EP0663656A1 - Monitoring sensor - Google Patents

Abstract

The monitoring probe monitors goods in danger of being stolen. It has a sensor element to monitor a usual contact of the probe (10) with the goods. A connection cable (12) connects the probe (10) to a monitoring circuit of a theft prevention system. If the usual contact between the probe (10) and the goods is removed, the probe (10) provides a signal which can be evaluated by the monitoring circuit as an alarm situation. The probe (10) includes a flexible contact element (14) which normally lies flat on the goods and makes contact. Its shape is thus determined by the shape of the goods. The sensor element of the probe (10) comprises a wire strain gauge on which a force is exerted when the usual contact between the goods and the probe (10) is broken. Thus a change in the shape of the contact element (14) results in a change in the output signal.

Description

The invention relates to a monitoring sensor for theft-prone goods with a sensor element for monitoring proper contact of the sensor with the goods to be secured and with a connecting cable for connecting the sensor to a monitoring circuit of an anti-theft system. When the proper contact with the goods to be secured is removed, the sensor provides an electrical measured variable, the change of which can be evaluated by the monitoring circuit as an alarm situation.

Such monitoring sensors with associated anti-theft systems are mainly used in shops for radio and television sets, video equipment, telephone systems, etc., in which a large number of exhibition devices are ready for use and ready for test use for customers. In order to avoid corresponding losses of such devices due to theft, the devices are connected to the anti-theft systems with the above-mentioned monitoring sensors, which trigger an alarm when the measured variable changes due to the removal of the proper contact with the goods and thus theft of the effectively prevent the secured device or the secured goods.

Monitoring sensors mentioned at the outset are known from DE 33 02 459 A1. These are each connected to a monitoring circuit of an anti-theft system via a connecting cable connected. As a sensor element for monitoring the proper contact of the sensor with the goods to be secured, the sensors have a large number of special plug connections which contain an integrated microswitch, the switching state of which changes when the sensor is removed from the goods. The change in the switching state is recognized by the monitoring circuit in the form of a measured variable and evaluated as an alarm situation.

In order to be able to use the monitoring sensors in a variety of ways, instead of the large number of plug-in connection parts, a so-called adhesive sensor has also been proposed, as is known from DE 42 21 686 A1. The sensor has a rigid housing that is glued to the goods, whereby when gluing a switch located on the underside of the sensor is actuated, which returns to its initial position and thus a measured variable in the form of an interrupted current conductor with infinite resistance for the monitoring circuit Provides when the proper connection between sensor and goods is broken again.

U.S. Patent 4,772,878 shows a similar adhesive probe with a rigid housing. It is glued to a product to be secured with a flat side surface, in which a piston movable vertically to it is integrated. Movement of the piston actuates an electrical switch so that an alarm situation can be recognized.

The disadvantage of these adhesive sensors is that they are difficult to permanently affix to curved surfaces of the goods to be secured and, in particular, because they are inadequate Attaching these sensors to the goods will always cause false alarms in the normal use of the exhibited goods by customers. This leads to the fact that the staff of the shops are repeatedly confronted with a false alarm, so that attention is reduced when an alarm is triggered and an actual alarm situation is no longer taken seriously.

In the course of the increasing importance of designing the high-priced goods and devices, it is becoming increasingly difficult to find a surface on the goods to be secured that is sufficiently flat for the conventional adhesive contacts, which guarantees a safe and permanent correct contact between the sensor and the goods.

Another disadvantage of all known sensors is that in these the sensor element monitors only a small part of the contact area of the sensor with the goods to be secured, so that these sensors are not tamper-proof. For example, a razor blade can be pushed between the feeler and the goods and then the feeler with the razor blade can be lifted off the goods without this being recognized by the sensor element.

In addition, the known adhesive contacts or adhesive sensors are relatively voluminous, so that in particular they cannot be attached to the underside of goods exhibited ready for operation without the functionality of the devices or their stability being impaired in some cases.

The object of the invention is to propose a generally usable monitoring sensor, which is manufactured in a flat construction and without problems also with curved or curved Areas can be connected to a proper and permanent contact and is largely tamper-proof.

This object is achieved according to the invention in a monitoring sensor with the features mentioned at the outset in that the sensor comprises a flexible contact surface element which, when the sensor and the product are properly in contact, lies flat against the product, and in that the sensor element is designed and arranged such that a force is exerted on the contact surface element and / or a change in shape of the contact surface element results in a change in the measured variable when the proper contact between the sensor and the goods is removed.

The flexibility of the contact surface element according to the invention allows the monitoring sensor to be precisely adapted to the surface of the goods to be secured. Therefore, a large-area adhesive contact between the sensor and the goods to be secured can be made, which is very durable and eliminates false alarms. In particular, the monitoring sensor can also be used to secure goods without even external surfaces, since the sensor can be adapted to any external contour.

It is also possible to make the contact surface element and the sensor element very flat, so that the overall height of the monitoring sensor is very low. This opens up a multitude of possibilities for attaching the sensor to the goods to be secured, which could not be used with the previously known adhesive sensors. In many cases, the monitoring sensor according to the invention can be glued to the underside of the goods to be secured become. This leads to the advantage that the appearance of the goods to be secured is not impaired.

Due to the fact that the sensor element detects a force acting on the contact surface element and / or a change in shape of the contact surface element that occurs in particular in edge regions, any manipulation of the sensor properly attached to the goods to be secured and thus any attempted theft is recognized.

Electrical signals or measured values are particularly suitable as the measured variable, but optical or magnetic measured variables can also be used to monitor proper contact between the sensor and the goods.

A very particularly preferred embodiment is characterized in that the contact surface element forms a flat side of the sensor and that the sensor element extends in edge regions of the contact surface element. This takes into account that detachment of the sensor from a product to be secured occurs from the edge of the sensor, the flexible contact surface element, which extends to the edge of the sensor, being particularly strongly deformed in the edge region. This change in shape is detected particularly reliably by the sensor element when it extends to edge regions of the contact surface element. This embodiment thus has the advantage that manipulation attempts on the sensor properly arranged on a product to be secured are recognized very reliably.

A particularly reliable detection of every change in shape of the contact surface element, even if this takes place only in part of the contact surface element, is also achieved by a large-area, supports the design of the sensor element, which is adapted to the surface extent of the contact surface element at least in one direction.

However, it is also possible for the sensor element to be arranged essentially only in edge regions of the contact surface element. This in turn results in an extremely reliable detection of theft attempts.

In addition, a second sensor element, such as a microswitch, e.g. be assigned in the area of the center of the contact surface of the contact surface element for monitoring the proper contact with the goods. This then results in a particularly high level of security against manipulation attempts. Such a measure can be appropriate especially when securing very high-priced goods.

A particularly reliable monitoring of the proper contact of the sensor and the goods over the entire contact area and a low design of the sensor is achieved in that the sensor element is deformable with the contact surface element to adapt to the shape of the goods and is essentially parallel to the contact surface of the contact surface element is arranged.

