Method and system for localizing objects using passive RFID tags which identifies the RFID with an LED

Method and system for localizing an object in a stack of objects. A passive radio frequency identification (RFID) tag is attached to each object. Each RFID tag includes a unique identifier of the object to which the RFID tag is attached. A RFID reader has multiple antennas. Each antenna is positioned to transmit a signal that can be detected by different RFID tags attached to the objects within a reading range of the transmitted signal from each antenna. The antennas are sequentially selected and powered to transmit to the stack a read signal including object's identifier for the object to be localized. If a response to the read signal is received by a selected and powered antenna from a RFID tag that matched the object's identifier with its unique identifier, then a light emitting diode (LED) is lighted to identify a location of the object being localized.

FIELD OF THE INVENTION

The present invention relates generally to the methods and systems for localizing objects and more specifically to a method and systems for localizing an object among a set of stacked similar objects equipped with passive RFID tags.

BACKGROUND OF THE INVENTION

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 thanks to their various formats, colors, sizes and materials. So 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 so 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 CD's or DVD's can record any type of information, not only text and images as books did, but also sound and video. The result is that state of the art mediatheques are now with shelves full of objects that follow the same format. Finding a given object within such a mediatheque becomes much more demanding as it was in the past.

To overcome this difficulty, the RFID technology provides an interesting capability allowing to uniquely identify an 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 Radio Frequency Identifier (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.

The current RFID technology allows to assign a unique identifier to an RFID tag, so that this tag can be uniquely identified when read by an RFID reader. Establishing a one-to-one relationship between the RFID tag and the object it is attached to, allows consequently to uniquely identify a given object among a set of objects. Thus, an obvious solution for localizing objects in shelves consists in sticking an RFID tag onto each object, to associate each object with the stuck RFID tag, and then to read the RFID tag identifier thanks to 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 either asks for a tedious and precise manual operation, or to put in place 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 important due to the fact that they have to include a power source (like a battery) bringing stringent form factor constraints.

Therefore, there is a need for a method and systems for identifying objects in mediatheques, using passive RFID tags.

SUMMARY OF THE INVENTION

Thus, it is a broad object of the invention to remedy the shortcomings of the prior art as described here above.

It is another object of the invention to provide improved RFID reader comprising several antenna that can be activated sequentially.

It is a further object of the invention to provide improved RFID reader comprising several antenna that can be activated sequentially, for localizing a passive RFID tag among a set passive RFID tags.

It is a further object of the invention to provide improved RFID reader comprising several antenna that can be activated sequentially, and adapted to provide enough energy to passive RFID tags equipped with visual indication means for activating such indication device.

It is still a further object of the invention to provide improved passive RFID tags having visual indication means for improving tag localization.

The accomplishment of these and other related objects is achieved by a method in an electronic tag reader for localizing an electronic tag having a predetermined identifier among a set of electronic tags, said electronic tag reader comprising a plurality of antennas, said method comprising the steps of,selecting at least one antenna among said plurality of antennas;powering said selected at least one antenna;interrogating the electronic tags localized in the reading range of said selected at least one antenna; and,if said electronic tag having said predetermined identifier is not detected within the reading range of said selected at least one antenna, stopping powering said selected at least one antenna, deselecting said selected at least one antenna, selecting at least one antenna of said plurality of antennas, different from the previously selected at least one antenna, and repeating the last two steps;

and by a passive electronic tag to be used in conjunction with the above method, said passive electronic tag comprising visual indication means adapted to indicate the position of said passive electronic tag when said passive electronic tag is activated.

Further embodiments of the invention are provided in the appended dependent claims.

Further advantages of the present invention will become apparent to the ones skilled in the art upon examination of the drawings and detailed description. It is intended that any additional advantages be incorporated herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As mentioned above, the proposed invention aims to address the problem of identifying a mediatheque object, with passive RFID tags that allow short reading range, typically less than 10 inches. According to a first embodiment, a new passive RFID tag comprises visual indication means e.g., Light Emitting Diode (LED). In the following description, this improved passive RFID tag is referred to as the “Led RFID tag”, or LRFID for short.

RFID Systems

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. An 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 reader. The system works basically as two separate antennas, one on the RFID tag and the other on the reader. The read data can either be transmitted directly to another system like a host computer through standard interfaces, or it can be stored in a portable reader and later uploaded to the computer for data processing. An RFID tag system works effectively in environments with excessive dirt, dust, moisture, and/or poor visibility. It generally overcomes the limitations of other automatic identification approaches.

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, they are powered by the reader using an induction mechanism (an electromagnetic field is emitted by the 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 reader, carrying the data stored in the RFID tag. Active RFID tags comprise a battery to transmit a signal to a reader. A signal is emitted at a predefined interval or transmit only when addressed by a reader.

