EP1285396A1

EP1285396A1 - Method for detecting simultaneous transmissions of electronic tags

Info

Publication number: EP1285396A1
Application number: EP01936572A
Authority: EP
Inventors: Richard Kalinowski; Michel Latteux; David Simplot
Original assignee: Tagsys SAS
Current assignee: Tagsys SAS
Priority date: 2000-05-25
Filing date: 2001-05-17
Publication date: 2003-02-26
Also published as: US20030122654A1; AU2001262453A1; WO2001091037A1; FR2809515B1; FR2809515A1

Abstract

The invention concerns a method for detecting simultaneous transmissions of electronic tags through an interrogation/reading/recording device comprising steps which consist in: (a) transmitting at the beginning of each electronic tag message a particular sequence of signals (90, 92, 94, 96), which is selected from a set of particular sequences; (b) identifying, on receiving messages through the interrogation/reading/recording device, said sequence of signals received at the beginning of each message; and (c) detecting the simultaneous transmission of at least two electronic tags if the identified sequence of signals (98) does not correspond to one of the particular sequences of said set.

Description

METHOD FOR DETECTING SIMULTANEOUS TRANSMISSIONS

ELECTRONIC LABELS

The invention relates to the field of electronic tags which are affixed to products to identify them according to their attributes and, more particularly, a method for detecting the simultaneous transmission of messages originating from at least two electronic tags to be identified. It is known practice to use "bar" codes which are printed on the products to identify them and which are read by an optical device during their passage at the checkout of the sales store. The bar code which is read makes it possible to interrogate a computing device which provides, for example, the price of the product. These barcodes have certain limitations which are mainly due to the fact that they are frozen when they are printed and therefore cannot be modified during the life of the product.

Also, it is now proposed to replace the bar codes by so-called electronic labels which contain electronic circuits such as a memory capable of recording a binary code with n bits. This binary code is representative of the bar code, in particular to indicate the type of product, but can also represent other information or attributes likely to be modified during the life of the product assuming, of course, that the memory is rewritable for at least some parts of the n-digit binary code. This can be the case for the date of sale, the identification of the store, the duration of the guarantee, the date of the end of the guarantee, etc. To interrogate the labels, an interrogation / reading / recording device is used, abbreviated as "ILE device", which can operate in the manner of a bar code reading device, that is to say that it dialogue only with one electronic label at a time, the one presented to it in its radiation volume.

However, the interrogation / reading / recording device is capable of simultaneously interrogating all the electronic tags situated in its radiation volume so that these also respond simultaneously and therefore cannot be identified one by one. To solve this problem, it has been proposed to interrogate the electronic labels according to different so-called anti-collision methods which make it possible to "manage" the labels which respond simultaneously and which are therefore in "collision". These anti-collision processes are divided into two classes: deterministic and non-deterministic.

In the first class, a first way of doing this is to interrogate the electronic labels on the basis, for example, of all or part of the product identification code until a single label responds to this code or part of code. This first way can lead to a high number of interrogations knowing that for a code with n = 64 bits there are more than 10 possibilities. A second way of doing this consists in making the interrogation / reading / recording device repeat what it receives from the electronic labels and this bit by bit or block of bits by block of bits. Electronic tags which recognize the repeated bit or block of bits know that they have been taken into account by the interrogation device and continue alone to transmit another bit or block of bits. These operations are repeated until a single electronic label is selected.

Deterministic processes assume that there are no two electronic tags with the same identification code, which is a major constraint in the event that. there is a high number of products of the same type, for example in a supermarket, because each article must be labeled differently to be recognized.

In addition, these deterministic methods only identify the electronic tags which are present at the start of the implementation of the method for a given set. Any new label in relation to this set may not be taken into account and must wait for the next cycle of the identification process. As a result, these deterministic processes are not directly applicable to the continuous identification of products, for example those parading on a treadmill.

