WO2013142962A1

WO2013142962A1 - Anti-tamper systems and methods

Info

Publication number: WO2013142962A1
Application number: PCT/CA2013/000270
Authority: WO
Inventors: Andrei Bucsa; Stephen D. W. Fosty
Original assignee: Tyco Safety Products Canada Ltd.
Priority date: 2012-03-28
Filing date: 2013-03-21
Publication date: 2013-10-03

Abstract

Systems and methods are disclosed for detecting tampering with an enclosure. A pair of glass sheets can enclose a gas filled space. If either sheet is breached the gas pressure reduction is sensed and an alarm condition signaled. One sheet can have openings with conductive mesh members on one surface and conductive elastomer members on another surface. Maintaining the space pressure at a predetermined value expands the elastomer members into the openings to contact the mesh members, completing a circuit. A drop in pressure drops due to a sheet breach causes the elastomer to contract, breaking the circuit, and signaling an alarm. One sheet can include a conductive trace which, if broken, can signal an alarm condition. Light can be transmitted through one of the sheets and a "flight time" of the light can be measured. An alarm is signaled if the "flight time" is outside of a predetermined value.

Description

ANTI-TAMPER SYSTEMS AND METHODS

Cross-Reference to Related Applications

[0001] This application claims priority to pending U.S. provisional patent application, serial no 61/616,648, filed March 28, 2012, the entirety of which is incorporated by reference herein.

Field of the Disclosure

[0002] The disclosure relates generally to the field of anti-tamper devices, and more particularly to anti-tamper devices employing thin glass sheets for use in security system alarm cabinets.

Background of the Disclosure

[0003] Alarm systems are prevalent in a variety of home and business applications. Such alarms (often referred to as "security systems" or "alarm systems") can include a number of sensors used to monitor unauthorized entry and other conditions at monitored premises, such as fire, smoke, toxic gases, high/low temperature (e.g. freezing) or flooding, at a premises. In response to sensing an alarm condition, a sensor provides a signal to an alarm panel that in turn may sound and notify the occurrence of the alarm to occupants of the premises and remotely signal a monitoring center or other third party.

[0004] Often the occurrence of an alarm is signaled to a remote monitoring center that may then dispatch capable authorities to intervene at the premises. For example, in the case of sensing an unauthorized entry to the premises, the monitoring center may dispatch security personnel, typically in the form of private security guards or police officers. [0005] Many alarm systems now allow audio communication with the monitoring center. Such audio communication may be one-way (e.g. from the premises to the station) or two-way (i.e. from and to the premises/monitoring center). In this way, the alarm system may be used to audibly monitor the premises during an alarm condition, or it may be used to dispatch a verbal request for help by an occupant at the monitored premises. To this end, some alarm systems are equipped with an audio interface, including a microphone and optionally a speaker.

[0006] Communication between the premises and the monitoring station normally takes place over a communications network, for example by way of the public switched telephone network (PSTN). Unfortunately, audio interfaces are susceptible to unauthorized modification or disconnection by intruders. Tampering can be deterred by encasing the audio interface in a robust structure that is resistant to breaching, however, such arrangements are costly and cumbersome.

[0007] Alternatively, tampering attempts can be detected and signaled as an alarm condition to the remote monitoring center. Known schemes for detecting tampering include use of vibration sensors and pattern recognition algorithms. Such tamper detection methods, however, are very sensitive to external vibration and can fail if combination tampering techniques (e.g., drilling plus thermite) are used.

Summary of the Disclosure

[0008] In view of the above, systems and methods are disclosed for providing tamper detection of alarm system components. [0009] A system is disclosed for detecting a tamper condition of an enclosure. The system may include first and second sheet members positioned to define an interior space

therebetween. The interior space may include a gas at a first pressure. The system may also include a sensor element for detecting a breach in at least one of the first and second sheet members. The breach may cause the pressure in the interior space to decrease below the first gas pressure. When the pressure in the interior space decreases below the first gas pressure the sensor element may signal a tamper condition.

