This invention relates to a receiving device for receiv¬ ing a magnetic field in connection with a goods monitor¬ ing system. In SE-PS 8404691-1 a system for detecting the presence of indicating devices is described which have the form, for example, of a narrow and thin, but relatively long strip of a highly permeable material, such as being commercial¬ ly available under the name Permalloy. The presence of such strips is detected by means of coils, which emit and receive magnetic fields, and by means of devices for the detection of signals received.

When such a strip is exposed to a relatively weak extern¬ al magnetic field, it assumes in its longitudinal direct- ion a magnetic flux density, the magnitude of which can be twentythousand times higher than in the external magn¬ etic field. The presence of such strips is detected by a detector device, in which variations in voltage induced in a receiver coil are detected. According to said patent, two coils are caused to emit a magnetic alternating field of high frequency, where a first one of the coils emits an alternating field with a fre¬ quency, which is different from the frequency of the field emitted by the second one of the coils ₃ and where at least one difference and/or sum frequency n • f. + m • f aris¬ ing by intermodulation by the indicating device, where n and m are positive or negative integers, is caused to be received by one or several coils. According to the patent, a third magnetic alternating field of low frequency is caused to be emitted in said zone, which has a frequency substantially lower than said frequencies f.,,f _p , where the low-frequency field is caused to have a field intens¬ ity sufficiently high for, independently of the fields with the frequencies f^ and f , putting the indicating device into- saturated condition in step with the frequency

of the low-frequency field. A field re-emitted from the indicating device is caused to be detected by means of a detector device as an intermodulation, occurring period¬ ically with the frequency of the low-frequency field, bet- ween the frequencies f. and f .

One problem with known theft-prevention systems is that the orientation of the strip in the space is of great importance for whether or not the presence of the strip in the investigation zone will be detected and, thus, whether or not alarm will be sounded. This problem normally has been solved in such a way, that emitting and receiving coils were positioned in different places relative to the in¬ vestigation zone. Another problem is that a simple receiving coil is relat- ively sensitive to interferences of various kind, originat¬ ing from other electric devices installed in shop premises.

The present invention provides a solution of the aforesaid problems. The present invention comprises receiving coils so arranged, that indication of the presence of a strip in the investigation zone substantially takes place independ¬ ently of the orientation of the strip in the space, and so arranged that they are insensitive to external inter¬ ferences. This implies that the number of expected false alarms is reduced, which is of utmost importance for a goods monit¬ oring system of the kind here concerned.

The present invention, thus, relates to a receiving device for detecting the presence of an indicating device in a restricted investigation zone, where coils and associated feeding and receiving equipment are provided for emitting and receiving magnetic alternating fields, in order there¬ by to detect the presence of an indicating device in the investigation zone, which indicating device is capable to receive alternating fields and to re-emit an alternating field. The invention is characterized in that said receiv-

ing coils, which are provided to receive said re-emitted alternating fields, comprise at least two coil sets, where each coil set is a so-called balanced coil wound to a configuration corresponding to two substantially rectangular eights located to the side of each other in the same plane, and that said coil sets are located in parallel with each other, preferably in the same plane, where each of the coil sets is offset, relative to an adjacent coil set, in a direction in parallel with said plane .through a distance corresponding to about one fourth of the distance in the offset direction between the outer opposed sides in a coil set.

The invention is described in greater detail in the foll¬ owing, with reference to an embodiment thereof shown in accompanying drawings and elucidating sketches, in which Fig. 1 shows schematically a coil, which is utilized acc¬ ording to the invention,and indicates different transport directions A-E for an indicating device in the form of said strip, Fig. 2 shows a co-ordinate system, in which the coil in Fig. 1 is located in the x-y plane, Fig. 3 shows a strip oriented in the x-direction, y-direction and z-direction, respectivel ,Figs. a- f show schematically the signal intensity (v), which is obtained at the passage of a strip past the coil in Fig. 1, depending on the ori- entation of the strip (x;y;z) and transport path (A-E), Fig. 5 is an elucidating Figure to illustrate a coil set comprising three coils according to Fig. 1, Fig. 6 shows a monitoring zone, and Fig. 7 shows schematically a com¬ plete device for indicating the presence of an indicating device in an investigation zone.

