DE3828691B4 - Electronic article surveillance system - Google Patents

Abstract

Monitoring system for determining the presence of articles provided with magnetic markers in a query or monitoring zone, in particular for theft protection, with a
- Transmitting device for establishing a magnetic field at a preselectable frequency in the interrogation or monitoring zone (13);
- Receiving device (17) for ascertaining faults in the magnetic field in the interrogation or monitoring zone (13) and for emitting a first signal characterizing this, said receiving means
- Have a frequency range channel (D), into which the first signal is fed and which generates a further signal, the content of which is characteristic of the energy levels in each of a predetermined number of frequency bands (62, 63, 64) of the first signal as well
- Have a time domain channel (C) into which the first signal is fed and which generates a third signal, the content of which contains the information as to whether a signal level of the first signal obtained by sampling lies within an acceptable signal-to-noise ratio such as...

Description

The The invention relates to electronic article surveillance systems or systems and particularly relates to electronic article surveillance systems or systems in which magnetic markers are used for monitoring purposes come.

In the present application, the following publications are cited regarding the prior art:
U.S. Patents 4,660,025 and 4,686,516 (both correspond to the DE-OS 35 41 536 );
DE-OS 37 15 387 ,

electronic Article surveillance systems or systems in which magnetic markers are attached to the monitoring underlying articles and goods for the purpose of theft protection are already known. With these systems or systems there is a query or monitoring zone with the help of an antenna system under the influence of a alternating magnetic field. If one with a suitable magnetic Marked article in this query or surveillance zone then the presence of the marker causes in a disturbance in this zone the magnetic field.

This Field in the query or monitoring zone is sensed by means of a receiving antenna, the output of which contains this interference. A detector analyzes that from the recipient coming signal to estimate or assess whether a disorder of the magnetic field has taken place and whether, if applicable such a disturbance resulted from a marker. If applicable, an alarm is triggered as Indication for the existence Marker the surveillance or query zone happens. In known electronic article surveillance systems, the interference generated by a magnetic marker in the magnetic field With the help of methods that are based on a frequency domain or time domain analysis (frequency domain or time domain analysis techniques). If in typical Way frequency domain methods are used, then the Frequency content of the received signal with respect to harmonics of the fundamental frequency, i.e. the driver frequency of the field used. This Harmonics are the result of a non-linear hysteresis characteristic of the magnetic marker. By comparing the relative Amplitudes of the detected harmonics are an indication of the frequency spectrum of the Receive signal. Using certain decision criteria this spectrum is then compared to a spectrum of a valid one Marker is expected and this will make a decision in terms of of the presence of the marker. The application of frequency domain analysis methods is especially desirable if conditions are characterized by noise or interference are to be expected, however, the response time of the known system too long due to the fact that filters with high Q factor are required to isolate the harmonics generated.

at a method based on a time domain analysis becomes a Time domain pulse (or a pulse train) of the received signal relative to its pulse shape and its time shift analyzed to a basic phase of the generated field. At this Kind of analysis is the shape of the signal considerably by the amplitude and phase characteristics of the one used in the acquisition process Filtering affects as well as through changes in the received signal due to variations in the generated one Driver field. Because this analysis uses amplitude thresholds that are above the Umgebungsstörpegel have to lie the application of this method is most desirable in those cases where who have a high signal / interference ratio.

Around estimate or to be able to assess whether Time domain or Frequency range analysis method within the monitoring system it is essential that the registration process in the Is able to distinguish between disturbances or changes to be differentiated in the field from valid markers, and such disorders or changes, that arise from other sources that are independent of the markers. On Failure of the system to make the necessary distinction leads to false alarm triggers, which the blamelessness and the usability of the whole system in extraordinary Question way.

The problem of reliably distinguishing between interference caused by magnetic markers and between external interference that is independent of markers is of particular importance in retail stores, in particular supermarkets or the like, in which metallic fittings, metallic counters, metallic shopping trolleys, noise or interference generators (e.g. laser scanners, digital scales, credit card and barcode readers, conveyor belts and the like) are abundant. This equipment creates quasi "rough" electronic environmental conditions and can cause disturbances in the detection system, which hide or cover signals originating from valid markers cover and / or appear as signals originating from valid markers. This seriously jeopardizes the reliability of the monitoring system.

The monitoring systems currently available or systems solve this reliability problem not completely and suffer about it also under other disadvantages. So are already spacious or large-scale Systems executed been having a river through the surveillance zone is caused. Also with these systems is a relatively strong field Made use of. In the end, however, these fields extend often about the query or monitoring zone addition, reducing the likelihood of interference, which are from marker-independent Sources or from outside the surveillance zone markers located, still increased becomes. Moreover is in these known systems as a result of their overreaches System sensitivity associated with electronic noise in considerable Scope has been reduced. This resulted often at a relatively low level Capture rate and an undesirable Number of false alarm triggers.

Therefore, a magnetic marker having properties that can alleviate some of the aforementioned problems encountered in the known plants or systems has recently been developed. A magnetic marker of this type emerges from the U.S. Patent 4,660,025 with the designation "Magnetic marker for article monitoring with a hysteresis loop with large Barkhausen discontinuities" as already known. The marker has a magnetic material with internal pent-up mechanical tension and has a magnetic hysteresis loop, which is characterized by large Barkhausen discontinuities. As a result, when this marker is exposed to an external magnetic field above a threshold, its magnetic polarization is regeneratively reversed.

There it possible is that this regenerative reversal takes place at a relatively low threshold, can also advantageously the required for the marker generated field can be kept relatively weak. Furthermore results in the occurrence of the mark in the form of a jump function Reversal of magnetic polarization in field disturbances, which are rich in high harmonics, which makes detection easier and becomes easier.

The marker according to the aforementioned US-PS also has the advantage that it can be deactivated with the help of various methods, as is shown in the U.S. Patent 4,686,516 with the designation "Procedure, system and device for article monitoring" is described in more detail. By mitigating the internal pent-up mechanical stress in the marker or by crystallizing part of the marker, it is easily deactivated so that it can pass through the interrogation or monitoring zone without causing any alarm.

A "marker" can also as "day" i.e. "Security tag" or "security tag" may be referred to.

An improved system for magnetic article monitoring is also in the DE-OS 37 15 387 (Union priority: June 30, 1986 US 880 138 ). In this known system, a magnetic shield is provided, which is arranged in the lateral edge areas of the monitoring zone and is capable of reducing the intensity of the magnetic field outside of this zone. The shield is also capable of easily distinguishing and blocking or suppressing the interference in the magnetic field caused by the shield itself.

to Achievement of the aforementioned Properties require that the magnetic material the shielding has a sufficiently high, specific electrical Resistance at a given permeability and frequency of the generated magnetic field, thereby a depth of penetration or to achieve skin depth that is significantly greater than the layer thickness of the magnetic Shielding is. Furthermore, the magnetic material has a saturation flux density have larger than the maximum flux density, depending in the shield of the positive and negative maximum amplitudes of the magnetic field is produced. Finally must magnetic material still have the property that it Peak values of the generated magnetic field respond to it, but nearby of zero crossings one has little or no sensitivity.

