GB2067804A - Glass breakage detector safety mounting - Google Patents

Abstract

A housing 1, 2 containing a security device and circuitry therefor is mounted to a window pane 3 via two bondings in series. The housing is attached to an element 6 exerting a magnetic field on the circuitry by a weak bonding 19, while the element is attached to the window pane by a strong bonding 18. There may be other bondings 17 directly between the housing 1, 2 and the window pane 3. An intruder forcing the security device and its housing from the window pane will automatically break the weak bonding 19 and therefore increase the distance between the element 6 and the housing, causing the magnetic field to alter. The circuitry may contain a magnetically operated reed switch 16 which will then initiate an alarm. <IMAGE>

Description

SPECIFICATION Breakage detector safety mounting This invention relates to mounting a device for detecting the breaking of a frangible object, onto the frangible object. The frangible object will typically be a window pane and the detecting device will often be part of a burglar alarm system.

The invention aims to defeat attempts to remove the device from its mounting on the object, so that breaking of the object, e.g. by an intruder entering through a window, will occur undetected.

If the device is a transducer mounted indirectly on the object, there may be two mountings to safeguard, i.e. those of the intermediary piece to the object and to a transducer or the like.

Disturbance of either mounting would frustrate the operation of the detector, so that such disturbances should themselves be detectable.

A typical window pane breakage detector is disclosed in the Specification 1,577,941 of RCA Corporation which describes detection of a certain combination of frequencies in ultrasonic waves picked up from the breaking glass.

According to features and embodiments of the present invention, a disturbance of the mounting of a detector for the breakage of a frangible object may be signalled by the attachment of an element influencing an electrical path more firmly to the object itself than to a housing or wall member of the detector. The electrical path may be defined by reed switch contacts and the path-influencing sensor element may be a permanent magnet. The magnet may be adhered to the object by a glue of greater strength than a glue fixing the magnet to a part of the detector housing. The weaker glue may be a latex based adhesive adequate to retain the magnet during shipment. The stronger glue may fix the element directly or indirectly (e.g. via a decorative plate or the like) to the frangible object.

The breakage detector may have a conducting path which becomes non-conductive to signify breakage condition. The magnet may hold switch contacts in this path together only while magnetically influencing the switch, so that relative distancing of the magnet causes the same alarm condition as a breakage. Some other type of path-influencing sensor element may be affixed by a weak adherent to the breakage detector housing, but a magnetic element and a reed switch are presently preferred.

US Patent 4,149,156 proposes attaching a sensor to an external surface by a weak adhesive.

Breaking this weak bond causes an alarm to be given. However, this proposal does not consider the advantages of the sensor having a strong bond to an external surface and a weak bond to an internal surface, i.e. concerning the detector unit itself. The weak bond may become broken accidentally or spontaneously in the course of time. This proposal uses a depressed plunger switch as the sensor, with a certain risk that an intruder could introduce a thin blade slid between the surface and the plunger in order to maintain the plunger depressed while he effects removal of the sensor unit.

Our invention preferably involves the detector housing also being bonded to the frangible surface by a strong glue, such as that attaching the magnet to the surface. The force required to break the bonding between the housing and the surface will reliably break the weak glue attachment between the magnet and the housing.

Embodiments of the invention tend to give greater security and cost effectiveness than mechanically actuated sensor switches.

French Patent Application 2,397,711 proposes a switch contact which is made by action of a permanent magnet causing a tilting when a carefully placed object is moved such as inadvertently to allow such tilting. The present invention as embodied proposes that a sensor element e.g. magnet influences an electric path, e.g. switch contacts by a field effect, so that the contacts may be enclosed to be protected longterm from corrosion etc. Careful mechanical handling cannot defeat the object of the safety mounting in embodiments of the present invention, because the strong glue will always cause the weaker glue bond to give, relatively moving the sensor and the housing of the sensitive electric path.

British Patent Specification 1,521 ,96 1 describes reed switch contacts which are operated by a person who wishes to signal an alarm condition such as fire manually breaking glass to which a permanent magnet adheres. Obviously this person is not an intruder, and nothing, such as the differential bonding of the instant invention, is disclosed which would frustrate an attempt to immobilize or remove a glass break detector.

Embodiments of the invention will be described by way of example in conjunction with the drawings, in which: FIGURE 1 shows essentials of a first embodiment of the invention, in a section through a glass sheet showing the thickness, a permanent magnet, a detect glass-break detector device housing, and bonds according to the inventive teaching: FIGURE 2 shows a section in more detail of another embodiment, including some details of a break glass detector; and FIGURE 3 shows a transducer arrangement detail of Figure 2.

Referring to Figure 1, a glass-break detector housing 1 houses an installation bolt 8 on which an ultrasonic wave pick-up transducer (not shown) is installed in contact with window pane 3 which is to be protected against breakage. Also not shown, in housing 1, is circuitry for evaluating possible breaking-glass signals from the transducer to determine whether a break-in through the window is occurring at any time. The glass break detector housing 1 is mounted on the window pane 3 via a plastic mounting member 2 which may be held to the housing also by bolt 8 or by other means such as adhesives.

Following the teachings of the invention the housing contains a switch member 16, which is illustrated end-on in the orientation ot the apparatus chosen for Figure 1. The switch 1 6 is responsive to the position of a sensing element 6.

The switch member 1 6 may comprise normally closed reed contacts which open when a permanent magnet constituting sensing element 6 is moved away from the reed switch.

