Abstract:
A tamper resistant magnetic contact switch apparatus for alarm systems that has a plurality of magnetic reed switches configured in a logic circuit and a magnetic pack for actuation. A magnetic shield disposed around the switches defines an actuation zone. In combination with the magnetic shield preventing magnetic fields from reaching the switches from locations outside the actuation zone, the magnet pack activates two of the five switches when positioned within the actuation zone to complete the logic circuit. The magnet pack does not, however, activate any of the other switches. If any of the other three switches are activated, or if either of the two activated switches are deactivated, the logic circuit is broken and the alarm circuit is activated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. provisional application serial No. 60/323,988 filed on Sep. 14, 2001, incorporated herein by reference. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
         REFERENCE TO A COMPUTER PROGRAM APPENDIX  
         [0003]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0004]    1. Field of the Invention  
           [0005]    This invention relates generally to high security, tamper resistant contact systems and more particularly to a logic circuit including magnetically actuated sensors that are resistant to foreign magnetic fields and that do not require custom tuning for monitoring door or window openings or the like as part of an electrically monitored physical security system.  
           [0006]    2. Description of the Background Art  
           [0007]    Thieves and intruders of increasing sophistication continue to challenge the peaceful security of businesses and homes alike. Because security personnel cannot be present at all potential points of entry to a structure, effective security systems require accurate indicators of the movement of doors and windows and other points of entry. Similarly, safety control systems on industrial machinery require accurate indicators as to whether a safety guard or cover is in place before activating or deactivating the machine.  
           [0008]    Magnetically actuated proximity switches known in the art typically have one or more reed switches mounted to a frame surrounding a window or doorway that are electrically connected to the security control unit. One or more permanent magnets are mounted to a door or window in predetermined positions. When the permanent actuating magnet is brought in proximity to the reed switch, as determined by the sensitivity of the reed switch and the strength of the permanent magnet, the reed switch is actuated by the permanent magnet by closing a set of magnetic contacts within the switch and completing the circuit. A signal is thereby sent to the control unit indicating that the door is in proper position.  
           [0009]    One deficiency with conventional magnetic proximity switches is that the permanent actuating magnets must be precisely positioned with respect to the reed switch, thereby requiring periodic adjustments to avoid false alarms or an ineffective system. Consequently, regular adjustments must be made as a result of seasonal changes in temperature and humidity, as well as frequent use of the door or window that may lead to misalignment of the permanent magnet and switch. Misalignment may result in false alarms as well as unscheduled service calls for manual realignment of the switch or magnet.  
           [0010]    It is therefore desirable to use a fairly sensitive switch so that the device has a greater tolerance to small deviations in the position of the permanent actuating magnets and the switch without setting off a false alarm. Sensitivity of the reed switches has been improved in the art by the placement of small biasing magnets near the reed switch to bias the response of the switch to external magnetic fields. The biasing magnet may be oriented in polar opposition to the actuating magnet thereby increasing the sensitivity of the reed switch. Thus, smaller and less expensive magnets may be used as actuating magnets.  
           [0011]    Another deficiency of conventional magnetically actuated proximity switches is that they may be subject to circumvention or manipulation by strong foreign magnetic fields. For example, a conventional magnetic switch can be defeated by the placement of an external magnet near the switch. A magnet may be used to defeat a conventional magnetic switch on the opposite side of the door if it produces a sufficient magnetic field. The intruder can open the door without activating the alarm because the strong external magnetic field caused the switch to stay in the same state as when the actuating magnets are in the proper position.  
           [0012]    Later “balanced” type switches were developed that may be sensitive to externally applied magnetic fields. One approach to solving the aforementioned deficiencies can found in U.S. Pat. No. 4,945,340 to Brill, incorporated herein by reference. The Brill patent discloses an apparatus comprising three switches, two of which are responsively adapted to a fixed magnetic field placed in proximity to the switches. The third switch is positioned to detect when another magnetic field, introduced by someone who is attempting to defeat the security system, is placed in proximity to the three switches and the fixed magnetic field. One apparent deficiency of the Brill approach, however, is that the third switch fails to detect when another magnetic field of the same form produced by the magnet pack assembly described therein is placed in proximity to the three switches. Therefore, Brill&#39;s approach is easily compromised by anyone placing another like fixed magnetic field in proximity to the three switches and thereby disabling the security system.  
