GB2287339A - Security sensors - Google Patents

Abstract

A security sensor comprising a tab 25 with an adhesive backing 56 for bonding the tab to an item which is to be protected and an electrical sensing element 53 interposed between the adhesive and the tab. In use, the tab cannot be separated from the item without stressing at least a portion of the sensing element, thereby producing a measurable change in the restrictive properties of the sensor. <IMAGE>

Description

SECURITY SENSOR This invention relates to a security sensor.

The invention finds a particular. but not exclusive. application in the provision of a security tag attached to consumer goods displayed in a retail outlet. Such tags are commonly used in retail outlets as a deterrent to the shoplifter. The tags are attached to the goods on display and form part of a circuit which is monitored by a loop alarm system. When the tag is separated from the goods without authorisation. the circuit becomes open and the alarm is triggered.

For electrical goods. a tag or sensor may plug into an output or input socket (for instance the aerial socket of a television). and the alarm system would monitor the internal resistance. Other sensors. for general use. include limpet-type micro switches which are attached to the goods being displayed. The micro switch may be closed when attached to the goods and open when removed.

A disadvantage with known sensors is that it is possible to carefully remove the sensor without setting off the alarm: it is also possible to cut the cable connecting the sensor to the alarm system while simultaneously shorting the wires in the cable so that the alarm system is not aware that the circuit has been broken. Furthermore, limpettype micro switches are difficult to attach to curved surfaces such as china or glassware. Another disadvantage is that limpet-type micro switches are relatively expensive to manufacture.

Another application of the invention is the provision of a security tag for sealed containers. packages or documents where the tag is attached across the seal.

According to the invention there is provided a security sensor comprising: a substrate with an adhesive backing for bonding the substrate to an item being protected, and a conductive sensing element interposed between the adhesive and the substrate. In use the sensing element may form a conductive iink in a monitor circuit. and the monitor circuit then monitors the resistance and/or continuity of the link. Any attempt to separate the substrate from the item being protected stresses the sensing element. and this is detected as a change of resistance or an open-circuit. There the sensor forms part of a security seal. the monitor will detect when the seal has been tampered with and/or replaced.

Preferably. the conductive sensing element includes at least one discontinuity or weak link at which the element is liable to fracture or separate when the substrate is subjected to a force tending to separate the substrate from its adhesive backing or from the item being protected.

The substrate is preferably flexible and may be in the form of a tab having a generally circular head portion and an elongate tail portion.

In this case only the head portion may have the adhesive backing.

The head portion may then carry an open loop portion of the conductive element and the tail portion may carry a pair of leads to the open loop.

The conductive element is preferably deposited as a conductive track on the substrate. and at least the open loop part of the element may comprise conductive graphite which can be printed on the substrate as carbon ink. The conductive element preferablv has a predetermined resistance which is monitored by the monitor circuit. when the element is stressed. the resistance changes and this is detected by the monitor circuit. The element may be fractured completely to produce an opencircuit.

then conductive leads are provided in a tail portion of the tag. the leads can be formed separately from the open loop in the head portion.

and may comprise conductive silver. the leads overlapping the respective ends of the open loop to form an electrical connection and the remaining exposed portions of the leads being protected by a dielectric strip to prevent the leads being short-circuited.

The adhesive backing is preferably a double-sided adhesive backing, one side of the backing being bonded to the item being protected and the other side to the flexible substrate. The conductive element is then sandwiched between the adhesive backing and the substrate. In addition. a part of the substrate may be masked from the adhesive. for example by applying a peelable mask pattern on to the substrate before printing with carbon ink. Only the unmasked portions of the substrate are then printed with the carbon ink and only these portions are bonded to the item being protected, the bond between the adhesive and the mask being stronger than the bond between the mask and the substrate.

Accordingly. the stress applied to the carbon ink is particularly concentrated in the unmasked regions of the substrate if attempts are 6 made to separate the tag from the item being protected.

The open loop portion of the conductive element can be continuous or it can be formed as discrete segments. some segments being attached to the substrate and other overlapping or bridging segments being carried by the adhesive backing. The open loop may also include a severable link which ruptures when an attempt is made to separate the substrate from the item being protected.

The resistance of the conductive element is monitored by the monitor circuit so that any change in the resistance is detected. If the sensor is being used as a security tag to prevent theft of a consumer product. the change of resistance detected by the monitor circuit may trigger an alarm.

Alternatively. the security tag may form part of a security seal which prevents unauthorised tampering of goods during transit. In this case, the change of resistance detected by a monitor circuit at the destination of the goods will indicate that the seal has been tampered with and/or replaced.

