DE4436977A1 - System for electronic article surveillance - Google Patents

Abstract

The electronic monitoring and alarm system is used to provide protection against the theft of articles. The system has a voltage controlled oscillator (2) that connects with a power amplifier (3) that outputs a signal to a transmitter (4) to create an electromagnetic field. The field produced interacts with a security hanger element (5) attached to the article that forms part of a resonator circuit. A receiver (6) responds to provide an input to a detector (7) coupled to an alarm (8) and responds to both amplitude and phase changes. If the article is passed through the field with the hanger attached the alarm is triggered.

Description

The invention relates to a system according to the Preamble of claim 1.

Electronic article surveillance systems covering the property unit with an oscillating circuit (coil and capacitor) equipped security trailer in a surveillance zone and especially in sales facilities Finding use are known in the art. There the respective resonance frequency of the individual fuse followers of a manufacturing tolerance of typical Subject to ± 10%, it is necessary to set the transmission frequency above to vary (sweep) a certain range to to be sure to prove all fuse tags can. To do this, there is usually a voltage controlled Oscillator (VCO) use that the desired Fre quenz range supplies. Its input is a sine, Triangle or stair tension applied, and on his Output is proportional to the input voltage Frequency. Alternatively, you can use digital-analog wall ler for frequency generation. The oscillator is in the Usually connected to an output stage that is in the pulse drove a transmitting antenna, which in turn a Magnetic field generated in the surveillance zone. A separate Receiving antenna is used to receive the signals which is a security tag in the surveillance zone testifies. This antenna is connected to a receiving circuit  closed, which is swept over the entire by the transmitter The frequency range is sensitive while the output of the receiver is connected to a detector which in the presence of a fuse tag in the Surveillance zone triggers an alarm so that the personnel the shop or the like, if applicable, theft can hinder. The detection is usually so real that if the received power changes - due to the presence of a fuse trailer - at a certain, constant transmission frequency, so constant time position during the sweep cycles Alarm is given.

A system of the generic type is known from EP 565 481 A1 known. On the one hand the Probability of proof of the safety trailer by to be able to improve an increase in sensitivity, on the other hand the probability of a false alarm increases reduce, the receiver has a mixer whose first input is connected to the receiving coil. The second input is charged with the transmission frequency strikes. The result is one at the outlet of the mixer DC signal, which is present in the presence of a fuse tag in the surveillance zone. This voltage is a bandpass filter and a ver stronger and finally an analog-to-digital converter of the Detector supplied.

EP 387 970 A1 discloses another, but similar one Investment. Here the mixer receives a reference signal from the transmitter gnal supplied with such a phase that a maximum output signal results if a fuse connection in the surveillance zone. However, if a Si gnal received, that by 90 ° relative to that of a Siche is out of phase, the output tension minimal.  

Mixers are therefore found in the known systems (Synchronous detectors) use that the - in the signal path subsequent - detection device only one Information about the product from the received ampli tude and a firm (although possibly fre sequence-dependent), from the phase shift between ge sent and received signal dependent factor lie remote. A disadvantage is that the Infor tion about the phase of the received signal unnecessary wisely lost, so that sometimes not by one Interference signals originating from fuse tags (their ampli tude, but not phase the signal of a resonance resonant circuit) to trigger one for the affected customers can cause embarrassing alarms.

Based on the prior art, the invention is Underlying problem, an attachment to electronic items to create surveillance that is characterized by low sensitivity sensitivity to interference signals and a high Distinguishes probability of detection.

According to the invention the object is achieved in that both the amplitude of the signals received and the Phase difference between the field of the transmitter and the received signals is evaluated.

The key idea is to make the decision about that Triggering an alarm using additional information tion - the phase shift between the broadcast and the received signal - to make it better between that generated by a fuse tag and other, from any sources of interference (such as transmitters, me metallic objects in the surveillance zone or the like.) differentiate originating output signals of the receiver to be able to.  

The main advantages of the invention are that the probability of triggering a false alarm mes is significantly reduced by sources of interference. Also the sensitivity, i.e. the detection probability Safety trailer safety has improved.

Due to the manufacturing tolerances of the resonance circles of fuse tags it proves to be necessary dig, the transmission frequency over a certain range vary to really all possibly in excess security zone contained in the safety zone can. This - known per se - variation of the broadcast frequency allows, not only at maximum ampli tude, so at the resonance frequency of a fuse signals detected by the receiving antenna, but also Si measured at other transmission frequencies gnale to decide whether to trigger an alarm Draw rate. In particular, the detected amplitude and phase measurements are used, that arise at transmission frequencies that are close to the Resonance frequency of a fuse trailer. At all other frequencies (except possible interference transmitter), however, the received amplitude is below of the noise level always present, so that there is no possibility here of securing trailers point.

