GB2064222A - Security alarm sensor element - Google Patents

Abstract

A sensor element (12) for use with a security alarm system comprises a collapsible and self- recoverable central layer (40) of electrically nonconductive material having a plurality of passages (42) therethrough defining air spaces, a pair of electrically conductive films (28, 30) positioned on opposite sides of the central layer, and a pair of dielectric outer layers (32, 34) attached to the conductive films, the conductive films (28, 30) being able to contact each other in the area of the passages (42) when pressure is applied to the element. The dielectric outer layers may have at least two opposing edge portions extending laterally beyond the central layer and the conductive films and sealingly bonded to each other (at 36). The conductive films (28, 30) are bonded (at 44) to opposite sides of the central layer (40) along lines located between the passages (42) to form a manifold defined by the conductive films and central layer and communicating with the air spaces (42). <IMAGE>

Description

SPECIFICATION Security alarm sensor element This invention relates to an improved sensor element responsive to pressure to trigger a security alarm system.

There have heretofore been proposed a variety of different types of elements for detecting the presence of an intruder on premises being secured. Prior elements for sensing changes in pressure as part of a security alarm system have generally included a series of ribbon switches wherein two steel strips or ribbons are spaced at regular intervals by a plurality of spacers. Inward pressure applied to one of the ribbons at a location between the spacers deforms the ribbons by forcing them together to close the circuit between the ribbons and activate the alarm system.

Removal of pressure from the element then allows the element to be returned to the initial open circuit condition by the "memory" of the steel ribbons. A series of such ribbon switches are generally enclosed between two vinyl or rubber sheets to form a pressure-sensitive protective element of a predetermined size and shape. These pressure sensors cannot be trimmed or cut down after assembly without damage. The sensors also involve substantial manufacturing costs due to the number of individual elements which must be accurately assembled and the number of soldering operations necessary electrically to connect the ribbon switches to each other, and are prone to malfunction due to broken connecting wires and solder joints.Furthermore, elements incorporating ribbon switches of this type are able to sense the application of pressure only in the vicinity of one of the ribbons, and are basically insensitive to the application of pressure at other points on the outer vinyl sheets. While the chances of an intruder avoiding detection by applying pressure only to the areas between the switches can be reduced by increasing the number of switches and decreasing the spacing between the switches, this construction greatly increases manufacturing costs.

The location of the individual ribbon switches in a particular installation can also be easily detected by an intruder using a compass by simply passing the compass over the floor covering and observing the movement of the compass needle. The steel or other ferrous material having sufficient "memory" to effect recovery of the ribbons will influence the needle of the compass and disclose the location of the switches. Once the locations of the switches are known, the intruder may simply step around them to avoid triggering the alarm.

A further type of pressure sensor useful in security alarm systems is disclosed in United States Patent Specification No. 1,658,848 which describes, among other things, a layer of springy insulating material having a plurality of perforations and sandwiched between a pair of metallic sheets which are electrically connected to an alarm system. The application of inward pressure compresses the insulating material and closes the alarm circuit by contact of the metal sheets at the locations of the perforations. The elements of this switch are simply superimposed on one another without physical connection. They also end abruptly at the edge and are in no way protected from the environment.Without any form of connection between the various layers, the insulating material of this structure would undoubtedly slide laterally relative to the metal sheets and therefore wrinkle or bunch up at one or more points. As the insulating material bunches up, the precise separation of the metal sheets is lost. The separation at some points is much greater than intended and at other points may be nonexistent due to the removal of the insulating material from those areas. When this happens, the device becomes unable to function in the desired manner. Any uniform application of a laminating adhesive or other compound between the metal sheets and the insulating material would render the device unable to operate. If the adhesive were applied to all points on the metal sheets, it would act as an insulator preventing contact of the conductors.If the adhesive were somehow applied to all points on the metal sheets other than those directly opposite the perforations, there would be no path for the air within the perforations to escape when the insulating material is compressed or to return when the insulating material is allowed to recover. This condition would either prevent operation of the device by maintaining the space between the metal sheets at all times, or cause damage to the device by forcing the air outwardly through the insulating material and thus rupturing the insulating material.

