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A Method of Searching for a Rotating Angle of LFDC Signals in NNTCM Algorithm

In eddy current testing (denoted as ECT), for restraining the support structure and reserving the flaw, one has invented a mixing-frequency method by adding signals in different channels. Later, our team has presented a NNT-cancellation mixing-frequency (denoted as NNTCM) method, in which one uses the information about the data about some heat transfer tube, but not about the calibration tube to realize cancellation, and the final result from a group of given data is stable and ideal intuitively. However, in that algorithm we didn't give the specific method of obtaining the rotating angle of the low-frequency differential channel (LFDC) signal to make the cancelled-structure energy minimize. To do this, in this paper we present a method of searching for the rotating angle of the LFDC signals, in which, we first determine the positions of these structures, and then give a rotating angle of the high-frequency differential channel (denoted as HFDC) signal when the phase angle of the through-hole is pointed to 40 degrees, and next, let the rotating angle of the LFDC signals change from 1 degree to 180 degrees, in the following we take these actions on high and low signals: rotating, removing the trend curve, cancelling, computing out all the sums of the norms of the signals at all structures after cancellation, giving the minimal index of the sum vector, which is the rotating angle of the LFDC signal we seek. More examples show this method is effective.