An alarm is only triggered if the course of the Phases and amplitudes of the received signals depending frequency at least approximately matches a specified curve shape. These Curve shape corresponds to the curve of a siche trailer in the surveillance zone is expected. The at the individual, different transmission frequencies received signals are therefore stored and it will  continuously checked whether they (as a function of the send fre quenz) of the defined curve.

Furthermore, an alarm is only triggered if the amplitudes of the received signals a certain one Exceed threshold.

Based on the measured amplitudes and phases, the Half width and half frequency of the resonance curve ei nes possibly in the surveillance area Resonant circuit are evaluated. Based on both values die (the quotient of resonance frequency and half value wide corresponding) quality of the resonant circuit is calculated will. Only if they are within a certain inter valls lies, which corresponds to the usual, in the Siche used in each case to be secured an alarm is triggered. The advantage is that other resonant circuits with higher or lower quality no undesirable error can cause alarm.

As a synchronous detector is preferably a per se known so-called IQ detector for use. He points an output on which the in-phase component of Send and receive signal is present. On a second out the gear is 90 ° (or any other Angle) shifted portion of the received signal.

The following are embodiments of the invention hand of the drawings explained in more detail. They show in nice mathematical representation in

Figure 1 is a block circuit of the parts of the system used for sending and receiving.

Fig. 2 is a block circuit of a detection device;

Fig. 3-6 examples of received signals;

Fig. 7 is a flow chart for the detection of fuse tags.

In Fig. 1 is a for generating an electromagnetic field's in the monitoring zone and for receiving the Nende circuit is shown. By means of a voltage applied to the input ( 1 ), the output frequency of a voltage-controlled oscillator ( 2 ), a VCO known per se, is varied. The frequency of the power amplifier ( 3 ) voltage is thus varied by a change in the voltage at the input ( 1 ) such that it successively sweeps the entire range in which the resonance frequency of a fuse trailer ( 5 ) could be. The transmission signal amplified by the power amplifier ( 3 ) is fed to a transmission antenna ( 4 ) which emits an electromagnetic field of a corresponding frequency into the monitoring zone. A receiving antenna ( 6 ) serves to detect the electromagnetic fields generated by the transmitting antenna ( 4 ) and by the safety device contained in the surveillance zone ( 5 ); its output signal corresponding to the received field strength is fed to an input of a synchronous detector ( 7 ). The second input of the synchronous detector ( 7 ) is acted upon by a reference signal which is taken from the power amplifier ( 3 ). At the two outputs of the synchronous detector ( 7 ), the detection device ( 8 ) is connected. At an output of the synchronous detector ( 7 ), a voltage is present which corresponds to the in-phase component between the output voltage of the power amplifier ( 3 ) and the voltage supplied by the receiving antenna ( 6 ); the proportion of both voltages shifted by 90 ° is present at the other output. The - also known under the name IQ detector - synchronous detector ( 7 ) thus delivers the product from the input voltages at the first output and the product of an input voltage and a 90 ° phase-shifted input voltage at the second output. Any alarm device is connected to the output ( 9 ) of the detection device ( 8 ).

Fig. 2 shows a block diagram of a detection device ( 8 ). It contains a digital-to-analog converter ( 15 ), the output ( 10 ) of which is connected to the input ( 1 ) of the voltage-controlled oscillator ( 2 ). The digital-to-analog converter ( 15 ) thus controls the frequency emitted by the transmitting antenna ( 4 ). As a converter ( 15 ), a 12-bit embodiment is generally used, which makes it possible to set the output ( 10 ) to one of 4096 possible voltage values and to choose between 4096 different transmission frequencies.

In a read-only memory (ROM) ( 12 ) Pro programs control a microprocessor ( 11 ) such that it in turn affects the digital-to-analog converter ( 15 ) via an intermediate digital input-output circuit ( 16 ) so that the transmission frequency is varied in corresponding steps.

