WO2002056246A1

WO2002056246A1 - Programmable tags

Info

Publication number: WO2002056246A1
Application number: PCT/GB2002/000124
Authority: WO
Inventors: Gordon Malcolm Edge; Michael David Crossfield
Original assignee: Scientific Generics Limited
Priority date: 2001-01-11
Filing date: 2002-01-11
Publication date: 2002-07-18
Also published as: GB0100751D0

Abstract

The invention relates to a remotely readable passive data tag (31) comprising an inductive antenna (L) for picking up an electromagnetic interrogation signal; means (32) responsive to energy generated by the picked-up interrogation signal to develop a response signal; and a micro-miniature switch (S) having a mechanical component for altering the response characteristic of the tag, and systems for programming and interrogating such tags.

Description

Programmable Tags

The present invention relates to remotely readable data tags including low-cost passive data tag implementations which allow variable data to be stored and retrieved remotely. Such devices have a wide variety of applications. These include inventory control, ticketing, automated shopping systems, monitoring work- in-progress, security tagging, access control and anti- counterfeiting.

There are many types of data tagging systems currently available. These fall into two broad categories: active tags, which contain a power source, and passive tags which are energised by the interrogation method. The present invention is concerned primarily with passive tags.

The most widely-used passive data tag technology is based on optically-read printed patterns of lines, popularly known as barcodes. The tag element of such systems is very low-cost, being typically just ink and paper. The readers are also relatively low cost, typically employing scanning laser beams. Optical barcodes are heavily applied in cost-sensitive application such as retailing. For retail, the cost benefit outweighs the drawback of requiring line-of-sight between the reader and the tag.

For applications where line-of-sight is not possible, systems not employing optical transmission have been developed. The most popular employ magnetic induction for coupling between the tag and the interrogator electronics. These typically operate with alternating magnetic fields in the frequency range of 50kHz to 20MHz, and generally employ integrated electronic circuits ("chips") to handle receive and transmit functions, and to provide data storage and manipulation. Tags may be "Read Only", or "Read-Write". Read Only tags have a serial number permanently programmed during manufacture. Read-Write tags can be re-programmed at any time by the user, in some cases remotely. In order to avoid the need for a battery, power for the chip is obtained by rectification of the interrogating signal received by an antenna coil. In order to increase the power transferred, and to provide discrimination against unwanted signals and interference, the coil is usually resonated with a capacitor at the frequency of the interrogation signal carrier frequency. A typical product of this type is the TIRIS system manufactured by Texas Instruments Ltd. The principal drawback to chip- based tags for high-volume applications such as retailing is their cost, which today is typically in the range $1 - $3.

Security systems used to prevent pilferage in the retail environment also make use of remotely readable tags . In this application it is tag presence which is detected, and the devices do not carry data as such. It is normal for retail security tags to be switchable, so that they can be disabled after the goods to which they are attached have been paid for.

Retail security tags are generally of two types : Magnetic and RF. Magnetic labels employ elements made from ultra- soft magnetic material. These re-radiate detectable harmonics when interrogated with an alternating magnetic field. Magnetic labels are generally disabled by permanently magnetising one or more auxiliary elements, which then bias the soft element into saturation. The process can be reversed by de-gaussing the label. This is useful in applications such as lending libraries, where it is desirable to re-use the label. RF labels contain a coil antenna and a capacitor connected as a resonant electrical circuit. When the tag is interrogated with the appropriate frequency of alternating magnetic field, energy is absorbed by the resonant circuit. This power drain can be detected in the transmitter, and used to signal an alarm. Cancellation involves either disrupting the electrical circuit by creating a short or open circuit, or shifting the resonant frequency of the tag by modifying the value of the capacitor. All of these processes can be effected remotely, and usually involve applying a higher than normal interrogating field to the device. They all involve physical changes which are intended to be irreversible, and it is not possible to use current RF tags in applications where cycling between active and inactive states is required.

A concern of the present invention is to improve passive data tags to extend their functionality.

In accordance with one aspect of the present invention there is provided a remotely readable passive data tag comprising an inductive antenna for picking up an electromagnetic interrogation signal; means responsive to energy generated by the picked- up interrogation signal to develop a response signal; and a micro-miniature switch having a mechanical component for altering the response characteristic of the tag.