The universal usability of the sensor is supported in that the sensor element with the contact surface element can be adapted to the shape of the goods. For this purpose, the sensor element is preferably also designed to be flexible, in order to be able to implement an overall very flexible sensor, in particular if the sensor element extends considerably in terms of area, so that the contact surface of the Contact surface element is optimally adaptable to any shape or surface of a product to be secured.

The arrangement of the sensor element parallel to the contact surface of the contact surface element allows a particularly flat design and thus a high flexibility of the sensor.

The sensor element is preferably arranged directly on the contact surface element or is at least partially cast into it. This results in a direct transfer of any change in shape or force that the contact surface element experiences to the sensor element. Accordingly, a particularly reliable detection of theft attempts is made possible.

In an alternative embodiment, the sensor element can have a very compact shape compared to the flat design of the contact surface element.

A rubber-elastic material is preferably used for the production of the flexible contact surface element, which allows the contact surface element to be particularly well adapted to curved or curved surfaces of the goods to be secured. A soft rubber material is particularly suitable for this, but plastics can also be used.

It has proven to be advantageous if the housing material has a Shore hardness of 20 to 50 and in particular 25 to 40. This results in good flexibility of the sensor with adequate protection of the components arranged in the housing, such as the sensor element.

A particularly preferred embodiment is characterized in that the sensor has a flexible housing and that the contact surface element is formed in one piece with the housing. This results in a particularly simple manufacture in which, for example, the sensor element including the necessary connecting parts and the beginning of the connecting cable are enclosed in the housing. Alternatively, however, the housing can also be made in several parts.

In any case, the housing is preferably very flat, so that, in addition to a low overall height, great flexibility of the sensor is ensured. Furthermore, the flat design leads to a generous dimensioning of the contact surface of the contact surface element if the contact surface is formed by a flat side of the housing, so that a particularly secure connection to the goods to be secured is made possible.

The contact surface element preferably comprises an adhesive layer for fastening the sensor to the goods. The adhesive layer forms a generously dimensioned adhesive surface by means of which the contact surface element can be attached directly to the goods to be secured. In this case, the entire surface facing the goods to be secured is preferably used for the adhesive contact in order to produce a particularly durable and permanent connection between the sensor and the goods. This results in a better connection compared to the prior art, since an adhesive surface of the adhesive layer corresponding to the entire contact surface of the contact surface element can be used for fastening and cutouts for actuating elements of switches or the like, as are provided in the prior art, are not necessary .

Furthermore, the provided adhesive layer enables the monitoring sensor to be attached very easily to the goods to be secured, without the need for separately provided fastening means. Alternatively, however, the sensor can also be attached to the goods using an adhesive or a separate adhesive element.

A first particularly preferred embodiment is characterized in that the sensor element is arranged between the adhesive layer and the contact surface element. In this case, the sensor element, in particular, is completely enclosed by the adhesive layer and the contact surface element, so that these form an envelope protecting the sensor element. A housing for accommodating the sensor element can thus be dispensed with, and a particularly low-profile design of the sensor is possible. This results in a very simple and inexpensive manufacture of the sensor.

In an advantageous embodiment it is further provided that the adhesive layer adheres more strongly to the goods than to the contact surface element, and that the sensor element adheres to the adhesive layer, so that when the proper contact between the sensor and the goods is removed, the sensor element at least partially with the adhesive layer from the contact surface element separated and thereby a measuring loop formed by the sensor element is interrupted. Due to the very easy to implement different adhesive properties or different adhesive forces, when the sensor is attempted to be detached from a product to be secured, the sensor element is destroyed in that it remains at least partially, like the adhesive layer, on the product. Through this A measuring loop formed by the sensor element is interrupted, so that it is very easy to detect when proper contact of the sensor with the goods is canceled.

A second alternative embodiment results from the fact that the adhesive layer adheres more strongly to the goods than to the contact surface element and the sensor element is integrated into the adhesive layer, so that when the proper contact between the sensor and the goods is removed, the sensor element at least partially with the adhesive layer from the contact surface element separated and thereby a measuring loop formed by the sensor element is interrupted. This again leads to the sensor element being destroyed when the sensor is detached from the goods. Due to the interruption of the measuring loop formed by the sensor element, the attempted theft is again detected very easily and reliably. An advantage of this embodiment variant is that it only has to be ensured that the adhesive layer adheres better to the goods than to the contact surface element and that any adhesive forces that may occur between the sensor element and the contact surface element are irrelevant. Accordingly, this embodiment is very reliable. A simple manufacture is made possible, for example, by the sensor element being embedded between two layers of adhesive material lying on top of one another.

A particularly simple implementation of the measuring loop formed by the sensor element is characterized in that the sensor element comprises an electrical, in particular metal foil-like conductor loop. An electrical conductor loop has the advantage that an interruption of the circuit formed by the conductor loop with a very little effort is detectable. The particularly metallic foil-like design of the conductor loop leads not only to inexpensive manufacture, but also to a particularly flat design of the sensor element. In addition, there is a flat contact of the conductor loop with the adhesive layer and, accordingly, a good adhesive connection of the conductor loop with the adhesive layer, so that if the sensor is detached from the goods to be secured, the conductor loop adheres to the adhesive layer detaching from the contact surface element. In the event of manipulation, this leads to the intended destruction of the conductor loop and thus to a reliable detection of the attempted theft.

Further advantages of the thin, film-like design of the sensor element can be seen in the fact that the sensor element is very flexible, so that a high degree of flexibility of the entire sensor can be achieved, and that a film is destroyed even with little force if the adhesive layer is removed from the sensor , so that there is a particularly reliable detection of theft attempts.

As an alternative to the foil-like formation of the conductor loop, this can also be formed by a vapor-deposited metal layer or a very thin wire. In addition, predetermined breaking points can be provided in order to ensure that the conductor loop is interrupted as intended when the sensor is detached from the adhesive layer.

A particularly simple contacting of the conductor loop is achieved in that at least two contact points are integrated in the contact surface element. This enables a very simple manufacture, for example by the conductor loop and the adhesive layer are applied in succession to the contact surface element, the electrical contact with the conductor loop being ensured by abutment at the contact points. However, contact can also be made via connecting wires.

Another preferred embodiment is characterized in that the sensor element comprises a strain gauge. This results in a very sensitive detection of every change in the shape of the contact surface element, since the strain gauge changes its electrical resistance significantly even with the slightest changes in shape or the effects of force. In addition, the strain gauge is very flat and therefore enables a low overall height of the sensor.

A preferred embodiment results from the fact that the strain gauge is at least partially integrated into the flexible contact surface element. In this way, any change in shape of the contact surface element is transferred and registered particularly securely to the strain gauge when the sensor is detached from the goods to be secured. However, the strain gauge can, for example, also be glued to a flat side of the contact surface element.

