[0001] The invention relates to an alarm system capable of detecting when an article is
removed from a predetermined position.
[0002] In known alarm systems used to deter or detect shop-lifters for example, each article
on an open display has been interconnected in a loop by means of a wire coupled to
an alarm. Such a system is obtrusive and tends to detract from the display. Adding
or removing articles with such a display requires disconnecting the interconnected
wires in the loop which can become tedious.
[0003] An alternative known system makes use of pressure pads which are connected to an
alarm and removal of an article triggers this alarm. In such a system it is necessary
to have a pressure pad for each article on display if removal of one is to be detected
and thus changes in the number of articles displayed requires addition or removal
of one or more pads and changes in the system connections. Thus such a system does
not lend itself to frequent changes in the display.
[0004] It has also been proposed to use the capacitive effect of displayed articles to trigger
an alarm by a system which uses a metal sheet as a conductor connected to an alarm
system and each displayed article is placed on this sheet which overlies a shelf.
In such a system effectively it was necessary to measure the capacitance of the articles
between this electrode and earth and such a system has been shown to be unreliable
due to stray capacitance effects and such a system could instigate an alarm condition
merely by a customer being in the vicinity of the display which proves to be an embarrassment
to both the customer and the shop-keeper.
[0005] The present invention is concerned with a system which overcomes the above drawbacks
to provide a reliable, flexible and typically unobtrusive system which can cope with
a number of articles together if required.
[0006] According to the invention there is provided an alarm system for indicating the removal
of an article from a predetermined position comprising:
a) sensing circuit means including
(i) two electrical conductors spaced from but adjacent each other and for positioning
adjacent to the'predetermined position to be occupied by at least one article when
in use, and (ii) signal generating means responsive to the capacitance present between
these conductors to provide a first output condition indicative of substantially constant
capacitance value present between the electrodes due to the undisturbed presence of
said at least one article and to provide a second output condition indicative of a
change in capacitance value present due to the disturbance of said at least one article,
and
b) alarm actuator means responsive to the output of said sensing circuit means to
provide an alarm indication as a result of a change in the output condition of said
senning circuit means whenever a change in capacitance due to the removal of an article
occurs.
[0007] According to a further aspect of the invention there is provided a method of detecting
and indicating when an article has been removed from a predetermined position comprising:
providing two electrical conductors spaced from but adjacent each other and adjacent
the predetermined position to be occupied by at least one said article,
sensing the capacitance present between these conductors to provide a first output
condition indicative of-substantially constant capacitance value present between the
electrodes due to the undisturbed presence of said at least one article and to provide
a second output condition when a change in capacitance value due to the disturbance
of said at least one article occurs,
and actuating an alarm indicator whenever a change in the output condition occurs
as a result of the removal of said article.
[0008] According to a third aspect of the invention there is provided an alarm system for
indicating the removal of an article from a predetermined position comprising:
sensing circuit means including
(i) two electrical conductors spaced from but adjacent each other and for positioning
adjacent to the predetermined position to be occupied by at least one article when
in use, and
(ii) signal generating means responsive to the capacitance present between these conductors
to provide a first output condition indicative of substantially constant capacitance
value present between the electrodes due to the undisturbed presence of said at least
one article and to provide a second output condition indicative of the rate of change
in capacitance value present due to the disturbance of said at least one article,
and
alarm actuator means responsive to the output of said sensing circuit means to provide
an alarm indication as a result of a change in the output condition of said sensing
circuit means whenever a rapid change in capacitance due to the removal of an article
occurs.
[0009] According to a fourth aspect of the invention .there is provided a method of detecting
and indicating when an article has been removed from a predetermined position comprising:
providing two electrical conductors spaced from but adjacent each other and adjacent
the predetermined position to be occupied by at least one said article,
sensing the rate of change in capacitance present between these conductors to provide
a first output condition indicative of substantially constant capacitance value present
between the electrodes due to the undisturbed presence of said at least one article
and providing a second output condition when a rapid change in capacitance value due
to the disturbance of said at least one article occurs,
and actuating an alarm indicator whenever a change in the output condition occurs
as a result of the removal of said article.