[0010] A system is disclosed for detecting a tamper condition of an enclosure. The system may include a first sheet member having first and second electrodes formed therein. A second sheet member may have a conductive ink trace formed on a first surface thereof. The second sheet member may be positioned with respect to the first sheet so that first and second ends of the conductive ink trace are in electrical contact with the first and second electrodes. A sensor element may be provided for sensing a breach in at least one of the first and second sheet members. The breach may cause a change in an impedance of the conductive ink trace. The breach may be representative of a tamper condition.

[0011] A system is disclosed for detecting a tamper condition. The system may include a first sheet member having a reflective coating disposed on a perimeter edge surface thereof, a light transmitting element for transmitting light within said first sheet member, a light sensing element for receiving the transmitted light reflected within the first sheet member, and a time of flight estimator for determining a time of flight for the light transmitted between the light transmitting element and the light sensing element and for outputting a signal representative of a tamper condition when the time of flight exceeds a predetermined value. [0012] A method is disclosed for detecting a tamper condition. The method may include detecting a pressure in a space formed between first and second sheet members, the first and second sheet members associated with a monitored enclosure; and signaling an alarm condition if the detected pressure decreases below a predetermined value. The alarm condition may be representative of a breach of the monitored enclosure.

[0013] A method is disclosed for detecting a tamper condition. The method may include detecting an impedance of a conductive ink trace disposed on a first sheet member, comparing the detected impedance with a reference impedance value, and signaling an alarm condition if the detected impedance departs from the reference impedance value by greater than a predetermined amount.

[0014] A method is disclosed for detecting a tamper condition. The method may include transmitting light through a first sheet member, where the light reflects off a plurality of edge surfaces of the first sheet member. The method may further include receiving the transmitted light at a receiver, comparing a time of flight for the transmitted light to a reference value, and signaling a tamper condition if the time of flight deviates from the reference value by a predetermined amount.

Brief Description of the Drawings

[0015] By way of example, specific embodiments of the disclosed device will now be described, with reference to the accompanying drawings, in which:

[0016] FIG. 1 is a block diagram of a premises monitored by an alarm system and an associated monitoring center; [0017] FIG. 2 is a block diagram of an exemplary tamper sensing system according to the disclosure;

[0018] FIGS. 3A and 3B are partial sectional views of an embodiment of the disclosed tamper sensing system;

[0019] FIG. 4 is an exemplary system for monitoring a tamper sensing systems of FIGS. 2, 3A and 3B;

[0020] FIG. 5 is a sectional view of an additional embodiment of the disclosed tamper sensing system;

[0021] FIG. 6 is a top plan view of the tamper sensing system of FIG. 5;

[0022] FIG. 7 is an exemplary latching circuit for use with the tamper sensing system of FIG. 5;

[0023] FIG. 8 is an exemplary system for monitoring the tamper sensing system of FIG. 5;

[0024] FIG. 9 is a further embodiment of the disclosed tamper sensing system;

[0025] FIGS. 10-12 show an exemplary system for monitoring the tamper sensing system of FIG. 9;

[0026] FIG. 13 is a flow chart illustrating an embodiment of the disclosed method;

[0027] FIG. 14 is a flow chart illustrating an additional embodiment of the disclosed method; and

[0028] FIG. 15 is a flow chart illustrating a further embodiment of the disclosed method. Detailed Description

[0029] The disclosed methods and arrangements may be applied alone or in conjunction with other anti-tampering methods to secure an enclosure. Although the description will proceed in relation to alarm system enclosures, it will be appreciated that the disclosed methods and arrangements can be applied for use with any of a variety of enclosures for which security is important, including without limitation safe deposit boxes, museum rooms and enclosures, and the like.

[0030] FIG. 1 illustrates premises 10 monitored by an exemplary alarm system 40. As illustrated, the alarm system 40 includes a central control panel 20 in communication with a plurality of sensors 18. The sensors 18 may be entry sensors, motion sensors, flood sensors, smoke sensors, gas sensors or any other appropriate sensor. The sensors 18 may be in communication with a control panel 20, either wirelessly or by hard wiring.

[0031] The alarm system 40 may include an audio interface 25. The audio interface 25 may take the form of an audio station remote from the panel 20, and may include a microphone, camera, keypad, display, speaker, electronic ringer or the like.

