The Method Of Signal Acquisition And Processing In Pulsed Eddy Current Detection

In recent years, as the crash occurred frequently, which is often caused by some minor defect in the surface or internal of the plane, detecting the corrosion of aircraft structures has become more and more popular in the aerospace NDT(nondestructive testing) field. PEC(Pulsed Eddy Current) testing is a new emerging NDT technology, which has been widely applied to defect quantitative evaluation and classification, because it can get more information about corrosion by using a broadband pulse signal as excitation signal. At present, the study of PEC detection is mainly focused on the time domain, ignoring the rich spectral information of PEC signal. But many interference factors exist in time domain, which cause larger detection error. In order to ensure the reliability of the PEC detection signal source and improve the accuracy in detection, the PEC detection signal acquisition and processing method are researched, in this paper. The specific research contents are as follows:1、The theory of PEC detection is studied in this paper. Firstly, the principle of PEC detection is analyzed by introducing the eddy current effect, which provides theoretical support for the design of signal acquisition system. Next, the characteristics of the excitation signal and PEC response signal are analyzed, which lay a foundation for later signal processing.2、The signal acquisition system of PEC detection is designed and implemented. In order to improve the sensitivity of detection, the induced magnetic field of different probe parameters was analyzed by building the PEC detection simulation model. And then, the design of probe module is completed. Next, the sequence pulse excitation signal is designed and realized by combining virtual instrument a nd data acquisition card. Finally, the program of data acquisition is compiled. And then the PEC testing signals are collected, at the same time, displayed by graph and stored in file. The design, which integrates signal collection and occurrence, improves the quality of the exaction signal and provides guarantee for the stability of signal.3、The detection signal is processed and analyzed. Firstly, to accurately extract the features of detection signal, a new preprocessing method based on EMD(Empirical Mode Decomposition) is applied to PEC testing signal, and then the eddy current differential signal is obtained, which has a high SNR(Signal to Noise Ratio). Next, the features of detection signal are extracted and analyzed, respectively, in time domain and frequency domain. Moreover, a method of time- frequency domain, WT-PSD(Wavelet Transform—Power Spectrum Density), is proposed. And then, the features of time-frequency domain are extracted.4、The defect classification and quantitative evaluation are researched. The PEC detection signal which include surface, subsurface defect and the corrosion of metal, is processing and analyzed in time, frequency and time-frequency domain. The result is proved that, compared to the features of time and frequency domain, the features of time- frequency domain is more effective and intuitive in defect quantitative evaluation and classification.