Another prior pressure sensing device is disclosed in United States Patent Specification No. 4,137,1 16 in the context of a sensor to be positioned within an automobile seat cushion to indicate whether the seat is occupied. This switch includes a pair of flexible outer layers positioned on opposite sides of an intermediate foam layer and bonded at opposite lateral edges to the foam layer. Positioned between the foam layer and each of the outer layers is a further flexible layer having a foil laminated to the inner side thereof. These intermediate flexible layers do not extend laterally as far as the flexible outer layers and the foam layer, and are not bonded in any way thereto. They are physically confined between the layers without being otherwise secured.In use as an undercarpet pressure sensor, the intermediate layers would thus have a tendency to shift about and bunch up in much the same manner as the insulating material of the device of Patent No. 1 ,658,848.

While some portions of the switch element would be inoperative in the open condition due to the absence of one of the foil contact elements therefrom, other portions could be inoperative in the closed condition due to the undesired compression of the foam layer by the bunched up intermediate layer or the projection of a portion of the intermediate layer through one of the openings in the foam layer and into permanent contact with the other intermediate layer. Any attempt uniformly to bond the foam layer to one or both of the intermediate layers would, of course, suffer the drawbacks discussed above in relation to the previous device. The bonding of the lateral edges of the outer layers to the foam layer would also yield a device unsuitable for undercarpet use due to the direct exposure of the foam layer to moisture and other contaminants.A single instance of liquid spillage on the carpet or shampooing of the carpet would thus cause the foam layer to absorb at least some liquid and would produce a permanent short circuit.

Therefore, in many applications it is desirable to provide a reliable pressure sensor which is inexpensive to manufacture and which can be supplied in bulk and trimmed to the desired size and shape on installation.

According to the present invention there is provided a sensor element for use with a security alarm system having an electrical sensing circuit, comprising: a collapsible and self-recoverable central layer of electrically nonconductive material having a plurality of passages therethrough defining air spaces; a pair of electrically conductive films positioned on opposite sides of said central layer; a pair of dielectric outer layers attached to said conductive films, the conductive films being held in a pre-determined spaced apart relationship preventing the passage of electrical signals therebetween for the central layer in a normal recovered condition and being allowed to contact each other in the area of said air spaces for the central layer in a collapsed condition; means for bonding the conductive films to the central layer at locations between the passages in the central layer; and means for electrically connecting the conductive films to respective leads in the sensing circuit of an alarm system, whereby the sensor will switch between said conditions to make and break the sensing circuit in response to predetermined changes in the inward pressure on the outer layer.

The means for bonding the conductive films to the central layer may comprise means for bonding the conductive films to opposite sides of the central layer on lines located between the passages to form a manifold defined by at least one of the conductive films and the central layer and communicating with the air spaces, such that air within the air spaces is freely redistributed through the manifold when the central layer is collapsed or allowed to self-recover. The means for electrically connecting the conductive films to respective leads in the sensing circuit of an alarm system may comprise a pair of eyelets, each of which is secured to one of the conductive films and to the corresponding upper layer to both make the electrical connection and to mechanically anchor the leads of the sensing circuit to the respective outer layers.The conductive films may comprise either a metallic material deposited on the inner surfaces of the outer layers or thin metallic foils laminated to the surfaces. The dielectric outer layers may have at least two opposing edge portions extending laterally beyond the central layer and the conductive films and bonded to each other to seal out the elements.

The remaining portion of the element's periphery may be sealed with an adhesive-backed tape to prevent the intrusion of water or other contaminants. In addition, the central layer may extend a relatively short distance laterally beyond the conductive films to eliminate the possibility of a short circuit between the conductive films at the lateral edges thereof. This distance may be on the order of 6 to 13 mm to account for manufacturing tolerances of material size and alignment.

It is also intended that in some instances a construction utilizing a second collapsible and self-rscoverable layer having conductive films on either side will be placed above or below the sensor element described above and bonded thereto. In this configuration, the device of the present invention may operate as a pair of independent pressure switches for connection to separate sensing circuits.

