| 摘要 |
PURPOSE:To form a highly sensitive output with respect to the change in magnetic permeability by a method wherein the excitation frequency is set low, a plurality of detecting coil bodies each having a small sectional area are employed and each, two of them is paired, and differential outputs are drawn out therefrom and detected at each given phase position of the excitation frequency. CONSTITUTION:An exciting coil body 20 is excited at a low frequency, and an object to be inspected is inserted between a coil 23 and coils 11-19 of a detecting coil body 10. When a minute metal to be detected remains in the object being inspected, the magnetic flux radiated from the coil 23 causes a change in the induced voltage in the coil of the detecting coil body 10 facing to the position of said metal. Said output change is detected by drawing out the differential output between the outputs of each two coils, 11 and 12, 12 and 13... by means of differential amplifiers 31-39, and the output change common to all the coils 11-19 is canceled out. Said differential outputs are detected at the phase where change becomes largest, and the outputs of detecting circuits 41-49 are drawn out through an OR circuit 50 and compared with a given level. Thereby, the existence of the metal to be detected can be confirmed. |
