DE3828691A1 - Magnetic marker anti-shoplifting system for supermarket - has evaluator responding to content of energy levels in band to indicate marker presence - Google Patents

Abstract

An interrogator creates a magnetic field at a selected frequency in a monitor zone. It includes a receiver establishing disturbances in the field of the zone to give out a characteristic signal. The receiver has a channel for giving out a signal content characteristic for the energy levels in one resp. band of a number of bands of the signal. An evaluator responds to the signal contents to compare the energy levels which are characterised by the signal content, with the object of evaluating whether there is an article present in the zone. There are pref. at least three bands, and the first signal content for a resp. band of the number of bands, has signal levels which are characteristic for the energy in that band. The evaluator, in the course of evaluation compares these levels.

Description

The invention relates to electronic article surveillance systems or systems and relates in particular on electronic article surveillance systems or systems in which magnetic markers for the Monitoring purposes are used.

The state of the art

In the present application, the following publications are cited regarding the prior art:
US-PS 46 60 025 and 46 86 516 (both correspond to DE-OS 35 41 536); DE-OS 37 15 387).

Electronic article surveillance systems or systems, in which magnetic markers are attached to those of surveillance underlying articles and goods for the purpose of theft protection are already known. At These systems or systems are available with the help of an antenna system a query or monitoring zone under the influence of an alternating magnetic field. When a with a suitable magnetic marker Items get into this query or surveillance zone, then causes the presence of the marker a disturbance in the magnetic field in this zone.  

This field in the interrogation or monitoring zone is sensed by means of a receiving antenna, the output of which contains this interference. A detector analyzes the signal coming from the receiver in order to estimate or evaluate whether a disturbance in the magnetic field has occurred and whether such a disturbance results from a marker. If applicable, an alarm is triggered as an indication that a marker is passing the monitoring or interrogation zone. In known electronic article surveillance systems, the disturbances generated by a magnetic marker in the magnetic field are detected with the aid of methods which use frequency domain or time domain analysis techniques. If frequency domain methods are typically used, then the frequency content of the received signal is examined with respect to harmonics of the fundamental frequency, ie the driver frequency of the applied field. These harmonics are generated as a result of a non-linear hysteresis characteristic of the magnetic marker. By comparing the relative amplitudes of the detected harmonics, an indication of the frequency spectrum of the signal is obtained. Using certain decision criteria, this spectrum is then compared to a spectrum that can be expected from a valid marker, and a decision regarding the presence of the marker is achieved in this way. The use of frequency domain analysis methods is particularly desirable when conditions characterized by noise or interference are to be expected, but the response time of the known system is too long due to the fact that filters with a high Q factor are required to match the harmonics generated isolate.

With a time domain analysis based Method becomes a time domain pulse (or a pulse train) of the received signal with regard to its Pulse shape and its time shift relative to one Basic phase of the generated field is analyzed. At this Kind of analysis, the form of the signal is considerable Way through the amplitude and phase characteristics of the one used in the acquisition process Filtering affected, as well as changes in the received signal due to variations in the generated driver field. Because this analysis amplitude thresholds used that above the ambient noise level the application of this procedure is on most desirable in those cases where a high signal / interference ratio is present.

In order to be able to estimate or assess whether or frequency domain analysis procedures within the Surveillance system should be applied, it is essential that the acquisition process is able to between glitches or changes in the field too differentiate from valid markers, and such disturbances or changes due to other sources independent of the markers. A system failure, the necessary distinction cause false alarm triggers, which the blamelessness and the usability of the whole Systems questioned in an extraordinary way put.  

The problem of a reliable distinction between interference caused by magnetic markers and between third-party interference independent of markers are particularly important in retail stores, especially supermarkets or the like, in which metallic fittings, metallic Counters, metallic shopping carts, intoxication or Interference generators (e.g. laser scanners, digital scales, Credit card and barcode readers, conveyor belts and the like more) are abundant. This equipment creates "rough" electronic environmental conditions and can malfunction in the detection system cause which originate from valid markers Hide or mask signals and / or as from signals from valid markers appear. Hereby is the reliability of the surveillance system in significantly questioned.

The currently available surveillance systems or Systems do not completely solve this reliability problem and also suffer from other disadvantages. So are already spacious or large-scale systems have been carried out at which caused a flow through the surveillance zone becomes. Also, with these systems, one is proportionate strong field. In the end result however, these fields often span the Interrogation or surveillance zone, reducing the likelihood the occurrence of disturbances caused by Marker-independent sources or from outside the surveillance zone markers located, still increased becomes. In addition, in these known systems Follow their overreaches in connection with a  electronic noise the system sensitivity in considerable Scope has been reduced. This often leads at a relatively low acquisition rate and an undesirable number of false alarm triggers.

It has therefore recently become a magnetic marker have been developed with properties that some the aforementioned, in the known plants or Systems encountered problems are mitigated can. A magnetic marker of this type emerges from the US-PS 46 60 025 with the designation "Magnetic marker for article monitoring with a hysteresis loop major Barkhausen discontinuities "as already known forth. The marker has a magnetic material internal pent-up mechanical tension on and has a magnetic hysteresis loop that passes through large Barkhausen discontinuities is marked. In As a result, this marker occurs when this an external magnetic field above a threshold is exposed to a regenerative reversal its magnetic polarization.

Since it is possible that this regenerative reversal at a relatively low threshold takes place, can also advantageously the required for the marker generated field kept relatively weak will. In addition, the result of the marker in in the form of a step function magnetic polarization in field perturbations, rich in are high harmonics, which makes detection easier and becomes easier.  

The marker according to the aforementioned US-PS also points still the advantage that he can with the help of various Methods can be disabled, as in the U.S. PS 46 86 516 with the designation "procedure, system and Device for article monitoring "described in more detail is. By mitigating the inner, pent-up mechanical tension in the marker or by Part of the marker becomes crystallized slightly disabled so that it can be queried or Surveillance zone can go through without doing so to cause any alarm trigger.

A "marker" can also be used as a "tag" d. H. "Security tag" or "security tag" will.

An improved system for magnetic article surveillance is also described in DE-OS 37 15 387 (Union priority: June 30, 1986 US 8 80 138). At this known system is a magnetic shield provided in the lateral edge areas of the surveillance zone arranged and capable of Intensity of the magnetic field outside this zone to reduce. The shield is also capable of: the interference caused by the shield itself easily distinguished and blocked in the magnetic field or can be suppressed.

It is to achieve the aforementioned properties required that the magnetic material of the shield a sufficiently high, specific electrical resistance at a given Permeability and a predetermined frequency of the generated  Magnetic field has a penetration depth or to achieve skin depth that is essential greater than the layer thickness of the magnetic shield is. Furthermore, the magnetic material must have a saturation flux density have greater than the maximum Flux density depending on the shielding of the positive and negative maximum amplitudes of the Magnetic field is generated. After all, it has to magnetic material still have the property that it at the peak values of the generated magnetic field this responds, but near the zero crossings little or no responsiveness having.

As can further be seen from DE-OS 37 15 387 have Shields made of ferrites or pressed Iron powder are made in the foregoing explained characteristics, so that a ratio the front, d. H. within the surveillance zone existing magnetic field to the rear, d. H. outside the surveillance zone and behind the existing magnetic field shield of at least 10: 1 is achievable. A special one Ferrite, through which this ratio of 10: 1 is achieved and which is also a maximum Response to the generated maximum field amplitudes and minimal response at the zero crossings has, for. B. a ferrite from the manufacturer TDK Corporation of Tokyo, Japan, which trades in Q 5 B referred to as. Similar characteristics are included pressed iron powder, which is preferably not sintered and is made of approximately 99% iron with approximately 1% consists of trace elements to which FeP, H₂, C, Mn and S  belong and maybe very small amounts of others Elements.

If the magnetic shield is a Laminate consists of a plurality of thin layers, which are glued together and electrically from each other are isolated, then the depth of penetration of everyone such a layer much larger than the layer thickness self.

The need for minimal shielding response near the zero crossings of the generated field arises from the fact that the magnetic Markers and in particular the markers according to US Pat. No. 4,660,025 maximum responsiveness in the Zero crossings or near the zero crossings of the generated field and minimal responsiveness at the peak values of this field. By choosing a magnetic shielding material with quasi-reversed properties interference caused by this shielding easily capture and eliminate without an overlay with the detection of disturbances caused by markers occurs. DE-OS 37 15 387 also shows that in combination with that discussed above magnetic shield an auxiliary shield electrically conductive material can be provided. This auxiliary shield is behind the magnetic Shielding arranged and damped by eddy current losses those interference from markers or external sources of noise or interference that arise are outside the query or surveillance zone.  

Although the magnetic markers discussed above as well as the magnetic shielding already significant Improvements for magnetic, electronic article surveillance systems represent systems there is still a need for further improvements and Perfections of such plants and systems, in particular in terms of reliability, compactness and Freedom from false alarm triggers.

The task

With regard to the one described above Situation of the present invention is thus as Main task based on an improved facility or a improved system for electronic article surveillance to accomplish.

In particular, the invention is based on the object an improved article surveillance system according to the Preamble of claim 1 to create the particular has the following characteristics: Increased Control functions; advanced and improved Possibilities of detection, d. H. of finding the Presence of magnetic markers Articles in a query or surveillance zone; opportunities the implementation of clear frequency range and unique time domain detection method; Possibilities of responding and self-adjustment due to changes in the environment including changes in noise or interference levels; Equipped with an improved antenna arrangement and an improved base or stand for the Antenna arrangement; Equipped with an improved  Comb bandpass filter and an improved comb notch filter; and finally the possibility of using it a magnetic marker according to US-PS 46 66 025 and a shield according to DE-OS 37 15 387.

solution

The task underlying the invention is based on from a surveillance system in accordance with the preamble of claim 1, according to the invention by combining the features according to the characteristic Part of claim 1 solved. Further advantageous embodiments of the invention result from claims 2-62.

The operation of a magnetic, electronic article surveillance system is preferred controllable with the help of a microcomputer.

In particular, a detector circuit is provided here which circuits for frequency and time domain display includes and which one received from an interrogation or surveillance zone Signal processed. There are filters including one improved comb bandpass filter provided that the received signal in the detector circuit to harmonics limit or restrict the generated field.

At least two frequency bands of the received signal are isolated by a frequency detection circuit and the signal content will be in each band certainly. At the same time it is done by the time domain circuit a sampling of the received  Signal to thereby create a digitized signal produce. The results of these operations are then through clear decision programs in the microcomputer processed further. These programs are on improved criteria for valid markers and have threshold values that indicate ambient noise as well as an undesirable response behavior are obtained from objects other than valid markers.