The two outputs of the synchronous detector ( 7 ) are each connected to an input ( 21, 22 ) of the detection device ( 8 ). The microprocessor ( 11 ) sets the digital-to-analog converter ( 15 ) to a constant output voltage for a period of, for example, one millisecond, in which the transmission frequency also remains the same. During this period, the two integrators ( 18 , 20 ) integrate the respective voltages at their - connected to the synchronous detector ( 7 ) - inputs ( 21 , 22 ). After this period, the microprocessor ( 11 ) initiates the two Analog-digital converter ( 17 , 19 ) to convert the output voltages of the integrators ( 18 , 20 ) into a digital value which is written into a memory (RAM) ( 13 ) via the digital input-output circuit ( 16 ). This process is successively repeated for all frequencies of the frequency range to be examined. After the end of a frequency run, the microprocessor ( 11 ) examines the data stored in the memory ( 13 ) to determine whether a backup trailer ( 5 ) is contained in the monitoring zone. The output ( 9 ) of the detection device ( 8 ) is connected to a bus ( 14 ) which connects the microprocessor ( 11 ) with the memories ( 12 , 13 ) and the digital input-output circuit ( 16 ).

As has already been established, the inputs ( 21 , 22 ) have information about the amplitude of the received signals and about the phase difference between the field of the transmitter and the received signals. For each frequency sweep, the values supplied by the two analog-digital converters ( 17 , 19 ) are therefore stored separately from one another.

The output voltage of the synchronous detector ( 7 ), which is present at the input ( 21 ) and is hereinafter referred to as "I", corresponds to the product of the transmitted and the received field strength, while at the other input ( 22 ) the voltage designated as "Q" lies, which corresponds to the product of the received field strength and a voltage that is 90 ° out of phase with the transmitted field strength. If a signal that is in phase with the transmitting field is received, a direct voltage is present at input ( 21 ), while there is no voltage at input ( 22 ). However, if the received signal is shifted by 90 ° relative to the transmitted signal, the input ( 21 ) becomes approximately zero and a DC voltage is present at the input ( 22 ).

FIG. 3 shows a typical course of the voltages at the inputs ( 21 , 22 ) as it results from a frequency sweep if no fuse tag ( 5 ) is contained in the monitoring zone. The voltage I at the input ( 21 ) is plotted on the vertical axis and the voltage Q at the input ( 22 ) is plotted on the horizontal axis. If the transmission frequency is varied, the run begins at the lowest frequency at point ( 23 ) and ends at the highest frequency at point ( 24 ). The artifact ( 25 ) is the result of an external interference that can originate from a radio station or a neighboring article surveillance system.

The amplitude of the received signal corresponds to the Ab stood from the origin, d. H. the length of the radius vector of the Point (Q, I) and is equal to the square root of the Sum of the square of I and the square of Q. The Pha senwinkel corresponds to the angle between the radius vector and the horizontal axis. The tangent of the angle ent thus speaks the quotient I / Q.

With reference to FIG. 3 can be seen that the amplitude of the received signal varies during the frequency sweep ((5) without a backup trailer). It is proportional to the degree of coupling between the transmitting antenna ( 4 ) and the receiving antenna ( 6 ). The phase of the received signal varies typically 300 ° during this cycle, which is a consequence of the signal's transit time through the system, ie transmitter and receiver. The phase shifts are mainly caused by the filter of the receiver. The transit time (ie delay) of the signal in the system shown here is approximately 400 ns, which results in a phase difference of 300 ° for signals between 7 and 9 MHz.

Fig. 4 shows the course of the voltages I and Q if a fuse tag ( 5 ) is included in the monitoring zone. The artifact ( 25 ) due to interference is still present. Added to this is a loop ( 26 ) generated by the safety trailer ( 5 ). The latter arises from the fact that the resonant circuit causes a rapid change in the phases and amplitudes (the field strength emitted by it again) when the transmission frequency is varied across its resonance frequency. This characteristic change in the phase and amplitude of the received signal forms the actual essence of the present invention. If any object - but not a security tag ( 5 ) - is in the monitoring zone, it also causes a change in the coupling between the receiving and transmitting antennas; however, the characteristic loop that creates a resonance circuit does not arise. This information makes it easy to decide whether a security tag is included in the monitoring zone. Interference signals such as the artifact ( 25 ) also do not produce a characteristic loop ( 26 ). This means that they, too, cannot generate an unwanted false alarm.

A preferred method according to which the detection device ( 8 ) can differentiate between fuse tags and other objects or interference signals on the basis of the information obtained is described in more detail below. In this embodiment, the microprocessor ( 11 ) is controlled by programs stored in the read-only memory ( 12 ) and processes the measurement values obtained in several frequency runs and stored in the memory ( 13 ).

