APPARATUS FOR DETECTING FERROMAGNETIC OBJECTS AT A PROTECTED DOORWAY ASSEMBLY

Abstract

An apparatus for protecting an entrance to a protected area comprises a magnetic sensor to measure an ambient magnetic field or gradient within a first zone of sensitivity at a non-magnetic sensor means adapted to detect the presence of objects within a primary detection zone, a signal processing circuit arranged in communication with the magnetic sensor apparatus and nonmagnetic sensor apparatus, and a warning device operable by an output signal from the signal processing circuit, the warning device adapted to provide an alarm. The signal processing circuit identifies temporal variations due to the movement of a ferromagnetic object within the ambient magnetic field and correlates them with instances in which the non-magnetic sensor means detects the presence of an object in its detection zone, and causes the alarm to operate in the event that the correlation is indicative of the presence of a ferromagnetic object in the primary detection zone. It also determines the direction from which an object is approaching the using signals from the non-magnetic sensor means and modifies the operation of the warning device dependent on the direction.

Claims

1. Apparatus for protecting an entrance to a protected area, the apparatus comprising: a magnetic sensor apparatus adapted to measure an ambient magnetic field or gradient within a localised volume of space defined by a first zone of sensitivity of at least one magnetic sensor, and to produce a corresponding measurement signal, a non-magnetic sensor means adapted to detect the presence of objects within a primary detection zone in the vicinity of the magnetic sensor apparatus, a signal processing circuit arranged in communication with the magnetic sensor apparatus and non-magnetic sensor means, and a warning device operable by an output signal from the signal processing circuit, the warning device adapted to provide within the vicinity of primary sensor apparatus at least one of an audible alarm and a visible alarm, and further in which: the signal processing circuit is configured to identify temporal variations in the measurement signal due to the movement of a ferromagnetic object within the ambient magnetic field and to correlate the identified temporal variations in the measurement signal with instances in which the non-magnetic sensor means detects the presence of an object in its detection zone, and in which the signal processing circuit is arranged to cause the alarm to operate in the event that the correlation is indicative of the presence of a ferromagnetic object in the primary detection zone, and further in which the signal processing circuit is adapted to determine the direction from which an object is approaching the doorway from the output signals from the non-magnetic sensor means and to modify the operation of the warning device dependent on the direction of approach.

2. Apparatus according to claim 1 in which the non-magnetic sensor means comprises a first non-magnetic sensor adapted to detect the presence of objects within a primary detection zone and a second non-magnetic sensor adapted to detect the presence of objects within a second detection zone.

3. Apparatus according to claim 2 in which the non-magnetic sensor means is arranged so that an object approaching the door from within the protected area enters the second zone of detection before the first, primary, detection zone.

4. Apparatus according to claim 2 in which the second non-magnetic sensor in a position of use is arranged to detect an object approaching the doorway from within the protected area prior to the object being detected by the first non-magnetic sensor so that the alarm is suppressed whenever an object approaches the doorway from within the protected zone.

5. Apparatus according to claim 2 in which the second non-magnetic sensor has a zone of detection that is at least partially different to that of the first non-magnetic sensor and the signal processing circuit in use is adapted to determine that the object has approached from one direction if the object has entered the second detection zone before it enters the first detection zone and has approached from a different direction if it enters the first zone of detection before the second zone.

6. Apparatus according to claim 1 which is adapted to suppress the alarm for a defined period of time sufficient to allow a person to move through the doorway entering and then leaving the detection zone of the first sensor apparatus.