[0032] The control panel 20 may be interconnected with a communications network to a monitoring center 14. In the illustrated embodiment, the control panel 20 is interconnected by a subscriber line 13 to a public switch telephone network "PSTN" 12. The subscriber line 13 terminates in a central office (not shown) of the PSTN 12. A monitoring center 14 is also interconnected with the PSTN 12. It will be appreciated that the panel 20 could be

interconnected with the monitoring center 14 by way of another communications network, like a cellular telephone network, packet switched data network (e.g. the internet), or the like. [0033] The monitoring center 14 is shown as a single monitoring station. However, it will be appreciated that the monitoring center 14 could instead be multiple monitoring stations, each in different physical locations. The monitoring center 14 may be associated with one or more PSTN telephone numbers that would be used by telephone control panel 40 to contact the monitoring center 14. As will be appreciated, audio interface 25 allows two way audio communication between the alarm system 10 and the monitoring center 14.

[0034] In operation, as a particular sensor 18 is tripped, signifying a sensed condition, the sensor provides a signal to a sensor interface 25 of control panel 20. The control panel 20 initiates a communication to monitoring center 14. For example, control panel 20 may place a telephone call to a pre-programmed telephone number to contact monitoring center 14, typically by way of PSTN 12.

[0035] As such, if an intruder wishes to defeat the alarm system 40, they may attempt to access the enclosure(s) that contain the sensor interface 25 and/or the control panel 20 to deactivate or destroy the electronic components therein. By providing a separate detection mechanism that alerts the monitoring center 14 when one of these enclosures has been breached, such tampering and circumvention of the alarm system 40 can be thwarted.

[0036] An embodiment of the invention described herein uses one or more glass sheets affixed to one or more internal walls of a security system cabinet. When the glass is broken, melted or otherwise compromised a tamper alarm may be generated and sent to the monitoring center 14.

[0037] Referring to FIG. 2, a tamper sensing arrangement 50 includes a double-sheet pressure- based arrangement. A "sandwich" of two sheet members 52, 54 are sealed gas-tight with a gasket 56 to create an interior space 58. The sheet members 52, 54 may be a breakable material such as glass. In some embodiments, the sheet member 52 positioned closer to a wall of the associated housing 62 may be thinner than the other sheet member 54. In other embodiments, the sheet members 52, 54 may be the same thickness. In still other embodiments, the sheet members 52, 54 can be made from the same material, or they may be made from different materials. The interior space 58 can then be pressurized, or held at a vacuum, and the pressure in the interior space monitored using a pressure sensor 60. Therafter, any attempt to tamper with the housing 62 (e.g., by drilling through a wall having the sheet members 52, 54 attached) will break one or both of the sheet members, causing an abrupt change in the pressure in the interior space 58 (either up or down, depending upon whether the space 58 is pressurized or held at a vacuum). Sensing the abrupt change, the pressure sensor 60 can generate a signal representative of an alarm condition.

[0038] The sheets 52, 54 can be attached to one or more interior walls of the associated housing 62 using double adhesive tape or other appropriate mechanical fixation technique, and the size of the tamper sensing system 50 can be adjusted to fit the particular dimensions of the housing 62. It will be appreciated that this arrangement may be used by itself or in addition to existing (e.g., vibration-based) anti-tamper systems.

[0039] FIGS. 3A and 3B show an alternative implementation of a tamper sensing arrangement 64 utilizing a pair of breakable sheets, in which one sheet forms a portion of a sensor element. First and second sheet members 66, 68 may be provided adjacent to each other so that an interior space 70 is formed therebetween. The first and second sheet members 66, 68 may be sealed gas-tight in a manner similar to that described in relation to FIG. 2 (note, the gasket has been omitted from the figures). One of the sheet members (in this case, the second sheet member 68) may be coupled to a wall of an alarm system housing using a suitable adhesive layer 71.

[0040] The first sheet member 66 may include first and second holes 72, 74, which may be sealed on one side with a layer of conductive rubber foil 76 which may be, but is not limited to, conductive silicon. As applied this layer of conductive rubber foil 76 can preserve the gas-tight seal between the first and second sheet member 66, 68. On a side of the first sheet member 66 - opposite the conductive rubber foil 76 - the first and second holes 72, 74 may be covered with a respective pair of electrodes 78, 80. In one embodiment, the electrodes 78, 80 are a non- corroding conductive mesh material such as copper, and are fixed in place using a suitable adhesive. One benefit of using a conductive mesh in lieu of a solid conductor for the electrodes is that the mesh allows gas to pass therethrough, for reasons that will be described later. The conductive mesh may also serve to protect the exposed rubber foil 76.

