Abstract:
A Radio Frequency Identification (RFID) structure. The RFID structures includes a tag stack of N essentially identical RFID tags and an object stack of N essentially identical objects. N is at least 2. Each tag is attached to a corresponding object and includes an electronic chip, an antenna, and at least one pair of electrical contacts. The N tags are daisy chained together in electrical contact via the at least one pair of electrical contacts in the tags. The tag stack is configured to receive power from an external power source located outside of the RFID structure. A method for localizing an object in the object stack includes receiving an external signal in each tag via the antenna, and enabling or not enabling a lighting of a lighting device in each tag in dependence upon data in the external signal.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a method and system for localizing objects and more specifically to a method and system for localizing an object among a set of stacked objects equipped with improved Radio Frequency Identification (RFID) tags. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the previous millennium, mediatheques were merely libraries with shelves full of books. Finding a book in a library was not always an easy task to do, but was nevertheless facilitated by their various formats, colors, sizes and materials. Thus, discriminating between a cook book, a dictionary, a comic book, an atlas, a schoolbook, a picture book, a prayer book, a cashbook, an account book, was not difficult. With the recent explosion of electronic media, it is today quite common to find all these different books recorded on a common media following worldwide standards in terms of physical form factor, size and even colors. Either CDs or DVDs can record various types of information, such as text and images as in books did, and also sound and video. The result is that state of the art mediatheques now have shelves full of objects that follow the same format. Finding a given object within such a mediatheque becomes much more demanding than it was in the past. 
         [0003]    To overcome this difficulty, the RFID technology provides an interesting capability to allow unique identification of a Radio Frequency Identification (RFID) tag, and subsequently the object it is attached to. For example, U.S. Pat. No. 6,693,539 discloses an article inventory control system for articles, such as books, using RFID tags attached to the articles. Each tag has a unique identification or serial number for identifying the individual article. An inventory database tracks all of the tagged articles and maintains circulation status information for each article. Articles are checked out of the library using a patron self-checkout system. Checked out articles are returned to the library via patron self-check in devices. The shelves are periodically scanned with a mobile RFID scanner for updating inventory status. 
         [0004]    The current RFID technology allows assignment of a unique identifier to a RFID tag, so that this tag can be uniquely identified when read by a RFID reader. Establishing a one-to-one relationship between the RFID tag and the object it is attached to, allows consequently unique identification of a given object among a set of objects. Thus, an obvious solution for localizing objects in shelves consists of attaching an RFID tag onto each object, to associate each object with the attached RFID tag, and then reading the RFID tag identifier by an RFID reader. To make such a solution affordable, the RFID tags have to be inexpensive, robust and thin, so that only passive RFID tags are considered. This limitation brings a cumbersome constraint as the reading range of passive RFID tags is quite limited, typically few inches. In order to locate a given object within a set of shelves, the reader will have to pass close to each shelf, scanning all of its width. This requires either a tedious and precise manual operation, or use of an expensive robot. Active RFID tags do not suffer from this short reading range, but are unfortunately not well suited, due to their price and more importantly due to the fact that they have to include a power source (such as a battery) which imposes stringent form factor constraints. 
         [0005]    Therefore, there is a need for RFID tags allowing long reading range while being equivalent in terms of size, form factor, and price to the passive RFID tags, for identifying objects in mediatheques. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a Radio Frequency Identification (RFID) structure comprising a tag stack of N essentially identical RFID tags and an object stack of N essentially identical objects: 
         [0007]    wherein N is at least 2; 
         [0008]    wherein each tag of the N tags is attached to a corresponding object of the N objects; 
         [0009]    wherein the N tags are denoted as T 1 , T 2 , . . . , T N ; 
         [0010]    wherein each tag comprises an electronic chip, an antenna, and at least one pair of electrical contacts, said at least one pair of electrical contacts of T 1 , T 2 , . . . , T N  respectively denoted as C 1 , C 2 , . . . , C N ; 
         [0011]    wherein the N tags are daisy chained together such that C i  is in electrical contact with C i+1  for i=1, 2, . . . , N−1; 
         [0012]    wherein in each tag, the antenna is configured to receive an external signal from outside the RFID structure and to transmit the external signal to the chip which is configured to receive the signal from the antenna via an electrical connection between the antenna and the chip; and 
         [0013]    wherein the tag stack is configured to receive power from an external power source located outside of the RFID structure. 