7. Apparatus for protecting an entrance to a protected area, the apparatus comprising: at least one magnetic sensor adapted to measure an ambient magnetic field or gradient within a localised volume of space defined by a first zone of sensitivity of the at least one magnetic sensor, and to produce a corresponding measurement signal, a non-magnetic sensor means adapted to detect the presence of objects within a primary detection zone in the vicinity of the at least one magnetic sensor, an RF receiver which is adapted to receive an identity signal from a transducer when the transducer is within a detection zone, a memory which stores one or more identity codes corresponding to respective identities of transducers, a signal processing circuit arranged in communication with the at least one magnetic sensor, the non-magnetic sensor means, and the RF receiver, and a warning device operable by an output signal from the signal processing circuit, the warning device adapted to provide within the vicinity of primary sensor apparatus at least one of an audible and a visible warning, and further in which: the signal processing circuit is configured to identify temporal variations in the measurement signal due to the movement of a ferromagnetic object within the ambient magnetic field and to correlate the identified temporal variations in the measurement signal with instances in which the non-magnetic sensor means detects the presence of an object in its detection zone, and in which the signal processing circuit is arranged to cause the alarm to operate in the event that the correlation is indicative of the presence of a ferromagnetic object in the primary detection zone but to modify the operation of the alarm in the additional event that the RF receiver detects a transducer which has an identity that is stored in the memory.

8. Apparatus according to claim 7 which is adapted to suppress the alarm for a defined period of time sufficient to allow a person to move through the doorway or entering and then leaving the detection zone of the first sensor apparatus, in which the time of suppression is varied according to the identity of the RF tag that is detected.

9. Apparatus according to claim 7 in which the RF receiver comprises an RFID tag reader, and the transducer radio frequency identify tags (RFID tags).

10. Apparatus according to claim 9 in which the modification comprises suppressing the alarm if the presence of an RFID tag of a known identity is detected by the RFID receiver.

11. Apparatus according to claim 9 in which the modification comprises modifying the threshold at which an alarm is triggered as a function of the magnitude of the temporal variations in magnetic field detected by the magnetic sensor when a known RFID tag is detected.

12. Apparatus according to claim 11 in which the memory stores alongside each identity in the memory a value indicative of a ferromagnetic profile associated with an RFID tag, the profile being used by the apparatus to set the threshold level of change of magnetic field which may trigger an alarm.

13. Apparatus according to claim 9 which includes at least one RFID tag which can be allocated to person who is to pass through the entrance, or can be stuck to, embedded into or otherwise attached to an object that might move through the protected entrance.

14. Apparatus for protecting an entrance to a protected area comprising: at least one magnetic sensor adapted to measure an ambient magnetic field or gradient within a localised volume of space defined by a first zone of sensitivity of the at least one magnetic sensor, and to produce a corresponding measurement signal, a non-magnetic sensor means adapted to detect the presence of objects within a detection zone in the vicinity of the at least one magnetic sensor, a signal processing circuit arranged in communication with the at least one magnetic sensor and the non-magnetic sensor means, and a warning device operable by an output signal from the signal processing circuit, the warning device adapted to provide within the vicinity of the at least one magnetic sensor at least one of an audible and a visible warning, wherein the signal processing circuit is configured to identify temporal variations in the measurement signal due to the movement of a ferromagnetic object within the ambient magnetic field and to correlate the identified temporal variations in the measurement signal with instances in which the non-magnetic sensor means detects the presence of an object in the zones, and to cause the warning device to raise an alarm in the event that the correlation is indicative of the presence of a ferromagnetic object in the vicinity of the at least one magnetic sensor; and further comprising: a door position sensing means which is adapted to monitor the angular position of a door of the entrance, and in which the signal processing circuit is adapted to modify the operation of the alarm in the event that output of the door position sensing means indicates that the door is moving.

15. Apparatus according to claim 14 which is arranged to modify the operation of the alarm by suppressing the alarm if the door is moving, or if the door is moving and one or more conditions are met.

16. Apparatus according to claim 14 which is arranged to modify the operation of the alarm by altering the threshold at which an alarm is raised according to the position at which the door is located and/or the rate at which the door is opening or closing.

17. Apparatus according to claim 14 that includes a memory which stores historical data information about the change in ambient magnetic field in the zone of sensitivity of the magnetic sensor, and in the event that the output of the door position sensing means indicates that the door is moving at the same time that the non-magnetic sensor indicates the presence of an object in the first detection zone, the apparatus is arranged to cause an alarm to be raised if the stored historical data indicates that the ambient magnetic field was changing prior to the start of the movement of the door.

