Abstract:
A sensor system is provided for use in detecting movement of an item, e.g. a picture  16 , suspended from a wall or other substantially vertical structure, or for detecting movement of an item displayed or stored on a horizontal or inclined surface, the sensor system including a piezoceramic element ( 22 ) which is mounted on a metallic disc ( 23 ) which, in use, is located between the item ( 16 ) and the structure or surface, and means for detecting the generation of an output by the piezoceramic element ( 22 ) as a result of deflection of the disc ( 23 ) in either of two opposite directions as a result of movement of the item ( 16 ).

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to sensor systems and to security alarms which incorporate sensor systems.  
           [0002]    In British Patent Specification No. 2 306 035, to which reference should be made, there is described a support system for a shelf or similar surface on which one or more articles can be placed, the system including one or more piezoelectric devices for partially or wholly supporting the weight of the shelf or similar surface, and means whereby an output voltage will be generated proportional to the change of weight when objects are placed on or taken from the shelf or similar surface.  
           [0003]    It is an object of the present invention to provide a sensor system which can be used to detect the attempted theft or damage of items displayed or stored by hanging them on walls, partitions or similar substantially vertical structures, as opposed to items which are displayed by placing them on shelves or similar substantially horizontal structures.  
           [0004]    It is another object of the present invention to provide an improved sensor system which can be used to detect the attempted theft of, or tampering with, small and lightweight items displayed or stored on horizontal or inclined surfaces.  
           [0005]    It is a further object of the present invention to provide a sensor system which includes improved means for filtering out the effects of environmental disturbances, such as vibration and wind-induced movement.  
         SUMMARY OF THE INVENTION  
         [0006]    According to a first aspect of the present invention there is provided a sensor system for use in detecting movement of an item suspended from a wall or other substantially vertical structure, or for detecting movement of an item displayed or stored on a horizontal or inclined surface, the sensor system including a piezoceramic element which is mounted on a plate which, in use, is located between the item and the structure or surface, and means for detecting the generation of an output by the piezoceramic element as a result of deflection of the plate in either of two opposite directions as a result of movement of the item.  
           [0007]    The plate may have a first edge portion which is attached to a mounting and is bent out of the plane of the remainder of the plate, the plate being positioned so that a second edge portion, diametrically opposite to the first edge portion, is in contact with the item, for example, adjacent the lower edge of an item in the case of an item suspended from a wall or other substantially vertical structure.  
           [0008]    The arrangement is thus such that, in use, the weight of a painting or other item suspended from the wall or other substantially vertical structure will create a small degree of deflection of the plate. Pressing the painting or other item towards the structure will alter the degree of deflection and thus generate an output, while moving the painting or other item away from the structure will also alter the degree of deflection and generate an output.  
           [0009]    The same effect will be obtained if an attempt is made to move an item displayed or stored on a horizontal or inclined surface.  
           [0010]    The plate may alternatively be in the form of a brass or other metal disc which is attached at its one edge to a mounting and has its diametrically opposite edge in engagement with a pad fixed to or in contact with the item, with the arrangement such that the disc is caused to deflect and an output is generated by the piezoceramic element in response to movement of the item in either direction.  
           [0011]    In a further alternative arrangement, the disc is mounted on a resiliently deformable metal strip or plate, for example, a spring steel strip, such that a part of the strip, typically an end of the strip, is in contact with the item, with the arrangement such that the strip is caused to deflect (and will transmit such deflection to the disc) in response to movement of the item in either direction and an output will be generated by the piezoceramic element.  
           [0012]    According to a second aspect of the present invention there is provided a sensor system for detecting movement of an item suspended from a wall or other substantially vertical structure, the sensor system including a piezoceramic element fixed to one side of a plate and a permanent magnet fixed to the other side of the plate.  
           [0013]    The plate is preferably in the form of a brass or other metal disc that is mounted within a housing in such manner that the periphery, or a portion of the periphery, of the disc is clamped while the centre portion of the disc is free to move.  
