shows the spectrum of the electromagnetic signal seen by a read terminal in the presence of tag  51  of  FIG. 5 . The spectrum comprises three lines  59   a  to  59   c,  respectively at frequencies on the order of 2.6 GHz, 2.2 GHz, and 4.4 GHz, respectively corresponding to the resonance frequencies linked to the dielectric strip portions having the same reference numerals. The shorter the length of a dielectric strip portion, the higher the associated resonance frequency. The read terminal can detect the presence of lines in the signal spectrum and determine the tag identification code. It should be noted that the spectrum peaks may also be used to code the identifier associated with the tag. 
         [0048]      FIG. 7 , substantially identical to  FIG. 5 , schematically shows tag  51  for three different identification codes. The three codes correspond to three different lengths  58   a   1 ,  58   a   2 ,  58   a   3  of conductive bridge  58   a,  thus affecting the length of dielectric strip portion  59   a,  as shown in dotted lines in the drawing. Dielectric strip portions  59   b  and  59   c  have the same length for the three codes. 
         [0049]      FIG. 8  schematically shows the superposition of the power spectrums of the electromagnetic signal seen by a read terminal in the presence of each of the tags of  FIG. 7 . When the length of dielectric strip portion  59   a  varies, the position of the corresponding strip  59   a  in the strip also varies. The spectrum superposition thus comprises three different strips  59   a   1 ,  59   a   2 , and  59   a   3 , corresponding to the three different lengths of dielectric strip portion  59   a.  According to an advantage of the present invention, a length modification of one of the dielectric strip portions has no influence upon the resonance frequencies associated with the other dielectric strip portions. Indeed, the spectrum superposition comprises a single line  59   b  corresponding to the resonance frequency linked to dielectric strip portion  59   b  and a single line  59   c  corresponding to the resonance frequency linked to dielectric strip portion  59   c.  As mentioned hereabove, the spectrum peaks may also be used to code the identifier associated with the tag. 
         [0050]    It may be provided to associate one or several bits of an identification code with each dielectric strip portion. As an example, in the case of tag  51  ( FIGS. 5 and 7 ), it may be provided to associate three bits of an identification code with each dielectric strip portion  59   a  to  59   c.  Each portion  59   a  to  59   c  may then take one of eight different lengths corresponding to eight different resonance frequencies. It will of course be ascertained that there is no overlapping between resonance frequency ranges associated with different dielectric strip portions. 
         [0051]      FIG. 9  schematically shows another alternative embodiment of a chipless RFID tag  91 . Tag  91  is similar to tag  51  of  FIG. 5 , except for the fact that the parallel conductive strips have the shape of concentric circle portions. The tag has substantially the same operating principle as tag  51 . 
         [0052]    An advantage of chipless RFID tags, U-shaped or in circle portions, of the type described in relation with  FIGS. 5 and 9 , is that they enable to store more data per surface area unit than tags of the type described in relation with  FIG. 2 . As an example, tag  51  of  FIG. 5  enables to store a nine-bit identification code (three bits per dielectric strip portion) on a 17.5×15-mm rectangular surface, for an operating frequency range from 2 to 5 GHz. The tag surface area can be strongly decreased by using higher identification frequencies. 
         [0053]      FIG. 10  is a top view schematically showing another alternative embodiment of a chipless RFID tag  101 . Tag  101  is formed on a dielectric support  103 . A surface of support  103  has parallel rectilinear conductive strips  105   a  to  105   f  formed thereon. Strips  105   a  to  105   f  have the same width and are spaced apart from one another by a same step. Pairs of neighboring strips have the same length. In the shown example, neighboring strips  105   a  and  105   b  have a first length, the next neighboring strips  105   c  and  105   d  have a second length greater than the first length, and the next neighboring strips  105   e  and  105   f  have a third length greater than the second length. Thus, conductive strips  105   a  to  105   d  delimit three rectilinear dielectric strips of different lengths  107   a  to  107   c,  respectively between conductive strips  105   a  and  105   b,    105   c  and  105   d,  and  105   e  and  105   f.  Conductive bridges  108   a  to  108   c  are formed, each at one end of one of dielectric strips  107   a  to  107   c,  interconnecting conductive strips of same length. Actually, the configuration of  FIG. 10  is similar to the configuration of  FIG. 3 , with the difference that parallel conductive strips have different lengths and that the conductive bridges are formed at the end of the dielectric strips. Each dielectric strip defines a resonant circuit determining a resonance frequency of tag  101 . The resonance frequencies of the tag altogether define an identification code. 
         [0054]    It may be provided to associate with each dielectric strip  107   a  to  107   c  a bit of an identification code or, as in the example described in relation with  FIG. 7 , several bits of an identification code. 
         [0055]    Specific embodiments of the present invention have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. 
         [0056]    In particular, chipless RFID tag patterns comprising three or four parallel conductive strips have been described hereabove in relation with  FIGS. 3 ,  4 ,  5 ,  9 , and  10 . The present invention is not limited to these specific examples. Patterns comprising a larger number of conductive strips may especially be provided. 
         [0057]    Further, the possibility of associating three bits of an identification code to each dielectric strip portion has been mentioned. The present invention is not limited to this specific case. It may especially be provided to associate a larger number of bits with each dielectric strip portion. However, this will decrease the interval, in the electromagnetic signal spectrum, between two resonance lines corresponding to two different lengths of a same dielectric strip portion. A sufficiently sensitive read terminal should thus be provided. 
         [0058]    Further, RFID tags  51 ,  91 , and  101 , described in relation with  FIGS. 5 ,  9 , and  10 , comprise dielectric strips not shorted by conductive bridges to avoid stray coupling phenomena between the resonant regions of the tag. The present invention is not limited to this specific case. It may be provided to use all dielectric strips for the identification code storage, as in the case of tag  41  of  FIG. 4 . It will then be ascertained to provide a sufficient distance between two dielectric strips to avoid stray coupling phenomena. 
         [0059]    Further, in chipless RFID tags described in relation with  FIGS. 3 ,  4 ,  5 ,  9 , and  10 , all the dielectric strips delimited by parallel conductive strips have the same width. The present invention is not limited to this specific case. One may in particular have dielectric strips of different lengths on a same tag. Similarly, the parallel conductive strips may have different widths. 
         [0060]    Further, although one of the advantages of chipless RFID tags provided hereabove is the possibility of doing away with any conductive ground plane, one may also, for certain uses, and especially in a metal environment, use patterns of the type described in relation with  FIGS. 3 ,  4 ,  5 ,  9 , and  10  in combination with a ground plane.