18. Apparatus according to claim 17 in which the signal processing circuit is adapted to correlate the stored historical data with baseline data indicative of the variation in the output of the magnetic sensor due to movement of the door alone and information indicative of the position of the door to remove from the data the effect of the movement of the door, and in the event that the correlated data indicates the presence of a moving magnetic object in the detection zone of the magnetic sensor to raise the alarm.

19. Apparatus according to claim 18 in which the baseline data comprise actual data obtained during installation of the apparatus as the door is opened and closed and the output of the magnetic sensor is sampled.

20. Apparatus according to claim 14 in which the memory in use stores historical data from a period of at least one second, or at least 5 seconds, before the door opening.

21. Apparatus according to claim 20 in which the memory includes at most 5 seconds, or at most 10 seconds, or at most 30 seconds of historical data.

22. Apparatus according to claim 14 in which the door position sensing means comprises a rotary encoder that connects the door and the door frame or infra-red or ultrasonic distance measurement sensor, linear encoder, potentiometer sensor, strain gauge, camera, pneumatic bladder with pressure sensor.

23. Apparatus for protecting an entrance to a protected area comprising: at least one magnetic sensor adapted to measure an ambient magnetic field or gradient within a localised volume of space defined by a first zone of sensitivity of the at least one magnetic sensor, and to produce a corresponding measurement signal, a non-magnetic sensing means adapted to detect the presence of objects within a detection zone in the vicinity of the at least one magnetic sensor, a signal processing circuit arranged in communication with the at least one magnetic sensor and the non-magnetic sensor means, and a warning device operable by an output signal from the signal processing circuit, the warning device adapted to provide within the vicinity of the at least one magnetic sensor at least one of an audible and a visible warning, wherein the signal processing circuit is configured to identify temporal variations in the measurement signal due to the movement of a ferromagnetic object within the ambient magnetic field and to correlate the identified temporal variations in the measurement signal with instances in which the non-magnetic sensor means detects the presence of an object in the zones, and to cause the warning device to raise an alarm in the event that the correlation is indicative of the presence of a ferromagnetic object in the vicinity of the at least one magnetic sensor; and further comprising: a door position sensing means which is adapted to monitor the angular position of the doorway, and and further comprising a memory which in use stores historical temporal variations in the measurement signal due to movement of a ferromagnetic object within the ambient field, and correlation means which is adapted to correlate the stored historical temporal variations in the measurement signal prior to the door moving with incidences in which the non-magnetic sensing means detects the presence of an object in its detection zone, and in which the apparatus is arranged to cause the warning device to raise an alarm in the event that the correlation, albeit separated in time, is indicative of the presence of a magnetic object in the vicinity of the at least one magnetic sensor.

24. Apparatus according to claim 10 in which modification to the alarm comprises suppressing the alarm completely, or partially.

25. Apparatus according to claim 24 in which the warning device is adapted to produce both visual and audible alarms and the modification comprises suppressing the audible but not the visible alarm.

Description

[0076] There will now be described by way of example only, several embodiments.

[0077] FIG. 1 is a schematic view of a first embodiment of the present invention which includes a second light beam that detects the approach of objects from inside of a room;

[0078] FIG. 2 is a schematic view of a second embodiment of the present invention which includes an RF id tag reader and at least one RF id tag attached to a person or object approaching a protected doorway;

[0079] FIG. 3 is a schematic view of a third embodiment of the present invention which includes a door position sensor and which suppresses an alarm if the door is moving;

[0080] FIG. 4 is a general view of a doorway of a protected MRI room showing a possible location for installing apparatus of the invention;

[0081] FIG. 5 is a view from above showing the location of the sensors and a stylised representation of the associated zones of detection for the embodiment of FIG. 1;

[0082] FIG. 6 is a view from above showing the location of the sensors and a stylised representation of the associated zones of detection for the embodiment of FIG. 2;

[0083] FIG. 7 is a view from above showing the location of the sensors and a stylised representation of the associated zones of detection for the embodiment of FIG. 3;