In Fig. 7 a device for detecting the presence of an indic¬ ating device is shown schematically, which indicating dev¬ ice, for example, has the form of a strip of a highly permeable material and is located in a restricted investig- ation zone 1, which is defined by two parallel screens 2,3, which comprise cυils for emitting and receiving magnetic

alternating fields. A feeding device 4 is provided for emitting a field of a certain frequency by means of emitt¬ ing coils. The indicating device 5 is provided to receive said fields and to re-emit an alternating field, which comprises a frequency other than that of the emitted fre¬ quency. A receiving equipment is provided for receiving a signal arising in the receiving coils due to the altern¬ ating field re-emitted by the indicating device. The receiving equipment 6 comprises a receiver 7 and a detector circuit 8 capable to detect and interpret the signal received. When the detector circuit 8 receives a signal, which agrees with the criterion or criteria apply¬ ing when an indicating device is present in the investigat¬ ion zone, the detector circuit 8 emits a signal to an alarming device 9 of known kind. The system described above can be similar to that described in said Swedish patent. The present invention, however, can be used at an indicating device of a different kind, for example an in¬ dicating device in the form of a resonance circuit. According to the present invention, the receiving coils are designed in a special way. According to the invention, the receiving coils comprise at least two coil sets, as described below. In Fig. 5 three coil sets are shown, where a first one 10 is shown by fully drawn lines, a second one 10' is shown by dash-dotted lines, and a third one 10" by dashed lines.

In Fig. 1 a coil set 10 is shown in greater detail. The coil set 10 is a so-called balanced coil wound to a con¬ figuration corresponding to two substantially rectangular eights, which are located to the side of each other in the same plane. As appears from Fig. 1, the coil set 10 has such a configuration, that the outer winding portions 11- 18 form a rectangle, and the remaining winding portions 19-22 divide the rectangle into four smaller rectangles ^' of eqαal ^" size- ^{~ ~} ■

The coil set 10 is wound so that from one pole 23 of it the conductor extends to the second pole 24 via the wind¬ ing portions as follows: 11,19,20,12,11,19, 20,13,14,21, 20,13,14,21,22,16,15, 21,22, 16,15, 21,22, 17,18,19, 22, 17, 18,19,20, 12. The winding portions 11-18, thus, com¬ prise two turns, and the winding portions 19-22 comprise four turns. The coil portions, thus, which are constituted by said smaller rectangles are wound so, that the rectangl¬ es in the first and third quadrant are wound in the same direction while the rectangles in the second and fourth quadrant are wound in opposite directions.

In Fig. 3 an indicating device is shown, which has the form of a strip of a highly permeable material in three differ¬ ent orientations x,y and z according to the co-ordinate system in Fig. 2. In the following reasoning the coil set 10 is assumed to be located in the x-y plane.

The letters A-E indicate five different imagined transport paths for a strip 5 when it is transported in the x-y plane past the coil set 10. A, thus, implies that the strip is transported alonr: the winding portions 18 and 1. B implies, that the strip is transported along a path located centrally between the winding portions 18, 11 and, respectively, 22, 20. C-E have a corresponding meaning. The dimensions of the strip 5 are only, for example, 25 x 2 x 0.1 mm while the coil set can have outer dimensions of, for example, 100 x 80 cm.

In Figs. 4a-4f the signal intensity (v) is illustrated, which is obtained when a strip 5 is moved in the x-direct- ion according to the paths A-E. At each diagram both the transport path (PATH) and the orientation (ORIENT) of the strip are indicated. In Fig. 4a, thus, the signal (v) is shown which is received when the strip is x-oriented and follows the path A in Fig. 1. The indication "-E" implies, that a signal corresponding to the one shown, but with opposite sign, is received when the strip is x-oriented and follows the path E. The Figs. 4b-4f show in a corres-

ponding way the signal for different orientations of the strip when it follows different paths.

When the strip is x-oriented or z-oriented, no signal or only a very weak one is received when the strip follows 5 the path C. When the strip is y-oriented, no signal or a very weak one is received when the strip follows one of the paths B or D.

