EP0319637B1 - Magnetic theft or intrusion security system and suitable metallic sensor element therefor - Google Patents

Abstract

Arrangement for safeguarding objects located in a room provided with at least one room outlet, particularly a (self-service) sales location such as, for example, a supermarket, against theft or against burglary, in which the objects to be safeguarded are in each case provided with at least one magnetically activatable metal sensor element, comprising a first device, to be arranged at each room outlet, for generating a magnetic field in the area of which a magnetically activated metal sensor element in each case arranged at an object to be safeguarded generates a signal when a limit field strength value is exceeded, and a second device associated with the first device, by means of which a signal is to be detected and to be supplied to an alarm device in order to trigger an alarm, the metal sensor elements (8) in each case being constructed in such a manner that they generate in the active state in an area of the magnetic field in which the field strength is greater than a lower limit value an atypical signal which is not generated by other metallic objects when magnetically excited but is only generated by a magnetisable object when the latter has been selectively subjected to a special treatment; and in that the first device consists of at least one (first) field coil (9) having a preferably horizontal centre axis (16) which annularly surrounds the relevant room outlet (1) and the height (H) and width (B) of which is constructed in such a manner that a person can pass it in the direction of its centre axis (16), and a metal sensor element suitable for this purpose and the use of a Wiegand wire as metal sensor element for a safeguarding system. <IMAGE>

Description

The invention relates to an anti-theft system according to the preamble of claim 1, as is known generically from DE 35 41 536 A1.

In this known anti-theft system, the sensor wire elements essentially consist of amorphous material, at least part of the wire being crystallizable when deactivated. The sensor wire elements are each driven symmetrically in the area of a periodic magnetic field generated by a field coil, whereby when the magnetic polarity is reversed in the hysteresis loop, a large Barkhausen discontinuity (large Barkhausen jump) arises due to known physical effects, that is to say a normal one "Metal objects atypical signal, but due to the type of control a symmetrical signal.

The anti-theft system according to DE 35 41 536 A1 trusts that comparable atypical signals from the induction coil assigned to the field coil are not generated by other objects, and moreover also provides qualitative detection of such an atypical signal for the signal caused thereby Resolution of an alarm is generally satisfied.

With this known anti-theft system, previously known anti-theft systems could be improved from certain points of view. It has been shown, however, that when using the system known from DE 35 41 536 A1, a not inconsiderable number of false alarms can still occur, which are always extremely unpleasant for everyone involved and very often in sales outlets the loss of at least the customer in question cause a customer to feel the inconvenience does not want to suspend a false alarm again. These false alarms can occur in particular because it is not uncommon for people to carry objects with them that trigger a qualitatively similar signal in the field of the field and induction coils, and because the signals generated are not quantified during detection, which at least leads to a reduction which could lead to false alarms.

In order to increase the accuracy of identification, one can examine the frequency spectra of the generated pulse signals and qualify them in a suitable manner, but this obviously requires a relatively complicated and complex apparatus with a receiver coil arrangement, the output of which is connected via a receiver device to a plotter and a spectral analysis device, so that a reduction in false alarms brought about in this way by improving the identification of the generated signals is generally eliminated in terms of the effort required, quite apart from the fact that such devices are relatively maintenance-intensive.

In addition, the deactivation of the sensor wire elements in the prior art according to DE 35 41 536 A1 for an anti-theft system, such as is particularly advantageous for use in particular in supermarkets or the like, appears to be technically complicated and therefore not very suitable . A first embodiment assumes that the sensor wire element is embedded between the substrate and cover layers, and that two contacts are provided which must penetrate insulation of the sensor wire element, one contact to one The power supply is connected and the other contact is connected to earth, so that a capacitor discharges into a section of the sensor wire element lying between the two contacts and thereby raises its temperature above the voltage relief temperature of the material.

In the case of a possibly non-mechanical-electrical configuration of the deactivation unit, a laser is provided, the rays of which are directed onto a part to be crystallized (or the entire wire element) in order to heat it (possibly locally).

