Pulsed Eddy Current: Feature Extraction Enabling In-Situ Calibration and Improved Estimation for Ferromagnetic Applicati
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Pulsed Eddy Current: Feature Extraction Enabling In-Situ Calibration and Improved Estimation for Ferromagnetic Application Faris Nafiah1,2
· Mohammad O. Tokhi1 · Shiva Majidnia2 · John Rudlin2 · Zhanfang Zhao1 · Fang Duan1
Received: 14 January 2020 / Accepted: 22 June 2020 © The Author(s) 2020
Abstract Steel pipes in process plant applications are often covered with insulation or weather protection that make inspection difficult because the additional layers need to be penetrated to inspect the pipes’ structure. The pulsed eddy current (PEC) method was devised as a means of inspection through the surface layers. However, the performance of a PEC system is dependent on the electrical and magnetic properties of the pipe material, which are generally unknown. Therefore, the use of a calibration block from a different steel will give inaccurate results. The concept of calibrating using τ0 values obtained during inspection has undoubtedly been discussed in the literature. However, no comprehensive work was dedicated to using |∇|−1 to carry out calibration on inspected structure. The linear relationship of the |∇|−1 feature with the thickness squared, d 2 , is first established using analytical solutions, and the calibration is carried out using the feature values obtained in air and the reference signal. The performance of this technique is assessed and compared with the conventional τ0 technique. Although both features exhibit similar immunity towards lift-off, τ0 technique requires normalisation procedure, which contributes to determining more configuration parameters. Experimental results also suggest the relative advantage of using |∇|−1 feature in both wall thickness estimation and influences of noises. Keywords Pulsed eddy current · Non-destructive testing · Feature extraction · Ferromagnetic application · Pipeline inspection
1 Introduction Pipe inspection using pulsed eddy current (PEC) poses various challenges; in addition to the variation in the value of lift-off/insulation thickness, electromagnetic properties differ from one pipe to another. Therefore, in order to predict the output from a PEC inspection, the calibration sample should have exactly the same properties as the pipe to be inspected. For on-site inspection, this is rarely possible, and the errors introduced by the difference in materials is tolerated, as no practical alternative is available. In most cases, to obtain a measure of the pipe thickness, the time at which the diffusion phase of the induced current ends, τ0 , is employed. The relation between the wall thickness, d, and τ0 is claimed to be quadratic, i.e., τ0 ∼ d 2 [1–3],
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Faris Nafiah [email protected]
1
School of Engineering, London South Bank University, London, UK
2
TWI Ltd, Cambridge, UK
which indicates the ability to directly assess the wall thickness information from the aforementioned parameter. The search for a better feature extraction technique can be seen in many previous works. One of the attempts is to make use of differential signal features, obtained by subtr
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