Decision information is stored in the microcomputer of the time and frequency domain programs for one generated every half period of the generated field. These Information is used for a time range and a frequency range counter to the latest To stand. Depending on that Counters reach preselected meter readings, alarm trigger signals are generated.

Another program in the microcomputer processes selected ones Values of successive digitized Signals in accordance with thresholds that are based on the expected characteristics of undesirable, high quality Secondary interference (electronic noise peaks) based. Another program in the microcomputer is due to changes in the fundamental frequency component lifted off, which is also by the Detection circuit is isolated. The exits these latter two programs are used to an alarm is triggered by the time and frequency domain counters to prevent.

In addition, the Noise or interference level is monitored as soon as the system is in  Has been put into operation, and periodically updated Updated, as well as the level of the fundamental frequency component. The computer programs are then with updated these values so that the facility changes according to the changes in the environment can change dynamically.

A comb filter provided in the system according to the invention uses a delay line in the form of an integrated circuit with a feedback in order to achieve the desired "comb response". This results in filtering with high Q values, while avoiding the need for individual capacities of conventional filters and the associated disadvantages (quantity, bulkiness).

The monitoring system according to the invention also closes Receiving and transmitting antenna arrangements with one in are trained in such a way that undesirable and zeros normally present in the generated field be reduced. One is in particular Receiving antenna arrangement provided, which first and second upper loops that interlock in one nested relationship are arranged, and further third and fourth lower loops, also in arranged in a nested relationship are, the first and second loops with the assigned third and fourth loops respectively form octagonal configurations. The loops are electrically in series and in phase switched that the upper loops the same first Phase and the lower loops the same second phase  which is opposite to the first phase. According to a preferred embodiment of this antenna arrangement is a section of each of these loops in the contact area of the respective eight-shaped configuration inclined obliquely to the horizontal.

It is also a transmit antenna arrangement as well with a nested arrangement of Grinding provided. This antenna arrangement runs the current in an inner loop in the same direction as the current in an outer Loop, with the axis referring to the inner loop on the axis of the outer loop by a preselected one Angular dimension is rotated, preferably by 45 °.

The antenna arrangements of the above explained Plant according to the invention are in pedestals or Stands stored, which are designed in such a way that the antenna arrays are immobilized and a physical or structural unity or wholeness (integrity) form. This is done through a manufacturing process reached, where the area between the Walls of the base frame are filled with a foam and during the curing of the foam on the entire arrangement of pressure is applied. The resulting, solidified foam encapsulates the antennas one, thus making them immobile while the foam the cavity between the walls of the base or stand essentially fills, which the Base the desired reinforcement and stiffening is awarded.  

It should also be noted that within the framework of the present invention of a marker use is made as described in US Pat. No. 4,660,025 and that in conjunction with those in the base housed antenna arrays a shield according to DE-OS 37 15 387 is used. In this The system according to the invention is also a trap capable of the shielding peak pulse discussed above in influencing the mode of operation to prevent the plant.

For a more detailed explanation of the invention, its further The following description serves features and advantages of preferred embodiments using the Drawings. In these shows:

Fig. 1 is a simplified block diagram of a magnetic, electronic monitoring system which is assigned to a query or surveillance zone;

Fig. 2A, 2B and 2C are more detailed block diagrams of the monitoring system of FIG. 1;

Fig. 3 is a diagram of signals with the waveforms of various in the monitoring system produced according to Figures 2A-2C.

FIG. 4 shows a tabular overview of a number of parameters which are decisive for the monitoring system according to FIGS. 2A-2C, representative numerical values for these parameters being indicated;

5A and 5B are flow charts relating to the programs 2A to 2C contained in a microcomputer of the monitoring system of FIG, namely on the one hand for a time-domain signal analysis and the other for a frequency domain signal analysis..;

Fig. 6 is a flowchart relating to a program included in the microcomputer of the monitoring system shown in Figs. 2A-2C for "peak" detection signal analysis;

FIGS. 7 and 8 are circuit diagrams of a comb bandpass filter to the block diagram in accordance with Figure 2A shown.

Fig. 9, 9A and 9B are circuit diagrams for a comb-notch or comb trap filter to the block diagram in accordance with Figure 2A shown.

FIG. 10 is a schematic plan view of a receiving antenna arrangement for the monitoring system according to Figures 2A-2C.

FIG. 11 is a schematic plan view of a transmitting antenna arrangement for the monitoring system according to Figures 2A-2C.

Fig. 12 (I) is a schematic diagram showing the zero zones of a conventional receiving antenna arrangement;

FIG. 12 (II) in a further schematic, graphic representation the zero zones of the receiving antenna arrangement according to FIG. 10;

Fig. 13 is a schematic, perspective view of a group consisting of pedestals or posts arrangement for accommodating the antenna arrays of FIGS. 10 and 11;

FIG. 14 shows a cross-sectional view through one of the pedestals or stands according to FIG. 13; and

Fig. 15 is a schematic graphical representation of the microcomputer of the monitoring system shown in FIGS. 2A-2C with meters and with a maximum amplitude stack.

In Fig. 1, a magnetic, electronic monitoring system is generally designated 1 . The monitoring system 1 has an alternating current driver 11 which supplies a sinusoidal alternating current of high polarity and a corresponding voltage at a fundamental frequency Fo to a transmitting antenna 12 . As a result, this transmitting antenna 12 generates an alternating magnetic field with the frequency Fo , which extends into an interrogation or monitoring zone 13 , through which articles or goods subject to monitoring, e.g. B. an article 14 must go through.

A marker 15 made of a magnetic material is attached to this article 14 . The marker 15 thus generates disturbances in the magnetic flux which have harmonic vibrations of the fundamental frequency Fo and are sensed by a receiving antenna 16 . This receiving antenna 16 converts the interference into an electrical signal which, according to FIG. 1, is passed on to a block designated as a control unit interface and receiver 17 . Control unit interface and receiver 17 derive frequency and time domain information from the received signal, which are made available by the interface of a central control unit 18 .

This central control unit 18 analyzes the information received in accordance with certain decision criteria. If a decision criterion indicates the presence of a valid marker, then the central control unit 18 addresses a downstream alarm unit 19 . As a result, this alarm unit 19 is activated to provide an indication that an article 14 is within the query or monitoring zone 13 .

The central control unit 18 also controls the AC driver 11 and receives information regarding the state of the AC current and the AC voltage for the transmitting antenna 12 . In addition, the central control unit 18 provides timing, address and other information to the control unit interface and receiver 17 , as will be described in more detail below.

According to a preferred embodiment of the monitoring system 1 , the marker 15 is a marker of the type according to US Pat. No. 4,660,025, which has a hysteresis loop with a large Barkhausen discontinuity or a large change in the level function in the flow, each time when the generated field reverses its polarity and exceeds a relatively low threshold. The disturbances in the field caused by the marker 15 therefore occur in the vicinity of the zero crossings of the generated field and are rich in harmonics of the fundamental frequency Fo . Furthermore, the signal to be expected to be ascribed to the marker is a pulse with an extremely narrow pulse width (less than 200 µsec).

In addition, in a preferred embodiment of the monitoring system 1, the transmitting antenna 12 and the receiving antenna 16 are provided with shields, as already explained above, a measure which leads to so-called "shielding peaks" in the received signal, to which the monitoring system 1 Takes into account and suppresses them.

Although according to a preferred embodiment of the invention Monitoring system from a marker according to US-PS 46 60 025 and a shield according to DE-OS 37 15 387 use can be made those on which the present invention is based Principles on other designs of markers and Antennas are applied. In such a case  the possibility of designing the monitoring system and the decision programs too modify to reflect the special properties of the Special markers and / or antennas used To take into account.

Further details of the central control unit 18 and control unit interface and receiver 17 will now be explained with reference to FIGS. 2A, 2B and 2C. The central control unit 18 is equipped with a microcomputer 21 , which is, for example, an 8031 microcomputer from the manufacturer Intel and which effects a primary sequence control with regard to the operating sequence of the monitoring system 1 . The microcomputer 21 communicates with a main program memory (EPROM) 22 which contains the main programs and the decision routines for the microcomputer. Furthermore, a second memory, ie a permanent memory (NOVRAM) 23, is provided for storing the operating parameters of the monitoring system 1 . This NOVRAM 23 records these parameters in the event of a loss of energy or power in the monitoring system 1 .

A programming interface 24 is also provided for external communication with the microcomputer 21 . The interface 24 makes it possible to set the operation parameters in the NOVRAM 23 for the purpose of adapting to the special, on-site conditions under which the monitoring system is currently being used.

Furthermore, the programming interface 24 permits a large number of diagnostic measures to be carried out for the purpose of checking the operation of the monitoring system 1 .

The microcomputer 21 delivers a signal relating to the status of the monitoring system to a status indicator 25 . A watchdog circuitry 26, upon turning on the power supply, applies a reset pulse to the microcomputer 21 , which commands the computer to begin executing the main program. A help interface 27 is also provided.

An address switch 28 allows the microcomputer 21 to also address other components both in the central control unit 18 and in the control unit interface and the receiver 17 . An analog / digital converter 29 is also provided, which serves to convert the analog data supplied by the receiver 17 into digital data which are processed by the microcomputer 21 . A general CPU clock generator 31 operating at 12 MHz supplies a primary clock signal for the monitoring system 1 .

The CPU clock 31 feeds both the microcomputer 21 and a programmable divider 32 . This generates a number of further clock signals which are synchronized with the main clock signal and are administered to the microcomputer and other components of the monitoring system.

The central control unit 18 also comprises a driver channel 30 (see FIG. 2B) for providing the driver current for the transmission antenna 12 . The driver channel 30 has low-pass filters 33 (see FIG. 2A), a stage setting multiplexer 32 and a buffer amplifier 35 (see FIG. 2B).

FIG. 2C also shows a power amplifier 79 of the AC driver 11 and a current sensor 81 which is connected to the transmission antenna 12 . FIG. 2C also shows an acoustic alarm device 82 , a visual alarm device 83 , an event counter 84 and an alarm interface circuit 85 of the alarm unit 19 .

As already explained above, the programming interface 24 is used to address the microcomputer 21 so that the special operating parameters desired in the monitoring system are set in the permanent memory (NOVRAM) 23 . In most cases, these operating parameters depend on the environment in which the monitoring system 1 is used. Representative operating parameters are seen from the Fig. 4, which are programmable in the NOVRAM 23, wherein exemplary values are given for these parameters.