           [0013]    Magnetically biased high security switches also have disadvantages due to changes in the magnetic strength of the biasing magnets. For example, due to their inherent sensitivity, they may malfunction and cause false alarms. Accordingly, the switches and magnets must be carefully adjusted and positioned during installation to avoid false alarms. Furthermore, magnetically balanced switches are difficult to manufacture and are costly because the magnets must be magnetically balanced very carefully, either during installation, or preset at the factory.  
           [0014]    Therefore there is a need for a switch apparatus that cannot be compromised by the placement of an additional magnetic field placed in proximity to the apparatus for the purpose of defeating the security system. There is also a need for an apparatus that will detect the presence of an additional magnetic field in proximity to the apparatus. There is also a need for a switch apparatus that will enter into an alarm state when an additional magnetic field is placed in proximity to the apparatus. There is also a need for a switch apparatus that can be precisely assembled and obviates the need to perform adjustments at the factory. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies found in existing equipment.  
         BRIEF SUMMARY OF THE INVENTION  
         [0015]    The present invention is an apparatus for use in a physical security-monitoring environment to activate an alarm circuit. In general terms, the apparatus comprises a plurality of magnetic reed switches configured in a logic circuit and a magnetic pack for actuation. A magnetic shield disposed around the switches defines an actuation zone. In combination with the magnetic shield preventing magnetic fields from reaching the switches from locations outside the actuation zone, the magnet pack activates two of the five switches when positioned within the actuation zone to complete the logic circuit. The magnet pack does not, however, activate any of the other switches. If any of the other three switches are activated, or if either of the two activated switches are deactivated, the logic circuit is broken and the alarm circuit is activated.  
           [0016]    By way of example, and not of limitation, the apparatus includes a sensor unit, having a common conductor, a guard conductor and preferably at least five switches, and an actuator unit associated with the sensor unit. Each of the switches is adapted to be placed in an activated state in response to exposure to a magnetic field of predetermined magnetic flux.  
           [0017]    The sensor unit also includes a logic circuit electrically interconnecting the switches and the common and guard conductors, the logic circuit completing a series circuit between the common conductor and the guard conductor whenever at least two predetermined switches are in a magnetically actuated condition.  
           [0018]    The associated actuator unit includes at least two permanent magnets and provides discrete magnetic fields of predetermined flux density and position sufficient to activate the two or more predetermined switches individually to complete the series circuit between the common conductor and the guard conductor when the actuator unit is located in predetermined position with respect to the sensor unit.  
           [0019]    The series circuit is interrupted and a shunt circuit to an alarm conductor is completed whenever the actuator unit is removed or an additional magnetic field is placed in proximity to the sensor unit switches.  
           [0020]    Preferably, the sensor unit and the actuator unit each include a sealed protective nonmagnetic housing. The sensor unit further includes a magnetically permeable shield in which the switches are disposed.  
           [0021]    In one embodiment, the sensor unit includes five switches and the actuator unit includes two permanent magnets, each permanent magnet being positioned to activate a corresponding switch such that only two of the five switches are activated to complete the series circuit between the common conductor and the guard conductor when the actuator unit is located in its predetermined position. In that embodiment, each switch is a single-pole-double-throw (SPDT) reed switch and the logic circuit includes a printed circuit board on which the reed switches are mounted. Three of the switches are placed in a row along the length of the sensor unit, and the switches at both ends of the row are activated when the actuator unit is located in its predetermined juxtaposition. One switch is placed in parallel to each switch located at both ends of the row respectively and is not activated when the actuator unit is located in its predetermined position. Two permanent magnets are poles so that the middle switch remains in a deactivated state.  
           [0022]    The apparatus includes an arrangement of switch elements and logic circuitry that will interrupt a guard circuit when it is disturbed by the presence of a foreign magnetic field. The switch units of the logic circuit and the magnets of the actuator unit are spaced apart in a coordinated array so that the flux pattern from the cooperating magnetic actuator will only actuate specific switch units.  