When a passive High Frequency (HF) RFID tag is to be read, the 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, using the energy stored within its capacitor as its power source. Generally, 128 bits, including error detection information, are transmitted over a period of 20 ms. This data is picked up by the receiving antenna and decoded by the reader. Once all the data has been transmitted, the storage capacitor is discharged, resetting the RFID tag to make it ready for the next read cycle. The period between transmission pulses is known as the ‘sync time’ and lasts between 20 ms and 50 ms depending on the system setup. The transmission technique used between the RFID tag and the reader is Frequency Shift Keying (FSK) with transmissions generally 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.

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. Its content can not be modified. The write-once RFID tags differ from the read-only RFID tags in that they 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 in the limit of the memory technology. Generally, the number of write cycles is limited to about 500,000 while the number of read cycles is not limited. A detailed technical analysis of RFID tag is disclosed e.g., in RFID (McGraw-Hill Networking Professional) by Steven Shepard, edition Hardcover.

FIG. 1depicts an example of the architecture of a passive HF or Ultra High Frequency (UHF) RFID tag100. As shown, the dipole antenna comprising two parts105-1and105-2is connected to a power generating circuit110that provides current from received signal to the logic and memory circuit115, to the demodulator120, and to the modulator125. The input of demodulator120is connected to the antenna (105-1and105-2) for receiving the signal and for transmitting the received signal to the logic and memory circuit115, after having demodulated the received signal. The input of modulator125is connected to the logic and memory circuit115for receiving the signal to be transmitted. The output of modulator125is connected to the antenna (105-1and105-2) for transmitting the signal after it has been modulated in modulator125.

The architecture of a semi-passive RFID tag is similar to the one represented onFIG. 1, the main difference being the presence of a power supply that allows it to function with much lower signal power levels, resulting in greater reading distances. Semi-passive tags do not have an integrated transmitter contrarily to active tags that comprise a battery and an active transmitter allowing them to generate high frequency energy and to apply it to the antenna.

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 is absorbed to power the tag and a small part 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, 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., it is modulated.

FIG. 2, comprisingFIGS. 2aand2b, shows an RFID system200. As depicted onFIG. 2a, RFID system200comprises a reader205having an antenna210. The antenna210emits a signal215that is received by an RFID tag220. Signal215is reflected in RFID tag220and re-emitted as illustrated with dotted lines referred to as225.FIG. 2billustrates the signal215emitted by the antenna210of the reader205and the signal225reflected by the RFID tag220. As shown onFIG. 2b, the reflected signal225is modulated.

Led RFID Tags for Identifying Objects in Shelves

The main characteristics of the LRFID tag are,short reading range, typically less than 10 inches;visual identification of a targeted LRFID tag, thanks to an imbedded tiny LED;convenient form factor allowing to stick to or imbed the LRFID tag in the objects;low production costs; and,power scheme based on energy received by the RFID antenna.

These features will be better understood by examiningFIG. 3, comprisingFIGS. 3ato3dand illustrating the LRFID tag and the LRFID tag attached to an object and to a set of objects.FIG. 3adepicts the LRFID tag itself whileFIGS. 3band3cshow Compact Disc (CD) boxes on which a LRFID tag is attached to andFIG. 3dillustrates the spine of CD boxes on which LRFID tags are attached to.

As illustrated onFIG. 3a, the LRFID300comprises an RFID chip305, an antenna310, and a LED315, or any equivalent lighting device. RFID chip305is connected to antenna310to receive data and/or control commands as well as to receive power as discussed above. LED315is controlled by RFID chip305so that it can be powered upon 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.

FIGS. 3band3cshow an example where a LRFID300is attached to a CD box320. LRFID300is preferably stuck on the spine of the CD box320so that LED315is visible when the CD box is stacked with others. As illustrated onFIG. 3d, when CD boxes320-1to320-nare stacked, the LED are visible e.g., LED315-1of LRFID tag300-1attached to CD box320-1. This arrangement allows to show on a column the set of LEDs, so that any searched LRFID excited by a RFID reader lights it for being easily identified.

With the arrangement described onFIG. 3, the identification of a given CD box becomes easy. The user must first select the CD to be searched. Then, he/she must identify the associated identifier, thanks to some defined relationship between a CD and an identifier. Such a relationship is beyond the scope of the present invention, but will typically correspond in a preferred 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, so that all LRFID in range receive a reading trigger. Each passive RFID receiving this reading trigger carrying the identifier compares the received identifier with its own one. If they do not match, the passive RFID does not react. If they match, the passive RFID reacts by responding to the reader and, if available, by lighting its LED. This allows the user to immediately identify the searched CD.