[0041] For embodiments in which the interior space 70 between the first and second sheet members 66, 68 is pressurized, the layer of conductive rubber foil 76 may expand so that it contacts the electrodes 78, 80 {see FIG. 3B) This contact creates a current path between electrodes 78, 80, placing the resulting circuit in an ON state. If, thereafter, one or both of the first and second sheet members 66, 68 is broken, such as by tampering, the interior space 70 will be depressurized and the conductive rubber foil 76 will contract such that it no longer contacts the electrodes 78, 80 {see FIG. 3A). In this condition, the sensor will assume an OFF state. Appropriate circuitry (e.g., FIG. 4) may be coupled to the electrodes 78, 80 in a known manner to sense the ON/OFF state of the sensing circuit and to generate an alarm signal as appropriate. It will be appreciated that an opposite condition could also be used, in which a normal condition of the sensing system 64 is represented by an OFF state, and a tamper condition is signaled when the sensor assumes an ON state.

[0042] In one embodiment, a desired pressure can be achieved in the interior space 70 after sealing of the first and second sheet members 66, 68 by using a compound or compounds that generate a gas which, as it evolves, increases the pressure in the interior space. For example, two different chemical compounds (e.g., one powder and the other one liquid) can be combined after the first and second sheet members 66, 68 have been sealed together. In one embodiment, a solid (powder) and a fluid can be placed in different zones on one of the sheet members 66, 68. After sealing the enclosure formed by the sheets, the enclosure may be shaken to combine the compounds. In the embodiment described in relation to FIGS. 3A, 3B, the developed gas may then press the conductive rubber foil 76 into engagement with the electrodes 78, 80.

[0043] The components may be selected to be environmentally friendly. For example, the components may be selected so that the chemical reaction generates carbon dioxide (C0₂), which may maintain a desired gas pressure within the sealed inter-sheet space. The amount of each compounds may be calculated based on glass sheet size, and the spacing between the sheets (i.e., the inter-sheet volume). A non-limiting example of appropriate compounds includes sodium bicarbonate (baking soda), which is a base, and acetic acid (vinegar), which react to produce water (H₂0) and Sodium Acetate. This reaction also creates C0₂ which is used to provide the desired inter-space pressure. The exemplary chemical reaction may be represented as follows:

[0044] CH3COOH (acetic acid) + NaHC0₃ (sodium bicarbonate) -> CH₃COONa (Sodium Acetate) + H₂C0₃ (Carbonic Acid). H₂C0₃ is not stable, however, and it breaks down quickly to form water and carbon dioxide: (H₂CO₃ -> H₂0 + C0₂). Thus the resulting equation can be stated as: CH₃COOH+ NaHC0₃ -> CH₃COONa + H₂0 + C0₂.

[0045] In addition to the disclosed alarm housing application, this embodiment may be used for tamper detection in large thermal insulation double glass panes that are filled with an inert gas such as argon, xenon or krypton. When filling an interior space with an inert gas, the first and second sheet members 66, 68 may be sealed together in a pressurized room.

[0046] As described in relation to the embodiment of FIG. 2, a pressure sensor 60 can be used to monitor pressure variations in the interior space 58, and the sensed pressure profile can be used to signal a tamper condition caused by breakage or damage to one or both of the glass sheets. In one embodiment, the pressure sensor 60 is a pressure switch configured to provide a tamper signal to the control panel 20 of the alarm system 40 if a measured pressure condition within the interior space drops below a predetermined value. In other embodiments, the pressure sensor 60 can be a linear differential pressure sensor capable of sensing discrete changes in the pressure profile within the interior space 58, related to environmental pressure and transmitting those sensed changes to appropriate monitoring circuitry.

[0047] It will be appreciated that a pressure sensor of this type may be used to detect a tamper condition of an alarm system housing, it may find additional uses. For example, if the disclosed system were installed in one or more exterior windows of a building, pressure trend information could be collected and analyzed, for example, from windows located on different side of the building. Wind speed and direction could then be estimated. This information, in turn, may be used to evaluate building vibration, which is an indicator of the safety factor of the building. In addition, the pressure in the interior space may also indicate if the thermal insulation remains effective (e.g., for double pane glass that includes a vacuum space between panes, or that includes a pressurized volume of inert gas).