The permanent memory (NOVRAM) 23 is also equipped with an initial set of parameter values, namely so-called standard specifications that are specified for a nominal or nominal environment. These parameters can be used to control the operating sequence of the monitoring system in the event that the so-called nominal ambient conditions are present.

Once the parameters of the monitoring system are set, the microcomputer 21 continues to set the current for the transmit antenna through the driver channel 30 . For this purpose, the microcomputer 21 generates a pulse-shaped signal with the desired driver frequency Fo . This pulse signal is derived from a further pulse signal SSB 1 , which is twice the desired frequency (2 Fo) and which is derived by the programmable plate 32 from the 12 MHz clock signal (CPU clock generator 31 ).

The pulse signal with the frequency Fo is then sent from the microcomputer 21 through the low-pass filter 33 and converted into a sinusoidal AC signal. The resulting AC signal then passes through the level adjustment multiplexer 34 which has been addressed by the microcomputer to set an initial current level. The AC signal then passes through the buffer amplifier 35 and is forwarded to the power amplifier 79 of the AC driver 11 for the transmission antenna 12 . The output of the power amplifier 79 applies the frequency Fo to the transmitting antenna 12 , so that a magnetic field with this frequency is built up in the interrogation or monitoring zone 13 .

The current sensor 81 senses the current in the coils of the transmission antenna 12 and delivers its output further to an input TXI of an analog multiplexer 75 , which is contained in block 17 (control unit interface and receiver). This input is addressed by the microcomputer 21 through the address switch 28 and the address input ADDR of the multiplexer 78 and the transmission antenna current is read via an analog / digital converter 29 .

The microcomputer 21 then compares this current value with the predetermined current value stored in the NOVRAM 23 and, if there is a difference between these two values, the microcomputer 21 addresses the stage setting multiplexer 34 via the address switch 28 to thereby change the To cause current to the predetermined value. This process continues until the current level is set to the predetermined level.

Once the current level is set, the microcomputer 21 adjusts the phase of the driver current via the Fo signal. This adjustment is made in such a way that the peak levels of the driver current are exactly in the zero-level intervals of the SSB 1 signal, while the zero-crossings of the driver current are exactly at the logical one-level intervals. This relationship, which is illustrated by the graphical representation of waveforms 401 and 402 in FIG. 3, allows the SSB 1 signal to be blanked out during the maximum amplitudes of the current destined for the transmitting antenna and, on the other hand, to have a signal pass nearby the intervals each containing a zero crossing. As a result, the signal SSB 1 can be used to blank the shielding peak, which can be seen in the waveform 403 in FIG. 3, which, due to the use of a shielding according to DE-OS 37 15 387, within the monitoring system 1 according to FIG . 1 is generated. This will be explained in more detail below.

As can further be seen from FIG. 2C, the monitoring system 1 is also able to ensure operation of the transmitting antenna arrangement when resonating. For this purpose, the multiplexer 75 receives, in addition to receiving a transmission current information from the current sensor 81 at the input TXI , at its further input TXV from an output of the power amplifier 79 data about the voltage of the transmitter. When the system is initialized, the microcomputer 21 reads and stores the information about the transmitter current and the transmitter voltage at the inputs TXI and TXV of the multiplexer 75 .

The microcomputer also compares the phases of these signals to determine if there is a phase difference. If the phases differ from one another, the microcomputer 21 detects a non-resonance state of the transmission antenna 12 and then adjusts the transmission frequency so that the transmitter current is brought into phase with the transmitter voltage so that the transmission antenna 12 operates at resonance becomes.

If it is now assumed that the phase and level of the transmitter current are set, the microcomputer 21 addresses the inputs PFO, PFB 1 , PFB 2 and PFB 3 of the analog multiplexer via the address switch 28 and the address input ADDR 75 . The receiver, which is contained in the "control unit interface and receiver" block 17 according to FIG. 1, supplies the aforementioned component inputs with the basic component level of the received signal and the respective received signal energy level in the three preselected ones Frequency bands FB 1 , FB 2 and FB 3 . The microcomputer 21 treats the levels in the last three bands as ambient noise levels. The average of these levels and the base level are stored by the microcomputer for future use in the monitoring system operation.

The microcomputer now begins its monitoring process. This process is repeated by the microcomputer every half period of the transmitter stream, such half period being referred to as a "frame" (see FIG. 3). During a first interval of this half-period, that is, during a so-called "marker window" interval, the receiver is caused by the microcomputer to collect the frequency domain and time domain information from the received signal which contains any interference which may occur in the Field occur in the query or monitoring zone. The microcomputer reads this information from the receiver and during the remaining interval of the frame, ie during the "processing interval", the microcomputer uses its decision programs to evaluate whether the information is indicative of the presence of a valid marker in the query or Monitoring zone 13 according to FIG. 1.

As can also be seen from FIG. 2A, the received signal is branched in the area of the receiver and thus sent through a first signal branching channel A and through a second signal branching channel B. In the first signal branching channel A , a fundamental signal detector 50 is arranged, which uses a bandpass filter to extract the fundamental frequency component, ie the component at the frequency Fo , of the signal. This level is then made available to the analog multiplexer 75 according to FIG. 2C at its input PFo in order to enable a subsequent analysis by the microcomputer.

In the second signal branching channel B according to FIG. 2A, two notch or trap filters 51 and 52 are arranged, each of which has the frequency components of the signal at the frequency Fo and at the frequency of the supply network for the monitoring system (ie frequencies which are in a range from 50 to 60 Hz). The extraction of these frequency components eliminates signal content that could otherwise lead to incorrect displays of markers.

After passing through the notch or trap filter 52 , the resulting signal is amplified in a preamplifier 53 and this amplified signal is sequentially passed through a high-pass filter 54 and a low-pass filter 55 , which effectively isolate the frequency band in which they are connected the harmonic vibrations used with the present monitoring system are expected. The particular frequency range indicated in FIG. 2B relates to a preferred marker according to US Pat. No. 4,660,025, which is capable of offering a substantial harmonic content over the indicated range of 1-8 kHz. By limiting the frequency band to this relatively high range, the effects of so-called "non-marker" disturbances and the noise on the detection process can be minimized.

After filtering, the signal is then passed through a pre-emphasis circuit 56 which serves to compensate for the attenuation of the signal caused by the subsequent passage through a comb bandpass filter 58 (see Fig. 2A). Starting from the pre-emphasis circuit 56 , the signal amplitude is first limited in a limiter 57 and then sent to the comb bandpass filter 58 already mentioned, which, as will be explained in more detail below, has a digital delay line, which at a multiple (1024) of the fundamental frequency Fo is clocked.

The comb bandpass filter 58 has narrow pass bands at the fundamental frequency Fo and at their harmonics, so that only harmonics of the frequency Fo pass through this filter. As a result, this filter continues to limit the signal only to those frequency components that are expected to result from the marker used. The design of the comb bandpass filter 58 also favors the compactness of the monitoring system according to the invention, since this filter does not use any separate, voluminous capacities.

After passing through the comb band-pass filter 58 , the sampling noise caused by it is eliminated by a low-pass filter 59 . The resulting signal coming out of the low-pass filter 59 is now sufficiently processed in terms of frequency and amplitude in order to be subsequently sent through time-domain and frequency-domain channels C and D of the receiver.

As shown in FIG. 2A also shows the signal passes in the frequency domain channel D first passed through a shielding pulse-peak blanker 61 which is controlled by the signal SSB 1, such that the signal content during a neutral or blanking interval of signal SSB 1 is suppressed and the signal is passed through during a non-blanking interval of signal SSB 1 . As already mentioned above, this blanking takes place at the maximum values of the transmitter current, that is to say when a shielding peak originating from the antenna shielding occurs. This blanking is required because the shield tip is frequency coherent and cannot be filtered out. These peaks are therefore eliminated from the signal by the shielding pulse peak blanker 61 .

Following blanking, the resulting signal branches into a number of frequency domain subchannels, as can be seen in detail in FIG. 2B. Each of these subchannels isolates a separate frequency band of the signal and determines the energy in this band. These energies are then used by the microcomputer in its frequency domain decision program for the purpose of evaluating whether an article or a marker is present in the query or monitoring zone 13 according to FIG. 1.

The number of frequency domain subchannels is selected from the viewpoint that it ensures that at least two frequency bands are obtained which are fairly certain to have the frequency content expected from a marker. According to a preferred embodiment of the invention, three subchannels D 1 , D 2 and D 3 are provided according to FIG. 2B, which correspond to a low frequency band FB 1 , medium frequency band FB 2 and a high frequency band FB 3 .

In detail, the frequency bands FB 1 , FB 2 and FB 3 are isolated by corresponding low-range, medium-range and high-range bandpass filters 62 , 63 and 64 and then rectified in full-wave rectifiers 65 , 66 and 67, respectively. Downstream synchronous integrators 68 , 69 and 71 then integrate the rectified signals, thereby generating DC voltage values that are representative of the energy in each of the bands FB 1 , FB 2 and FB 3 . These DC voltage values then appear at the corresponding inputs PFB 1 , PFB 2 and PFB 3 of the analog multiplexer 75 according to FIG. 2C for the purpose of subsequent processing by the microcomputer 21 .

The operation of the synchronous integrators 68 , 69 and 71 is synchronized with the microcomputer operation by a second synchronization signal SSB 2 , this signal SSB 2 being passed to the aforementioned integrators via a level shifter 76 . The SSB 2 signal has the same frequency (2 FO ) as the SSB 1 signal, but has a slight phase shift in order to take into account the signal delay introduced by the bandpass filters 62 to 64 . It should also be noted that the amplification factors of the bandpass filters 62 to 64 are selected in such a way that they normalize the outputs of the synchronous integrators 68 , 69 and 71 , on the basis of a preselected property which is related to the the expected response behavior is representative of the markers used. Therefore, if a marker 15 is present in the query or monitoring zone 13 according to FIG. 1, the outputs of the synchronous integrators 68 , 69 and 71 will be the same or have increasing levels.

For the frequency band envisaged in the exemplary embodiment explained here, the bands of FB 1 , FB 2 and FB 3 are centered around frequencies of 1.5 2.5 and 3.5 kHz. In addition, the bandwidth of each band is 600 Hz.