It is an object of the present invention to provide a pressure sensing structure which is extremely reliable and durable.

It is another object of the present invention to provide a pressure sensing structure which is inexpensive both to manufacture and install.

It is a further object of the present invention to provide a pressure sensing structure which may be easily trimmed to a desired shape or size on installation.

The sensing structure of the present invention may be manufactured as a continuous sheet which may be cut to a desired size or shape by an installer. The electrical connections are made with a pair of simple metallic eyelets or grommets crimped or otherwise applied by the installer to the respective dielectric outer layers and the conductive films attached thereto. The cut edges may be taped or otherwise sealed, if desired, to prevent water intrusion. The present invention thus eliminates the expensive fabrication of sensor elements to precise sizes and shapes, and further eliminates the need to solder connecting wires between the metallic contacts of a great many ribbon switches. The connection of the lead wires achieved with the pair of metallic eyelets provides both the electrical connection to the conductive films and a secure mechanical connection to the outer layers. This structure is able to withstand substantial forces on the connecting wires of the alarm circuit without damage.

Self-recoverability of the switch is effected by resilience of the central layer, rather than any "memory of the conductive films. The films can therefore be made of aluminium copper or any other non-ferrous conductor, making them undetectable by magnetic means. This is a significant advantage over ribbon switches from the standpoint of security.

The bonding of the conductive films to opposite sides of the central layer as discussed herein prevents any relative sliding movement between the central layer and the conductive films while at the same time forming a manifold communicating with the air spaces to allow redistribution of the air within the spaces when the central layer is collapsed or allowed to self-recover. The many problems of relative sliding motion present in prior devices are thus entirely eliminated while at the same time providing for the conduction of air between various parts of the sensor to facilitate collapse and self-recovery of the central layer. The sensor is able to operate indefinitely in this way without damage.Confinement of the bonding pattern to a plurality of spaced lines also enables portions of the sandwich structure to be cut away for making electrical connections to the sensing circuit as with the eyelets described above.

The location of the bond lines between the passages is accomplished by application of an adhesive or other bonding agent to the conductive films such that upon lamination the agent contacts the central layer only between the passages. No adhesive is present on the portions of the conductive films which close off the passages, and thus the films are free to make contact with each other at the passages when the central layer is compressed.

The bonding of the opposite lateral edges of the outer layers to each other where they extend laterally beyond the central layer and the conductive films effectively seals the interior of the sensor from entry of contaminants along those edges. As the bulk laminated material comes off the roll, these are the lateral side edges of the mat.

The installer then simply tapes the cut edges where the piece used is cut from the roll to entirely seal the sensor from the elements.

The many structural features described herein, including thin conductive films carried by flexible outer layers and bonded in the disclosed manner to a central compressible layer, yield a pressure sensing structure which is uniquely adapted to operate continuously in the intended manner without malfunction. Even if the conductive films become damaged, the device will continue to operate in the intended manner due to the fixed relative positions of the various components and the simple theory of operation. The thin conductive films will always return to the initial open circuit condition under the influence of the control layer due to their general lack of resilience.

This is not true of ribbon switches, which can become kinked or bent to permanently close the sensing circuit.

The exemplary device of the present invention also serves to enhance the thermal insulation of the surface to which it is applied. Fire retardant foam material is now available to meet current Underwriters Laboratories specifications and is ideal for use as the central layer of the sensor element. In the preferred form of the present invention, the flexible outer layers and the adhesives used are also fire retardant.

The present invention may be more fully understood from the following detailed description given by way of example with reference to the accompanying drawings, wherein FIG. 1 is a perspective view of a typical undercarpet installation of a pressure sensor constructed in accordance with the present invention, in conjunction with an alarm system which is shown somewhat diagrammatically; FIG. 2 is a perspective view of a roll of bulk pressure sensing mat material of the type illustrated in FIG. 1; FIG. 3 is a vertical sectional view of the mat material of FIG. 2 in its initial and recovered condition, taken along the line 3-3; FIG. 4 is an enlarged fragmentary vertical sectional view of the sensor shown in FIG. 1 taken along the line 4-4;; FIG. 5 is a vertical sectional view illustrating the structure of FIG. 3 in its compressed condition; FIG. 6 is a vertical sectional view of a second embodiment of the present invention; FIG. 7 is a perspective view of the device constructed in accordance with the present invention in use as a typewriter undermat; and FIG. 8 is a diagrammatic perspective view illustrating the manufacturing process of the device of the present invention.