[0084] FIG. 8 is an illustration of the logic used within the signal processing circuit of the first embodiment of FIG. 1;

[0085] FIG. 9 is an illustration of the logic used within the signal processing circuit of the second embodiment of FIG. 2;

[0086] FIG. 10 is an illustration of the logic used within the signal processing circuit of the third embodiment of FIG. 3;

[0087] FIG. 11 is an illustration of the logic used within the signal processing circuit of a modified form of the third embodiment of FIG. 3; and

[0088] FIG. 12 is a set of plots of signal against time for the different sensors of the apparatus of FIG. 2 showing how historic signal information is used in the event that the door is moving to determine whether an object is approaching the protected area.

[0089] Referring to FIG. 1, a first embodiment of a protection apparatus for use in protecting an entrance to a protected area is shown. For the purpose of this example, the protected area is an MRI room, typically a shielded space in which a magnetic resonance imaging machine (MRI machine) is located. These rooms are often found in larger hospitals or universities. The entrance comprises a doorway which is fitted with a magnetically shielded, side hung, door in a doorframe. The apparatus protects the doorway of the room by raising an alarm, in appropriate circumstances, if it detects that a ferromagnetic object is passing or about to pass through the doorway.

[0090] The apparatus includes a primary magnetic sensing apparatus comprising a magnetic sensor 4, such as a fluxgate sensor, a magneto-resistive sensor, a magneto-impedance sensor, a Hall Effect sensor, or a galvanic coil sensor, that outputs a signal that is a measurement of the magnetic field incident upon the sensor 4. Sensor 4 may be an arrangement of more than one sub-sensor such as a balanced pair or an array of magnetometers. Since the apparatus will typically be fixed in position proximate a doorway that is being protected, for most of the time the sensor will register a largely unchanging magnetic field due to the earth or where the door is an entryway to and MRI machine, the combination of the earth's magnetic field with the fringe field of the MRI's magnet. This constitutes a large offset on the output of the sensor. This constant offset can be removed using a high pass filter. The sensor will also likely measure regular changes in the magnetic field associated with the power supply for electrical equipment located near the doorway which will cause the output to vary at the supply frequency and its harmonics. This can also be filtered out using a low pass filter. The filters collectively constitute a band-pass filter 6 to perform these functions.

[0091] It is known that it takes between 0.3 seconds and 3 seconds, typically, for a person to pass through a doorway. The reciprocal of these times defines the frequencies of interest in the output of the sensor 4, i.e. 0.3 to 3 Hz.

[0092] If a ferromagnetic object carried, or pulled or pushed, by a person passes close to the sensor 4, the ambient magnetic field will be altered causing a change in the output of the sensor 4. That change will pass through the filter 6 and be amplified by an amplifier 8. In order to trigger an alarm the signal size is compared with a preset threshold.

[0093] Because the signal may be positive or negative, the threshold detector consists of a rectification stage 10 followed by a comparator 12 that has a circuit 14 to provide a threshold voltage. Alternatively, separate comparators are used for positive and negative signals with the outputs combined to give a single alarm signal instead of a rectifier 10 and a single comparator 12. An optional latch 12a may be provided which holds the value of the signal output from the comparator for a predetermined period—perhaps up to 1 second. The output of the latch is therefore a digital signal, with either a logic “zero” value or a logic “one” value. The presence of a logic “one” value at the output of the latch will, in some circumstances as outlined below, cause the alarm to be raised.

[0094] To raise the alarm the output of the latch 12a is fed into a signal processing circuit 16 whose output 18 is fed to one or more warning devices such as an audible alarm 20 and a visual alarm 22, and an optional external alarm 24. It has been found to be beneficial, although not essential, that both a visual and audible alarm is provided.