[0048] An example of an arrangement that may be used to monitor the pressure sensor 60 (either the pressure switch or linear sensor embodiments) is shown in FIG. 4. The pressure sensor 60 may be coupled to a comparator 82 for sensing a predetermined threshold level from the sensor as being an "alarm" signal, a latch 84 for maintaining the sensed alarm signal, and a processor 86 for receiving the alarm signal from the latch. The processor 86 may also be configured to reset the latch 84. In one embodiment, the processor is part of the control panel 20. For embodiments in which building analytics are desired (e.g., vibration and safety factor calculations) the pressure sensor 60 may have a separate coupling path to a pressure trend estimator 88 and a decision block 90. The decision block 90 may itself have a connection to the processor 86. Because the sheet members may be flexible, any vibration or tension of the sheets with respect to each other will cause a pressure variation. By analyzing this variation using the pressure trend estimator 88 and the decision block 90, an estimate of the force variation (i.e., pressure/surface) applied to the external window can be estimated. It will be appreciated that the pressure trend estimator 88 and decision block 90 are not required.

[0049] Referring now to FIGS. 5-8, an embodiment of a tamper detection system 92 will be described. In this embodiment, the tamper detection system 92 may employ a double-sheet conductive ink arrangement. First and second sheet members 94. 96 may be provided. The first sheet member 94 may serve as a protective layer and an electrode support, while the second sheet member 96 may be covered with a conductive ink region 98, which, in the illustrated embodiment, is a conductive ink trace. In one embodiment, the conductive ink region is printed on the second sheet member 96 using an appropriate printing technique. First and second electrodes 100, 102 may be disposed through the first sheet member 94, and the first and second sheet members 94, 96 may be pressed together so that the electrodes 100, 102 contact opposite ends of the conductive ink region 98. The first and second sheet members 94, 96 may then be edge sealed using an appropriate gasket and/or sealant 99. In addition, one of the sheet members (in this case, the second sheet member 96) may be coupled to a wall of an alarm system housing using a suitable adhesive layer 104.

[0050] Thus arranged, the conductive ink region 98 and the electrodes 100, 102 form a sensing circuit similar to that described in relation to the embodiment of FIGS. 3 A, 3B. That is, a current path is created between the electrodes 100, 102, which places the resulting circuit in an ON state. If, thereafter, any portion of the conductive ink region 98 is broken, such as by tampering, the circuit will be broken and the sensor will assume an OFF state. Appropriate circuitry (e.g., FIG. 7) may be coupled to the electrodes 100, 102 in a known manner to sense the ON/OFF state of the sensing circuit and to generate an alarm signal as appropriate.

[0051] In some embodiments the conductive ink region 98 should be arranged so that it traverses as much of the surface of the second sheet member 96 as possible to maximize the chance that a breach in the second sheet member will cause a tamper condition to be sensed.

[0052] FIG. 7 shows an optional latching circuit 106 for use with the tamper detection system 92 illustrated in FIGS. 4 and 5. It will be appreciated that the latching circuit 106 may be used to maintain a "tamper" condition in the event of a short, temporary, interruption of the conductive ink region 98. In a normal operating mode, Tl (conductive ink region 98) has a low resistance and D2 is OFF. Q2 and Ql are also OFF. If the conductive ink region 98 is interrupted, Q2 is biased through R2 and D2 and is turned ON. This will turn ON Ql . Ql will continue to bias Q2 even if the conductive ink region 98 circuit is restored. The tamper I/O will remain indefinitely at a LOW level (in one non-limiting embodiment, about 0.65 V). To reset the latch (after Tl (conductive ink region 98) is replaced) a LOW logic (0V) is applied on the Tamper I/O. Dl will discharge the base capacitance. After reset, the Tamper I/O microcontroller GPIO is switched back as an input. The level will be set HIGH. After powering-up, a latching circuit reset cycle will ensure proper functionality of the tamper circuit.

