NL9300991A - Theft detection system. - Google Patents

Description

Ir. T.W.H. Fockens Theft detection system.

The invention relates to a shoplifting detection system.

The systems are known based on the detection of a resonant LC circuit, as mentioned in USA patent no. 5051727 (European patent application no. 90200630.3). This refers to an air coil, which is tuned to the resonant frequency by means of a parallel capacitor. Labels made for this technique are relatively cheap to produce in large numbers. It is also possible to make these labels in the form of adhesive labels for a very low price, intended for one-time use, if desired deactivatable.

A drawback of this technique is that conductive objects in the immediate vicinity of the coil can tune and damp this coil. Therefore, this technique is less suitable for securing objects containing metal.

Another technique uses strips of saturable magnetic material. European Patent Application No. 92200765.3 (Granovsky) gives an example of a detection system based on this technique. Such magnetic strips still function if they are attached to metal-containing objects. However, the drawback of this technique is that the detection distance is very limited, so that wide passages cannot be secured.

One technique that makes it possible to detect metal-containing objects at larger passage widths is to use mechanical resonance of a plate of magnetic material, which is vibrated in a magnetic alternating field using the magnetostriction effect. U.S. Patent No. 4,510,489 gives an example of such a detection system.

The fundamental problem with this technique is that the plate in question must vibrate mechanically, for which it must be suspended in a chamber, so that no mechanical damping occurs.

However, by manually deforming the chamber from the outside, the resonance effect can be suppressed and thus effectively disable the label. Such a label can also be rendered ineffective by means of a permanent magnet.

In the theft detection system described in applicant's European patent no. 0084400, a response signal is generated from an interrogation signal using a frequency divider. A reliable and unambiguous detection has proved possible by means of frequency modulation of the interrogation signal and synchronous demodulation of the thereby also frequency-modulated response signal. Since the operating frequencies are selected low (138 kHz or 17.25 kHz, respectively) and the coils are wound on ferrite rods, the presence of metal in the objects to be protected does not affect the functioning of the label.

However, because the frequency of the interrogation signal and of the response signal are very different (at least a factor of 2, a factor of 8 in the above-mentioned system, two separate ferrite rod antennas are necessary.

This number of two antennas, together with the need to use multiple components, including tuning capacitors, various components in the power circuit, makes such a label relatively expensive to produce.

It is the object of the present invention to provide a theft detection system that combines the non-influence of metal with a large detection distance and a low cost of the label.

The invention is based on the principle that the tag derives a high sensitivity detectable response signal from the interrogation signal without the risk of false alarms, and then transforms this signal in frequency into a frequency band close to and on either side of the interrogation frequency.

The invention will be described in more detail below with reference to figures.

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

Figure 2 shows a number of signal shapes that occur in a label according to Figure 1.

Figure 3 shows the spectra of the response signal (Figure 3a) and the frequency-transformed response signal (Figure 3b).

Figure 4 shows the block diagram of the interrogation and detection unit.

Figure 5 shows the block diagram of the detector circuit for detecting the response signal associated with the invention.

Figure 6 shows the block diagram of the quadrature detector from the detector circuit.

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

Figure 8 shows the modulation process from a bit pattern in the memory to a response signal.

Figure 9 shows how the bit patterns with the hexadecimal value 0 are translated into a response signal.

Figure 10 shows how the bit patterns with the hexadecimal value F are translated into a response signal.

Figure 11 shows this translation for a sequence of bit patterns 0 and F.

Figure 1 shows the block diagram of a label according to the invention. Resonant circuit 1, consisting of coil L and capacitor C, is tuned to the interrogation frequency f. This frequency is, for example, 120 kHz. Rectifier circuit 2 forms a direct voltage from the alternating voltage across the circuit to supply power to the entire label circuit. The interrogation signal is also applied to a chain of frequency dividers 3, 4, and 5. In the first frequency divider circuit 3, the interrogation frequency f is divided by a factor N. In the practical example, N = 64. 7) by divisor 4 divided by M (eg M = 2). The output signal thereof, 8, is finally divided again by factor P in divider 5, with the output signal 9. In the practical example, P = 8.

In Figure 2 the signals are shown for the case N = 64, M = 2 and P = 8. Signal 9 operates a switch 6. Output signal 10 of this switch is a combination of the signals 7 and 8, so that a frequency-switched signal (FSK, Frequency Shift Keying) is created. This signal 10, which can be regarded as an FSK modulated subcarrier, forms the above-mentioned response signal and is applied to switch S.

Switch S switches a resistor R parallel to circuit 1, so that the circuit losses increase sharply and the Q factor decreases. As a result, the absorption of energy from the alternating primary magnetic field of the interrogation signal decreases, the alternating current through coil L decreases, and thus the emitted field strength of the label coil secondary field decreases. Switch S thus modulates the secondary magnetic field formed by coil L. If S is driven at a particular rhythm, the frequency of that rhythm, f, is transformed by the switch and the interrogation signal flowing through it (subcarrier) into two new frequency components, equal to the sum of f and f and the difference of f and f.

o r

After all, it is known from radio communication technology that amplitude modulation with the aid of a switching element can be described as a frequency transformation process.

Figure 3 shows the spectra of the response signal 10 (Figure 3a) and of the frequency-transformed response signal with frequencies f, and f t, fQt and fot, across circuit 1 (Figure 3b).

Figure 3b also shows that these high-frequency response signal components fall within the resonance curve RC of circuit 1. Therefore, these signal components with the greatest possible strength are represented in the secondary magnetic field of antenna coil L.

The manner in which this label signal can be received and transformed back to a baseband response signal, both for an absorption system and for a transmission system, has already been described in detail in NL patent application no. 9202158 of the applicant.