Description of the Preferred Embodiments Referring now to the drawings, there is illustrated in FIG. 1 thereof, a preferred embodiment of the present invention generally designated 10. The embodiment 10 includes a composite mat 12 connected to an alarm system 14 through electrical leads 16 anda releasable connector 1 8. The mat 12 is positioned beneath a carpet or other floor covering 20 and is sealed at its cut edges by adhesive-backed tape 22.

The alarm system 14 may be any conventional electrical alarm system having a normally open sensing circuit including the leads 24 and designed to produce an alarm signal when the sensing circuit is closed. In the embodiment of FIG. 1, the sensing circuit is closed whenever a predetermined amount of downward pressure is applied to the carpet 20, compressing the mat 1 2.

The mat 12 is shown in greater detail in FIGS.

2, 3 and 5. Referring first to FIG. 3, it can be seen that the mat 1 2 comprises a central core layer 26 sandwiched between upper and lower conductive films 28 and 30, respectively, and the entire structure is enclosed between upper and lower flexible outer layers 32 and 34. Each of the conductive films 28 and 30 is bonded to the corresponding flexible outer layer such that the conductive films and the outer layers move and flex together. The upper and lower conductive films 28 and 30 are continuous across the face of the core layer 26 and terminate a relatively small distance short of the lateral edges thereof. The flexible outer layers 32 and 34, however, extend laterally beyond both the core layer 26 and the conductive films to form an overlapping heat sealed edge 36 at opposite sides thereof.As shown in FIG. 2, the heat sealed edges 36 form the lateral edges of the composite mat 12 as it comes off a roll 38 of bulk material. The sides of the mat 12 are thus presealed in their bulk condition, reducing the number of edges which must be manually sealed with tape 22. The central core layer 26 is a sheet of compressible and selfrecoverable material having a pattern of circular passages 40 extending therethrough. The passages 40 bounded on either side by the conductive films 28 and 30 thus form a plurality of air spaces 42. Each of the conductive films 28 and 30, and the respective outer layers attached thereto, are bonded to opposite sides of the core layer 26 by lines of adhesive compound 44.The conductive films 28 and 30 are free of the core layer 26 at all points other than the lines 44, enabling the air spaces 42 to communicate with one another on either side of the core layer.

The preferable manner of connecting the electrical leads 1 6 to the mat 12 is illustrated in detail in FIG. 4. The outer layer 34, the conductive film 3Q and the core layer 26 are cut at a first location to form a cutout 46 extending upwardly from the lower side of the mat 12, while the upper layer 32, the conductive film 28 and the core layer 26 are cut at a second location to form a cutout 48 extending downwardly into the end portion of the mat 12. A pair of metallic eyelets or grommets 50 made up of first and second interfitting annular portions 51 and 53 engage respective openings 55 in the mat 12 at the cut-outs 46 and 48.Each of the annular portions 53 carries a ring terminal or lug 57 having a connector sleeve 59 crimped about one of the leads 1 6 and projects into one of the openings 55 from the side of the corresponding conductive film. The annular portions 51 extend through the respective openings 50 and through oppositely located annular portions 53, and are flared outwardly at points beyond the annular portions 53. This tightly clamps the grommets 50 about the respective outer layers and conductive films, with the lugs 57 contacting the conductive films. The flaring may be accomplished by crimping the portions 51 and 53 together, providing a secure pressure connection of the leads 1 6 to the mat 12 both electrically and mechanically.The electrical leads 1 6 are connected to the relatively durable flexible outer layers 32 and 34 as well as to the conductive films 28 and 30, substantially increasing the strength of the connection to the mat 12. The device of the present invention is thus able to survive continued use and rough handling without being damaged. In fact, it has been found that the structure described herein for connection of the electrical leads 1 6 to the mat 12 will survive a ten pound pull test required for Underwriters Laboratories approval.