[0053] Thus arranged, when the glass is broken the conductive ink is interrupted and a tamper condition is generated. To detect a tamper condition, the conductive trace impedance (i.e., the impedance of the trace formed by the conductive ink region 98) may be compared with a reference impedance. If the trace impedance passes a threshold, an error is generated which will turn ON a latch. Any momentary "impedance jump" should create a permanent tamper condition. An exemplary system architecture is shown in FIG. 8. As illustrated, conductive ink region 98 may be coupled via first and second electrodes 100, 102 to a trace impedance comparator 108 for comparing the conductive trace impedance with a reference impedance. The comparator 108 may be coupled to the latching circuit 106 for maintaining a sensed alarm signal if the conductive trace impedance is determined to have passed the predetermined threshold. The latching circuit 106 may be coupled to a processor 110, which may be part of the control panel 20 of the alarm system 40. The processor may also be configured to reset the latch 106 as appropriate.

[0054] It will be appreciated that in some embodiments the disclosed latching circuit 106 can be replaced by a hardware interrupt which will set a tamper flag. [0055] The embodiments described in relation to FIGS. 1-8 may find use in a variety of applications in the security alarm field, as well as other applications as previously mentioned. For example, some of the embodiments may find use in safe-deposit box applications, while other embodiments may be used as to detect tampering in large thermal insulation double glass windows, such as large thermal-pane windows filled with inert gasses such as argon, xenon and/or krypton, or sealed with a vacuum glazing.

[0056] Referring now to FIGS. 9-12, an embodiment of a tamper detection system 1 12 will be described. The disclosed system 1 12 may employ a single-sheet optical reflection evaluation technique, in which light beam path length estimation can be used to determine if a tamper condition has occurred. With this embodiment, a transmitter portion 122 creates a light mesh inside a sheet member 1 16 using multiple light reflection from the side walls 1 18 of the sheet member 1 16. In one non-limiting embodiment the sheet member 1 16 is a glass panel and the side walls 1 18 are coated with a reflective coating such as silver. The path length of the light may be continuously measured by evaluating light time traveling inside the sheet member. Thus arranged, if the sheet member 1 16 is breached at any location (e.g., hole, crack, break), the light path will be altered due to the change in the light path travel time. Detection of this alteration can be used to generate a tamper condition.

[0057] A tamper sensor 120 may be mounted into the edge of the sheet member 1 16 (as shown in FIG. 9), along the horizontal center line of the sheet member. A transmitter portion 122 of the sensor 120 may be aimed so that the reflection points along the horizontal edge (L) of the sheet member are separated into equal segments. After multiple reflections from the edge of the sheet member, the light beam may be detected by the receiver portion 124 of the sensor

120. The light-travel time will be related to the total number of reflections and the size of the sheet member 1 16. It will be appreciated that a dense light mesh will increase the tamper detection probability.

[0058] For the embodiment of FIG. 9, in which "L" is divided into 4 segments, the light path length can be estimated as:

[0059] If the incident beam reflection angle is to be decreased (for better surface coverage) then splitting L into (2*k) - odd number of equal segments for symmetry - is desired, and the first reflection will be located at L/(2*k). It will be appreciated that "k" is the number of equal intervals that segment the length "L" of the glass side. The distance from the sensor wall and the first reflection point will also be "k." The light path length will be:

delay

[0060] Incident light angle "a" depends on "k" and can be determined as:

— arctg

[0061] Where ci = speed of light in glass.

[0062] A non-limiting exemplary implementation using this technique will be described in relation to FIGS. 10-12. FIG. 10 shows an exemplary system architecture, including the transmitter and receiver portions 122, 124 of the sensor 102 (FIG. 9), both of which may be coupled to an oscillator 126. The oscillator 126 may be connected to a "time of flight" estimator 128 (which evaluates the phase between the reference and received signal), which, in turn, may be coupled to a decision block 130 which, in turn, may be connected to a processor 132. In one embodiment, the processor 132 may be part of the control panel 20 of the alarm system 40.

[0063] In one embodiment, the oscillator 126 may be a reference oscillator, the transmitter portion 122 may be a light emitting diode (LED) or laser diode, and the receiver portion 124 may be a photo-detector (PHD). The transmitted signal may be buffered and applied to the first input of an XOR gate (FIG. 11). The frequency of the reference oscillator may be selected to allow a delay (t) of about 1/4T (where T is the period of the reference oscillator signal) (FIG. 12). In this way, the XOR signal will have a duty cycle of about 50% and the output level (after filtering) will be about 50% of the XOR signal peak to peak value.

