Detection of multiple articles

Abstract: An identification system comprises an interrogator and a plurality of transponders. The interrogator includes a transmitter which transmits an interrogation signal and an inhibiting signal to the transponders, and a receiver for receiving response signals from the transponders. The interrogator also includes processor means for identifying the transponders from data in the received response signals and means for disabling transponders. The transponders which are disabled remain disabled whilst they continue to receive the inhibiting signal. Each transponder comprises receiving means, a code generator, and transmitter means connected to the code generator. On receipt of the transmitted interrogation signal the transponder transmits a response signal containing data which identifies it.

Description: BACKGROUND OF THE INVENTION This invention relates to an identification system comprising an interrogator and a plurality of transponders. South African Patent application No. 92/0039 describes an identification system comprising an interrogator and a number of individual transponders which may be attached to or associated with articles to be identified. The articles to be identified may be, for example, items of stock in a supermarket or warehouse. It is an object of the invention to increase the probability of transponder identification in a system of the kind referred to above. PRIOR ART EP 494114 A discloses an interrogator/transponder system in which an interrogator broadcasts an interrogation signal to a plurality of transponders present in the interrogation field. One example of the identification system comprises an interrogator or reader which transmits interrogation signals at a power of approximately 15W and at a frequency of approximately 915 MHZ to a number of passive transponders. The transponders derive a power supply from energy in the interrogation signal, and modulate a portion of the energy received from the interrogator with an identification code to generate a response signal, which is transmitted back to the interrogator. GB 2152335 A discloses an inventory control which employs tags which resonate at the unique pair of frequencies and reradiate simultaneously at a third frequency. An interrogator sweeps stepwise through a programmed set of all the pertinent pairs frequencies to interrogate a plurality of tags and the receiver of the interrogator is tuned to the third frequency. The amplitude of third frequency is a function of distance, receiver antenna pattern and the number of tags present. WO 8200910 A discloses an ultrasonic communication and security system. A plurality of Intrusion sensors can be interrogated from a central data unit by signals which are propagated at two different frequencies. The frequencies are sufficiently separated that the locations of nulls due to multipath propagation are spatially separate. SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided an identification system comprising an interrogator and a plurality of transponders, the interrogator including transmitter means for transmitting an interrogation signal to the transponders, receiver means for receiving response signals from the transponders, and processor means for identifying the transponders from data in the response signals; each transponder comprising receiving means for receiving the transmitted interrogation signal, a code generator, and transmitting means connected to the code generator, the transponder being adapted to transmit a response signal containing data which identifies the transponder on receipt of the interrogation signal, and to be disabled by a turn-off instruction in the interrogation signal on successful identification of the transponder, characterised in that the interrogator transmits the interrogation signal and an inhibiting signal, and that a disabled transponder remains disabled while it continues receiving the inhibiting signal. Preferably, the interrogation signal is a relatively directional signal, and the inhibiting signal is a relatively widely dispersed signal. The inhibiting signal has a beam width which is preferably at least 5 times greater than that of the interrogating signal. Preferably, the interrogation signal is a relatively high power signal and the inhibiting signal is a relatively low power signal. The turn-off instruction to the transponder may be transmitted in both the interrogation signal and the inhibiting signal. The system may include processor means for recording data received from each identified transponder and for relating the received data to stored data corresponding to the received data. The processor means may be adapted to store price or identification data of articles to which different transponders are attached, and to relate the identification codes of identified transponders thereto. The system may include display means for generating a display in which the articles to which respective transponders are attached are associated with price data. The system may further include printer means for generating a printout of the display. According to a second aspect of the invention there is provided a method of identifying a plurality of transponders comprising the steps of transmitting an interrogation signal to the transponders, each transponder receiving the interrogation and transmitting a signal, containing data, in response, receiving the response signals from the transponders, identifying transponders from data in the response signals, detecting successful identification of any transponder, disabling any transponder; and characterised in that the interrogation signal and an inhibiting signal are transmitted, and that a disabled transponder remains disabled while it continues receiving the inhibiting signal. According to a third aspect of the invention there is provided a transponder comprising receiving means for receiving an interrogation signal, a code generator, and transmitting means connected to the code generator, the transponder being adapted to transmit a response signal containing data which identifies the transponder on receipt of the interrogation signal, and to be disabled by a turn-off instruction in the interrogation signal, characterised in that the transponder receives the interrogation signal and an inhibiting signal, and that a disabled transponder remains disabled while it continues receiving the inhibiting signal. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram illustrating the creation of a null in an interrogation zone as a result of a reflected signal; Figure 2 is a schematic diagram illustrating a first embodiment of the invention; Figure 3 is a diagram illustrating the effect of using different frequencies for the interrogation signal; Figure 4 is a schematic diagram of a second embodiment of the invention: Figure 5 is a schematic illustration of an alternative embodiment of the invention; and Figure 6 is a block schematic diagram illustrating the operation of the embodiment of Figure 5. DESCRIPTION OF EMBODIMENTS Figure 1 illustrates a problem which occurs in identification systems of the kind referred to when there is a reflecting surface close to the interrogator 10 and/or the interrogation zone in which it is desired to detect transponders. A primary interrogation signal 12 is transmitted directly from the antenna 14 of the interrogator 10 to the interrogation zone, while a secondary interrogation signal 16 is reflected from the reflecting surface. At certain distances from the interrogator, the direct and reflected signals 12 and 16 will be half a wavelength out of phase, causing nulls in the electric field of the interrogation signal. This results in zones 20 of the interrogation zone having a weak interrogation signal, with insufficient RF energy to power up the transponders. As a result of this, certain transponders may go undetected by the interrogator. Figure 2 illustrates schematically a first solution to the problem. In Figure 2, the interrogator 10 is provided with first and second antennas 22 and 24, which are spaced one half wavelength apart, and which can be selected by means of switch means 26. Due to the different spacing of the antennas, the nulls or zones 20 of low power occur at different locations. In use, the interrogator 10 is first connected to the antenna 22 and scans articles in the interrogation zone, recording the identity codes received from the various transponders attached to the articles. The switch means 26 then connects the interrogator 10 to the antenna 24, and the process is repeated. The identification codes recorded during both interrogation procedures are compared, and duplicated codes are discarded. In this way, all of the articles in the interrogation zone can be identified, despite some of them lying in portions of the interrogation zone which are in an RF null of one of the antennas 22 or 24. The above system is adequate for identifying articles which each have a transponder with a unique identification code. However, where a number of articles are provided with transponders all having the same identification code, it is not possible to count the number of articles accurately using the system of Figure 2, since it is not possible to compare the results of the first and second interrogation procedures in such a way as to discard duplicate readings. The system described in South African patent application No. 92/0039, includes a number of identical transponders, which are attached to articles of the same kind, to allow automatic stocktaking. Each transponder is disabled after it has successfully communicated its presence to the interrogator, and remains in a disabled state until the RF field caused by the interrogation signal has been removed completely. Clearly, a system which has deep RF nulls in its interrogation zone would not be suitable for use with this type of tag, as individual tags might interpret the lack of RF power in a null as the turning off of the interrogator. As a result, a transponder which had been disabled after successful identification could be turned on again when the position of the null moves, providing an extra signal and thus causing an incorrect count. The present invention is illustrated in figures 4, 5 and 6. This embodiment of the invention can be applied in stock taking, for example in a book store or a store selling compact discs and cassettes. In such an application, each item of stock in the store, whether it be a book, a compact disc or a cassette, has a transponder fixed to it which identifies the article in question. Using the identification system described in South African patent applications 92/0039 and 93/6267, and using a hand-held interrogator unit or scanner, the articles can be interrogated, and will be identified and counted. As the transponder of each article communicates successfully with the interrogator, it receives a turn-off instruction and shuts down. After a while, the shut-down transponders reset themselves and are ready to respond again to an interrogation signal. A system for implementing this embodiment of the invention is illustrated schematically in Figure 4. In this system, the interrogator comprises an interrogator/controller unit 28, a first transmitter 30 with an