[0095] In addition, a visual indicator 11 of the magnitude of the magnetic signal may be included. For instance, a series of amber light “bars” may be provided which are illuminated sequentially as the output signal increases in strength up to the threshold, and once the threshold is reached a red light may be illuminated. The output of the comparator may be arranged to have logic level zero for the state where the signal does not exceed the threshold, and level ‘one’ for the state when the signal has exceeded the threshold. Once an object has passed out of range of the magnetic sensor 4 the logic level returns to zero once the signal level has dropped below the threshold. In practice, it may be preferable that the alarm continues for an elapsed time defined by a reset delay and a latch such as a flip-flop that maintains the output at logic zero until the button is pressed.

[0096] To reduce false alarms, the apparatus further includes a secondary, complimentary, non-magnetic sensing means 30 that senses when a person passes through the doorway or is about to pass through the doorway. The non-magnetic sensing means in this example comprises a non-magnetic sensor 30 having a distinct zone of protection. This zone should, of course, lie at least in part within the region over which the primary sensing apparatus is sensitive to ferrous objects. The extent of the zone is shown stylised form in FIG. 5 of the drawings for a typical installation of the apparatus to a door as shown in FIG. 4 of the drawings.

[0097] In the example of the first embodiment the non-magnetic sensor 30 comprises a photo-electric sensor (or any other device that is sensitive to incident light) which is arranged to detect when a person, or other object, passes through a beam of light directed at the sensor. The light beam may be visible or invisible. The beam of light is generated by a light source 31 such as an LED, associated with the photo detector. To protect the doorway it passes fully across a width of the doorway about 1 foot to 2 feet above the ground. The region of space through which the beam passes defines the zone of sensitivity for the sensor. If an object is in the path of the beam, it is in the zone, and will break the beam. Generally speaking a person or large object will break the beam as it tries to pass through the doorway. For better protection a number of beams may criss cross the doorway along different paths. The light sensor and light source are located within a rotatable turret 110 at the base of a main body 100 of the apparatus.

[0098] If the beam is unbroken, the output of the sensor 30 is a logical zero and if it is broken by movement of an object into the path of the beam, the output changes to a logical one level. Of course, the logic could be reversed in an alternative embodiment. The output of each sensor may be held for a period, say up to 1 second, using a latch (not shown).

[0099] The output of the sensor 30 is fed to one input of the signal processing circuit 16, which in a simple arrangement may comprises a digital logic circuit built using discrete digital logical elements. In its simplest form as shown in FIG. 8 this will comprise an AND gate 16a with the output of the sensor 30 fed to one input of the AND gate and the output of the gate 16a fed to a digital latch 17. The other input of the AND gate 16a is fed with the output of the magnetic sensor comparator 12.

[0100] The signal processing circuit 16, in use, performs a correlation function between the magnetic and non-magnetic sensors. If there is a correlation showing the beam is broken and a large change in ambient magnetic field is present, the circuit will typically output a signal which is passed to the alarm circuit (as shown the direct output from the latch is connected to the alarm circuit).

[0101] In addition the embodiment of FIG. 1 the non-magnetic sensor means includes a second non-magnetic sensor 32. This comprises a photo-electric sensor (or any other device that is sensitive to incident light) which is arranged to detect when a person, or other object, passes through a beam of light directed at the sensor in a similar manner to the first non-magnetic sensor apparatus. Again the light beam may be visible or invisible. The beam of light is generated by a light source 33 such as an LED, associated with the photo detector 32. Importantly for this embodiment, the second beam extends across a region close to the inside of the room that is being protected as shown in FIG. 5. The significance of this is that the order in which the beams are broken allows the signal processing circuit to determine the direction in which an object is moving.

[0102] The beam of the second photo-electric sensor is arranged to be broken by an object that is approaching the doorway from a position inside the MRI room, the protected area. before the beam of the first non-magnetic sensor apparatus could be broken. The output of the second sensor is fed to the digital circuit 16 and in the event that it indicates that an object has broken the second beam before the first beam has been broken the digital logic circuit causes a suppression of the alarm signal 18. This signal is combined with the output of the latch using a logical AND 16b gate so that as long at the suppression signal is at the logic “one” value the output of the AND gate will be low and the alarm suppressed by holding the signal 18 at a low value.

