Method and system for detecting an indicating device
First Claim
1. A method of detecting the presence of an indicating device in a restricted investigation zone, comprising the steps of detecting an indicating device (31) of a highly permeable material by emitting and receiving a magnetic field, wherein two coils (1,2) are caused to emit a high-frequency magnetic alternating field, and a first one of the coils (1) emits an alternating field with a frequency (f1) differing from the frequency (f2) of the field emitted by the second one of the coils (2), and at least one positive difference and/or positive sum frequency n·
- f1 +m·
f2 occuring by intermodulation by means of the indicating device (31), where n and m are positive or negative integers, is caused to be received by one of several coils (9), characterized in that a third low-frequency magnetic alternating field is caused to be emitted into said zone with a frequency (f3) on the order of ten to fifty times lower than said frequencies (f1, f2) which low-frequency field is caused to have a field intensity sufficiently high, independently of the fields with the frequencies f1 and f2 to put the indicating device (31) into a saturated condition in step with the frequency of the low-frequency field, and a field re-emitted from the indicating device (31) is caused to be detected by means of a detector device as an intermodulation between the frequencies f1 and f2, which occurs periodically with the frequency of the low-frequency field.
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Accused Products
Abstract
An indicating device (31) of a highly permeable material is detected by emitting and receiving a magnetic field, wherein two coils (1,2) are caused to emit a high-frequency magnetic alternating field, and a first one of the coils (1) emits an alternating field with a frequency (f1) different from the frequence (f2) of the field emitted by the second one of the coils (2), and at least one difference and/or sum frequency n·f1 +m·f2 occurring by intermodulation by means of the indicating device (31), where n and m are positive or negative integers, is caused to be received by one or several coils (9). A third low-frequency magnetic alternating field is caused to be emitted in said zone with a frequency (f3) substantially lower than the frequencies (f1,f2), which low-frequency field has a field intensity sufficiently high, independently of the fields with the frequencies f1 and f2 to put the indicating device (31) into a saturated condition in step with the frequency of the low-frequency field. A field re-emitted from the indicating device (31) therefore is detected by means of a detector device as an intermodulation between frequencies f1 and f2 occurring periodically with the frequency of the low-frequency field.
77 Citations
14 Claims
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1. A method of detecting the presence of an indicating device in a restricted investigation zone, comprising the steps of detecting an indicating device (31) of a highly permeable material by emitting and receiving a magnetic field, wherein two coils (1,2) are caused to emit a high-frequency magnetic alternating field, and a first one of the coils (1) emits an alternating field with a frequency (f1) differing from the frequency (f2) of the field emitted by the second one of the coils (2), and at least one positive difference and/or positive sum frequency n·
- f1 +m·
f2 occuring by intermodulation by means of the indicating device (31), where n and m are positive or negative integers, is caused to be received by one of several coils (9), characterized in that a third low-frequency magnetic alternating field is caused to be emitted into said zone with a frequency (f3) on the order of ten to fifty times lower than said frequencies (f1, f2) which low-frequency field is caused to have a field intensity sufficiently high, independently of the fields with the frequencies f1 and f2 to put the indicating device (31) into a saturated condition in step with the frequency of the low-frequency field, and a field re-emitted from the indicating device (31) is caused to be detected by means of a detector device as an intermodulation between the frequencies f1 and f2, which occurs periodically with the frequency of the low-frequency field. - View Dependent Claims (2, 3, 4, 5, 6)
- f1 +m·
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7. Apparatus for detecting the presence of an indicating device in a restricted investigation zone, comprising coils (1,2) and associated feeding equipment for emitting and receiving magnetic alternating fields and thereby detecting an indicating device (31) of a preferably highly permeable material, wherein two coils (1,2) are provided, one (1) of which is capable of emitting a high-frequency alternating field with a frequency (f1) and a second one (2) of the coils is capable of emitting a high-frequency alternating field with a frequency (f2) differing from said first mentioned frequency, and a detector device including a receiver coil capable of detecting the occurrence of at least one positive difference of positive sum frequency n·
- f1 +m·
f2, where n and m are positive or negative integers, which are formed by intermodulation by means of said indicating device (31), characterized in that the feeding equipment (4-8) comprises means for emitting a third alternating field, which has a low frequency with a frequency (f3) substantially lower than said frequencies (f1, f2), which means are capable of emitting the low-frequency field with a field intensity sufficiently high, independently of the fields with the frequencies f1 and f2 to put the indicating device (31) into saturated condition in step with the frequency (f3) of the low-frequency field, and the detector device (10-13;
15-23) is capable by means of a receiver coil (9) of receiving and thereby detecting a field re-emitted by the indicating device (31), which comprises an intermodulation between the frequencies f1 and f2 occurring periodically with the frequency (f3) of the low-frequency field, and at the occurrence of such a periodically occurring intermodulation to emit a signal to an alarm device. - View Dependent Claims (8, 9, 10, 11, 12, 13, 14)
- f1 +m·
Specification