How continue from the DE-OS 37 15 387 shields made from ferrites or from pressed iron powder have the characteristics explained above, so that a ratio of the front magnetic field, ie inside the surveillance zone, to the rear, ie outside the surveillance zone and behind the shield, existing tip Magnetic field of at least 10: 1 can be reached. A special ferrite, by means of which this ratio of 10: 1 is achieved and which also has a maximum response at the generated maximum field amplitudes and a mini has a fine response at the zero crossings, is for example a ferrite from the manufacturer TDK Corporation of Tokyo, Japan, which is known in the trade with Q5B. Similar characteristics exist for pressed iron powder, which is preferably not sintered and consists of approximately 99% iron with approximately 1% trace elements, which include FeP, H ₂ , C, Mn and S, and perhaps very small amounts of other elements.

If the magnetic shield is a laminate of one Majority of thin Layers that are glued together and electrically from each other are isolated, then the depth of penetration of each such layer much larger than the layer thickness itself.

The need for minimal response to the shield near the zero crossings of the generated field arises from the fact that the magnetic markers, and particularly the markers according to the U.S. Patent 46,660,025 have maximum responsiveness at or near the zero crossings of the generated field and minimal responsiveness at the peak values of that field. By selecting a magnetic shielding material with quasi-reversed properties, the interference caused by this shielding can be easily detected and eliminated without being overlapped with the detection of the interference caused by markers. From the DE-OS 37 15 387 it also follows that, in combination with the magnetic shield described above, an auxiliary shield made of electrically conductive material can be provided. This auxiliary shielding is placed behind the magnetic shielding and, through eddy current losses, dampens those disturbances which result from markers or external noise or disturbance sources which are located outside the interrogation or monitoring zone.

Even though that explained above magnetic brands as well as magnetic shielding already significant Improvements for magnetic, electronic article surveillance systems or systems represent, 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.

Out DE 31 28 980 C2 a method and a device for determining the unauthorized passage of protected objects through a surveillance zone is known. This method and the device make use of an electromagnetic field which is generated in the relevant monitoring zone. Furthermore, the protected objects are provided with a target, which lead to disturbances in the electromagnetic field, which are converted into electrical signals during evaluation, from which a detector signal is generated after evaluation, taking into account the influence of interference signals, in particular noise signals. The electrical signals are fed in parallel to at least three frequency-selective channels of different frequencies. The amplitudes of the output signals of these channels are compared with one another and the detector signal is then generated from the comparison result if the amplitudes have a specific relationship to one another within tolerance ranges.

Out US 3,820,103 the result is an anti-theft monitoring system in which interference caused by a magnetic marker in the form of harmonics of the transmission frequency is evaluated in an interrogation zone and triggers an alarm. The circuit for signal evaluation contains a series connection of low-pass filter and high-pass filter, which in this combination defines the bandwidth of the frequency band to be processed. Only low-order harmonics and high-frequency components are filtered out. Through subsequent integration, the energy level of the signal is determined and processed. The integration over a frequency band not only evaluates the amplitude but also the temporal extent of the emergency signal.

With consideration the present situation lies on the facts described above Invention therefore as the main task, an improved system or an improved system for electronic article surveillance to accomplish.

In particular, the invention has for its object to provide an improved article surveillance system according to the preamble of claim 1, which has in particular the following properties: increased control functions; expanded and improved possibilities of detection, ie the detection of the presence of articles provided with magnetic markers in an interrogation or monitoring zone; Possibilities of realizing unique frequency domain and unique time domain detection methods; Possibilities of responding and self-tuning due to changes in the environment including changes in the noise or interference level; Equipped with an improved comb bandpass filter and an improved comb notch filter; and finally the possibilities of using a magnetic marker according to the U.S. Patent 4,660,025 and a shield according to the DE-OS 37 15 387 ,

The solution:

The The task underlying the invention is based on a monitoring system in accordance with the preamble of claim 1, according to the invention the combination of features according to the characteristic Part of claim 1 solved. Further advantageous embodiments of the invention result from the claims 2-34.

The Operation of a magnetic electronic article surveillance system can preferably be controlled with the aid of a microcomputer.

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

At least two frequency bands of the received signal are through a frequency detection circuit isolated and the signal content in each band is determined. At the same time, the time domain circuit turns on Sampling of the received signal to thereby digitize it Generate signal. The results of these operations are then through clear decision programs processed in the microcomputer. These programs are focused on and improved criteria for valid markers have threshold values that indicate ambient noise as well an undesirable Response behavior are obtained from objects other than valid markers.

In the microcomputer receives decision information from the time and frequency domain programs for each half period of the generated Field generated. This information is used to and a frequency domain counter to bring it up to date. Dependent on assuming that these counters each preselected Reach meter readings Alarm trigger signals generated.

On another program in the microcomputer processes selected values of successive digitized signals in accordance with Thresholds based on the expected characteristics of undesired, high quality secondary interference (electronic noise peaks) based. Another program in the microcomputer is on changes in the fundamental frequency component lifted, which is also isolated by the detection circuit becomes. The exits These two latter programs are used to trigger an alarm to prevent the time and frequency domain counters.

Furthermore the noise or interference levels are monitored by the microcomputer, as soon as the system is put into operation and periodically on the updated, as well as the level of the fundamental frequency component. The computer programs are then updated with these values, so the facility themselves according to the changes to change dynamically in the environment can.

On in the system according to the invention Comb filter provided uses a delay line in the form of an integrated circuit with a feedback, to achieve the desired "comb response". This results in high Q filtering, the need for individual capacities conventional filter and the associated disadvantages (quantity, Volatility) avoided become.

The monitoring system according to the invention includes also receive and transmit antenna arrangements with one in the Are trained that unwanted and Normally existing zeros in the generated field are reduced become. In particular, a receiving antenna arrangement is provided which has first and second upper loops that are interlocked nested relationship are arranged, and further third and fourth lower loops, which are also nested in one another Relationship are arranged with the first and second loops with the associated third and fourth loops of eight-shaped configurations form. The loops are electrically one behind the other and in the Way in phase that the upper loops the same first phase and lower loops have the same second phase opposite to the first Phase is. According to one preferred embodiment of this antenna arrangement is a section of each this grinding in the touch area the respective eight-shaped Configuration opposite the horizontal obliquely inclined.

It is also a transmitting antenna arrangement also with one another nested arrangement of loops provided. With this antenna arrangement extends the current in an inner loop in the same direction as the current in an outer loop, where the axis of the inner loop is relative to the axis of the outer loop a selected one Angle measure rotated is, preferably by 45 °.

The explained in the foregoing Antenna arrangements of the system according to the invention are in pedestals or stands stored, which are designed in such a way that the antenna assemblies immobile be made and a physical or form a structural unit or wholeness (integrity). this will achieved by a manufacturing process in which the area between the walls of the pedestal filled with a foam and during which the curing pressure is applied to the entire assembly. The resulting, solidified foam encapsulates the antennas and thus making them immobile while the foam the cavity between the walls of the base or stand essentially fills out whereby the base the desired one reinforcement and stiffening is conferred.