An additional embodiment is characterized in that the sensor element comprises a flexible light transmission element with an associated light transmitter and light receiver. This embodiment variant is based on the idea that the shape of the light transmission element is also changed when the shape of the contact surface element changes, as a result of which the light transmission properties of the light transmission element are influenced. Accordingly, it depends Light signal received by the light receiver depends on the shape of the contact surface element. This has the result that changes in shape of the contact surface element are reflected in a change in a measured variable output by the light receiver. This embodiment variant is not limited to the use of light or, more generally, electromagnetic waves, but it is also conceivable that, for example, ultrasonic waves are used, the transmitter, receiver and transmission element being adapted accordingly.

In any case, the last-mentioned embodiment variant enables a highly sensitive detection of changes in shape of the contact surface element and therefore a very reliable detection of theft attempts.

The light transmitter is preferably designed as a gallium arsenide diode and the light receiver as a phototransistor. These components are available inexpensively and accordingly enable the monitoring probe to be manufactured at low cost. In addition, these components are very reliable and robust, so that there is a very reliable monitoring sensor.

It has proven to be advantageous if the light transmitter can be controlled in such a way that it regularly emits light signals, in particular with a frequency of 5 to 50 Hz. The frequency is selected so that a sufficiently rapid detection of changes in shape of the light transmission element and thus also of the contact surface element is ensured. By sending out flashes of light instead of constantly operating the light transmitter, a significant energy saving is achieved, which is particularly noticeable with a large number of monitoring sensors.

It is preferably provided that the light transmission element is arranged such that the main direction of transmission of the light runs essentially parallel to the contact surface of the contact surface element. This has the result that manipulation attempts on the contact surface element, which usually result in a movement of the contact surface element transversely to its contact surface, have a particularly marked influence on the light conduction in the light transmission element and thus on the measured variable supplied by the light receiver. This results in a very reliable detection of manipulation attempts. Alternatively, the light transmission element can be arranged, for example, to run perpendicular to the contact surface.

A particularly preferred embodiment is characterized in that the light transmission element comprises a waveguide which is formed in one piece with the contact surface element and which forms at least one light channel from the light transmitter to the light receiver. In addition to simple manufacture, the one-piece design results in a direct coupling of the light transmission element to the contact surface element, so that a change in shape of the contact surface element is transmitted directly to the light transmission element. This means that changes in shape of the contact surface element can be detected particularly reliably by the sensor element.

It is preferably provided that the wall of the light channel is not or only slightly reflective. For example, the contact surface element and possibly the entire housing of the monitoring sensor are made of a matt black material. Accordingly, the light intensity detected by the light receiver depends on the measured variable of the sensor element directly from the cross section of the direct, straight line of sight between light transmitter and light receiver. Changes in shape of the contact surface element then lead to changes in the side walls of the light channel formed by the contact surface element and thus to a change in the measured variable supplied by the light receiver. Accordingly, there is a simple and reliable detection of changes in shape of the contact surface element.

Alternatively, the light transmission element can, for example, also be delimited by reflecting walls. In this case, changes in shape result, for example, in influences on the phase position or the intensity distribution, which are recognized by the light receiver or a corresponding detector element.

It is further provided that the light channel is at least partially arranged in a section of the housing with a reduced cross section. A particularly great flexibility of the sensor is achieved in at least one area of the light channel. Accordingly, any attempt to remove the contact between the monitoring sensor and the goods leads to the sensor and thus also the light channel being deformed particularly easily in the region of the section with a reduced cross section. This results in a particularly sensitive detection of manipulation attempts.

The monitoring sensors according to the invention can also be equipped with an optical display for the operating state and the function of the sensor. The very small-sized LEDs are particularly suitable here.

The optical display is preferably operated in the function in which it permanently tampering with the connection of the goods and the monitoring sensor, i.e. until the alarm is cleared by authorized personnel. In the event of an alarm, this makes it easier for the sales staff to locate the goods that have been attempted to be stolen. If necessary, proper contact between the goods and the sensor can be restored very quickly.

Alternatively or in addition, the optical display is operated in such a way that it emits an optical signal, such as a blinking signal, in the state properly attached to the goods to be secured, in order to draw the attention of theft to potential thieves and to deter theft. In this case, the optical display can also be formed by the light transmitter provided for the sensor element in an embodiment variant, the flashing of which is visible when the sensor is in the armed state through a correspondingly translucent area of the sensor housing.

In addition, the optical display can of course also be used to communicate other information, such as for the visual display for a properly re-established connection of the sensor to the monitoring circuit. For example, the display may briefly light up for this purpose.

Switching between different display modes of the optical display is also possible. Such a switch is preferably carried out centrally controlled by the anti-theft system.

A particularly reliable protection against false alarms is achieved in that the sensor comprises a rigid connection part for the connecting cable. As a result, forces acting on the connecting cable are largely decoupled from the flexible contact surface element, so that no or only minimal forces are exerted on the sensor element by movement of the connecting cable, for example when the customer is inspecting the goods to be secured. Of course, the cable connection area, the rigid connection part and the flexible contact surface element can be formed with a one-piece jacket.

In cases in which the monitoring sensor according to the invention is used with a novel monitoring circuit for anti-theft devices to be described below, the measured variable supplied by the sensor element can be made available directly to the monitoring circuit without preparation and conversion. The measured variable is then processed and evaluated within the monitoring circuit of the anti-theft system.

However, in order to be able to use the monitoring sensor with conventional anti-theft systems, it can be provided that a circuit is integrated in the sensor, which processes the measured variable supplied by the sensor element, in particular by the strain gauge or light receiver, for the monitoring circuit. The task of this circuit integrated in the sensor is to provide an output signal that can be used for a conventional anti-theft system, it being possible, for example, to provide a digital signal.

Accordingly, a preferred embodiment is characterized in that the circuit comprises an evaluation circuit for processing the measured variable of the sensor element and converting it into a digital value, and a memory for temporarily storing the digital value, the circuit preparing the measured variable at predetermined time intervals and converting it to a digital value and outputs an alarm-indicating signal to the monitoring circuit if the temporarily stored digital value deviates from a later digital value by a predeterminable value. The monitoring sensor can be used particularly flexibly, i.e. regardless of the shape of the flexible contact surface element and thus regardless of an initial value of the measured variable supplied by the sensor element, which depends on the shape of a product to be secured. When the sensor is connected to the monitoring circuit for the first time, the measured variable, for example the resistance value of the strain gauge or the measured variable supplied by the light receiver, is then processed and saved, and a separate output signal for the monitoring circuit is made available when the measured variable changes later. The monitoring circuit detects an alarm situation on the basis of this signal. Instead of integrating the evaluation circuit into the sensor itself, the circuit can of course also be installed in a plug contact with which the connecting cable of the sensor can be connected to the anti-theft system.

It is also conceivable that a connection of a conventional anti-theft system is equipped with an interface which makes it possible to connect a monitoring sensor without a processing circuit for the measured variable of the sensor element.

