Abstract:
An intrusion alarm system in which intrusion into an protected space is detected as a variation in air pressure. The variable pressure detector uses a membrane and a displacement detector. One side of the membrane is exposed to the protected space and the opposite side of the membrane is enveloped by an enclosure with a limited pressure coupling to the protected space. A signal from the displacement detector is analyzed by a processor to identify rapid changes in air pressure to activate the security alarm. The same type of a variable pressure detector may be used to control electric lights and other devices in response to people entering into a room.

Description:
[0001]    The invention relates to alarm systems, and in particular to an alarm system designed to protect an enclosed space and give warning that the space has been penetrated by an intruder. It is based on U.S. Provisional Patent Application No. 60/842,522 filed on Sep. 6, 2006. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    An intrusion alarm is typically intended to protect an enclosed space from intrusion. The space may be a domestic dwelling or commercial building, a room in such a building, a safe, a vault, or the interior of a vehicle. 
         [0003]    It is a well known fact that air pressure in an enclosed space will remain unchanged as long as that space remains fully enclosed. When the space develops an opening, air pressure changes depending on the outside air pressure. If the enclosed space is a room in a building, air pressure inside will remain either constant or will change slowly in accordance with the outside atmospheric pressure. Opening of doors and windows would result in a rapid fluctuation of the air pressure in the room. This can be detected by an appropriate sensor. 
         [0004]    In U.S. Pat. No. 3,947,838 there is described an alarm system comprising a moving vane sensor responsive to air pressure within an enclosed space, the sensor providing electrical signals related to the sensed air pressure, and a signal processor to which the electrical signals are supplied and operative to initiate an alarm indication when the signal supplied by the sensor is indicative of an intrusion into the enclosed space. 
         [0005]    The U.S. Pat. No. 4,692,734 issued to Holden et al. describes the signal processing in the alarm system based on a comparison of the current signal with the reference set. 
         [0006]    The prior art relies on use of either complex pressure sensors, or the pressure sensors are not sufficiently sensitive to detect as small pressure variations as few mm H 2 O. 
         [0007]    It is therefore the object of this invention to develop a sensor for the security alarm system that is sensitive to detect small changes in pressure; 
         [0008]    It is another object of this invention to make pressure sensor insensitive to slow changing air pressure. 
         [0009]    And another object of this invention is to reduce a complexity and cost the air pressure sensor. 
       SUMMARY OF THE INVENTION 
       [0010]    According to this invention an alarm system comprises a sensor responsive to air pressure changes within an enclosed space. The sensor contains a thin and relative large membrane with one side exposed to the air in a monitored enclosed space, while the opposite side of the membrane is enveloped by an enclosure having a small hole that is exposed to the same monitored enclosed space. The hole restricts the air flow between the interior of the enclosure and the outside, thus resulting in a delay between the variations in pressure inside and outside of the enclosure. The delay causes a temporary disbalance of pressures across the membrane and thus the membrane deflection. The deflection is measured by the displacement sensor, for example, optical. The output signal of the displacement sensor is further compared with a predetermined threshold whose output, in turn, controls the alarm. 
     
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]    This invention will now be described by way of example with reference to the drawings, in which: 
           [0012]      FIG. 1  is an example of an enclosed space with a door and windows; 
           [0013]      FIG. 2  shows variations in air pressure within the enclosed space; 
           [0014]      FIG. 3  is a cross-sectional view and block-diagram of the differential air pressure sensor and the alarm system; 
           [0015]      FIG. 4 . shows operation of the optical displacement detector; 
           [0016]      FIG. 5  depicts a timing diagram of pressures across the membrane, and 
           [0017]      FIG. 6  shows an opening in the enclosure with a variable aperture. 
           [0018]      FIG. 7  illustrates a capacitive option of the displacement sensing, and 
           [0019]      FIG. 8  depicts a corrugated membrane. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    The system shown in  FIG. 1  comprises a sensor  5  which is arranged in an enclosed space  1  to be monitored and responsive to the air pressure in that space to provide electrical signals indicative of the air pressure variations at any time. The sensor  5  is connected to the monitor  6  that may be comprised of a microprocessor, alarm, power source and other components. The enclosed space  1  has windows  3  and one or more doors  4 . The interior air pressure is P h  and the exterior pressure is atmospheric P atm . Generally, these pressures are somewhat different, primarily due to a temperature gradient between the enclosed space and the outside. When the doors and windows are closed, still some air leaks may be present and pressure P h  would change rather slowly along with P aim . When doors  4  or windows  3  are opened and closed, air flow (draft) occurs and pressure P h  changes more rapidly towards equalization with P aim . The same effect occurs when people enter the enclosed space and move within the space. This is illustrated in  FIG. 2  that shows the internal air pressure. When it changes slowly, the changes Δ a  are smaller than Δ b  which occur during the rapid pressure variations. The time to is a fixed interval to measure the pressure variations. The purpose of sensor  5  is to respond to faster changes in pressure and not to respond to slower changes in pressure. It also should be noted that air drafts caused by movement of intruders may be quite small—typically not greater than few mmH 2 O. 
         [0021]    A differential pressure sensor  5  is shown in  FIG. 3 . Unlike the conventional differential pressure sensors that respond to constant and changing pressures, the illustrated sensor responds only to relatively fast changes in the gas pressure differential and is not sensitive to slow changing pressures. A goal of the sensor is to convert the differential air (gas) pressure changes to the output electrical signal that can be processed by the signal conditioner  20 , processor  22  and activate the alarm  23 , if needed. In this example of the design, the printed circuit board (PCB)  10  supports membrane  13  which is air-tight sealed to the PCB  10  all around the circumference at areas  14  and  15 . The PCB acts as a support structure. The membrane is fabricated of any suitable material, such as Mylar, aluminum or brass foil and is stretched reasonably tight. It must be flexible enough to respond to small variations in pressure across its thickness. A shape of the membrane  13  may be a disk having a diameter from 0.25 to 4 inch and thickness between 0.0005 and 0.005 of an inch. The membrane may be flat or corrugated as shown in  FIG. 8  where the creases  46  may have a circular shape. 