It should also be pointed out that within the scope of the present invention use is made of a marker as used in the U.S. Patent 4,660,025 and that in connection with the antenna arrangements housed in the base frame a shield DE-OS 37 153 87 applied. In this case, the system according to the invention is also capable of preventing the shielding peak pulse explained above from influencing the mode of operation of the system.

to closer explanation The following, the invention, its further features and advantages serve Description of preferred embodiments using the attached Drawings. In these show:

1 a simplified block diagram of a magnetic, electronic monitoring system which is assigned to an interrogation or monitoring zone;

2A . 2 B and 2C each detailed block diagram of the monitoring system according to 1 ;

3 a diagram with the waveforms of different at the monitoring system according to the 2A - 2C generated signals;

4 a tabular overview of a number of parameters that are required for the monitoring system according to the 2A - 2C are decisive, whereby representative numerical values for these parameters are given;

5A and 5B in each case flow diagrams relating to those in a microcomputer of the monitoring system according to 2A to 2C programs included, on the one hand for a time domain signal analysis and on the other hand for a frequency domain signal analysis;

6 a flowchart relating to a in the microcomputer of the monitoring system according to 2A - 2C included programs for a "peak" detection signal analysis;

7 and 8th Circuit diagrams of a comb bandpass filter according to the in 2A block diagram shown;

9 . 9A and 9B Circuit diagrams for a comb-notch or comb-trap filter according to the in 2A block diagram shown;

10 schematically shows a plan view of a receiving antenna arrangement for the monitoring system according to the 2A - 2C ;

11 schematically shows a plan view of a transmitting antenna arrangement for the monitoring system according to the 2A - 2C ;

12 (I) in a schematic graphic representation the zero zones of a conventional receiving antenna arrangement;

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

13 is a schematic, perspective view of an arrangement consisting of pedestals or stands for accommodating the antenna arrangements according to the 10 and 11 ;

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

15 the microcomputer of the monitoring system according to the 2A - 2C with counters and with a maximum amplitude stack.

In the 1 is a magnetic, electronic surveillance system in general with 1 designated. The surveillance system 1 has an AC driver 11 on a sinusoidal alternating current of high polarity and a corresponding voltage at a fundamental frequency Fo to a transmitting antenna 12 supplies. This transmitting antenna 12 consequently generates an alternating magnetic field with the frequency Fo, which is in an interrogation or monitoring zone 13 spreads through the articles or goods under surveillance, for example an article 14 , have to go through.

On this article 14 is a marker 15 attached from a magnetic material. The marker 15 thus generates interference in the magnetic field which has harmonic vibrations of the fundamental frequency Fo and by a receiving antenna 16 be sensed. This receiving antenna 16 converts the interference into an electrical signal that according to 1 to one with control unit interface and receiver 17 designated block is forwarded. Control unit interface and receiver 17 derive frequency and time domain information from the received signal through the interface of a central control unit 18 to provide.

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 addresses 18 a downstream alarm unit 19 , This alarm unit 19 is consequently put into operation to provide an indication that an article 14 within the query or surveillance zone 13 located.

The central control unit 18 also controls the AC driver 11 and receives information regarding the state of the alternating current and the alternating voltage for the transmitting antenna 12 , In addition, the central control unit delivers 18 Time calculation (timing), address and other information at 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 is the marker 15 around a marker of the type according to the U.S. Patent 4,660,025 which has a hysteresis loop with a large Barkhausen discontinuity or a large step function change in the flux every time the generated field reverses its polarity and exceeds a relatively low threshold. The through the marker 15 Interference caused by the field therefore occurs in the vicinity of the zero crossings of the generated field and is 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 200μsec.).

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

Although according to a preferred embodiment of the monitoring system according to the invention from a marker according to the U.S. Patent 4,660,025 as well as a shield according to the DE-OS 37 15 387 If use is made, the principles on which the present invention is based can also be applied to other designs of markers and antennas. In such a case, there is the possibility of modifying the design of the monitoring system and the decision programs in order to take into account the special properties of the special markers and / or antennas used.

More details from the central control unit 18 and control unit interface and receiver 17 are now based on the 2A . 2 B and 2C explained. The central control unit 18 is with a microcomputer 21 equipped, which is, for example, an 8031 microcomputer from the manufacturer Intel and which is a primary sequence control with regard to the operating sequence of the monitoring system 1 causes. The microcomputer 21 communicates with a main program memory (EPROM) 22 which contains the main programs as well as the decision routines for the microcomputer. There is also a second memory, ie a permanent memory (NOV-RAM) 23 for storing the operational parameters of the monitoring system 1 intended. This NOVRAM 23 holds these parameters in the event of a loss of energy or power in the monitoring system 1 firmly.

It is still a programming interface 24 for external communication with the microcomputer 21 intended. the interface 24 allows the operation parameters in the NOVRAM 23 for the purpose of adapting to the special, on-the-spot conditions under which the monitoring system is currently being used.

The programming interface also allows 24 a variety of diagnostic measures to check the operation of the monitoring system 1 are to be carried out.

The microcomputer 21 delivers to a status indicator 25 a signal regarding the status of the monitoring system. A watchdog circuit 26 (watchdog circuitry) acts on the microcomputer 21 after turning on the power supply with a reset pulse that commands the computer to begin executing the main program. An auxiliary interface 27 is also provided.

An address switch 28 allow that microcomputer 21 , also other components both in the central control unit 18 as well as in the control unit interface and the receiver 17 to address. It is still an analog / digital converter 29 provided that is used by the recipient 17 convert analog data supplied into digital data by the microcomputer 21 are processed. A general CPU clock operating at 12 MHz 31 provides a primary clock signal for the monitoring system 1 ,

The CPU clock 31 feeds both the microcomputer 21 as well as 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 includes a driver channel 30 (please refer 2 B ) to provide the driver current for the transmitting antenna 12 , The driver channel 30 has low-pass filters 33 (please refer 2A ), a level setting multiplexer 32 as well as a buffer amplifier 35 (please refer 2 B ) on.

From the 2C also goes a power amplifier 79 of the AC driver 11 as well as a current sensor 81 the one with the transmitting antenna 12 connected is. In the 2C are also an acoustic alarm 82 , a visual alarm 83 , an event counter 84 and an alarm interface circuit 85 the alarm unit 19 shown.

As already explained above, the programming interface is used 24 the microcomputer 21 to be addressed so that in the non-volatile memory (NOVRAM) 23 the special operating parameters desired for the monitoring system are set. In most cases, these operating parameters depend on the environment in which the monitoring system is located 1 is used. From the 4 representative operating parameters can be seen in the NOVRAM 23 are programmable, with exemplary values for these parameters being given.

Permanent storage (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 have been set, the microcomputer drives 21 so that the current for the transmitting antenna through the driver channel 30 is set. 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 SSB1, which is twice the desired frequency (2Fo) and which is by the programmable divider 32 from the 12 MHz clock signal (CPU clock 31 ) is derived.

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

The current sensor 81 feels the current in the coils of the transmitting antenna 12 and delivers its output to an input TXI of an analog multiplexer 75 who is in the block 17 (Control unit interface and receiver) is included. This input is through the microcomputer 21 through the address switch 28 addressed and the address input ADDR of the multiplexer 78 and the transmit antenna current is via analog / digital converter 29 read.

The microcomputer 21 then compares this current value with that in the NOVRAM 23 stored predetermined current value and, if there is a difference between these two values, the microcomputer addresses 21 the stage setting multiplexer 34 via the address switch 28 to cause the current to change 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 resets 21 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 SSB1 signal, while the zero-crossings of the driver current are exactly at the logical one-level intervals. This relationship, based on the graphical representation of the waveforms 401 and 402 according to 3 is explained in more detail allows the SSB1 signal to be blanked out during the maximum amplitudes of the current intended for the transmitting antenna and, on the other hand, to pass signals close to the intervals each containing a zero crossing. As a result, the signal SSB1 can be used to shield the waveform 403 in 3 can be seen, which is due to the use of a shield according to the DE-OS 37 15 387 within the surveillance system 1 according to 1 is produced. This will be explained in more detail below.