associated antenna 32, and a second transmitter 34 with an associated antenna 36. Tags or transponders 38 are shown spaced about within an interrogation zone which is adjacent to a reflective surface 40. Nulls or areas of low RF field intensity 42 and 44 which are spaced apart from one another and do not overlap are shown schematically. A problem may arise in that transponders which have already been interrogated and shut down are reset in this way, and are then accidentally re-illuminated by the beam of the interrogator, so that they are recounted, resulting in stock taking errors. To deal with this possible problem, a hand-held interrogator 108 is provided which transmits a first, narrow interrogator beam (beam 1) and a second wide angle inhibiting beam (beam 2) simultaneously, at different frequencies. The interrogating beam (beam 1) typically has a beam width of approximately 15cm at its maximum operating range of 4 m and is transmitted at a frequency of 915 MHZ and at a power of 6 watts. The inhibiting beam (beam 2) is much broader or more widely dispersed, and has a beam width of approximately 4 m at the same operating distance. The ratio between the width of the inhibiting beam and the interrogating beam is at least 5 to 1, and preferably greater than 10 to 1. In the above example, the ratio is greater than 20 to 1. The inhibiting beam is transmitted at a lower power, typically 3 watts, and at a different frequency (in the prototype system, at 900 MHZ. Figure 6 shows, in a simplified schematic diagram, part of the circuitry used by the hand-held interrogator 108. The circuitry includes a 915 MHz transmitter 110 with a directional transmitting antenna 112 which transmits the interrogating beam (beam 1). The interrogator includes a directional receiving antenna 114 (which maybe separate or which may be the same antenna as the antenna 112) which feeds a receiver 116 with received transponder response signals, which are fed to the decoding and control circuitry 118 of the interrogator. This circuitry also controls a second transmitter 120 which an associated antenna 122 which is less directional than the antenna 112, to transmit the inhibiting beam (beam 2). Due to the higher power and narrower beam width of the interrogating beam (beam 1), this beam provides sufficient power to the transponders illuminated by it to power them up and to cause them to transmit response signals. The inhibiting beam (beam 2) has a lower power and is transmitted over a much greater volume, so that its power is normally insufficient to activate transponders which are currently inactive. However, it transmits sufficient power to prevent successfully detected, disabled transponders from powering down completely, thus preventing them from resetting when the interrogation beam (beam 1) moves away from them. Since the transponders modulate their identity codes by either changing the reflectivity of their receiving antenna, or by reradiating a percentage of the received interrogation signal energy, modulated with the identification code, this data will be transferred on both frequencies. A number of practical applications of the present invention present themselves. In one application, the system is used in a supermarket to automate the check-out procedure, obviating the need for manual scanning or entry of prices using a cash register. In another application, the contents of a store room, warehouse or a truck, for example, can be determined without unloading or unpacking. In another application, articles such as books in a bookstore or library, or compact discs in a music store can be identified and counted, in an automated stock-taking process. It will be appreciated that these examples are merely exemplary, and many other applications of the invention are possible. The circuitry of the transponder is designed to change the effective input impedance of the transponder circuit when the transponder is transmitting its identification code at its onboard oscillator clock rate (typically 10 KHz), thereby changing the termination and reflectivity of the transponder antenna accordingly. Thus, a portion of the received interrogation signal is reflected back to the interrogator antenna, modulated with the transponder's own output signal. Transponders which are very close to the interrogator unit 108 may receive sufficient power from the inhibiting beam to be powered up and to transmit response signals as a result. However, these response signals will comprise the 900 MHz carrier of the inhibiting beam, modulated with the transponder response code, which will not be detected by the 915 MHz narrow band receiving antenna 114. When a transponder is successfully detected, the control circuit 118 generates a "gap" or turn-off signal which is applied to both the transmitter 110 and the inhibiting transmitter 120, so that the outputs of both transmitters are briefly interrupted to turn off the relevant transponder. The turn-off signal is applied to the transmitter 120 as well, to cater for the possibility that the inhibiting beam (beam 2) is sufficiently strong due to the closeness of the transponder to the interrogator 108, to prevent the transponder from turning off as intended. Frequently, the transmissions from individual transponders will be "jammed" by overlapping transmissions from other transponders, so that the received signals will not pass one or other of the checking/verification steps. However, when a transponder signal is received during a "quiet" period when other transponders are not transmitting simultaneously, it will be verified, and the resulting data is fed to a microprocessor for