If we now turn to the time-domain channel C (see FIG. 2A), it should first be mentioned that this channel also receives the output of the low-pass filter 59 , whereupon its output signal passes through a subsequent high-pass filter 72 runs (see FIG. 2B). The lower edge of the high-pass filter 72 is chosen from the point of view that high harmonics of the signal are let through sufficiently so that a good image of expected changes in a marker pulse can be obtained. The desired signal content is adequately ensured by a filter edge which is approximately at the center of the high-bandpass subchannel D 3 , ie at 3.5 kHz in the present exemplary embodiment.

The outgoing signal from the high-pass filter 72 is then rectified in a full-wave rectifier 73 and continues to be sampled in a sample and hold circuit 74 in order to obtain a digitized signal version. This digitized signal is then made available to the microcomputer 21 via a further input PTD of the analog multiplexer 75 (cf. FIG. 2C).

The sampling interval and the correct timing or setting (timing) of the sample and hold circuit 74 are controlled by the microcomputer 21 via a sequential logic 77 . During each sampling interval, sample and hold circuit 74 samples the signal and holds previous samples. The microcomputer 21 controls this circuit so that, as soon as the circuit provides a new sample, the microcomputer reads and stores the previous sample contained in the holding circuit.

At the end of the so-called "marker window" interval, the microcomputer 21 has then stored in its memory a digitized signal corresponding to the filtered and rectified received signal. At this time, the microcomputer has access to the terminals PFB 1 , PFB 2 and PFB 3 of the analog multiplexer 75 and thus reads and stores the DC levels with respect to the frequency bands FB 1 , FB 2 and FB 3 .

The microcomputer 21 then initializes its phase of processing, in which it analyzes this time-domain and frequency-domain information by means of its decision programs. On the basis of the results of these programs, the microcomputer updates the time domain counter 21 a and the frequency domain counter 21 b (see FIG. 15) and checks the counter status of each of these two counters. Only when these counters 21 a and 21 b are both at preselected counter readings which are indicative of the presence of a marker over a number of frames (see FIG. 3), does the microcomputer 21 make a decision that a marker is present is. The result of this decision is that the microcomputer 21 activates alarms 82 and 83 via an alarm interface circuit 85 unless the alarm decision is a result of further peak detection and fundamental frequency detection programs such as described in the present application, is prevented.

FIGS . 5A and 5B each show flow diagrams with respect to the time domain program 100 and the frequency domain program 200 of the microcomputer 21 . These programs are experimentally determined frequency domain and time domain data of so-called "non-marker objects", such as. B. shopping trolley, as well as similar data regarding markers of the type according to US Pat. No. 4,660,025. These programs are also based on data acquired in an experimental way, in particular with regard to noise level and minimally detectable signal level. Finally, it is assumed in these programs that a frame (see FIG. 3) contains 64 equal intervals and that the marker window contains 24 intervals, each of which is assigned to a sample of the stored, digitized signal.

The time domain program 100 is initialized first, and when it is complete, the frequency domain program 200 continues. In the present preferred case, both programs are always executed for every frame of the processing sequence.

According to FIG. 5A, the sample peak value PS (peak sample value) of the 24 samples of the digitized signal is determined in the marker window interval in steps 101 and 102 of the time domain program 100 . By checking the signal only in this interval, some content due to antenna shielding is eliminated and the microcomputer effectively blankets out the so-called shielding tips.

In step 101 , the maximum sample value Ps is compared with a threshold value of 0.2 V and, if it is less than this threshold value, the program ends, the time range counter 21 a is reduced or reduced and the frequency range program starts. If the maximum sample is above the 0.2 V threshold, then step 102 is passed, where the level at half the maximum sample value (the 6 dB threshold) is compared to the ambient noise level that previously determined and stored by the microcomputer. If the 6 dB threshold is lower than the noise level, then the program ends, the time domain counter 21 a is reduced or reduced, and the program moves to the frequency domain program.

If the 6 dB threshold is greater than the noise level, then step 103 is started, in which it is averaged whether more than six samples of the 24 samples are above the noise level in the marker window interval. If more than 6 samples are above this noise level, the program ends, both the time domain counter and the frequency domain counter are reset to 0 and the frequency domain program is started. If no, then step 104 begins.

In this step 104 , the position of the sample maximum value Ps in the frame is compared with the position of the sample maximum value in the frame that is two frames earlier. These positions are designated as phase positions PH 1 and PH 2 of the samples in their respective frames and, if these phases differ from one another by more than 2.8 ° (ie a sample interval), the program ends, both the time domain and the frequency range counter are reset to 0 and the frequency range program begins.

If the difference between these two phases is less than 2.8 °, then step 105 is proceeded to, in which it is determined whether the number of samples above the 6 dB threshold is greater than 3. If so, the program ends, the time domain counter is decreased by 1 and the frequency domain program is started. If not, then the program proceeds to step 106 .

In step 106 , all samples above the 6 dB threshold are compared to determine if they are within 2 sample intervals. If not, the program ends, the time range counter is reduced by 1 and the frequency range program begins. If so, the time domain counter is incremented by 2 and the process moves to step 107 .

In step 107 it is determined whether the number of samples above a 10 dB threshold value (that is one third of the sample maximum value Ps) is greater than 4. If so, the program ends and the frequency domain program is initiated. If not, the program proceeds to step 108 . In this step it is determined whether all samples are above the 10 dB threshold within three samples. In the affirmative, the time domain counter is increased by 1, the program ends and the frequency domain program is initiated. If no, this program ends and the frequency domain program begins.

In summary, it can be said that the time domain program 100 checks the digitized signal samples to first determine whether the signal level is within the acceptable accuracy level of the receiver equipment (step 101 ) and then whether the Signal level is within an acceptable signal-to-noise ratio (step 102 ). If one of these conditions is not met, then the ability to determine a marker is not given and the time range counter 21 a is therefore reduced by 1.

If the above conditions exist, then the samples are checked to estimate the relative pulse width of the signal (step 103 ). Therefore, if a relatively large number of samples (in the present case 6) are above the noise level, either an excessive pulse width, not expected by a valid marker, is displayed, or too much noise during the marker window than that a decision could be made regarding a valid marker. In any case, this is considered to be a significant deficiency, where the determination of the presence of a marker makes it highly unlikely. Both the time domain counter and the frequency domain counter are therefore reset to 0.

If the pulse width test is performed, then a phase test follows in step 104 . This test examines the position of the maximum sample in relation to the maximum sample two frames beforehand. Specifically, the difference between the positions of these samples in their respective frames is required to be within a sample interval. This requirement is based in part on the fact that a valid marker should deliver a signal at substantially the same point in each frame, while such a requirement is not to be expected from pulses derived from so-called non-marker objects. The specific position error of a sample interval is based on a phase error of the sample and hold circuit and on the marker arrangement in the field.

In addition, the influence of the earth's magnetic field on the analysis is eliminated by comparing the maximum samples in every other frame. This is because, depending on the orientation of a marker in the interrogation or monitoring zone 13 with respect to the generated field and the earth's magnetic field, the field disturbances caused by the marker and the resulting marker signal are not in the same position in the marker -Windows in successive frames (ie alternating polarities of the driver current) will occur. In addition, in the case of a weak marker drive, the signal will not be present in any other frame. Testing alternate frames therefore eliminates erroneous decisions that could occur due to the earth's magnetic field.

Failure of this phase test also means determining that the presence of a marker is highly unlikely. This failure is treated as if it was the failure of the pulse width test, and the two counters 21 a and 21 b are reset to 0.

When the phase test in step 104 is over, a refined pulse width test in steps 105 and 106 must be performed. If the samples exceed 3 above the 6 dB threshold, this test fails. In addition, if such samples above the 6 dB threshold do not occur within 2 sample intervals, the test also fails. Although these two test failures are characteristic of a pulse width which is greater than that to be expected from a valid marker, the test failures do not mean that they completely exclude the probability of a marker. The counter 21 a is therefore reduced only by 1 to indicate this condition.

If this second pulse width test meets the requirements, then there is a high probability for the presence of a marker and the counter 21 a is increased by 2 in order to register this fact.

The final test according to the two steps 107 and 108 represents a further, more refined pulse width test, after the expiry of which there is a high degree of probability that a marker is present. If this test fails, however, it does not lead away from the earlier determination of the likelihood of a marker presence in the query or surveillance zone. Going through this test further increases the counter 21 a by 1, but its failure does not result in a change in this counter.

When the time domain program 100 ends , the frequency domain program 200 is initiated (see FIG. 5B). Within the scope of the frequency range program, the energy is checked in the frequency bands FB 1 , FB 2 and FB 3 , which are presented by the DC levels at the synchronous integrators 68 , 69 and 71 (see FIG. 2B). This program then determines whether the course of the frequency spectrum of the received signal is as it is to be expected from a valid marker and not as it is to be expected from so-called non-marker objects.

As already explained above, the bandpass filters 62 , 63 and 64 are designed so that the expected, integrated outputs of the bands FB 1 , FB 2 and FB 3 for the markers result in the same or increasing direct current values. Therefore, the frequency range program 200 examines such a rise in the DC level.

As outlined in detail in the flow chart of results in Fig. 5B, in the first step 201 of the frequency is queried range program, whether the 6 dB-threshold value for the DC level of the high frequency band FB 3 is lower the noise level. If so, then this test fails, the frequency range counter 21 b is reduced and the program ends. If not, the program proceeds to step 202 , in which the DC level of the frequency band FB 1 is checked in order to determine whether it exceeds a predetermined threshold value which is 3 volts. This predetermined value is based on system constraints, which are provided to prevent the DC level assigned to this frequency band from exceeding this level with a valid marker and under normal operating conditions of the monitoring system. When the predetermined level is exceeded (YES output of step 202), then the probability before ensure that a valid marker is not present, and the frequency domain counter 21 b is reduced, and therefore the program ends.

If, however, the level is not exceeded, then the further steps 203 and 204 are initiated, in which the direct current level of the frequency band FB 1 is compared with the direct current level of the frequency band FB 2 and the direct current level of the frequency band FB 2 is compared with the DC level of the frequency band FB 3 . If either the DC level of band FB 1 is greater than the DC level of band FB 2 or the DC level of band FB 2 is greater than the DC level of band FB 1 , then this test fails. The frequency range counter 21 b is then reduced and the program ends.

However, if the direct current level of the band FB 1 is less than the direct current level of the band FB 2 and furthermore the direct current level FB 2 is lower than the direct current level of the band FB 3 (in each case outputs NO of steps 203 and 204 ), then the presence of a valid marker is likely and the frequency range counter 21 b is increased by 2 and the program ends.