[0064] Thus arranged, the output level will vary if the light path is modified (i.e., if the sheet member 1 16 is breached). A 2*fl detector is used to validate the receiving signal. If the signal is not present at the PHD side, the XOR output frequency will be only fl instead of 2*fl . Thus, the oscillator can transmit the signal, and the phase of the received signal can be compared with a reference value that represents the light "time of flight." Any change in this time of flight can be sensed and used to generate an alarm.

[0065] Referring now to FIG. 13, an exemplary method in accordance with the disclosure will be described. At step 1000, first and second sheet members are provided. The sheet members are separated by an interior space. The interior space is filled with gas at a first pressure. At step 1 100, the gas pressure in the interior space is monitored using a pressure sensor. At step 1200, the monitored pressure information is transmitted to a processor. At step 1300, pressure trend information is developed using the received pressure information. Alternatively, or in addition, at step 1400, an alarm condition is signaled if the detected pressure decreases below a predetermined value, indicating a breach of at least one of the first and second sheet members.

[0066] Referring to FIG. 14, another method in accordance with the disclosure will be described. At step 2000, first and second sheet members are provided. The first sheet member may have first and second electrodes while the second sheet member may have a conductive ink trace formed on a surface thereof such that first and second ends of the trace contact the first and second electrodes. At step 2100, an impedance of the conductive ink trace is detected. At step 2200, the detected impedance is compared to a reference impedance value. At step 2300, an alarm condition is signaled if the detected impedance departs from the reference impedance value by greater than a predetermined amount.

[0067] Referring to FIG. 15, a further method in accordance with the disclosure will be described. At step 3000, light is transmitted through a first sheet member. At step 3100, the light reflects off a plurality of edge surfaces of the first sheet member. At step 3200, the reflected light is received at a receiver. At step 3300, a time of flight for the transmitted light is compared to a reference value. At step 3400, a tamper condition is signaled if the time of flight deviates from the reference value by a predetermined amount.

[0068] It will be appreciated that this method can be used to secure a building's large plate glass windows, display cases or as a sensitive light curtain to secure vault or museum rooms. In some embodiments the glass sheet may be replaced with a secure sheet, such as "bullet proof glass made from polycarbonate, thermoplastic and layers of laminated glass, for safe / bank vaults and other similar applications.

[0069] As used herein, an element or step recited in the singular and proceeded with the word "a" or ''an" should be understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. Furthermore, references to "one embodiment'^" of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

[0070] While certain embodiments of the disclosure have been described herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.

Claims

Claims What is claimed is:

1. A system for detecting a tamper condition, comprising:

first and second sheet members positioned to define an interior space therebetween, the interior space including a gas at a first pressure; and

a sensor element for detecting a breach in at least one of the first and second sheet members, the breach causing the pressure in the interior space to decrease below the first gas pressure;

wherein when the pressure in the interior space decreases below the first gas pressure the sensor element signals a tamper condition.

2. The system of claim 1 , wherein the sensor element comprises a pressure switch.

3. The system of claim 1 , wherein the sensor element comprises a linear pressure detector.

4. The system of claim 3, further comprising:

a comparator coupled to the sensor element for receiving the tamper condition signal from the sensor element,

a latching circuit coupled to the comparator for receiving a second signal from the comparator and for transmitting a third signal to a processor, the processor configured to recognize the third signal as a tamper condition and to send an alarm to a monitoring center.

5. The system of claim 4, wherein the sensor element comprises a linear pressure detector, the system further comprising:

a pressure trend estimator for receiving a pressure level signal from the linear pressure detector; and

a decision block for receiving a pressure trend estimator signal, the decision block coupled to a processor to provide the processor with information about the pressure condition in the interior space.

