Electronic identification system

Abstract: An electronic identification system 100 comprises an interrogator 10 and a plurality of transponders 12, 14 and 16. The interrogator comprise a transmitter 11 for transmitting an interrogation signal to the transponders; a receiver 13 for receiving response signals from the transponders; and a controller 19 for processing response signals received, to identify the transponders by their respective response signals. Each transponder comprises a signature generator 35, 32 for generating a unique signature characteristic of the transponder and intermittently transmits, in response to the interrogation signal, a response signal including the signature. The interrogator further comprises an acknowledgement signal generator 21 for generating upon reception of a response signal from one of the transponders, an acknowledgement signal to be transmitted by the transmitter 11. The acknowledgement signal comprises the signature, thereby to acknowledge reception of the response signal.

Description: INTRODUCTION AND BACKGROUND THIS invention relates to electronic identification systems and more particularly to such a system comprising an interrogator and a plurality of transponders. In an electronic identification system of the aforementioned kind known to the applicant, the interrogator comprises a transmitter for transmitting an interrogation signal to a plurality of transponders and a receiver for receiving intermittently repeated discrete response signals from the transponders. A microprocessor in the interrogator reads and identifies a particular transponder from characteristic data in a data stream in the response signal received from that transponder. In a time window immediately after transmission of the response signal, the transponder awaits an acknowledgement from the interrogator indicative of the fact that the transponder has been read by the interrogator. The acknowledgement is non-discriminative or non-specific and is provided by the interrogator by a brief interruption of the energizing signal. If the interruption is detected by a transponder during the aforementioned time window associated with that transponder, that transponder accepts that it has been read and it then changes to a sleep mode wherein it does not reply to the interrogator, to enable the interrogator to read other transponders not yet read. The probability of collisions in response signals from the plurality of transponders is reduced by utilizing for each transponder a respective randomly generated hold off time after energization by the interrogator has commenced and before the first response signal is transmitted by the transponder; and by causing the response signal to be repeated after randomly generated time periods. In spite of these measures designed to avoid collisions and to facilitate accurate reading and subsequent switching of transponders already read, errors still occur. For example, because the acknowledgement signal is non-specific, any and all transponders the time window of which is active at the time when the acknowledgement signal is transmitted will be switched off, whether they have been read or not. EP-A-0467 036 describes a cyclic reading system wherein ID's of tags read during a cycle are first collected in a database and wherein acknowledgement signals comprising the ID's are then transmitted in a batch by the interrogator/reader to all those transponders, but only at the end of each cycle. Thereafter a new reading cycle is started with a new "Hello" signal. US-A-4691202 describes an identification system including an interrogator which transmits an interrogation signal to a plurality of transponders. The interrogator receives the reply signals from the transponders, these comprising a characteristic of the originating transponder. The interrogator process the reply signals and retransmits the reply signals back to the transponder. If the signals do not correspond, the transponder continues the transmission; if the signals do correspond, the transponder discontinues the transmission. OBJECT OF THE INVENTION Accordingly it is an object of the present invention to provide an alternative system and method with which the applicant believes the aforementioned disadvantages may at least be alleviated. SUMMARY OF THE INVENTION According to the invention there is provided an electronic identification system comprising an interrogator and a plurality of transponders the interrogator comprising: transmitter means for transmitting an interrogation signal to the transponders; each transponder comprising: a controller; signature generating means for generating a unique signature that is characteristic of the respective transponder; transmitter means for transmitting in response to the interrogation signal, intermittent response signals comprising the signature; the interrogator further comprising acknowledgement signal generating means for generating upon reception of a response signal from one of the transponders, an acknowledgement signal comprising the unique signature of the one transponder to be transmitted by the transmitter means of the interrogator, thereby to acknowledge reception of the response signal to said one transponder; and the system being characterized in that each transponder comprises a time window generating means to generate a time window immediately after transmission of each of the intermittent response signals; and the controller being operative to cause the transponder to change from a normal operational mode to another mode, if an acknowledgement signal comprising the signature of the transponder is received during said time window. The transmitter means of the transponder may comprise modulator means for backscatter modulating the interrogation signal with the response signal. In other embodiments the transmitter means may include a local carrier generator for generating and transmitting the response signal. The signature generating means may comprise a random number generator and the signature