As already explained further above, after the frequency range program 200 has ended, the two counters 21 a and 21 b are checked in order to transmit whether their respective counter readings exceed the preselected values set for the presence of a valid marker in the query. or surveillance zone are characteristic. In the present exemplary embodiment, the two counters 21 a and 21 b are set in such a way that they can count up to a maximum of 12 and downwards to a minimum of 0. In addition, a count of 10 in each counter results in a decision that a valid marker is present.

If the checking process with regard to the two counters 21 a and 21 b shows that they do not have the preselected counter readings, then the microcomputer 21 initiates the information gathering phase for the next frame of the generated field and the procedure described above is repeated.

If the check performed by the Mirocomputer indicates that the preselected meter readings have been reached an alarm signal is triggered, provided, however, that this is not by two further test programs of the microcomputer prevented becomes.

One of these test programs is a peak detection program to check the likelihood that the received signal is from electrical noise peaks, and the other test program is to check the fundamental frequency component to determine if the component of the received signal with the fundamental frequency has not exceeded a predetermined threshold value, which is assigned to large metallic objects in the query or monitoring zone 13 according to FIG. 1.

The flow diagram of the peak detection program 500 is shown in FIG. 6. As already mentioned above, this program is used to selectively distinguish the signals from valid markers from electrical noise peaks caused by voltage peaks in the supply network, as a result of electrical discharge, by closing switches, by motor switch noise, fluorescence. and neon lamps and the like arise. Such noise sources produce transient voltage spikes in the form of electrical pulses or "spikes" which are not distinguished by the time domain and frequency domain programs 100 and 200 because these spikes have some properties similar to those of magnetic markers on which the monitoring system is sensitive 1 should respond according to FIG. 1.

The noise peaks can be caused in particular by the following features and characteristics be:

• (a) Basically speaking, the Spikes around a pulse-shaped or pulse-like type speaking behavior, as is also the case with the signal from one valid marker is the case, and therefore time can area response to be a bit similar that of a valid marker;
• (b) Point peaks often very short rise and fall times on and therefore generate a frequency spectrum which is similar is that of a valid marker;
• (c) Depending From the source, the peaks can have an amplitude have, which are not by the monitoring system is automatically rejected;
• (d) because of the "echo" effect of the comb bandpass filter 58 of FIG. 2A, a single spike may appear in multiple marker windows;
• (e) in general, several of the properties of the noise peaks are similar enough to those of a valid marker so that the filtering parts of the receiver 17 have little effect on attenuating the peaks.

The peak detection program depends on the inventory management of a memory grouping or matrix, which according to FIG. 15 is to be referred to as the “maximum amplitude stack”. The storage locations of this stack contain maximum values of the sample and hold circuit 74 shown in FIG. 2B during the successive "marker window" intervals. On a continuous basis, memory location 1 is updated with the maximum value assigned to the current or applicable frame, with the previous contents being reset to location 0. The first two storage locations of the stack therefore contain a data record relating to the marker pulse or a marker-like pulse.

Once the time domain counter reaches 21 a before the selected with respect to a valid marker characterizing count and the operation of the maximum amplitude stack changes. Now maximum amplitude values of successive frames (see FIG. 3) are placed in memory locations 2-15 in the stack. When the frequency range counter 21 b now also reaches its preselected value and the microcomputer 21 is about to address an alarm trigger, the computer branches to the peak detection program 500 according to FIG. 6. It should be noted that the stack is not necessarily filled at this point. The stack stops charging at the moment the time domain and frequency domain counters have reached their preselected counts.

In step 501 , the microcomputer 21 checks the contents of the stack from memory location 2 to that memory location indicated by the stack pointer to determine the largest maximum value. Once found, the microcomputer determines whether this maximum value occurred in an odd or even frame. If the maximum maximum has occurred in an odd-numbered frame, then the microcomputer proceeds to step 503 with the index that the frame pointer is to be placed in stack position 1 as shown in FIG. 15; otherwise, the microcomputer proceeds to step 502 , in which the frame pointer is to be set to the stack position 0. The reason for this action lies in the physics of the magnetic system. Due to the influence of the earth's magnetic field on the monitoring location 1 according to Fig. 8186 00070 552 001000280000000200012000285913807500040 0002003828691 00004 38067 <1 two things can happen:
(1) Depending on the orientation of the antennas of the monitoring system, the earth's magnetic field can alternately overlap, so to speak, to support and counteract the driver field of the monitoring system when this driver field changes its polarity, causing different amplitudes of the marker signal in alternating windows;
(2) The earth's magnetic field acts as a DC bias on the material of the marker and, depending on the marker orientation, the marker will "switch" and generate a signal at different times in successive windows. In extreme cases, a true marker signal can only appear in every second window.

A third reason for this ODD / EVEN method in the peak detection program is that a noise peak input is reflected by the comb bandpass filter 58 in every other frame.

Once the ODD or EVEN state has been determined, the microcomputer 21 proceeds to step 504 where only the appropriate frames of the maximum amplitude stack are considered. First of all, it should now be assumed that this is the EVEN-NUMBER case, the stack pointer being stopped in frame No. 13 according to FIG. 15.

In step 504 , the rise time of the peak signal is determined. In most cases the rise time of a peak will be shorter than that of a marker or marker signal. The frame pointer is now set more to No. 0. An initial increase is determined by dividing the value in frame # 2 by the value in frame # 0. The microcomputer then proceeds to step 505 , in which an evaluation of the determined increase is carried out. If the calculated ratio or rise is equal to or greater than 4, then the signal is treated as a peak and the microcomputer proceeds to step 517 , ie the output of the peak detection program; otherwise the microcomputer proceeds to step 506 .

If it is determined in this step 506 that X = 0 or = 1, the microcomputer proceeds to step 515 in which the frame pointer is incremented. A spike will often occur in a frame, but not in the previous frame, while a marker entering the magnetic field of the monitoring system will have a gradual build-up or increase in amplitude from frame to frame. In this test it is determined whether or not there was a signal in frame No. 0 or No. 1 that did not fully qualify and passed the time domain criteria.

If the frame pointer is not at # 0 or # 1 at step 506 , the microcomputer proceeds to step 507 where a test is made to determine if the maximum value in frame X is equal to or higher than 6 dB is above the noise level (2 × as large). If no, then the signal is too small or too weak to make a decision and the microcomputer proceeds to step 515 where the frame pointer is incremented by two. If the test is passed or positive, the microcomputer proceeds to step 508 where a test is performed to determine if the maximum in frame X is equal to or higher than 10 dB above the noise level ( 3 × as large).

If not, then the microcomputer proceeds to step 509 where the slope is tested again. If the increase is equal to or greater than 1, then a "possible marker" counter is increased in step 510 . If the increase is not greater than 1, then the counter is decremented in step 512 . If the S / N ( S / N ) ratio is equal to or greater than 10 dB, then the microcomputer proceeds to step 511 , where the slope is tested again. If the increase is equal to or greater than 0.85, then the "possible marker" counter is increased in step 510 . If no, the counter is decremented in step 512 and then the process continues to step 513 .

In step 513 the "possible marker" counter is checked and, if the count is equal to 2, then the microcomputer proceeds to step 514 , in which at least five successive marker signals have been received which increase the correct rate of the amplitudes and the program is then terminated with the decision that the signals are from a valid marker. If the "possible marker" counter is less than 2, the microcomputer proceeds to step 515 where the frame pointer is incremented, then step 516 is proceeded to.

In step 516 , the new position of the frame pointer is compared to the position of the stack pointer (in this case No. 13). If the frame pointer goes beyond the stack pointer, there are no more slopes to compare and the maximum or peak values are assumed to be peaks. The program is therefore ended and no alarm is generated (step 517 ). If the frame pointer is lower than the stack pointer, then more information needs to be processed and a new increase is calculated in step 504 .

Once the tip detection program is complete, when the decision is made that a tip was present. The time range counter 21 a and the frequency range counter 21 b are therefore reset to zero, an alarm is not triggered and the monitoring system 1 continues its monitoring function. If the decision has been made that a marker is present, the microcomputer 21 performs a final test before the alarm is triggered. In this test, the microcomputer 21 compares the fundamental frequency component of the received signal, which it receives from the input PFo of the analog multiplexer 75 , with its stored value and, if the fundamental frequency component has changed beyond a predetermined threshold value, then made the decision that a valid marker does not exist. The microcomputer 21 therefore does not trigger an alarm, the two counters 21 a and 21 b are reset to zero and the monitoring is continued. If the aforementioned threshold value is not exceeded, then an alarm is triggered, which indicates the presence of a marker in the query or monitoring zone 13 according to FIG. 1.

As already mentioned further above, after initialization of the operation, the microcomputer 21 carries out a determination of the noise levels in the frequency bands FB 1 , FB 2 and FB 3 and uses these levels to provide an average noise level for the monitoring system 1 and for one Form use within its decision making programs. The microcomputer updates this noise level by periodically referring frames as the noise update frame. During this frame, the microcomputer treats the DC levels developed by the subchannels as characteristic of the noise level within the monitoring system and forms average values from these levels in order to obtain an average noise value.

After a preselected number of noise update frames have been passed, the microcomputer forms an average value from the stored average noise values. This average is then treated by the microcomputer as the new ambient noise level and used in the subsequent decision programs. The monitoring system 1 therefore updates itself dynamically in order to adapt to changing environmental conditions, which would otherwise impair the ability of the monitoring system to reliably detect markers.

In Fig. 7 is a block diagram of the comb-band pass filter 58 is shown. This filter has a delay line 601 in the form of an integrated circuit, further input and output connections 602 and 603 and a clock connection 604 . The output connection 603 of the delay line 601 is fed back to the input connection 602 by means of a feedback device 605 with resistors R 1 and R 2 .

A clock signal generator 606 supplies the clock signals to the clock terminal 604 of the delay line 601 . Typically, delay line 601 may be an "Reticon R 5107" integrated circuit. In this case it contains 512 delay stages or monolithic C elements and requires two clock cycles to shift from stage to stage.

The comb bandpass filter 58 in the present embodiment passes harmonics of a fundamental frequency Fo when the clock rate is set to a value equal to the number of delay stages times the frequency Fo times that required for a shift from one stage to the next clock cycles is. It is therefore necessary for a 512-stage line with the two clock cycles required for a shift and for the basic frequency Fo of the monitoring system, a clock rate of 1024 Fo , which is the signal with which the comb bandpass Filter 58 is opened by the level shifter 76 according to FIG. 2C. In the embodiment of the filter of FIG. 7 is set its Q-factor by setting the reflection object R1.