A program contained in the read-only memory ( 12 ) calculates the average values of the voltages at the inputs ( 21 , 22 ) - that is, I and Q - at all frequencies of the variation range. The average is averaged over the last three (or any other number) runs. These average values thus contain information about the averaged amplitude and phase of the voltages received during the last frequency sweeps; they represent a saved background.

The amplitudes and phases of each subsequent run are compared to the stored averages. In concrete terms, the difference from the current and the mean measured values is calculated for each frequency and in turn stored in the memory ( 13 ). The differences make it very easy to see any differences between the current and the previous runs. Such differences can in particular be a consequence of the introduction of a security trailer ( 5 ) or another object into the monitoring zone, or can arise from short-term interference signals.

Fig. 5 shows the amplitudes and phases of a frequency run, in which a fuse tag ( 5 ) is included in the monitoring zone after the stored average values of the previous runs have been subtracted. The characteristic loop ( 26 ) of the resonance resonant circuit has thereby merged into a characteristic circle ( 27 ) which touches the origin of the coordinate system and whose center lies outside the origin.

Obviously the difference is from the current and always approximate the average measured values Zero, except for near the resonance frequency frequencies of a fuse trailer.  

If an object is brought into the monitoring zone that does not contain a resonance circuit, the coupling between the transmitting ( 4 ) and the receiving antenna ( 6 ) changes - and thus the received voltage - over the entire frequency range passed. The result is that the curve shown in Fig. 3 is enlarged, reduced or rotated. The differences arising in such a case from the current and the average measured values are shown in FIG. 6. As with a safety trailer ( 5 ), a characteristic circle is created, which, however, has fundamentally different properties. It has its center in the origin of the coordinate system and does not touch the origin. However, the center of the circle of a fuse tag ( 5 ) lies outside the origin and touches the latter. The programs contained in the read-only memory ( 12 ) differentiate between fuse tags ( 5 ) and other objects on the basis of these characteristic differences.

A disadvantage of using the difference between the current len and the average measured values is that the shape of the resulting circles is not exactly circular. The reason for this is that the degree of coupling between the transmitting ( 4 ) and receiving antenna ( 6 ) varies with the respective transmission frequency. These changes in the coupling distort the shape of the characteristic circles ( 27 , 28 ). It is therefore advisable to avoid the distortions by normalizing the measured values. The respective difference between the last measured (i.e. current) and the average measured values is calculated and then divided by the average measured values. Thus, at frequencies with strong coupling - at which high measured values arise - is divided by large numbers and divided by smaller numbers at small coupling. As a result, the amplitudes of the standardized measured values are approximately independent of frequency.

In particular, the standardized measured values make it possible to decide on the basis of the bandwidth of the resonant circuits detected whether a safety tag ( 5 ) is contained in the monitoring zone. As is known, conventional fuse trailers ( 5 ) have a quality between 50 and 200. The quality determines the bandwidth of the detectable signals, which corresponds to the number of measuring points of a circle in the IQ diagram. If an object creates a circle with too many or too few points, the conclusion suggests that it is not a security tag.

Finally, FIG. 7 shows a flow diagram according to which the microprocessor ( 11 ) can operate. First, the transmission frequency is run ( 29 ) through the frequency range to be examined. Then the calculation ( 30 ) of the difference to the previously recorded, average measured values takes place, wherein a standardization (division by the average measured values) can optionally be carried out. In the next step (31) it is checked whether the measured values in the IQ diagram can be represented by one or more circles. It is therefore examined whether the standardized measured values (or the differences between the current and the average measured values), which are referred to below as I '(f) and Q' (f), as a function of the transmission frequency f at least approximately the following equation can be represented:

I ′ (f) + i · Q ′ (f) = U₀ + U₁ exp (i · c · f).