As mentioned before, reader antenna used with passive RFID must be very close to the RFID antenna and has a very short range coverage. Consequently, according to the invention, the RFID reader325comprises several antennas330-1to330-p, used to cover all subsets of the set of CD boxes e.g., antenna330-1is adapted to access the LRFID of CD boxes320-1to320-5. The antennas are powered sequentially. When a passive RFID responds, the reader determines the strength of the received signal and memorizes it with the corresponding antenna. Then, the reader powers the neighboring antennas to determine which antenna is the nearest from responding RFID i.e., which antenna receives the most powerful response from the RFID tag. The energy is sent to the determined antenna so that the responding RFID still receives power for powering its LED. It is to be noticed that since RFID's identifiers are unique, only one RFID tag can respond to a specific request. As EN 302 208-1 standard allows 2 Watts ERP emission in Europe (4 Watts EIRP allowed in US) this allows to provide enough energy to the passive RFID to light its LED. Alternately, the antenna powering sequence can be stopped as soon as an RFID tag responds, as illustrated onFIG. 4b(for sake of illustration).

FIGS. 4aand4b, illustrate the behaviour of the RFID reader for activating sequentially its antennas, when no LRFID responds and when an LRFID responds, respectively. The horizontal axis represents time while the vertical axis represents the antenna references. For sake of illustration, the RFID reader comprises three antennas. Turning toFIG. 4a, during period400, the first antenna is powered. After a setup period, TS(440), the RFID reader transmits a read command (405) for activating the LRFID tag having a predetermined identifier and waits for a response during a response period, TR(445). If not any LRFID tag is responding during the period TR, the powering of the antenna is stopped, the next antenna is selected and powered. Similarly during period410, the second antenna is powered. After a setup period, the RFID reader transmits a read command (415) and waits for a response for a predetermined response period. If not any LRFID tag is responding, the powering of the antenna is stopped and the next antenna is selected and powered. Likewise during period420, the third antenna is powered. After a setup period, the RFID reader transmits a read command (425) and waits for a response for a predetermined response period. If not any LRFID tag is responding, the powering of the antenna is stopped and the next antenna i.e., the first antenna, is selected and powered. The process loops for powering sequentially each antenna, returning to the first antenna after the last one has been selected and no LRFID has responded.

Turning toFIG. 4b, during period400′, the first antenna is powered. After a setup period, the RFID reader transmits a read command (405′) for activating the LRFID tag having a predetermined identifier and waits for a response during a response period. If not any LRFID tag is responding during the response period, the powering of the antenna is stopped, and the next antenna is selected and powered. Similarly during period410′, the second antenna is powered. After a setup period, TS(440′), the RFID reader transmits a read command (415′) and waits for a response for a predetermined response period, TR(445′). If the LRFID tag having the predetermined identifier is responding (450), the powering of the antenna is maintained for a period420′ and the process continues without selecting a different antenna. After the setup period, the RFID reader transmits a read command (425′) and waits for a response for a predetermined response period. Again, the LRFID tag having the predetermined identifier is responding (455). The process can be stopped manually by the user or after a predetermined delay, determined so that the user has enough time to see the lighted LED.

FIG. 5illustrates an example of the algorithm implemented within the RFID reader for activating sequentially its antennas, searching for a specific LRFID tag identified by an identifier “X”. This method is based on the following functions and messages,SearchRFID(X) is the request specifying that a LRFID tag with identifier X is searched. This function, when specifying a null parameter X, is used to stop any former outstanding search;RFIDfound(X, FoundA) is the response to the former function, specifying that the LRFID tag with identifier X has been found on the antenna having index “FoundA”;ReadRFID(X) is the message sent onto an antenna for interrogating the LRFID tags within the range of the antenna for localizing the LRFID tag having identifier “X”;RFIDAnswer(X, newRSSI) is the response to the former message, specifying that a LRFID tag with identifier “X” has replied, and that the reply was received with a signal strength equal to newRSSI.