         [0022]    Next to the membrane  13 , the PCB  10  has an opening  11  which is smaller than the membrane overall size. An inlet tube  12  is attached to the PCB  10  to allow air pressure P h  to access the membrane  13  through the opening  11 . At the opposite side of the PCB  10 , there is an enclosure  16  which is air-tightly attached to the PCB  10 . The membrane  13  has two sides: side  50  is exposed to the protected space, while side  51  is exposed to enclosure  16 . In other words, membrane  13  at the left side  50  is exposed to the monitored pace air pressure P h , while at the right side  51  it is exposed to the air pressure P 2  inside the enclosure  16 . The enclosure  16  has at least one hole  17  whose aperture may be either fixed or adjusted by a moving cover  34  as illustrated in  FIG. 6 . The cover  34  may be rotated around pivot  35 . In general, the area of aperture of the hole  17  shall be at least 100 times smaller than the overall inner surface area of the enclosure  16  or the membrane  13 . 
         [0023]    In the first preferred embodiment, at one of the sides of the membrane  13 , for example at side  51 , there is a displacement sensor  18  as illustrated in  FIG. 3 . The purpose of the displacement sensor  18  is to detect the membrane  13  displacement, that is, to convert distance  19  to the membrane  13  into electrical signal that can be processed by the signal conditioner  20 . The membrane  13  displacement is the measure of a differential pressure ΔP. 
         [0024]    Since the enclosure  16  is connected to the protected space only through a small hole  17 , changes in air pressure P h  are not immediately reflected by the internal pressure P 2 . In other words, there is a phase shift between the outside and the inside pressures, as illustrated in  FIG. 5 . When P h  changes slowly, a small hole  17  allows P 2  to follow P h  very closely so pressures at both sides of membrane  13  are nearly the same and the membrane is substantially flat and not moving. During faster changes in P h , the hole  17  slows down the pressure equalization and the internal pressure P 2  (dotted line in  FIG. 5 ) lags behind and also is somewhat smoother. A differential pressure ΔP across the membrane  13  is shown at the bottom portion of  FIG. 5  as pressure  32 . When the differential pressure  32  is near zero, the membrane remains substantially flat and the distance  19  is at its base level. When pressure  32  deflects from zero, the membrane  13  flexes inwardly or outwardly, thus modulating distance  19 . 
         [0025]    The displacement sensor  18  monitors this distance  19  and provides a signal to the signal conditioner. When the pressure differential ΔP and, subsequently, the distance  19  are sufficiently large to reach the preset threshold  33 , the processor  22  detects the threshold crossing  36  and indicates the alarming event. 
         [0026]    There are numerous ways of designing a displacement sensor.  FIG. 4  illustrates one possible way of designing the displacement sensor  18 . It is comprised of an opto-coupler  27  with the photo emitter  28  and photo detector  29 . The membrane  13  is shown in two states: the base state  25  which corresponds to a zero differential pressure, and a flexed state  26  when P h  is higher than P 2 . The right side of membrane  13  is made reflective. For example, if the membrane is made of a plastic film, like Mylar, at least one side can be metallized. When the membrane  13  is in state  25 , the emitted light L e  is reflected from the membrane and goes to the detector  29  as the beam L r0 . The output signal from the opto-coupler  27  is the strongest. When the membrane  13  moves to the state  26 , the reflected light beam L p  is diverted from the detector  29 , causing the opto-coupler&#39;s output signal to drop. To minimize the opto-coupler power consumption, the emitted light doesn&#39;t need to be continuous, it can be emitted as short pulses with a small duty cycle. For example, a light pulse can have a duration of 10 microseconds and the pulses are emitted with a rate of 100 pulses per second. This corresponds to a duty cycle of 0.001 which results in a significant reduction in power consumption without compromising reliability of the intrusion detection. 
         [0027]    In the second embodiment, the function of a displacement sensor may be assumed by the signal conditioner  20  that should be responsive to changes in a capacitance. In this case, the enclosure  16  is replaced by a substantially flat and rigid plate  40  shown in  FIG. 7 . The disk has at least one and possibly several small holes  41  whose combined area of aperture shall be at least 100 times smaller than area of the plate  40  adjacent to the membrane. The plate  40  is positioned close to membrane  13  and is separated from it by a spacer  43 ,  44 . The gap  42  between the membrane  13  and plate  40  should be no larger than 0.1 of an inch. The plate  40  shall be electrically conductive and at least one side of membrane  13  also shall be electrically conductive. An electrical capacitance is formed between the membrane  13  and plate  40 . A value of this capacitance will change when pressure P h  varies with respect to the air pressure P 2  inside the gap  42 . The capacitance variations are measured by the signal conditioner  20  and presented as the output  45  reflecting the differential pressure ΔP. 
         [0028]    One should not overlook other potential applications of the above described differential pressure detector. These may include turning on electric lights in a room in response to an intrusion or walking near the detector. This can be exemplified by a stairway that needs to be illuminated. Traditional infrared motion detectors that are used for this purpose respond only when there is a direct vision of the intruder, while the differential air pressure detector would have a coverage not limited by a direct line of view. In such applications, an alarm  23  of  FIG. 3  is replaced with an electric switch. 
         [0029]    Without further elaboration, the foregoing will so fully illustrate our invention that others may, by applying current or future knowledge, readily adopt the same for use under various conditions of service.