identification and counting of the article to which the transponder is attached. The above described system exploits the fact that low cost transponders of the kind in question use wide tolerance components, which allow good yields in manufacture. These transponders do not include tuned circuits and comprise a single integrated circuit produced in a conventional integrated circuit foundry. The antenna of the transponder determines its frequency response characteristics, and can be designed for a relatively wide bandwidth. These transponders can then be interrogated on several different frequencies simultaneously, using relatively narrow bandwidth interrogator transmit/receive antennas, so that the transponders modulate one or several interrogation signals simultaneously when transmitting a response. When all of the transponders have been successfully identified, which can typically take less than one second, a microprocessor may pass the data to a till, which generates a print out which can take the form of the sample print out shown in Figure 12, by associating the received transponder codes with information in a price look-up table. The nature of each article is indicated, as well as the price per article and the number of articles. The microprocessor or the till itself can store the price look-up data, which can be updated periodically, for example daily. Alternatively, the microprocessor or the till can be connected online to a central computer, which provides updated price look-up data on an ongoing basis. The information in the sample print out of Figure 12 can be displayed on a screen of the till and is reflected on a paper print out generated by a printer. It can be seen that the above described system also exploits the relative frequency insensitivity of the transponders to provide a more accurate reading or interrogation operation. It is not essential that the inhibiting beam or signal be transmitted from the portable interrogator 108. In some installations, it may be convenient to provide a completely separate transmission system for the inhibiting signal.

Claims: An identification system comprising an interrogator (28) and a plurality of transponders (38), the interrogator (28) including transmitter means (30, 34) for transmitting an interrogation signal to the transponders (38), receiver means (114, 116) for receiving response signals from the transponders (38), and processor means (118) for identifying the transponders (38) from data in the response signals; each transponder (38) comprising receiving means for receiving the transmitted interrogation signal, a code generator, and transmitting means connected to the code generator, the transponder (38) being adapted to transmit a response signal containing data which identifies the transponder (38) on receipt of the interrogation signal, and to be disabled by a turn-off instruction in the interrogation signal on successful identification of the transponder, characterised in that the interrogator (28) transmits the interrogation signal and an inhibiting signal, and that a disabled transponder remains disabled while it continues receiving the inhibiting signal. An identification system according to claim 1 wherein the interrogation signal is a relatively directional signal, and the inhibiting signal is a relatively widely dispersed signal. An identification system according to claim 2 wherein the inhibiting signal has a beam width which is at least 5 times greater than that of the interrogating signal. An identification system according to any one of claims 1 to 3 wherein the interrogation signal is a relatively high power signal and the inhibiting signal is a relatively low power signal. An identification system according to any one of the claims 1 to 4 wherein the turn-off instruction to the transponder is transmitted in both the interrogation signal and the inhibiting signal. An identification system according to any one of claims 1 to 5 wherein the interrogation signal and the inhibiting signal have respective different frequencies which are selected to fall within the reception bandwidth of the receiving means of the transponders. An identification system according to claim 1 wherein the interrogation signal and the inhibiting signal are relatively narrow bandwidth signals, the receiving means of each transponder having a relatively broad reception bandwidth within which the respective different frequencies of the interrogation signal and the inhibiting signal falL An identification system according to claim 7 wherein the interrogation signal is modulated with data, the data modulation bandwidth of the interrogation signal being less than the spacing between the respective different frequencies of the interrogation signal and the inhibiting signal. An identification system according to any one of claims 1 to 8 wherein the transmitting means of the transponder comprises an antenna and means for modulating the reflectivity of the antenna, so that the response signal of the transponder comprises the interrogation signal carrier modulated with the data which identifies the transponder. An identification system according to any one of claims 1 to 9 wherein the interrogation signal and the inhibiting signal have respective different frequencies which are selected so that there are no overlapping nulls in the electric fields of the interrogation signals within a predetermined distance of the transmitter means of the interrogator. An identification system according to any one of claims 1 to 10 including processor means for recording data received from each identified transponder and for relating the received