GB2332645A - Piezoelectric security thread detection apparatus - Google Patents

Abstract

Apparatus for detecting a poled piezoelectric security thread in documents eg banknotes, comprises a pair of electrodes 18,20 (Fig 1) that stimulate the thread 12 with an alternating voltage. The resultant heating of the thread is detected when the electrodes are switched to receive mode. The electrodes may be in the form of fingers or strips 18<SP>1</SP>,20<SP>1</SP> (Fig 6) that extend transversely across the thread. The thread is poled positively or negatively at 14 and unpoled at 16 so as to provide a binary or tertiary code.

Description

METHOD OF DETECTING A PIEZOELECTRIC SECURITY THREAD This invention relates to a method of detecting a security thread for protecting documents, banknote or identification cards, cards, security security comprising a piezoelectric material.

It is known to provide piezoelectric film in security documents as described in US 4,763,927 or US 4, 792, 667, the presence of piezoelectric material being detectabie by mechanical or pyroelectric testing means.

In US 4, 792, 667, films of polymeric material mace from polyvinylidene fluoride (PVDF) or other polymeric piezoelectric material are fixed to documents for security. The polymeric material has poled regions to create a piezoelectric effect.

In US 4, 763, 927, a variety of methods for testing the presence of piezoelectric thread in a document are described. Mechanical means of testing include stimulating the thread by bending, and ultrasound waves. Pyroelectric testing means include emission of m. crowave or infrared rays to heat the thread. The electrical response of the excisez piezoelectric film is read by direct conta~. therewith or by capacitive coupling with an electrode. A further method of testing involves excitir. c the piezoelectric material with an alternating electrical signal, tu produce ultrasound due to vibration o~ the piezoelectric material, which may be detected with appropriate microphones.

One e of the problems of the prior testing methods is that the stimulating effect is different from the detection effect thereby rendering the testing device more complex (for example heading with infrared rays, but measuring the electrical response). In addition, some methods may not be appropriate for a piezoelectric thread embedded within a document such as a banknote, where there is no external access to the thread. wt would be desirable to provide a simple and reliable method of testing a piezoelectric security film or thread. It would also be desirable to provide a reliable detecting means for detecting an enhanced security thread, in particular a security thread with a code formed by a secuence of poled and unpoled regions that form a binary or tertiary code.

Objects of this invention have been achieved by providing a method of detecting a piezoelectric security film according tc claim 1. Disclosed herein is a method of detecting presence of a security film element embedded in a document, the security film comprising a piezoelectric polymer having at least one poled region susceptible of producing electrical charges under mechanical or pyroelectric stimulation, comprising the steps of : placing the security film in an alternating electrical field adapted to heat the piezoelectric material due to the dielectric loss facfor of the piezoelectric polymer ; and detecting the electrical charge resulting from thermal stresses of the poled region. The thread or film may have a plurality of poled and unpcled regions, which may either provide a binary code, or a tertiary code by poing some poled regions in one direction and poling other pole regions in another direction (positive or negative fields). The code may be detected by detecting the response with a plurality of receiver electrodes. Electrodes may be provided on opposing sides of the security film, by positioning on a opposing sides of a document within which the film is embedded, one of the electrodes for example being the ground electrode conductively or capacitively coupled to a ground metalisation layer of the piezoelectric polymer. Alternatively, the electrodes may al'be positioned on one side of the security film, whereby the electrical field is generated between adjacent separate electrode portions, or between the electrodes and an opposed conductive layer on the piezoelectric polymer film. The stimulation (excitation) and detection (receiver) electrodes may be the same electrodes coupled tc a switch for switching from emit to receive modes, or the receiver may comprise separate electrodes for example in the form of branches in an interdigital fashion with the emit electrodes. In providing a coded security thread with poled and unpcled regions, and a receiver with a plurality of spaced electrodes, it is possible to read the binary code of the thread by detecting which of the receiver electrodes is opposite a poled region (emission of electrical charge due to thermal stresses) or opposite a non-poled region (where there is no electrical charge). The electrodes may be provided for example on the surface of a board (as circuit traces), the reverse side of the circuit board used for positioning electrical components for processing the signals. A simple and reliable detecting method is therefore achieved. Only electrical currents need to be processed for both stimulating and detecting the presence of the security thread. In addition, the means of stimulating the film depends on the dielectric loss factor of piezoelectric polymer material, irrespective of whether the film is poled or unpoled. An electrical alternating field, which is easy to generate, may thus be used tc create thermal stresses in the piezopolymer.