I corresponds to the imaginary unit number. U₀ is a complex number that corresponds to the center of the circle in the IQ diagram. U₁ is the radius of the circle and exp is the known exponential function. c is real and a measure of the number of measuring points that a circle encompasses. If it contains many measuring points, c is larger than if it contains only a few measuring points. The microprocessor ( 11 ) ver tries in step (31) to evaluate the numbers c, U0 and U1, which result in a minimal deviation between the measured values I 'and Q' and the above equation. In the next step (32), possibly found circles - several of them can be found in one run, for example if several security tags with different resonance frequencies are contained in the monitoring zone - are examined separately. The first query ( 33 ) checks whether the center of the circle is at the origin, that is, whether U₀ is less than a specified value. If the result is yes, the process jumps to step (41) and determines that there is no safety tag ( 5 ). However, if the result in step (33) is no, a check ( 34 ) is carried out to determine whether the circle runs through the origin (ie whether the difference between the amount of U₁ and the amount of U₀ is less than a defined value). If the result is no, there is no fuse tag and the process continues with step (41). However, if the circle runs through the origin, a calculation ( 35 ) of the distance of its center from the origin (ie the amount of U₀) and a check ( 36 ) is carried out to determine whether this distance exceeds a threshold value. If the result is no, the process is also continued at step (41). If the threshold is exceeded, the quality of the fuse tag ( 5 ) is calculated ( 37 ) (it corresponds to the quotient of the resonance frequency and half-width). It is then checked in step (38) whether the quality is greater than a certain amount (e.g. 50). If the result is no, the process continues with step (41), otherwise a check ( 39 ) is carried out to determine whether the quality is less than a second value (e.g. 200). If the result is yes, alarm ( 40 ) is triggered, otherwise the process proceeds to step (41). After it has been determined in step (41) that the circuit examined does not correspond to a safety tag ( 5 ), it is checked in step (42) whether all circles identified in step (31) have been processed (examined). If this is not the case, the process jumps back to step (32) and the next circle is made available for further investigation. On the other hand, if all circles have been processed, the average measured values are updated in step (43), ie the most recent measurements are replaced by the most recent ones. A new run ( 29 ) then takes place over the frequency range.

Obviously, many other methods are conceivable, according to which detection of the characteristic circles of a Resonant circuit and the differentiation from others Objects that produce other circles is possible. Should it be for cost or speed reasons prove to be advantageous, the calculations with di gitalen (as described) or analog circuits or a combination of both.

As a result, you get a system that is much better between fuse tags and other objects below can divorce. It allows the detection to be relatively strong noisy environments where con conventional systems could not work.

Claims (11)

1. System for electronic article surveillance, with a transmitter which emits an electromagnetic field of variable transmission frequency by means of a transmission antenna ( 4 ) into a monitoring zone, and a receiver provided with a reception antenna ( 6 ), the output signals of which are fed to a detection device ( 8 ), which activates an alarm signal in the presence of a fuse tag ( 5 ) in the monitoring zone, the fuse tag ( 5 ) containing a resonant circuit, characterized in that both the amplitude of the received signals and the phase difference between the field of the transmitter and the received one Signals is evaluated. 2. System according to claim 1, characterized in that signals are evaluated at several different transmission frequencies, which are in particular in the vicinity of the Re resonance frequency of the fuse trailer ( 5 ). 3. Plant according to claim 2, characterized in that an alarm is only triggered if the course of the Phases and amplitudes of the received signals depending frequency at least approximately matches a specified curve shape.   4. Plant according to claim 3, characterized in that an alarm is only triggered if the amplitudes of the received signals a fixed threshold exceed. 5. Plant according to one of claims 3 or 4, characterized marked that an alarm is only triggered if the quality calculated on the basis of the received signals of a detected resonant circuit within a specified range. 6. Plant according to one of claims 1 to 5, characterized ge indicates that at each transmission frequency difference measurement values are determined by the difference between the Measured values of the last pass of the transmission fre quence and from the average values of a fixed number previous runs are calculated. 7. Plant according to claim 6, characterized in that Measured values normalized at each transmission frequency be true that the difference between the measured values of the each last pass of the transmission frequency and from the Averages of a fixed number of previous ones Runs through the respective average values divi is dated. 8. Installation according to one of claims 1 to 7, characterized in that two measured values are evaluated at each transmission frequency, the first measured value (I) being proportional to the proportion of the signals received by the receiving antenna ( 6 ) which is used for Field of the transmitter is the same phase, and the second measured value (Q) is proportional to the part of the signals received by the receiving antenna ( 6 ), which are shifted by a fixed phase angle of 90 ° relative to the field of the transmitter. 9. Plant according to claim 8, characterized in that it is checked whether the differential measured values or the normalized ten measured values (I 'and Q') at the transmission frequencies f at at least approximately by the equation I ′ (f) + i · Q ′ (f) = U₀ + U₁ exp (i · c · f), can be represented, where U₁ and c are real numbers and U₀ is a complex number. 10. Plant according to claim 9, characterized in that an alarm is only triggered if the amount of U₀ is approximately equal to the amount of U₁. 11. Plant according to claim 9 or 10, characterized indicates that an alarm is only triggered if the amount of U₀ and / or U₁ a fixed Threshold exceeded.