Theft detection system

Abstract

 A theft 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. The electronic labels generate 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. 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 are located 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.

Description

[0001] 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.

[0002] Well known are the systems based on the detection of an LC circuit brought into resonance as described in U.S. Patent No. 5,051,727. This reference mentions an air coil which is tuned to the resonance frequency by means of a capacitor connected in parallel. Detection labels that are suitable for this technique are relatively inexpensive to produce in large numbers. These labels can also be produced at a very low price in the form of adhesive labels intended to be used only once. If desired, the labels may be deactivatable.

[0003] 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.

[0004] According to another prior art technique, strips of saturable magnetic material are employed. European patent application no. 92200765.3 (Granovsky) gives an example of a detection system based on this technique. Such magnetic strips also function when they are stuck onto metallic articles. However, the disadvantage of these magnetic strips is that the detection distance is very limited, so that wide passageways cannot be protected.

[0005] One technique enabling detection of metallic articles in the case of larger passage widths involves making use of mechanical resonance of a plate of magnetic material which is caused to vibrate by means of the magnetostriction effect in a magnetic alternating field. U.S. Patent No. 4,510,489 gives an example of such a detection system.

[0006] 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.

[0007] However, by manually deforming the chamber from outside, 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.

[0008] In the theft detection system described in applicant's European patent no. 0084400, starting from an interrogating signal, a reply signal is generated by means of a frequency divider. Through 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.

[0009] However, because the frequency of the interrogating signal and that of the reply signal are markedly different (a factor of 2 at a minimum and a factor of 8 in the example described in the above-mentioned patent), two separate ferrite rod antennas are needed in the label (the responder).

[0010] This number of two antennas, together with the necessity of using several components, inter alia tuning capacitors, and various components in the supply circuit, renders such a label relatively expensive to produce.

[0011] 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.

[0012] 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.

[0013] According to the invention, 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.

[0014] Hereinafter the invention will be further described with reference to the accompanying drawings of some examples of embodiments of the invention.

Fig. 1 shows a block diagram of a first embodiment of a label according to the invention;

Fig. 2 shows a number of signal forms occurring in a label according to Fig. 1;

Fig. 3 shows the spectra of the reply signal (Fig. 3a) and the frequency-transformed reply signal (Fig. 3b);

Fig. 4 shows a block diagram of an interrogation and detection unit;

Fig. 5 shows a block diagram of a detector circuit for detecting a reply signal which can occur in a theft detection system according to the invention;

Fig. 6 shows a block diagram of a quadrature detector from the detector circuit of Fig. 5;

Fig. 7 shows a block diagram of a second embodiment of a label according to the invention;

Fig. 8 illustrates a modulation process of a bit pattern in the memory for forming a reply signal;

Fig. 9 illustrates in what way bit patterns with the hexadecimal value 0 can be translated into a reply signal;

Fig. 10 shows in what way bit patterns with a hexadecimal value F can be translated into a reply signal; and

Fig. 11 shows this translation for a succession of bit patterns 0 and F.

[0015] 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. In the first frequency divider circuit 3, the interrogation frequency f₀ is divided by a factor N. In a practical example, N may be equal to 64. Then the frequency of the signal 7 which has thus arisen is divided by a factor M (for instance, M = 2) in divider 4. 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. In a practical example, P may be equal to 8.

[0016] In Fig. 2 the various signals are drawn for the case where N = 64, M = 2, and P = 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.

[0017] 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. 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.

[0018] Fig. 3 draws the spectra of the reply signal 10 (Fig. 3a) and of the frequency-transformed reply signal with frequencies f₇, and f_7'', f_8' and f_8'' across the circuit 1 (Fig. 3b), where

and

.

[0019] 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.

[0020] The manner in which this label signal can be received and transformed back to a baseband reply signal, both for an absorption system and for a transmission system, has been discussed extensively in applicant's Dutch patent application no. 9202158.

[0021] 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.

[0022] Where the absorption principle is utilized, one and the same antenna is connected to both the transmitter and the demodulator. In that case 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.