The invention further relates to a novel monitoring circuit for an anti-theft system, which is especially designed for use with a monitoring sensor of the present invention, in that this monitoring circuit comprises an evaluation circuit for processing the measured variable of the sensor element and converting it into a digital value, and a memory for temporarily storing this digital value . In this case, the monitoring circuit will prepare the measured variable supplied by the sensor element at predetermined time intervals and convert it to a digital value and trigger an alarm if the temporarily stored digital value deviates from a later-obtained digital value by a predetermined value. In the simplest case, storing a one-digit digital value is sufficient.

By using the digital filter formed in this way for triggering the alarm, there is the advantage that minimal changes in the measured variable of the sensor element, such as can be caused, for example, by rough handling of the goods secured with the monitoring sensor according to the invention, are not taken into account when triggering the alarm, so that an alarm is only triggered if there is serious manipulation of the contact between the monitoring sensor and the goods. Furthermore, it can be provided that the predetermined value can be set by the operating personnel so that the sensitivity of the system can be adapted to the respective application.

Furthermore, the evaluation circuit mentioned has the advantage that sensors with different sensor elements, for example very different electrical resistance values supply as measured variables, can be connected to the same type of monitoring circuit.

The invention further relates to an anti-theft system with a monitoring circuit described above. Such an anti-theft system can work with extremely small-sized surveillance sensors and essentially manages with a single type of surveillance sensor, since the surveillance sensors according to the invention work satisfactorily with practically every product to be secured and can be attached to it without preventing the functionality of the product.

In general, the monitoring sensors are intended for attachment to goods with solid surfaces. However, the monitoring sensors, whose sensor element only shows a significant change in the measured variable when the sensor is detached from a secured product, as is the case, for example, in the embodiment with the conductor loop arranged between the contact surface element and the adhesive layer, also to secure goods with flexible surfaces.

In addition, the monitoring sensors according to the invention do not disturb the appearance of the goods to be secured in any way, since they can be designed very inconspicuously, and can therefore also be used in cases where an optically particularly demanding presentation of the goods is important.

In a particularly preferred embodiment of the anti-theft system according to the invention it is provided that this security system comprises an activation circuit which, when a connection is first occupied with a connection cable of a monitoring sensor, is connected to the connection assigned monitoring circuit activated, which for the first time prepares the measured variable of the sensor at a predetermined time interval for activation and temporarily stores it in the memory.

With the automatic activation of the monitoring circuits when a connection to the anti-theft system is occupied for the first time, there is no need for dummy plugs for the unused connections, which is a great advantage for handling the anti-theft system. In addition, the predetermined time interval for activation, in which the measured variable of the sensor is processed, converted and temporarily stored in the memory, can be selected such that a secure attachment of the sensor to the goods is possible and thus a constant measured variable of the sensor in the correct manner condition of the goods is achieved before a digital value is saved.

Here, too, the operating personnel can be given a certain leeway again, so that in cases where the sensor is first attached to the goods before the connecting cable is connected to the anti-theft system, a relatively short interval can be selected. On the other hand, in cases where the connecting cable is first connected to the monitoring circuit due to inexperienced personnel before the actual sensor is properly connected to the product, the evaluation circuit can trigger a correspondingly larger time interval for evaluating the first digital value False alarms can be avoided.

Fig. 1

eine schematische Darstellung einer erfindungsgemäßen Diebstahlsicherungsanlage;

Fig. 2

eine Schnittansicht einer ersten Ausführungsform eines erfindungsgemäßen Überwachungsfühlers;

Fig. 3

eine Ansicht des Fühlers nach Fig. 2 von unten;

Fig. 4

eine Schnittansicht einer zweiten Ausführungsform eines erfindungsgemäßen Überwachungsfühlers;

Fig. 5

eine schematische Darstellung einer in den Fühler nach Fig. 4 integrierten Schaltung mit einem angeschlossenen Verbindungskabel und einem angeschlossenen Dehnungsmeßstreifen;

Fig. 6

eine Draufsicht auf eine dritte Ausführungsform eines erfindungsgemäßen Überwachungsfühlers;

Fig. 7

einen Längsschnitt des Fühlers nach Fig. 6 gemäß Linie I-I; und

Fig. 8

einen Querschnitt des Fühlers nach Fig. 6 gemäß Linie II-II.

The invention is explained in more detail below with reference to the drawing of three exemplary embodiments. Show it:

Fig. 1

a schematic representation of an anti-theft system according to the invention;

Fig. 2

a sectional view of a first embodiment of a monitoring sensor according to the invention;

Fig. 3

a view of the sensor of Figure 2 from below.

Fig. 4

a sectional view of a second embodiment of a monitoring sensor according to the invention;

Fig. 5

a schematic representation of a circuit integrated in the sensor according to FIG. 4 with a connected connecting cable and a connected strain gauge;

Fig. 6

a plan view of a third embodiment of a monitoring sensor according to the invention;

Fig. 7

a longitudinal section of the sensor of Figure 6 along line II. and

Fig. 8

a cross section of the sensor of FIG. 6 along line II-II.

1 shows an anti-theft system 10 according to the invention, which has at least one, but preferably a plurality of monitoring circuits 12 and a common alarm device 14.

A monitoring sensor 20 can be connected to each monitoring circuit 12 via a plug connection 16 and a connecting cable 18. 1, a sensor 20 is connected to the monitoring circuit 12 via the connecting cable 18 and is also attached to a product 22 to be secured.

If a thief manipulates the sensor 20 or the connecting cable 18, this is recognized by the associated monitoring circuit 12 and a signal is output to the alarm device 14 of the anti-theft system 10. The alarm device 14 then displays the alarm, for example by means of an acoustic signal or a visual display.

FIG. 2 shows a first embodiment of a sensor 20 according to the invention. This has a flexible, very flat housing 24 made of a rubber-elastic material. The housing 24 has an integrated contact surface element 26 with a flexible, smooth contact surface 28, which forms a flat side of the housing 24.

In the embodiment shown, the housing 24 is essentially designed as a flat cuboid with rounded edges. The contact area 28 corresponds to a base area of the cuboid. However, the housing 24 can also have the shape of a flat section of a circular cylinder or an elliptical cylinder or some other shape. It is important that a flexible contact surface element 26 is formed.

A rigid connecting part 30, to which the connecting cable 18 is connected, is cast into the housing 24. The connection part serves on the one hand a fixed mechanical Connection of the connecting cable 18 to the sensor 20 as a strain relief and on the other hand an electrical connection of the sensor 20 to the connecting cable 18. Since the connecting part 30 is fully covered by the housing 24, the connecting cable 18 is also partially cast into the housing 24 and thus mechanically connected to this.

At its free end, the connecting cable 18 has a schematically indicated plug 32 for forming the plug connection 16 and for connecting the sensor 20 to a monitoring circuit 12 in the anti-theft system 10.

The connecting part 30 is very flat and extends with its main plane of extension substantially parallel to the contact surface 28. The connecting part 30 has two contact points 34 in the form of contact pins, each of which extends through the contact surface element 26 to the contact surface 28 and possibly somewhat over the contact surface 28 are also increased.