may be in the form of a randomly generated number. The generator may be adapted to generate a new signature each time the interrogation signal is received again, after it has been interrupted. The random number generator may also be operative to determine a random delay or hold off period after first reception of the interrogation signal and before a first of said intermittent response signals is transmitted. Furthermore, the random number generator may also be operative to determine random repetition periods for the intermittent response signals. The random number generator may also be operative to generate a divisor for a programmable clock frequency divider of the transponder, from time to time to yield a randomly selected clock frequency for the controller of the transponder. The aforementioned acknowledgement signal may also include a command to cause the transponder to switch from the normal operational mode to a selected one of a plurality of other modes. Also included within the scope of the present invention is a transponder comprising: receiver means for receiving an interrogation signal; signature generating means for generating a signature characteristic of the transponder; transmitter means for transmitting in response to the interrogation signal a response signal comprising the signature characteristic of the transponder; time window generating means; and a controller; characterized in that the controller is adapted to cause the transponder to switch from one mode of operation to another, if an acknowledgement signal comprising the signature is received in a time window generated by the time window generating means immediately after transmission of the response signal. The signature generating means may comprise a random number generator and the signature may thus be in the form of a number. The number generator may be operative to determine a delay or hold off period after first reception of the interrogation signal and before the response signal is transmitted. The random number generator may be operative to determine random repetition periods for the response signal. The random number generator may be operative to generate a divisor for a programmable clock frequency divider of the transponder, from time to time to yield a randomly selected clock frequency for the controller of the transponder. According to another aspect of the invention there is provided a method of operating an electronic identification system comprising an interrogator and a plurality of transponders, the method comprising the steps of: transmitting from the interrogator an interrogation signal to the transponders; causing the transponders to receive the interrogation signal and each intermittently to respond with a respective response signal comprising a signature characteristic of the transponder; causing the interrogator to receive the response signals and to process a response signal from at least one of the transponders; characterized in that the method comprises the steps of causing the interrogator to transmit an acknowledgment signal comprising the signature immediately after processing of the response signal of the transponder; and if the acknowledgement signal is received within a time window immediately following transmission of the response signal, causing the transponder upon reception of the acknowledgement signal to switch from one mode of operation to another mode of operation. BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein Figure 1 is a functional block diagram of an electronic identification system according to the invention comprising an interrogator and a plurality of transponders; Figure 2 is a more detailed block diagram of one of the transponders forming part of the system; and Figure 3 is a diagrammatic representation against time of responses from transponders to an interrogation signal and acknowledgement thereof by the interrogator. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION In figure 1 there is shown an electronic identification system 100 comprising an interrogator 10 and a plurality of transponders, transponder #1, transponder #2 and transponder #3 designated 12, 14 and 16 respectively which are mounted on or associated with articles (not shown) to be identified or counted. The interrogator 10 comprises a transmitter 11 for transmitting an interrogation or energizing signal to the transponders and a receiver 13 for receiving response signals from the transponders. A duplex arrangement 15 connects either the transmitter 11 or receiver 13 to antenna 17. The interrogator further comprises a controller 19 and an acknowledgement signal generator 21, the function of which will be described hereinafter. The transponders 14, 16 and 18 are similar in configuration and therefore only transponder 14 will be described in more detail hereinafter, with reference to figure 2. Each transponder includes a receiver 23 including virtual battery 25, detector 27 and demodulator 29. The virtual battery captures and stores energy from the energizing signal, to power other circuitry of the transponder. In other embodiments other receivers such as super-regenerative, superheterodyne and homodyne receivers may be utilized. The aforementioned other circuitry includes a controller 31 including a time window generator 33 which will be referred to hereinafter and a random number generator 35 which will also be referred to hereinafter. The random number generator 35 forms part of, or serves or functions as a signature generator 37 for generating a unique signature for the transponder, a repeat period generator 39 for generating a random repeat period (RP) between subsequent repetitions of response signals, a hold-off time generator 41 for generating a random hold-off time after first reception of the interrogation signal and first transmission of the response signal and a divisor generator 43 for intermittently generating a random divisor for a divider 45 of a clock frequency (f c ) generator 47 forming part of the transponder. The