A DEVICE FOR ACTIVATING OR DEACTIVATING AN ANTITHEFT DEVICE

Abstract: A device for activating or deactivating an antitheft device and comprising a strip (1) with a layer of magnetic, soft material and a layer of magnetic, hard material, whereby a characteristic magnetizing pattern is provided by an activation/deactivation of the strip (1), said characteristic magnetizing pattern being removable by means of an erasing magnetic field. According to the invention the erasing magnetic field is formed by the outer field of a relatively long and flat electromagnet (3). The flat structure of the electromagnet has the effet that an almost homogenous field is obtained across a relatively large area. Such an electromagnet can for instance be inserted in a counter top. Alternatively, the relatively long electromagnet can be mounted in a portable scanner in such a way that the electromagnet presents a well-defined distance to the strip (1) to be acted upon by the field of the electromagnet.

Description: Title: A device for activating or deactivating an antitheft device Technical Field The invention relates to a device for activating or deactivating an antitheft device, preferably in form of at least one strip with at least one layer of magnetic, soft material and at least one layer of magnetic, hard material, said device upon an activation and a deactivation, respectively, of the strip being capable of providing a characteristic magnetizing pat¬ tern which is removable by means of the erasing AC magnetic field pro¬ duced by said device. Background Art A device of this type is known, where the magnetising pattern can be removed by means of a horseshoe-formed electromagnet, the field between the poles of the magnet being utilized. This field is, however, not particularly homogenous. In connection with a relatively long electro- magnet of a cross section in form of a horseshoe, the field is substan¬ tially homogenous in the longitudinal direction of the pole shoe. Such an erasing magnet is produced by SIGMA. Such an electromagnet requires relatively much iron for the production of the core, and the complete structure is large and unhandy. In addition, the field is not homogenous transverse to the pole shoes. Description of the Invention The object of the invention is to provide a device of the above type, whereby the amount of iron for the production of the core is substan¬ tially lower than hitherto known, and the field is more homogenous than previously known. The device according to the invention is characterised by said device being formed by a relatively long electromagnet, where the erasing AC magnetic field is the outer field at one long side of the electromagnet. The resulting field turned out to be sufficient for activating a strip. The field is almost homogenous, and the production of the core does not require much iron. Danish Patent Application No. 2646/84 discloses a device for activating and deactivating, respectively, a strip by means of a flat permanent magnet. It is, however, not possible to perform a demagnetizing by means of the DC-field. A first embodiment of the device is preferably intended to be built into a counter, and it may according to the invention be characterised by the relatively long electromagnet being relatively flat. In this manner a prede¬ termined concentration of the field above the relatively flat electromag- net is obtained, and an almost homogenous field is obtained across the relatively large area. The flat structure of the electromagnet has the effect that the amount of iron for the production of the core can be additionally reduced. Furthermore according to the invention the relatively flat electromagnet may comprise a core of a substantially rectangular cross section, where¬ by the core is easy to manufacture. Moreover according to the invention, the relatively flat electromagnet may comprise a core of an oval cross section, whereby a particularly advantageous embodiment is obtained. In addition according to the invention the relatively flat electromagnet may have an ampere-turn of about 1 ,900. The resulting magnetic field is sufficient for activating the strip. Another embodiment of the device is preferably to be built into a port¬ able scanner and is according to the invention characterised by the relatively long electromagnet being mounted in the scanner in such a manner that said electromagnet in an activation/deactivation situation has a well-defined distance to the strip to be activated/deactivated by the AC-field of the electromagnet. The well-defined distance to the strip ensures an almost homogenous magnetic field. Finally according to the invention, the electromagnet of the portable scanner may have an ampere-turn of about 400, whereby a magnetic field is obtained which is sufficient for activating the strip. Brief Description of the Drawings The invention is described in greater detail below with reference to the accompanying drawings, in which Fig. 1 a illustrates a known device for deactivating an antitheft device as well as the antitheft device, Fig. 1 b illustrates the antitheft device in the moment the deactivation is performed, Fig. 1 c illustrates the antitheft device after deactivation, and after the device has been removed from the deactivation device, Fig. 2 illustrates a first embodiment of a device according to the inven¬ tion for activating an antitheft device, Fig. 3 illustrates the device of Fig. 2 when used in connection with a counter at an exit for customers, Fig. 4 is a sectional view through a second embodiment of the device for activating an antitheft device and built into a portable scanner, Fig. 5 is a bottom view of the device of Fig. 4, and Fig. 6 illustrates a diagram of a detected placed at the exit for detecting an activated antitheft device. Best Mode for Carrying Out the Invention Fig. 1 illustrates a known device 2 for deactivating an antitheft device in form of a strip 1 of magnetic material. The device 2 is formed by a plurality of juxtaposed permanent magnets, where the north pole and the south pole are alternately facing upwards. The strip 1 comprises at least one layer of magnetic soft material, i.e. amorphous material, such as of the type Allied 17 MA, and at least one layer of magnetic hard material. The strip 1 can for instance be fastened to a book which should not be illegally removed from a library or a bookshop. When the strip 1 has not been deactivated, it will be detected when passing an electromagnetic detector at the exit of the library or the bookshop, cf. Fig. 3. This detector produces an electromagnetic field and can respond to a material with a sharp-edged hysteresis corresponding to the layer of amorphous material. As a result, harmonics are generated to the frequency of the emitted electromagnetic field. The strip 1 can be deactivated by being "cut" to pieces figuratively speaking, cf. Fig. 1 , where the strip 1 in a manner known per se is moved towards a plurality of permanent mag¬ nets 2, where the north pole and the south pole alternately face the strip 1. As a result, the strip 1 is provided with a particular magnetizing pat- tern corresponding to a situation where the strip has been cut into sev¬ eral pieces magnetized in different directions, cf. Fig. 1 b . . The above permanent magnets 2 can for instance be placed below a counter top, cf. Fig. 3. As an alternative, they can be mounted on a roller across wnich the strip 1 is moved. The strip 1 can be activated by means of a device capable of generating a sufficiently strong and nevertheless almost homogenous magnetic field across an area of such a size that it can cover the strip 1 ; also when said strip is spaced up to 4 cm from the device. Fig. 2 illustrates a device to be built into a counter. The device is formed by an electromag¬ net 3 placed below the counter top. The electromagnet 3 is flat and rela¬ tively long. The core 3' is of a rectangular cross section and of a height substantially smaller than the width. A plurality of relatively thick wind- ings 3" are wound on the core 3', said windings standing a current of for instance 6 A. The ampere-turn is about 1 ,900. According to the invention the outer magnetic field of one long side of the electromagnet 3 is utilized. This magnetic field is almost homogenous across an area corresponding to the product of the width and the length of the core 3'. The flat structure provides in addition a predetermined concentration of the outer field above the magnet. The outer field at one long side of the electromagnet is sufficient for erasing the magnetizing pattern shown in Fig. 1 a of the layer of amorphous material when the strip 1 is moved across said electromagnet 3. The current fed to the coil 3" is preferably AC. The current supply can, of course, be interrupted when the activ¬ ating device is not used. The current supply is typically 6A at a voltage of about 48 V. A blower for cooling the coil 3" can be included. As an alternative, the current supply to the coil 3" can be pulse-modulated. The preferably flat electromagnet can alternatively comprise a core of an oval cross section. The strips 1 can advantageously be inserted in books at a library or a bookshop, in which case a bar code detector 4 can be provided in front of the coil 3", said bar code detector 4 communicating with a computer registering for instance the returning of a book when said book is moved across the bar code detector 4 and the electromagnet 3 in a direction of the arrow shown in Fig. 2. Then the book cannot pass the detector 12, 13 of Fig. 3 and be removed from the library without activating an alarm, unless the strip inserted in the book has been deactivated, cf. Fig. 1. According to an alternative embodiment, the activating device is built into a portable scanner, cf. Fig. 4. This portable scanner comprises a housing with a gripping member. The housing is bent and comprises at the end an opening, cf. Fig. 5, containing at least one electromagnet 6 of a substantially circular cross section. The electromagnet 6 is of a length of about 7 cm. The gripping member can be provided with a contact for the current supply to the electromagnet 6. The current sup¬ ply is typically 200 mA at a voltage of 15 V. The ampere-turn of the electromagnet 6 is about 400. The electromagnet 6 is mounted inside the scanner in such a manner that in an activation situation it has a well- defined distance to the strip 1 to be activated. The scanner can further¬ more be provided with a bar code detector registering the returning of a book in a computer through a cable 5 connected to the computer. The detector 12, 13 at the exit of the library or the bookshop comprises a transmitter and a receiver. The transmitter transmits a sinus-shaped signal of for instance 1 kHz via a frame aerial. A likewise frame receiver aerial communicates with a receiver detecting one or more harmonics of the transmitted signal frequency. The detector activates an alarm if the intensity and/or the composition of the harmonics correspond to the harmonics generated by a passage of an activated strip. Fig. 6 illustrates an example of how the detector can be shaped. An oscillator 11 transmits a sinus-shaped signal of a frequency of 1 kHz. This signal is amplified and transmitted to a transmitter aerial 12. From a receiver aerial 13, the received signal is transmitted to a detector 14 and a bandpass filter 15. The bandpass filter 15 has a centre frequency corresponding to the ninth harmonic, i.e. 9 kHz. The signal from the bandpass filter 15 is then mixed at 16 with the ninth harmonic of the output signal of the oscillator 1 1 . When the signal from the mixing step 16 exceeds a predetermined value, the exceeding indicates the presence of an activated strip 1 in the area being observed by the detector.