According to the present invention, therefore, the receiv¬ ing device comprises at least two coil sets 10, 10', of the

JO kind shown in Fig. 1, where each coil set 10, 10', 10" is a balanced coil wound to a configuration corresponding to two substantially rectangular eights located to the side of each other in the same plane. The coil sets 10, 10', 10" are located in parallel with each other and preferably in

15 the same plane. Each of the coil sets is offset, relative to an adjacent coil set, in a direction in parallel with said plane through a distance (a/-) corresponding to about one fourth of the distance (2a) in the offset direction between the outer opposed sides l8,ll;l4,15 in a coil set

2010.

It is hereby obtained, that the paths A-E of the second coil set 10' are offset relative to the paths A-E of the first coil set. Thus, the path B of the coil 10 corresponds to the path A of the coil 10' ^■ the path C of the coil 10 5 corresponds to the path B of the coil 10' , a.s.o. This implies, that a signal (v) is received from one or the other or from both of the coil sets 10, 10', irrespective of which orientation the strip has and which path it foll¬ ows.

30 It is obvious that between the paths indicated as well as at an orientation of the strip in a direction between the x-,y-z-directions a signal will be received in a corresp¬ onding way. According to a preferred embodiment, each coil set parti-

35 ally overlaps an adjacent coil set, so that two respective

opposed winding portions 16,17; 12,13, and 25,26; 27,28, respectively, of each of the coil sets 10, 10' are locat¬ ed along two respective straight lines, i.e. as illustrat¬ ed in Fig. 5.

5 According to another preferred embodiment, a set of rec¬ eiving coils comprises at least four coil sets in a vert¬ ical plane where they are offset relative to each other in the way stated above in a vertical plane, and where the total height of the coil sets exceeds 1,5 meters, pref-

10 erably 1,7 meters, and the total width of the coil sets exceeds 0,5 m and preferably is about 0.8 m. A vertical screen 2;3 of such design is shown in Fig. 6. In the embodiment shown in Fig. 6, the uppermost located coil set extends from the indicated level 3 to the level

1532. The winding portions according to Fig. 1 are marked in Fig. 6. In a corresponding manner, the second coil set extends between the levels 35 and 31, the third coil set between the levels 3 and 30, and the fourth coil set betw¬ een the levels 33 and 29.

20 According to a further preferred embodiment, two sets of receiving coils, i.e. screens 2,3, are positioned in par¬ allel with each other and at a distance from each other of about 0.5 to 1.0 in, as illustrated in Fig. 6, whereby a restricted investigation zone 1 is formed between the

25 same.

The receiving device preferably is capable to receive alternating fields of such frequency, that the wave length is long compared to the height and width of the coil sets, especially a frequency below 100 MHz, preferably below

3030 MHz.

This implies, that the coil sets act as fully balanced coils to interferences arising from a place located at a distance from the coils, which distance corresponds to a minimum distance of about 4a to 6a, see Fig. 5. Hereby 35 so-called remote interferences, from electric devices in the premises, arc effectively oppressed.

The said coil sets are connected vi conductors 37,38 to the receiver 7, where emf generated in the coils are sensed.

The said emitting coils,(not shown) preferably extend 5 along the periphery 41-44; 45-48 formed by the coil sets. These emitting coils are connected via conductors 39,40 to the feed equipment 4.

According to another further preferred embodiment of the invention, the winding portions of the coil sets extend in ° tubular portions, only five of which are marked in Fig. 6 by the numeral 4y. The ends of the tubular portions are attached to each other so, that a frame is formed, whibh has the configuration of a grid forming a number of rect¬ angular squares, as illustrated in Fig. 6. This embodiment 5 provides the advantage, that the staff easily can monitor persons passing the investigation zone 1. It is obvious that the present invention eliminates in a simple way the disadvantages referred to in the introduct¬ ory part. 0 The present invention, of course, can be varied in differ¬ ent ways without abandoning the invention idea, that at least two coil sets are used which are offset relative to each other. The present invention, thus, must not be regardedurestrict- 5 ed to the embodiments described above, but can be varied within the scope defined in the attached claims.