data to stored data corresponding to the received data. An identification system according to claim 11 wherein the processor means is adapted to store price or identification data of articles to which different transponders are attached, and to relate the identification codes of identified transponders thereto. An identification system according to claim 10 including display means for generating a display in which descriptions of the articles to which respective transponders are attached are associated with price data. An identification system according to claim 13 including printer means for generating a printout of the display. An identification system according to any one of claims 1 to 14 wherein the interrogator is adapted to detect transponder response signals transmitted on the frequency of the inhibiting beam and to disregard them. A method of identifying a plurality of transponders (38) comprising the steps of transmitting an interrogation signal to the transponders (38), each transponder (38) receiving the interrogation and transmitting a signal, containing data, in response, receiving the response signals from the transponders (38), identifying transponders (38) from data in the response signals, detecting successful identification of any transponder (38), disabling any transponder (38); and characterised in that the interrogation signal and an inhibiting signal are transmitted, and that a disabled transponder (38) remains disabled while it continues receiving the inhibiting signal. A method according to claim 16 wherein the interrogation signal is relatively directional, and the inhibiting signal is relatively widely dispersed. A method according to claim 17 wherein the width of the inhibiting signal is at least 5 times greater than the width of the interrogating signal. A method according to any one of claims 16 to 18 wherein the interrogation signal is relatively high power and the inhibiting signal is relatively low power. A method according to any one of the claims 16 to 19 wherein a turn-off instruction is transmitted in both the interrogation signal and the inhibiting signaL A method according to any one of claims 16 to 19 wherein the respective different frequencies of the interrogation signal and the inhibiting signal are selected to fall within the reception bandwidth of the transponders. A method according to claim 27 wherein the interrogation signal and the inhibiting signal have relatively narrow bandwidth and the transponders have a relatively broad reception bandwidth within which the respective different frequencies of the interrogation signal and the inhibiting signal fall. A method according to claim 22 wherein the interrogation signal is modulated with data, the data modulation bandwidth of the interrogation signal being less than the spacing between the respective different frequencies of the interrogation signal and the inhibiting signal. A method according to any one of claims 16 to 23 wherein the response signal of a transponder comprises the interrogation signal carrier modulated with data which identifies said transponder. A method according to any one of claims 16 to 24 wherein the interrogation signal and the inhibiting signal have respective different frequencies which are selected so that there are no overlapping nulls in the electric fields of the interrogation signals within a predetermined distance of the transmitter means of the interrogator. A method according to any one of claims 16 to 25 including recording data received from each identified transponder and relating the received data to stored data corresponding to the received data. A method according to claim 26 wherein the price or identification data of articles to which different transponders are attached are stored, and the identification codes of identified transponders are related thereto. A method according to claim 27 including displaying descriptions of the articles to which respective transponders are attached and associating said articles with price data. A method according to claim 28 including printing a printout of the display. A method according to any one of claims 16 to 29 wherein transponder response signals transmitted on the frequency of the inhibiting beam are disregarded. A transponder (38) comprising receiving means for receiving an interrogation signal, a code generator, and transmitting means connected to the code generator, the transponder (38) being adapted to transmit a response signal containing data which identifies the transponder (38) on receipt of the interrogation signal, and to be disabled by a turn-off instruction in the interrogation signal, characterised in that the transponder (38) receives the interrogation signal and an inhibiting signal, and that a disabled transponder remains disabled while it continues receiving the inhibiting signal. A transponder according to claim 31 wherein the transmitting means of the transponder comprises an antenna and means for modulating the reflectivity of the antenna, so that the response signal of the transponder comprises the interrogation signal carrier modulated with the data which identifies the transponder.

Inventors: LENARCIK ANDRZEJ, MARSH MICHAEL JOHN CAMILLE, OOSTHUIZEN MARTHINUS JACOBUS R, VAN SCHALKWYK ANDRIES CHRISTOF, VAN ZYL CLINTON AIDEN, LENARCIK, ANDRZEJ, MARSH, MICHAEL JOHN CAMILLE, OOSTHUIZEN, MARTHINUS JACOBUS RUDOLPH, VAN SCHALKWYK, ANDRIES CHRISTOFFEL, VAN ZYL, CLINTON AIDEN
Applicants: BTG INT LTD, BTG INTERNATIONAL LIMITED
Application year: 1993
Main classification: G01S13
IPC codes: H01Q21, G01S13, G07C9, A47F9, G07G3, G06K17, G06K7, G08B13, H01Q1, G07G1, G06Q50, H04B1