Fig. 8 shows an actual embodiment of the circuit shows the comb bandpass filter 58. In this case the comb bandpass circuit is used to emphasize multiples of 73 Hz. The components R 220 , R 221 and U 203 A form a reverse buffer amplifier, which is used to drive the comb-bandpass circuit. The parallel connection of the components R 222 , R 223 and R 224 corresponds to the resistor R 1 according to FIG. 7, while the component R 225 corresponds to the resistor R 2 . The three resistors connected in parallel allow the Q factor of the circuit to be optimized simply by removing and removing the appropriate resistor or the appropriate resistors. In most devices, however, all three components will remain in the circuit.

The components C 208 , R 226 and U 203 B serve to feed the signal into the delay line IC U 201 . The component C 208 serves as a DC block capacitor, while the component U 203 B is a non-inverting buffer with one gain and high input impedance. The combination of high impedance with a fairly small capacitance results in a small phase delay in the DC block capacitor. Resistor R 226 forms a DC line to ground for component U 203 B.

As already mentioned above, component U 201 represents a delay line in the form of an integrated circuit (IC). The IC used in the present circuit implementation is a Reticon R 5107. This IC contains 512 monolithic C elements and is therefore clocked at a rate of 74.752 kHz (1024 × 73 Hz).

The remaining circuit component is designated U 203 C. Since the Reticon R 5107 has a low output drive capability, a buffer must be used at its output. The component U 203 C is designed as a non-inverting buffer and is able to deliver a sufficient output drive, so that the component R 225 is not a too large load.

In the context of the above description of the monitoring system 1 according to FIG. 1, it has already been pointed out that the second signal branching channel B (cf. FIG. 2A) is equipped with a notch or trap filter, which the supply network frequency component from the received signal removed before this signal propagates into subsequent channels. In order in which the network frequency noise components are very strong, the monitoring system can also be designed such that not only the network frequency but also all of its harmonics are suppressed. As can be seen in detail from FIG. 2A, a comb notch or trap filter 58 a with stop bands at the network frequency and at harmonics of this network frequency can be provided for this purpose , which passes the comb band pass filter within the receiver 58 is connected upstream to eliminate the harmonics of the mains frequency.

As already explained above, is the use of the comb-notch or -Fallenfilters 58 a particular order in environments of interest, in which network interferences occur of considerable size. However, where this size is not significant, the filter 58 a can be switched off in order to avoid any possible increase in noise for the monitoring system.

Fig. 9 is a block diagram showing a comb-notch filter in accordance with the present invention. The comb-notch filter comprises a comb-bandpass filter 702 , which contains a digital delay line 703 . From the output terminal 703 b of the delay line, the signal is fed back via a buffer 707 , a resistor R 2 , a capacitor C 1 and a further resistor R 3 to the input terminal 703 a of the digital delay line 703 . The input signal is coupled to the input terminal 703 a via a reversing buffer 708 , a resistor R 1 , the capacitance C 1 and the resistor R 3 .

The input signal and the input at the input terminal 703 a are coupled to a summing circuit 704 . Because the latter two inputs are out of phase due to the reverse buffer 708 , the utility frequency and its harmonics in the input are subtracted and effectively removed from the input. This results in a comb-notch filter characteristic.

A clock signal for the digital delay line 703 is derived abge from a supply network reference frequency, which is passed through a circuit for conversion into square waves 701 to a PLL circuit 709 (phase locked loop) operating at 2, the Ver n times sorgungsnetzfrequenz, where n is the number of stages of the delay line. A divider 711 supplies a 2 n division of the output of the PLL circuit 709 in order to be able to offer the phase locked loop a clean reference signal.

From FIGS. 9A and 9B, the realization of an electric circuit is a comb-notch filter or notch filter for a monitoring system 1 can be seen. The circuit for conversion into square waves consists of the components R 25 - 29 C 27 and U 8 A (see upper part of Fig. 9B.). Resistors R 25 and R 27 function as a voltage divider to ensure that the input amplitude of the mains sine wave does not exceed the safe input level of component U 8 A. Resistors R 25 and R 27 also act as low-pass filters with respect to the input signal. R 28 and R 29 serve as resistors for gain adjustment, which ensure that there is a rectangular pulse train at the output of this circuit. This high gain causes the pulse train to move rapidly between its positive and negative levels. R 26 provides the hysteresis for the circuit, but R 26 also serves to shorten the transition time. This is necessary because the subsequent PLL circuit requires short transition times. The phase locked loop consists of the components U 2 , U 3 , C 17 , C 18 , R 22 and R 33 . This phase locked loop is used as a frequency multiplexer in order to provide the delay line (IC U 1 ) with the required clock signal. The voltage controlled oscillator (VCO) of the phase locked loop operates at a frequency which is 2048 times the supply network frequency. A 2048 divider is looped so that the VCO can be phase controlled with respect to the utility signal.

Because a shielding tip is an undesirable signal that additionally contributes to noise at the output of the comb-notch filter, the components U 5 and U 6 generate a blanking control signal from SSB 1 and 64 Fo . This signal actuates the electronic switch U 4 C so that the shielding tip does not pass through the component U 7 A. In addition to its function as a blanker, component U 7 A plays a second role as a reversing buffer at the input of the comb-bandpass part of the comb-notch filter circuit.

The comb bandpass section consists of the components U 1, U 7 B, D 7 D, R 3, R 4, R 7-10, R 21 and C. 13 The operation of this part of the circuit is the same as that previously described for the comb-bandpass filter, with the exceptions that the delay line IC is now a Reticon R 5108 and that the clock frequency is 2048 times the filter Fundamental frequency is.

The summing circuit 704 according to FIG. 9 is shown by the components R 5 , R 6 and U 8 B. R 5 and R 6 set the proportion in which the signal passing through the comb-notch filter and the input signal are to be added together. They are not added in the same proportion because of the blanking that has been done on the signal passing through the comb-notch filter. The signals are subtracted because the signal passing through the comb-notch filter has been inverted by the component U 7 A.

Following the summing circuit, a low pass filter is used to suppress any circuit noise generated by component U 1 . The filter edge frequency is 8 kHz, so that all of the marker harmonics of interest are passed through. After this filter, an amplifier is used to bring the remaining signal level to the same amplitude that the signal levels would have had the filter not been used.

In accordance with a further aspect of the present invention, the transmit and receive antenna arrangements for the monitoring system 1 according to FIG. 1 are designed in such a way that the usual zeros that exist in the query or monitoring zone 13 are present Field must be reduced.

Fig. 10 shows a correspondingly configured receiving antenna array shows 90th This antenna arrangement has first and second upper loops 91 and 92 , which are arranged in a nested relationship, and third and fourth lower loops 94 and 93 , which are also arranged in a nested relationship. This nesting is such that the center C ₂ of the loop 92 lies away from the center C ₁ of the loop 91 , while the center C ₄ of the loop 94 lies away from the center C ₃ of the loop 93 . Furthermore, the first, upper loop 91 together with the fourth, lower loop 93 forms a first, eight-shaped configuration, while the second, upper loop 92 forms together with the third, lower loop 94 a second eight-shaped configuration.

The lying in the same plane loops 91-94 are all wound in the clockwise direction as shown in Figure 10 indicated by the arrows, and all are electrically connected in series.. Furthermore, it is provided that the upper loops 91 and 92 have the same first phase and the lower loops 93 and 94 have the same second phase, the first and the second phases being opposite to one another.

According to a preferred embodiment of the receiving antenna arrangement 90, it is provided that the section 91 A of the first loop 91 and the sections 93 A of the fourth loop 93 , which sections are located in the mutual contact area of the first eight-shaped configuration, are oblique to the horizontal are inclined, while the section 92 A of the second loop 92 and the section 94 A of the third loop 94 are likewise inclined obliquely with respect to the horizontal in the mutual contact area in the second eight-shaped configuration. A typical value for this angle of inclination is 20 °. According to the preferred embodiment of the antenna arrangement shown in FIG. 10, the two outer loops, ie the first upper loop 91 and the fourth lower loop 93 are mirror images of one another, in the same way the two inner loops, ie the second upper loop 92 and the third lower loop 94 mirror images of each other.

In Fig. 12 (I) the zero zones of a receiving antenna arrangement with an eight-shaped configuration are shown schematically. As can be seen, this antenna arrangement has zero zones 11-1, 11-2 and 11-3 in three different bands in the direction of its vertical axis. The zero zones of a receiving antenna arrangement 90 according to FIG. 10 are shown comparatively from FIG. 12 (II).

As can be seen, in this improved antenna arrangement, the zero zones are reduced to only one zero zone 11-1 ' , which is inclined at an angle to a horizontal. Because of this inclination, however, the zero zone 11-1 'will advantageously be linked or coupled with vertically oriented markers along the X axis, so that their effect is minimized to a considerable extent.

FIG. 11 schematically shows an improved transmit antenna arrangement 801 for the monitoring system 1 according to FIG. 1, this transmit antenna arrangement being particularly suitable for use in combination with the receive antenna arrangement 90 according to FIG. 10. The transmit antenna arrangement 801 has a first, individual loop 802 , in which a second loop 803, which is in the same plane, is nested. The major axis of the second loop 803 is rotated with respect to the major axis (Y axis) of the loop 802 . In this case, both loops are wound clockwise and are in the same phase.

Due to the presence of the second, internally arranged loop 803 of the transmitting antenna arrangement 801 , there is a field in the Y direction along the X axis of the first loop 802 , but this would not be the case if the second loop 803 were missing. In view of this situation, the reception Antennanord voltage is shown in FIG. 10 is now capable of coupling with fields along the horizontal or X axis so that the monitoring according to the invention in the present system 1 of FIG. 1 transmit and receive used Antenna assemblies enable detection of the presence of markers that are moved along the horizontal direction through the interrogation or monitoring zone 13 .

Exemplary embodiments of pedestals or stands for accommodating the transmitting and receiving antenna arrangements of the monitoring system 1 according to FIG. 1 are shown in FIGS. 13 and 14.

In each case a base or stand 301 has first and second cladding walls 302 and 303 with a mutual spacing, which preferably consist of plastic and are connected to one another in such a way that they enclose a cavity in the manner of a shell, which is used to receive the transmit and receive signals. Serves antenna arrangements, which are shown in FIG. 14 by coils 304 and 305 . These antenna arrangements are housed between stiffening rods 306 , which are each arranged on the lateral edges of the cavity between the two cladding walls 302 and 303 .

