Note: Descriptions are shown in the official language in which they were submitted.
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AN P~Nq~M~NT RELATING TO E~ECTRONIC ~T-~RM SYSTEN8 THAT DELIVER
CONTROL SIGNALS 8UC~ AS TO DISPLACE THE x~ .l TR~N~T~ION
FREQUENCY AND T~EREWITH AVOID D~STURBANCE8
The present invention relates to an arrangement pertaining to
electronic alarm systems of the kind used in goods monitoring
systems.
Many different types of goods monitoring alarm systems are
described in the patent literature. These systems are intended to
prevent the removal of unpaid goods from stores and like shopping
facilities. These alarm systems normally include some sort of
alarm element that is fastened to the goods to be protected, and
detection means that are permanently installed at store exits and
that generate an alarm with the aid of remote sensors when goods
carrying an alarm element are brought into the proximity of the
store exit.
Remote sensing is normally achieved by generating an alternating
magnetic field in which the presence of an alarm element can be
detected as a result of a change in the alternating field that is
characteristic of the alarm element.
The alarm element may have the form of a long, narrow and thin
strip of highly permeable material whose characteristic property
is found in the transmission of high order harmonics when exposed
to a magnetic alternating field. This known basic principle
enables the presence of narrow and inexpensive alarm elements to
be detected with the aid of complicated and relatively expensive
detection devices. This type of alarm system is particularly
suited for use in daily shopping complexes and is described in
European Patent Specification EP 0 153 286, among others.
,
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Known alarm elements can also consist of a simple electric
resonance circuit. The detector arrangement can therewith be made
relatively simple and inexpensive, provided that the coil in the
resonance circuit is made relatively large and that a relatively
good Q-value can be easily obtained together with a large
coupling to an external field. In this regard, the coil is placed
in an alarm plate which is fastened to the goods to be protected,
with the aid of some kind of locking device.
Such alarm systems may be constructed in a manner in which the
detector arrangement includes two mutually spaced alarm arched
coils that delimit the sensing zone. Several such alarm arches
may be placed parallel with one another so as to form two or more
parallel sensing zones.
When large departmental stores and like service complexes include
two or more mutually independent alarm systems of the kind that
transmit on essentially the same frequency and that are set to
detect the occurrence of approximately the same frequency
resulting from the presence of an alarm element, the mutually
independent alarm systems are liable to interfere with one
another by virtue of the fact that the transmitted alternating
fields give rise to frequency differences that correspond to the
frequency or frequencies to be detected by the system, thereby
resulting in over-driving of the amplifiers in the detectors due
to the strength of the signal greatly exceeding the strength of
a signal that originates from an alarm element. This makes it
impossible to detect the presence of an alarm element.
One known method of solving this problem is to synchronize the
fields transmitted from the various systems so that the fields
will be transmitted synchronously, by drawing cables between the
different systems so as to synchronize the transmitterS.
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The different systems can also be synchronized by allowing each
system to operate intermittently over short time intervals and to
activate the systems successively so that only one system will
operate at any one time. Synchronization of the systems is
effected by means of a cable connection also in this case.
Departmental stores and shopping centres often have a multiple of
systems that are located at relatively short distances from each
other and that belong to different owners. Furthermore, new
systems are installed and systems are moved.
Consequently, the problem solution achieved by cable connections
can only be applied in practice in the case of a few systems and
when the systems can be mutually coordinated, for instance when
the systems have only one owner.
Accordingly, the present invention relates to an arrangement
pertaining to electronic alarm systems of the kind used in goods
monitoring systems, wherein the system includes an alarm element
which is adapted to receive a magnetic alternating field that is
transmitted from a transmitter by means of a transmitter coil and
the frequency of which is swept, wherein the alarm element is
adapted to retransmit a magnetic alternating field, wherein a
receiver that includes a receiver coil is adapted to receive and
detect the retransmitted alternating field, wherein the electron-
ic alarm system is characterized in that the system transmitter
is adapted to vary the transmitted frequency; wherein the system
receiver is adapted to receive and detect the presence of a
signal deriving from a disturbing other system; wherein the
system includes a control circuit that analyzes the frequency and
phase position of a signal transmitted from a disturbing system
relative to the frequency and phase position of the alternating
field transmitted from the first-mentioned system; and wherein
the control circuit is adapted to deliver a control signal to the
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transmitter of the first-mentioned system such as to displace its
sweeping frequency and therewith prevent disturbance of the
first-mentioned system by the disturbing signal.
The invention will now be described in more detail with reference
to an exemplifying embodiment thereof and also with reference to
the accompanying drawing, in which
- Figure l is a block schematic illustrating an inventive
arrangement.
The present invention relates to an arrangement pertaining to
electronic alarm systems of the kind used in goods monitoring
systems.
Such a system includes a known alarm element l that is adapted to
receive a magnetic alternating field transmitted from a transmit-
ter 2 by means of a transmitter coil 3. The frequency of the
transmitted alternating field is swept. For instance, there is
transmitted an alternating field having a frequency of 8.2 MHz
+/- O.5 MHz. The alarm element 1 is adapted to retransmit a
magnetic alternating field. A receiver 4 that includes a receiver
coil 5 is adapted to receive and detect the retransmitted
alternating field. To this end, the transmitter 2 delivers the
transmitted signal to the receiver 4 via a conductor 6 and the
received signal is down-mixed in a known manner.
Depending on the type of alarm element used, i.e. the type of
retransmitted field generated by the alarm element, the system
will include adapted circuits for the detection of the signal
received by the receiver with respect to fields that are generat-
ed by an alarm element. It can be said generally that known alarm
elements retransmit a field periodically, i.e. at given time
intervals contingent on the periodically sweeping field transmit-
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ted by the transmitter, thereby enabling the occurrence of such
a periodically occurring signal to be detected. The arrangement
triggers an alarm signal when an alarm element is located in the
monitoring zone.