How to continue from the 2C is the monitoring system 1 also able to ensure operation of the transmit antenna arrangement when resonating. The multiplexer receives for this purpose 75 , in addition to receiving transmitter 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 reads and stores 21 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, the microcomputer recognizes 21 a non-resonance state of the transmitting antenna 12 and then adjusts the transmit frequency so that the transmitter current is brought into phase with the transmitter voltage so that the transmitter antenna 12 is operated at resonance.

If it is now assumed that the phase and level of the transmitter current are set, then the microcomputer addresses 21 via the address switch 28 and the address input ADDR the inputs PFO, PFB1, PFB2 and PFB3 of the analog multiplexer 75 , The receiver in the block "Control unit interface and receiver" 17 according to 1 is included, supplies to these aforementioned respective inputs the basic component level of the received signal and the respective received signal energy level in the three preselected frequency bands FB1, FB2 and FB3. 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 carried out repeatedly by the microcomputer at every half-period of the transmitter stream, such a half-period being referred to as a "frame" (cf. 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 surveillance zone 13 according to 1 are.

How continue from the 2A can be seen, 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 is a fundamental detector 50 arranged, which extracts the fundamental frequency component, ie the component at the frequency Fo, of the signal by means of a bandpass filter. This level is then the analog multiplexer 75 according to 2C PFo provided at its entry to enable subsequent analysis by the microcomputer.

In the second signal branching channel B according to 2A are two notch or trap filters 51 and 52 arranged, each eliminating 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 in a preamplifier 53 amplified and this amplified signal is sequentially through a high pass filter 54 and a low pass filter 55 sent, which effectively isolate the frequency band in which the harmonic vibrations that are possible for markers used in connection with the present monitoring system are expected. The Indian 2 B indicated special frequency range refers to a preferred marker according to the U.S. Patent 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 minimize the noise on the detection process.

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

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 bandpass filter 58 is the sampling noise caused by this through a low pass filter 59 eliminated. That from the low-pass filter 59 The resulting resulting signal 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.

How 2A further shows, in the frequency domain channel D the signal first passes through a shielding pulse peak blanker 61 which is controlled by the signal SSB1, such that the signal content is suppressed during a zero or blanking interval of the signal SSB1 and that the signal is passed through during a non-blanking interval of the signal SSB1. 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 extracted from the signal by the shielding pulse peak blanker 61 eliminated.

Following blanking, the resulting signal branches into a number of frequency domain subchannels, as detailed in the 2 B can be seen. 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 in the query or monitoring zone 13 according to 1 is available.

The number of frequency domain subchannels is selected from the viewpoint that it is ensured 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, according to 2 B three subchannels D1, D2 and D3 are provided which correspond to a low frequency band FB1, a medium frequency band FB2 and a high frequency band FB3.

In detail, the frequency bands FB1, FB2 and FB3 are replaced by corresponding low-range, medium-range and high-range band-pass filters 62 . 63 and 64 isolated and then in full-wave rectifiers 65 . 66 and 67 rectified. Downstream synchronous integrators 68 . 69 and 71 then integrate the rectified signals, thereby producing DC voltage values representative of the energy in each of the bands FB1, FB2 and FB3. These DC voltage values then appear at the corresponding inputs PFB1, PFB2 and PFB3 of the analog multiplexer 75 according to 2C for subsequent processing by the microcomputer 21 ,

Operation of the synchronous integrators 68 . 69 and 71 is synchronized with the microcomputer operation by a second synchronization signal SSB2, this signal SSB2 via a level shifter 76 is directed to the aforementioned integrators. The signal SSB2 has the same frequency (2Fo) as the signal SSB1, but on the other hand a slight phase shift by the bandpass filter 62 to 64 introduced signal delay to take into account. It should also be noted that the gain factors of the bandpass filter 62 to 64 are chosen in such a way that these are the outputs of the synchronous integrators 68 . 69 and 71 normalize based on a preselected property representative of the response expected from the markers used. Therefore, if a marker 15 in the query or surveillance zone 13 according to 1 is present, the outputs of the synchronous integrators 68 . 69 and 71 be the same or have increasing levels.

For that in the one explained here embodiment envisaged Frequency band are the bands of FB1, FB2 and FB3 around frequencies of 1.5, 2.5 and 3.5 KHz centered. Moreover is the bandwidth of each band 600 Hz.

If we now turn to time domain channel C (cf. 2A ), it should first be mentioned that this channel is also the output of the low-pass filter 59 records, whereupon its output signal through a subsequent high-pass filter 72 runs through (cf. 2 B ). The bottom edge of the high pass filter 72 is chosen from the point of view that sufficiently high harmonics of the signal are let through so that a good image of expected changes in a marker pulse can be obtained. The desired signal content is adequately guaranteed by a filter edge which is approximately at the center of the high-bandpass subchannel D3, ie at 3.5 kHz in the present exemplary embodiment.

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

The sampling interval and the correct timing or timing of the sample and hold circuit 74 are through the microcomputer 21 through sequential logic 77 controlled. The sample and hold circuit samples during each sampling interval 74 the signal off and holds the previous sample. 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 hold circuit.

At the end of the so-called "marker window" interval, the microcomputer has 21 then a digitized signal corresponding to the filtered and rectified received signal is stored in its memory. At this time, the microcomputer has access to the PFB1, PFB2 and PFB3 ports of the analog multiplexer 75 and thus reads and stores the DC levels with respect to the frequency bands FB1, FB2 and FB3.

The microcomputer then initializes 21 his phase of processing, in which he analyzes this time domain and frequency domain information using his decision programs. Based on the results of these programs, the microcomputer brings the time domain counter 21a as well as the frequency domain counter 21b (see. 15 ) up to date and checks the counter reading of each of these two counters. Only if this counter 21a and 21b both are at preselected counter readings that indicate the presence of a marker over a number of frames (cf. 3 ) are characteristic, the microcomputer hits 21 a decision that a marker is present. The result of this decision is that the microcomputer 21 via an alarm interface circuit 85 alarm device 82 and 83 activated unless the alarm decision is prevented as a result of further peak detection and fundamental frequency detection programs as described in the present application.

In the 5A and 5B are flowcharts once with respect to the time domain program 100 and on the other hand regarding the frequency range program 200 of the microcomputer 21 shown. These programs contain experimentally determined frequency domain and time domain data of so-called "non-marker objects", such as shopping carts, as well as similar data regarding markers of the type according to the U.S. Patent 4,660,025 based. 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, these programs assume that a framework (cf. 3 ) 64 contains equal intervals and that the marker window 24 Contains intervals, each of which is assigned to a sample of the stored, digitized signal.

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

According to 5A will in the steps 101 and 102 of the time domain program 100 the sample maximum value PS (peak sample value) 24 Samples of the digitized signal determined in the marker window interval. 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 domain counter 21a is reduced or reduced and the frequency range program starts. If the maximum sample is above the 0.2 V threshold, the step becomes 102 passed in which the level at Half of the maximum sample value (the 6dB threshold) is compared to the ambient noise level previously determined and stored by the microcomputer. If the 6dB threshold is less than the noise level, the program, the time domain counter, ends 21a is reduced or reduced and the program switches to the frequency domain program.

If the 6dB threshold is greater than the noise level, then the step 103 started, in which it is determined whether more than six samples of the 24 samples in the marker window interval are above the noise level. 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 not, then the step begins 104 ,

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 each other by more than 2.8 ° (ie a sample interval), the program ends, both the time domain and the frequency range counter is also reset to 0 and the frequency range program begins.