Other aspects of the invention include systems for programming and systems for interrogating tags of the type just set out hereinbefore.

In order that the invention may be more readily understood embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which

Figure 1 is a block diagram of a basic resonant frequency RF tag;

Figures 2 and 3 are block diagrams of more complex variants of the tag of Figure 1,

Figures 4 , 5 and 6 are block diagrams showing those different embodiments of programmable tags;

Figure 7 is a section through a micro-machined switch;

Figure 8 is a block diagram of an embodiment of a programmable shift register, Figure 9 is a block diagram of another embodiment of a RF tag in accordance with an aspect of the present invention which can utilise the shift register of Figure 7 ,

Figure 10 is a block diagram of a system for programming tags, and

Figure 11 is a block diagram for interrogating tags.

A basic resonant RF tag consists of a coil antenna and a parallel capacitor. The resonant frequency f of the tag is approximately equal to l/2pOLC, where L is the value of the inductor , and C the value of the capacitor. The "quality factor" or "Q" of the circuit, defined by f/Df, determines the sharpness of the resonance, Df being the bandwidth of the resonance. Q also defines how much voltage amplification the circuit will produce compared with an un-tuned coil, and can be calculated for a given circuit using the equation Q = 2πfL/R, where R is the total circuit a.c. resistance. As an example, a good Q for a typical resonant tag operating at 4 MHz is 100. 250 would be exceptionally good. Referring now to the accompanying drawings, Figure 1 illustrates a programmable RF tag comprising a coil antenna LI, a main capacitor Cl forming a resonant circuit with the antenna LI, and a capacitor C2 selectable by a switch S. With the switch S open, the resonant frequency of the tag is fi . With the switch closed the resonant frequency of the tag drops to f₂,, the change being proportional to the square root of the change in total circuit capacitance. As will be appreciated, this scheme can be readily extended as shown in Figure 2 by adding more capacitors C3, C4, ...C(_n+1) and switches, the number of unique frequencies possible being 2ⁿ, where n is the number of switches.

This type of tag is suitable for application where a relatively small number of different tags is required. It has the merit that if a number of tags are simultaneously present, they can all be identified without confusion.

The same configuration shown in Figure 2 is also suitable for implementing a low-cost retail security tag which can be routinely enabled and disabled e.g. for application in lending libraries . In this case the tag resonant frequency could be shifted outside the normal range of operation. Alternatively the tag could be disabled by using a switch similar to switch S to short out all or part of the inductor L. The switch could also be alternatively be placed in series with the resonant circuit, allowing it to be opened when it was required to disable the tag.

Figure 3 illustrates a programmable RF tag employing 2 separate resonant circuits L1,C1,C2 and L2,C3,C4, each of which can operate at one of two different frequencies by means of associated switches S1,S2. Of course, this scheme can readily be expanded to include more resonant circuits and/or operating frequencies for each circuit. In the simple example shown 4 different codes are possible (2 possible operating frequencies for each of 2 resonant circuits), and in general with m resonant circuits, the number of different tags possible is given by n^m, where n is the number of switches in each resonant circuit. Employing multiple resonators in this way makes much better use of available bandwidth than the example shown in Figure 1, at the expenses of a more complex tag, and the loss of the ability to read several tags simultaneously present.

In the case of both types of tag, the number of usable frequencies will be influenced by the Q of the tag, and the bandwidth available, taking account of regulatory as well as practical constraints. The benefit of using switches with low on-resistance is therefore significant.

Tag programming can be accomplished either directly, by making temporary direct electrical connection to the switches on the tag as shown in the embodiment of Figure where an external programmer is shown at 10, or remotely, by utilising the inductive communications path. Direct electrical connection can be simplified for tags incorporating many switches by including within the tag a serial to parallel conversion device. This is the arrangement shown in Figure 5 in which the serial-to- parallel conversion device is provided by a single silicon chip 11 powered via by connection to an external programmer 10. The chip 11 is only utilised during programming.