In the non-deterministic methods, the electronic tags are provided for sending a message after a time interval of random duration counted from a starting point given by the interrogation / reading / recording device. The electronic label will consider that its message has been recognized if it receives an acknowledgment from the interrogation / reading / recording device. In the absence of an acknowledgment, the unrecognized electronic tag sends its message during the next interrogation cycle after the expiration of a new time interval of random duration. In such a non-deterministic process, it is likely that several electronic tags emit simultaneously if their total number is significantly greater than the number of random durations planned in a cycle, which scrambles the message of the electronic label which issued first. Also, it has been proposed to silence them. electronic tags upon detection of one emission from another, detection which can be done by the electronic tags themselves or by the interrogation / reading / recording device. The performance of non-deterministic methods is determined by the maximum number NsMAX of windows of transmission duration during a cycle with respect to the number N of electronic tags, this number NsMAX also defining the number of possible random waits. Thus, in the case of N electronic labels, the method is optimal for an Nsl value of NsMAX but in the case of 2N electronic labels, the method is optimal for a 2Nsl value of NsMAX. This has the consequence that the method is not suitable for any number of electronic labels.

In patent application No. 99 08181 filed on June 25, 1999 by the Applicant, there is described a method for adapting the number of random waits or transmission windows to the number of electronic tags remaining to be identified. This adaptation is carried out by the ILE device as a function of the identification results of the previous cycle. This method of identifying electronic labels includes the following steps:

(a) indicate on electronic labels the number Ns of consecutive emission windows of a first cycle or round,

(b) counting, during the cycle of Ns windows, the messages received from the electronic tags for determine the number or identifications, the number n _v of empty windows or boxes and the number n _c of collisions,

(c) stop the process if n _c = 0 or go to step (d) if n _c ≠ 0,

(d) calculate a number Nsl of transmission windows for the following cycle as a function of the values of Ns, n ^, n _v and n _c ,

(e) return to step (a) with Ns = Nsl calculated. In each electronic label, the method further comprises the following steps consisting in: (m) receiving the number Ns of windows in each cycle or round, (n) randomly choosing a window _. emission among Ns,

(o) send a message for the duration of the selected window,

(p) wait for a message from the interrogation / reading / recording device, (q) receive the acknowledgment of the message sent from the interrogation / reading / recording device in the absence of a collision,

(r) return to step (m) in the event of a failure to acknowledge receipt, that is to say in the event of a collision or of an empty window.

This identification process, known as adaptive rounds, considerably improves the efficiency of identification because it adapts, as and when labels are recognized, the ratio of transmission and silence times by modifying the duration of the next round, in terms of windows or slots contained in the round at the end of each round. However, this identification process with adaptive rounds leads to an identification duration of duration equal to at least Ns.d if d is the duration of a window. However, among the Ns windows, some are empty and others have at least one collision, which reduces the time efficiency of the process. Also, the aforementioned patent application plans to move to the next window as soon as an empty window or a collision is detected. While detecting an empty window is easy, detecting a collision window is more difficult. The object of the present invention is therefore to implement a method for detecting simultaneous transmissions of messages from electronic tags, that is to say the detection of collisions. This method consists in preceding the message sent by each label with a group of signals having a particular format so that the ILE device can detect as quickly as possible after the start of a window the simultaneous emission of several labels and therefore detect the existence of collisions. In the following description, we will consider that this group of signals is an integral part of the message sent by each label and constitutes an anti-collision sequence or frame, called ACF for the English acronym "Anti-Collision Fra e", which is arranged at the beginning of the message.

The format of the ACF anti-collision frame in each message is chosen from a set of formats which is such that the superimposition of ACF frames originating from two or more simultaneous messages leads to a group of signals whose format does not belong to the above set.

The set of formats can include N different formats, each label choosing one of the N formats in a random or pseudo-random manner during each message transmission by a label. In order to allow the recognition of an ACF anti-collision frame by. the ILE device, this can be preceded by a particular signal, such as a so-called "code violation" signal, indicating the start of the ACF frame.