The operation of the mat 12 is seen most clearly from a comparison of FIGS. 3 and 5. FIG. 3, as described above, illustrates the mat 12 in either its initial or recovered (open circuit) condition. FIG.

5 illustrates the mat 12 in its compressed (closed circuit) condition. In the condition of FIG. 3, the upper and lower conductive films 28 and 30 are spaced at all points either by the central core layer 26 or the air spaces 42. The relatively low voltage differential between the conductive films is insufficient to produce an arc across the air spaces 42. The circuit between the conductive films of the mat 12 is thus held open when the mat is left undisturbed. Upon application of a predetermined inwardly directed force to one of the outer layers 32 or 34, the core layer 26 is compressed to the condition shown in FIG. 5. The volume of each air space 42 in the area of the applied force is thus greatly reduced, bringing the upper and lower conductive films 28 and 30 into direct contact with one another at the passages 40.This closes the circuit of the mat 12, and thus the circuit of the alarm system 14, causing the alarm system to produce a warning signal. The air within the spaces 42 adjacent the applied force is driven from the spaces 42 as the core layer 26 collapses.

This air passes into the region between the conductive films 28 and 30 and the core layer 26, causing the conductive films and the outer layers bonded thereto to move slightly away from the core layer 26. The conductive films and the core layer thus act as a form of manifold for conducting the expelled air away from the compressed region.

This manifold action is made possible by the lines of adhesive compound 44 which serve to eliminate any relative sliding motion between the components of the mat 12 without unduly restricting relative movement of the components in a direction perpendicular to the mat 12. Due to the relatively small volumes of air held within the air spaces 42, the desired air flow is achieved with a relatively small outward movement of the conductive films. This movement is generally imperceptible once the mat 12 has been installed beneath a carpet or other floor covering, but is sufficient to prevent bursting or other damage to the mat 1 2 adjacent to the air spaces 42 and to enable the mat 1 2 to be readily compressed by a force of predetermined magnitude.Upon removal of the inwardly directed force, the core layer 26 self-recovers to its initial thickness, restoring the air spaces 42 to their initial volume. This again separates the conductive films at the passages 40 to reopen the circuit of the mat 12 and the sensing circuit of the alarm 14. The air previously expelled into the manifold formed between the conductive films and the core layer 26 is drawn back into the air spaces 42, allowing the conductive films to lie back down against the core layer 26. The mat 12 thus completely self-recovers to the condition of FIG. 3, in which it is prepared to again sense the application of inwardly directed pressure. The device disclosed herein is reusable on an indefinite basis.

A second embodiment of the present invention.

is illustrated in FIG. 6, wherein a composite mat 1 2a is provided with a core layer 26a sandwiched between conductive films 28a and 30a of slightly lesser lateral dimension than the core layer 26a and also between flexible layers 32a and 34a. The mat 1 2a is provided at its lower surface with a third conductive film 52, a second core layer 54, a fourth conductive film 56 and a third flexible layer 58. The third conductive film 52 is bonded to the lower side of the flexible-layer 34a while the fourth conductive film 56 is bonded to the upper side of the third flexible layer 58. The core layer 54 is provided with a pattern of openings 60 similar to openings 40a in the core layer 26a.The conductive films 52 and 56 are of slightly lesser lateral dimension than the core layer 54 and are attached to the core layer 54 by parallel lines of adhesive compound 62 similar to lines of adhesive 44a connecting the films 28a and 30a to the core layer 26a. The peripheral edges of the flexible layers 32a, 34a and 58 extend outwardly beyond the core layers 26a and 54 to the edge 36a, at which the layers 32a and 58 are heat sealed to opposite sides of the flexible layer 34a. The mat 1 2a is thus similar in structure to the mat 12, however, it includes a second core layer 54 sandwiched between third and fourth conductive films'52 and 56, respectively.The mat 12a may thus be wired into two independent sensing circuits either for the purpose of providing a system with a built-in double check on itself or for sensing the application of two distinct levels of pressure. In the case where different pressure levels are to be sensed, the core layers 26a and 54 will differ in density or other physical variables to cause the upper and lower portions of the mat 1 2a to switch at different pressures. It is also possible to provide a short circuit between the conductive film 30a and conductive film 52, if desired. The short circuit may take the form of a common ground, rendering the mat 1 2a essentially a three electrode pressure switch having a grounded center electrode.It is, of course, possible for these purposes to eliminate the flexible layer 34a entirely and form the conductive films 30a and 52 of a single conductive sheet.