If the sensor is connected to an alarm system through a cable. the alarm cannot be circumvented by cutting the cable and shorting the wires to the sensor because the monitor ciiWüit will detect the change in resistance.

Since the sensor cannot be removed from the item without changing the resistance of the conductive element and raising the alarm, it provides additional security compared to the shown limpet-type of sensor. It has the further advantage of being easily applied to curved surfaces.

To assist a fuller understanding of the above and other aspects of the present invention. some security tags embodying the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1(a) shows a plan view of a first embodiment with the top layer removed: Figure 1(b) shows an exploded schematic cross-section of the first embodiment: Figure 2 (a)-(e) shows plan views of the different layers of the first embodiment: Figure 3 shows a plan view of a second embodiment of the security tag; Figure 4 shows a schematic cross-section of the tag of Fig. 2; Figure 5 shows a plan view of the substrate of the tag of Fig. 2; Figure 6 shows a plan view of the backing to the substrate of Fig. 5t Figure a shows an exploded perspective view of a locking connector; Figure 7b shows a perspective view of the locking connector and security tag: Figure 8 shows a plan view of a third embodiment of the security tag; and Figure 9 shows a schematic cross-section of the third embodiment.

Referring to Figures 1 and 2, a security tag 10 is shown comprising a flexible polyester substrate 11 having a circular head portion 25 and an elongate stem 19 forming a tag which can be adhesively bonded to the surface of a retail product. The substrate 11 can be made from HS polyester with a thickness of about 0.125mm, each face being printed with black ink (not shown). The end of the elongate stem 19 has a shoulder 16 with a pair of electrical terminal pads 5sa. 58b for connection to a cable which forms part of a conventional loop alarm system.

The circular head portion 25 of the substrate 11 has a double-sided adhesive backing 56 for bonding the tag to an item being protected.

The adhesive 56 is provided with a release backing (not shown). A conductive track is interposed between the adhesive backing and the substrate. the track consisting of two parallel leads 12a. 12b connected to the respective terminal pads 58a. 58b and an open sensing loop 53. The leads 12a. 12b are formed. for example of conductive silver. while the open sensing loop 53 and the terminal pads are formed of conductive graphite (carbon ink) which is printed on to the substrate and overlaps the silver leads to provide an electrical connection. The remaining exposed portions of the silver leads are then covered by a dielectric strip 54, for example a dielectric ink, to prevent the tail from short-circuiting. The width of the track forming the open sensing loop 53 is approximately 4mm and its thickness is determined by the printing process. The dimensions of the track determine its resistance and its susceptibility to fracture and/or change of resistance.

A peelable mask pattern 52 is interposed between the substrate 11 and the open sensing loops 53. The pattern 52 has three discrete arcuate segments spaced apart from one another so that only portions of the substrate 11 aligned with gaps 52b between the segments are exposed to the conductive graphite 53 and the adhesive backing. The mask pattern is preferably deposited by screen printing a peelable ink.

Ulhen depositing the loop 53 of conductive graphite. there will be a discontinuity in the uniformity of the loop at each point where the graphite overlies a gap Sob in the peelable ink pattern 52. The discontinuities represent weak links in the conductive track which are liable to fracture when the tag is stressed. In particular. since the peelable ink 52 will adhere to the adhesive backing 56 more positively than it does to the polyester substrate 11. the ink will detach itself from the substrate if an attempt is made to separate the tag from the item being protected. On the other hand. the unmasked portion of the substrate 11 aligned with the gaps 52b in the peelable ink will be bonded to the adhesive backing 56 through the overlying portions 53b of the conductive graphite 53 so that the graphite layer will tend to fracture at these points. This in turn produces a change of resistance and the conductive loop will eventually open-circuit.

In use. after peeling away a release backing (not shown) from the adhesive 56 and bonding the tag directly to an item to be protected.

the loop alarm system monitors the resistance to the tag and remains on standby while measuring a closed-circuit for constant resistance. The flexible nature of the substrate increases the susceptibility of the conductive graphite loop 53 to fracture and change resistance. If an attempt to separate the tag from the item is made. the stresses applied to the conductive graphite loop 53 increase the resistance of the conductive path through the sensor and may even break the circuit. The loop alarm system detects the change in resistance and sets off the alarm.

The sequence of steps in a method of manufacturing the security tab will now be described with reference to Figure 2a-e. The steps are as follows: (a) The tab shaped flexible polyester substrate 11 is surface printed each side with black ink (not shown).

(b) A peelable ink pattern 52 is then screen printed on to the circular head portion 25 of the flexible substrate to form a mask pattern.

(c) Conductive silver leads l2a. 12b are then applied to the stem 19 of the flexible substrate.

