Fourier Descriptor Classification of Differential Eddy Current Probe Impedance Plane Trajectories

A technique involving the use of Fourier descriptors for characterizing impedance plane trajectories to facilitate defect classification is presented. The Fourier descriptors are obtained by expanding the complex contour function in a Fourier series. Functions of Fourier coefficients which are invariant under transformation of the trajectory are derived and incorporated into a feature vector. Defect classification is obtained by using the K-Means algorithm to cluster the feature vectors. The principal advantage of the approach lies in the ability to reconstruct the curve from the coefficients. Other advantages include the insensitivity of the descriptors to drift in the eddy current instrument as well as variations in probe speed. Experimental evidence attesting to the ability of the approach to discriminate between trajectories and hence identify defects is presented.