Abstract:
A position monitoring device according to one embodiment includes a passive module including a magnetic field generating device, and an active module including a magnetic field sensing device. The device is operable in a configuration mode and a monitoring mode. In the configuration mode, the device stores configuration data representative of the output of the magnetic field sensing device when the passive and active modules are aligned. In the monitoring mode, the device compares the output to the configuration data, and generates an alarm signal when the output deviates from the configuration data.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a position monitoring device and particularly relates to a position monitoring device intended for security applications. 
       BACKGROUND TO THE INVENTION 
       [0002]    Electronic building security systems sometimes include sensors on doors or windows or the like which can be used to detect if the door or window is opened. One such sensor is a reed switch. Usually, a reed switch is mounted in the door frame to align with a permanent magnet mounted in the door itself. The reed switch is held closed by the magnetic field of the magnet. When the door is opened, the magnet moves out of proximity to the reed switch which opens which indicates an alarm condition to a security panel or building alarm system. 
         [0003]    It has been found that a reed switch type of position monitoring system can be defeated by an intruder by them placing their own permanent magnet in proximity to the reed switch. 
         [0004]    There remains a need to provide improved position detection systems. 
       SUMMARY OF THE INVENTION 
       [0005]    In a first aspect the present invention a position monitoring device including: a first module including at least one magnetic field transducers; a second module including at least one magnetic field generating device; the first and second modules are arranged to be mounted to fixed and moveable objects respectively; the device is operable in a configuration mode wherein it stores configuration data representative of the output of the at least one magnetic field transducers when the first and second modules are aligned; the device is operable in a monitoring mode wherein the output of the at least one magnetic field transducer is compared to the configuration data; if the outputs of the at least one magnetic field transducer deviates from the configuration data by more than a pre-determined amount then the device is arranged to provide an output signal that is indicative of an alarm condition. 
         [0006]    The first module may include two or more magnetic field transducers. 
         [0007]    The at least one magnetic field transducer may be provided in the form of a hall effect sensor. 
         [0008]    The second module may include two or more magnetic field generating devices. 
         [0009]    The at least one magnetic field generating device may be provided in the form of a permanent magnet. 
         [0010]    The second module may include regions for removably receiving the at least one magnetic field generating device and for allowing the magnetic field generating device to be arranged in either of two polarities. 
         [0011]    The first module may include at least one anti tamper switch to detect disassembly or removal of the first module. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  is an exploded view of a position monitoring device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    Referring to  FIG. 1 , a position monitoring device  10  is shown for monitoring the position of a door or a window or the like. Device  10  includes a first active module  20  which is intended for mounting in the frame of a door or window and a second passive module  30  which is intended for affixing to the door or window so that, when the door or window is in its normally closed position, the modules  20 ,  30  align with one another. 
         [0015]    Module  20  includes magnetic field transducers in the form of three hall effect sensors  26  which are mounted to a potted PCB  23 . Also mounted to the PCB  23  is an anti-tamper switch  24 , a program switch  25 , a signal relay (not visible) and a microcontroller (not visible). Potted PCB  23  is mounted inside housing  21  along with an optional end of line module  29  which enables encrypted communication with a building security system in a known manner. Power and communications are connected to module  20  by way of a multi strand cable (not shown) which is connected to the building security system. Housing  21  and cover  22  are formed from a non-ferrous metal or plastic material. 
         [0016]    When module  20  is assembled, the leaf of anti-tamper switch  24  protrudes from aperture  28  in housing  21  to detect removal of module  20  from a door frame. When module  20  is installed in a door frame the anti-tamper switch is depressed. If module  20  is removed from the door frame, such as is necessary to gain access to the program switch  25 , then this is detected by the anti-tamper switch  24 . 
         [0017]    Module  30  includes a housing  31  and cover  32 , both formed from a non-ferrous metal or plastic material. Housing  31  includes apertures  34  for mounting three magnetic field generating devices in the form of magnets  33  in approximate alignment with three hall sensors  26 . 
