| 摘要 |
PURPOSE:To enable the discrimination of the metal placed in a magnetic field, by extracting a higher harmonic signal corresponding to the kind of a metal. CONSTITUTION:The oscillation signal from an oscillator 1 is applied to a series resonance circuit 3 through an amplifier 2 and the voltage induced in a detection coil 7 by the change in magnetic flux is applied to a filter circuit 9 through an amplifier 8 and fundamental frequency is removed while a higher harmonic component is sent out to filter circuits 10, 11. An eighth higher harmonic and a tertiary higher harmonic are respectively extracted by the circuits 10, 11 to be inputted to rectifier circuits 12, 13 and the outputs of the circuits 12, 13 are respectively applied to a differential amplifier 4 and the differential pressure thereof is amplified by the differential amplifier 14 to be inputted to a comparator 15 to be compared with reference voltage and, when said differential voltage is higher, a display signal is outputted to a gate circuit 6. The output of the circuit 13 is also inputted to a comparator 17 to be compared with set voltage and, when said output is lower, a signal is outputted to a hold circuit 18 which, in turn, outputs an prohibiting signal to the gate circuit 16 for a definite time. The display signal from the comparator 15 is outputted to a display apparatus 19 when there is no prohibiting signal. |
