WO2000022569A1

WO2000022569A1 - Passive rf identification tag and method of manufacture and use thereof

Info

Publication number: WO2000022569A1
Application number: PCT/IL1999/000536
Authority: WO
Inventors: Michael Braiman; Valery Iline
Original assignee: Inforay Technologies Ltd.
Priority date: 1998-10-13
Filing date: 1999-10-12
Publication date: 2000-04-20
Also published as: IL126550A; AU6119399A; IL126550A0

Abstract

A passive RF identification tag (10) comprising a piezoelectric transducer (11) for receiving a pulse of RF radiation and generating an acoustic pulse (12), and an arrangement of staggered reflectors (13) each displaced a different distance from the transducer. The reflectors receive the acoustic pulse and reflect back as a respective reflected acoustic pulse (14) toward the piezoelectric transducer, so as to generate therein a respective RF pulse train wherein successive pulses are separated in time according to the respective displacement of the reflectors from the transducer. The resultant RF pulse train is characteristic of the arrangement of reflectors.

Description

Passive RF identification tag and method of manufacture and use thereof

FIELD OF THE INVENTION

This invention relates to passive identification devices.

BACKGROUND OF THE INVENTION

Passive identification devices address the requirement to allow remote identification of a portable transponder without the need to self-power the transponder by means of an internal battery. Of the various common approaches, radio-frequency (R.F.) identification is particularly popular because it does not require direct line-of-sight between the transceiver and transponder as is required when infrared is used and it is not limited to short range.

In order to obviate the requirement to provide a battery in the transponder, a passive transponder is used. Thus it is known to use a piezoelectric crystal which responds to an RF signal and produces a corresponding surface acoustic wave. Piezoelectric crystals operate in both directions: that is to say, when struck by a surface acoustic wave they produce an RF signal. Typically, an RF pulse is directed to the piezoelectric crystal thereby producing therein a surface acoustic wave which may be modulated and transduced back by the piezoelectric crystal, to an RF pulse which may be received remote from the transponder. In order that different transponders may be uniquely identified, it is necessary that the outgoing RF signal be different for different transponders. One way in which this may be achieved employs multiple aligned reflectors disposed on one side only of the transducer each of which receives the acoustic pulse at different times, according to the respective displacement of each reflector from the transducer. Each reflector reflects the acoustic pulse back to the transducer at a different time, whereby an outgoing RF pulse train is generated by the piezoelectric crystal comprising a series of RF pulses displaced in time from one another by amounts which are a function of the mutual displacements of the reflectors. It is thus possible to associate a unique code with the transponder by varying the spacing between adjacent reflectors. It appears that in such a device, the reflectors are so arranged that the phase of the acoustic pulse is modified when it is reflected, in order to produce phase modulation of the outgoing RF pulse train enabling the unique identity code to be decoded.

There are several drawbacks with such an approach. First, the reflectors must also be able partially to transmit the acoustic pulse so that it can be received by an adjacent aligned reflector remote from the transducer. This reduces the intensity of the reflected component of the acoustic pulse. Secondly, the acoustic pulse produced by the transducer is directed on both sides of the transducer. Since the reflectors are disposed on one side only of the transducer, not all of the acoustic pulse is exploited by such a device. Moreover, such an approach is not easily amenable to mass-production because the spacing between adjacent reflectors must be varied from one transponder to another in order that the outgoing pulse train be different for different transponders. This, in turn, requires customization of each transponder during manufacture which militates against mass-production and again increases per-unit costs. SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improved RF identification tag which is highly efficient and amenable to mass-production and wherein the drawbacks associated with hitherto-proposed devices are reduced or eliminated.

According to one aspect of the invention there is provided a passive RF identification tag, comprising: a piezoelectric transducer for receiving a pulse of RF radiation and generating an acoustic pulse, an arrangement of staggered reflectors each displaced a different distance from said transducer for receiving the acoustic pulse and reflecting back as a respective reflected acoustic pulse toward the piezoelectric transducer so as to generate therein a respective RF pulse train wherein successive pulses are separated in time according to the respective displacement of said reflectors from the transducer; whereby the RF pulse train is characteristic of said arrangement of reflectors.