In operation, the mat 1 2a compresses and recovers in a manner similar to that of the mat 12 described above, with the various conductive films forming manifolds with the different core layers to facilitate the passage of air from and to the air cavities within the core layers during compression and seif-recovery of the mat.

FIG. 7 illustrates an embodiment of the present invention designated generally lOb. Embodiment 1 Ob comprises a mat 1 2b supporting a typewriter 64 and connected to an alarm system 1 4b having a normally closed sensing circuit 24b.

Embodiment 1 Ob is designed to produce a warning signal upon the removal of an object such as a typewriter 64 from the mat 12b. The mat 1 2b may have the structure of either the mat 12 or the mat 12a, and functions in generally the same way.

The unique feature of the mat 1 2b is that on installation an object of sufficient weight to close the circuit of the mat 1 2b is intentionally placed thereon. The mat 1 2b is thus closed when it is installed, maintaining the sensing circuit 24b of the alarm system 1 4b in its normally closed condition. It is only on the removal of the typewriter or other object that the one or more core layers within the mat 1 2b are allowed to selfrecover to separate the conductive films of the mat 1 2b and open the sensing circuit 24b. When the circuit 24b opens, the alarm system 1 4b emits a signal indicating that the object has been removed.

The manufacturing process of the mat 1 2 is illustrated diagrammatically in FIG. 8. A central roll 66 carrying a web 67 of core layer 26 is flanked on top and bottom by rolls 68 and 70, respectively. The web 67 is a flexible and compressible material having a plurality of circular passages 40, as described above. The roll 68 carries a composite web 72 comprising the upper conductive film 28 laminated to the lower surface of the upper flexible outer layer 32. Similarly, the roll 70 holds a lower composite web 74 comprising the lower conductive film 30 laminated to the upper surface of the lower flexible outer layer 34. The composite webs 72 and 74 engage opposite surfaces of the web 67, with which they are fed between a pair of first rollers 76.As the webs 72 and 74 are drawn from their respective rolls, parallel lines of adhesive compound 44 are applied to the conductive films thereof by adhesive applicators 78 and 80. The adhesive applicators 78 and 80 are supplied by an adhesive reservoir (not shown) and may be provided with a series of wicks or other elements 84 for transferring the adhesive to the conductive films. The first rollers 76 press the webs 72 and 74 against the web 67 to cause them to be bonded to the web 67 along the lines 44. The lines 44 are positioned to fall on the web 67 in the regions between the passages 40 to prevent any interference with the closing and opening of the switch formed.The application of adhesive to the conductive films 28 and 30, rather than directly to the web 67, is crucial to the operation of the mat 12 when the web 67 is made of foam material which has a tendency to absorb liquids.

Absorption of adhesive applied directly to the foam causes an excessive amount of adhesive to be transferred to the foam, filling the pores of the foam and seriously interfering with the collapsibility and self-recoverability of the core layer. The conductive films 28 and 30, however, are non-porous and therefore present no problems of adhesive application. Once the surfaces of the conductive films carrying the adhesive are pressed against the surface of the web 67, bonding is achieved with precisely the desired amount of adhesive. In effect, the adhesive is "printed" onto the web 67 via the conductive films 28 and 30 rather than being applied directly thereto. After passing through the first rollers 76 and being sealed together along the lines 44 the composite mat 12 is drawn through a pair of edge sealing rollers 86 having recessed center regions 88. The rollers 86 thus engage the edges of the mat 12 in the area where the outer layers 32 and 34 extend beyond the conductive films and the core layer.

