A feature extraction technique based on principal component analysis for pulsed Eddy current NDT Ali Sophian a, * , Gui Yun Tian a , David Taylor a , John Rudlin b a The University of Huddersfield, Queensgate, Huddersfield HD1 3DH, UK b Structural Integrity Department, TWI, Granta Park, Great Abington, Cambridge CB1 6AL, UK Received 17 June 2002; revised 9 September 2002; accepted 10 September 2002 Abstract Pulsed Eddy current (PEC) is a new emerging NDT technique for sub-surface defect detection. The technique mainly uses the response peak value and arrival to detect and quantify the defects. This could suffer from noise and be not sufficient to extract more information about the defects, e.g. depth information of defects. This paper introduces the application of principal component analysis in extracting information from PEC responses. A comparative test carried out shows that the introduced technique has performed better than the conventional technique in the classification of defects. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Pulsed Eddy current sensor; Feature extraction; Principal component analysis (PCA) 1. Introduction Eddy current sensors are generally robust and small in size [1]. Due to robust to the medium, non-contact Eddy sensor can be used for precision engineering to displace- ment and geometric measurement by using novel sensor design and blind signal separation [1–6]. By employing advanced signal processing techniques, the measurement resolution for displacement can be down to 0.1 mm [2,5]. The resolution varies for different target materials, and lift- off can be compensated by the nominal gap. Eddy current sensors have been widely used for non-destructive evalu- ation (NDE) [7], crack detection in particular. However, the measurement accuracy and resolution is much behind than the surface geometric measurement because of the feature of subsurface ‘blinding’ measurement [4]. Conventional Eddy current techniques use single fre- quency sinusoidal excitation and detect flaws as impedance or voltage changes on an impedance plane display with inspectors interpreting magnitude and phase changes. However, these techniques are sensitive to a variety of parameters that are inherent in the flaws [3]. Additionally multiple frequency measurements have been combined to provide a more rigorous assessment of structural integrity by reducing signal anomalies that may otherwise mask the flaws [8]. Pulsed Eddy current (PEC) sensing is a new and emerging technique [9] that has been particularly developed and devised for subsurface crack measurements, with some success at Iowa State University in USA [10], 1 DERA in UK [11] and the Cegely Laboratorium in France [8]. PEC techniques excite the probe’s driving coil with a repetitive broadband pulse, usually a square wave. The resulting transient current through the coil induces transient Eddy currents in the test piece, which are associated with highly attenuated magnetic pulses propagating through the material. The probe provides a series of voltage–time data pairs as the induced field decays, and since the produced pulses consist of a broad frequency spectrum, the reflected signal contains important depth information. Physically, the pulse is broadened and delayed as it travels deeper into the highly dispersive material, and flaws or other anomalies close to the surface affect the Eddy current response earlier than deeper flaws. Peak values and peak times have been used for flaw detection and identification. However, these systems cannot currently be readily used because of difficulties in calibration and the lack of suitable response signal processing algorithms [12]. 0963-8695/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0963-8695(02)00069-5 NDT&E International 36 (2003) 37–41 www.elsevier.com/locate/ndteint * Corresponding author. Tel.: þ44-1484-472-165; fax: þ 44-1484-472- 252. E-mail address: a.sophian@hud.ac.uk (A. Sophian). 1 http://www.cnde.iastate.edu/cnde.html, 2002.