         [0018]    To install device  10 , three magnets  33  are randomly selected from a range of various possible strengths of magnet. The magnets may be inserted into housing  31  in either polarity, that is, with either their North or South pole facing the hall effect sensors. In this way, a large number of possible combinations of strengths and orientations of magnets  33  is made possible. The hall effect sensors output a voltage which is proportional to the strength of magnetic field that they are being subjected to. 
         [0019]    When module  20  is powered up for the first time it enters a configuration mode during which it waits to detect a stable magnetic field pattern produced by the magnets  33  in module  30  by reading and storing the outputs of the hall effect sensors as configuration data in non-volatile memory of the microcontroller. Thereafter, the module  20  operates in a monitoring mode during which it compares the ongoing outputs of the hall effect sensors with the stored configuration data. 
         [0020]    If the outputs of the hall effect sensors match the stored output values then the signal relay of module  20  activates. The activation of the relay is in turn monitored by the building security system. If the activation of the relay is detected by the building system, then this indicates that the relevant door or window is closed. 
         [0021]    If the outputs of the hall effect sensors deviate from the stored configuration data by more than a pre-determined threshold, then module  20  signals an alarm condition by deactivating the signal relay and optional buzzer  27  may sound. The deactivation of the signal relay is detected by the building security system and appropriate responsive action can be initiated. 
         [0022]    By allowing a threshold difference between the monitored hall sensor outputs and the stored sensor output values before an alarm condition is indicated the device can accommodate gradual changes in magnetic field strength which may arise over time from door “drop” or minor door misalignment or warpage. The threshold may be set to accommodate door drop of around 10 mm. 
         [0023]    Should the device  10  needed to be reprogrammed to an alternative magnetic field pattern the module  20  is removed from its mounted position in a door frame (activating the anti-tamper switch  24  and buzzer  27 ) and cover  22  is removed. The internal program switch  25  is then depressed which causes the module  20  to re-enter the configuration mode. The action will also provide an “event alarm” condition. Security personnel can verify that maintenance on the device is authorised. 
         [0024]    In the event of a power failure the signal relay becomes deactivated. 
         [0025]    Whilst the above described embodiment utilised an arrangement of three hall effect sensors and three permanent magnets, a different number of hall effect sensors and magnets may be used. In some embodiments only one hall effect sensor and magnet are used. 
         [0026]    Whilst the embodiment described above used the same number of hall effect sensors as magnets, in other embodiments the number of hall effect sensors may differ from the number of magnets used. For instance, one magnet may be used with two hall effect sensors. 
         [0027]    In some embodiments the magnets  33  are affixed to the passive module at the time of manufacture and are sealed with a potting mix or the like. In such an embodiment, the person installing the device is unaware of the combination of magnets used. 
         [0028]    The embodiment shown in  FIG. 1  is intended for flush mounting into recesses cut into a door and a door frame. In other embodiments the modules of the device are configured for surface mounting, which obviates the need to cut recesses. 
         [0029]    Although the embodiment described above was intended for monitoring the opening or closing of a door or window, the invention has other applications including at least the following:
       The passive module can be mounted to a moveable item of value, such as a personal computer or the like to detect unauthorised removal of the item. The active module can be mounted in a counter top or the like. If the item of value is moved from its position by more than a predetermined distance then this causes an alarm condition to be indicated by the active module. This implementation may be useful in retail environments.   Embodiments of the invention can be used to detect opening of pool safety gates or balcony windows by children to assist in accident prevention.       
 
         [0032]    It can be seen that embodiments of the invention have at least one of the following advantages:
       Device cannot be defeated with a single permanent magnet as a complex and unknown magnetic field pattern is required to avoid an alarm condition.   Any combination of magnets can be randomly used when the device is programmed at installation time. Therefore, no record need be kept of the configuration of the device.   A device with a set threshold which will compensate for some limited movement of the protected object   Should object misalignment occur out of the programmed range the device can be simply reprogrammed, saving costly door realignment       
 
         [0037]    Any reference to prior art contained herein is not to be taken as an admission that the information is common general knowledge, unless otherwise indicated. 
         [0038]    Finally, it is to be appreciated that various alterations or additions may be made to the parts previously described without departing from the spirit or ambit of the present invention.