The rollers 86 are heated and press the outer layers together in a manner sealing the edges of the mat 1 2 against the intrusion of water or other contaminants. As the mat 12 passes from the rollers 86, it is taken up to form the roll 38 of bulk material illustrated in FIG. 2. This is the form in which the bulk material is supplied to users for installation relative to a particular structure.

To install the mat 12, a worker need only cut a desired length of material from the roll 38, make the cutouts described in relation to Figure 4 with a pair of scissors, and apply the wire carrying grommets 50 at the locations of the cutouts. The mat 12 is then connected into the circuit of the desired alarm system and may be sealed by application of tape 22 to the cut edges thereof. If the area to which a pressure sensor is to be applied is wider than the roll 38, more than one strip of the material can be cut from the roll. On the other hand, a section of the mat 12 may be cut in any desired shape or size less than the width of the roll 38 with a pair of scissors and installed in the same manner. The tape 22 is then applied to every cut edge, since part or all of the heat sealed edges 36 will have been cut off.The mat 12 supplied in rolls 38, is thus an extremely versatile material able to be inexpensively manufactured and installed in any desired configuration.

The conductive films 28 and 30 are preferably made of aluminum or other non-ferrous material and may comprise either metallic elements vacuum deposited onto the surfaces of the flexible outer layers 32 and 34, respectively, or metallic sheets laminated thereto. The conductor is preferably on the order of one thousandth of an inch thick and is applied to flexible outer layers made of polyvinyl chloride or other suitable plastic material. The combination of each conductive film and the respective outer layer is flexible and selfsustaining to avoid inadvertent short circuits. The overal thickness of the mat 12 is preferably no greater than 2.4 mm and optimally approximately 1.6 mm.The core layer 26 of the mat 12 and the core layers 26a and 54 of the mat 1 2a may be made of any suitable open or closed cell foam material having the desired dielectric properties and the necessary compressibility and self recoverability. Foamed plastics are the most common of the materials meeting these criteria. In choosing such a material, it is important from a practical standpoint to satisfy the various Underwriters Laboratories specifications, particularly for fire retardance. A suitable closed cell foam material is cross-linked polyethylene foam. The use of foam materials has the added advantage of increasing the thermal insulation factor of the surface to which the mat 12 is applied.

The passages 40 and 40a are preferably 9.5 mm in diameter and arranged in rows 1.6 mm apart, with the lines of adhesive 44 being parallel thereto and positioned between each row or pair of rows. However, it will be understood that the passages may be arranged in any pattern allowing contact between the conductive films on the application of pressure to the mat. Similarly, other patterns of adhesive compound will function satisfactorily so long as sliding motion between the sheets is prevented and the adhesive does not close the air spaces off from each other.

It will be understood that the method of manufacture of the mat 1 2a is essentially the same as that of the mat 12, except for the fact that a greater number of layers are used.

Claims (20)

1. A sensor element for use with a security alarm system having an electrical sensing circuit, comprising: a collapsible and self-recoverable central layer of electrically nonconductive material having a plurality of passages therethrough defining air spaces; a pair of electrically conductive films positioned on opposite sides of said central layer; a pair of dielectric outer layers attached to said conductive films, the conductive films being held in a predetermined spaced apart relationship preventing the passage of electrical signals therebetween for the central layer in a normal recovered condition and being allowed to contact each other in the area of said air spaces for the central layer in a collapsed condition; means for bonding the conductive films to the central layer at locations between the passages in the central layer; and means for electrically connecting the conductive films to respective leads in the sensing circuit of an alarm system, whereby the sensor will switch between said conditions to make and break the sensing circuit in response to predetermined changes in the inward pressure on the outer layer.

2. A sensor element as claimed in claim 1, wherein said dielectric outer layers have at least two opposing edge portions extending laterally beyond the central layer and the conductive films and bonded to each other to seal out the elements.

3. A sensor element according to claim 1 or 2, wherein the central layer comprises a sheet of flexible and compressible plastic foam.