The invention relates to, in a contactless electronic tag identification system comprising a plurality of electronic tags associated with an interrogation / reading / recording device, said electronic tags sending messages at randomly selected times or pseudo-random during a time interval or round, a method for detecting, in the interrogation / reading / recording device, the simultaneous emissions of at least two electronic tags, characterized in that it comprises the following steps consisting at :

(a) transmit, at the start of each electronic label message, a particular sequence (or frame) of signals, which is selected from a set of particular sequences,

(b) recognizing, on receipt of the messages by the interrogation / reading / recording device, said sequence of signals received at the start of each message, and c) detecting the simultaneous emission of at least two electronic tags if the sequence of recognized signals does not correspond to one of the particular sequences of said set. According to another characteristic of the invention, the particular signal sequence is selected randomly from the sequences of the set. This particular sequence can be randomly composed from a plurality of elementary sequences or words constituting a vocabulary. The number of words in a sequence can be fixed or chosen at random.

According to another characteristic of the invention, the particular sequence of signals can be preceded by a start of sequence signal of the code violation type, this signal not corresponding to any of the sequences in the set.

The particular signal sequence may be followed by a code violation type end of sequence signal which does not correspond to any of the sequences in the set and to that of the start of the sequence when it exists.

Other characteristics and advantages of the present invention will appear on reading the following description of particular embodiments, said description being made in relation to the accompanying drawings in which:

FIG. 1 is a functional diagram of a contactless electronic label and of an ILE device of such a label to which the method according to the invention applies,

- Figure 2 is a diagram showing the composition of a message sent by an electronic label implementing the method according to the invention, - Figure 3 is a diagram showing examples of anti-collision sequences or frames and the result of their overlap corresponding to a collision,

FIGS. 4 and 5 are diagrams showing sequences of symbols constituting words of a vocabulary which are combined to constitute a sequence or frame, and

- Figure 6 are diagrams showing sequences of words to constitute sequences or frames and the result. of their overlap corresponding to a collision. The method for detecting simultaneous transmissions of messages from electronic tags applies to a system comprising (FIG. 1) a plurality of contactless electronic tags 10 associated with an ILE device 30.

The contactless electronic label 10 comprises, for example, an antenna 12 consisting of a tuned circuit which comprises an induction winding 14 and a capacitor 16. The tuning frequency F ₀ of the antenna 12 is for example of 13.56 megahertz.

This tuned circuit of the antenna is connected to different circuits which each perform a particular function. Thus, a circuit 24 performs the full-wave rectification function of the signal at the terminals of the tuned circuit, for example, by a diode bridge 8. This rectification circuit 24 is followed by a filtering capacitor 26 of the rectified signal which provides the supply voltage V _dd of all the other circuits of the electronic label, in particular those represented in rectangle 10 in FIG. 1.

A circuit 18 performs the so-called clock function and the synchronization of the latter from the frequency F ₀ . The signals at different frequencies supplied by this Clock circuit 18 are applied to the other circuits of the label, whether or not represented, except for the antenna 12 and the rectifying circuit 24. The circuit 20 performs the function of demodulation and decoding of the signals which modulate the signal at the carrier frequency F ₀ , signals which constitute the information received by the label.

The information relating to the product to which the label is associated is recorded in a memory 22 which is addressable by a recording / reading circuit 30. This recording / reading circuit 30 is under the control of the signals detected and decoded by the circuit 20 and provides signals which are applied to a message synthesis circuit 32. The messages provided by the synthesis circuit 32 are applied to a antenna load modulation circuit which has been shown diagrammatically by a circuit 28 and by a switch 38 controlled by the circuit 28. A load resistor 36 has been shown in series with the switch 38.

The tag interrogation / read / write device 10 comprises for example an antenna 42 constituted by a tuned circuit which comprises an induction winding 44 and a capacitor 66, the tuning frequency being F ₀ . The two antennas 12 and 42 are magnetically coupled as shown by the arrow 46.

