US 8,008,913 B2Granted

Automatic lift-off compensation for pulsed eddy current inspection

AssigneeGE Inspection Technologies, LP

InventorsXiaoyu Qiao, John Hansen

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Abstract

Variations in the lift-off separation between a probe and the surface of a structure to be tested often mask the detection of defects in the structure. A method and apparatus for automatically classifying and compensating for variations in the lift-off is described. A reference signal at a known lift-off may be weighted by a corresponding calculated ratio parameter and subtracted from a test signal to compensate for lift-off. A number of reference signals are preferably obtained and the largest magnitude gradient for each reference signal is preferably determined. The largest magnitude gradient for subsequent test signals is also obtained and the corresponding reference signal with the closest largest magnitude gradient to the test signal is identified and the corresponding reference signal is selected in the related compensation procedure. Such a method has been found to restore the signal such that lift-off is removed and defects are easily identified.

Claims12

Dependent claims

A method for compensating for the effect of separation between an eddy current probe and a surface of a conductive specimen being tested, the method comprising

obtaining set of reference data by locating the eddy current probe at a substantially defect free area and a known reference separation from a surface of the specimen, the probe being excited to generate eddy currents in the specimen and sensing a time-varying reference signal generated by the eddy currents, the set of reference data containing the amplitude of the reference signal at one or more times
determining a largest magnitude gradient value of the reference signal;
obtaining a set of test data by locating the eddy current probe at an unknown separation from the surface of the specimen the probe being excited to generate eddy currents in the specimen and sensing a time-varying test signal generated by the eddy currents, the set of test data containing the amplitude of the test signal at one or more times;
determining largest magnitude gradient value of the test signal
calculating a compensation ratio, the compensation ratio being the largest magnitude gradient value of the test signal divided by the largest magnitude gradient value of the reference signal;
and
determining an amplitude of a compensated signal for one or more corresponding times in the sets of reference and test data by subtracting the amplitude of the reference signal multiplied by the compensation ratio from the amplitude of the test signal.

4 more elements

Dependencies are inferred from the claim text.