4. A sensor element according to claim 1,2 or 3, wherein the means for electrically connecting the conductive films to respective leads in the sensing circuit of an alarm system includes means for mechanically anchoring the leads of the sensing circuit to the respective outer layers.

5. A sensor element according to claim 4, wherein the connecting means comprises a pair of eyelets, each of which is secured to one of the conductive films and to the corresponding outer layer.

6. A sensor element according to claim 5, wherein one of the outer layers, one of the conductive films and the central layer are cut away adjacent the intended location of each of said eyelets to provide access for securement of the eyelets to the remaining elements.

7. A sensor element according to any preceding claim, wherein the outer layers are made of a plastics sheet material.

8. A sensor element according to claim 7, wherein the conductive films comprise a metallic material deposited on the inner surfaces of the outer layers.

9. A sensor element according to claim 7, wherein the conductive films comprise thin metallic foils laminated to the inner surfaces of the outer layers.

1 0. A sensor element according to any preceding claim, wherein said passages are arranged in rows and the means for bonding the conductive films to the central layer comprise bond lines between said rows and substantially parallel thereto.

11. A sensor element according to any preceding claim, which includes adhesive-backed tape applied to the unsealed portions of the periphery of the sensor element to prevent the intrusion of water.

12. A sensor element according to any preceding claim, which includes a third electrically conductive film attached to the outer surface of one of the outer layers, a second collapsible and self-recoverable layer having a plurality of passages therethrough and bonded to said third film, a fourth conductive film bonded to the outer side of said second collapsible and selfrecoverable layer, a third dielectric outer layer attached to said fourth conductive film, and means for electrically connecting the third and fourth conductive films to respective leads in a second sensing circuit of an alarm system; whereby the sensor element will independently make and break each of the sensing circuits in response to predetermined changes in the inward pressure on the outermost dielectric layers.

1 3. A sensor element according to any preceding claim, wherein the means for bonding the conductive films to opposite sides of the central layer is so arranged that a manifold is formed defined by at least one of the conductive films and the central layer and communicating with the air spaces, such that air within the spaces is freely redistributed by conduction through said manifold when the central layer is collapsed or allowed to self-recover.

14. A sensor element according to any preceding claim, wherein the means for bonding the conductive films to opposite sides of the central layer comprises an adhesive compound.

1 5. A sensor element according to any preceding claim, which. has been made by a method comprising simultaneously feeding a continuous collapsible and self-recoverable plastics foam web having a plurality of passages therethrough and a pair of continuous flexible webs of dielectric material each having an electrically conductive film bonded to the surface thereof, into a juxtaposed relationship with the conductive films facing the foam web; applying an adhesive compound to at least one of the conductive films as it and the flexible web to which it is bonded are fed toward the foam web; and causing said at least one conductive film carrying said adhesive compound to contact the foam web in face to face relationship; whereby the foam web is anchored relative to said at least one conductive film and the corresponding flexible web to which it is bonded, against sliding movement relative thereto.

16. A sensor element according to claim 1 5, wherein the edges of the flexible webs of dielectric material extend laterally beyond the electrically conductive films and the foam web, and the method of making the sensor element further comprised the step of sealing said edges of said flexible webs to each other to prevent intrusion of moisture and contaminants.

17. A sensor element according to claim 16, wherein the manufacturing step of sealing the edges of the flexible webs to each other comprised passing said flexible webs between a pair of rollers with said foam web sandwiched therebetween, said rollers engaging the edges of the flexible webs in a nipping relationship to heat seal the edges to each other.

18. A sensor element according to claim 17, wherein the adhesive compound was continuously applied to said at least one conductive film in at least one longitudinally directed line such that the adhesive contacts the foam web at locations between the passages.

19. A sensor element according to claim 18, wherein the adhesive compound has been applied by wick means contacting said at least one conductive film as said at least one conductive film and the flexible web to which it is bonded were fed toward the foam web.

20. A security alarm sensor element constructed and arranged substantially as herein described with reference to and as illustrated in the accompanying drawings.