[0103] The digital circuit includes a timer circuit which produces a time delay, holding the suppression signal at a high logic level for a preset time of, say, 1-3 seconds. This gives enough time for an object to pass through the doorway with the alarm suppressed.

[0104] Notably, if an object approaches the doorway from outside of the room the first beam will always be broken before the second, and so the suppression of the alarm will not occur. The apparatus of the first embodiment therefore provides suppression of the alarm for objects leaving the protected room, whilst maintaining full alarm for objects entering the room.

[0105] In a modified example, not shown, the signal processing circuit does not rely upon a timer to produce a set time delay for the suppression of the alarm. Instead, the alarm is suppressed until such time as the object has broken the first beam and subsequently the first beam has been remade. This gives an indication that the object has cleared the doorway.

[0106] It will be appreciated that different sensors could be provided. Instead of optical sensors the embodiment could be implemented using ultrasonic or radar sensors. These may be arranged to provide both position information, detecting objects in defined zones, or movement information, allowing movement of objects in the field of view to be detected. A single transmitter and emitter could be used to provide both position and movement information to the signal processing apparatus. Typically sensors that can detect movement exploit the Doppler effect, and can discriminate between objects approaching or moving away from the sensor.

[0107] A second embodiment of the present invention is illustrated in FIG. 2 of the drawings. This is generally similar to the first embodiment, and for clarity the same reference numerals have been used to denote like parts of the apparatus. In fact, the apparatus includes all of the parts of the first embodiment, but not the second beam that enables the alarm to be suppressed for objects leaving an MRI room.

[0108] As an alternative to the second beam, a different type of additional sensor 34 is provided that takes the form of an RF identification tag reader 34, a memory 50 in which identities of known RFID tags associated with the apparatus may be stored, and some modifications to the logical operation of the signal processing circuit as shown in FIG. 10. The apparatus is intended to be used along with a number of RFID tags 40, and these may be supplied along with the apparatus. FIG. 6 shows possible locations of the sensors around a protected doorway along with stylised representations of the zones of detection of the sensors.

[0109] RFID readers are well known in the art and as such will not be described in detail here. Suffice that the reader is arranged to provide as an output a digital signal providing an identity of any RFID tag that is detected within a zone of detection. For optimal operation this zone should extend across a region that is further away from the doorway that the first beam, and at least overlaps with the zone of sensitivity of the magnetic sensor. Ideally, the range of detection of the reader should be quite low, perhaps only a few tens of cms around the doorway, so as to only read tags that are close to the doorway.

[0110] Before the apparatus is used, the memory is loaded with the identities of the tags that are supplied with the system. For this example, there are two RFID tags and the memory stores the two Identities of the tags. These may be loaded into the memory by placing the apparatus into a learning mode, and moving each tag in turn into range of the reader.

[0111] In use, upon an object entering the range of the RFID tag reader, the identity of the tag will be output from the reader and fed to the signal processing circuit. The circuit compares the tag with the identities in the memory. If there is a match a flag is raised (moved from logic low to logic high value) to indicate that one of the known tags is near to the doorway. The alarm is then raised or suppressed according to the correlation of the output signals from the magnetic sensor and the photodetector in the same manner described for the first embodiment. The alarm signal is then fed to an AND gate along with the inverted value of the flag so that the alarm is suppressed if a known RFID tag has been detected.

[0112] The applicant believes that this system may help remove unhelpful alarms caused by operators passing through the doorway with ferromagnetic objects but who can be trusted to only have done so if it is safe to do so. For instance, an operator who has been trained in when it is safe to move objects, and so can be assumed to not be taking any unsafe objects into a room, may carry an RFID tag and so never trigger the alarm. This might be desirable where the operators might be wearing underwired bras which have become magnetised, whereas otherwise they will either always trigger an alarm or be required to change their clothing.