One method of remotely programming the switches involves modulating the switch setting data onto the tag interrogating signal, and including additional circuitry within the tag to extract the data, and route it to the appropriate switches. Figure 6 shows one possible arrangement. An auxiliary tuned circuit comprising L_a and C_a resonates at frequency f_a. An interrogating signal of this frequency is only transmitted in "programming" mode, and is modulated with setting data for the switches Si to S_n. The data demodulator 13, which is powered by rectification of the interrogating signal, extracts the data from the interrogating signal, and routes it to the parallel outputs which drive the switches. In normal operating mode the tag resonates at a frequency determined by the capacitors selected by switches Si to S_n. A similar scheme would apply in the case of a multiple resonator tag of the types indicated in Figures 2 and 3.

All the embodiments of the invention relate to variants of this basic configuration which are selected by means of one or more electrically operated switches which require no power to retain their settings. As will be appreciated from the above, in order to achieve satisfactory Q, the additional resistance introduced by each switch must be very low, typically a few ohms or less at 4 Mhz. Furthermore, in view of the small size of the tag, and the criticality of overall cost for many high-volume applications, the switches used must be very small and inexpensive. Such considerations rule out conventional electro-mechanical relays and semiconductor switches .

Accordingly, the type of switch employed in all the embodiments being described is a micro-machined sub- miniature bistable mechanical switch operated by electrostatic attraction.

Referring to Figure 7, the micro-switch comprises an element 1 which consists of a number of layers which are produced by deposition and/or etching techniques the type common in IC production. This micro-switch and variants thereof are fully described in the specification of International Patent Application No PCT/GB94/00977 the contents of which are accordingly incorporated in the present specification. The layers of the micro-switch are formed on a substrate 2 which can be made from any non-conductive material. A base contact 3 is positioned centrally on the substrate 2. Positioned on either side of the base contact are deflection electrodes 4, 5 which are also positioned on the surface of the substrate 2, but which are enclosed within a non-conductive layer 6. On top of the non-conductive layer 6 is a further non- conductive layer 7 which forms two spacers 7. On top of the spacers 7, and extending form one spacer to the other, is a bridging contact 8 which is formed from a flexible and electrically conductive material. The bridging contact 8 is under compression, which is introduced at the manufacturing stage in one of a number of ways. One method of manufacturing the bridging contact 8 is by forming on a bulging resist layer which is then etched away. Alternative methods such as flexing the substrate 2 whilst applying the bridging contact layer 8, using a metal which naturally goes under compression under thermal evaporation on specific substrates, or by forming the bridging contact layer at a greatly different temperature to that of the device operating temperature and employing the difference in thermal expansion between substrate 2 and bridging element 8 to introduce compression into bridging element 8 are also possible.

As the bridging contact 8 is under compression, it has two stable states, in one of which it is flexed away from the base contact 3, and in the other of which it is flexed toward the base contact 3. The contact 8 can be moved from one state to the other by application of voltage to the two deflection electrodes 4, 5. In this example, the bridging contact 8 has a positive voltage applied to it and so, when negative voltage is applied to both of the deflection electrodes, the bridging contact 8 is attracted towards and then brought into contact with the base contact 3 by electrostatic force. In order to move the bridging contact 8 to its uncontacted state, a positive voltage is first applied to one of the deflection electrodes 5, moving part of the bridging contact 8 in a direction away from the substrate 2. A positive voltage is then applied to the other deflection plate 4 and the rest of the bridging contact 8 moves away from the substrate 2 and into its other stable state. The bridging contact 8 can be brought back into contact with the base contact 3 by reversing the above procedure. Employing stepped repulsion and attraction requires less energy than employing a single attracting/repelling plate and thus the power consumption of the device when switching is reduced.

It is possible to introduce further spacing layers and insulating layers 2 to the device of Figure 3 and to add a further two deflection electrodes. Again, switching of the device is in a stepped fashion.

This new type of switch just described can be fabricated using processes similar to, and simpler than, those used for integrated circuit manufacture. The switches just described can be extremely small, and inexpensive to produce. Because the switch uses metal contacts, the ON resistance can be sufficiently low to meet the criteria specified above thus enabling their use as the switches described with reference to Figures 1 to 6.

