EP0628937A1 - Theft detection system - Google Patents

Theft detection system Download PDF

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Publication number
EP0628937A1
EP0628937A1 EP94201631A EP94201631A EP0628937A1 EP 0628937 A1 EP0628937 A1 EP 0628937A1 EP 94201631 A EP94201631 A EP 94201631A EP 94201631 A EP94201631 A EP 94201631A EP 0628937 A1 EP0628937 A1 EP 0628937A1
Authority
EP
European Patent Office
Prior art keywords
signal
frequency
circuit
detection system
detection
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP94201631A
Other languages
German (de)
English (en)
French (fr)
Inventor
Tallienco Wieand Harm Fockens
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nederlandsche Apparatenfabriek NEDAP NV
Original Assignee
Nederlandsche Apparatenfabriek NEDAP NV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nederlandsche Apparatenfabriek NEDAP NV filed Critical Nederlandsche Apparatenfabriek NEDAP NV
Publication of EP0628937A1 publication Critical patent/EP0628937A1/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/22—Electrical actuation
    • G08B13/24—Electrical actuation by interference with electromagnetic field distribution
    • G08B13/2402—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting
    • G08B13/2405—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used
    • G08B13/2422—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used using acoustic or microwave tags
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/22—Electrical actuation
    • G08B13/24—Electrical actuation by interference with electromagnetic field distribution
    • G08B13/2402—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting
    • G08B13/2405—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used
    • G08B13/2414—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used using inductive tags
    • G08B13/2417—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used using inductive tags having a radio frequency identification chip
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/22—Electrical actuation
    • G08B13/24—Electrical actuation by interference with electromagnetic field distribution
    • G08B13/2402—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting
    • G08B13/2428—Tag details
    • G08B13/2431—Tag circuit details