According to a third embodiment of the deactivation device in the known anti-theft system, the sensor element with internal bracing is enclosed in laminate layers which can be shrunk by heat, the laminates being acted upon by hot air for deactivation, so that the sensor element can relax and thus lose its internal bracing.

The present invention has for its object to significantly improve the known generic anti-theft system, in particular with regard to its signal identification accuracy and thus to reduce the risk of false alarms in such a way that they are practically excluded. In addition, the anti-theft system should not only be simplified and made cheaper with regard to its sensor elements. Finally, the (authorized) deactivation of the sensor elements should also be possible considerably more easily and with less effort.

This object is achieved by the characterizing features of claim 1.

It should be noted that a Wiegand wire is known per se. It's a Wiegand wire is a rotating element made of ferromagnetic material, which due to certain treatment during manufacture has a soft magnetic core and a hard magnetic jacket (shell), such as this in US Pat. No. 4,247,601 or the Z. "measure + check / automatic", May 1984, pp. 236-239.

If a pre-magnetized Wiegand wire rectified (positively or negatively oriented) by a sufficiently strong magnetic field with a saturation field strength rectified (positive or negative orientation) is applied to a core and shell, which (starting with a field strength output value 0 or a relatively low field strength output value) in the opposite direction increases towards the bias field, the magnetization state of the Wiegand wire initially remains in its bias state. If the field strength exceeds a threshold or limit value, also known as the ignition field strength, only the magnetization direction of the soft magnetic core changes when the ignition field strength is reached; this flips up spontaneously.

If the Wiegand wire is in an induction coil when the magnetization direction of the soft magnetic core is flipped over, the flux change in the induction coil associated with flipping over the core magnetization direction generates a voltage pulse, also known as a reset pulse, with a - due to the magnetic coupling between the core and the shell - relatively small amplitude.

The field strength of the magnetic field applied to a Wiegand wire then remains below a second limit value, which, if exceeded, also leads to the magnetic reversal of the hard magnetic shell and if, after the direction of magnetization of the soft magnetic core has been flipped over and the ignition field strength has been exceeded, the applied magnetic field rises again in the original direction, the magnetization direction of the soft magnetic core flips over again when the ignition field strength is reached. Since the magnetic field of the shell is in this state the applied external field and supports it, as it were, the magnetic coupling between the shell and core causes a relatively high flow rate and thus a relatively strong signal, which is quasi-static with an applied periodic alternating field.

It can be seen with one and the same Wie gand wire that either a symmetrical or an asymmetrical signal is generated, and that results in a symmetrical signal when the Wiegand wire is saturated in both directions with respect to the core and the sheath. If, on the other hand, the nucleus and the shell are positively saturated during each passage through the first quadrant due to a correspondingly high field strength and if only the nucleus is saturated in the third quadrant, an asymmetrical signal results from such asymmetrical excitation The type or shape therefore depends on the control or excitation of the Wiegand wire, as described in US Pat. No. 4,247,601.

Due to the fact that the anti-theft system according to the invention works with an asymmetrical control of the sensor wire elements and thus with asymmetrical pulse signals that are not generated by "normal" metal objects, there is already a high degree of identification accuracy with relatively simple means and thus an extremely small number of False alarms.

Since, in addition to this qualitative identification, a quantitative signal identification is also provided, in which the amplitude of the positive signal is compared with the amplitude of the negative signal (and not only by forming the difference, which is already highly useful in itself, but by forming the quotient) and only then Alarm is triggered when the amplitude ratio of the generated pulses exceeds a predetermined limit value of preferably 3: 1, the proportion of false alarms can be reduced to practically 0 with the anti-theft system according to the invention, since it is practically impossible that any other contradiction an atypical, asymmetrical signal generated, the amplitude ratio then also exceeds a predetermined limit value of 3: 1, for example.

Preferred embodiments of the present invention are described in subclaims.