It will be appreciated that in very many commercial situations the cost of tagging is a prime consideration given that it is a direct cost on each article sold. In order to reduce cost one aspect of the present invention proposes the use of organic semiconductor technology. This technology, also referred to as integrated plastic circuits or polymer electronics, enables integrated circuits using field effect transistor devices to be fabricated cheaply using much simpler processes than those required for conventional silicon integrated circuits. The semiconductor material employed is an organic material such as poly (3-alkylthiophene) . This material is typically applied using a wet coating process to a plastic substrate carrying an electrode structure. The integrated circuit is completed by an over-coating of an insulting layer, and a further electrode pattern. The electrodes may be metal or conducting inks of various kinds Several of the steps can employ process developed for high resolution printing, giving very high production rates. Several major electronics companies are developing organic semiconductor technology. These include Philips (Netherlands) and Siemens (Germany). Typical devices are described in e.g. Ullmann, Ficker, Fix, Rost, Clemens, "High Performance Organic Field- Effect Transistors and Integrated Inverters" published in Mat. Res. Soc. Proc. Vol.. 665 . 2001 and Gelinck, Geuns, de Leeuw, "High Performance all-polmer integrated circuits" App. Phs. Lett., 77 p 1487, 2000. There has also been considerable patent activity in this field and US Patents Nos. US 625293 and US 5079595 are examples of this from a large number of published patent specifications. In some fields a drawback to organic semiconductor technology is that the device feature sizes are large compared to conventional silicon integrated circuits, typical feature sizes being 10s of microns, compared with sub-micron features currently achieved with high-performance silicon. This limits the circuit complexity which can be accommodated in a given area of device. However organic semiconductor technology can provide remotely readable data tags with costs in high volume of a few cents each or less.

A tag based on integrated circuits employing organic semiconductors will now be described with reference to Figures 8 and 9. Figure 8 is a block diagram of a shift register which comprises a series of simple bi-stable circuits 15 (flip- flops) connected in series. The initial state of each bi-stable circuit is pre-set, so that when the chain of flip-flops is driven by a square wave "clock" signal on line 16, the pulse stream at the output 17 of the shift register is a serial digital representation of the initial settings of the flip flops 15. The example shown in Fig 8 has 6 stages, producing a data word containing 6 bits of data, but this can be extended to any desired length. In a typical remotely readable tag application the pulse stream generated as described is used to modulate the absorption of a tuned circuit containing the tag antenna coil. This modulation can be sensed by the interrogation system, thereby transferring the data encoded as the initial settings of the flip-flops in the tag to the interrogation system. This arrangement is shown in fig 9, The modulation scheme indicated involves switching the antenna coil L in and out of circuit using a solid state switch S such as a field effect transistor or a micro-machined switch as already described. The output of coil L is rectified by a diode 24 and used to provide power for the organic semiconductor circuit 25 which controls the operation of switch S. In prior art systems tag data content is determined by the flip-flop settings which are set by interconnection patterns defined during manufacture. This arrangement is very restrictive, in that it is only cost effective to produce large numbers of tags with the same data content. It is also not possible to change the data content after manufacture.

In the present invention the micro-machined sub miniature switches of the type previously described are applied to the programming of remotely readable data tags employing organic semiconductors. Such switches would be used in positions SI to S6 in the example shown in Figure 8. The micromachined switches may be manufactured separately, and combined with the organic integrated circuit as a hybrid assembly, or preferably manufactured on the same substrate as the organic integrated circuit by additional process steps. The switches are used to set the initial states of the bi-stable elements in the chain, thereby determining the structure of the serial data stream produced when the chain is driven by a repetitive clock sequence. By techniques analogous with those described above for the Resonant LC tags, programming, that is the initial setting of the micromachined switches, can be carried out either via a direct electrical connection to the tag, or remotely, as will be described hereinafter..

Conventional RF security tags as widely applied for retail anti-theft purposes need to be switchable, so that they may be disabled when the products to which they are attached are purchased. This prevents unwanted alarms if the purchased goods are subsequently taken through an interrogation system. Existing RF security tags are disabled by changing the state of the resonant L-C circuit. This typically involves either permanently shorting out or opening a part of the circuit, or permanently changing the value of the capacitor so that the tag resonant frequency is shifted to no longer match the interrogation signal. The first approach involves an additional component such as a fusible link which is ruptured during a special cancellation process. The components involved often add significant cost to the tag, and can be unreliable. The second "detuning" approach is relatively inexpensive, but is prone to unreliability - a common phenomenon is a full or partial reversal of the cancellation process over time, the tag becoming active again.