Definitions

  • This invention relates to a shoplifting detection system comprising an interrogation unit and a detection unit for generating an electromagnetic interrogation field with a predetermined frequency and detecting the presence of electronic labels belonging to the system in the interrogation field, these electronic labels generating a reply signal in the interrogation field by means of a resonant circuit which comprises at least a coil and a capacitor and which also functions as receiving circuit for the interrogation field.
  • a drawback of this known technique is that conductive articles in the immediate surroundings of the coil may detune this coil and dampen it. As a consequence, this technique is less suitable for protecting articles that contain metal.
  • a fundamental problem of this method is that the plate in question has to vibrate mechanically and for that purpose has to be freely suspended in a small chamber, so that no mechanical damping occurs.
  • the resonance effect can be suppressed and thereby the label can be effectively inactivated.
  • Such a label can also be rendered ineffective by means of a permanent magnet.
  • a reply signal is generated by means of a frequency divider.
  • frequency modulation of the interrogating signal and synchronous demodulation of the likewise frequency-modulated reply signal a reliable and uniform detection has been found to be possible. Since the operating frequencies have been chosen to be low (138 kHz and 17.25 kHz, respectively) and the coils are wound on ferrite rods, the presence of metal in the articles to be protected does not have any disturbing influence on the functioning of the label.
  • the object of the present invention is to provide a theft detection system which combines the non-influenceability by metal with a greater detection distance and a low cost price of the responder labels.
  • the invention is based on the principle that the label derives from the interrogating signal a reply signal detectable with high sensitivity without risk of false alarms and subsequently frequency-transforms this signal to a frequency band close to and on opposite sides of the interrogation frequency.
  • a theft detection system of the type described is characterized in that the labels comprise means which provide a frequency-switched signal with the aid of which the signal generated by the interrogation field in the resonance circuit is modulated via a first switching element in such a manner that at least two high-frequency signal components arise which lie within the resonance curve of the resonance circuit, in such a manner that these signal components are strongly present in the secondary magnetic field formed by the coil.
  • Fig. 1 shows a block diagram of a responder label for a theft detection system according to the invention.
  • a resonance circuit 1 comprising a coil L and a capacitor C, is tuned to an interrogation frequency f0.
  • This frequency can for instance be 120 kHz.
  • Rectifier circuit 2 forms from the alternating voltage prevailing across the ciruit if the label is located in an interrogation field, a direct voltage which can supply the entire label circuit with supply voltage.
  • the signal prevailing across the LC circuit is further supplied to a circuit of frequency dividers 3, 4 and 5.
  • the interrogation frequency f0 is divided by a factor N.
  • N may be equal to 64.
  • the output signal of divider 4 is indicated by 8 and is finally frequency-divided by a factor P in divider 5.
  • the output signal of divider 5 is indicated by 9.
  • P may be equal to 8.
  • Signal 9 controls a switching element 6 to which the signals 7 and 8 are supplied as well.
  • Output signal 10 of the switch 6 is a combination of the signals 7 and 8, so that a frequency-switched signal (FSK, Frequency Shift Keying) arises.
  • This signal 10 which can be seen as an FSK modulated subcarrier, constitutes the above-mentioned reply signal and is supplied to a switching element S.
  • Switching element S in closed condition switches a resistor R parallel to the circuit 1, so that the circuit losses increase strongly and the Q factor decreases. As a result, the absorption of energy from the primary magnetic alternating field of the interrogating signal decreases and the alternating current through coil L decreases. As a result, the field strength of the secondary field generated by the responder coil L decreases. Switching element S thus modulates the secondary magnetic field formed by the coil L.
  • switch S If switch S is driven in a particular rhythm, the frequency of that rhythm, f r , is transformed by the switching element and the interrogating signal (subcarrier) flowing through it, to two new frequency components, equal to the sum of f0 and f r and to the difference of f0 and f r , it being known from the radiocommunications theory that amplitude modulation by means of a switching element can be described as a frequency transformation process.
  • Fig. 3b also shows that these high-frequency reply signal components fall within the resonance curve RC of circuit 1. As a consequence, these signal components are represented with the largest possible strength, and in any case significantly, in the secondary magnetic field of antenna coil L.
  • Fig. 4 shows the block diagram of the interrogation and detection unit, which comprises a transmitter circuit Tx, one or more antennas 11, a demodulator circuit 12, a detector circuit 13, and an alarm circuit 14.
  • the transmitter generates the high-frequency interrogating signal of frequency f0, for instance 120 kHz.
  • Antenna 11 consists of a coil, in most cases an air-core coil, consisting of one or more windings, but a coil wound on a ferrite rod can also be used.
  • the demodulator is an envelope detector circuit, which recovers the baseband reply signal, i.e., signal 10, from the transmitted signal and the received label signal.
  • the transmitter and the demodulator circuit are each connected to an antenna 11.
  • the demodulator is then a product detector circuit which multiplies the label signal by a reference signal which is separately supplied from the transmitter circuit to the demodulator.
  • the product again contains the baseband reply signal.
  • the received reply signal is further processed in the reply signal detector 13.
  • a block diagram of an example of a suitable detector circuit is depicted in Fig. 5.
  • a reference signal of the same frequency as signal 9 is supplied from the transmitter circuit T x .
  • the actual FSK demodulation takes place by means of the filters 16 (tuned to signal 7) and 17 (tuned to signal 8), together with the envelope detectors 19 and 20.
  • Comparator 23 determines at which of the two frequencies the most signal energy is present, whereby the FSK modulation is recovered.
  • Filter 18 is tuned to a frequency f m between f7 and f8. In the practical example it is tuned to 1406 Hz. With filter 18, in conjunction with envelope detector 21, the noise level such as it is received by the reply signal detector, is measured.
  • An automatic amplification control circuit 22 controls the amplification of input amplifier 15, starting from the strongest signal from the detectors 19, 20 or 21.
  • comparator 25 the signal level from the quadrature detector 24 is compared with the noise level. The outcome thereof controls integrator 26. As soon as the output voltage thereof exceeds a threshold value, alarm circuit 27 gives an alarm.
  • the quadrature detector 24 is presented in more detail in Fig. 6.
  • the signal 9 recovered after the FSK demodulation is supplied to two product detectors 29 and 30, each receiving a reference signal, these reference signals being shifted 90° in phase relative to each other.
  • Low-pass filters 31 and 32 are connected with the outputs of the product detectors.
  • the low-pass filter have a low cut-off frequency of, for instance, 10 Hz, and determine the eventual effective noise bandwidth of the reply signal detector 13 and thus the detection sensitivity.
  • An RMS combiner 33 combines the output signals of the two channels in such a manner that the output signal of the quadrature detector 24 is proportional to the amplitude of the input signal but independent, or nearly independent, of the phase relation between the input signal and the reference signal.
  • the desired operation of the reply signal detector 13 can also be obtained by the use of a suitable algorithm in a DSP unit (Digital Signal Processor). That algorithm can be designed analogously to the above-described circuit.
  • DSP unit Digital Signal Processor
  • Fig. 7 shows a block diagram of this embodiment.
  • the antenna circuit 1, the supply circuit 2, load resistor R and switching element S are identical to those in the label of Fig. 1.
  • the frequency dividers have been replaced with an address counter 35, a programmable memory block 36, and a biphase modulator 37.
  • Fig. 8 indicates how a bit sequence 38 can be converted by the modulator 37 into a differential biphase-modulated reply signal 39.
  • a 1-bit gives rise to a complete period of a square-wave voltage of a frequency f b of, for instance, 1875 Hz.
  • Two 0-bits in succession then give rise to a complete period.
  • Figs. 9 and 10 This is depicted in Figs. 9 and 10.
  • the same reply signal is generated as in the label of Fig. 1: 00FF00FF00FF00FF00FF00FF00FF00FF00FF00FF Fig. 11 shows this for a part of the above pattern.
  • the signal of Fig. 11 has the same form as the signal 10 of Fig. 1 and likewise can be used to control a switching element connected with the resonance circuit of the identification label.
  • the output signal of the memory 36 could be supplied to the switching element S either directly, whether amplified or not, or via the biphase modulator 37 shown or a different processing circuit.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Automation & Control Theory (AREA)
  • Computer Security & Cryptography (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Burglar Alarm Systems (AREA)
EP94201631A 1993-06-09 1994-06-08 Theft detection system Withdrawn EP0628937A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
NL9300991A NL9300991A (nl) 1993-06-09 1993-06-09 Diefstaldetectiesysteem.
NL9300991 1993-06-09

