METHOD AND APPARATUS FOR EDDY CURRENT TESTING OF METALLIC PRODUCTS AND APPLICATION OF THE METHOD AND APPARATUS

摘要

A metal body is inspected for defects by relatively longitudinally displacing it and a plurality of pairs of coils connected in respective bridge circuits that are excited with alternating current. The coils are also excited with alternating current so that eddy-current variations caused by defects in the body become impedance variations of the coils which in turn become imbalance signals in the respective bridges. The imbalance signals are demodulated to form respective imbalance voltages which are periodically sampled and the levels of the sampled demodulated voltages are stored. The sampled demodulated voltages of each bridge are averaged to produce respective base lines which are compared with the respective stored levels of each sampled voltage for generation of respective error signals corresponding to the differences. These error signals are compared with a first relatively low threshold corresponding to the smallest defect to be detected and with a second relatively high threshold. When one of the error signals exceeds only the first threshold it indicates a small defect at the respective location on the body and when one of the error signals exceeds the second threshold it indicates a large defect. A longitudinal succession of at least abutting zones is established on the body and the error signals for each zone are summed. Each of these sums is compared with a predetermined third threshold and when one of the sums exceeds the third threshold it indicates a long defect in the respective zone.