An aspect of the present invention is to use a micromachined sub miniature switch of the type described above to either short out or open the resonant tag circuit. The state of the switch would be set either by making a direct electrical connection to the tag, or by rectifying the RF signal applied during the deactivation procedure, and using the dc signal developed to operate the switch. The RF level used during deactivation would be considerably greater than that used for normal interrogation, so that there would be no risk of accidental deactivation. This solution is less expensive than the switch elements currently used, and more reliable than either current switching or de-tuning methods. Importantly it has the added benefit of being easily reversible, unlike current methods. This makes RF tags suitable for application such as tagging public library books, where repeated activation/cancellation is required, depending upon whether books are on shelves or legitimately removed from the library.

Referring now to Figure 10 of the accompanying drawings, this shows in diagrammatic form a system for programming tags of the kind previously described in this specification. Tag data is held at 20 so that for every class of article with which a tag is to be associated there is a corresponding individual identifying number. In one variant this data is transmitted by a transmitter 21 using an RF signal of appropriate frequency and intensity to the tag 22 to be programmed, the programming signal pickup-up by the tag coils (not shown in Figure 10) and converted into parallel data by a serial-to- parallel converter 23 which is used to set the micro- switches SI to S6.

It is of course possible, as already described, to directly link the tag interface to the data source.

A system for reading the tags is shown in Figure 11. In this Figure an interrogator 30 sends an RF signal to the tag generally indicated at 31 where the signal is picked up by the inductive coil L, rectified by diode 32 and used to drive the organic integrated circuit 33 so as to generate as serial data the pre-programmed tag identity which is modulated onto the response of the conductive coil antenna L.

Claims

1. A remotely readable passive data tag comprising an inductive antenna for picking up an electromagnetic interrogation signal; means responsive to energy generated by the picked- up interrogation signal to develop a response signal; and a micro-miniature switch having a mechanical component for altering the response characteristic of the tag.

2. A data tag according to claim 1, wherein said switch has a bi-stable memory element movable by deflection electrodes with respect to a base contact so as to either be in or out of contact with the base contact.

3. A data tag according to claim 1 or claim 2, and including a capacitor forming a resonant circuit with the antenna, and a logic circuit in series with the output of the antenna.

4. A data tag according^' to claim 3, wherein the logic circuit is operable in response to a setting electromagnetic signal to set the state of the switch.

5. A data tag according to claim 4, wherein the logic circuit comprises a multi-bit shift register.

6. A data tag according to claim 5, wherein the shift register comprises a series of connected bi-stable circuits which can be pre-set so that when the series of flip-flops is driven by a square wave a pulse stream at the output of the shift register is a serial digital representation of the initial settings of the flip/flops.

7. A data tag according to claim 5 or claim 6, wherein the logic circuit is an organic semiconductor circuit.

8. A data tag according to claim 3, and including a plurality of capacitors connected in parallel with said antenna and wherein at least some of the capacitors have associated micro-switches by means of which they can be switched out of circuit.

9. A data tag according to claim 3, and comprising at least two separate resonant circuits each of which can operate at one of two different frequencies in accordance with the settings of micro-switches associated with each of the resonant circuits.

10. A data tag according to any one of claims 4 to 9 and adapted to be programmed by a temporary direct electrical connection.

11. A data tag according to claim 10, including an integrated circuit providing a serial-to-parallel conversion device for coupling a programming signal to the switches.

12. A data tag according to any one of claims 4 to 9 and including an auxiliary tuned circuit responsive to a signal modulated with setting data for setting said plurality of switches; and a data demodulator circuit powered by the output of said auxiliary tuned circuit and adapted to extract the data and route it appropriately to the inputs of each of the micro-switches.

13. A system for programming a tag as claimed in any one of claims 3 to 12, comprising means for setting the state of the or each switch in a tag.

14. A system for interrogating a tag as claimed in any one of claims 3 to 12, including an interrogator for generating a RF signal, picking up the response signal generated by the inductive coil antenna of a tag and extracting the data component from the tag.