Patent Application
20070018816
The present invention relates to an antenna for detecting a magnetic field which detects a detection tag or the like by detecting a change in magnetic field, a magnetic field detector employing the antenna, and a gate for detecting detection tag. More particularly, the present invention relates to an antenna for detecting a magnetic field which is constituted by a plurality of loop antennas and gives a high signal/noise ratio (S/N ratio), a magnetic field detector employing the antenna, and a gate for detecting detection tag.
Detection tags employing a magnetic field as a detection means are known. They are adhered to goods, etc., are carried with the goods and, when passing through particular gates, are detected by gates; thereby, the circulation of goods is controlled and the theft of goods is prevented. (Claim 1 of JP 1994-342065A)
On the lower side of the soft magnetic substance layer 20 is adhered a release liner 29 via a pressure-sensitive adhesive layer 28.
In using this detection tag, the release liner 29 is peeled and the release liner-removed detection tag is adhered to goods or the like to be controlled.
In
For example, when the goods or the like purchased normally is in a state that it can be carried out, the detection tag 34 adhered to the goods or the like is deactivated beforehand. Owing to this deactivation operation, there occurs no distortion of magnetic field when the detection tag 34 adhered to the goods or the like is passed between gates 30 and 32. Consequently, the detection tag adhered to the goods or the like is not detected during the passing between these gates and the goods or the like is carried outside.
Meanwhile, when the goods or the like is carried out illegally, the detection tag 34 adhered thereto is in a state not deactivated. Therefore, when the goods or the like containing the detection tag 34 not deactivated is passed between gates 30 and 32, distortion of magnetic field takes place. Detection of this distortion of magnetic field can detect illegal take-out of goods or the like.
Deactivation of detection tag can be achieved by magnetizing the ferromagnetic substance layer 25 of detection tag shown in
When the detection tag shown in
Inside the coil 62 for magnetic field generation are arranged, in upper and lower lines, a first antenna 64 for detecting a magnetic field and a second antenna 66 for detecting a magnetic field, both formed by winding an electric wire in an approximate 8 shape. Since the antennas 64 and 66 are formed each in a large, approximate 8 shape, the voltage induced by the magnetic field generated by the coil 62 for magnetic field generation is made small and further the detection area for detection tag is made large.
Since the formation of each antennas 64 and 66 are made in a large 8 shape, the antennas 64 and 66 detect an external noise which is generated in a large area. Consequently, there is a problem that the signal of small detection tag may not be detected.
The present inventor made a study in order to alleviate the above-mentioned problem. As a result, the present inventor thought of an idea of connecting in series a plurality of relatively small loop antennas wound in opposite directions and placing them in a plane. This antenna has been found to be able to cancel the generated external noise between the individual loop antennas while promising a large detection area and resultantly be able to detect a detection tag at a high S/N ratio. This finding has led to the completion of the present invention.
Hence, the present invention aims at solving the above-mentioned problems and providing an antenna of high S/N ratio for detecting a magnetic field, a magnetic field detector employing the antenna, and a gate for detecting detection tag.
The present invention which has achieved the above aim, is as described below.
[1] An antenna for detecting a magnetic field, wherein a plurality of loop antennas wound in opposite directions are connected in series and placed in a plane.
[2] A magnetic field detector comprising a plurality of antennas for detecting a magnetic field each set forth in [1] and an output circuit for taking out a differential output between the outputs of the plurality of antennas for detecting a magnetic field.
[3] A magnetic field detector according to [2], wherein the output circuit is a differential amplifier circuit.
[4] A magnetic field detector according to [2], wherein the output circuit is a circuit formed by connecting the plurality of antennas for detecting a magnetic field in series so that the polarities of antennas become opposite from each other.
[5] A gate for detecting detection tag comprising a coil for magnetic field generation and a magnetic field detector set forth in [2].
[6] A gate for detecting detection tag according to [5], wherein the distance between each loop antenna and the coil for magnetic field generation is 10 to 40 cm.
In the antenna for detecting a magnetic field according to the present invention, a plurality of loop antennas (which are small as compared with conventional 8-shaped antenna) are arranged in a state dispersed in a large area and are connected to each other; therefore, a magnetic field can be detected in a large area. In this case, each two adjacent loop antennas are reversed in the direction in which the electric wire constituting each loop antenna is wound. With this way of winding, the magnetic flux directions of two adjacent loop antennas are reversed, which cancels the external noise generated and consequently heightens the ratio of detection of target signal, resulting in a high S/N ratio.
Further, in the case of conventional 8-shaped antenna, the external noise generated in the vicinity of the antenna center (where intersection of electric wire takes place) is cancelled owing to the unique structure of the antenna; however, the signal generated is cancelled as well. Furthermore, in the 8-shaped antenna, the area in which the external noise is cancelled, is smaller than the area which is affected by the noise; therefore, the 8-shaped antenna gives a low S/N ratio.