Further advantageous aspects of this invention bill be described in the claims or will be apparent from the following description and drawings.

An embodiment of this invention will now be described by way of example with reference to the figures in which; figure 1 is a schematic cross-sectional view through a document with a security thread of film and a pair of electrodes for testing the presence of the security thread ; figure 2 is a simplified cross-sectional view through a document comprising a security thread, the document having a different layer structure from that of figure 1; figure 3 is a schematic cross-sectional view through a document with security thread, and ar. otner embodiment of a testing apparatus ; figure 4 is a schematic top view of the document showing the security thread and the electrodes of the tesng apparatus of figure 3 ; figure 5 is yet another embodiment of a testing apparatus ; figure 6 is a schematic top view of the security thread and the electrodes of figure 5.

Referring to figure 1, a portion of cocument 2 comprises a plurality of layers 4, for example a first layer of caner 6, a second layer of polyester 8, a third layer of conductive material 10 and a fourth Layer of piezopolymer 12. The layer cf piezopolymer may be for example polyvinylidene fluoride (PVDF) whereby certain regions 14 are poled positively or negatively, and other regions 16 unpoled, thereby providing the piezoelectric film, thread or layer 12 with a binary code of pcled and d unpoled regions. A tertiary code may be provided by providing the poled regions 14 either with positive or with negative poling, such that in addition to the presence, the sign of the electrical field generated by the piezoelectric pcled areas 14 is detectable. The conductive layer 10 may for example be thin metalisation layer provided on the piezopolymer layer 12.

The testing apparatus comprises a pair of electrodes 18, 20, a first electrode 13 positioned on a first side 19 of the document 2, and a second electrode 20 positioned against a second side 22 of the document.

The first side 19 corresponds to the side on which the conductive layer 10 is positioned with respect to the piezoelectric layer 12, the second side 21 corresponding to a side of the piezoelectric layer opposite the conductive layer 10. Between the électrodes ; C, 18 a voltage V may be appliedordetected.Stimulationofthe piezoelectric layer 12 is effected by generatinc an alternating voltage between the electrodes 18, 2C. The first electrode 18 capacitively couples to the conductive layer 10 whereas the second electrode 20 and conductive layer 10 form a capacitor with the piezopolymer layer 12 acting as a dielectric therebetween.

Piezopolymer material has a high dielectric Loss factor typically in the range of 0. 015 to C. 020 at room temperature and lkHz measurement frequency. The Loss factor increases with frequency and may reach or exceed 0. 20 in the MHz. The dielectric loss factor is sometimes known as "tan $". These high dielectric loss factors result at least in part from the piezoelectric molecu'ar structure of piezopolymer, and are not encountered in other materials such as paver or polyester, which may form other layers of the document 2. Thus, a burst of rapid alternating voltage between the electrodes 18, 20, for example sinusoidal or square waves, will cause a loss of electric energy in the piezopolymer layer 12, which subsequently heats up thereby creating thermal stresses in the layer 12. Only the poled regions14, however, will emit any subtantial electrical charge resulting from the trermal stresses. The voltage generated between the electrodes 18, 20 due to the variation of thermal stresses in the piezoelectric polymer film 12 can subsequently be detected by switching the electrodes from emit to receive mode. The response of the piezoelectric layer 12 to a short burst of alternating voltage, will follow a characteristic curve related to the stress variation in teh layer 12 dring cooling thereof. In the receive mcde a number o' samples over time, cf the resultant voltage between the electrodes, may be taken depending on requirements of precision and reliability.

Referring to figure 2, the above described detection method also applies to documents with different layer arrangements, for example as shown in figure 2 an additional layer of paper 24 is positioned over the piezoelectric film 12 for protection thereof.

The piezoelectric layer 12 in the document arrangement of figure 2 is capacitively coupled to the upper electrode rather then direct contact therewith as shown in the arrangement of figure 1.

In the embodiment of figure 1, as the thermal stresses generate a piezoelectric respor. se only substantially in the poled areas 14, the detection response will depend on the ratio of the poled to unpoled surface areas, however a particular binary or tertiary coding may not be identifiable.