Fig. 1

eine schematische Darstellung eines an einer Kasse eines Supermarktes angeordneten Kontroll-Gates, wobei im oberen linken Teil von Fig. 1 ein gesicherter Gegenstand teilweise perspektivisch vergrößert herausgezeichnet ist, in Richtung des Pfeiles I in Fig. 2 gesehen;

Fig. 2

eine Draufsicht auf die Darstellung gemäß Fig. 1 in Richtung des Pfeiles II in Fig. 1 gesehen;

Fig. 3

einen Schnitt durch die Darstellung gemäß Fig. 1 in Richtung der Schnittlinie III-III gesehen, also eine seitliche Draufsicht auf das einen Raumauslaß bildende Kontroll-Gate in Ausgangsrichtung gesehen;

Fig. 4

eine Draufsicht auf ein Preisschild (etwa in Originalgröße) für die Waren, wobei das Preisschild zugleich das Sicherungsetikett bildet und ein als Wiegand-Draht ausgebildetes Sensor-Metallelement enthält, in Richtung des Pfeiles IV in Fig. 5 gesehen; und

Fig. 5

eine Seitenansicht des Preis-/Sicherungsetikettes gemäß Fig. 4 in Richtung des Pfeiles V in Fig. 4 gesehen.

In particular, the function and mode of operation is further explained below using an exemplary embodiment with reference to a highly schematic drawing. It shows:

Fig. 1

a schematic representation of a control gate arranged at a checkout of a supermarket, with a secured object being drawn out partially enlarged in perspective in the upper left part of FIG. 1, seen in the direction of arrow I in FIG. 2;

Fig. 2

a plan view of the illustration of Figure 1 seen in the direction of arrow II in Fig. 1.

Fig. 3

seen a section through the representation of Figure 1 in the direction of section line III-III, that is, a side plan view of the control gate forming a space outlet seen in the exit direction.

Fig. 4

a plan view of a price tag (approximately in original size) for the goods, the price tag also forms the security label and contains a metal sensor element designed as a Wiegand wire, seen in the direction of arrow IV in FIG. 5; and

Fig. 5

a side view of the price / security label shown in FIG. 4 in the direction of arrow V in Fig. 4 seen.

1 to 3 of the drawing show the use of a security system according to the invention as an anti-theft security system in a supermarket, of which only one space outlet designed as a control gate 1 is shown as an example.

The objects to be secured, which in the present case are goods 2, which have been taken from a corresponding stock of goods by a customer 3 (e.g. shelves) in order to purchase them, will be purchased by the customer 3 when approaching one of the Check-gate 1 placed in front of cash register 4 on a conveyor belt 5 and conveyed to cashier 6, who types the corresponding prices, which can be found in each case on a price tag 7, into cash register 4 and settles the purchase amount with customer 3.

The price tags 7 are shown in FIGS. 4 and 5 in their original size. They each consist of a paper label that can be firmly affixed to the product 2 in question, on the underside of which a section of a Wiegand wire serving as a sensor metal element 8 is fixed. Already at this point it should be pointed out that the sensor metal elements 8 could possibly also be arranged within the goods 2 or their packaging, so that they are not recognizable by customers 3 at all.

In the area of the control gate 1 forming a space outlet, a field coil 9 is arranged, which is indicated in FIG. 1 with a dash-dotted line. As can be seen in particular from FIG. 3, the field coil 9 surrounds the control gate 1 in a ring. Its height H is 2.2 m and its width B 0.8 m, so that a customer 3 can easily pass the control gate 1 after the billing process.

The goods 2 held for sale in the supermarket and offered for purchase are each provided with at least one security label which also forms the price tag 7. This is the case for goods 2.2 and 2.3 (see FIGS. 1 and 2), while goods 2.1 are provided with three security labels 7 (according to FIGS. 4 and 5), the sensor consisting of a Wiegand wire -Metal element 8 in its longitudinal extent according to the arrows x or y or z recognizable in the upper left in FIG. 1 (there the goods 2.1 are partially drawn out in perspective in an enlarged view), the directions x, y and z in each case are perpendicular to each other.

