NL9500397A - Item security system with a pseudo-random generator - Google Patents

Abstract

An item security system of the pulse-detection type, in which the time intervals between consecutive interrogation signals are defined by a pseudo-random code and in which a digital signal processor synchronously accumulates the responses of a label. With this, detection sensitivity is improved, vulnerability to noise signals is reduced, and a plurality of detection systems of the same type can be positioned next to each other with no need for reciprocal synchronization.

Description

T.W.H. Fockens

Article protection system with a pseudo-random generator.

The invention relates to article protection systems based on resonant electronic labels, and in particular those article protection systems, in which a transmitter emits a short, pulse-shaped interrogation signal, and in which a resonant label is flung by this interrogation signal, such that after the under-question signal the resonating label continues to loop independently for some time, with that winding slowly fading out.

This rerolling is then detected in a receiver circuit, after which the presence of such a label is demonstrated and an alarm can be generated.

This category of article protection systems is referred to as pulse naslin ger, or pulse listening, systems.

International patent application WO84 / 04191 of 2 M Security System Aps (Denmark) gives an example of such an article security system. This system uses a resonance label consisting of an air coil and a capacitor. The resonant frequency is in the range of a few mHz and therefore belongs to the so-called radio frequency labels.

Another category of resonant labels are the so-called Accousto-Mechanical labels. They use the mechanical resonance of a vibration plate. This vibration plate is made of a metal alloy which exhibits the so-called magnetostriction effect. The magnetic constriction effect, together with a pre-magnetization plate, creates a coupling between an external alternating magnetic field and the mechanical vibration of the vibration plate. This type of label therefore also exhibits the post-cranking effect as in the above label with coil and capacitor.

European Patent Application No. 0219618 to Allied Corporation (USA) gives an example of an article security system using an Accou-sto-Mechanical label. The operating frequency of this type of label is in the range of 30 - 150 kHz.

Figure 1 shows a block diagram of an article protection system of the pulse listening principle according to the prior art. A transmitter circuit 1 momentarily transmits an interrogation signal in the form of a wave packet (burst), therefor controlled by transmitter pulse 2, which is generated by control circuit 3. The magnetic field generated by transmitter coil 4 causes label 5 to oscillate. The secondary magnetic field generated thereby induces a label signal in receiver coil 6. Receiver circuit 8 amplifies and rectifies this label signal, so that a voltage 9 is output at the output of 8, which is proportional to the instantaneous amplitude of the received label signal. Receiver circuit 8 is active only during post-label run. This is indicated by the activating signal 10, which is generated by receiver control circuit 11, after reception of transmit pulse 2. Sampling switches 12 and 13, controlled by sampling pulses 14 and 15, each take a sample from signal 9, which samples in comparison circuit 16 are compared with each other, and compared with two fixed threshold values.

Figure 2 shows a time diagram associated with the circuit of Figure 1. Signal 17 indicates the high-frequency interrogation signal as emitted by headpiece 4. The label signal 7 increases in amplitude as long as the interrogation signal is present (from time t0 to time t1). Then the amplitude gradually drops to zero.

During the interrogation signal, the receiver circuit 8 is not active, and thus the activation signal 10 is low. After the instant t2, the activating signal becomes high, thereby activating the receiver, so that the output signal 9 shows the amplitude of the label signal 7 until the instant t5, when the amplitude of the label signal has practically approached zero.

It is necessary that receiver circuit 8 is not active during the transmission of the interrogation signal, because the interrogation signal induces a high voltage across receiver coil 6, which would disturb the proper functioning of the receiver circuit after a long time.

Sampling switch 12 is momentarily closed at a time t3, whereby a sample is taken of the amplitude of the label signal at a time when that amplitude is still high. At time t4, a second sample is taken by sampling switch 13. At this time, the amplitude has decreased. The relationship between the two amplitudes depends on the difference in time, t4 - t3, and the Q factor of the resonance in the label. By now testing in comparison circuit 16 for a fixed ratio of the received signal levels at times t3 and t4, it can be tested whether the received signal indeed comes from a label. Circuit 16 will also test whether both signal samples are stronger than a fixed minimum signal threshold. If these conditions are met, an alarm signal will be generated by beeper 18. This entire cycle repeats as soon as control circuit 3 issues a new transmit pulse.

However, if, in addition to the label signal, disturbing signals are also received, for example radio signals, which also use this frequency band, other article protection systems in the vicinity, or interference signals from lighting systems or other sources of electromagnetic interference, the output signal of the receiver circuit 8 does not approach zero. The measured amplitude ratio between times t3 and t4 will then deviate, with the result that a label is no longer recognized as such, and an alarm is therefore missed.

It is therefore known from practice that article protection systems, based on the pulse-listening principle, function very effectively in situations without interference signals, but that this effectiveness decreases rapidly if interference signals are present, even if these interference signals are still relatively have a low signal level.

In article protection systems of this type, which are the state of the art, these transmit pulses are given in a fixed rhythm, in many practical embodiments the transmit pulses are derived from the 50 Hz mains voltage, using three or four phases for one period of to be able to generate a transmission pulse at three or four different moments. If a number of detection systems are lined up in a wide outlet of a store, it is necessary that all three systems send their interrogation signal at different times. That moment must not coincide with a period of time in which the receiver of another system is actually active, because the reception will then be disturbed by that interrogation signal. Simultaneous transmission of the interrogation signal in all systems in the output is also not possible, because it is no longer easy to determine when a label is passed, through which detection system the label passed.