Referring now to the embodiment of figures 3 and 4, the second electrode in the embodiment of figure 2 is replace with a plurality of electrode pads 20'that extend transversely across the piezoelectric film. The film may be provided in any shape, but in these examples it is shown as an elongate thread as best in figure 4, as is typical in banknotes. In the emit (stimulation) mode, the electrodes 20'can be fired simultaneously to produce an effect similar to a single electrode 2C as in the embodiment of figure 1, for heatina piezoelectric film 12, When switched to the subsequer. t receive mode, the various individual electrodes 2G'may be read separateiy by provision of a multiplexer 28.

Certain of the electrodes positioned above ur. poled regions 16 wili not detect any significant voltage whereas the electrodes positioned above the poled regions 14 will detect a voltage dependant en the thermal stresses generated. In this manner in in possible to determine the code of the security film 12.

It may be noted that positively poled regions will give a voltage of different sign to negatively poled regions and thereby allow tertiary rather than binary codingif desired.

In another embodiment according to figures and 6, the first electrode 18 for capacitively coupling to the conductive layer 10 in the embodiment of figure 1 and 3 is replace by the first electrode 18'which, is positioned on the upper side 21 of the document 2, on the same side as the second electrodes 20'. As in the embodiment of figures 3 and 4, and best seen ir. figure 6, the second electrodes 20'are in the form of a plurality of elongate fingers extending transversely across the elongate security threan 12. The security thread may for example correspond to the security thread extending across the width of a banknote.

The first electrode 18'is commoned and comprises a plurality of fingers 19 that interleave with the plurality of second electrode fingers 20'. The first electrode 18'couplestheconductivelayer10 capacitively in a similar manner to the coupling of the electrode 18 to the conductive layer 10 in the embodiment of figures 1 and 3, except from the upper (or second ; side 21. The second electrodes 20'may function in a manner similar to that already described for the embodiment of figure 3, the electrodes being multiplexed in the receive mode in order to determine the coding cf the piezoelectric layer 12. In the receive mode, the ground electrodes 18, 18' may be set to ground.

The arrangement according to figure 5 would enable testing in a particularly simply manner, by provision of all the electrodes on a single pad or surface that can be pressed against the document to be tested. The testing arrangement of figure 5 couid be duplicated for abutment against both sides 19, 21 of the document, such that testing can be effected without regard to o orientation of the document.

The testing apparatus may be simple and d costeffective by provision of the electrodes 18, 18', 20, 20'as conductive pads on a printed circuit boar on one side thereof, and by placing the signal processing electronics on the other side thereof.

Claims (10)

1. CLAIMS 1. A method of detecting the presence of a security film embedded in a document, the security film comprising a piezoelectric polymer (12) having at least one pcled region (14) susceptible of producing electrical charges under mechanical or pyroelecric stimulation, comprising the steps of : placing the security film in an alternating electrical field adapted to heat he piezcelectric material due to the high dielectric Loos facotr of the piezolectric polymer ; and detecting the electrical charge generated by thermal stresses of the poled region.
2. 2. The method of claim 1 wherein the electrical field ~s created by the provision of first and second eiectrodes (18, 18', 20, 20') whereby an alternating voltage between the first and second electrodes is produced.
3. 3. The method of claim 2 wherein the first and second electrodes (18, 20) are biased against opposed first and second surfaces (19, 21) of the document.
4. 4. The method of claim 2 whereir. the first and second electrodes (18', 2C') are positioned on a same second side (21) of the document.
5. 5. the method of claim 4 wherein the first electrode (18) comprises a plurality of fingers (19) arranged in a juxtapose manr. er and commence together, and the second electrodes (20') are in the form of a plurality of fingers interleaving with the fingers (19) of the first electrode.
6. 6. The method of anyone of claims2-5whereina plurality of second electrodes (20') are provided in a spaced juxtaposed arrangement for detecting the presence of poled (14) or unpoled (16j regions of the security film.
7. 7, re method of anyone of claims 2-6 wherein the second electrodes (20') comprise a plurality of fingers for positioning transversely across the seurity film (12) for detecting the presence of poled (14) and unpoled regions (16) therealong.
8. 8. The method of anyone of claims 2-7 wherein the second electrodes (20') are connected to a multiplexer (28) for individually reading said electrodes in the detection mode.
9. 9. The method of anyone of claims 2-8 wherein the electrodes for creating the alternating field are also used for detecting the electrical response of the poled piezoelectric regions (14), by switching electrodes from emit to receive mode after emitting a short burst of alternating voltage.
10. 10. The method of detecting the presence of a security film embedded in a document substantially as hereinbefore described with reference to the accompanying drawings.