           [0014]    The piezoceramic element is preferably in a pre-stressed state when the sensor system is fitted to the item being protected.  
           [0015]    According to a third aspect of the present invention there is provided a security alarm system which includes a sensor system as defined above for generating an output, a processing circuit for processing the output from the sensor system, and an alarm, the processing circuit including a slew rate limited anti-vibration low pass filter for differentiating between true alarm and false alarm signals.  
           [0016]    Any suitable form of alarm may be employed. For example, it may be an audible siren, a radio or infra red transmitter, or a relay which changes the state of a switch in an external circuit.  
           [0017]    The slew rate limited anti-vibration low pass filter is preferably arranged to filter out signals which have a duration less than a predetermined length of time and is preferably arranged in series with a pulse stretcher to ensure that the length of time for which the alarm is operated is greater than the length of time for which an output is generated by the slew rate limited anti-vibration low pass filter.  
           [0018]    The arrangement may be such that, for example, signals having a duration of less than one second are filtered out by the slew rate limited anti-vibration low pass filter. Normal filters are amplitude dependent. The slew rate limited anti-vibration low pass filter is, however, not amplitude dependent.  
           [0019]    The alarm may form part of a unit which, in the case of a wall-hung item, is located between the item and the wall. Alternatively, the alarm may be at a remote location with a signal transmitted to the alarm by means of an induction loop device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a sectional view of a first form of sensor,  
         [0021]    [0021]FIG. 1A is a plan view of the sensor shown in FIG. 1,  
         [0022]    [0022]FIG. 2 shows the sensor of FIGS. 1 and 1A installed behind a painting,  
         [0023]    [0023]FIG. 3 is a front view of a second form of sensor,  
         [0024]    [0024]FIG. 3A is a side view of the sensor of FIG. 3,  
         [0025]    [0025]FIG. 4 shows the sensor of FIGS. 3 and 3A installed behind a painting,  
         [0026]    [0026]FIG. 5 shows an electronic circuit for use with either form of sensor,  
         [0027]    [0027]FIG. 6 is a sectional view of a sensor installed beneath an item displayed or stored on a horizontal supporting surface,  
         [0028]    [0028]FIG. 7 is an underneath plan view of the arrangement shown in FIG. 6, which is a section along the line A-A of FIG. 7,  
         [0029]    [0029]FIG. 8 is a sectional view of a sensor installed beneath an item stored or displayed on an inclined surface,  
         [0030]    [0030]FIG. 9 shows an alternative sensor mounting arrangement,  
         [0031]    [0031]FIGS. 10 a ),  b ) and  c ) show different conditions of a further form of sensor mounting arrangement,  
         [0032]    [0032]FIGS. 11 a ),  b ) and  c ) are views from behind a painting, above a painting and to one side of a painting showing an additional form of sensor mounting arrangement, and  
         [0033]    [0033]FIGS. 12 a ),  b ) and  c ) are further views from behind a painting, above a painting and to one side of a painting showing another form of sensor mounting arrangement. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    The sensor  10  shown in FIGS. 1 and 1A includes a piezoceramic element  11  fixed to one side of a disc  12  of brass or some other elastically deformable material. A permanent magnet  13  is fixed to the centre of the opposite side of the disc  12  and the disc  12  is fitted into a housing  14  that includes clamping elements (not shown) for clamping the edge of the disc  12 , leaving the centre of the disc  12  free to move. The sensor is connected to an electronic control circuit  15  (shown in detail in FIG. 5) by means of flexible leads.  
         [0035]    In use, as shown in FIG. 2, the sensor  10  is positioned between a picture frame  16  (or other hung item) and a wall  17  from which the picture frame  16  is suspended by means of a cord  18  which fits over a hook  19 . The permanent magnet  13  is arranged so that it attaches itself to a small ferrous-based item  20  fixed to or incorporated in the picture frame  16 . The item  20  can be a small screw, washer, staple, or length of wire.  