6 (d) An open loop 53 of conductive graphite (carbon ink) is printed over the peelable ink pattern and over the end portions of the silver leads.

(e) A dielectric ink strip 54 is then screen printed on to the remaining exposed portions of the silver leads. and (f) A double-sided adhesive backing 56 is applied to the circular head portion 25 of the flexible substrate overlaying the conductive graphite loop 53 and the peelable ink pattern 52.

The embodiment of the tag shown in Figs. 3-7 will now be described, using like reference numerals to denote like parts in the various embodiments.

Referring to Figure 3. a security tag 10 is shown comprising a flexible substrate 11 having a circular head portion 25 and an elongate stem 19.

Figure 4 shows more clearly how the circular head portion of the substrate 11 is provided with a double-sided adhesive backing 217.

Angularly spaced around the circular head portion 25 of the substrate are three conducting bridges 21Sa. 21Sb, 21sue which link four separated conductive quadrants 224a, 24b. 224c. 224d formed on one side of the adhesive backing 217.

The four separated quadrants 224a. 224b. 224c. 224d are within a circle having a smaller diameter than the circular head portion of the substrate so that the conductive quadrants are bordered by an annular ring of adhesive. A conductive sensing loop is formed through a terminal pad 221a lead 12a. quadrant 224a. bridge 21spa quadrant 224b, bridge 218b, quadrant 224c, bridge 218c, quadrant 224d. lead 12b and terminal pad 221b.

A release backing 222 is provided over the adhesive backing.

In use. after peeling away the release backing 222 and bonding the tag 10 to a product convenientlv where it will not be directly visible. the loop alarm system monitors the resistance of the tag and remains on standby while measuring a closed circuit or a constant resistance. If an attempt to remove the product is made, the tension in the connecting cable is transmitted to the tag. The resulting force may be great enough to fracture a part of the conducting layer 224 and therefore increase the resistance of the conductive path through the sensor. The loop alarm system detects the change in resistance. thereby setting off the alarm.

Simultaneously with or alternatively to the fracturing of the conducting layer 224. that part of the backing 217 that is not bonded to the substrate (i.e. the part covered by the conducting quadrant 224) and its connection with the bridges 218) will be pulled away from the substrate breaking the conductive path. This gives double protection against removal of the sensor. Furthermore, the small surface area of the bridges 21S ensure that the resistance change of the conducting layer 224 due to a fracture in one of the quadrants is maximised.

A further feature is the locking connector 226 used to connect the security sensor 10 to the cable. This prevents accidentally disengagement which would set off a false alarm.

The locking connector 226 comprises a two-part moulding having a top part 227 and a bottom part 228 and zero insertion force (ZIF) connector 229. The cable is attached to the ZIF connector 229 at one end and the pads 221 of the tag 10 are push-fitted into the socket of the connector 229. The bottom part 228 holds the ZIF connector 229 which is connected to the cable, the cable being held in position by means of a cable grip 31 moulded into the bottom part 228. The top part 227 comprises a cover having an opening 232 including a narrow part 232a and a wide part 232b which is positioned adjacent the socket of the ZIF connector 229. The wide part 23orb is large enough to accommodate the shoulder portion 16 of the flexible membrane 11 and the narrow part is only large to accommodate the narrower elongate stem part 19 whereby the shoulder may be pushed through the wide part but may not be pulled through the narrow part.

In use. the top part is attached to the bottom part by means of a screw so that the socket of the ZIF connector 229 is accessible only through the narrow part of the opening. The top part 227 may be resiliently displaced relative to the bottom part so that the socket on the ZIF connector is accessible through the wide part and the shoulders may fit through and be inserted into the ZIF connector; the cover may be released then thereby trapping the shoulder part in the connector.

The embodiment of the tag shown in Figs. S and 9 will now be described.

Referring to Figure 8 a plan view of a security tag 10 of the third embodiment is shown comprising a flexible substrate 11 supporting an electrically conducting track 12. The shaded part of Figure 8 indicates a region of adhesive 214 on the flexible substrate 11.

The substrate 11 has a circular head portion 25 with an elongate stem 19 forming a tag which can be adhesively bonded to the surface of a retail product. The head portion 25 is formed in two parts. The first part is a central disc 317. and the second part is an annular band 316 bounding the disc 317. the two parts being joined by a pair of opposed severable links 318 which bridge the gap 326 separating the two parts.

A pair of electrical connectors 321 are crimped or soldered to the respective ends of the conducting track 12 at the end of the stem 19.