6. The system of claim 1, wherein the first sheet member includes first and second recesses, the system further comprising:

a conductive elastomer layer overlying the first and second recesses on a first surface of the first sheet member; and

first and second conductive mesh elements overlying the first and second recesses on a second surface of the first sheet member,

wherein the sensor element is coupled to the first and second conductive mesh elements;

wherein when the pressure in the interior space is at or above the first gas pressure the conductive elastomer layer expands into the first and second recesses to engage the first and second conductive mesh elements to enable current to flow between the first and second conductive mesh elements; and

wherein when the pressure in the interior space below the first gas pressure the conductive elastomer layer contracts so as not to be in contact with the first and second conductive mesh elements.

7. The system of claim 6, wherein the first and second sheet members comprise opposing glass panes of a double pane window.

8. The system of claim 1 , further comprising a gas generating component coupled to the interior space, the gas generating component comprising an acid component and a base component which, when mixed, generate a gas that will pressurize the interior space to at least the first gas pressure.

9. A system for detecting a tamper condition, comprising:

a first sheet member having first and second electrodes formed therein,

a second sheet member having a conductive ink trace formed on a first surface thereof, the second sheet member positioned with respect to the first sheet so that first and second ends of the conductive ink trace are in electrical contact with the first and second electrodes, respectively; and

a sensor element for sensing a breach in at least one of the first and second sheet members, the breach causing a change in an impedance of the conductive ink trace, the breach representative of a tamper condition.

10. The system of claim 9, wherein the sensor element comprises:

a trace impedance comparator for comparing a measured impedance of the conductive ink trace with a reference impedance value; a latching circuit for receiving a comparator signal from the trace impedance comparator when the measured impedance of the conductive ink trace departs from the reference impedance value by a predetermined amount; and

a processor for receiving a latching signal from the latching circuit and for signaling an alarm condition to a monitoring center.

1 1. The system of claim 9, wherein the first and second sheet members comprise glass panels sealed about their respective perimeters via a sealing gasket.

12. The system of claim 9, wherein the second sheet member is coupled to an alarm system housing along a second surface thereof.

13. A system for detecting a tamper condition, comprising:

a first sheet member having a reflective coating disposed on a perimeter edge surface thereof,

a light transmitting element for transmitting light within said first sheet member, a light sensing element for receiving the transmitted light reflected within the first sheet member; and

a time of flight estimator for determining a time of flight for the light transmitted between the light transmitting element and the light sensing element and for outputting a signal representative of a tamper condition when the time of flight exceeds a predetermined value.

14. The system of claim 13, wherein the time of flight estimator is connected to a processor for signaling an alarm condition to a monitoring center when the signal representative of a tamper condition is received.

15. The system of claim 14, further comprising an oscillator coupled to the time of flight estimator.

16. A method for detecting a tamper condition, comprising:

detecting a pressure in a space formed between first and second sheet members, the first and second sheet members associated with a monitored enclosure; and

signaling an alarm condition if the detected pressure decreases below a predetermined value;

wherein the alarm condition is representative of a breach of the monitored enclosure.

17. The method of claim 16, wherein signaling an alarm condition comprises sending an alarm signal to a monitoring facility.

18. The method of claim 17, further comprising providing first and second gas producing compounds in the space, wherein the first and second gas producing compounds, when combined, produce sufficient gas in the space to result in a gas pressure about the

predetermined value.

19. A method for detecting a tamper condition, comprising:

detecting an impedance of a conductive ink trace disposed on a first sheet member, comparing the detected impedance with a reference impedance value; and

signaling an alarm condition if the detected impedance departs from the reference impedance value by greater than a predetermined amount.

20. The method of claim 19, wherein detecting an impedance is performed by coupling to first and second electrodes disposed in a first sheet member, the first and second electrodes connected to first and second ends of a conductive ink trace formed on a surface of a second sheet member.

21. The method of claim 19, wherein signaling an alarm condition comprises sending an alarm condition to a monitoring facility, the alarm condition indicating a tamper condition in an alarm housing associated with the first sheet member.

22. A method for detecting a tamper condition, comprising:

transmitting light through a first sheet member, the light reflecting off a plurality of edge surfaces of the first sheet member;

receiving the transmitted light at a receiver;

comparing a time of flight for the transmitted light to a reference value; and

signaling a tamper condition if the time of flight deviates from the reference value by a predetermined amount.

23. The method of claim 22, wherein the first sheet member is associated with an alarm system housing, the tamper condition indicating a breach of the alarm system housing.