The contact points 34 serve for an electrical connection of a sensor element 36 arranged on the contact surface 28. The sensor element 36 is arranged parallel to the flat contact surface element 26 and lies flat on the contact surface 28 in the exemplary embodiment shown in FIG. 2.

3, the sensor element 36 in the first exemplary embodiment of the sensor 20 is formed by a conductor loop 38 resting on the contact surface 28. The conductor loop 38 is formed here by a metal foil strip made of aluminum, which is spaced apart and approximately is arranged parallel to the edge of the contact surface 28 of the contact surface element 26. In the exemplary embodiment shown in FIG. 3, the conductor loop 38 essentially forms an open U and covers one of the contact points 34 of the connecting part 30 with the free leg ends.

When the conductor loop 38 rests on the contact points 34, the conductor loop 38 is electrically contacted. In order to ensure good contact between the contact points 34 and the conductor loop 38, the contact points 34 are preferably slightly raised relative to the contact surface 28. Furthermore, the surfaces of the contact points 34 are preferably coated with gold in order to ensure a low contact resistance to the conductor loop 38.

Alternatively, the conductor loop 38 is replaced by a correspondingly vapor-deposited metal sheet, e.g. made of silver, which can also be evaporated onto the contact points 34 for electrical contacting. If the contact points 34 are resilient, the conductor loop 38 or the measuring loop can be applied to the adhesive layer 42.

It should be noted that the measuring loop or conductor loop 38 is preferably applied to the side of the adhesive layer 42 facing the contact surface 28 before the adhesive layer 42 is attached to the contact surface element 26, in order to ensure a good adhesive connection between the measuring loop and the adhesive layer 42.
In Fig. 3 it is indicated that a connecting line 40 leads from the connecting cable 18 to the contact points 34, so that a closed circuit through the conductor loop 38 and via the contact points 34, the connecting lines 40 and the connecting cable 18 for the monitoring circuit 12 can be formed.

2 that the sensor element 36 on the contact surface 28 of the contact surface element 26 is completely covered with an adhesive layer 42. This adheres both to the contact surface 28 and to the sensor element 36. In FIG. 3, the illustration of this adhesive layer 42 has been omitted for reasons of illustration.

The adhesive layer 42 is formed by a double-sided adhesive tape which is covered on its adhesive surface 44 facing away from the contact surface element 26 by a protective film (not shown) until it is attached to the goods 22 to be secured.

In the limit case, the housing 24 is so flat that it is identical to the contact surface element 26. The electrical connection of the sensor element 36 can then take place in that the connecting cable 18 is glued between the contact surface element 26 and the adhesive layer 42 or alone in the latter and connected to the sensor element 36. An extremely small thickness of the sensor 20 of only a few millimeters can thus be achieved.

The connection of the sensor 20 to the anti-theft system 10 and the function of the security device are described below.

The sensor 20 is fixed with its contact surface element 26 facing the product 22 on a surface of the product 22, this being done with the aid of the one not shown Protective film freed adhesive surface 44 of the adhesive layer 42 takes place. Due to the flexible design of both the sensor element 36 and the contact surface element 26 including the housing 24, the sensor 20 can be adapted to any curved or curved surface of the goods 22. The sensor 20 is glued over the entire surface to a surface, preferably the underside, of the goods 22 in order to make the proper contact of the sensor 20 and the goods 22 with the adhesive surface 44. The essentially parallel arrangement of the sensor element 36 to the contact surface element 26 is retained even when the sensor 20 is curved.

After the sensor 20 has been attached to the item 22 to be secured, the sensor 20 is connected to the anti-theft system 10 via the connecting cable 18 by plugging the plug 32 into a plug connection 16 and thus establishing the connection to an associated monitoring circuit 12.

The first connection of the sensor 20 to the monitoring circuit 12 is recognized by an activation circuit 46 of the monitoring circuit 12. With a predefined or adjustable delay time, this arm the subsequent evaluation circuit 48. This means that after the delay time has elapsed, the evaluation circuit 48 monitors the sensor 20 connected via the plug connection 16 and the connecting cable 18 with a view to proper contact with the goods 22 and the electrical connection to the sensor 20 for other attempts at manipulation. For this purpose, for example, a current flowing through the sensor element 36 of the sensor 20 or the electrical resistance formed by the sensor element 36 is used as a measured variable evaluated by the evaluation circuit 48. This converts the measured variable into a digital value which, after activation by the activation circuit 46, is temporarily stored in a memory 50. Subsequently, the measured variable is regularly measured and converted by the evaluation circuit 48, a current measured value always being compared with the stored value. If a deviation is found that exceeds a predetermined amount, an alarm situation is present and a corresponding signal is output to the alarm device 14.

If the sensor 20 is removed from the goods 22 during an attempted theft, the adhesive layer 42 sticks to the goods 22 with the sensor element 36 or the conductor loop 38. At most, the contact surface element 26 can be removed from the adhesive layer 42. For this purpose, the adhesive layer 42 has a higher adhesive or adhesive force on the adhesive surface 44 than compared to the contact surface element 26. Accordingly, when the sensor 20 is removed from the goods 22, the measuring loop formed by the sensor element 36, such as the electrical conductor loop 38, is interrupted. If necessary, the measuring loop can additionally have predetermined breaking points, such as tapering or perforations, for the defined interruption.

This destruction of the sensor 20 in the event of manipulation represents a very essential property of this embodiment according to the invention, which ensures particularly reliable detection of an attempted theft.

The interruption leads to a significant change in the measured variable, which the evaluation circuit 48 recognizes as an alarm situation. The evaluation circuit 48 gives in this case, an alarm triggering signal to the alarm device 14.

Furthermore, the evaluation circuit 48 also detects changes in the measured variable which are due to manipulations on the connecting cable 18 or on the plug connection 16, such as short-circuiting or breaking the connection, and an alarm is triggered.

The evaluation method described represents a preferred method variant. However, another evaluation is also possible, for example using only analog measured values. There is also the possibility of regularly updating the initial value stored in the memory 50 to the current measured value when the measured variable changes within the permitted tolerance range by storing it again. Slow changes in measured values, such as those caused by temperature influences, can be taken into account without triggering a false alarm.

In the sensor 20 according to the first embodiment described, the contact points 34 can alternatively also be formed by contact springs. These are arranged and are held, for example, by the contact surface element 26 or by the connection part 30 such that a spring section, such as a free end, projects over the contact surface in the unloaded state. The adjacent electrical conductor loop 38 is then particularly reliably electrically contacted via the contact springs due to the spring force, even if the conductor loop 38 should be at a slight distance from the contact surface 28, for example due to manufacturing tolerances.