The conductive track 12 consists of a pair of silver leads 12a.12b extending along the stem 19 and connected to the respective connectors 221. and a zig-zag resistive section 312 of doped carbon carried on the central disc 17. The resistive section 312 is joined to the leads 12alb through the severable links 1S such that. if either link is severed. an open circuit appears across the terminal connectors 21.

i Referring to Figure 9. it can be seen that the conductive track 12 is sandwiched between the flexible substrate 11 and the adhesive coating 214. This helps to prevent unauthorised removal of the security tag 10 from a product. .4 release backing 222 is provided over the adhesive region 214.

In use. after peeling away the release backing 22 and bonding the tag 10 discretelv to an item, the loop alarm system monitors the resistance of the tag and remains on standby while measuring a constant resistance. If a fraudulent attempt to remove the item is made, the tension in the connecting cable is transmitted to the tag. The resulting force may be great enough to remove the stem 19 and the annular part 16 of the head 25 from their adhesive attachment to the item. but would then sever the structurally weak links 31S so that the central disc 317 remains attached to the item. The loop alarm system would detect the change in resistance, thereby setting off the alarm.

Claims (26)

1. A security sensor comprising a substrate with an adhesive backing for bonding the substrate to an item being protected and an electrical sensing element interposed between the adhesive backing and the substrate. the arrangement being such that. in use. the substrate cannot be separated from the item being protected without stressing at least a portion of the sensing element.

2. A sensor according to claim 1 wherein the stressing of the element changes an electrical characteristic of the element.

3. A sensor according to claim 1 or 2 wherein the sensing element includes at least one discontinuitv or weak link at which the element is liable to fracture or separate when the element is stressed.

4. A sensor according to any one of the claims 1. 2 or 3 wherein the substrate is flexible.

5. A sensor according to claim 4 wherein the substrate is in the form of a tab having an enlarged head portion and an elongate tail portion.

6. A sensor according to claim 5 wherein the sensing element is conductive and the head portion carries an open loop portion of the sensina element.

7. A sensor according to claim 6 wherein the elongate tail portion of the tab carries conductive leads that connect a pair of terminals to the open loop portion of the sensing element.

8. A sensor according to claim 7 wherein the tail portion of the tab further includes a dielectric strip superposed on a portion of the conductive leads.

9. A sensor according to claim 2 in which the sensing element is a conductive path having a predetermined resistance and connected across a pair of terminals.

10. A sensor according to claim 9 wherein the conductive path comprises a layer of graphite.

11. A sensor according to claim 10 wherein the graphite is printed on the substrate as carbon ink.

12. A sensor according to ang one of the preceding claims further comprisina a mask pattern interposed between the adhesive backing and the substrate such that only unmasked portions of the substrate contact the adhesive backing.

13. A sensor according to claim 12 wherein the bond formed between the adhesive backing and the mask is stronger than the bond formed between the substrate and the mask. whereby the substrate peels awav from the mask to stress the sensing element.

14. A sensor according to any one of the claims 9-11 wherein the conductive path is formed as discrete segments. at least one segment being attached to the substrate and at least one segment being carried b the adhesive backing.

15. A sensor according to claim 14 wherein at least one of the segments is a bridging segment spanning a gap between a pair of overlvin segments.

16. A sensor according to any of the preceding claims wherein the sensing element includes a severable link which is severed in response to attempted separation of the substrate from the item being protected.

1-. A security tag attached across the seal of a sealed container or pac.kta2e. rhe tag including a sensor as claimed in any one of the preceding claims.

1S. A method of manufacturing a security sensor, the method comprising: applying an open loop conductive sensing element to one side of a flexible non-conductive substrate. and applying a double-sided adhesive backing to the substrate such that the sensing element is interposed between the adhesive and the substrate.

19. A method according to claim 1S further comprising: initially applying a mask pattern to the substrate before applying the conductive sensing element, the bond formed between the mask pattern and the adhesive being stronger than the bond between the substrate and the mask pattern whereby the substrate can be peeled awày from the mask pattern.

20. A method according to claim 19 wherein the mask pattern is printed on the substrate.

21. A method according to claim 20 wherein the mask pattern is screen printed on the substrate.

22. A method according to any one of the claims 19-21 wherein the conductive sensing element is printed on the substrate over the mask pattern before applying the adhesive backing.

23. A security sensor system including a security sensor according to anv one of the claims 1-16 and a monitoring circuit for monitoring the resistance of the conductive sensing element.

24. A security sensor according to claim 1 and substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.

25. A security sensor according to claim 1 and substantially as herein described with reference to Figs. 3-6 of the accompanying drawings.

26. A security sensor according to claim 1 and substantially as herein described with reference to Figs. 8 and 9 of the accompanying drawings.