           [0023]    One embodiment also includes a pry tamper system that has a wall-mounted magnet and a reed switch assembly positioned above the wall-mounted magnet. The reed switch is preferably placed in proximity to a window in the magnetic shield to allow the magnetic flux of the permanent magnet to reach the reed switch. An attempt to pry and separate the sensor unit from the actuation unit will activate the alarm.  
           [0024]    An object of the invention is to provide a magnetic contact apparatus that cannot be compromised by the introduction of an additional magnetic field placed in proximity to the system for the purpose of defeating the system.  
           [0025]    Another object of the invention is to provide a magnetic contact apparatus that will detect the presence of an additional magnetic field placed in proximity to the security system.  
           [0026]    Still another object of the invention is to provide a magnetic contact apparatus that will enter into an alarm state when an additional magnetic field is placed in proximity to the security system.  
           [0027]    A further object of the invention is to provide an alarm contact apparatus that can detect tampering or the removal of the contact from its original position of installation.  
           [0028]    Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    The invention will be more fully understood by reference to the following drawings that are for illustrative purposes only:  
         [0030]    [0030]FIG. 1 is a perspective view of a sensor unit and an actuator unit in accordance with the present invention.  
         [0031]    [0031]FIG. 2 is a perspective view of the sensor unit, the actuator unit and the tamper plate in accordance with the present invention.  
         [0032]    [0032]FIG. 3 is an exploded perspective view of the tamper plate shown in FIG. 2.  
         [0033]    [0033]FIG. 4 is an exploded back view in perspective of the sensor unit shown in FIG. 2.  
         [0034]    [0034]FIG. 5 is a back view in perspective of the sensor unit and the tamper plate in accordance with the present invention as shown in FIG. 2.  
         [0035]    [0035]FIG. 6 is an exploded perspective view of the actuator unit shown in FIG. 1 and FIG. 2.  
         [0036]    [0036]FIG. 7 is a top view of the sensor unit printed circuit board in proximity to the actuation unit and magnet positions.  
         [0037]    [0037]FIG. 8 is a schematic diagram of the sensor unit with the actuator unit and tamper plate in the proper position.  
         [0038]    [0038]FIG. 9 is a perspective side view of the sensor unit printed circuit board shown in FIG. 4 rotated 90 degrees about the long axis.  
         [0039]    [0039]FIG. 10 is a schematic diagram of the sensor unit with the actuator unit and tamper plate out of the proper position. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]    Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus generally shown in FIG. 1 through FIG. 10 wherein like references denote like parts. It will be appreciated that the apparatus may vary as to configuration and as to details of the parts without departing from the basic concepts as disclosed herein.  
         [0041]    Referring now to FIG. 1, the high security alarm sensor apparatus  10  of the present invention is generally shown. The apparatus has a sensor unit  12  and a cooperating actuator unit  14 , each of which is mounted in a predetermined position with respect to each other on points of entry to a building such as a door or window. The sensor unit  12  has an armored alarm control cable  18  that is preferably electrically connected to a central alarm control panel (not shown). It will be seen that whenever the actuator unit  14  is placed in the proper proximity  16  to the sensor unit  12 , the sensor unit remains in a guarded condition. If the actuator unit is removed from the proximity of the sensor unit  12 , or if an additional magnetic field is placed in proximity to the sensor unit  12 , the sensor unit will enter into an alarm condition.  
         [0042]    In one embodiment, a tamper plate  20  is coupled with the sensor unit  12  as seen in FIG. 1 and FIG. 2. Referring also to FIG. 3, the tamper plate  20  is affixed to the desired location with screws or other fasteners disposed in mounting holes  26   a  and  26   b  located at the center section  24  of the tamper plate  20 . The sensor assembly  12  is mounted to the peripheral section  22  of tamper plate  20 . Tamper plate  20  is preferably manufactured from a strong material such as ABS plastic. Accordingly, any attempt by an intruder to pry the sensor unit  12  from its installed location will result in the separation of the central section  24  from the peripheral section  22  of tamper plate  20  leaving central section  24  in its original location.  
         [0043]    A permanent magnet  28  is preferably disposed in a slot in the central section  24  of tamper plate  20 . In one embodiment, the magnetic field of permanent magnet  28  is shielded with a magnetic shield  30 . The tamper plate magnetic shield  30  is preferably made from zinc-plated steel that will permit an essentially constant magnetic field regardless of the nature of the mounting surface such as wood or steel.  