[0010] The invention will now be described by way of example with reference to the accompanying
drawings, in which:
FIGURE 1 shows a block diagram of the system of the invention,
FIGURE 2 shows one configuration for arranging the electrodes adjacent to an object
to be protected by the alarm,
FIGURE 3 shows one embodiment for realising the system of Figure 1,
FIGURE 4 shows an alternative arrangement to Figure 1 which detects rate of change
in capacitance,
FIGURE 5 shows an embodiment for realising the system of Figure 4, and
FIGURE 6 shows in more detail a suitable configuration for providing the elements
of the Figure 5 arrangement.
[0011] In the basic system of Figure 1, the item or items to be displayed will be positioned
such that they will overlie the electrodes 10, 11 which will typically be provided
on top of a display stand but not necessarily in the straight configuration indicated.
The effective capacitance of the items so placed is indicated by capacitor c. The
electrodes 10 and 11 are connected to the sensor 12 which produces en output level
indicative of the change in the capacitance c. Any object removed from across the
electrodes 10 and 11 will cause a rapid change in capacitance and thus the sensor
12 will detect this change and the output thereof will rise. The output of sensor
12 is received by alarm block 13 which compares the sensor level with a predetermined
threshold and effects an alarm condition when the threshold is exceeded.
[0012] The way in which the electrodes can be arranged is shown in Figure 2.
[0013] The shelf 1 is provided on its upper face with two metallic electrode strips 10,
11 which are arranged typically in an interleaved pattern so that, irrespective of
the position of placing an article 15 on the shelf, the base of the article will overlie
the two electrodes 10 and 11. The article is assumed to be metallic, or to incorporate
metal at or near its base, or to carry a sticker or tag 5 which is made of or incorporates
metal.
[0014] The strips are advantageously self-adhesive, and may be for example i inch apart,
and one-eighth to three- eighths of an inch in width. Thin wire could also be used.
The sticker could be a small disc of silver paper, for use in the case of glass, china
etc. When the disc is placed over the strips, there is an increase of capacitative
bridging of the strips 10 and 11.
[0015] The two electrode strips in this example are connected by the inner and outer portions
of a coaxial cable 4 to the sensor arrangement.
[0016] Although the electrodes 10 and 11 are shown on the upper surface of the shelf in
contact with the article 15 (or metallic sticker 5), it would also be possible to
disguise the electrodes by covering these with a thin dry material (e.g. cloth), and
placing the article on the cloth. Alternatively if the shelf were thin it may be possible
to provide the electrodes on its lower surface, although in these latter configurations
the sensitivity of the system would be somewhat reduced.
[0017] An arrangement for realising the system of Figure 1 is shown in Figure 3. The electrodes
10 and 11 are shown interleaved by way of example and suitable positions for a number
of articles 15 - 18 are shown thereon. The electrodes are connected via cable 4 to
points A and Bo The sensor 12 consists of a high impedance oscillator employing a
field effect transistor (FET) TR1 (e.g. 2N3819). Components associated with the oscillator
include inductance T
1, capacitors C
1, C
2, C
3 and C
5 and resistors R
1 and R
2. The circuit is arranged to go into, and out of, oscillation by the change of capacity
occurring between the strips.
[0018] The setting of the oscillator is controlled by the variable capacitor C1 in the gate
circuit of the FET, and which is affected by the loading of the shelf; when the shelf
is loaded with articles, the capacitor C
1 is adjusted so that the oscillator is just not oscillating, When an article is removed
from the shelf, the capacity at the shelf decreases, and allows the oscillator to
start working.
[0019] The output of the oscillator is typically selected to be at R.F. frequency, e.g.