According toFIG. 5, the antenna activating method corresponds to the following steps,step501: the method starts;step502: the parameters are initialized,parameter N holds the number of antennas;parameter A, identifying the active antenna, is set equal to 0 (this parameter can vary between 0 and N−1)parameter FoundA, identifying on which antenna the RFID tag has been found, is set equal to null;parameter RFIDfound, a Boolean specifying if the search RFID tag has been found or not, is set equal to the value FALSE;parameter RSSI, specifying the received signal strength of the RFID tag answer, is set equal to zero; and,the RFID parameter, identifying the searched RFID tag, is set equal to null;step503: a timer is started, with a time-out duration equal to TS;step504: the antenna with index A is powered, so that all the RFID tags within the range of this antenna receive radiated energy;step505: a waiting state is entered, waiting for events to proceed, corresponding either to step506or to step510;step506: the time-out event is received, informing that the timer started at step503has lasted for a time duration TS;step507: a timer is started, with a time-out duration equal to TR;step508: a test is performed to check if the parameter RFID is equal to null. If it is the case, then control is given to step512; otherwise control is given to step509;step509: the message ReadRFID(X) is issued on the current antenna, interrogating the RFID tag with identifier X. Then control is given to step512;step510: the SearchRFID(X) message is received, asking to search for a RFID tag with identifier X;step511: some local variables are updated. The parameter FoundA, identifying on which antenna the RFID tag has been found, is set equal to null. The parameter RFIDfound, a Boolean specifying if the search RFID tag has been found, is set equal to the value FALSE. The parameter RSSI, specifying the received signal strength of the RFID tag answer, is set equal to zero. The RFID parameter, identifying the searched RFID tag, is set equal to X;step512: a waiting state is entered, waiting for events to proceed, corresponding either to step513or to step515;step513: the time-out event is received, informing that the timer started at step507has lasted for a time duration; TRstep514: a test is performed to check if the Boolean parameter RFIDfound is TRUE and if the parameter FoundA is equal to null. If it is the case, then control is given to step503; otherwise control is given to step521;step515: the RFIDAnswer(X,newRSSI) message is received, specifying that a RFID tag with identifier “X” has replied, and that the reply was received with a signal strength equal to newRSSI;step516: the timer started at step507is stopped;step517: a test is performed to check if the Boolean parameter RFIDfound is TRUE and if the parameter FoundA is equal to null. If it is the case, then control is given to step503; otherwise control is given to step518;step518, the Boolean parameter RFIDfound is set equal to TRUE;step519: a test is performed to check if the value of the parameter newRSSI is greater than the value of the parameter RSSI. If it is the case, then control is given to step520; otherwise control is given to step521;step520: the parameter FoundA is set equal to the parameter A and the parameter RSSI is set equal to the parameter newRSSI;step521: the parameter A is incremented by one modulo N. This means that if A was before equal to N−1, its next value becomes 0;step522: a test is performed to check if the parameter FoundA is equal to the parameter A. If it is the case, then control is given to step523; otherwise control is given to step503;step523: the message RFIDfound(X, FoundA) is issued, specifying that the RFID tag with identifier X has been found on the antenna with index “FoundA”;step524: the parameter FoundA is set equal to null. Then control is given back to the step503;

According to the algorithm disclosed above, the different antennas of the RFID reader are sequentially powered until the searched RFID tag is identified and the received signal is the strongest. When the searched RFID tag is identified with the strongest signal, the sequential process of powering the antennas is stopped and the power of the antenna having identified the searched RFID tag with the strongest received signal is maintained.

ALTERNATE EMBODIMENT

To reduce the cost of RFID attached to CD boxes (LRFID in the first embodiment), or more generally to reduce the cost of RFID attached to the searched objects, a simple, low cost and classical passive RFID may be used. In such case, the RFID reader comprises visual indication means associated to each reader's antenna.

Basically, a user utilizes a multi-antennas RFID reader, fed with the identifier, so that all passive RFID tags in the reading range of the antennas sequentially receive a reading trigger. Each passive RFID receiving this reading trigger carrying the identifier compares the received identifier with its own one. If they do not match, the passive RFID does not react. If they match, then the passive RFID reacts by responding to the reader. The reader, when powering each antenna record received RFID tag signal level to determines which antenna is the nearest from responding RFID and light the LED associated to this antenna. This allows the user to immediately identify the area where is the searched object.

FIG. 6depicts a set of CD boxes each equipped with a standard passive RFID tag, and a RFID reader having several antennas that can be activated sequentially according to the algorithm described above. The RFID reader is connected to a set of LED, or any equivalent lighting device, adapted to identify each subset of CD boxes, corresponding to each reader's antenna. When the RFID having the searched identifier is detected by an antenna of the RFID reader, the response signal strength is memorized with the corresponding antenna. Then, the reader powers the neighboring antennas to determine which one is the nearest from responding RFID i.e., which antenna receives the most powerful response from the RFID tag, and the LED associated to this antenna is lighted. If the received signal strength is approximately the same for two antennas, the LED associated to these two antenna can be both lighted. The lighted LED does not give the exact position of the responding RFID, it gives an indication of where it is.

It is to be understood that both disclosed embodiments (standard passive RFID tags and improved RFID tags having a LED or any equivalent lighting device) are fully compatible and may be both implemented together.

The present invention provides an apparatus comprising means adapted for carrying out each step of the methods of the present invention. The present invention provides a computer-like readable medium comprising instructions for carrying out each step of the methods of the present invention.

Naturally, 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, however, are included within the scope of protection of the invention as defined by the following claims.