Security Glazing and Alarm System

This invention relates to security glazing units and to alarm systems particularly suitable for detecting attempts to break or tamper with glazing installations. The invention may be used as a burglar alarm or as a security system in penal institutions or the like.

Security alarm systems of the prior art conventionally include a frangible conductor, such as a foil tape or thin wire, typically in a serpentine or circumscribing path, affixed to the window glazing and through which an electric current is passed. When the window is broken, the conductor also breaks, thereby opening the circuit and triggering an alarm. A typical system employing thin wires may be seen in U.S. Pat. No. 1,223,583, and a typical system employing foil tape may be seen in U.S. Pat. No. 3,409,886. Such arrangements possess certain disadvantages however. In order to keep power consumption low, the conductor can be made relatively large in width or diameter so as to reduce its resistance, but the use of such obstrusive conductors can be unsightly, particularly if the center portion of a window is to be protected. Alternatively, the path of the conductor may be shortened in order to lower the resistance of the circuit, but large areas of a window may then be left unprotected, making it possible to partially break or cut through the window without disturbing the conductor. An attempt to avoid this dilemma is shown in U.S. Pat. No. 3,441,925, where a transparent conductive coating is applied to the entire window area. This arrangement still requires a relatively large break in the window in order to register a definite change in the resistance. Small holes and cracks in the glass do not produce discrete, predictable changes in the resistance, and when laminated glazing is used in such an arrangement (as is normally required to protect the conductive coating) major gaps in the circuit are not likely to be opened without an extraordinarily forceful and wanton attack. This is especially true in the case of laminated units designed to resist penetration.

Another disadvantage of most prior art security systems is that once one of the conductors has been broken, the system cannot be reset until the damage is repaired, which can involve a considerable lapse of time, particularly if a window must be reglazed. In the meanwhile no security is provided for the window where the break occurred, even though the damage may have been caused accidentally and may be slight. A system is disclosed in U.S. Pat. No. 1,766,500 that includes two independently actuatable circuits, so that if one wire is broken, a second wire still provides security. But that arrangement requires extra electrical components and provides only minor reset capability, limited to the case of only one wire being broken. If two wires happen to be broken instead of just one, the system will be put out of commission just the same. The circuit of another prior art system, shown in U.S. Pat. No. 2,423,649, includes two resistances in parallel, but there is no suggestion therein to extend a plurality of frangible conductors across an area of entry so as to provide backup protection after an attempted breach of security. Another related circuit is shown in U.S. Pat. No. 3,138,792 in a system that does not include frangible wires.

Accordingly, there is a need for a security system that can reliably detect security breaches in the early stages and then be reset to quickly re-establish security. At the same time, it is desirable that visual aesthetics not be sacrificed in security glazing installations.

In accordance with the present invention, laminated security glazing is provided with a resistance grid comprised of a plurality of thin, frangible conductor wires embedded in each unit. The wires extend across the face of each unit in generally parallel relationship and are affixed to bus bars at either end to create parallel electrical paths across the unit. A low voltage current is passed through the unit, and the total resistance across the unit is monitored by means of electrical circuitry connected thereto. When a wire is broken, the increase in resistance is detected by the circuit and an alarm is triggered. Subsequently, the circuit may be reset so that the resistance of the remaining unbroken portion of the grid can be monitored. In the preferred embodiments, changes in resistance are measured by comparing the resistance of one group of wires with the resistance of a second group of wires in an adjacent glazing unit or in a different portion of the same unit. A novel arrangement for resiliently attaching the fine wires to the bus bars so as to permit movement of the wires relative to the bus bars, thereby avoiding breakage due to thermal expansion, forms another aspect of the invention.

The invention will be fully understood from the following detailed description, taken together with the drawings in which:

FIG. 1 is a schematic diagram of one embodiment of the alarm system of the present invention;

FIGS. 2, 3, 4, and 5 are cross-sectional views taken along lines 2--2 in FIG. 1, showing different embodiments of the composite security glazing unit of the present invention;

FIG. 6 is an elevational view of a portion of a single ply of the inventive glazing unit prior to lamination, showing details of the bus bar connection;

FIG. 7 is a cross-sectional view of the ply portion of FIG. 6, taken along lines 7--7 in FIG. 6 showing bus bar details;

FIG. 8 is a schematic diagram of a preferred embodiment of the alarm system of the present invention which includes a glazing unit having two resistance grids; and

FIG. 9 is a schematic illustration of an alternate arrangement wherein two glazing units are paired in the circuitry.