         [0014]    The present invention provides a method for localizing an object in a Radio Frequency Identification (RFID) structure that comprises a tag stack of N essentially identical RFID tags and an object stack of N essentially identical objects, said method comprising: 
         [0015]    receiving by the N tags an external signal from outside the RFID structure,
       wherein N is at least 2,   wherein each tag of the N tags is attached to a corresponding object of the N objects,   wherein the N tags are denoted as T 1 , T 2 , . . . , T N ,   wherein each tag comprises an electronic chip, an antenna, and at least one pair of electrical contacts, said at least one pair of electrical contacts of T 1 , T 2 , . . . , T N  respectively denoted as C 1 , C 2 , . . . , C N ,   wherein the N tags are daisy chained together such that C i  is in electrical contact with C i+1  for i=1, 2, . . . , N−1,   wherein the tag stack is receiving power from an external power source located outside of the RFID structure,   wherein each tag comprises a lighting device such that the chip in each tag is electrically connected to the lighting device in each tag,   wherein in each tag, the antenna receives the external signal; and in each tag,   transmitting the external signal from the antenna to the chip via an electrical connection between the antenna and the chip,   receiving, by the chip, the external signal transmitted by the antenna,   extracting, by the chip, data in the external signal received by the chip, and enabling or not enabling, by the chip, a lighting of the lighting device in dependence upon the extracted data.       
 
         [0027]    The present invention advantageously provides RFID tags allowing long reading range while being equivalent in terms of size, form factor, and price to the passive RFID tags, for identifying objects in mediatheques. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  depicts an example of the architecture of a passive RFID tag. 
           [0029]      FIG. 2A  shows a Radio Frequency Identification (RFID) system with a RFID reader having an antenna and a RFID tag having a dipole antenna. 
           [0030]      FIG. 2B  illustrates the signal emitted by the antenna of the RFID reader and the modulated signal reflected by the RFID tag. 
           [0031]      FIGS. 3A ,  3 B, and  3 C, illustrate the daisy chained RFID tag of the present invention. 
           [0032]      FIG. 4  illustrates several CD boxes, each CD equipped with a daisy chained RFID tag according to the present invention and arranged properly in a shelf. 
           [0033]      FIGS. 5 and 6  depict further examples of the design of daisy chained RFID tags according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    The present invention provides improved powerless Radio Frequency Identification (RFID) tags providing long reading ranges. 
         [0035]    The invention provides improved RFID tags having embedded visual indication means. 
         [0036]    The invention provides improved low cost RFID tags providing long reading ranges. 
         [0037]    The invention provides improved RFID tags of which the power scheme is based on electrical contacts for receiving power from external sources through electrical connections. 
         [0038]    The proposed invention aims to address the problem of identifying a mediatheque object, with an innovative RFID tag which allows long reading range while being equivalent in terms of size, form factor and price to the passive RFID tags. In the following description, DCRFID stands for “Daisy Chained RFID”. 
         [0039]    The core of any RFID system is the ‘Tag’ or ‘Transponder’, which can be attached to or embedded within objects, wherein data can be stored. A RFID reader, generically referred to as reader in the following description, sends out a radio frequency signal to the RFID tag that broadcasts back its stored data to the RFID reader. The system works basically as two separate antennas, one antenna on the RFID tag and the other antenna on the RFID reader. The read data can either be transmitted directly to another system like a host computer through standard interfaces, or the read data can be stored in a portable reader and later uploaded to the computer for data processing. A RFID tag system works effectively in environments with excessive dirt, dust, moisture, and/or poor visibility and generally overcomes the limitations of other automatic identification approaches. 
         [0040]    Several kinds of RFID, such as piezoelectric RFID and electronic RFID, are currently available. For example, passive RFID tags do not require battery for transmission since generally, passive RFID tags are powered by the reader using an induction mechanism (i.e., an electromagnetic field is emitted by the RFID reader antenna and received by an antenna localized on the RFID tag). This power is used by the RFID tag to transmit a signal back to the RFID reader carrying the data stored in the RFID tag. Active RFID tags comprise a battery to transmit a signal to a RFID reader. A signal is emitted at a predefined interval or transmitted only when addressed by a RFID reader. 