The conclusion is that if multiple pulse listening type article protection systems are located in the same environment, and if these systems are pulses with a fixed pulse repetition frequency, it is necessary to synchronize these systems to one another or to a common synchronizing signal source.

It is the object of the invention to adapt the above-mentioned systems such that systems which, when placed in close proximity to each other, do not require mutual synchronization, and that these systems are less vulnerable to external interference signals.

Figure 3 shows a block diagram of an article security system according to the invention, and Figure 4 shows the associated time diagram.

The transmitter circuit 1 briefly transmits an interrogation signal 17 via transmitter coil 4. via resonant label 5 and receiver coil 6, receiver circuit 19 receives the label signal 7. This receiver circuit can be made active with activating signal 10. The output signal of receiver circuit 19 is uniform but amplified with respect to of the input signal, or it is frequency shifted, amplified, and contains a uniform envelope as the input signal. This signal is converted by Analog-Digital converter 20 into a stream of numerical data, which is processed by the Digital Signal Processor 21 using a detection algorithm.

Transmit coil 4 and receive coil 6 can also be combined into one transmit / receive coil. In that case, both transmitter circuit 1 and receiver circuit 19 are connected to this transmit / receive coil via a transmit / receive switch.

This is not indicated in figure 3 and is not important for the operating principle of the system according to the invention.

The time t0, at which the transmit pulse becomes high and at which the interrogation signal begins, is determined by the variable delay circuit 23. The pseudo-random generator 24 generates a code word prior to each transmit pulse, which delays the delay in the variable delay circuit 23 determines. This pseudo-random generator can generate different pseudo-random code sequences. The current sequence is selected by the code select input. Transmit pulse 2 causes a subsequent interrogation signal to be sent via fixed delay 22 and variable delay 23. This feedback maintains the generation of transmit pulses, the time span between two consecutive pulses always differing and determined by the pseudo-random code. Before starting the cycle of transmit pulses, and in case the generation of the transmit pulses stops for any reason, the watchdog circuit 25 delivers a start pulse to delay circuit 22 or delay circuit 23, thereby restarting the cycle. Transmit pulse 2 generates activation pulse 10 via receiver control circuit 11 similar to that which occurs in the prior art systems described above.

Figure 4 shows the time diagram associated with the circuit according to the invention. Signal 17 is the broadcast interrogation signal and signal 7 is the response of the label thereto. Transmit pulse 2 is also indicated, coinciding in time with interrogation signal 17, and starting at time t0 and ending at time t1. The amplitude of label signal 7 is maximum at time t1, but the receiver can only be switched on after all transmission energy has disappeared from the headpiece. This is the case at time t2. At time t5, the amplitude of the label signal has fallen to such an extent that it can no longer contribute to the detection of the signal, so that at that time the receiver can be switched off again. This is indicated by the activating signal 10. Signal 26 is the output signal of receiver circuit 19. As already mentioned, signal 26 is a modulated carrier wave which may or may not have been transformed in frequency into an intermediate frequency signal according to the methods known for this in radio engineering.

It is this signal 26 which is fed to one or more Analog-Digital converter 20 which supplies a digital data stream to Digital Signal Processor 21. Line 27 indicates the delay that the fixed delay circuit 22 generates. This delay lasts from time t0 to t6, where t6 comes later than t5. From the moment t6, a new interrogation signal may be transmitted. The timing of the next interrogation signal, t7, is determined by the pseudo-random delay of the variable delay circuit 23, which delay is indicated by line 28.

Analog-digital conversion circuit 20 and Digital Signal Processor 21 are restarted after each interrogation signal by the activating signal 10, which has a constant delay t2 - t0 with respect to the transmit signal 17, and thus with respect to the tag signal.

The digital signal processor is therefore able to synchronously accumulate the received responses from a label as a result of a series of consecutive interrogation signals.

Also, received noise and interfering signals, and noise produced by the receiving circuit itself, are not accumulated in this way, because these signal components are asynchronous. In this way it is possible the signal / noise or. significantly improve the signal-to-interference ratio, thereby improving sensitivity to weak label signals, and significantly reducing the negative effects of interference signals such as radio signals and interference signals from adjacent article protection systems.

This operating principle can also be used to make two similar article protection systems function side by side. By setting the pseudo-random code generators to different codes, so that firstly the probability of an interrogation signal from one system falling into the reception period of the other system is small, and secondly, if this happens, the resulting interference signal is asynchronous, so that it does not contribute to the accumulated signal energy.

The result is therefore that two or more similar systems can be placed next to each other, without having to synchronize them mutually.

Claims (4)

An article protection system of the pulse listening type, characterized in that the time intervals between successive interrogation signals are variable. An article security system according to claim 1, characterized in that the time intervals between successive interrogation signals are determined by a pseudo-random code. An article security system according to claim 1 or 2, characterized in that the received label signal is converted into a digital data stream. An article security system according to claim 3, characterized in that the digital data stream in a digital signal processor is processed on an algorithm such that only those signals which are synchronous with both the timing and carrier frequency of the interrogation signals are accumulated.