The Wiegand wires serving as sensor metal elements 8 are magnetized in their active state. In the case of proper billing, this activation state is thereby converted into a deactivated state (since the theft protection system is not supposed to trigger an alarm when properly purchased) by demagnetizing the Wiegand wires serving as sensor metal elements 8 in the area of the cash register 4. For this purpose, an initially relatively strong alternating magnetic field is generated in the area of the conveyor belt 5 by means of a field coil 10 indicated by a dash-dotted line in FIG. 1, which is directed towards the control gate 1 in FIG its strength is continuously reduced, so that the deactivation of the sensor metal elements 8 takes place automatically when properly purchased. In order to prevent goods 2 carried by a customer 3 from being deactivated with respect to their sensor metal elements (s) 8, the field coil 10 is shielded from the passage 11 running parallel to the cash register 4.

The entire security device is (only) switched on when a customer 3 approaches the area of the cash register 4. If he traverses a light barrier 12 (see FIG. 2), this causes a one-board computer 13 to be switched on, which works for several (possibly all) control gates 1 of the supermarket in question and first of all causes one Frequency generator 14, which also works for several (possibly all) control gates 1 of the supermarket, is put into operation. The frequency generator 14 draws its power supply from the three-phase network and generates a sinusoidal voltage, which is a maximum of 67 V in the illustrated embodiment. It delivers a peak current of 15 A. The alternating frequency generated by the frequency generator 14 is approximately 600 Hz. The frequency automatically regulates the system to the resonance frequency of the field coil 9.

The inductive resistance of the field coil 9 is compensated for by an appropriately dimensioned capacitance, a value of 600 Hz corresponding to the frequency generator 14 being selected as the resonance frequency. Since the capacitance is composed of discrete components which are subject not only to a certain tolerance but also to a certain aging, the resonance frequency cannot always be set precisely.

In order to nevertheless achieve a minimum resistance and to generate a maximum magnetic field, it has been provided - as has already been indicated - that the frequency generator 14 automatically settles to the exact resonance frequency actually present.

The induction coils 15, 15 arranged at a height h of 0.9 m on the two vertical legs of the field coil 9 are also oriented in such a way that they implement changes in the magnetic field, the vector of which points in the direction of the arrow 16. The induction coils 15, 15 are designed so that they have the lowest possible own capacitance and do not contain a metal core.

If an object 2 provided with an active sensor metal element 8 is not deactivated in the area of the cash register 4 (i.e. by means of the magnetic alternating field generated by the field coil 10 in the area of the conveyor belt 5), since the customer concerned does not place it on the conveyor belt 5 for billing , but for example in a bag, under a coat or the like. illegitimately, this customer 3 crosses the control gate 1 and thereby reaches an area in which the alternating magnetic field generated by the field coil 9 exceeds the intended ignition field strength of 17 A / cm (on this occasion it should be noted that the leg cross section of the field coil 9 is approximately 8 cm 2, and that the field coil 9 has 150 copper wire windings which can easily be loaded with a peak current of 15 A, the internal resistance being 4 Ω, the magnetic field in the center of the coil 17 (see FIG. 3) is the weakest at about 1.1 m above ground 18 and about 0.4 m from the respective side edge), so is the frequency generator 14 Via the computer 13 set in motion by the light barrier 12 and the Wiegand wire (= sensor metal element) 8, which is in the magnetized, active state, supplies 600 positive and negative pulses due to its activation with an alternating field of 600 Hz per second due to its remagnetization, which is constantly carried out with the appropriate frequency. The induction coils 15, 15, in which the pulses are generated, feed them to filter electronics 19, in which the signal applied to the induction coils 15, 15 is amplified and filtered out in such a way that a signal is present at the output of the filter electronics 19, which is sent to the computer 13 is supplied. The computer 13 first checks the probability of whether this signal is also certainly Wiegand pulses that were generated by a Wiegand wire serving as a sensor metal element 8. For this purpose, the computer 13 calculates the amplitude ratio of the amplitude of a positive pulse and a subsequent negative pulse, which are very different if the field strength does not exceed an upper limit value, which in the exemplary embodiment shown is approximately 25 A / cm. If the amplitude ratio in the present exemplary embodiment is at least 3: 1, then the computer 13 assumes, on the basis of appropriate programming, that the pulses determined by it originate from an active, non-deactivated Wiegand wire, and triggers an optical and a flashing light 20 an audible alarm on a signal horn 21, the entrance and exit doors of the supermarket optionally being able to be locked automatically if desired. The customer in question can then be subjected to a corresponding check.