         [0041]    When a passive High Frequency (HF) RFID tag is to be read, the RFID reader sends out a power pulse (e.g., a 134.2 KHz power pulse) to the RFID antenna. The magnetic field generated is ‘collected’ by the antenna in the RFID tag that is tuned to the same frequency. This received energy is rectified and stored on a small capacitor within the RFID tag. When the power pulse has finished, the RFID tag immediately transmits back its data to the RFID reader, using the energy stored within its capacitor as its power source. In one embodiment, 128 bits, including error detection information, are transmitted over a period of 20 ms. This transmitted data from the RFID tag is picked up by the receiving antenna of the RFID reader and decoded by the RFID reader. Once all the data has been transmitted from the RFID tag, the storage capacitor of the RFID tag is discharged, resetting the RFID tag to make the RFID tag ready for the next read cycle. The period between transmission pulses is known as the ‘sync time’ and may last between 20 ms and 50 ms depending on the system setup. A transmission technique that may be used between the RFID tag and the reader is Frequency Shift Keying (FSK) with transmissions in one embodiment comprised between 124.2 kHz and 134.2 kHz. This approach has comparatively good resistance to noise while also being very cost effective to implement. Many applications require that RFID tag attached to objects be read while traveling at specific speeds by a readout antenna. 
         [0042]    RFID tags can be read-only, write-once, or read-write. A read-only RFID tag comprises a read-only memory that is loaded during manufacturing process. The content of read-only RFID tag cannot be modified. The write-once RFID tags differ from the read-only RFID tags in that the write-once RFID tags can be programmed by the end-user with the required data (e.g., part number or serial number). The read-write RFID tags allow for full read-write capability, allowing a user to update information stored in a tag as often as possible subject to the limit of the memory technology. Generally, the number of write cycles may be limited (e.g., to about 500,000) while the number of read cycles is not limited. A detailed technical analysis of RFID tag is disclosed in RFID (McGraw-Hill Networking Professional) by Steven Shepard, edition Hardcover. 
         [0043]      FIG. 1  depicts an example of the architecture of a passive High Frequency (HF) or Ultra High Frequency (UHF) RFID tag  100 . As shown, the dipole antenna comprising two parts  105 - 1  and  105 - 2  is connected to a power generating circuit  110  that provides current from a received signal to the logic and memory circuit  115 , to the demodulator  120 , and to the modulator  125 . The input of demodulator  120  is connected to the antenna ( 105 - 1  and  105 - 2 ) for receiving the signal and for transmitting the received signal to the logic and memory circuit  115 , after having demodulated the received signal. The input of modulator  125  is connected to the logic and memory circuit  115  for receiving the signal to be transmitted. The output of modulator  125  is connected to the antenna ( 105 - 1  and  105 - 2 ) for transmitting the signal after the signal has been modulated in modulator  125 . 
         [0044]    The architecture of a semi-passive RFID tag is similar to the one represented in  FIG. 1 , the main difference being the presence of a power supply that allows the semi-passive RFID tag to function with much lower signal power levels, resulting in greater reading distances. Semi-passive tags do not have an integrated transmitter in contrast with active tags that comprise a battery and an active transmitter allowing the active tags to generate high frequency energy and to apply the high frequency energy to the antenna. 
         [0045]    As disclosed in “A basic introduction to RFID technology and its use in the supply chain”, White Paper, Laran RFID, when the propagating wave from the reader collides with tag antenna in the form of a dipole, part of the energy of the propagating wave is absorbed to power the tag and a small part of the energy of the propagating wave is reflected back to the reader in a technique known as back-scatter. Theory dictates that for the optimal energy transfer, the length of the dipole must be equal to half the wave length λ, or λ/2. Generally, the dipole is made up of two λ/4 lengths. Communication from tag to reader is achieved by altering the antenna input impedance in time with the data stream to be transmitted. This results in the power reflected back to the reader being changed in time with the data; i.e., the signal is modulated. 