Furthermore, the measuring loop or the conductor loop 38 can form one or more electrical connections, which can also be rectilinear, on the contact surface 28 or on the adhesive layer 42, which when the sensor 20 is removed from a product 22 to be secured by the contact surface element 26 releasing adhesive layer 42 are at least partially interrupted. The intended interruption of the electrical connection can also be enforced in that the conductor loop 38 has cuts, incisions or punchings, particularly in the case of a film-like configuration, which facilitate tearing.

An essential idea of the invention is also to be seen in the fact that the adhesive layer 42 can be pre-cut or punched, in particular with the measuring loop, in such a way that the adhesive layer 42 is torn by a product 22 to be secured when the probe 20 is released, so that sections of the adhesive layer 42, which also carry sections of the measuring loop, remain on the goods 22 to be secured. This results in a particularly reliable interruption of the measuring loop and a correspondingly reliable detection of manipulations on the sensor 20.

A second and a third embodiment of the monitoring sensor according to the invention are described in more detail below. In this case, parts that are the same or have the same effect are generally designated with the same reference numerals as above, even if an explicit designation is omitted. In addition, the same handling, the same application and the same advantages as in the first embodiment result in general in the further embodiments of the monitoring sensor.

The monitoring sensor 20 according to the invention according to the second embodiment according to FIG. 4 in turn has a housing 24 made of a rubber-elastic material, which forms an integrated, flexible contact surface element 26. The housing 24 is essentially flat, the contact surface element 26 forming a substantially flat contact surface 28 in the state in which the sensor 20 is not attached to a product 22.

In the housing 24, a sensor element 36 is arranged to run essentially parallel to the contact surface element 26. The sensor element 36 is in turn connected to the connecting cable 18 of the sensor 20 via a rigid connecting part 30 inside the housing 24.

A circuit 52 is integrated into the rigid connection part 30 and will be described in more detail later. Furthermore, an optical display 54 in the form of a light-emitting diode is arranged on the connecting part 30 and penetrates the housing 24 on the side of the sensor 20 facing away from the contact surface element 26 and is visible.

On the contact surface 28 of the contact surface element 26 there is in turn an adhesive layer 42 which serves to fasten the sensor 20 to the goods 22 to be secured.

In the second embodiment, the sensor element 36 is designed as a strain gauge 56. This is preferably located directly on the side of the contact surface element 26 facing away from the contact surface 28, a certain distance between the strain gauge 56 and the contact surface element 26 being shown in FIG. 4 For the sake of clarity. Due to this assignment of the sensor element 36 to the contact surface element 26, each change in shape of the flexible contact surface element 26 results in a change in shape or the effect of force on the sensor element 36 formed by the strain gauge 56. Accordingly, this leads to a change in a measured variable, which is formed here in particular by the electrical resistance or a current flowing through it.

Also the sensor 20 according to the second embodiment is secured to the article 22 to secure it by the proper contact of the sensor 20 and article 22 being established in that the sensor 20 with its adhesive layer 42 is glued over its entire surface to a surface of the article 22 and so with this is firmly connected. Due to the flexible design of the sensor 20, the contact surface element 26 can be adapted to any curvature of the surface of the product 22, so that the sensor 20 can be fastened to the product 22 in a manner which is highly resilient and thus secure.

Then the sensor 20 is connected to a monitoring circuit 12 of the anti-theft system 10 via the connecting cable 18.

An alternative to the first embodiment results for the processing and evaluation of measured variables by the circuit 52, which is integrated in the sensor 20 and, in the second exemplary embodiment, is arranged in the rigid connecting part 30. This alternative signal evaluation is described below for the second embodiment of the sensor 20, but can also be carried out without problems in the other embodiments of the sensor 20, and vice versa, the type of signal processing and alarm detection of the first Embodiment can be applied to the second and third.

5 schematically shows the electrical construction and connection of the sensor 20 according to the second embodiment to a monitoring circuit 12. The sensor element 36 formed by the strain gauge 56 is connected to the circuit 52 in the rigid connection part 30 via connecting lines 40. These components are all arranged in the flexible housing 24 of the sensor 20, preferably even cast into it.

The circuit 52 is supplied with power by the monitoring circuit 12 connected to the anti-theft system 10 via the connecting cable 18 and the plug connection 16.

The circuit 52 has an evaluation circuit 48 with an associated memory 50, which perform the same functions as in the method variant described with reference to FIG. 1. Accordingly, the evaluation circuit 48 prepares a measured variable of the sensor element 36, in particular the electrical resistance or a current flowing through the strain gauge 56, so that manipulations on the sensor 20, in particular a change in shape of the contact surface element 26 when the sensor 20 is detached from of the goods to be secured 22 are detected. For this purpose, the evaluation circuit 48 converts the measured variable into a digital value, which is temporarily stored in the assigned memory 50. The measured variable is then measured regularly and converted into a digital value, which is compared with the temporarily stored digital value. If one exceeds a predetermined amount Deviation is determined, the evaluation circuit 48 outputs a signal that indicates an alarm situation. This signal is output via the connecting cable 18 and the plug connection 16 to the monitoring circuit 12.

The monitoring circuit 12 has a detector circuit 58 which receives the output signal from the evaluation circuit 48 and detects an alarm situation. In the event of an alarm situation, the detector circuit 58 outputs an alarm signal to the alarm device 14 for triggering the alarm. In addition, the detector circuit 58 can monitor the electrical connection to the circuit 52 in the sensor 20 and can output an alarm signal to the alarm device 14 in the event of any manipulations, such as a short-circuiting or disconnection of the connection.

It goes without saying that also in this described alternative embodiment according to FIG. 5, an activation circuit 46 can also be arranged in the monitoring circuit 12, which activates an automatic arming after the plug connection 16 has been established to a sensor 20. This activation circuit 46 can, for example, only switch on the electrical supply to the circuit 52 after a preselectable delay time, so that until then it is possible to attach the sensor 20 to the goods 22 to be secured, with a resultant change in the measured variable of the sensor element 36, without a False alarm is triggered.

The display 54 connected to the circuit 52 is briefly switched on when the sensor 20 is properly connected to the monitoring circuit 12 and when the monitoring circuit 12 is armed. in the display 54 then remains off during normal operation. If the proper contact between the goods 22 and the sensor 20 is lost even briefly, the display 54 is permanently activated and emits, for example, a flashing signal until the monitoring circuit 12 is reset by authorized personnel and the alarm situation is thus ended.

It is also possible that the visual display 54 flashes regularly on the goods 22 in order to deter potential thieves.

As described above, the integrated circuit 52 can itself check the measured variable of the sensor element 36 for a change and indicate to the monitoring circuit 12 whether the monitoring sensor 20 detects the normal state of proper contact between the sensor 20 and the goods 22 or whether a situation exists, in which the proper contact between sensor 20 and goods 22 is or has been canceled. However, the circuit 52 can also provide the monitoring circuit 12 with a digital value, for example, which reflects the current measured variable of the sensor element 36.

The connecting cable 18 can generally be cast onto the housing 24, connected to the housing 24 by a strain relief, such as the connecting part 30, or in exceptional cases can be connected to the latter in a pluggable manner.