If the difference between these two phases is less than 2.8 °, then the step becomes 105 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 goes to step 106 ,

In the step 106 all samples above the 6dB threshold are compared to determine whether they are within 2 sample intervals of each other. If not, the program ends, the time domain counter is decreased by 1 and the frequency domain program begins. If so, the time domain counter is incremented by 2 and it becomes the step 107 passed.

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 goes to the step 108 about. In this step it is determined whether all samples are above the 10dB threshold within three samples of each other. 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 domain counter 21a is therefore reduced by 1.

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

When the pulse width test is carried out, a phase test is carried out 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, by comparing the maximum samples in every other frame, the influence of the earth's magnetic field on the analysis is demonstrated Untitled. This is because, depending on the orientation of a marker in the query or surveillance 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 will not occur in the same position in the marker window in successive frames (ie alternating polarities of the driver current). 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 pulse width test failure, and the two counters 21a and 21b are reset to 0.

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

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

The final test according to the two steps 107 and 108 represents another, more refined pulse width test, after which a high degree of probability is to be found 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 also increases the counter 21a by 1, but its failure does not change this counter.

At the end of the time range program 100 becomes the frequency domain program 200 initiated (see 5B ). As part of the frequency range program, the energy in the frequency bands FB1, FB2 and FB3 is checked by the DC level at the synchronous integrators 68 . 69 and 71 (see. 2 B ) are represented. 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 are 62 . 63 and 64 determined that the expected, integrated outputs of the bands FB1, FB2 and FB3 for the markers result in the same or increasing DC values. Therefore, the frequency domain program examines 200 such an increase in the DC level.

As can be seen in detail from the flow chart 5B results in the first step 201 the frequency range program queried whether the 6dB threshold value for the DC level of the high-frequency band FB3 is lower than the noise level. If so, then this test, the frequency domain counter, fails 21b is reduced and the program ends. If not, the program goes to step 202 over, in which the DC level of the frequency band FB1 is checked to determine whether it exceeds a predetermined threshold, which is 3 volts. This predefined value is based on system constraints, which are provided in order to prevent the direct current level assigned to this frequency band from exceeding this level with a valid marker and under normal operating conditions of the monitoring system. If the specified level is exceeded (YES output in step 202 ), then there is the probability that a valid marker is not present and the frequency domain counter 21b is reduced and the program therefore ends.

But if the level is not exceeded, then the next steps 203 and 204 initiated in which the direct current level of the frequency band FB1 is compared with the direct current level of the frequency band FB2 and furthermore the direct current level of the frequency band FB2 is compared with the direct current level of the frequency band FB3. If either the DC level of band FB1 is greater than the DC level of band FB2 or the DC level of band FB2 is greater than the DC level of band FB3, then this test fails. The frequency domain counter 21b is then reduced and the program ends.

However, if the DC level of the tape FB1 is less than the DC level of the band FB2 and furthermore the direct current level of the band FB2 is lower than the direct current level of the band FB3 (in each case outputs NO of the steps 203 and 204 ), then the presence of a valid marker is likely and the frequency domain counter 21b is increased by 2 and the program ends.

As already explained above, after the end of the frequency range program 200 the two counters 21a and 21b checked to transmit whether their respective counter readings exceed the set preselected values which are indicative of the presence of a valid marker in the query or monitoring zone. In the present embodiment, the two counters 21a and 21b set in such a way that they can count up to a maximum of 12 and down 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 verification process regarding the two counters 21a and 21b shows that these do not have the preselected counts, then the microcomputer conducts 21 the information gathering phase for the next frame of the generated field and the above-described procedure is repeated.

If the check performed by the microcomputers indicates that the preselected counts have been reached are triggered an alarm signal, provided, however, that it is not replaced by two more Test programs of the microcomputer is prevented.

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

The flow chart of the peak detection program 500 is in the 6 shown. 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" that are generated by the time domain and frequency domain programs 100 and 200 are not differentiated because these peaks have some properties that are similar to those magnetic markers on which the monitoring system 1 according to 1 should address.

The noise peaks can be characterized in particular by the following properties and features:
(a) Basically, the peaks are pulse-like or pulse-like responses, as is the signal from a valid marker, and therefore the time domain response of a peak can be quite similar to that of a valid marker; (b) Peaks often have very short rise and fall times and therefore generate a frequency spectrum that is similar to that of a valid marker; (c) depending on the source, the peaks can have an amplitude which is not automatically rejected by the monitoring system; (d) because of the "echo" effect of the comb bandpass filter 58 according to 2A a single tip can appear in multiple marker windows; (e) in general, various of the properties of the noise spikes are similar enough to those of a valid marker that the filtering parts of the receiver 17 have a slight effect on damping the tips.

The peak detection program depends on the inventory management of a memory array or matrix, which according to 15 should be referred to as "maximum amplitude stack". The locations of this stack contain maximum values of the sample and hold circuit 74 according to 2 B 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 in the stack therefore contain a data record relating to the marker pulse or a marker-like pulse.

Once the time range counter reaches 21a the preselected counter reading, which characterizes a valid marker, and the operation of the maximum amplitude stack change. Now maximum amplitude values of successive frames (cf. 3 ) placed in locations 2-15 in the stack. If the frequency domain counter 21b now also reaches its preselected value and the microcomputer 21 is about to address an alarm trigger, then a branch to the tip detection program is found in the computer 500 according to 6 instead of. It should be noted that the stack is not necessarily filled at this point. The stack stops charging the moment the time domain and frequency domain counters have reached their preselected counts.

In step 501 the microcomputer checks 21 the contents of the stack from location 2 to that indicated by the stack pointer to determine the largest maximum. 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 frame, then the microcomputer proceeds to the step 503 with the index that the frame pointer in the stack position 1 according to 15 to be set; otherwise the microcomputer continues 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 position 1 according to 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, as it were, in a supportive and opposing manner when the driver field of the monitoring system changes its polarity, as a result of which different amplitudes of the marker signal in alternating windows caused; (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 there is a noise peak input through the comb bandpass filter 58 is reflected in every second frame.

Once the Odd or Even state has been determined, the microcomputer proceeds 21 continue to step 504 in which 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 in frame No. 13 according to 15 is stopped.

In step 504 the rise time of the maximum value 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 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, to the exit of the peak detection program; otherwise the microcomputer continues to step 506 ,

If in this step 506 If it is determined that X = 0 or = 1, the microcomputer proceeds to step 515 by increasing the frame pointer. A spike will often occur in one 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 in the step 506 is not at No. 0 or No. 1, the microcomputer proceeds to step 507 , in which a test is carried out to determine whether the maximum value in frame X is equal to or higher than 6 dB above the noise level (2 × as large). If not, then the signal is too small or too weak to make a decision and the microcomputer proceeds to step 515 by increasing the frame pointer by two. If the test has passed or passed, the microcomputer proceeds to the step 508 , in which a test is carried out to determine whether the maximum value in frame X is equal to or higher than 10 dB above the noise level (3 × as large).

If not, the microcomputer proceeds to step 509 , in which the increase is tested again. If the increase is equal to or greater than 1, then a "possible marker" counter in the step 510 elevated. If the increase is not greater than 1, then the counter in step 512 reduced. If the S / N (S / N) ratio is equal to or greater than 10dB, the microcomputer proceeds to step 511 , in which the increase is tested again. If the increase is equal to or greater than 0.85, then the "possible marker" counter in step 510 elevated. If no, the counter will step 512 is decreased and then it goes to the step 513 moved on.