'5
According to the invention, the system transmitter 2 is designed
to be able to vary the transmitted frequency, i.e. the centre
frequency about which the field sweeps. The frequency at which
the transmitted frequency sweeps is preferably synchronized with
the frequency of the transmitted signal. This can be achieved,
for instance, by generating the sweeping frequency by dividing
the frequency 14 generated by the transmitter and applied to the
amplifier 10. This causes the centre frequency to control the
sweep frequency.
The system receiver 4 is adapted to receive and to detect the
occurrence of a signal from a disturbing other system. There is
provided to this end a control circuit 8 which functions to
analyze the frequency and phase position of a signal transmitted
from a disturbing system in relation to the frequency and phase
position of the alternating field transmitted from the first-
mentioned system. The disturbing system may be a system of the
same kind as the inventive system or of a kind different thereto.
The transmitted frequency of the disturbing system will interfere
with the present system when the frequency lies so close to the
transmitted frequency of the inventive system that reception of
the disturbing frequency will cause the receiver amplifier to be
over-driven and thereby prevent detection of an alarm system.
r Detection of the disturbing frequency can be effected by includ-
ing a quadrature detector 7 in the receiver. The signals generat-
ed in the quadrature detector can be delivered to a known control
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circuit 8 which functions to analyze the frequency and phase
position of the received signal.
According to the invention, the control circuit 8 also functions
to deliver a control signal to the transmitter 2 of the first
system and to displace the sweeping frequency of said transmitter
so as to prevent the disturbing signal from disturbing the first-
mentioned system.
According to a first embodiment of the invention, the control
circuit 8 is adapted to deliver a control signal 9 to the
transmitter 2 of the inventive system so as to cause the trans-
mitter to transmit a frequency sweeping alternating field whose
frequency and phase position coincide with the disturbing signal
received at each moment in time.
The control circuit is suitably adapted to change the transmitted
frequency so as to bring said frequency successively closer to
the disturbing frequency.
Thus, the inventive system will adapt itself to transmit a field
that is fully synchronous with the disturbing field.
In Figure l, the arrow ll indicates the field transmitted by the
inventive system and the arrow 12 indicates the disturbing
signal.
Thus, when two systems of the present kind are installed, both
systems will individually adjust their phase position and
frequency in accordance with the respective disturbing signal.
The two systems will thus finally transmit a frequency that lies
somewhere between the original frequencies. It will be understood
that three or more mutually disturbing systems will also approach
one another until they transmit on the same frequency.
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Thus, all coordination systems are effected in a wireless
fashion.
According to a second embodiment of the invention, the control
circuit 8 is adapted to deliver to the transmitter 2 of the
first-mentioned system a control signal which displaces its
sweeping frequency so that said frequency will differ from the
sweeping frequency of the disturbing signal by a difference
frequency that is greater than the bandwidth of the signal that
the first-mentioned system is intended to receive.
According to this second embodiment, the control circuit is
conveniently adapted to change the transmitted frequency so as to
bring said frequency successively closer to the disturbing
frequency.
This second embodiment of the invention thus also causes the
frequencies of two or more mutually disturbing systems to be
displaced so that the systems will no longer disturb one another.
Thus, when the different systems transmit frequencies that do not
disturb one another at a given time point, one of the systems may
be driven slowly in frequency so that the driving system will
begin to disturb another system after a given period of time. In
this case, the respective control circuit of the mutually
disturbing system may be adapted to displace respective frequen-
cies directly upon the occurrence of disturbance, so that said
disturbance will no longer occur.
According to one highly advantageous embodiment of the invention,
the transmitter 2 includes a digital direct synthesis circuit to
which the control signal 9 is delivered. The digital circuit is
~ adapted to generate digitally the frequency and its sweep to be
transmitted. The digital circuit is also adapted to transmit the
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frequency and its sweep in a phase position given by the control
signal. Such digital synthesis circuits, or chips, are known and
commercially available, as is also the simple logic required to
effect a sweep that is correlated to the centre frequency. This
can be achieved, for instance, with the aid of the clock that
controls the digital direct synthesis circuit also counting in a
frequency counter that controls the frequency generated by the
digital direct synthesis circuit. The signal generated by the
digital direct synthesis circuit is thus amplified in an amplifi-
er 10 and transmitted via the coil 3.
According to one preferred embodiment of the invention, the
control circuit 8 includes a microprocessor that is adapted to
carry out said analysis, preferably by means of a Fourier
analysis and adapted to generate said control signal.
The frequency and phase position of a disturbing signal can be
analyzed by a Fourier analysis.
The control circuit 8 is conveniently adapted to activate an
alarm device 13 when the presence of an alarm element is detect-
ed.
According to one embodiment, the digital direct synthesis circuit
is constructed to enable the frequency of its internal oscillator
to be influenced by the control signal. This enables the frequen-
cy setting to be finely adjusted by slightly displacing the
frequency of the internal oscillator, therewith displacing
correspondingly the frequency generated by the digital direct
synthesis circuit.
The control signal may, however, contain information that causes
the digital direct synthesis circuit to generate the desired
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frequency while retaining the frequency of the internal oscilla-
tor.
It will be evident that the problems mentioned in the introduc-
tion are solved by means of the present invention, wherewith the
systems become self-adjusting in relation to one another.
Although the invention has been described with reference to
various embodiments thereof, it will be obvious that these
embodiments can be modified in many different ways. For instance,
detection can be effected in ways other than with the aid of a
quadrature detector.
The invention is therefore not restricted to the afore-described
exemplifying embodiments, since modifications and variations can
be made within the scope of the following Claims.