If, on the other hand, the computer 13 detects Wiegand pulses, but on the other hand determines that their amplitude ratio is below the amplitude limit ratio of 3: 1 provided for triggering an alarm, the computer assumes that the label is deactivated, and therefore does not trigger an alarm. This is particularly the case when a Wiegand wire serving as a sensor metal element 8 has been deactivated in the area of the conveyor belt 5 by an initially very strong and then continuously decreasing alternating magnetic field. In this demagnetized state, the amplitudes of positive and negative pulses are the same.

It is readily apparent that false alarms can practically not be triggered in the security system according to the invention. For one thing, such Wiegand impulses are only generated by Wiegand wires, which are almost certainly not carried by customers. Even if this should be the case contrary to expectations, the corresponding Wiegand wire would still have to be in an activated state and the ratio of its positive and negative impulses (or vice versa) would have to exceed the limit value of 3: 1. All of this is practically impossible, so that an alarm is only triggered if a customer actually leads an appropriately secured item 2 past the checkout 4 to steal it.

As has already been explained above, the Wiegand wires can easily be glued onto the goods 2 to be secured together with a conventional price tag 7, an approximately 1 cm long Wiegand wire being completely sufficient, and the diameter of which can be, for example, only 0.25 mm. Despite these small dimensions, a sensor metal element 8 designed in this way is not, for example, scratched with a fingernail or the like. to render ineffective.

As has already been explained above, such a Wiegand wire 8 then generates a relatively strong, easily evaluable pulse if its longitudinal axis does not deviate too much from the magnetic vector (see arrow 16) of the magnetic field generated by the field coil 9. If a thief carries a correspondingly secured item 2, the sensor metal element 8 of which, for example, happens to be in the vertical direction when crossing the control gate 1, this would not induce a sufficiently strong pulse in the induction coils 15. This can possibly be accepted if, on the one hand, it can be assumed that the goods 2 to be secured are not very high-quality goods, and, on the other hand, it is assumed that a thief will visit such a sales outlet more or less regularly, so that it can then be determined on average every second or third theft according to statistical principles and, after appropriate punishment, can be banned from the home.

If, on the other hand, you want to further increase security (e.g. because the goods to be secured are at least partially higher-quality goods), a second field coil 9 and possibly even a third field coil 9 could possibly - as already explained above installed, the magnetic vector axes of which run at an angle (possibly at right angles) to one another, in which case the security of theft detection is increased accordingly. Another option is there however, it can also be seen in the fact that the installation is not correspondingly complex, but that higher-quality goods are provided with two or three sensor metal elements 8, the longitudinal axes of which are each at an angle, preferably at right angles, to one another. In this way, for example, the object 2.1 indicated in FIGS. 1 and 2 is secured, which is drawn out again on the upper left in FIG. 2, partially enlarged. It can be seen that the goods 2.1 are provided with three security labels 7, the sensor metal element 8 of which is designed as a Wiegand wire and extends at right angles to the two other Wiegand wires 8, 8. With such a security, a thief has no chance of crossing the field coil 9 without triggering an alarm, since one of the Wiegand wires 8 is always in a parallel or quasi-parallel position to the magnetic vector 16 of the field coil 9. Such a fuse with several Wiegand wires 8 is easily justifiable in price, since such sensor metal elements 8 do not cost 6 to 25 Pfg. As in the known prior art, but 0.5 Pfg., So that obviously a material expenditure of less than half a penny is economically viable if practically one hundred percent theft protection can be achieved.

In order to increase the display security even further, the Wiegand wires serving as sensor metal elements 8 can optionally also be integrated into the goods 2 or their packaging, so that a potential thief who is familiar with such a security system has practically no possibility has to track down the sensor metal elements and, if necessary, remove them if he wants to steal a certain item 2.