transponder further includes a response signal generator 49 for generating a response signal to the interrogation signal and a memory arrangement 53 for storing data. The response signal generator forms part of a transmitter 51 for backscatter modulating the interrogation signal via modulator 25, to transmit the response signals to the interrogator. In other embodiments the transponder may comprise a local carrier generator (not shown) for generating and transmitting the response signal. The energizing signal is shown at A in figure 3 and consequent responses from transponders #1 to #3 are shown at B to D respectively. As shown at B to D, upon reception of the energizing signal, each transponder reflects back to the interrogator intermittent signals 18 comprising some of the energy in the energizing signal by what is known as backscatter modulation and which energy is modulated with data shown in more detail in figure 3. The balance of the energy is utilized by virtual battery 25 to power the transponder circuitry, to enable it to perform its functions as herein described. The response from each transponder comprises intermittently repeated response signals 18, each signal comprising start bits 20, synchronization bits 22, data bits 24, a signature or ID code 26, a CRC error correction code 28 and stop bits 30. The repetition period RP2 or RP3 of the signals 18 in the response of each transponder is randomly generated by generator 39 and is not constant for a transponder. Furthermore, each transponder has a respective randomly generated hold-off time (HOLD-OFF 1 to HOLD-OFF 3 ) generated by generator 41 after energization commenced at 32 and before the transponder commences with the response. Immediately following each response signal 18, there is a time window 34 generated by generator 33 during which the respective transponder awaits and is responsive to an acknowledgement signal from the interrogator modulated on the energizing signal in A, that the response signal has been received and read by the interrogator. If the acknowledgement signal is received by the transponder within the time window 34 (see TW 1 in B for transponder #1), the transponder is switched to a mode wherein it does not further respond to the interrogation signal. However, if an acknowledgement signal is not received within the time window 34 associated with that transponder (see TW 2 and TW 3 for transponders #2 and #3) , the transponder, after the aforementioned randomly generated periods RP 2 in the case of transponder #2 and RP 3 in the case of transponder #3, repeats the response signal as shown at 18.1 in C and D , until it is received as hereinbefore described. In the system according to the invention, and upon reception of the energization signal, each transponder, after its hold-off time, transmits the response signal 18. The interrogator 10 locks onto the strongest signal received from the plurality of transponders and utilizes the synchronisation bits 22, to synchronize a clock (not shown) of the interrogator with the clock 47 of the relevant transponder. The data 24 is received and stored. The interrogator also receives the signature or ID code 26 associated with that transponder. During the time window 34 immediately following the response signal 18 received, the interrogator broadcasts the acknowledgement signal generated by acknowledgement signal generator 21 by modulating the energizing signal A with the ID code 26 just received as shown at 36 in A . The transponder associated with that ID code then receives and interprets the acknowledgement and accepts that it has been read by the interrogator and it then switches to a mode wherein it no longer responds to the interrogation signal as shown at B. Since each transponder has a unique ID code, only the transponder actually read will be switched off. As stated hereinbefore, respective hold-off times (HOLD-OFF 1 to HOLD-OFF 3 ) for transponders #1 to #3 are randomly generated by respective random number generators 35, 41 forming part of each transponder. Furthermore, the random number generator 35, 39 also determines the signal repetition period for each transponder, so that the signal repetition period for a particular transponder (RP 2 in the case of transponder #2 and RP 3 in the case of transponder #3) is not constant, but varies randomly. Still furthermore, the random number generator 35, 37 also generates the signature or ID coded 26 for the transponder. The signature remains constant while the transponder remains energized but upon re-energization after an interruption in the energization, the generator generates another random signature. To facilitate the interrogator locking onto a particular transponder and to alleviate interference between concurrent responses from transponders, the clock speeds of respective transponders may be varied by varying a divisor for a divider 45 in a respective programmable clock signal generator forming part of the respective transponders. The divisor may also be determined by the aforementioned respective random number generator 35,43. Together with the acknowledgement 36 sent by the interrogator 10, the interrogator may also transmit a command or signal to cause the transponder just read, to change from its normal operational mode to a selected one of a plurality of other modes, such as a sleep mode for a predetermined short period of time; a sleep mode for a longer period of time; and a mode wherein it no longer responds to the energizing signal, but is able to be switched to the normal operational mode again at any time. Although this description is focussed on so-called passive transponders (i.e. transponders with a virtual battery which derives its power from the energizing signal), the claims also include within their scope so-called active transponders which have on-board power supplies. It will be appreciated that there are many other variations in detail on the system and method according to the invention without departing from the scope of the appended claims.