A solidified foam 307 of high density, such as. B. a urethane foam, is used to fill the free cavity and to surround the two coils 304 and 305 . This high-density, solidified foam 307 immobilizes the two antenna arrangements, ie the coil 304 (= transmit antenna arrangement) and the coil 305 (= receive antenna arrangement), and gives the finished base or stand 301 the necessary reinforcement and stiffening.

The edges of the two cladding walls 302 and 303 , which e.g. B. are each formed as a thin plastic cladding, are each formed into hook-shaped end portions 308 , which serve to carry an extruded edge protective strip or protective buffer 309 (see FIG. 13). Finally, a support base 311 is seen per foot frame or stand 301, which allows a firm and safe installation of the foot frame or stand on the floor.

The pedestals or posts as shown in FIGS. 13 and 14 are preferably made in such a way that the internal components including the transmitting and receiving coils are introduced 304 and 305 to the next in one of the Panel walls 302nd These components are then with the help of an adhesive such. B. hot melt, attached to the places provided for them. The two substantially plastic shell-shaped first and second cladding walls 302 and 303 are then filled with a predetermined amount of chemically reactive foam of the desired density, such as. B. polyurethane, consisting of various combinations of foams based on polyolisocyanates. The shell-like cladding walls 302 and 303 continue to be joined together and thus practically closed and, in this state, placed in a plate press or deck press for the purpose of a short-term curing (5-10 minutes), so that ultimately a solid, compact construction is obtained. Finally, the rest of the components of the base or stand are added to complete it.

The invention is not limited to the illustrated and described exemplary embodiments. This also includes all professional modifications and further training as well as partial and sub-combinations of the described and / or illustrated features and measures. For example, the monitoring system 1 according to FIG. 1 can be equipped with a further analog multiplexer 78 as shown in FIG. 2C for collecting diagnostic information relating to the components of the receiver. This diagnostic information can then be read by the microcomputer 21 to diagnose any problems associated with the components of the receiver.

Reference symbol list

1 monitoring system
11 AC drivers
11-1 zero zone
11-1 ′ zero zone
11-2 zero zone
11-3 zero zone
12 transmitting antenna
13 Interrogation or monitoring zone
14 items
15 markers
16 receiving antenna
17 Control unit interface and receiver
18 central control unit
19 alarm unit
21 microcomputers (or microprocessors)
21 a time domain counter
21 b Frequency range counter
22 main program memory (EPROM)
23 permanent memory (NOVRAM)
24 programming interface
25 status indicators
26 watchdog circuitry
27 Auxiliary interface
28 address latch
29 analog / digital converter
30 driver channel
31 CPU clock
32 Programmable divider
33 low pass filter
34 level adjuster multiplexer
35 buffer amplifier
50 fundamental detector
51 notch or trap filter
52 notch or trap filters
53 preamplifiers
54 high-pass filters
55 low pass filters
56 Pre-emphasis circuit
57 delimiter
58 comb bandpass filters
58 a comb notch or trap filter
59 low pass filter
61 shielding spike blanker (shield-spike-blanker)
62 low-range bandpass filter
63 Mid-range bandpass filters
64 high-range bandpass filters
65 full wave rectifier
66 full wave rectifier
67 full wave rectifier
68 Synchronous integrator
69 synchronous integrator
71 synchronous integrator
72 high-pass filters
73 full-wave rectifier
74 Sample and hold circuit
75 analog multiplexers
76 level shifter
77 Sequential logic
78 analog multiplexers
79 power amplifiers (of 11 )
81 current sensor (connected to 12
82 alarm device (acoustic)
83 alarm device (visual)
84 event counters
85 Alarm interface circuit
90 receiving antenna arrangement
91 first upper loop
91 A section of the loop
92 second upper loop
92 A section of the second loop
93 fourth lower loop
93 A section of the fourth loop
94 third lower loop
94 A section of the third loop
100 time range program
101 step
102 step
103 step
104 step
105 step
106 step
107 step
108 step
200 frequency range program
201 step
202 step
203 step
204 step
301 base or stand
302 cladding wall
303 cladding wall
304 coil (or transmitting antenna arrangement)
305 coil (or receiving antenna arrangement)
306 stiffening bar
307 solidified foam
308 hook-shaped end section (from 302 and 303 )
309 Edge protection strips or buffers
311 support base
401 waveform
402 waveform
403 waveform
500 spike detection routine
501 step
502 step
503 step
504 step
505 step
506 step
507 step
508 step
509 step
510 step
511 step
512 step
513 step
514 step
515 step
516 step
517 step
601 digital delay line
602 input connector
603 output connector
604 clock connection
605 feedback device
606 clock signal transmitter
701 Square wave conversion circuit
(signal squaring circuit)
702 comb bandpass filter
703 digital delay line
703 a input connector
703 b output connector
704 summing circuit
705 buffers
707 buffers
708 reverse buffer
709 PLL circuit (phase locked loop)
711 dividers
801 transmit antenna arrangement
802 first loop
803 second loop
A first signal branching channel
B second signal branching channel
C time domain channel
D frequency range channel
D 1 subchannel
D 2 subchannel
D 3 subchannel
C ₁ center of the loop 91st
C ₂ center of loop 92
C ₃ center of loop 93
C ₄ center of loop 94

Claims (62)

1. Monitoring system for determining the presence of articles provided with magnetic markers in a query or monitoring zone, in particular for theft protection, characterized by the combination of the following features:

• - Transmitting means for establishing a magnetic field at a preselectable frequency in the interrogation or monitoring zone ( 13 );
• - Receiving means ( 17 ) for ascertaining disturbances in the magnetic field in the interrogation or monitoring zone ( 13 ) and for emitting a first signal characterizing this.
  • - wherein these receiving means have a first channel for delivering a first signal content, which is characteristic of the energy levels in each of a number of bands of said first signal.
• - And evaluation means responsive to the first signal content for comparing the energy levels, which are characterized by the first signal content, for the purpose of evaluating whether an article is present in the query or monitoring zone.

2. Plant according to claim 1, characterized, that the number of tapes is at least three. 3. Plant according to claim 1 or 2, characterized in that

• - The first signal content for each band from the number of bands has signal levels which are characteristic of the energy in the respective band, and
• - The evaluation means compare these signal levels in the course of the evaluation.

4. Plant according to claim 3, characterized, that the evaluation means are designed so that they the comparison of the signal levels as characteristic of an item in the query or surveillance zone handle if the signal levels are the same stay or move on or move on from a signal level of a lower band to a signal level of a higher band. 5. Plant according to one of the preceding claims, characterized in

• that the first channel has sub-channels (D 1 , D 2 , D 3 ) for each of the bands mentioned,
  • - wherein each of these sub-channels (D 1 , D 2 , D 3 ) has the following components: a bandpass filter ( 62, 63, 64 ) which comprises the band assigned to the sub-channel; a rectifier ( 65, 66, 67 ) connected downstream of the bandpass filter; and an integrator ( 68, 69, 71 ) connected downstream of the rectifier.

6. Installation according to one of the preceding claims, characterized in that each of the magnetic markers ( 15 ) is of a type which generates high harmonics of the preselected frequency, namely at low threshold values of the field in the interrogation or monitoring zone ( 13 ) . 7. Plant according to claim 6, characterized in that the magnetic marker ( 15 ) consists of a material with an internal, pent-up mechanical tension, which has a large Barkhausen discontinuity in its characteristic hysteresis loop. 8. Plant according to one of the preceding claims, characterized in that

• - The transmission means have an antenna arrangement ( 12 ) and driver means ( 11 ) which generate a current for operating the antenna arrangement at the preselected frequency;
• - At least one magnetic shielding device for limiting the magnetic field to the interrogation or monitoring zone ( 13 ) is provided, this shielding device generating disturbances in the magnetic field at the peaks of the driver current;
• - each magnetic marker ( 15 ) generates disturbances in the magnetic field in the vicinity of the zero crossings of the driver current; and
• - Means ( 61 ) for blanking the first signal near the peaks of the driver current are provided.

9. Monitoring system for determining the presence of articles provided with magnetic markers in an interrogation or monitoring zone, in particular for theft protection, characterized by:

• - Transmitting means for establishing a magnetic field at a preselectable frequency in the monitoring zone ( 13 );
• - Receiving means ( 17 ) for detecting disturbances in the magnetic field in the monitoring zone and for emitting a first signal which characterizes this, wherein
  • - These receiving means have a first channel for generating and delivering a first signal content, which is characteristic of samples of the first signal, and evaluation means, which respond to the first signal content, for the presence of a marker in the monitoring zone to determine.

10. Plant according to claim 9, characterized, that each half period of the preselected frequency forms a frame and the first signal content during each frame samples of the first Signal during this frame. 11. Plant according to claim 10, characterized, that the evaluation means for the purpose of evaluation one or more during a particular framework Samples of the first, assigned to the particular frame Signals with one or more samples the first, assigned to a different frame Compare signals. 12. Plant according to claim 11, characterized, that the other frame is two frames earlier the particular frame.   13. Plant according to claim 11 or 12, characterized, that the evaluation means the position of the tip sample in the particular frame with the position of the top sample in the other frame mentioned to compare. 14. Installation according to one of claims 11 to 13, characterized, that the evaluation means for the purpose of evaluation with regard to a particular framework, one or several samples of the first signal with a preselected one Compare threshold. 15. Plant according to claim 14, characterized, that the preselected threshold with respect to the ambient noise level is characteristic in the system. 16. Plant according to claim 14 or 15, characterized, that the comparison is whether a preselected one Number of samples the preselected threshold exceeds.   17. Plant according to claim 10, characterized, that the evaluation means in the course of the evaluation process during a particular framework or several samples of the first signal first frame with one or more samples of the first signal of another frame or more compare other frame. 18. Plant according to claim 17, characterized in that

• the samples being compared are the top samples of each frame;
• - The first frame is the frame that has the largest or highest peak sample;
• - and the one or more other frames is or are an odd or odd frame if the first frame is an odd frame and is or are an even or even frame if the first frame is an even frame.