         [0036]    If pressure is applied to the picture frame  16  to press it against the wall  17 , such pressure will produce deflection of the piezoceramic element and hence generation of an output signal which will be received by the electronic control circuit  15 . If, on the other hand, an attempt is made to pull the picture frame  16  away from the wall  17 , this will cause the permanent magnet  13  to break contact with the ferrous item  20  on the frame  16  and create a sufficiently large deflection of the piezoceramic element  11  to induce an output voltage pulse to activate an alarm (as described below).  
         [0037]    The design of the sensor system is such that it uses the attraction between the permanent magnet  13  and the ferrous item  20  to amplify the change in the strain in the piezoceramic element  11  when the bond between the magnet  13  and the ferrous item  20  is broken, and hence produce a sufficiently large output signal for transmission to the electronic control circuit  15 .  
         [0038]    External disturbances, such as vibration and wind-induced movement, are taken up by the flexible leads, as well as being filtered out by the electronic control circuit  15  (as described below) to avoid or reduce false alarms.  
         [0039]    The sensor  21  shown in FIGS. 3 and 3A comprises a piezoceramic element  22  fixed to one side of a disc  23  of brass or some other elastically deformable material. One edge of the disc  23  is bent so that it is inclined to the main body portion of the disc  23 , as shown in FIG. 3A, and is connected to the electronic control circuit  15 .  
         [0040]    In use, as shown in FIG. 4, the edge of the disc  23  rests against the frame  16  or against the rear of the painting or other hanging item, the weight of which results in the application of sufficient force to the disc  23  to produce a small deflection of the disc  23 , as indicated in FIG. 4. Pressing the frame  16  or the painting against the wall  17  will increase the deflection and cause the sensor  21  to send a signal to the electronic control circuit  15  to generate an alarm signal. Equally, if the painting or the frame  16  is moved away from the wall  17 , the elasticity of the material of the disc  23  will return the sensor  21  to its undeflected condition and, in the process, will produce an output to initiate the alarm signal.  
         [0041]    The sensor system shown in FIGS. 3, 3A and  4  is thus based on pre-stressing the piezoceramic element  22  and it is the relief of this stress (or an increase in the stress) that generates a sufficiently large output signal to the electronic control circuit  15 . The effects of vibration and wind-induced movement are filtered out by the electronic control circuit  15  (as described below) to avoid false alarms.  
         [0042]    As shown in FIG. 5, the electronic control circuit  15  includes an amplifier  30  which receives the output signal from the sensor  10  or  21 . The amplified output is then applied, via either a positive direction comparator  31  or a negative direction comparator  32  to a slew rate limited anti-vibration low pass filter  33 . The filter  33  is connected, via a threshold comparator  34 , to a pulse stretcher  35  which is connected to an output  36  and thence to a relay  37  and an audible or other alarm  38 , which is operated for at least a specified minimum period of time.  
         [0043]    The slew rate limited anti-vibration low pass filter  33  is not amplitude-dependent (unlike the majority of filters) and serves to filter out all incoming signals which have a duration less than a predetermined length of time, which may be, for example, of the order of a second in length, but can be selected as required. The pulse stretcher  35  then serves to extend the length of time of the output pulse from the filter  33 . If, for example, the output from the sensor  10  or  21  had a duration of 1.5 seconds, the output from the filter  33  would have a duration of 0.5 seconds and this might be multiplied by a factor of twenty by the pulse stretcher  35  to produce an output having a duration of ten seconds. All short duration pulses from the sensor  10  or  21 , which might arise, for example, as a result of vibration of the wall  17  caused by a person knocking on a door, will thus be filtered out.  
