Research On Multifrequency Eddy Current Testing Based On Frequncy Modulated Excitation

As a new approach of eddy current testing, multi-frequency eddy current testing uses multiplecomponents of different frequencies as excitation, inheriting merits of traditional eddy currenttechnology, such as low-cost, no need of coupling agent, non-contact detection, and a high sensitivityof conductive material near-surface defects, etc. as well as the advantages of interference suppressionin detection process and multi-parameters detection, making it a most suitable method for theinspection of near-surface cracks of aluminum material.This paper describes the basic principles and theoretical basis of multi-frequency eddy currenttesting technology, and a multi-frequency eddy current testing system based on frequency modulationis built. The system uses a single detection channel to detect defects at various depths at the sametime, and has a known limited frequency range, making it convenient for frequency domainanalysis.The signal generator based on voltage-controlledvoltage source is designed, whichcangeneratethetriangular wave modulationsignal andfrequency modulated excitation signal. Adifferential probe is designed as tranducer, which can effectively suppress slow change of sampleshapeand nature while have a highsensitivity for near-surface defects.AD637RMS/DC converter ischosen as a spectrum amplitude calculator to realize the spectrumanalysis in time domain, thusgreatly reducing the amount ofcomputationof the spectrum analysis.In addition, the power amplifiermoduleof frequency modulated signal generator, the conditioning circuit of output signalof thedifferentialprobe, designof data acquisition module, and defect recognition in Matlab has also beenelaborated.Using the designed detection system, some experiments research on surface and subsurfaceslotted defects of aluminum specimen have been done, the influence of the incentive frequency on theinduced eddy current field has been anylised, the normalized defect signals have been extracted, andthus both static test and dynamic scan of defects have been successfully fulfilled. Spectrum analysisof characteristic normalized defect signal is done, realizing location and chatacterization of defects.