[0023] Where the transmission principle is utilized, 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.

[0024] 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. The frequency switch is first demodulated and the thus recovered signal 9 (in the above-mentioned example 120000/(64*2*8) = 117.1875 Hz) is modulated in quadrature in a quadrature detector 24. For that purpose, via connection 28 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.

[0025] 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.

[0026] 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.

[0027] In 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.

[0028] 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.

[0029] Divider circuit 34 divides the interrogating signal of the transmitter by factor R, with

, in such a manner that two reference signals arise which are shifted 90° in phase relative to each other.

[0030] 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.

[0031] There is another possible embodiment of the label, different from the embodiment described above and depicted in Fig. 1. To generate the same reply signal it is also possible to start from an identification label, such as described in applicant's EP-A-0576100.

[0032] 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, however, have been replaced with an address counter 35, a programmable memory block 36, and a biphase modulator 37.

[0033] 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. A 0-bit then corresponds with a half period of a square-wave voltage of a frequency of 1/2 f_b = 937.5 Hz. Two 0-bits in succession then give rise to a complete period. A sequence of four 0-bits, also designated by the hexadecimal digit 0, then gives two periods of a 937.5 Hz square-wave voltage, and a sequence of four 1-bits, also designated by the hexadecimal digit F, then gives four periods of a 1875 kHz square-wave voltage.

[0034] This is depicted in Figs. 9 and 10. By now loading the memory 36 with the hexadecimal digit pattern shown hereinbelow, the same reply signal is generated as in the label of Fig. 1:
   00FF00FF00FF00FF00FF00FF00FF00FF
   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.

[0035] Accordingly, by programming a special code in the memory of a coded responder designed for identification purposes, of the type described in EP-A-0576100 and making use of the full custom integrated circuit used therein, it is possible to use such an identification label as a theft detection label. This makes it possible to use the same integrated circuit for both applications, which is attractive for reasons of economy.

[0036] It is noted that after the foregoing, various modifications will readily occur to those skilled in the art. As an address counter, for instance a frequency divider can be used. Further, depending on the programming of the memory, 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.

[0037] It is further noted that in Figs. 7-10, for clarity, square-wave signals have been drawn. In reality, however, these may also be differently formed signals, for instance sine-shaped signals, which may or may not be amplitude-limited.

[0038] These and similar modifications are understood to fall within the scope of the present invention.

Claims

1. A theft 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, said 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, 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.

2. A theft detection system according to claim 1, characterized in that the electronic label comprises at least three frequency divider circuits and an electronic switching element which in operation is operated by the output signal of one of said frequency divider circuits and which alternately transmits an output signal of the other frequency divider circuits to the first switching element.

3. A theft detection system according to claim 1, characterized in that the labels comprise a programmable memory circuit in which a predetermined data pattern has been programmed, as well as an address device for reading out the memory, the data pattern providing a memory output signal which, directly or indirectly, constitutes said frequency-switched signal.

4. A theft detection system according to any one of the preceding claims, characterized in that the detection unit comprises a reply signal detection circuit comprising a demodulation circuit for demodulation of an FSK signal, followed by a quadrature detector for providing a frequency switch signal which corresponds with the frequency switch of the frequency-switched signal.

5. A theft detection system according to any one of the preceding claims, characterized in that the detection unit comprises a reply signal detection circuit comprising band filters tuned to the at least two high-frequency signal components as well as an additional narrow-band signal channel whose frequency lies between the frequencies of the high-frequency signal components, said channel providing a detection threshold signal representing the noise and interference signal level.

6. A theft detection system according to any one of the preceding claims, characterized in that the detection unit comprises a reply signal detection circuit comprising a digital signal processor programmed with such an algorithm that first the frequency switch of the frequency-switched signal is modulated and then synchronous quadrature detection of that switch signal occurs.

7. A theft detection system according to claim 6, characterized in that in the detection algorithm further a third signal channel functions for providing a detection threshold signal representing the noise and interference signal level.

Drawing