In contrast, the present invention antenna is constituted by relatively small loop antennas; therefore, the noise generated can be cancelled and signal cancellation can be prevented by arranging individual loop antennas apart from each other. In other words, it is easy to control a balance between the adverse effect of noise and the reception of signal. Reduction in the voltage (noise) induced by the coil for magnetic field generation is possible even with the 8-shaped antenna and there is a certain effect in the center of the antenna. When a plurality of small loop antennas are arranged as in the present invention, it is possible to arrange them apart from each other and accordingly there can be obtained a S/N ratio higher than in the 8-shaped antenna.
In the conventional large 8-shaped coil, a noise of large area is detected and, therefore, there is a case in which cancellation of noise by the large antennas is difficult. However, when relatively small coils are used as in the present invention, a probability of noise cancellation by coils is high.
20 is a soft magnetic substance layer; 22 is an adhesive layer; 23 is a through-hole; 25 is a ferromagnetic substance layer; 27 is a protective layer; 28 is a pressure-sensitive adhesive layer; 29 is a release liner; 30 and 32 are each a gate; 34 is a detection tag; S is a magnetic field; 40 and 42 are each a harmonic; 50 is a deactivation machine; 52 is a support; 54 is an N pole; 56 is an S pole; 60 is a conventional gate; 62 is a coil for magnetic field generation; 64 is a first antenna for detecting a magnetic field; 66 is a second antenna for detecting a magnetic field; 100 is a gate for detecting detection tag; 102 is a floor; 104 is a coil for magnetic field generation; 106, 108, 120 and 122 are each a loop antenna; 110 is a first antenna for detecting a magnetic field; 112 and 124 are each a leader line for terminal; 114 and 126 are each one end; 116 is an output circuit; T is a distance; and 118 is a second antenna for detecting a magnetic field.
An embodiment of the present invention is described in detail below with reference to the accompanying drawings.
In
In the same plane as that formed by the coil 104 for magnetic field generation, a plurality (two in
As to the distance T between the loop antenna 106 or the loop antenna 108 and the coil 104 for magnetic field generation, there is no particular restriction. However, the distance is preferably about 10 to 40 cm.
Below the first antenna 110 for detecting a magnetic field is placed a second antenna 118 for detecting a magnetic field, having the same constitution as that of the first antenna 110 for detecting a magnetic field. That is, loop antennas 120 and 122 wound in opposite directions are connected in series and a leader line 124 for the terminal of the loop antenna 122 is earthed. One end 126 of the loop antenna 118 is connected to the input side of the output circuit 116.
The output circuit 116 is constituted so that the voltage difference between the output of the first antenna 110 for detecting a magnetic field and the output of the second antenna 118 for detecting a magnetic field can be taken out.
An example of the output circuit 116 is shown in
Another example of the output circuit 116 is shown in
Incidentally, in the above explanation, the antennas 110 and 118 both for detecting a magnetic field have been constituted each by two loop antennas; however, more than two loop antennas may be combined. In this case, each antenna for detecting a magnetic field is preferred to be constituted by an even number of loop antennas, from the standpoint of noise cancellation. Further, it is possible to use more than two antennas for detecting a magnetic field. It is preferred to use an even number of antennas for detecting a magnetic field, from the standpoint of efficient noise cancellation. In the above explanation, the antennas for detecting a magnetic field have been placed inside the coil for magnetic field generation; however, they can be placed in any desired site as long as the aim of the present invention is not impaired.
The present invention is described more specifically below by way of Example and Comparative Example.
A gate shown in
Loop antennas 120 and 122 having the same constitution as the loop antennas 106 and 108 were fitted below the loop antennas 106 and 108. The distance between the two loop antennas 120 and 122 and the distance between the coil 104 for magnetic field generation and the loop antenna 120 or 122 were the same as above. The loop antennas 106, 108, 120 and 122 and the coil 104 for magnetic field generation were placed in the same plane.
An alternate current of 300 Hz and 100 V was supplied to the coil 104 for magnetic field generation.
The outputs of the above two antennas 110 and 118 for detecting a magnetic field were sent to an output circuit 116 [constituted as a differential amplifier circuit (b)] and differential amplification was made. The differential amplifier output was subjected to A/D conversion and then sent to a personal computer (not shown) for data processing regarding the conversion of time axis to frequency axis. The data obtained was stored in a memory. With respect to the detection frequency, the main frequency was 300 Hz. Incidentally, the amplification ratio K of the differential amplifier circuit was 10,000.
An electromagnetically responding detection tag shown in
As a result, the gate could detect passing of all of the 100 tags.
Then, 100 detection tags deactivated by a deactivation machine were passed along the gate under the same conditions as above. As a result, none of the tags was detected.
A gate shown in
Then, 100 detection tags deactivated by a deactivation machine were passed along the gate under the same conditions as above. As a result, 34 tags were detected. It became clear from this result that the gate of Comparative Example was easily affected by noise.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-199323 | Jul 2003 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP04/08406 | 6/9/2004 | WO | 1/17/2006 |