Preferably, the reflectors are arranged in a regular linear configuration in order to facilitate mass-production. Such a configuration causes a regular pulse stream to be emitted comprising pulses of equal period separated by identical gaps. In order to provide discrimination between different identification tags, some of the reflectors are inhibited so that the inter-pulse gaps between selected reflectors may be increased. This allows for the creation of a varying pulse train, each characteristic of a respective tag. The staggered reflectors may be disposed on either one only or both sides of the transducer. BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic representation of an identification tag according to a first embodiment of the invention;

Figs. 2a and 2b are schematic representations of a reflector suitable for use in the identification tag shown in Fig 1 ; Fig. 3 is a schematic representation of an identification tag according to a second embodiment of the invention; and

Fig. 4 is a flow diagram showing a method according the invention for generating a variable pulse train characteristic of an identification tag.

DETAILED DESCRD7TION OF A PREFERRED EMBODIMENT Fig. 1 shows schematically an identification tag depicted generally as

10, comprising a piezoelectric transducer 11 for receiving a pulse of RF radiation (constituting an interrogation signal) and generating an acoustic pulse 12. It is to be noted that, in fact, a corresponding acoustic pulse 12' is also emitted on the opposite side of the piezoelectric transducer 11 as will be explained in greater detail below with reference to Fig. 3 of the drawings.

Disposed within the identification tag 10 is an arrangement of staggered reflectors 13 each displaced a different distance from the piezoelectric transducer 11 for receiving the acoustic pulse 12 and reflecting back as a respective reflected acoustic pulse 14 toward the piezoelectric transducer 11. As shown, the arrangement of the reflectors 13 is linear and regular such that the respective displacement of each reflector 13 from the piezoelectric transducer 11 increases as an arithmetic progression, i.e. d, 2d, 3d and so on, whereby the gaps between successive pulses are identical in width. When the reflected acoustic pulses strike the piezoelectric transducer 11 they are converted back into corresponding RF pulses which are emitted by the piezoelectric transducer 11 as an RF pulse train wherein successive pulses are separated in time according to the respective displacement of the reflectors 13 from the piezoelectric transducer 11.

In order that the resulting pulse train be characteristic of the identification tag 10, the gaps between successive pulses must be modified. This may be achieved by inhibiting selected ones of the reflectors so as to prevent an acoustic pulse from being reflected by the selected reflectors back to the piezoelectric transducer 11 thereby extending the respective time interval between successive pulses in the RF pulse train. Such inhibition of the reflectors may be effected by disposing an acoustically opaque marker on a surface of the desired reflectors. The acoustically opaque marker constitutes a reflection inhibitor and is preferably disposed on the selected reflectors during manufacture of the identification tag by painting an acoustically opaque pigment on the surface of the reflector using an ink-jet technique or other conventional printing or marking methods. Figs. 2a and 2b show schematically details of a reflector 20 employing the well-known Bragg effect which may be employed as the reflectors 13. The reflector 20 (referred to hereinafter as a "Bragg reflector") comprises a plurality of taps 21 having a mutual spacing of λ/2 and each positioned on the peak of the surface acoustic wave produced by the piezoelectric transducer 11, where λ is the wavelength corresponding to the fundamental frequency of the RF pulse. Each tap 21 must have a length equal to at least 8λ in order that the all of the incident energy be reflected back to the piezoelectric transducer 11 , while shorter lengths result in the energy being dissipated almost circularly. Assuming that each tap 21 reflects 1% of the incident energy, 100 taps allow, theoretically, for almost 70% of full energy reflection of the surface acoustic wave towards the piezoelectric transducer 11. Use of such a Bragg reflector 20 thus allows highly efficient reflection of the incident surface acoustic wave, whilst being easily amenable to inhibition as described above.

