Abstract: Multi-frequency eddy current test (MFECT) is an important NDT technology, and it can realize multi-parameter detection and interference suppression effectively. Aiming at the problem that normal single frequency eddy current test can only detect flaws but can’t realize flaw classification, a new MFECT method which adopts linear frequency modulation (LFM) signal exciting and detection signal spectrogram analysis is proposed. The probe-coil in AC bridge is excited by LFM signal, and the output signal of AC bridge is amplified and sampled to identify flaw characteristic by spectrogram method. The flaw can be detected according to spectrogram energy variation, besides, with the purpose of flaw classification, a new method called spectrogram barycenter excursion is proposed to realize flaw classification. Compared with traditional multi-frequency eddy current test method, it has the merits of short detection time and low system cost. The agreement between theory and experiment verifies the correctness of the employed method.

Key words: Flaw classification, Linear frequency modulation, Multi-frequency eddy current test, Spectrogram analysis, Driving safe boundary , Dynamic coordinated control factor, EPS/ESP , Vehicle dynamics features