         [0044]    Referring now to FIG. 4, the preferred components of the sensor unit  12  of the invention can be seen. The sensor unit  12  preferably has an outer housing  32  made of anodized aluminum or similar material. Sensor unit  12  preferably has a printed circuit board assembly  44  that is held in place within an upper and lower printed circuit board holders  38  and  42  respectively. Upper and lower board holders  38  and  42  preferably enclose the printed circuit board  44  and permit access of alarm control cable  18  comprising a wire bundle  46  and an armored cable sheath  48 . Armored cable sheath  48  is preferably stainless steel flex cable or the like that is commercially available.  
         [0045]    Board  44 , upper holder  38  and lower holder  42  are disposed in a generally “C” shaped sensor magnetic shield  34 . Magnetic shield  34  is preferably made from zinc-plated steel or like material to shield the circuit board from external magnetic fields. Shield  34  is configured to be contained within outer housing  32 .  
         [0046]    Upper holder  38  and lower holder  42  are preferably made of ABS plastic or like material and precisely hold circuit board  44  in a predetermined position within the housing  32 . A slot  40  is located on the upper holder  38  above one of the reed switches  60  of the printed circuit board  44 . A corresponding slot  36  is preferably located in the magnetic shield  34 . Referring also to FIG. 5, it can be seen that permanent magnet  28  of tamper plate  20  is positioned over the slot  36  of magnetic shield  34  and slot  40  of the upper holder  38  and tamper reed switch  60  on the printed circuit board  44 . Tamper reed switch  60  may be raised from the plane of the surface of printed circuit board  44  with a tamper switch mount  64  as seen in FIG. 9.  
         [0047]    Accordingly, when tamper plate  20  is mounted with sensor assembly  12 , the magnetic field of wall mounted magnet  28  can reach the tamper reed switch  60  through the slots but the magnetic field does not reach any of the other reed switches located on printed circuit board  44 . When reed switch  60  is within the magnetic field of magnet  28  the system is normal. However, if an intruder attempts to remove the sensor assembly  12  that has been installed with the tamper plate  20 , the outer section  22  of the plate will remain with the sensor housing  32  while the central section  24  with magnet  28  will remain fixed to the wall. The magnetic field of magnet  28  will thereby be removed from the proximity of reed switch  60  and an alarm will be activated.  
         [0048]    Turning now to FIG. 6, the preferred configuration of the actuator unit  14  can be seen. The actuator unit  14  preferably has a pair of permanent magnets  66  and  68  that are held in the proper position by magnet holder  72 . Optionally, the north pole of the magnets  66 ,  68  may be marked with a mark  70  to indicate the polarity of the magnets.  
         [0049]    In the embodiment shown, magnet holder  72  has retaining brackets  74  and  76  that are configured to precisely position the magnets  66 ,  68  and their magnetic fields. Magnet holder  72  is preferably made of ABS plastic and is dimensioned to fit within actuator housing  78 . Magnet housing  78  is preferably made of anodized aluminum. In one embodiment, holes  80  are preferably drilled after potting.  
         [0050]    In one embodiment, each magnet  66  and  68  is composed of neodymium-iron-boron that preferably has a coercive force of about 12,000 oresteds, a diameter of about 1.3 centimeters and a height of about 0.6 centimeters.  
         [0051]    Further details of sensor unit  12  may be seen with reference to FIG. 7 through FIG. 10. The state of the sensor unit  12  when the tamper plate  20  and actuation unit  14  are in the proper position is shown in FIG. 7 and FIG. 8. In contrast, FIG. 10 shows the state of the sensor unit when the tamper plate  20  and actuation unit  14  are out of position.  
         [0052]    Printed circuit board  44  carries an array of printed circuit conductors that are connected to the conductors of cable  46  and on which are mounted five magnetically actuated reed switches  50 ,  52 ,  54 ,  56 ,  58  and optionally one tamper reed switch  60 . Each reed switch  50 ,  52 ,  54 ,  56  and  58  is preferably of the single-pole-double-throw (SPDT) type and has a common terminal, a normally closed contact terminal, and a normally open contact terminal as shown in FIGS. 8 and 10. In addition, reed switch  60  is preferably of the single-pole-single throw (SPST) variety having a normally open contact.  