Claims: Claims. 1. A device for activating or deactivating an antitheft device, pre¬ ferably in form of at least one strip with at least one layer of magnetic, soft material and at least one layer of magnetic, hard material, said device upon an activation and a deactivation, respectively, of the strip being capable of providing a characteristic magnetizing pattern which is removable by means of the erasing AC magnetic field produced by said device, c h a racte rised by said device being formed by a rela¬ tively long electromagnet (3), where the erasing AC magnetic field is the outer field at one long side of the electromagnet (3). 2. A device as claimed in claim 1 and preferably to be built into a counter, characterised by the relatively long electromagnet (3) being relatively flat. 3. A device as claimed in claim 2, c h a r act e ri s e d by the relatively flat electromagnet (3) comprising a core of a substantially rectangular cross section. 4. A device as claimed in claim 2, c h a ra c te r i s e d by the relatively flat electromagnet comprising a core of an oval cross section. 5. A device as claimed in claim 2, c h a ra cte r i s e d by the relatively flat electromagnet (3) having an ampere-turn of about 1 ,900. 6. A device as claimed in claim 1 and preferably to be built into a portable scanner, cha racterised by the relatively long electro¬ magnet (6) being mounted in the scanner in such a manner that said electromagnet (6) in an activation/deactivation situation can present a well-defined distance to the strip (1) to be activated/deactivated by the AC field of the electromagnet (6). 7. A device a claimed in claim 6, characterised by the elec¬ tromagnet (6) in the portable scanner having an ampere-turn of about 400.

Inventors: HANSEN LARS, HANSEN, LARS
Applicants: ACTRON ENTWICKLUNGS AG
Application year: 1993
Main classification: H01F13
IPC codes: G08B13, H01F13