Furthermore, the connecting cable 18 can be configured with two, three or even multiple wires, depending on the scope of functions that is implemented in the monitoring sensor 20 itself. A two-wire connecting cable 18 is sufficient to merely tap the measured variable of the sensor element 36. This can also be sufficient if the integrated circuit 52 takes over the complete evaluation of the measured variable available from the sensor element 36 and provides the monitoring circuit 12 with a signal representing the correct contact and a signal representing the removal of the correct contact.

If the connecting cable 18 is to be equipped with a separate line for testing short circuits, or an activation current is to flow via a current flow via the plug connection for connecting the monitoring sensor 20 to the anti-theft system 10, which activates the monitoring circuit 12 assigned to the sensor 20 with the aid of the activation circuit 46 then it is advisable to choose a three-wire or four-wire connection cable 18.

The third embodiment of the sensor 20 according to the invention is shown in a top view in FIG. 6. The sensor 20 in turn has a housing 24 made of a rubber-elastic material. Here, the connecting cable 18 is firmly connected to the sensor 20 via an extension 60 of the housing 24.

The housing 24 essentially has the shape of a flat cuboid, the base side of which is designed as a contact surface element 26 with a flat contact surface 28 when the sensor 20 is not fastened, as is the case in the longitudinal section according to FIG. 7 along the line II in FIG. 6 can be seen.

Furthermore, the housing 24 is reduced in cross section in a central section 62, transverse to the longitudinal extension of the cuboid. This can also be seen in FIG. 8, the one Cross section of the sensor 20 according to the line II-II of Figure 6 shows. Accordingly, the housing 24 and the contact surface element 26 in the tapered section 62 are particularly flexible.

In the third embodiment, the sensor element 36 comprises a light transmitter 64 in the form of a GaAs diode, a light transmission element 66 and a light receiver 68 in the form of a phototransistor. In this exemplary embodiment, the light transmission element 66 is formed as a hollow conductor 70 by a circular-cylindrical cavity running in the longitudinal axis of the cuboid. The light transmitter 64 and the light receiver 68 are arranged opposite one another at the two ends of the waveguide 70. Accordingly, the light transmission element 66 forms a light channel 72 between the light transmitter 64 and the light receiver 68.

In this embodiment of the sensor 20, a matt black rubber material is preferably used for the housing 24 and the contact surface element 26 for the production. Accordingly, the cylindrical wall of the light channel 72 in the housing 24 is not or only slightly reflective for light signals emitted by the light transmitter 64. The intensity received by the light receiver 68 is therefore directly dependent on the clear cross section of the light channel 72. The light receiver 68 outputs a signal corresponding to the received light as a measured variable of the sensor element 36.

In the third embodiment, the electrical connections for the light transmitter 64 and the light receiver 68 are not shown in FIGS. 6, 7 and 8 for reasons of simplification.

The longitudinal section according to FIG. 7 clearly shows that in the preferred arrangement of the optical axis of the light transmission element 66 parallel to the contact surface 28, the lateral wall of the waveguide 70 is at least partially formed by the contact surface element 26. As a result, changes in the shape of the contact surface element 26 in the event of attempts to remove the proper contact between the sensor 20 and the article 22 lead directly to a change in the clear cross section of the light channel 72, so that the resultant change in the measured variable output by the light receiver 68 is carried out by the monitoring circuit 12 the anti-theft system 10 is detectable.

To reduce the energy consumption, it has proven useful if the light transmitter 64 emits light pulses at a frequency of 10 Hz, which are received by the light receiver 68 and evaluated for their intensity. Either a synchronized measurement by the light receiver 68 or an evaluation of the measured intensity maxima can take place in a continuous measurement.

It should be mentioned that the sensor 20 according to the third embodiment also has an adhesive layer 42 on its contact surface 28 in order to be able to be attached to the product 22 to be secured with the aid of the adhesive layer 42.

Otherwise, measurement value processing and evaluation can optionally be provided as in the first embodiment or in the second embodiment.

In the light transmission element 66 described, the light channel 72 formed runs in a straight line from the light transmitter 64 to the light receiver 68. However, it is also possible for the light transmission element 66 so that a curved light channel 72 is formed. In particular, a circular or ring-shaped configuration is appropriate, so that the light transmitter 64 and the light receiver 68 can be arranged close to one another and opposite in such a way that the circular light channel 72 forms a curved optical connection from the light transmitter 64 to the light receiver 68.

Here, the wall of the light channel 72 is designed to be only slightly reflective, and the light transmission element 66 is preferably formed by a very thin and flexible tube, such as a capillary made of a little elastic plastic, the interior of which can practically not be squeezed together. For example, such a tube is arranged in the edge region of a flexible sensor 20 designed as a circular disk. This has the advantages that the sensor 20 does not generate a false alarm in the state properly attached to the product 22 under pressure, since the interior of the tube is not changed, but that pulling the sensor 20 off the product 22 changes the curvature of the tube, due to its flexibility, and thus leads to a change in the light transmission properties and accordingly to a change in the measured variable output by the light receiver 68. This results in reliable detection of theft attempts.

In all embodiments, the sensor elements 36 can be arranged in particular in the edge region of the contact surface element 26 in order to register the particularly strong change in the shape of the contact surface element 26 in the edge region when the sensor 20 is detached from a product 22 to be secured. This leads to a particularly reliable detection of theft attempts.

In addition to the previously described applications of the monitoring sensor 20 for the anti-theft protection of goods 22, the sensors according to the second and third embodiment, which provide constantly changing measured variables as a function of changes in shape of the contact surface element 26, are generally suitable for measuring displacements or the like. In particular, the sensors 20 mentioned can be used for measuring dilatations in the field of construction. For example, a slowly progressing crack formation can be monitored with the aid of a sensor 20 glued over the crack and, if necessary, an alarm can be given if a tolerance range is exceeded.

It is particularly important for the present invention that the monitoring sensor 20 itself does not have to have a rigid structure in order to rule out unintentional false alarms. Rather, the sensor housing 24 can be made very flexible overall and receives a stable position by being glued to a rigid surface in firm contact. In this way, false alarms can be reliably excluded even with very flexible, in particular rubber-elastic sensor housings 24 of small thickness. However, it is also possible to attach the sensor 20 with its flexible contact surface element 26 to a flexible product 22 if the sensor element 36 is designed, for example, as a flexible conductor loop 38 arranged between the adhesive layer 42 and the contact surface element 26, the measurement variable provided of which is only available when the Removing the sensor 20 from the goods 22 changes significantly.

In any case, the flexible design of the sensor 20 leads to a better connection between the sensor 20 and of the goods 22 can be produced, since when the sensor 20 is glued to the goods 22, the pressure force is not distributed over the entire contact surface 28, but rather a higher pressure is generated in some areas. Optionally, the sensor 20 is pressed several times at various points by an operator with his finger on the goods 22 to be secured. This results in a much stronger and more durable adhesive contact between the sensor 20 and the goods 22 to be secured than in sensors with a rigid housing.