Depicted in FIG. 1 is a security glazing unit, designated generally as 10, which consists of a composite of transparent plies having a plurality of fine, conductive wires 11 forming a resistance grid sandwiched between the plies. Each wire is affixed at each end to bus bars 12 which are also placed between the plies. Electrical leads 13 and 14 are connected to the bus bars and extend beyond the edges of the glazing unit.

The structure of the glazing unit 10 may include at least one glass ply and at least one transparent plastic ply in a variety of combinations, three examples of which are illustrated in cross-section in FIGS. 2, 3, and 4. A simplified embodiment, shown in FIG. 2, is comprised of a glass ply 21 combined with a plastic ply 22. The glass may be any suitable glazing product including tempered or untempered, clear or tinted, and coated or uncoated glass. The plastic ply may be comprised of any polymeric material suitable for inclusion in a laminated transparency, such as polyvinylbutyral, polyurethane, acrylic, polyester, or polycarbonate resins. Polycarbonates are preferred because of their toughness. The plies may be bonded together using conventional laminating techniques such as autoclaving, or they may be loosely sandwiched in face-to-face relationship. Examples of the latter type of composite structure can be seen in U.S. Pat. No. 3,821,071. When laminating acrylics or polycarbonates to glass, it is usually desirable to employ an interlayer material between the plies. In the laminating process the wires 11 become embedded in the plastic ply or plies. The laminated structure is designed such that a substantially intact barrier remains even after the glass ply or plies have been broken, and such that it is improbable that all of the wires would be broken in a single attempted breach of security. The diameter and alloy of the wires will vary in accordance with the resistance requirements for each particular installation, which in turn depends upon the length of wire required to extend across a given glazing unit and the spacing of the wires. Because a plurality of wires are provided in parallel, the wires may be made so fine as to be barely visible without incurring excessive power consumption when passing a current through the unit. Copper-nickel alloy wires having a diameter of about 2 mils or less may be employed satisfactorily, but larger diameters may be used if greater visibility of the wires can be tolerated. A 3-inch space between adjacent wires is typical.

Because plastics are susceptible to scratching, it is preferred to protect the plastic with at least one glass ply. For this reason, a more practical embodiment is that shown in FIG. 3, where a second glass ply 23 is provided so as to leave no plastic surface exposed.

A preferred embodiment is depicted in FIG. 4, where two additional plastic plies 24 and 25 of an interlayer material such as polyurethane are interposed between the plastic ply 22 and the glass plies 21 and 23 to improve the bonds therebetween. In this case, laminating tends to press the wires into the interlayer material. As a specific example, the FIG. 4 embodiment may be comprised to two 1/4 inch tempered glass plies, one 1/4 inch polycarbonate ply, and two 0.060 inch polyurethane interlayer plies, all of which are laminated by autoclaving at 275.degree. F. and 200 psi. Details of such a laminating process may be seen in U.S. Pat. No. 3,388,032.

It is also possible to fabricate the glazing unit from a plurality of plastic plies alone if scratch resistance is not a matter of concern. Such an embodiment is shown in FIG. 5, where two plastic plies 26 and 27, have been fused together. An interlayer of a more ductile plastic may optionally be placed between the plastic plies to form a three-layered lamination. Alternatively, the wires 11 may be pressed into one of the plastic plies and the two plies loosely retained in a face-to-face relationship by a common framing means. Plies 26 and 27 may be comprised of any rigid plastic material suitable for glazing applications. It may also be advantageous to apply coatings to the exposed plastic surfaces as is known in the art to improve their abrasion resistance.

The attachment of wires 11 to the bus bars 12 is illustrated in detail in FIGS. 6 and 7. The wires and bus bars are arranged in the desired configuration on the surface of one of the plies prior to lamination and are retained in place temporarily with adhesives until the wires are permanently embedded by the laminating process. As shown in FIGS. 6 and 7, plastic ply 22 is the substrate preferred for assembling the wires and bus bars. Similar glazing units in the automotive and aircraft fields that incorporate fine wires for defrosting purposes conventionally impress the wires onto the plastic ply in an undulatory or randomly varied pattern so as to prevent undesirable concentrations of light reflection from the wires. Since the spacing of the wires for security purposes is wider than for defrosting units, and since the requirements for optical distortion are less stringent in the case of architectural glazing than with automotive or aircraft glazing, the wires in the present invention may be laid straight across the glazing unit, thereby avoiding the complex and costly machinery used to lay defrosting wires. Each bus bar may be a composite structure built up from conductive strips 30 and 31 and adhesive layers 33 and 34 as shown in FIG. 7 (the thicknesses shown being greatly exaggerated for the sake of clarity). Strip 30 is made of a conductive material, e.g., copper foil, and may be adhered to ply 22 by means of a pressuresensitive adhesive 33. Suitable adhesive-coated foil strips are commercially available, such as "Scotch" brand tape number "X-1245" sold by the Minnesota Mining and Manufacturing Company, St. Paul, Minn., which is comprised of copper foil 3/8 inch wide and 0.0015 inch thick carrying a 0.002 inch thick layer of acrylic-based adhesive.

