Sensors and Actuators A 135 (2007) 675–679 Development of differential probes in pulsed eddy current testing for noise suppression Li Shu * , Huang Songling, Zhao Wei State Key Lab of Power Systems, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China Received 10 May 2005; received in revised form 25 September 2006; accepted 9 October 2006 Available online 9 November 2006 Abstract Differential probes are developed to suppress noises and improve the sensitivity and robustness of pulsed eddy current (PEC) testing. As the close magnetic circuit is used to concentrate the magnetic field, three-core probe and U-shape probe gain high signal to noise ratio (SNR) with limited excitation current, the latter also reduces the influence caused by probe tilting. Lift-off effect is a great obstacle for practical application of pulsed eddy current testing, so probes developed with two-stage differential design can carry out the effective suppression of the lift-off effect. Verifying experiments are also presented, by which the performance of these differential probes are tested and compared in this paper. © 2006 Elsevier B.V. All rights reserved. Keywords: Differential probe; Close magnetic circuit; Probe titling; Lift-off effect; Two-stage subtraction; EMC 1. Introduction In eddy current testing, the excitation coil fed with alternat- ing current generates eddy current in conductive materials. A defect in materials will perturb the distribution of eddy current, and changes of magnetic field can be detected. In contrast to eddy current testing employs sinusoidal excitation at one or sev- eral discrete frequencies [1], pulsed eddy current (PEC) testing applies pulses as excitation to generate a magnetic field which propagates into the materials detected to induce eddy current, and consequently, the sum of excitation field and the magnetic field due to induced eddy current is detected by pick-up coils [2]. PEC testing measures the transient response of voltages instead of the impedance and reactance of coils used in conventional eddy current testing. One advantage of PEC testing is its broadband property. When Fourier transformation is applied, in frequency domain, there will be a signal peak for responses of conventional eddy current testing, while a broadband response for PEC testing [3–7], so a broadband response results in simultaneous multiple frequencies in test piece. According to skin-depth relationship for penetration depth, for a certain test piece, conductivity and * Corresponding author. Tel.: +86 10 62773070; fax: +86 10 62783057. E-mail address: lis4@rpi.edu (L. Shu). permeability are fixed, and consequently, multiple frequency comments reflect defect information pertaining to many depths. Meanwhile, advantage of PEC testing lies in pulses can be eas- ily generated and controlled by the intensity of excitation and starting time for data synchronization and interpretation. Fur- thermore, responses of a pulse often come when step excitation is over and no overlap between excitation and response will occur, so PEC testing is more robust to interference [4]. As the PEC testing yields a transient signal with broad fre- quency contents that reflects richer information about the mate- rials detected, it can conduct digital signal analysis and extract subtle differences from geometric variations and quantify more parameters. However, changes of magnetic field due to the defect perturbation are so weak that precise measurement of small vari- ations in relatively large impedance is difficult [6]. Therefore, in PEC testing the differential probe is often adopted, and transient signal is gained by subtracting a reference from the defect signal. As long as the probe moved above parts with similar character- istics and structures, a zero signal will be observed. If the probe moved above a flaw, a defect signal will be detected [8,9]. Three features of differential signal mainly employed to quantify the defect are peak amplitude, time to peak amplitude and time to zero crossing [3]. For the detection of surface-breaking defects, peak amplitude is the main feature to judge whether there is a defect and evaluate its condition. Unfortunately, the lift-off effect is a main obstacle to the sensitivity of PEC testing, and 0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.10.013