         [0044]    The electronic control circuit shown in FIG. 5 can be used not only in connection with paintings and other items hung on a wall but also for the protection of items stored or displayed on horizontal or sloping surfaces, as shown in FIGS.  6  to  8 . Thus, FIGS. 6 and 7 show an arrangement in which an item  40  is positioned above a support surface  41  on which there is an electronic control circuit  15  which is arranged to receive inputs from any one of four sensors  42  each of which includes a flexible disc on which a piezoceramic element is mounted, such that movement of the item  40  out of its normal position, either towards or away from the support surface  41 , will produce deflection of the disc of at least one of the sensors  42  to generate an output and initiate operation of the alarm.  
         [0045]    [0045]FIG. 8 shows an arrangement in which an item  50  is resting on an inclined support surface  51  with its upper edge in contact with the support surface  51  and its lower edge spaced from the support surface  51 . An electronic control circuit  15  is provided beneath the item  50  and is arranged to receive an input from a sensor  52  which includes a brass or other metal disc on which a piezoceramic element is mounted, such that movement of the item  50  out of its normal position, either towards or away from the support surface  51 , will produce deflection of the disc of the sensor  52  to generate an output and initiate operation of the alarm.  
         [0046]    [0046]FIG. 9 shows a further arrangement in which the item  60  being protected rests against a pad  61 . The pad  61  is attached to one edge of a sensor  62  which includes a brass or other metal disc on which a piezoceramic element is mounted. The opposite edge of the sensor  62  is connected to a fixed mounting  63 . The arrangement is such that, in the normal position of the item  60 , the disc of the sensor  62  is in a deflected condition (as described above with reference to FIGS. 3, 3A and  4 ) such that movement of the item  60  either towards or away from the item will produce a change in the degree of deflection of the disc and hence the generation of an output to initiate operation of an alarm.  
         [0047]    [0047]FIGS. 10 a ),  b ) and  c ) show a further arrangement in which the item to be protected is normally in engagement with the free end or edge of a strip or plate  71  of spring steel or other resilient material on which a sensor  72  is mounted. The opposite end of the spring steel strip or plate  71  is clamped or otherwise fixed to a mounting  73 . The spring steel strip or plate  71  is normally in a deflected condition and the arrangement is such that, if the item is moved either towards or away from the mounting  73 , the degree of deflection of the spring steel strip or plate  71  will be changed and an output will be generated.  
         [0048]    [0048]FIGS. 11 a ),  b ) and  c ) show an arrangement similar to that of FIG. 4, except that two sensors  21  are employed, each sensor  21  including a disc  23  on which a piezoceramic element is mounted, with one edge of the disc  23  attached to the electronic control circuit  15  and a diametrically opposite edge of the disc  23  in contact with the rear of the painting  16 . As shown, each disc  23  is in a deflected condition such that, any movement of the painting  16 , whether towards or away from the wall on which the painting  16  is hung, will result in a change in the degree of deflection of the disc  23  and hence the generation of an output by the piezoceramic element to initiate operation of the alarm.  
         [0049]    Turning next to FIGS. 12 a ),  b ) and  c ), these show an arrangement similar to FIGS. 11 a ),  b ) and  c ), where the electronic control circuit  15  is suspended from a cord between the rear of the painting  16  and the wall. The two sensor discs  23  are arranged so that one edge of each disc  23  is fixed to the electronic control circuit  15  and the diametrically opposite edge is fitted with a pad  63  which rests against the wall on which the painting  16  is hung. Each disc  23  is again in a deflected condition such that, any movement of the painting  16 , whether towards or away from the wall on which the painting  16  is hung, will result in a change in the degree of deflection of the disc  23  and hence the generation of an output by the piezoceramic element to initiate operation of the alarm.  
         [0050]    The sensor systems described above can be used in conjunction with the control circuit described in British Patent Specification No. 2 306 035, and the electronic control circuit of FIG. 5 can be used in conjunction with the sensor system described in Patent Specification No. 2 306 035.  
         [0051]    The alarm may form part of the electronic control circuit  15 . Alternatively, the alarm may be at a remote location with a signal transmitted to the alarm by means of an induction loop device.