In step 513 the "possible marker" counter is checked and, if the count is equal to 2, the microcomputer proceeds to step 514 , in which at least five successive marker signals have been received which have the correct rate of increase in amplitude, 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 , in which the frame pointer is raised, whereupon to step 516 is passed over.

In step 516 the new position of the frame pointer is compared with 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 terminated 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 will occur in the crotch 504 calculated.

Once the tip detection program is complete, when the decision is made that a tip was present. The time domain counter 21a and the frequency domain counter 21b are therefore reset to zero, an alarm is not triggered and the monitoring system 1 continues its monitoring function. When the decision is made that a marker is present, the microcomputer guides 21 a final test before the alarm is triggered. In this test, the microcomputer compares 21 the fundamental frequency component of the received signal, which it receives from the input PFo of the analog multiplexer 75 receives, with its stored value and, if the fundamental frequency component has changed beyond a predetermined threshold value, then the decision is made that a valid marker is not present. The microcomputer 21 therefore does not trigger an alarm, the two counters 21a and 21b are reset to zero and monitoring continues. If the above-mentioned 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 1 displays.

As already mentioned above, the microcomputer performs 21 after the operation has been initialized, the noise levels in the frequency bands FB1, FB2 and FB3 are determined and these levels are used to determine an average noise level for the monitoring system 1 as well as for use within its decision-making programs. The microcomputer updates this noise level by periodically referring frames as the noise update frame. During these frames, the microcomputer treats the DC levels developed by the subchannels as indicative of the noise level within the monitoring system and averages them to obtain an average noise level.

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 surveillance system 1 therefore dynamically updates itself to adapt to changing environmental conditions, which would otherwise impair the monitoring system's ability to reliably detect markers.

In the 7 Figure 3 is a block diagram of the comb bandpass filter 58 shown. This filter has a delay line 601 in the form of an integrated circuit, furthermore input and output connections 602 and 603 as well as a clock connector 604 , The output connector 603 the delay line 601 is to the input port 602 by means of a feedback device 605 fed back with resistors R1 and R2.

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

The comb bandpass filter 58 in the present embodiment, harmonics of a fundamental frequency Fo pass if the clock rate is set to a value equal to the number of delay stages times the frequency Fo times that required to shift from one stage to the next clock cycles. It is therefore for one 512 -Stage line with the two clock cycles required for a shift and a clock rate of 1024 Fo required for the basic frequency Fo of the monitoring system, which is the signal with which the comb bandpass filter 58 through the level shifter 76 according to 2C is applied. When executing the filter according to 7 its Q factor is set by adjusting the resistor R 1.

8th shows an actual implementation of the circuit of the comb bandpass filter 58 , In this case, the comb bandpass circuit is used to emphasize multiples of 73 Hz. Components R220, R221 and U203A form an inverse buffer amplifier which is used to drive the comb bandpass circuit. The parallel connection of the components R222, R223 and R224 corresponds to the resistor R 1 according to 7 while the component R225 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 resistors. In most devices, however, all three components will remain in the circuit.

The Components C208, R226 and U203B are used to convert the signal into the Delay line IC Feed in U201. Component C208 serves as a DC block capacitor, while the component U203B a non-inverting buffer with one gain and high input impedance. The combination of high impedance with a fairly small capacity gives a small phase lag in the DC block capacitor. Resistor R226 forms for the component U203B a direct current line to earth.

As already mentioned above, the component U201 represents a delay line in the form of an integrated circuit (IC). The IC used in the present circuit implementation is a Reticon R5107. This IC contains 512 monolithic C-links and is therefore clocked at a rate of 74.752 KHz (1024 × 73 Hz).

The remaining circuit component is designated U203 C. Since the Reticon R5107 has a low output performance (output drive capability) must be on a buffer can be used at its output. The component U203C is designed as a non-inverting buffer and is able to to provide a sufficient output drive, so that the Component R225 is not a too large load.

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

As already explained above, the use of the comb notch or trap filter 58a particularly of interest in environments where there is significant network interference. However, where this size is not significant, the filter can 58a be switched off in order to avoid a possible increase in noise for the monitoring system.

9 Figure 3 shows a block diagram of a comb-notch filter in accordance with the present invention. The comb-notch filter has a comb-bandpass filter 702 on that's a digital delay line 703 contains. From the output port 703b the delay line gets the signal through a buffer 707 , a resistor R 2, a capacitor C 1 and a further resistor R 3 to the input terminal 703a the digital delay line 703 fed back. The input signal is with the input connector 703a via a reversal buffer 708 , a resistor R 1, the capacitance C 1 and the resistor R 3 coupled.

The input signal as well as the input at the input connection 703a are with a summing circuit 704 coupled. Because the latter two inputs because of the reverse buffer 708 are out of phase, 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 from a utility grid reference frequency that passes through a square wave conversion circuit 701 to a PLL circuit 709 (Phase locked loop), which operates at 2n times the supply network frequency, where n is the number of stages of the delay line. A divider 711 provides a 2n 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 the 9A and 9B is the realization of an electrical circuit of a comb notch filter or trap filter for a monitoring system 1 seen. The square wave conversion circuit consists of the components R25-29, C27 and U8A (see upper part of 9B ). Resistors R25 and R27 act 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 8A. Resistors R25 and R27 also act as low-pass filters with respect to the input signal. R28 and R29 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. R26 provides the hysteresis for the circuit, but R26 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 U2, U3, C17, C18, R22 and R 33. This phase locked loop is used as a frequency multiplexer in order to provide the delay line (IC U1) with the required clock signal. The voltage controlled oscillator (VCO) of the phase locked loop works 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 shield tip represents an unwanted signal that contributes to the noise at the output of the comb-notch filter the components US and U6 a blanking control signal from SSB1 and 64Fo. This signal actuates the electronic switch U4C so that the shield tip is not passes through component U7A. In addition to their function as A blanker plays the component U7A a second role Reversal buffer at the input of the comb-bandpass part of the comb-notch filter circuit.

The Comb bandpass section consists of the components U1, U7B, U7C, U7D, R3, R4, R7-10, R21 and C13. The operation of this part of the circuit is the same of those who have previously worked for the comb bandpass filter has been described is, with the exceptions that now the delay line IC is a Reticon R5108 and that the Clock frequency is 2048 times the basic filter frequency.

The summing circuit 704 according to 9 is formed by components R5, R6 and U8B, such as 9A shows. R5 and R6 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 component U7A.

in the Connection to the summing circuit uses a low pass filter to any To suppress circuit noise, that is generated by component U1. The filter edge frequency is at 8 kHz, so that all of the marker harmonics that are of interest become. After this filter an amplifier is used to control the remaining one Bring signal levels to the same amplitude as the signal levels would have if the filter had not been used.

In accordance with another aspect of the present invention, the transmit and receive antenna arrangements for the surveillance system 1 according to

1 trained in such a way that the usual zeros in the in the interrogation or monitoring zone 13 existing field can be reduced.

10 shows a correspondingly designed receiving antenna arrangement 90 , This antenna arrangement has first and second upper loops 91 and 92 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 C2 of the loop 92 from the center C1 of the loop 91 lies while the center C4 of the loop 94 from the center C3 of the loop 93 dista. Furthermore, the first, upper loop forms 91 together with the fourth, lower loop 93 a first, eight-shaped configuration, while the second, upper loop 92 together with the third, lower loop 94 forms a second eight-shaped configuration.