450-470 Kcs., and this output is passed via capacitor C
4 to the amplifier comprised of transistor TR2 with associated resistors R
3 and R
4. The output from this transistor passes via capacitor C
6 and is rectified by the diode Dl, and the DC content is fed to the driving circuit
comprising power transistor TR3 and associated resistor R
5 and capacitor C
7 which circuit actuates the relay circuit (RL
1 and C
8) when this DC level exceeds a predetermined threshold. The contacts of the relay
are used to switch on any convenient alarm system shown here as a bell 6 powered by
battery B
l.
[0020] The oscillator radiates only micro-watts, and only does so when an article is removed.
The oscillator is not in a state of oscillation, when everything is normal at the
shelf. When in this normal state, the supply current used by the equipment is very
low. The maximum current is used when an article is removed from the shelf, and tnis
current mainly consists of that which is consumed by the power transistor and the
relay used.
[0021] It will be appreciated that the oscillator circuit and the relay could be miniaturised
and encapsulated. The relay although shown as an electro-mechanical device, this could
alternatively comprise a solid state switch.
[0022] By use of a display device coupled to respective relays for each of a plurality of
shelves, there may be obtained an indication of which shelf, amongst many, is subject
to the removal of an article.
[0023] When the alarm or indicator means installed at a point remote from the shelf, e.g.
at a supervisory post or assistant's location in a shop, the device provides an automatic
safeguard against unobserved removal of articles, and moreover dependent on the type
of alarm (i.e. visual or audible) can give notification of an alarm condition without
giving any indication of that fact to the person removing the article.
[0024] The placing of the electrodes as a pair in close proximity to each other and over
which the article is placed so as to bridge the electrodes, ensures that stray capacitance
effects due to customers or assistants moving in the vicinity of the display are minimised
and thus reduce the possibility of false alarm conditions.
[0025] Although the system has been described generally for use with an 'article' comprising
an object for display, the system is also capable of detecting the movement of other
'articles' away from a predetermined position for example such as in opening of a
door or window, a face of which could be provided with a metallic strip for example
in the vicinity of the electrodes and any movement thereof causing actuation of the
alarm.
[0026] In the alternative and preferred arrangement of Figure 4 a system is shown which
rather than sensor 12 detecting a change in capacitance per se, detects the rate of
change in capacitance, so that a relatively large rate of change in capacitance as
a result of removal of of an article causes an alarm condition but a small rate of
change, caused by slight stray capacitance or circuit drift for example will not cause
false alarm conditions to be instigated.
[0027] This rate of change function will now be described with reference to the embodiment
of Figure 5. Although not shown, electrodes 10 and 11 could again be coupled via coaxial
cable to the sensor 12.
[0028] Oscillator 20 is a free running oscillator (typically 200 kHz). The oscillator output
is received by buffer 22 to prevent loading of the oscillator. A filter 21 is provided
to remove any r.f. present. The buffered oscillator output passes via pulse shaper
24 to a phase locked loop configuration provided by blocks 26 - 29 inclusive. The
shaped pulses are received by the first comparator 26 which produces an error voltage
which passes via low pass filter 27 to control the oscillator 28. The output of the
voltage controlled oscillator (VCO) is used as the other input to comparator 26 so
as to allow this comparator to determine the error and adjust the error voltage. Similarly
comparator 29 receives the pulses from shnper 24 and the output from VCO 28, so as
to again produce an error voltage. This error voltage passes via driver 30 to an integrator
31 where it is integrated. The integrator output is received by alarm trip unit 13
which instigates an alarm condition when this output exceeds a predetermined level.
[0029] The low pass filter 27 effectively determines the rate at which a change in capacitance
c affecting oscillator 20 will cause the VCO to come out of lock and so cause the
error voltage at the output of comparator 29 to continuously change as the VCO is
searching for a locked situation. Thus this comparator output on integration will
effectively cause the alarm to actuate. Thus the low pass filter 27 and integrator
31 provide the facility for determining the rate of change rather than change per
se in capacitance c.