It has been noted that in the identification tag 10 shown in Fig. 1, an acoustic pulse 12' is also emitted on the opposite side of the piezoelectric transducer 11. Clearly, it is desirable to exploit this emission in order that the strength of the RF pulse train emitted by the identification tag 10 may be increased.

Fig. 3 shows an identification tag 30 wherein corresponding arrays 31 and 32 of Bragg reflectors are disposed on opposite first and second sides 33 and 34, respectively, of a piezoelectric transducer 35. The Bragg reflector 36 in the first array 31 on the first side 33 of the piezoelectric transducer 35 which is most remote therefrom reflects the acoustic pulse back to the piezoelectric transducer 35 before the acoustic pulse reaches the Bragg reflector 37 in the second array 32 on the second side 34 of the piezoelectric transducer 35 which is closest thereto. By such means, the Bragg reflector 37 and the other Bragg reflectors in the array 32 reflect the incident surface acoustic wave produced by the piezoelectric transducer 35 after all of the Bragg reflectors in the array 31 have reflected the incident surface acoustic wave. This permits fewer Bragg reflectors to be provided in each array in order to achieve a pulse train of desired bit length and, since less of the incident RF energy is wasted, the efficiency of the identification tag 30 is significantly greater than that of the identification tag 10 described above with reference to Fig. 1 of the drawings.

Fig. 4 is a flow diagram showing a method according the invention for generating a variable pulse train characteristic of an identification tag. On at least one side of a piezoelectric transducer is disposed a staggered array of reflectors each for reflecting toward the piezoelectric transducer a respective acoustic pulse a corresponding time delay following transmission of an RF pulse to the piezoelectric transducer. The time delay between adjacent acoustic pulses is adjusted so that a resulting pulse train is characteristic of the identification tag. Such adjustment is preferably effected by inhibiting selected ones of the reflectors so as to increase the time delay between the respective pulses reflected by active reflectors on opposite sides of the transducer. The reflectors may be inhibited by disposing an acoustically opaque material on the selected reflectors during manufacture of the identification tag. This may be done using an inkjet technique or other conventional printing method. Alternatively, selected ones of the reflectors may be shorted during manufacture so as to render them ineffectual.

It will be appreciated that a similar approach may be employed for manufacturing the identification tags shown in Figs. 1 and 3, respectively.

It will further be understood that the configuration according to the invention is particularly advantageous when a linear, regular array of reflectors is employed since this is most simply adapted to mass-production. In a first stage, all the identification tags are identical and the encoding of each tag is effected in a second stage wherein some of the reflectors are inhibited. However, non-linear configurations are also envisaged providing, of course, that the decoder (not a feature of the invention) knows the corresponding transducer-to-reflector displacements.

Furthermore, whilst, a regular array is most amenable to mass- production, it is to be noted that the encoding is achieved by virtue of an irregular array of reflectors whose successive displacements from the piezoelectric transducer do not follow a straight arithmetic progression, d, 2d, 3d and so on. By such means, the gaps between successive pulses may be varied so as to be characteristic of the tag's identity code. In the preferred embodiment, an initially regular array of reflectors is rendered irregular by inhibiting some of the reflectors. However, it will readily be understood that the identification tag 10 can be manufactured with an initially irregular reflector array, albeit thereby decreasing production yield. Although the manner in which the identification tag 10 is interrogated is not per se a feature of the invention, it should be noted that the interrogation signal can be generated in the form of a single pulse or as a pulse train including a number of pulses with pseudo random phase distribution.

Furthermore, it will be noted that a unique ID Pattern (or RF Image) is thereby created characteristic of the RF transponder. The ID Pattern is an overall AFP (Amplitude, Frequency and Phase) characteristic of the reflected signal and function of the:

• type of interrogation signal (single pulse or pulse train)

• the physical structure of the identification tag, • substance characteristics of the identification tag, and

• fabrication accuracy deviation of the identification tag.