REFERENCE SIGN LIST

10th

Anti-theft system

12th

Monitoring circuit

14

Alarm device

16

Connector

18th

connection cable

20th

(Monitoring) sensor

22

Would

24th

casing

26

Contact surface element

28

Contact area

30th

Connector

32

plug

34

Contact point

36

Sensor element

38

Conductor loop

40

Connecting line

42

Adhesive layer

44

Adhesive surface

46

Activation circuit

48

Evaluation circuit

50

Storage

52

circuit

54

display

56

Strain gauges

58

Detector circuit

60

approach

62

section

64

Light transmitter

66

Light transmission element

68

Light receiver

70

Waveguide

72

Light channel

Claims (28)

Monitoring sensor for theft-prone goods (22) with a sensor element (36) for monitoring proper contact of the sensor (20) with the goods (22) and with a connecting cable (18) for connecting the sensor (20) to a monitoring circuit (12) Anti-theft system (10), the sensor (20) providing a measured variable when the proper contact with the goods (22) is removed, the change of which can be evaluated as an alarm situation by the monitoring circuit (12),
characterized,
that the sensor (20) comprises a flexible contact surface element (26) which, when the sensor (20) and product (22) come into proper contact, lies flat against the product (22), and that the sensor element (36) is designed and arranged in such a way that a force effect on the contact surface element (26) and / or a change in shape of the contact surface element (26) results in a change in the measured variable when the proper contact of the sensor (20) and the goods (22) is removed. Monitoring sensor according to claim 1, characterized in that the contact surface element (26) forms a flat side of the sensor (20) and that the sensor element (36) extends in edge regions of the contact surface element (26). Monitoring sensor according to claim 1 or 2, characterized in that the sensor element (36) with the Contact surface element (26) to adapt to the shape of the goods (22) is deformable and arranged essentially parallel to the contact surface (28) of the contact surface element (26). Monitoring sensor according to one of the preceding claims, characterized in that the contact surface element (26) is made of a rubber-elastic material. Monitoring sensor according to one of the preceding claims, characterized in that the sensor (20) comprises a flexible, preferably flat housing (24), and in that the contact surface element (26) is formed in one piece with the housing (24). Monitoring sensor according to one of the preceding claims, characterized in that the contact surface element (26) comprises an adhesive layer (42) for fastening the sensor (20) to the goods (22). Monitoring sensor according to claim 6, characterized in that the sensor element (36) is arranged between the adhesive layer (42) and the contact surface element (26). Monitoring sensor according to claim 7, characterized in that the adhesive layer (42) adheres more strongly to the goods (22) than to the contact surface element (26), and that the sensor element (36) adheres to the adhesive layer (42), so that when the proper contact of the sensor (20) and the goods (22), the sensor element (36) at least partially with the adhesive layer (42) of the contact surface element (26) separated and thereby a measuring loop formed by the sensor element (36) is interrupted. Monitoring sensor according to claim 6, characterized in that the adhesive layer (42) adheres more strongly to the goods (22) than to the contact surface element (26) and the sensor element (36) is integrated in the adhesive layer (42), so that when the correct one is removed Contact of sensor (20) and goods (22), the sensor element (36) at least partially separated from the contact surface element (26) with the adhesive layer (42), thereby interrupting a measuring loop formed by the sensor element (36). Monitoring sensor according to claim 8 or 9, characterized in that the sensor element (36) comprises an electrical, in particular metal foil-like conductor loop (38). Monitoring sensor according to claim 10, characterized in that the conductor loop (38) is contacted via at least two contact points (34) integrated in the contact surface element (26). Monitoring sensor according to one of claims 1 to 9, characterized in that the sensor element (36) comprises a strain gauge (56). Monitoring sensor according to claim 12, characterized in that the strain gauge (56) is at least partially integrated in the flexible contact surface element (26). Monitoring sensor according to one of claims 1 to 6, characterized in that the sensor element (36) comprises a flexible light transmission element (66) with an associated light transmitter (64) and light receiver (68). Monitoring sensor according to claim 14, characterized in that the light transmitter (64) is a GaAs diode. Monitoring sensor according to claim 14 or 15, characterized in that the light transmitter (64) can be controlled so that it regularly emits light signals, in particular with a frequency of 5 to 50 Hz. Monitoring sensor according to one of claims 14 to 16, characterized in that the light receiver (68) is a phototransistor. Monitoring sensor according to one of Claims 14 to 17, characterized in that the light transmission element (66) is arranged in such a way that the main direction of transmission of the light runs essentially parallel to the contact surface (28) of the contact surface element (26). Monitoring sensor according to one of Claims 14 to 18, characterized in that the light transmission element (66) comprises a cavity (70) which is formed in one piece with the contact surface element (26) and which has at least one light channel (72) from the light transmitter (64) to the light receiver (68) forms. Monitoring sensor according to claim 19, characterized in that the wall of the light channel (72) is not reflective. Monitoring sensor according to claim 19 or 20, characterized in that the light channel (72) is at least partially arranged in a section (62) of the housing (24) with a reduced cross section. Monitoring sensor according to one of the preceding claims, characterized in that the sensor (20) comprises an optical display (54) for the operating state of the sensor (20). Monitoring sensor according to one of the preceding claims, characterized in that the sensor (20) comprises a rigid connecting part (30) for the connecting cable (18). Monitoring sensor according to one of the preceding claims, characterized in that a circuit (52) is integrated in the sensor (20) which processes a measured variable for the monitoring circuit (12) supplied by the sensor element (36). Monitoring sensor according to Claim 24, characterized in that the circuit (52) comprises an evaluation circuit (48) for processing the measured variable of the sensor element (36) and for conversion into a digital value, and a memory (50) for temporarily storing the digital value, the circuit ( 52) the measured variable is prepared at predetermined time intervals and converted into a digital value and an alarm signal is sent to the Monitoring circuit (12) outputs when the temporarily stored digital value deviates from a later digital value by a predeterminable value. Monitoring circuit for anti-theft systems for use with a monitoring sensor according to one of Claims 1 to 24, characterized in that the monitoring circuit (12) has an evaluation circuit (48) for processing the measured variable of the sensor element (36) and converting it into a digital value and a memory (50 ) for temporarily storing the digital value, the monitoring circuit (12) processing the measured variable supplied by the sensor element (36) at predetermined time intervals and converting it to a digital value and triggering an alarm if the temporarily stored digital value deviates from a later digital value by a predeterminable value. Anti-theft system with a monitoring circuit (12) according to claim 26. Anti-theft system according to claim 27, characterized in that the anti-theft system (10) comprises an activation circuit (46) which, when a connection is first occupied with a connecting cable (18) of a monitoring sensor (20), activates the monitoring circuit (12) assigned to the connection, whereby This first processes the measured variable of the sensor (20), converts it and stores it temporarily in the memory (50) at a predetermined time interval for activation.

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