[0030] An arrangement for realising the system of Figure 5 is shown in Figure 6. The oscillator
20 comprises coil L
1 (e.g. a standard I.F. coil), resistors R
1-R
3, capacitors C
1, C
2 and integrated circuit element 40. C
8 is a decoupling capacitor. Filter 21 comprises R
4 and C
3. Buffer 22 comprises C
4, R5 and element 41. Pulse shaper 24 comprises R
12 and element 46. Driver 30 comprises elements 47, 50, 51 and resistor R
9. Integrator 31 Comprises element 52, R10, Rll, C
7 and diode D1. The phase locked loop 26 - 29 comprises integrated circuit chip 44
and associated components R
6-R
8, C
5,C
6 and relay contact AI. The alarm trip 13 comprises elements 42,
43,
48, 49, resistors R13, R14, diodes D
2, D
3, light emitting diode 56, relay A and loudspeaker 55 (e.g. Piezo transducer U35R).
Elements 42 and 43 operate in parallel to provide the necessary current handling capability
due to the relatively low value of R13.
[0031] Elements 40 - 43 can conveniently comprise 4 dual input Rand gates on a single chip
(e.g. type 4011). The inputs of each of these elements 40 - 43 are strapped together
so that each functions as an inverter. Chip 44 is a phase locked loop chip (e.g. type
4046).
[0032] Elements 46 - 49 can typically comprise 4 Schmitt triggers on a single chip (e.g.
type 4093).
[0033] Elements 50 - 52 can conveniently comprise 3 out of 4 of the Nand gates of an I.C.
chip (e.g. type 4011), each with inputs strapped to function as inverters. It is convenient
to use NANDs since these are also used elsewhere although a normal inverter chip could
be used.
[0034] In the system of Figure 3 the output from the integrator is normally low. When a
rapid capacitance change occurs, then C
7 charges up and the elements 42, 43 go low (when C
7 reaches about 1/2 rail voltage). The Schmitts 48, 49 positively select the alarm
condition causing the transducer 55 to operate.
[0035] The relay A is also energised and contact A
1 closes and thus R
8 is placed in parallel.with R
7 to relock the phase locked loop more quickly, (R
7 and C
6 forming the low pass filter). C
5 and R
6 define the center frequency of the VCO.
[0036] On relocking of the VCO the integrator output will fall and the alarm condition will
cease thus de-energising relay A, opening its contact A
1 and disconnecting R
S thus returning the sensitivity back to the normal condition to again await changes
caused by any disturbance of capacitance c.
[0037] Thus in this embodiment the oscillator is free running whether or not an alarm condition
is occuring. Any slight changes in capacitance will cause the oscillator frequency
to vary but this variation can be followed by the phase locked loop system. A rapid
capacitance change however will cause the loop error to become large and thus on integration
cause an alarm condition to be instigated. The system will automatically relock into
a standby condition on de-energisation of relay A. This locking into a standby condition
will also occur automatically a short time after articles have been placed on the
electrodes when making up the display and thus typically no manual adjustment or setting
up of the system is required, other than initially arranging the electrodes in a suitable
configuration.
[0038] Although in this embodiment the system has been generally described as for use with
an article comprising an object for display, this system can also be used with 'articles'
such as doors or windows which typically cculd be provided on one face with a metallic
strip in the vicinity of the electrodes to allow any movement of the door or window
by opening to be detected and an alarm condition actuated.
1. An alarm system for indicating the removal of an article from a predetermined position
comprising:
(a) sensing circuit means including
(i) two electrical conductors spaced from but adjacent each other and for positioning
adjacent to the predetermined position to be occupied by at least one article when
in use, and
(ii) signal generating means responsive to the capacitance present between these conductors
to provide a first output condition indicative of substantially constant capacitance
present between these conductors to provide a first output condition indicative of
substantially constant capacitance value present between the electrodes due to the
undisturbed presence of said at least one article and to provide a second output condition
indicative of a change in capacitance value present due to the disturbance of said
at least one article, and
(b) alarm actuator means responsive to the output of said sensing circuit means to
provide an alarm indication as a result of a change in the output condition of said
sensing circuit means whenever a change in capacitance due to the removal of an article
occurs.