Thus, the ID Pattern will be unique even for identically coded transponders having nominally identical reflector configurations, and varies for different interrogating signal patterns. By such means, a system utilizing identification tags according to the invention is able to register an ID Pattern of the tags, thereby preventing frauds. In many cases the ID Pattern can be used instead of traditional coding, thus simplifying the transponder design and significantly decreasing the size of the identification tag.

Claims

CLAEVIS:

1. A passive RF identification tag (10), comprising: a piezoelectric transducer (11) for receiving a pulse of RF radiation and generating an acoustic pulse (12), an arrangement of staggered reflectors (13) each displaced a different distance from said transducer for receiving the acoustic pulse and reflecting back as a respective reflected acoustic pulse (14) toward the piezoelectric transducer so as to generate therein a respective RF pulse train wherein successive pulses are separated in time according to the respective displacement of said reflectors from the transducer; whereby the RF pulse train is characteristic of said arrangement of reflectors.

2. The passive RF identification tag according to Claim 1, wherein said arrangement of reflectors (13) is linear and irregular such that selected adjacent reflectors give rise to an extended time interval between successive pulses in the RF pulse train.

3. The passive RF identification tag according to Claim 1, wherein said arrangement of reflectors (13) is linear and regular and there are further included: reflection inhibitors associated with selected ones of the reflectors so as to prevent an acoustic pulse from being reflected by the selected reflectors back to the piezoelectric transducer thereby extending the respective time interval between successive pulses in the RF pulse train; whereby the RF pulse train is characteristic of said arrangement of reflectors and associated reflection inhibitors.

4. The passive RF identification tag according to Claim 3, wherein the reflection inhibitors comprise an acoustically opaque marker disposed on a surface of the reflectors.

5. The passive RF identification tag according to any one of the preceding claims, wherein the reflectors comply with or operate in accordance with the Bragg effect.

6. The passive RF identification tag according to any one of the preceding claims, wherein a reflector on a first side of the transducer which is most remote therefrom is adapted to reflect the acoustic pulse back to the transducer before the acoustic pulse reaches a reflector on a second, opposite side of the transducer which is closest thereto.

7. The passive RF identification tag according to any one of Claims 1 to 6, wherein the staggered reflectors are disposed on either one only or both sides of the transducer.

8. A method for generating a variable pulse train characteristic of an identification tag, said method comprising the steps of:

(a) disposing on at least one side of a piezoelectric transducer a staggered array of reflectors each for reflecting toward the piezoelectric transducer a respective acoustic pulse a corresponding time delay following transmission of an RF pulse to the piezoelectric transducer, and

(b) adjusting the time delay between successive acoustic pulses such that a resulting pulse train is characteristic of the identification tag.

9. The method according to Claim 8, wherein step (b) includes inhibiting selected ones of the reflectors so as to increase the time delay between the respective pulses reflected by active reflectors on opposite sides of the selected reflectors.

10. The method according to Claim 9, wherein the step of inhibiting includes disposing an acoustically opaque material on the selected reflectors during manufacture of the identification tag.

11. The method according to any one of Claims 8 to 10, further including associating with the identification tag a unique Amplitude, Frequency and Phase characteristic of the reflected signal said unique characteristic being a function of: (a) a physical structure of the identification tag,

(b) substance characteristics of the identification tag, and

(c) fabrication accuracy deviation of the identification tag.

12. The method according to any one of Claims 8 to 11, wherein step (b) includes disposing the staggered reflectors on either one only or both sides of the transducer.

13. A method for manufacturing an identification tag, said method comprising the steps of:

(a) disposing on at least one side of a piezoelectric transducer a staggered array of reflectors each for reflecting toward the piezoelectric transducer a respective acoustic pulse a corresponding time delay following transmission of a RF pulse to the piezoelectric transducer, and

(b) inhibiting selected ones of the reflectors so as to adjust the time delay between successive acoustic pulses whereby a resulting pulse train is characteristic of the identification tag.

14. The method according to Claim 13, wherein the step (b) of inhibiting includes disposing an acoustically opaque material on the selected reflectors during manufacture of the identification tag.