The loops lying in the same plane 91 - 94 are all wound clockwise as in 10 indicated by the arrows, and all electrically connected in series. It is also provided that the upper loops 91 and 92 the same first phase and the lower loops 93 and 94 have the same second phase, the first and second phases being opposite to each other.

According to a preferred embodiment of the receiving antenna arrangement 90 it is envisaged that section 91A the first loop 91 as well as the sections 93A the fourth loop 93 which sections are in the mutual contact area of the first eight-shaped configuration which are inclined obliquely to the horizontal during the section 92A the second loop 92 and the section 94A the third loop 94 are also inclined in the same way 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 in the 10 The antenna arrangement shown are the two outer loops, ie the first upper loop 91 and the fourth bottom loop 93 The two inner loops, ie the second upper loop, are mirror images of one another 92 and the third lower loop 94 Mirror images of each other.

In 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 in the direction of its vertical axis 11-1 . 11-2 and 11-3 in three different bands. Out 12 (II) are comparatively the zero zones of a receiving antenna arrangement 90 according to 10 shown.

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

In the 11 is schematically an improved transmit antenna arrangement 801 for the surveillance system 1 according to 1 schematically shown, this transmit antenna arrangement in particular for an application in combination with the receive antenna arrangement 90 according to 10 it is a possibility. The transmit antenna arrangement 801 has a first, single loop 802 on, in which a second loop lying in the same plane 803 is nested. The main axis of the second loop 803 is in relation to the main axis (Y axis) of the loop 802 turned. In this case, both loops are wound clockwise and are in the same phase.

Due to the presence of the second loop inside 803 the transmit antenna arrangement 801 there is a field in the Y direction along the X axis of the first loop 802 , but what if there is no second loop 803 would not be the case. In view of this, the receiving antenna arrangement is in accordance with 10 now capable of coupling with fields along the horizontal or X axis, so that the monitoring system according to the present invention 1 according to 1 used transmit and receive antenna arrangements enable the presence of markers to be determined, which are along the horizontal direction through the interrogation or monitoring zone 13 be moved through.

Embodiments of pedestals or stands for accommodating the transmit and receive antenna arrangements of the monitoring system 1 according to 1 are in the 13 and 14 shown.

One base or stand each 301 has first and second cladding walls 302 and 303 at a mutual distance, which are preferably made of plastic and are connected to one another in such a way that they enclose a cavity in the manner of a shell, which serves to accommodate the transmit and receive antenna arrangements used in the 14 by coils 304 and 305 are shown. These antenna arrays are between stiffening bars 306 housed, each on the side edges of the cavity between the two cladding walls 302 and 303 are arranged.

A stuck foam 307 of high density, such as 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 makes the two antenna arrays, ie the coil 304 (= Transmit antenna arrangement) and the coil 305 (= Receiving antenna arrangement), immobile and gives the finished base or stand 301 the necessary reinforcement and stiffening.

The edges of the two cladding walls 302 and 303 For example, each of which is designed as a thin plastic cladding, each have hook-shaped end sections 308 molded to serve as an extruded edge protective strip or protective buffer 309 (see. 13 ) to wear. After all, is per pedestal or stand 301 another carrier base 311 provided, which allows a firm and safe installation of the stand or stand on the floor.

The bases or stands according to the 13 and 14 are preferably made in such a way that first in one of the cladding walls 302 the internal components including the transmit and receive coils 304 and 305 be introduced. These components are then attached to the places provided for them with the help of an adhesive such as hot melt. The two essentially plastic shell-shaped first and second cladding walls 302 and 303 are then chemically reacted with a predetermined amount greedy foam filled with the desired density, such as polyurethane, consisting of various combinations of foams based on polyolisocyanates. The shell-like cladding walls 302 and 303 are still put together and thus practically closed and in this state placed in a plate press or multi-stage press for the purpose of short-term curing (5-10 minutes), so that ultimately a solid, compact construction is obtained. Finally, the remaining components of the base or stand are added to complete it.

The invention is not restricted to the exemplary embodiments shown and described. This also includes all professional modifications and further training as well as partial and sub-combinations of the features and measures described and / or shown. For example, the monitoring system 1 according to 1 with another analog multiplexer 78 as in 2C be equipped to collect diagnostic information regarding the components of the receiver. This diagnostic information can then from the microcomputer 21 read to diagnose any problems related to the components of the receiver.

1

monitoring system

11

AC drive

11-1

zero zone

11-1 '

zero zone

11-2

zero zone

11-3

zero zone

12

transmitting antenna

13

Query- or surveillance zone

14

items

15

marker

16

receiving antenna

17

Control unit interface and receiver

18

central control unit

19

Alarm unit

21

microcomputer (or microprocessor)

21a

Time domain counter

21b

Frequency range counter

22

Main program memory (EPROM)

23

permanent memory (NOVRAM)

24

Programming interface

25

status indicator

26

Watchdog circuit (watchdog circuitry)

27

Auxiliary interface

28

Address switch (address latch)

29

Analog / digital converter

30

Driver Channel

31

CPU clock

32

programmable divider

33

Low-pass filter

34

Stage setting multiplexer (level adjuster multiplexer)

35

Buffer amplifier

50

Fundamental detector

51

notch or trap filter

52

notch or trap filter

53

preamplifier

54

High-pass filter

55

Low-pass filter

56

Pre-emphasis circuit

57

limiter

58

Comb bandpass filter

58a

Comb-notch or trap filter

59

Low-pass filter

61

Shield-Spike-Blanker

62

Depth range bandpass filter

63

Midrange bandpass filters

64

High-range band-pass filter

65

Full wave rectifier

66

Full wave rectifier

67

Full wave rectifier

68

Synchronous integrator

69

Synchronous integrator

71

Synchronous integrator

72

High-pass filter

73

Full wave rectifier

74

sample and hold circuit

75

Analog multiplexer

76

Level shifting means

77

sequential logic

78

Analog multiplexer

79

Power amplifier (from 11 )

81

Current sensor (with 12 connected)

82

alarm device (acoustically)

83

alarm device (visually)

84

Event counter

85

Alarm interface circuitry

90

Receive antenna arrangement

91

first top loop

91A

section the first loop

92

second top loop

92A

section the second loop

93

fourth lower loop

93A

section the fourth loop

94

third lower loop

94A

section the third loop

100

Time-domain program

101

step

102

step

103

step

104

step

105

step

106

step

107

step

108

step

200

Frequency-domain program

201

step

202

step

203

step

204

step

301

pedestal or stand

302

Panel Wall

303

Panel Wall

304

Kitchen sink (or transmit antenna arrangement)

305

Kitchen sink (or receiving antenna arrangement)

306

stiffening rod

307

solidified foam

308

hook-shaped end section (from 302 and 303 )

309

Edge protective strip or buffer

311

support base

401

waveform

402

waveform

403

waveform

500

Peak detection program (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 port

603

Output terminal

604

Stroke connection

605

Feedback device

606

Clock timer

701

circuit for conversion into square waves

(signal squaring circuit)

702

Comb bandpass filter

703

digital delay line

703a

Input port

703b

Output terminal

704

Summing circuit

705

buffer

707

buffer

708

inverting buffer

709

PLL (Phase Locked Loop)

711

divider

801

Transmitting antenna array

802

first loop

803

second loop

A

first Signal branching channel

B

second Signal branching channel

C

Time domain channel

D

Frequency domain channel

D1

subchannel

D2

subchannel

D3

subchannel

C1

Center of the loop 91

C2

Center of the loop 92

C3

Center of the loop 93

C4

Center of the loop 94

Claims (34)