19. Plant according to claim 18, characterized, that the comparison is forming the relationships of the Includes peak values. 20. System according to claim 9, characterized in that the receiving means ( 17 ) have a second channel for emitting a second signal content, which is characteristic of the energy levels in each of a plurality of bands of the first signal, the evaluation means being responsive to the second signal content. 21. Plant according to claim 20, characterized, that the evaluation means in the course of the evaluation process the first signal content in agreement with a first pre-selected decision criterion and the second signal content in accordance with a second pre-selected decision criterion to process.   22. Installation according to claim 21, characterized, that the evaluation means the second signal content process every time they receive the first signal content to process. 23. Installation according to claim 22, characterized, that each half period of the preselected frequency forms a frame. 24. System according to claim 22 or 23, characterized in that the processing of the first and the second signal content takes place in each frame and that the evaluation means have first and second counters ( 21 a , 21 b) , which are in agreement updated with the processing of the first and second signal content. 25. Plant according to claim 24, characterized in that in the event that the first and second counters ( 21 a , 21 b) each reach first and second preselected values, the evaluation means create the conditions for triggering an alarm. 26. Plant according to claim 25, characterized in that

• - Have the receiving means ( 17 ) for extracting or for obtaining the component of the first signal at the preselected frequency; and
• - The evaluation means compare the aforementioned component with a preselected value and prevent the alarm from being triggered if the preselected value is exceeded.

27. Plant according to one of claims 20-26, characterized in that the evaluation means contain a microcomputer ( 21 ). 28. Monitoring system for determining the presence of articles provided with magnetic markers in a query or monitoring zone, in particular for theft protection, characterized by the combination of the following features:

• - Transmitting means for establishing a magnetic field at a preselected frequency in the interrogation or monitoring zone ( 13 );
• - Receiving means ( 17 ) for detecting disturbances in the magnetic field in the interrogation or monitoring zone and for emitting a first signal, which characterizes this; and
• - Evaluation means responsive to the receiving means ( 17 ) for evaluating whether an article is present in the query or monitoring zone, these evaluation means containing a microcomputer ( 21 ).

29. System according to claim 28, characterized in that the microcomputer ( 21 ) has preselected decision programs for carrying out the evaluation. 30. System according to claim 29, characterized, that the decision programs time domain and Include frequency domain analyzes. 31. System according to claim 28, 29 or 30, characterized in that an interface ( 24 ) for external communication with the microcomputer ( 21 ) is provided. 32. System according to one of claims 28-31, characterized in that a permanent memory ( 23 ) communicating with the microcomputer ( 21 ) is provided for storing the operating parameters for the system. 33. Surveillance system for determining the presence of articles provided with magnetic markers in a query or surveillance zone, in particular for theft protection, characterized by the combination of the following features:

• - Sending means for establishing a magnetic field at a preselected frequency in the interrogation or monitoring zone ( 13 ), said transmitting means having an antenna arrangement ( 12 ) and a device ( 11 ) for operating this antenna arrangement;
• - Receiving means ( 17 ) for detecting disturbances in the magnetic field in the interrogation or monitoring zone and for emitting a first signal, which characterizes this;
  Evaluation means, which respond to the receiving means ( 17 ) and serve to deliver the presence of a marker in the query or monitoring zone as the evaluation result;
• - Detection means for detecting the voltage and the current for the antenna arrangement ( 12 );
• - Means for comparing the phase of the voltage and the current for the antenna arrangement ( 12 ); and
• - Means for adjusting or adjusting the antenna driver device for the purpose of reducing the phase shift to zero.

34. Receiver for use in an article monitoring system for determining the presence of articles in an interrogation or monitoring zone, in which system a magnetic field is built up at a preselected frequency and an antenna arrangement is provided which detects interference in the magnetic field, in particular for theft protection, characterized in that the receiver ( 17 ) has the following components:

• Means adapted to respond to the antenna arrangement and generate a first signal; and
• - Means for processing the first signal, said means comprising a comb bandpass filter ( 58 ) with pass bands at harmonics of the preselected frequency.

35. Receiver according to claim 34, characterized in that the comb bandpass filter ( 58 ) has the following components:

• - A digital delay line ( 601 ) with a number of delay stages and input and output connections ( 602, 603 );
• - a clock connector ( 604 ); and
• - A feedback device ( 605 ) for coupling the input and the output connections.

36. Receiver according to claim 35, characterized in that a clock signal transmitter ( 606 ) is provided for supplying a clock signal to the clock connection ( 604 ), this clock signal having a frequency which is equal to the product of the preselected frequency, the Is the number of clock cycles required to carry the information from stage to stage in the delay line ( 601 ). 37. Receiver for an article surveillance system, in particular for theft protection, in which the presence of articles is determined in an interrogation or surveillance zone in which a magnetic field has been built up at a preselected frequency, the system being antenna arrangements for detecting interference in the magnetic field and the system being subjected to signals of a predetermined frequency from a supply network, characterized in that the receiver ( 17 ) has the following components:

• Means adapted to respond to an antenna arrangement and to generate a first signal; and
• - Means for processing the first signal, these means having a comb-notch filter or comb-trap filter ( 58 a) with stop bands at the harmonics of the predetermined network frequency.

38. Receiver according to claim 37, characterized in that the comb-notch filter or trap filter ( 58 a) has the following components:

• - A comb bandpass filter ( 702 ) with bands at harmonics of the predetermined network frequencies;
• - means for applying the first signal to the comb bandpass filter ( 702 ) and for outputting a filtered signal; and
• Means for subtracting the first signal and the filtered signal.

39. Receiver according to claim 38, characterized in that the comb bandpass filter ( 702 ) has the following components:

• - A digital delay line ( 703 ) with a number of delay stages and with input and output connections;
• - a clock connector; and
• - A feedback device for coupling the input and output connections.

40. comb bandpass filter, characterized by a digital delay line ( 601 ) with a number of delay stages, input and output connections ( 602, 603 ) and a clock connection ( 604 ), and a feedback device ( 605 ) for coupling the input and output connections. 41. Comb bandpass filter according to claim 40, characterized by clock means ( 606 ) for supplying a clock signal to the clock connection. 42. Comb-notch filter or trap filter, characterized by the combination of the following features:

• - a digital delay line ( 703 ) with a number of delay stages;
• - Input and output connections and a clock connection;
• - Feedback means for connecting the input to the output connections;
• - A first connection for receiving an input signal, this first connection being coupled to the input connection ( 703 a) of the delay line ( 703 ); and
• - A differentiating circuit with first and second inputs, respectively, which are coupled to the first connection and to the input connection of the delay line ( 703 ).

43. Antenna arrangement for a system for article surveillance, in particular for theft protection, characterized by the combination of the following features:

• - first and second upper loops ( 91, 92 ) arranged in a nested relationship;
• - third and fourth lower loops ( 94, 93 ) arranged in a nested relationship;
• - The first ( 91 ) and the fourth loop ( 93 ) form a first eight-shaped configuration; and
• - The second ( 92 ) and the third loop ( 94 ) form a second eight-shaped configuration.

44. Antenna arrangement according to claim 43, characterized in that the first ( 91 ) and the second loop ( 92 ) have the same first phase and that the third ( 94 ) and the fourth loop ( 93 ) have the same second phase, which are opposite to the first phase. 45. Antenna arrangement according to claim 44, characterized in that the first to fourth loops ( 91, 92, 94, 93 ) are electrically connected in series. 46. Antenna arrangement according to one of claims 43 to 45, characterized in that all loops ( 91 to 94 ) are wound in the same direction. 47. Antenna arrangement according to one of claims 43 to 46, characterized in that the sections ( 91 A , 94 A) of the first and fourth loops in the contact area of the first eight-shaped configuration are inclined obliquely with respect to the horizontal and the sections ( 92 A , 94 A) of the second and third loops in the contact area of the second eight-shaped configuration are also inclined obliquely with respect to the horizontal. 48. Antenna arrangement according to claim 47, characterized in that the inclinations of the inclined loop sections ( 91 A , 92 A , 93 A , 94 A) are the same. 49. Antenna arrangement according to one of claims 43-48, characterized in that the center (C ₁) of the first loop ( 91 ) from the center (C ₂) of the second loop ( 92 ), while the center (C ₄) of third loop ( 94 ) from the center (C ₃) of the fourth loop ( 93 ). 50. Antenna arrangement according to one of claims 43-49, characterized by a nested arrangement of the second eight-shaped loop configuration ( 92, 94 ) in the first eight-shaped loop configuration ( 91, 93 ). 51. Antenna arrangement according to one of claims 43-50, characterized in that the two lower loops ( 93, 94 ) are mirror images of the two upper loops ( 91, 92 ). 52. Antenna arrangement for a system for article surveillance, in particular for theft protection, characterized in that a first loop ( 802 ) is provided and a second loop ( 803 ) is arranged nested within the first loop ( 802 ), the axis of the second loop ( 803 ) is rotated with respect to the axis of the first loop ( 802 ). 53. Antenna arrangement according to claim 52, characterized in that the loops ( 802, 803 ) lie in the same plane. 54. Antenna arrangement according to claim 52 or 53, characterized in that the angle between the axes of the two loops ( 802, 803 ) is approximately 45 °. 55. Base or stand for at least one antenna arrangement for a system for article surveillance, in particular for theft protection, characterized by the combination of the following features:

• - First and second cladding walls ( 302, 303 ) at a mutual distance;
• - at least one antenna arrangement ( 304, 305 ), which is accommodated within the said distance; and
• - Solidified foam ( 307 ) to fill the cavity between the walls ( 302, 303 ) and to surround the antenna arrangement ( 304, 305 ), whereby it is immovable and the base frame or the stand are stiffened and reinforced.

56. Base frame according to claim 55, characterized in that the first and second walls ( 302, 303 ) are each formed in the form of thin plastic panels. 57. Base frame according to claim 56, characterized in that the edges of the cladding walls ( 302, 303 ) are hook-shaped and for carrying protective elements, such as. B. edge protective strips or protective buffers ( 309 ) are used. 58. Process for the production of a pedestal or stand for use within a system for article surveillance, in particular for theft protection, characterized by the following process steps:

• a) introducing at least one antenna arrangement into a cavity of a first wall;
• b) filling the cavity of the first wall with a curable foam in an initially unconsolidated state;
• c) filling a cavity of a second wall with a curable foam in its initially unconsolidated state;
• d) joining the two first and second walls in such a way that the two foam-filled cavities adjoin one another; and
• e) allowing the foam filled into the cavities to harden.

59. The method according to claim 58, characterized, that during the curing of the foam on the pressure exerted on both first and second walls becomes.   60. System according to claim 28, characterized in that the microcomputer ( 21 ) stores information relating to the ambient noise level, this information being updated during the operation of the system. 61. System according to claim 28, characterized in that the microcomputer ( 21 ) monitors the receiving means ( 17 ) for diagnostic purposes. 62. System according to claim 28, characterized in that the microcomputer ( 21 ) generates alarm signals for indicating the presence of a marker in the query or monitoring zone.