         [0046]      FIGS. 2A and 2B , shows an RFID system  200 . As depicted in  FIG. 2A , RFID system  200  comprises a reader  205  having an antenna  210 . The antenna  210  emits a signal  215  that is received by an RFID tag  220 . Signal  215  is reflected by RFID tag  220  as illustrated with dotted lines referred to as  225 .  FIG. 2B  illustrates the signal  215  emitted by the antenna  210  of the reader  205  and the signal  225  reflected by the RFID tag  220 . As shown in  FIG. 2B , the reflected signal  225  is modulated. 
         [0047]    RFID tags are autonomous electronic devices of which data can be accessed without any physical contact. Due to their internal power, the reading distance of active or semi-passive RFID tags is greater than the reading distance of passive RFID tag receiving power from their antenna. However, active or semi-passive RFID tags present drawback due to the internal power source that increases costs and reduces life cycle. 
         [0048]    The DCRFID tag of the present invention encompass the architecture of active or semi-passive RFID tags in combination with with external power sources, offering the advantages of the active or semi-passive RFID tags without the drawbacks resulting from the internal power source. 
         [0049]    The main characteristics of the DCRFID tag are: long reading range (typically up to 10 meters); visual identification of a targeted DCRFID tag due to an imbedded tiny Light Emitting Diode (LED); convenient form factor enabling the DCRFID tag to be attached to or embedded in the objects; low production costs; and a power scheme based on electrical contacts enabled by proper stacking of DCRFIDs. 
         [0050]      FIGS. 3A ,  3 B, and  3 C illustrate the DCRFID tag.  FIG. 3A  depicts the DCRFID tag itself while  FIGS. 3B and 3C  show the front view and the rear view, respectively, of a Compact Disc (CD) box on which the DCRFID tag is attached. 
         [0051]    As illustrated on  FIG. 3A , the DCRFID tag  300  comprises two pairs of electrical contacts ( 305 ,  310 ) and ( 315 ,  320 ), an electronic RFID chip  325 , an antenna  330 , and a LED  335  or any equivalent lighting device. The two pairs of electrical contacts ( 305 ,  310 ) and ( 315 ,  320 ) enable receiving and transmitting power from an external source (not represented). For example, contact  305  receives power or current from the external source, contact  315 , being connected to contact  305 , transmits received power or current, and contacts  310  and  320 , being connected together, are also connected to ground. RFID chip  325 , which may be of the active or semi-passive type, is connected to the pairs of electrical contacts ( 305 ,  310 ) and ( 315 ,  320 ) for receiving power. RFID chip  325  is connected to antenna  330  to receive data and/or control commands from a RFID reader. LED  335  (or an equivalent lighting device) is controlled by RFID chip  325  so that LED  335  can be powered according to conditions determined by received instructions and data stored therein. For example, if the received data match the stored data, the LED is powered during a predetermined delay. 
         [0052]    Electrical contacts  305 ,  310 ,  315 , and  320  are arranged in such a way so that the contacts  315  and  320  of a first DCRFID are respectively touching (i.e., in mechanical and electrical contact with) the contacts  305  and  310  of a second DCRFID when these two DCRFID are attached to stacked objects, as described below by reference to  FIG. 4 . 
         [0053]      FIGS. 3B and 3C  show an example where a DCRFID  300  is attached to a CD box  340 . DCRFID  300  is preferably attached to the edge of the CD box so that LED  335  is visible when the CD box is stacked with other CD boxes so that electrical contacts can be established with neighboring DCRFIDs. 