Monitoring system for ascertaining the presence of articles provided with magnetic markers in an interrogation or monitoring zone, in particular for theft protection, with a transmission device for establishing a magnetic field at a preselectable frequency in the interrogation or monitoring zone ( 13 ); - receiving device ( 17 ) to detect faults in the magnetic field in the interrogation or monitoring zone ( 13 ) and for emitting a first signal characterizing this, these receiving means having a frequency range channel (D), into which the first signal is fed and which generates a further signal, the content of which is characteristic of the energy levels in each of a predetermined number of frequency bands ( 62 . 63 . 64 ) of the first signal and - have a time domain channel (C), into which the first signal is fed and which generates a third signal, the content of which contains the information as to whether a signal level obtained by sampling the first signal within an acceptable signal Noise ratio lies and - evaluation device which determines the presence of an article in the query or monitoring zone on the basis of the output signals of the time domain channel (C) and frequency domain channel. Installation according to claim 1, characterized in that the number the tapes is at least three. System according to claim 1 or 2, characterized, that - the first Signal content for each band has signal levels from the number of bands, characteristic of are the energy in each band, and - the evaluation device compares these signal levels in the course of the evaluation. System according to Claim 3, characterized in that the evaluation device is designed in such a way that it treats the comparison of the signal levels as characteristic of an article in the query or monitoring zone if the signal pe gel remain the same or increase from a lower band to a higher band. Installation according to one of the preceding claims, characterized in that - the first channel has subchannels (D1, D2, D3) for each of the said bands, - each of these subchannels (D1, D2, D3) having the following Components comprises: a bandpass filter ( 62 . 63 . 64 ) which comprises the band assigned to the subchannel; a rectifier connected downstream of the bandpass filter ( 65 . 66 . 67 ); and an integrator downstream of the rectifier ( 68 . 69 . 71 ). 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, with low threshold values of the field in the interrogation or monitoring zone ( 13 ). Installation according to claim 6, characterized in that the magnetic marker ( 15 ) consists of a material with internal, pent-up mechanical tension, which has a large Barkhausen discontinuity in its characteristic hysteresis loop. System according to one of the preceding claims, characterized in that - the transmitting device has an antenna arrangement ( 12 ) and drivers ( 11 ) that 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 interference in the magnetic field at the peaks of the driver current; - each magnetic marker ( 15 ) Disturbances in the magnetic field in the vicinity of the zero crossings of the driver current are generated and - shielding pulse peak blanker ( 61 ) are provided for blanking the first signal near the peaks of the driver current. Plant according to one of the preceding claims, characterized characterized that a every half-period of the selected Frequency forms a frame and the first signal content during each Frame has samples of the first signal during this frame. System according to claim 9, characterized in that the Evaluation device for the purpose of evaluation during a particulate frame one or more samples of the first, the particular frame assigned signal with one or more samples of the first, compares signals associated with another frame. Installation according to claim 10, characterized in that the other frames are two frames earlier than the particular frame located. Plant according to claim 10 or 11, characterized in that that the Evaluation device the position of the tip sample in the particulate frame with the position of the peak sample in said other Frame compares. System according to one of claims 10 to 12, characterized in that that the Evaluation device for the purpose of evaluating a particular frame one or more samples of the first signal with a preselected threshold compares System according to claim 13, characterized in that the preselected Threshold regarding the Ambient noise level is characteristic in the system. Plant according to claim 13 or 14, characterized , that the Comparison is whether a preselected number of samples the preselected Threshold exceeded. System according to claim 9, characterized in that the Evaluation device in the course of the evaluation process during a particular framework one or more samples of the first signal of a first frame with one or more samples of the first signal of another frame or several other frames. System according to claim 16, characterized, that - the samples, which are compared, the peak samples of each frame are; - the first frame is the frame that has the largest or highest peak sample; - and the one or more other frames an odd or odd number Frame is or are if the first frame is an odd frame and is or are an even or even frame, if the first frame is an even-numbered frame. Installation according to claim 17, characterized in that the Compare making up the relationships which includes peaks. System according to one of the preceding claims, characterized in that the evaluation device in the course of the evaluation process processes the first signal content in accordance with a first preselected decision criterion and the second signal content in accordance with a second preselected decision criterion. Installation according to claim 19, characterized in that the Evaluation device processes the second signal content each time, when processing the first signal content. Installation according to claim 20, characterized in that the processing of the first and the second signal content takes place in each frame and that the evaluation device first and second counters ( 21a . 21b ), which are updated in accordance with the processing of the first and second signal content. Installation according to claim 21, characterized in that in the event that the first and second counters ( 21a . 21b ) each reach first and second preselected values, the evaluation device creates the conditions for triggering an alarm. Installation according to claim 22, characterized in that - the receiving device ( 17 ) Has means for extracting or for obtaining the component of the first signal at the preselected frequency and - the evaluation device compares the aforesaid component with a preselected value and prevents the alarm from being triggered if the preselected value is exceeded. System according to one of claims 19 to 23, characterized in that the evaluation device comprises a microcomputer ( 21 ) contains. Installation according to claim 24, characterized in that the microcomputer ( 21 ) has pre-selected decision programs for carrying out the evaluation. Installation according to claim 25, characterized in that the Decision programs include time domain and frequency domain analyzes. System according to claim 24, 25 or 26, characterized in that an interface ( 24 ) for external communication with the microcomputer ( 21 ) is provided. System according to one of claims 24 to 27, characterized in that a with the microcomputer ( 21 ) communicating permanent storage ( 23 ) is provided for storing the operating parameters for the system. Installation according to one of the preceding claims, characterized by a receiver for use in an article monitoring system for determining the presence of articles ( 14 ) in a query or surveillance zone ( 13 ), a magnetic field being built up at a preselected frequency in this system and an antenna arrangement ( 16 ) is provided, which detects disturbances in the magnetic field, in particular for theft protection, the receiver ( 17 ) has the following components: - an evaluation device which is adapted to adapt to the antenna arrangement ( 16 ) and to generate a first signal and - switching elements for processing the first signal, these switching elements being a comb bandpass filter ( 58 ) with pass bands at harmonics of the preselected frequency. Installation according to claim 29, characterized in that the comb bandpass filter ( 58 ) has the following components: - A digital delay line ( 601 ) with a number of delay stages as well as input and output connections ( 602 . 603 ); - a clock connection ( 604 ) and - a feedback device ( 605 ) for coupling the input and output connections. System according to claim 30, characterized in that a clock signal transmitter ( 606 ) for supplying a clock signal to the clock connection ( 604 ) is provided, this clock signal having a frequency which is equal to the product of the preselected frequency, the number of stages of the delay line ( 601 ) and the number of clock cycles required to transfer the information from stage to stage in the delay line ( 601 ) to transport. System according to claim 31, characterized in that the receiver ( 17 ) has the following components: - an evaluation device which is adapted to adapt to an antenna arrangement ( 16 ) to address and generate a first signal and - switching elements for processing the first signal, these switching elements a comb-notch filter or comb-trap filter ( 58a ) with stop bands at the harmonics of the specified network frequency. Installation according to claim 32, characterized in that the comb-notch filter or pleated filter ( 58a ) has the following components: - A comb bandpass filter ( 702 ) with ribbons Harmonics of the specified network frequency; Means for applying the comb bandpass filter ( 702 ) with the first signal and for delivering a filtered signal and - means for subtracting the first signal and the filtered signal. Installation according to claim 33, 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 connectors.