Technical Field

The present invention relates to a position monitoring device and particularly relates to a surface mounted position monitoring device intended for security applications.

Background to the Invention

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.

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.

There remains a need to provide improved position detection systems.

Summary of the Invention

In a first aspect the present invention provides a position monitoring device including: a first module including at least one magnetic field transducer; a second module which generates a magnetic field; the first and second modules are arranged to be mounted to fixed and moveable objects respectively; the device is operable in a monitoring mode wherein the output of the at least one magnetic field transducer is compared to configuration data which is representative of the output of the at least one magnetic field transducer when the first and second modules are aligned; if the output 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; the second module includes a fixture arrangement for fixing the second module to a surface; and releasing the fixture arrangement of the second module involves separation of a part from the second module which alters the magnetic field produced by the second module.

The device may be operable in a configuration mode wherein it stores the configuration data.

The first module may include two or more magnetic field transducers.

The at least one magnetic field transducer may be provided in the form of a hall effect sensor.

The second module may include two or more magnetic field generating devices.

The at least one magnetic field generating device may be provided in the form of a permanent magnet.

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.

The first module may include at least one anti tamper switch to detect disassembly or removal of the first module.

The second module may include a cover which must be removed in order to release the fixture arrangement.

The cover may include a region of ferrous material.

The cover may incorporate a permanent magnet which is affixed to the cover.

Brief Description of the Drawings

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is an exploded view of a prior art position monitoring device; and Figure 2 is an exploded view of an embodiment of a magnetic module intended for use in surface mount applications.

Detailed Description of the Preferred Embodiment

Referring to figure 1, a prior art 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.

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.

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.

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.

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.

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.

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.

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.

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 lOmm.

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.

In the event of a power failure the signal relay becomes deactivated.

The embodiment shown in figure 1 is intended for flush mounting into recesses cut into a door and a door frame. Referring to figure 2, a modified form of second module 130 is shown. This module has been developed for use in surface mount applications which obviates the need to cut recesses into a door or window frame.

Module 130 includes a main housing 131 and a cover 132. A number of permanent magnets 133 are arranged at the upper region of housing 131. In use the magnets may be obscured from view by a stainless steel cover plate. Housing 131 and cover 132 are formed from nickel. Cover 132 includes a lug which holds a permanent magnet 138. Module 133 is intended for use with a module similar to module 20 described above, which may or may not also be modified for surface mounting.

To install module 132 to a door or window or the like, a fixture arrangement in the form of screws 136 are inserted through mounting holes (not visible) provided in the rear wall of housing 131. These screws are then driven into the surface that the module is being attached to. Cover 132 is then installed onto housing 131 and is held in place with four screws which are inserted through mounting holes 137 and are received in corresponding threaded holes provided in the housing 131. A corresponding detection module is then installed in the frame of the door or window in close facing proximity to module 131 when the door or window is closed.

The combination of the detection module and the magnetic module 132 is then commissioned in the same way as described above in relation to figure 1. The detection module enters a configuration mode during which it waits to detect a stable magnetic field pattern produced by module 130. Importantly, the magnetic field produced by module 130 is influenced by both the magnets 133 as well as magnet 138. Thereafter, the detection module operates in a monitoring mode during which it compares the ongoing outputs of the hall effect sensors with the stored configuration data.

In the monitoring mode, if the outputs of the hall effect sensors match the stored output values then the signal relay of the detection module 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.

If the outputs of the hall effect sensors deviate from the stored configuration data by more than a pre-determined threshold, then the detection module signals an alarm condition by deactivating the signal relay and an optional buzzer may sound. The deactivation of the signal relay is detected by the building security system and appropriate responsive action can be initiated.

Should a malevolent individual wish to compromise the position sensor by releasing the magnetic module 130 then they must first remove cover 132 to gain access to the mounting screws 136. The act of removing the cover causes withdrawal of magnet 138 from the housing, thus altering the overall magnetic field produced by module 130. This change in magnetic field is detected by the detection module. Thus, removal of cover 132 to gain access to the mounting screws will cause an alarm condition and thus alert building security.

In some embodiments the cover 137 may itself be formed from a ferrous material which affects the overall magnetic field produced by the magnetic module.

In some embodiments the fixture arrangement may include a magnet which is affixed to the door or window behind the housing 131 and which is fixed to the door or window independently of the housing 131. In these embodiments the separation of the housing 131 away from the magnet portion will affect the overall magnetic field detected by the sensing module and give rise to an alarm condition.

In this specification the term magnetic field transducer is used to refer to devices which provide a variable output signal in response to being exposed to a magnetic field including hall effect sensors and magnetometers.

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.

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.