         [0054]      FIG. 4  illustrates several essentially identical CD boxes equipped each with a DCRFID and arranged properly in a shelf. With reference to the  FIG. 4 , a set of ten CD boxes  340 - 1  to  340 - 10  are stacked one above the other, forming a vertical stack. Each CD box is equipped with a DCRFID as described above by reference to  FIG. 3 . DCRFID  300 - 1  is attached to CD box  340 - 1 , DCRFID  300 - 2  is attached to CD box  340 - 2 , and so on. According to this arrangement, the set of LEDs of the DCRFIDs (e.g., LED  335 - 1 ) are aligned in a column, so that any DCRFID tag identified by a reader will light the LED for easy identification of the DCRFID. The DCRFIDs  300 - 1 ,  300 - 2 , . . .  300 - 10  are essentially identical DCRFIDs arranged in a stack of DCRFIDs. This stack of CD boxes sits above a pedestal base  400  comprising two electrical contacts  405  and  410  which receive power from an external power source  415 . These contacts  405  and  410  are aligned with the contacts of the DCRFID, to establish the received voltage between the contacts  305 - 10  and  310 - 10  of the bottom CD box. Since contacts  305 - 10  and  310 - 10  are respectively connected to contacts  315 - 10  and  320 - 10 , and since contacts  315 - 10  and  320 - 10  are respectively connected to the contacts  305 - 9  and  310 - 9 , forming an electrical continuity between contacts  315 - 10  and  305 - 9 , and between contacts  320 - 10  and  310 - 9 , the same voltage is applied to contacts  315 - 10  and  305 - 9 , and so on along this “daisy chain” of contacts, resulting in the same voltage on each CD box from the bottom CD box up to the top CD box. 
         [0055]    With the arrangement described on  FIG. 4 , the unique identification of a given CD box is easy. The user first selects the CD to be searched. Then the user identifies the associated identifier, due to some defined relationship between a CD and an identifier. Such a relationship is beyond the scope of the present invention, but it typically corresponds, in one embodiment of the present invention, to an association with an Electronic Product Code (EPC). Then the user utilizes an RFID reader, fed with the identifier in a signal, so that all DCRFIDs in range receive a reading trigger in the signal from the RFID reader. Each DCRFID receiving this reading trigger carrying the identifier compares the received identifier with its own identifier. If the identifier of the reading trigger and the DCRFIDs own identifier do not match, the DCRFID does not react to the reading trigger. If they match, then the DCRFID reacts by lighting its LED, which allows the user to immediately identify the searched CD. If this CD is pulled out of the vertical stack, then the remaining CD&#39;s, if any, will still be powered due to the propagation of energy from the pedestal base up to the top CD. 
         [0056]      FIGS. 5 and 6  illustrate further examples of DCRFID designs. In  FIG. 5 , the DCRFID  300 ′ comprises a single pair of contacts  305 ′ and  310 ′, a RFID chip  325 ′, an antenna  330 ′, and a LED  335 ′ or any equivalent lighting device. The contacts  305 ′ and  310 ′ are designed in such a way that they can establish electrical contacts with neighboring DCRFIDs as described by reference to  FIG. 4 . To that end, the contacts  305 ′ and  310 ′ extend on each side of the object (e.g., CD) on the edge of which the DCRFID is attached. 
         [0057]      FIG. 6  depicts an embodiment of the DCRFID according to the present invention, further comprising a power controller  600 . Like the DCRFID  300  of  FIG. 3 , the DCRFID  300 ″ of  FIG. 6  comprises two pairs of electrical contacts ( 305 ″,  310 ″) and ( 315 ″,  320 ″), an RFID chip  325 ″, an antenna  330 ″, and a LED  335 ″ or any equivalent lighting device. Each contact  305 ″,  310 ″,  315 ″, and  320 ″ is connected to the power controller  600 . Outputs of the power controller  600  are connected to the RFID chip  325 ″ for powering this chip. The power controller  600  determines which pair of contacts ( 305 ″,  310 ″) or ( 315 ″,  320 ″) is powered and the polarity of the received power. The received power is transmitted to the non-powered pair of contacts ( 305 ″,  310 ″) or ( 315 ″,  320 ″) and to the RFID chip  325 ″ according to a predetermined polarity (i.e., depending upon the received power polarity, the output polarity of the power controller  600  is reversed or not). 
         [0058]    The DCRFID according to  FIG. 6  enables stacking the objects on which the DCRFID is attached on one side or on the other side. For example, some CD boxes of  FIG. 4  equipped with the DCRFID of  FIG. 6  can be stacked upside down. 
         [0059]    Without departing from the spirit of the proposed invention, some enhancements can be proposed along the following points. The stack of CD boxes can be arranged horizontally, while spring means ensure that all CD boxes remain in contact. The layout and the number of the contacts may vary, provided that stacked objects present their contacts as being electrically connected. The power source can be located in the pedestal base, using for instance a battery. 
         [0060]    In order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many modifications and alterations, all of which are included within the scope of protection of the invention.