2. A system as claimed in claim 1, wherein the signal generating means comprises an
oscillator capable of providing a first output condition in which no oscillation occurs
during normal operation and a second output condition in which oscillation occurs
on movement of an object due to a change in capacitance in the oscillator circuit
as sensed between the two conductors, and
adjustment means are provided for selecting the first output condition to compensate
for the type and number of articles present, when in use.
3. A system as claimed in claim 2, wherein the alarm actuator means includes a driver
circuit for rectifying the oscillator output and actuating an alarm element when the
rectified oscillator output exceeds a predetermined level.
4. A system as claimed in claim 1, 2 or 3, wherein the two electrical conductors of
the sensing circuit means comprise a substantially parallel adjacent pair of metallic
electrodes.
5. A method of detecting and indicating when an article has been removed from a predetermined
position comprising:
providing two electrical conductors spaced from but adjacent each other and adjacent
the predetermined position to be occupied by at least one said article,
sensing the capacitance present between these conductors to provide a first output
condition indicative of substantially constant capacitance value present between the
electrodes due to the undisturbed presence of said at least one article and to provide
a second output condition when a change in capacitance value due to the disturbance
of said at least one article occurs, and
actuating an alarm indicator whenever a change in the output condition occurs as a
result of the removal of said article.
G. An alarm system for indicating the removal an article from a predetermined position
comprising:
(a) sensing circuit means including
(i) two electrical conductors spaced from but adjacent each other and for positioning
adjacent the predetermined position to be occupied by at least one article when in
use, and
(ii) signal generating means responsive to the capacitance present between these conductors
to provide a first output condition indicative of substantially constant capacitance
value present between the electrodes due to the undisturbed presence of said at least
one article and to provide a second output condition indicative of the rate of change
in capacitance value present due to the disturbance of said at least one article,
and
(b) alarm actuator means responsive to the output of said sensing circuit means to
provide an alarm indication as a result of a change in the output condition of said
sensing circuit means whenever a rapid change in capacitance due to the removal of
an article occurs.
7. A system as claimed in claim 6 wherein said signal generating means comprises an
oscillator capable of providing ar oscillation frequency dependent on the value of
capacitance present between the conductors and integration means for integrating a
signal dependent on the output of said oscillator to provide an output to said alarm
actuator means dependent on the rate of change of the capacitance value.
8. A system as claimed in claim 7, wherein the signal generating means includes a
phase locked loop arrangement for providing an error voltage dependent on the frequency
variation due to a change in capacitance between the conductors so as to compensate
for any small variation in the capacitance value present to prevent an alarm condition
occuring due to stray capacitance variation, but to provide on integration by said
integration means a sufficient voltage to allow operation of said actuator means whenever
a relatively large change in capacitance occurs due to removal of one or more of said
articles.
9. A system as claimed in claim 8, wherein the phase locked loop arrangement includes
comparator means, a voltage controlled oscillator for providing an output signal dependent
on the output of said comparator means, which comparator means compares the voltage
controlled oscillator output with the signal derived from the oscillator of said signal
generating means to provide an error voltage dependent on any detected change in frequency.
10. A system as claimed in claim 9, wherein said comparator means comprise a first
comparator for providing an output for said voltage controlled oscillator second comparator
for providing an output for said integrating means.
11. A method of detecting and indicating when an article has been removed from a predetermined
position comprising:
providing two electrical conductors spaced from but adjacent each other and adjacent
the predetermined position to be occupied by at least one said article,
sensing the rate of change in capacitance present between these conductors to provide
a first output condition indicative of substantially constant capacitance value present
between the electrodes due to the undisturbed presence of said at least one article
and providing a second output condition when a rapid change in capacitance value due
to the disturbance of said at least one article occurs, and
actuating an alarm indicator whenever a change in the output ccndition occurs as a
result of the removal of said article.
12. An alarm system substantially as described herein with reference to Figures 1
to 3 or Figures 4 to 6 of the accompanying drawings.
13. A method of detecting and indicating when an article has been removed from a predetermined
position substantially as claimed in claim 5 or 11.