[0113] The memory 40 may, optionally, also store a value alongside each identity in the memory which determines a sensitivity level at which the latch will be triggered. For instance, a more trusted user may be provided with a tag which is associated with a higher threshold and a less experienced user given a tag associated with a lower threshold. Then, when a tag is identified by the circuit the level at which an alarm will be triggered can be set according to the corresponding value stored alongside it in the memory.

[0114] This later feature in particular is envisaged as useful where a tag is secured to a ferromagnetic object such as a trolley. The object will have a ferromagnetic profile as seen by the magnetic sensor when it is moved into the detection range of the magnetic sensor. In particular the profile dictates the peak change in magnetic field that is expected to be observed by the magnetic sensor due to the presence of the object. This can be used to set the threshold at which the latch triggers, so that the object alone cannot set off the alarm but the presence of any magnetic object in addition to the object will take cause the threshold to be exceeded and so an alarm may be raised.

[0115] A still further embodiment is illustrated in FIG. 3. Again those parts which are the same as the first and second embodiments have been represented by like reference numerals. The apparatus includes a magnetic sensor 4 and a first photodetector 30 that detects a break of a light beam that extends across the doorway of a room being protected.

[0116] In addition, a door position sensor 36 is provided in this embodiment which produces an output signal indicative of the angular position of the door. The location of the door sensor is shown in FIG. 7 of the drawings. The output of this sensor 36 is fed into a signal processing circuit, such as that shown in FIG. 10, which processes the signal to determine the position of the door at a given time and also whether the door is moving or stationary. In this example it does so by comparing the position at the given moment in time with the position shortly prior to this. Feeding the door position signal into a first in last out (FILO) buffer in memory provides enough information to achieve this.

[0117] The signal processing circuit generates a door moving signal with a logical high value if the door is moving and logical low value if it is still. This is then combined with the output of the latch using a logical AND gate to provide a signal that is fed to the alarm device. The alarm is therefore raised if the latch output is high, indicating that a ferromagnetic object has broken the beam of the photoelectric sensor, but is suppressed if the door is moving.

[0118] Suppressing the alarm if the door is moving allows the apparatus to be used with ferromagnetic doors which might otherwise cause false alarms as they move.

[0119] In a modification, shown in FIG. 11, the apparatus includes an additional memory which stores historical ambient magnetic field information from the magnetic sensor. The signal processing circuit, shown schematically in FIG. 11, upon the door moving signal going high to indicate that the door has started to move, stops using the output of the magnetic field sensor for the current time as the basis for detecting the presence of a ferromagnetic object moving in the zone of sensitivity and instead looks at historical values stored in the memory prior to the door moving signal going high. If this provides an indication that a ferromagnetic object was moving in the zone of sensitivity just before the door moved, the circuit will then override the logical high value of the door moving signal so that the alarm is no longer suppressed, the alarm instead being raised.

[0120] By looking at the historical data, the alarm can remain active but effect of the moving door on the operation of the alarm is removed because the apparatus relies instead on the information obtained prior to the door moving.

[0121] FIG. 12 illustrates the signals from the magnetic sensor, the optical beam, the door position sensor and the RF receiver when a ferromagnetic object approaches a door that then starts to move, as would be the case where a person carrying the object approaches the door and then pushes it open.

[0122] As can be seen, when the signal processing circuit detects that the door is moving it looks back in time at the magnetic signal before the door started to move. If this indicates that a ferromagnetic object was moving in the detection zone of the magnetic sensor as shown the alarm condition is raised. The RFID tag condition is then checked to see if the alarm should be suppressed. If there is a known RFID tag detected it may suppress the alarm or modify it according to information held about the RFID tag.

[0123] The skilled person will understand that any of the features of these embodiments can be introduced into any other embodiment. For instance, the door moving suppression can be implemented in conjunction with the second optical beam to suppress the alarm as an object leaves a room.

[0124] It should also be understood that the simplified discrete logic circuits that have been illustrated are not to be construed as limiting. An almost limitless set of possible digital circuits could be readily implemented that achieve the same overall logical function, using combination of OR, AND, NOR, NAND and XOR gates, either as discrete elements or as logical steps in a computer implemented program executed on digital processing device.