Method of and apparatus for testing a metal body by means of eddy currents
US4631688A · kind A · utility
Inventors
Key dates
| Filing date | Jun 6, 1980 |
| Grant date | Dec 23, 1986 |
| Priority date | — |
| Expiry date | Jun 6, 2000 |
Classification
- Technology area (CPC G)Physics
- CPC primaryG01N27/9046
- WIPO fieldMeasurement
- WIPO sectorInstruments
Abstract
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…
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.