Claims: An electronic identification system (100) comprising an interrogator (10) and a plurality of transponders (12, 14, 16) the interrogator comprising: transmitter means (11) for transmitting an interrogation signal (32) to the transponders; each transponder comprising: a controller (31) ; signature generating means (37) for generating a unique signature that is characteristic of the respective transponder; transmitter means (51) for transmitting in response to the interrogation signal, intermittent response signals (18) comprising the signature (26); the interrogator further comprising acknowledgement signal generating means (21) for generating upon reception of a response signal from one of the transponders, an acknowledgement signal (36) comprising the unique signature of the one transponder to be transmitted by the transmitter means of the interrogator, thereby to acknowledge reception of the response signal to said one transponder and the system being characterized in that each transponder comprises a time window generating means (33) to generate a time window (34) immediately after transmission of each of the intermittent response signals; and the controller being operative to cause the transponder to change from a normal operational mode to another mode, if an acknowledgement signal (36) comprising the signature of the transponder is received during said time window (34). A system as claimed in claim 1 wherein the transmitter means of the transponder comprises modulator means for backscatter modulating the interrogation signal with the response signals. A system as claimed in claim 1 or claim 2 wherein the signature generating means (37) comprises a random number generator (35). A system as claimed in claim 3 wherein the random generator (35) is operative to determine a random delay or hold off period (HOLD-OFF 1 ) after first reception of the interrogation signal and before a first of said intermittent response signals is transmitted. A system as claimed in claim 3 or claim 4 wherein the random number generator is operative to determine random repetition periods (RP 2 ) for the intermittent response signals. A system as claimed in any one of claims 3 to 5 wherein the random number generator (35) is operative to generate a divisor for a programmable clock frequency divider (45) of the transponder to yield a randomly selected clock frequency (f c ) for the transponder. A system as claimed in any one of the preceding claims wherein the acknowledgement signal (36) also includes a command to cause the transponder (TRA#1) to switch from a normal operational mode to a selected one of a plurality of other modes. A transponder (12) comprising: receiver means (23) for receiving an interrogation signal (32); signature generating means (37) for generating a signature characteristic of the transponder; transmitter means (51) for transmitting in response to the interrogation signal a response signal comprising the signature characteristic of the transponder; time window generating means (33); and a controller (31); characterized in that the controller is adapted to cause the transponder to switch from one mode of operation to another, if an acknowledgement signal comprising the signature is received in a time window (34) generated by the time window generating means (33) immediately after transmission of the response signal. A transponder as claimed in claim 8 wherein the signature generating means (37) comprises a random number generator (35) and wherein the signature is in the form of a number. A transponder as claimed in claim 10 wherein the number generator (35) is operative to determine a random delay or hold off period (HOLD-OFF,) after first reception of the interrogation signal (32) and before the response signal (18) is transmitted. A transponder as claimed in any one of claims 9 and 10 wherein the random number generator is operative to determine random repetition periods (RP 2 ) for the response signal. A transponder as claimed in any one of claims 9 to 11 wherein the random number generator (35) is operative to determine a clock frequency (f c ) for the transponder to yield a randomly selected clock frequency for the transponder. A transponder as claimed in any one of claims 9 to 12 wherein the random number generator (35) is operative to generate a divisor for a programmable clock frequency divider (45) of the transponder. A method of operating an electronic identification system (100) comprising an interrogator (10) and a plurality of transponders (12, 14, 16), the method comprising the steps of: transmitting from the interrogator (10) an interrogation signal (32) to the transponders; causing the transponders to receive the interrogation signal and each intermittently to respond with a respective response signal (18) comprising a signature (26) characteristic of the transponder; causing the interrogator (10) to receive the response signals and to process a response signal from at least one of the transponders; characterized in that the method comprises the steps of causing the interrogator to transmit an acknowledgement signal comprising the signature immediately after processing of the response signal of the transponder; and if the acknowledgement signal is received within a time window immediately following transmission of the response signal, causing the transponder upon reception of the acknowledgement signal to switch from one mode of operation to another mode of operation.

Inventors: KRUGER JOHN DAWID, TURNER CHRISTOPHER GORDON GERV, KRUGER, JOHN DAWID, TURNER, CHRISTOPHER GORDON GERVASE
Applicants: SUPERSENSOR PROPRIETARY LTD, SUPERSENSOR (PROPRIETARY) LIMITED
Application year: 1999
Main classification: G06K19
IPC codes: G01S13, G06K7, H04B1