The antenna 42 is supplied with electrical signals at the carrier frequency F ₀ which are modulated by low frequency digital signals conveying the information to be transmitted to the label 10. These modulated electrical signals are produced by a modulator 50 which receives on the one hand, a signal at the frequency F ₀ of an oscillator 48 and the modulation signals of a message generator 52. The output signals of the modulator 50 are applied to a power amplifier 54 whose terminal output is directly connected to the antenna 42. The signals received by the antenna 42 are applied to a reception circuit 56 which performs their detection, demodulation and decoding. The decoded signals are applied to a microprocessor 58 which interprets them and provides the control signals for the message generator 52. The identification system for electronic labels can either be of the type in which the labels send a message as soon as they are supplied, or of the type in which they send a message after having received the order from the ILE device. In both types of operation, the label message includes for example (Figure 2):

a message start signal 70 for example constituted by a particular series of binary digits which indicates, for example, how the data are presented,

a time interval 72 whose constant duration L is known to the interrogation device,

- a binary data message 74, and - an error correcting code 76, better known by the acronym C.R.C. for "Cyclic Redundancy Chec".

According to the invention, this conventional message from the labels of the prior art is preceded by a group of signals 80 which mainly comprises a sequence or frame 82 called of anti-collision, known by the acronym A. CF. for "ANTI-COLLISION FRAME". This frame 82 can be preceded by an ACF frame start signal (reference 84) which is for example constituted by a particular sequence of signals analogous to that of the ACF frame. It is preferably followed by an end of frame ACF signal (reference 86) similar to that of the ACF frame. The composition of each ACF frame of a message of a label is determined randomly or pseudo-randomly, for example as a function of information contained in the electronic label. In the case of a random composition, none of the ACF sequences or frames will be identical to another, so that the overlay according to a combination two ACF frame logic will not result in a possible ACF frame. This characteristic makes it possible to detect a collision in the ILE device. Indeed, when the ACF frame is received, a collision will be detected as soon as the received ACF frame signals do not correspond to a possible ACF frame.

In the case of a pseudo-random composition, there is a certain probability that the ACF frames of two labels transmitting simultaneously are identical, but such a probability is very low. The composition of ACF frames can be achieved in different ways in electronic labels and some examples will be described below in relation to Figures 3, 4, 5 and 6. First, by way of example, we will not use as frame signals which have two symbols denoted H for a high level and B for a low level. The succession of these two symbols makes it possible to produce an ACF frame and FIG. 3 shows the different ACF frames 90, 92, 94 and 96 of four labels 1, 2, 3 and 4. When these labels transmit simultaneously, the signals of the four ^• ACF frames received by the interrogation device are superimposed and their combination according to a certain logic rule gives the signals of diagram 98.

This logical rule is for example that, if at least one label emits a symbol B, then the ILE device understands the symbol B and that it does not understand a symbol H only if all the labels emit an symbol H. The diagram 98 is different of any of the diagrams 90, 92, 94 and 96 and indicates that there are at least two tags which emit simultaneously, that is to say that there is a collision. It is possible to carry out the method according to the invention by using only the four compositions in FIG. 3, each label randomly choosing one of the four compositions on each transmission. However, the probability would be high that two labels simultaneously transmit the same ACF frame.

To reduce this probability, one of the solutions consists in increasing the number of possible compositions, which leads to increasing the size of the memory for storing them in the label and at a higher cost of the latter.

Also, the invention provides tools so that the composition of each frame is made from simple elements which are combined randomly to constitute a sequence or frame. According to a first characteristic, the symbols H and B are alternated according to given lengths to constitute words such as those represented by the diagrams of FIGS. 4 and 5, each group of words constituting a "Vocabulary". The Vocabulary VI of FIG. 4 comprises three words M1, M2 and M3 each extending over three lengths of symbols, the symbol B taking a different position from one word to another. Vocabulary V2 of figure 5 includes four words M'I, M'2, M'3 and M '4 each comprising two levels B at the end of each word preceded respectively by two, three, four and five levels H according to the word considered. These words are the simple elements that are combined randomly in a sequence to compose an ACF frame.