Publications (1)

Publication Number Publication Date
EP0628937A1 true EP0628937A1 (en) 1994-12-14

Family

ID=19862509

Family Applications (1)

Application Number Title Priority Date Filing Date
EP94201631A Withdrawn EP0628937A1 (en) 1993-06-09 1994-06-08 Theft detection system

Country Status (2)

Country Link
EP (1) EP0628937A1 (nl)
NL (1) NL9300991A (nl)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8587489B2 (en) 2007-06-08 2013-11-19 Checkpoint Systems, Inc. Dynamic EAS detection system and method
US8933790B2 (en) 2007-06-08 2015-01-13 Checkpoint Systems, Inc. Phase coupler for rotating fields

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3984835A (en) * 1974-06-03 1976-10-05 Rca Corporation Homodyne communication system
FR2371128A7 (fr) * 1976-11-15 1978-06-09 Nedap Nv Systeme de detection a circuit resonnant non lineaire
EP0084400A2 (en) * 1982-01-14 1983-07-27 N.V. Nederlandsche Apparatenfabriek NEDAP Detection system
FR2598006A1 (fr) * 1986-04-28 1987-10-30 Signalisation Detecteur de presence, a proximite d'un poste de controle, d'un repondeur associe a un porteur
EP0441237A1 (de) * 1990-02-05 1991-08-14 Anatoli Stobbe Tragbares feldprogrammierbares Detektierplättchen

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3984835A (en) * 1974-06-03 1976-10-05 Rca Corporation Homodyne communication system
FR2371128A7 (fr) * 1976-11-15 1978-06-09 Nedap Nv Systeme de detection a circuit resonnant non lineaire
EP0084400A2 (en) * 1982-01-14 1983-07-27 N.V. Nederlandsche Apparatenfabriek NEDAP Detection system
FR2598006A1 (fr) * 1986-04-28 1987-10-30 Signalisation Detecteur de presence, a proximite d'un poste de controle, d'un repondeur associe a un porteur
EP0441237A1 (de) * 1990-02-05 1991-08-14 Anatoli Stobbe Tragbares feldprogrammierbares Detektierplättchen

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8587489B2 (en) 2007-06-08 2013-11-19 Checkpoint Systems, Inc. Dynamic EAS detection system and method
US8933790B2 (en) 2007-06-08 2015-01-13 Checkpoint Systems, Inc. Phase coupler for rotating fields

Also Published As

Publication number Publication date
NL9300991A (nl) 1995-01-02

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