Figure 4 shows the block diagram of the interrogation and detection unit, which consists of 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 interrogation signal with frequency f, for example 120 kHz. Antenna 11 consists of a coil, usually an air coil, consisting of one or more windings, but a coil wound on a ferrite rod is also possible.

If the absorption principle is used, one antenna is connected to both the transmitter and the demodulator. In this case, the demodulator is an envelope detector circuit, which recovers the baseband response signal from the transmit signal and the received label signal.

In case the transmission circuit is utilized, the transmitter and demodulator circuit are connected, each to an antenna 11. The demodulator is then a product detector circuit, which mixes the label signal with a reference signal, which is separated from the transmitter circuit to the demodulator. linked. The product again contains the baseband response signal.

The received reply signal is further processed in the reply signal detector 13. The block diagram of this detector circuit is shown in figure 5. The frequency switch is first demodulated and the signal 9 thus recovered (in the example 120000 / (64 * 2 * 8)) = 117.1875 Hz) is quadrature demodulated in quadrature detector 24. For this purpose, a reference signal having the same frequency as signal 9 is supplied from the transmitter circuit via connection 28. 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 on which of the two frequencies the most signal energy is present, with which the FSK modulation is recovered.

Filter 18 is tuned to a frequency between f and fD. In

/ O

the practical example at 1406 Hz. Together with envelope detector 21, this measures the noise level as received by the answer signal needle detector.

The automatic gain control circuit 22 controls the gain of amplifier 15 according to the strongest signal from detectors 19, 20, or 21.

In comparator 25, the signal level from the quadrature detector 24 is compared to the noise level. The result of this controls integrator 26. As soon as its output voltage rises above a threshold value, arm circuit 27 already issues an alarm.

The quadrature detector 24 is shown in more detail in Figure 6. The signal 9 recovered after the FSK demodulation is applied to two product detectors 29 and 30, each of which is given a reference signal which are mutually shifted in phase by 90 °. The low-pass filters 31 and 32, with a low cut-off frequency of, for example, 1.0 Hz, determine the final effective noise bandwidth of the response signal detector 13 and thus the detection sensitivity. RMS combiner 33 combines the output signals of both channels such that the output signal of the quadrature detector 24 is proportional to the amplitude of the input signal, independent or nearly independent, of the phase relationship between the input signal and the reference signal.

Sub-circuit 34 divides the interrogation signal of the transmitter by factor R, where R = N * Μ * P, so that two reference signals are created, which are mutually shifted in phase by 90 °.

The reply signal detector 13 can also be implemented as an algorithm in a DSP unit (Digital Signal Processor). That algorithm can be structured in the same manner as the circuit described above.

A different embodiment of the label is possible than that described above and indicated in figure 1. In order to generate the same response signal, it is also possible to start from an identification label, as described in NL patent application no. 9201116 of the applicant.

Figure 7 shows the block diagram of this. The antenna circuit 1, power circuit 2, damping resistor R and switch S are identical to those in the former label of Figure 1. The frequency dividers have been replaced by an address counter 35, a programmable memory block 36, and a bi-phase modulator 37.

Figure 8 shows how a bit sequence 38 stored in memory is converted by modulator 37 into a differential biphase modulated response signal 39. A '1 bit' gives rise to a full period of an 1875 Hz block voltage. A '0 bit' to half a period of a 937.5 Hz block voltage. Two consecutive 0 bits then give rise to a full period. A series of four 0 bits, also indicated by the hexadecimal digit 0, then gives, two periods of a 937.5 Hz block voltage, and a series of four 1 bits, also indicated by the hexadecimal digit F, then gives four periods of a 1875 kHz block voltage.

Figures 9 and 10 show this. By now loading the memory with the hexadecimal number pattern below, the same response signal is generated, as in the label of figure 1:

OOFFOOFFOOFFOOFFOOFFOOFFOOFFOOFF

Figure 11 shows this in part of the above pattern.

It is thus possible, by programming a special code in an identification label, as described in NL patent application no. 9201116 and the full custom integrated circuit used therein, to use this identification label as theft detection label. This makes it possible to use the same integrated circuit for both applications, which is attractive for economic reasons.

Claims (7)

A theft detection system, consisting of an interrogation and a detection unit and an electronic label, which generates an answer signal, characterized in that the answer signal consists of a frequency-switched subcarrier and which signal is produced by means of a modulation process converted into two high-frequency signal components, lying within the resonance curve of antenna circuit 1, such that these signal components are present in the secondary magnetic field of antenna coil L. The theft detection system according to claim 1, characterized in that the response signal in the electronic label is generated by a combination of at least three frequency divider circuits and an electronic switch, which is operated by the output signal of one of those frequency divider circuits . A theft detection system according to claim 1, characterized in that the reply signal is generated by means of a memory circuit in which a data pattern is programmed such that the reply signal referred to in claim 1 is formed. The theft detection system according to one or more of the preceding claims, characterized in that the response signal detection circuit consists of a circuit for demodulating an FSK signal, followed by a quadrature detector with a synchronous reference signal for detecting the frequency switching signal 9. The theft detection system according to one or more of the preceding claims, characterized in that the response signal detection circuit of the interrogation and detection unit comprises an additional narrow-band signal channel (18, 21), the frequency of which is situated between f , and f_, and which channel is used to determine the noise and interference signal level, based on which a detection threshold is determined. A theft detection system according to one or more of the preceding claims, characterized in that the response signal detection circuit 13 of the interrogation and detection unit is formed by a digital signal processor, programmed with an algorithm such that the frequency switching of the subcarrier is first demodulated. and then synchronous quadrature detection of that shift signal takes place. A theft detection system according to claim 6, characterized in that a third signal channel also functions in the detection algorithm, with which a detection threshold is determined on the basis of the noise and interference signal level.