In spite of this unusually high alarm reporting security, a false alarm is practically impossible, since the computer 13 and the filter electronics 19 can easily be designed or programmed in such a way that other pulses, which originate from atmospheric disturbances or any electromagnetic interference pulses, are recognized or at which Filtering can be filtered. Even if any metallic objects are brought into the area of the field coil 9, which emit any pulses when magnetically excited, a distinction is easily possible and given here, since - as stated - an alarm is ultimately only given with an asymmetrical amplitude ratio (certain size) is triggered, but the asymmetry of the positive and negative impulses is a special feature of the Wiegand effect or the Wiegand wire working according to or with this effect, which does not occur with any other metallic materials. A false alarm is accordingly only conceivable if a customer carries an active Wiegand element which is not a sensor metal element 8 of the security system. However, since Wiegand wires are only used in speed and flow meters, and it is not common to use such devices when shopping in a supermarket, visiting an exhibition or the like. a false alarm is practically impossible.

Virtually all conceivable goods can be secured with the security system according to the invention, only securing highly magnetic products (for example magnetic tin cans) being problematic, since they can possibly prevent excitation of the Wiegand wire.

In addition to the exemplary embodiment described above and shown schematically in the accompanying drawing, which relates to a supermarket, it is of course also possible to provide all possible other sales or exhibition facilities, libraries, etc. with an inventive security system in an advantageous manner.

As has already been explained above, the security system according to the invention is obviously also suitable as a burglar security system, for example also for private households. If the outlets leading to the outside such as windows and doors, floor and cellar hatches etc. are each provided with at least one field coil, which is then expediently embedded in the wall surrounding the room outlet or in the floor already during construction (possibly also later) and if the objects to be secured are each provided with a Wiegand wire (in as invisible a place as possible) (in this case, of course, attaching numerous Wiegand wires is also inexpensive to carry), an alarm is then triggered in an analogous manner, if an object secured with at least one Wiegand wire is conveyed to the outside through a room outlet, since then, when the field coil is activated, it triggers a Wiegand pulse in at least one induction coil, which leads to an alarm being triggered when it is detected. A burglar alarm system designed in this way is even more advantageous compared to known burglar alarm systems because it cannot be recognized as such and practically cannot be switched off by unauthorized burglars, since the frequency generator that is preferably to be provided is located somewhere hidden in a house, apartment or the like . so can be installed in such a way that it is practically undetectable, and since it is practically impossible for burglars, especially with larger objects, to search for Wiegand wires, whereby, moreover, they can never be sure whether they are attached to a specific object (e.g. a valuable piece of furniture, a carpet or the like) have actually found all the Wiegand wires if they know the security system as such and have already removed three or four Wiegand wires from an object, for example.

REFERENCE SIGN LIST

¹'kapazitiven' (capacitive) in the German text is adjectival, indicating a missing noun, presumably 'Widerstand' ('reactance').
²Translator's Note: 'Igel' in German generally means 'hedgehog', 'porcupine', 'urchin', i.e. anything of which the spikes/spines extend in all directions.(LIST OF REFERENCE NUMERALS)

¹'capacitive 'in the German text is adjectival, indicating a missing noun, presumably' resistance '(' reactance ').
²Translator's Note: 'Igel' in German generally means 'hedgehog', 'porcupine', 'urchin', ie anything of which the spikes / spines extend in all directions.

Claims (17)

1. An anti-theft system for protecting objects in a room - more particularly a self-service shop such as a supermarket - against unauthorized theft, wherein

   a) the objects to be protected are each provided with at least one sensor wire element with which, in the region of a periodic magnetic field generated by a field coil, when a field strength limit value, oppositely directed to the magnetization condition of the sensor wire element, in an induction coil is exceeded, a pulsed signal is to be produced;

   b) the induction coil is connected to a detecting device, by means of which the signals produced in the induction coil are to be verified and if appropriate passed to an alarm device in order to actuate an alarm; and

   c) there is a deactivation station at which sensor wire elements of objects entitled to be taken out of the room are to be deactivated,