Figure 6 shows two ACF frames composed for label 1 (diagram 100) of the words M'I, M'3, M'I and M '4 of Vocabulary V2 and for label 2 (diagram (102) of words M'2, M'3, M'3 and M'I of vocabulary V2 The superposition of these two ACF frames by the. ILE device according to the aforementioned logic rule leads to the series of symbols in diagram 104, this series being unable to be generated by any sequence of the words M'I, M'2, M'3 and M '4 of Vocabulary V2. As a result, there are at least two labels in collision.

The example in FIG. 6 includes sequences of four words but the sequences can include more than four words and their number can be fixed or determined randomly. Vocabularies other than VI or V2 can be imagined without making use of invention but the words must be chosen so that two sequences of different words cannot be superimposed to give a possible sequence of words.

As indicated above, the ACF frame can be preceded and / or followed by a start and / or end signal of an ACF frame, this signal preferably being of the code violation type. In this case, the code violation must be different from any of the random sequences or a random sequence overlay.

In the case where the lengths of the sequences are not fixed, the code violation sequence 86 can be followed by a white space without signal, for example the part 72 which has a determined duration L, so that if the duration of the blank is less than the expected duration L, the collision is detected. This characteristic can be particularly taken advantage of in the case where the Clock signals of the electronic labels are not synchronized with the signals of the ILE device, that is to say that one is in the case of an asynchronous system.

Claims

1. In a contactless electronic label identification system comprising a plurality of electronic labels (10) associated with an interrogation / reading / recording device (40), said electronic labels transmitting messages at selected times of randomly or pseudorandomly over a time or round interval, a method for detecting, in the interrogation / reading / recording device (40), the simultaneous emissions of at least two electronic tags, characterized in that that it comprises the following stages consisting in:

(a) transmitting, at the start of each electronic label message, a particular sequence of signals (82) which is selected from a set of particular sequences (90, 92, 94, 96),

(b) recognizing on reception of the messages by the interrogation / reading / recording device, said sequence (90, 92, 94, 96) of signals received at the start of each message, and c) detecting the simultaneous emission of at least two electronic tags if the recognized signal sequence (98) does not correspond to one of the particular sequences (90, 92, 94, 96) of said set.

2. Method according to claim 1, characterized in that, in step (a), said particular sequence "of signals is selected randomly from the sequences of said set.

3. Method according to claim 1, characterized in that, in step (a), said sequence of signals particular is composed randomly from a plurality of elementary sequences or words (Ml, M2, M3, M'I, M'2, M'3, M'4) constituting a vocabulary (VI, V2).

4. Method according to claim 3, characterized in that the number of words (Ml, M2, M3, M'I, M '2, M' 3, M '4) of a sequence is fixed.

5. Method according to claim 3, characterized in that the number of words (Ml, M2, M3, M'I, M '2, M' 3, M '4) of a sequence is chosen randomly.

6. Method according to any one of the preceding claims 1 to 5, characterized in that step (c) consists in:

(Ci) compare the particular sequence recognized during step (b) with each sequence of said set, (c ₂ ) signal the existence of a simultaneous transmission if the comparison is negative.

7. Method according to any one of the preceding claims 1 to 5, characterized in that step (c) consists in: (c'i) comparing the particular sequence recognized during step (b) with each sequence d 'a second set of sequences resulting from all the logical combinations of the sequences of said set, (0 * ₂ ) signal the existence of a simultaneous transmission if the comparison is positive.

8. Method according to any one of the preceding claims 1 to 7, characterized in that in step (a), the particular signal sequence is preceded by a start of sequence signal (84) of the violation type. of code, said start of sequence signal not corresponding to any of the sequences of said set.

9. Method according to any one of the preceding claims 1 to 8, characterized in that in step

(a), the particular sequence of signals is followed by an end of sequence signal (86) of the code violation type, said end of sequence signal corresponding to neither of the sequences of said set nor to that of the start of the sequence.

10. Method according to claim 9, characterized in that the end of sequence signal (86) is followed by a time interval (72) of constant duration (L) without transmission signal and, in that the step (c) further comprises the operations consisting in:

- measure the time interval (72),

- signal the existence of a simultaneous transmission if the measurement of the time interval without signal is different from the value (L).