| 摘要 |
Process in order to detect the presence of an indicated device in a limited investigation zone, including the detection of an indicated device (31) in a highly permeable material through transmitting and receiving of a magnetic field, where two coils (1, 2) are made to transmit a high-frequency magnetic alternating field, where the first of the coils (1) transmits an alternating field with a frequency (f1), differentiating itself from the frequency (f2) of the transmitted field of the other coil (2), and where at least one, through intermodulation while the indicated device (31), occurring differentiating and/ or summation frequency n x f1 + m x f2 , where n and m are positive or negative integers, are received by one or more coils (9). According to the invention, a third low-frequency magnetic alternating field is brought to transmission in the given zone, which has a frequency (f3) essentially below the given frequencies (f1, f2), where the low-frequency field is brought to a field strength that is sufficiently high enough in order to, independent of the field with the frequencies f1 and f2 continue the indicated device (31) in its saturated condition in tact with the frequency of the low frequency field. A retransmitted field is brought from the indicated device (31) to detect that with the low-frequency field's frequency periodically occurring intermodulation between the frequencies f1 and f2 by means of a detection device.<IMAGE> |
