| 摘要 |
PURPOSE:To make construction simple and inexpensive by providing a pair of through-type excitation coils which are disposed coaxially and are connected to reversed polarities and a detecting coil between these excitation coils. CONSTITUTION:When a test material M is inserted into excitation coils 1, 2, and an excitation electric current is supplied to the coils 1, 2, magnetic fluxes phi, phi' in mutually reverse directions are generated and these pass through a detecting coil 3, thereby generating an induced voltage. If there is a defect F such as crack in the material M, the intensities and distributions of eddy currents E, E' change, and the induced voltage of the coil 3 changes as well. This induced voltage signal is fed to a bridge circuit 14, by which it is made into the signal of only the change component by the defect. This signal is fed to a phase detector 13, which generates the DC voltage corresponding to the phase difference between the reference voltage from a phase shifter 12 and the bridge output voltage. The DC voltage is inputted to a display device 16, by which the size and phase of the defect are displayed. In this way, the change in the eddy currents is detected locally, and the construction is made simple and inexpensive. |