         [0053]    Turning now to FIG. 7, it can be seen that when the actuation unit  14  and tamper magnet  28  are in the proper position, the logic circuit is complete on the normally closed (guard) loop. Magnets  66  and  68  of the actuator unit  14  activate only two reed switches  52  and  56 . The gap  16  between the sensor unit  12  and the actuator unit  14  is preferably between approximately 0.2 inches and approximately 0.6 inches.  
         [0054]    Each magnet  66 ,  68  of the actuator unit  14  are magnetized in the axial direction, and are preferably positioned in the same magnetic orientation opposite reed switches  52  and  56 . Consequently the magnetic fields of magnets  66 ,  68  oppose each other in the region centered between them so that the net flux is zero in that region and the center reed switch  54  is not affected. Furthermore, the magnetic flux field generated from magnets  66  and  68  does not extend to reed switches  50  and  58  and these switches are therefore not affected since they exist in a region of lower magnetic flux.  
         [0055]    The circuit of FIG. 8 shows the reed switches  50 ,  52 ,  54 ,  56  and  58  in the guard or secure position (e.g. when actuator unit  12  is properly spaced from sensor unit  10 ). When the sensor unit  10  and tamper plate  20  are properly installed on a door frame, for example, and the door to which the actuator unit  12  is attached is in a closed position, magnets  66  and  68  actuate reed switches  52  and  56 , respectively, closing normally open contacts in switches  52  and  56 . Thus, the closed contacts of switches  52  and  56  and the normally closed contacts of switches  50 ,  54  and  58  form a series circuit in a closed guard loop in the presence of the actuator unit  14 .  
         [0056]    Similarly, the normally open reed switch  60  of the optional tamper circuit is closed in the presence of magnet  28  of the tamper plate  20 . The system is in the armed state in the embodiment shown when the tamper and actuator magnets are in the proper position.  
         [0057]    Turning now to FIG. 9 and FIG. 10, the removal of the actuator  14  or other alteration of the magnetic field around any of the reed switches  50 ,  52 ,  54 ,  56 ,  58  causing the activation state of a reed switch to change, the normally closed loop will be broken and the normally open alarm loop will be closed. For example, as seen in FIG. 10, an intruder opening the door would remove the magnetic fields of magnets  66  and  68  of the actuator  14  thereby causing normally open switches  52  and  56  to open and completing the normally open alarm loop circuit.  
         [0058]    Likewise, if an attempt is made to defeat the sensor unit  12  by using one or more strong magnets at points along the housing, one or more of the reed switches will change state.  
         [0059]    By creating a logic circuit comprised of a series of reed switches  50 ,  52 ,  54 ,  56 , and  58  on a printed circuit board  44 , the precise location of reed switches can be maintained without custom tuning efforts. In addition, the magnetic shield  34  prevents magnetic fields from reaching the reed switches  50 ,  54  and  58  and switches  52  and  56  on all but the intended side of the actuator unit  14 . If any of the other reed switches  50 ,  54  or  58  are activated or if either of the two reed switches  52  and  56  are deactivated, the logic circuit is broken and the alarm circuit is triggered.  
         [0060]    Thus, unlike the prior art “balanced magnetic field” alarm switches, the logic circuit does not need to be fine tuned and will activate the alarm if a stronger magnetic field is introduced at points surrounding the sensor unit  12  or actuator unit  14 . The magnetic shield  34  provides protection from compromise from magnetic field exposures above, behind and below the sensor unit  12 . The center reed switch  54  protects against single magnet compromise and polarized pack compromise from the front of the sensor unit  16 . The aft reed switches  50  and  58  protect against defeat by exposure to large magnetic fields and magnets placed on the sides of the sensor unit.  
         [0061]    Accordingly, it will be seen that this invention provides a security system and apparatus for use in a physical security-monitoring environment that is resistant to being defeated by the presence of strong magnetic fields near the apparatus or removal of portions of the apparatus from the point of installation.  
         [0062]    Although the description above contains much specificity, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.”