The contacts of switch member 1 6 maybe in a separate alarm circuit loop (not shown), or in a loop e.g. 20 of the break-glass detector circuitry.

This may be as in Application 21133/76.

Mounting member 2 is attached to window 3 by one or more strong bonds 17, e.g. of strong glue such as "Permabond MPP 4Z". Another strong bond 1 8 attaches permanent magnet sensing element 6 to the glass. Element 6 is attached more weakly to the housing 1, in this embodiment to the mounting member 2 by a weak bond 19 of a material such as a latex based adhesive.

If, in an attempt to defeat the break-glass detector action, anyone attempted to detach the detector device housing 1 from its mounting, whether by unscrewing a screw such as 8 holding it to mounting member 2 or by force, or if anyone attempts to force the mounting itself from its attachment to the window, the magnet 6 remains attached by its strong bond to the window, while the reed switch will be removed at least partially from the permanent magnetic field. The reed contacts will open and the alarm is raised. The weak magnet bonding 19 to the mounting member 2 will be broken automatically, e.g. by anyone breaking the strong bonds 17, and strong bond 1 8 will survive.Thus the differential bondings 1 8 and 19 in series between the detector housing and the glass and separated by the sensor element, preferably with bonds such as 1 7 between the housing and the glass, provide the safety mounting of the detector.

Alternatively, the section through the transducer and its mounting may be as in Figure 2 which does not show the glass or the strong mounting bonds 1 7, 18, only the weak bond 19 between the magnet 6 and the mounting member 2. In Figure 2, like parts are similarly referenced to Figure 1, but there is also seen a piezo-electric pick-up transducer body 9 mounted on bolt 8 for contacting a frangible object, e.g. a window.

The body 9 is held resiliently with an O-ring 15, and a cyanoacrylate adhesive pad 7 may be used for a strong mounting 17 to a window. A transducer plate 5 forms the front wall of the housing, which may house the break glass detector circuitry on a printed circuit board 10, electrical access cable 20 exiting the housing through a nitrile grommet 4. The magnet is attached for shipment etc on its inside to the plastics mounting 2 by a weak glue, as for Figure 1. The transducer 8 may be as detailed in Figure 3.

The reed Switch 1 6 is mounted to printed circuit board 10 in such manner as to be located as close as possible to be sensitive to magnet 6.

Referring to Figure 3, a flange part 21 for attaching to a frangible object, e.g. glass has projecting from it a shank part 22, carrying the actual piezo-electric element (not shown) which is vibrated by waves picked up on the flange part 21, and carrying an internal thread 23 for mounting on bolt 8 (e.g. see Figure 1 or 2). The shank part 22 also carries an annular groove 24 for receiving an O-ring (e.g. see at 15, Figure 2).

Thus a frangible object breakage detector has its housing, in the above embodiments, bonded weakly to a sensor element, which is bonded strongly to the frangible object. The sensor element influences an electric alarm path in the housing to a degree varying with their relative location.

This path may be in the circuitry described in RCA Corporation Specification 1,577,941.The transducer and other glass break detection apparatus may also be as therein described.

Intrusion detectors using permanent magnets cooperating with reed-switches are proposed in British Patent 1,143,450 and 1,089,075. Neither of these uses such cooperation to prevent another intrusion detector from being removed, as in the present embodiment. Also these two proposals have the magnet mounted on a member moved by the intruder, whereas the herein described devices have the reed switch moved, and the magnet fixed by the differential bonding e.g. gluing arrangement.

Claims (9)

1. A mounted housing comprising.

an element located within the housing and defining an electrical path or not according to the nearness of a path influencing member; the path-influencing member being attached to the outside of the housing by means of a first adhesive; said housing being mounted in having the pathinfluencing member attached to a support by means of an adhesive stronger than the first adhesive, whereby the member tends to remain attached to the support if the housing becomes dismounted; and means to give an alarm according to whether the electrical path is defined or not.

2. A mounted housing according to claim 1 and containing a security system, which system comprises:- a transducer having an output influenced by a condition of said support; and electronic circuitry to process the transducer output whereby to recognise an alarm condition in the support and to give said alarm indication accordingly.

3. A mounted housing according to claim 2, in which: said path-defining element is a reed-switch and the influencing member is a permanent magnet: and said reed-switch is connected in said electronic circuitry.

4. A mounted housing according to any of claims 1-3 having an area directly mounted to the support by an adhesive stronger than said first adhesive.

5. A security unit comprising: means mounting it to a support: a housing, containing a transducer adapted to and positioned to sense an alarm condition relating to the support; electronic circuitry contained in the housing for indicating an alarm condition sensed by the transducer; said electronic circuitry being arranged also to indicate a further alarm condition set up by the absence of a field; and a field producing member having an attachment to the outside of the housing and a stronger attachment to the support; wherein said attachments constitute at least a part of the mounting means.

6. A security unit according to claim 5 wherein the housing has also direct attachments to the support stronger than the first-mentioned attachment; and said direct attachments include one generally on either side of the first-mentioned stronger attachment.

7. A security unit according to claim 5 or 6 wherein: the support is frangible and the transducer will sense a breaking of said support.

8. A security unit according to claim 5, 6 or 7 wherein the electronic circuitry inciudes a magnetically-operated make-or-break device.

9. A security unit arranged and mounted substantially as hereinbefore described with reference to any Figure.