Referring again to FIG. 6, each wire 11 is preferably provided near its end with a bent portion 32 extending in a generally transverse direction with respect to the main body of the wire and terminating in a soldered connection 33 to strip 30 at the distal end of the bent portion. The bent portions may be on the order of 3/4 inch long. The bent portions are desirable for preventing heatinduced breakage of the wires during the autoclaving step, as well as after installation, due to differing coefficients of thermal expansion within the units. Strip 31 and adhesive 34 serve to preserve the shape of the bent portions until the unit is autoclaved and provide resilient retention of the bent portion 32 between the strips. Strip 31 and adhesive 34 may be the same as that described above in connection with strip 30 and adhesive 33, but could take the form of any compatible pressure-sensitive adhesive tape, since electrical conductivity need not be required for strip 31. Adhesive 34 should be of a type that will remain deformable or become deformable when heated so as to allow some movement of the bent portions 32 between strips 30 and 31, thereby relieving thermally-induced stresses on wires 11.

It will be evident to those of skill in the art that a variety of circuit means may be employed to detect changes in the resistance grid of the security glazing unit. One suitable circuit is shown in FIG. 1, connected to glazing unit 10 by way of leads 13 and 14. A reference resistor 40 is connected in series with the resistance grid of the glazing unit, and a pair of voltage sources 41 and 42 (preferably a low voltage, on the order of about ten volts each) are connected in parallel across the grid and resistor 40. A variable voltage divider 43 is also connected in parallel across the voltage sources. A bridge circuit, including leads 44 and 45 and an amplifier 46, is wired across the tap of voltage divider 43 and the junction between the grid and reference resistor 40. Resistors 50, 51 and 52 determine the gain of amplifier 46. When the voltage divider is set to match the ratio of the grid resistance to the reference resistance 40, the circuit is balanced and no current will flow through the bridge circuit. If one or more of the wires 11 in the grid are then broken, the change in grid resistance will cause the circuit to become imbalanced and a current will pass through the bridge circuit, whereby amplifier 46 will produce an output voltage in lead 53. Output lead 53 is connected to the coil of a relay 54, so that the output voltage from the amplifier energizes the relay, causing contacts 55 to close, which in turn activates an alarm device such as a bell 56. When security has been restored, the system can be quickly and easily reset by merely adjusting voltage divider 43 so as to again match the ratio of the resistance of the unbroken portion of the grid to the reference resistance.

Referring now to FIG. 8, there is shown a preferred embodiment wherein a glazing unit 60, otherwise having the same laminated structure as any of the previously described embodiments, is provided with two electrically independent grids of wires 11a and 11b. The wires are connected at one side to a bus bar 12 as in the embodiment of FIG. 1, but along the opposite side two separate bus bars 12a and 12b are provided. Three leads 61, 62 and 63 extending from the glazing unit are connected to the bus bars. The resistance-monitoring circuitry is identical to that of FIG. 1 except that reference resistor 40 has been omitted and grid 11b has been substituted in place thereof. Thus the resistance of each grid is measured against the resistance of the other grid and an unequal change in either grid will trigger the alarm. Because the wires grids are embedded in a tough plastic ply, it is highly improbable that the same number of wires could be severed in both grids at the same moment. Not only does this arrangement avoid the need for a separate reference resistor for each glazing unit, but it also prevents false alarms stemming from resistance changes that can be caused by temperature fluctuations. Since both grids are exposed to essentially the same ambient temperature conditions, any resulting change in resistance is the same in both grids, and therefore the sensing circuit remains balanced.

A variation of the two-grid arrangement is shown in FIG. 9, where two separate adjacent glazing units 10a and 10b, each the same as unit 10 in FIG. 1, are connected in series so as to be the equivalent of the two-grid single unit in FIG. 8. Leads 71, 72 and 73 may be connected to the FIG. 8 circuit in place of leads 61, 62 and 63, respectively.

Although the description of the invention has heretofore dealt specifically with window glazing units made from glass and transparent plastics, it should be apparent that the inventive security system may be associated with opaque materials as well for use as wall panels, doors, and the like. Other modifications and variations as are known to those of skill in the art may be resorted to without departing from the spirit and scope of the invention as defined by the appended claims.