   characterized by the use of (at least) one section of Wiegand wire, having a soft magnetic core and a hard magnetic casing, as the sensor wire element (8), which can be activated by premagnetization, wherein in order to produce, as is known per se, an asymmetric signal by means of an activated sensor wire element (8) in the region of the field coil (9, 10) and the induction coil (15), the field strength amplitude of the alternating magnetic field generated by the field coil (9, 10) is greater than the triggering field strength of the core, but smaller than the triggering field strength of the casing; and wherein the signal produced by a sensor wire element (8) is first verified by the detecting device (13) as to whether it is an asymmetric Wiegand wire signal, in which the amplitude of the positive and negative pulse is compared, and an alarm is only actuated when the amplitude ratio of the pulses produced exceeds a predetermined limit value.
2. An anti-theft system according to claim 1, characterized in that the Wiegand wire sections used as the sensor wire element (8) are each about 5 to 20 mm long.
3. An anti-theft system according to claim 2, characterized in that the Wiegand wire sections (8) are each about 1 cm long.
4. An anti-theft system according to one of claims 1 to 3, characterized in that the diameter of the Wiegand wire sections (8) is about 0.15 to 0.4 mm.
5. An anti-theft system according to claim 4, characterized in that the diameter of the Wiegand wire sections (8) is 0.25 mm.
6. An anti-theft system according to one or more of the preceding claims, characterized in that at each room exit (1) of a room to be protected there are installed at least one field coil (9) surrounding the room exit (1) in a ring shape and at least one induction coil (15).
7. An anti-theft system according to claim 6, characterized in that associated with a room exit (1) are at least two field coils (9, 9) whose central axes (16) or magnetic vectors extend at an angle to one another.
8. An anti-theft system according to claim 7, characterized in that when two field coils (9, 9) are installed at a room exit (1), their central axes (16) or magnetic vectors extend substantially at right angles to one another.
9. An anti-theft system according to one or more of the preceding claims, characterized in that the field coil(s) (9, 9) is (are) excited by a frequency generator (14) with an (excitation) frequency of about 400 to 800 Hz.
10. An anti-theft system according to claim 9, characterized in that the frequency generator (14) is to be switched on automatically when a person (3) approaches it, and/or is to be switched off automatically when a person (3) has moved away from it.
11. An anti-theft system according to one or more of the preceding claims, characterized in that connected to each field coil (9) is a capacitive reactance, wherein the resonance frequency of the resonance system created by the inductance of the field coil (9) and the connected capacitance [is] essentially equal to the excitation frequency of the frequency generator (14); and in that the frequency generator (14) is designed in such a way that its (excitation) frequency automatically adjusts to the respective resonance frequency of the resonance system formed from the inductance of the field coil (9) and the connected capacitive [reactance]¹ .
12. An anti-theft system according to one or more of the preceding claims, characterized in that respective induction coils (15) are installed on two parallel sides of each field coil (9), wherein the induction coils (15, 15) are oriented in such a way that they convert changes of the magnetic field generated by the field coil (9), the vector thereof extending parallel to the central axis (16) of the field coil (9).
13. An anti-theft system according to one or more of the preceding claims, more particularly according to claim 12, characterized in that connected downstream of the at least one induction coil (15) is an electronic filter device (19), by means of which signals produced by a sensor wire element (8) are to be filtered out; in that connected downstream of or associated with the induction coil(s) (15, 15) and an amplifier and the filter device (19) respectively is a computer (13) which is to be activated when a person (3) approaches a room exit (1); in that the computer (13) then activates the frequency generator (14); and in that the computer (13), as detecting device, upon receiving a signal then carries out the verification thereof.
14. An anti-theft system according to one or more of the preceding claims, more particularly according to claim 13, characterized in that the detecting device (13) actuates an alarm only when the amplitude ratio of the pulses produced is (at least) about 3:1.
15. An anti-theft system according to one or more of the preceding claims, characterized in that an object (2) to be protected is protected with at least two Wiegand wire sections (8, 8) arranged at an angle to one another.
16. An anti-theft system according to claim 15, characterized in that the Wiegand wire sections (8, 8) extend substantially at right angles to one another.
17. An anti-theft system according to claim 15, characterized in that more than two Wiegand wire sections (8) are provided, arranged if appropriate in a hedgehog-like² shape.

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