Automatic Differential Lift-Off Compensation (AD-LOC) Method In Pulsed Eddy Current Inspection

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1 17th World Conference on Nondestructive Testing, Oct 2008, Shanghai, China Automatic Differential Lift-Off Compensation (AD-LOC) Method In Pulsed Eddy Current Inspection Joanna X. QIAO, John P. HANSEN, Ian MAYES GE Inspection Technologies Ltd., St. Albans, UK Tel: , Fax Web: Abstract: In the state of art Pulse Eddy Current inspection application, we present a novel method of Automatic Differential Lift-Off Compensation (AD-LOC). The lift-off key features were defined and extracted by the discovery of a strong relationship between the differential peak value of the sensed PEC signal and the lift-off distance. The corresponding compensation formula was then created. A practical implementation scheme was proposed based on the described method. Functions such as automatic reorganization, classification and optimal compensation regarding the randomly occurring of lift-off disruption were implemented. Keywords: pulsed eddy current (PEC), lift-off compensation, feature extraction, automatic classification, AD-LOC, corrosion detection. 1. Introduction Eddy current offers important advantage for defect detection in conductive structure and has a very broad range application. Hidden defects such as corrosion or cracks buried beneath the surface not only threaten serviceability, but also make the inspection task more challenging. The conventional eddy current technique based on sine wave coil excitation can provide non-destructive inspection facilities for invisible flaw detection. However, the shortcomings are: 1. The limited amount of information available from a small, fixed number of frequencies can frustrate the identification, sizing and positioning of hidden defects and other artefacts such as air gaps and changes in materials thickness. This is particularly important because the positioning, type and size of indication can lead to very different remedial actions being taken especially when the object under test has a variable and complicated structure. 2. At the low frequencies required for deep penetration, conventional sine wave coil excitation becomes increasingly difficult. In contrast with conventional eddy current inspection, Pulsed Eddy Current (PEC) technology has shown its great potential for inspecting complex structures, since a pulse is rich in frequency content. Further when a pulse is applied to a coil the energy spectrum is biased towards low frequencies due to the coil inductance and resistance forming a low pass filter.

2 The broad frequency spectrum contained in the PEC pick-up signal responses, enables PEC inspection data to have a much better capability to provide more information about defects, such as a defect s location and size. By the means of state of the art signal and image processing technologies, some interesting PEC inspection results have been achieved. PEC technology has emerged as a promising option to tackle difficult inspection tasks such as sub-surface defect detection in complex structures such as aircraft. The distance between the probe and the surface of specimen is called lift-off. One of main challenges for PEC inspection is how to handle this. It is generally caused by the presence of uneven coating thickness or the presence of rivets. Lift-off produces false indications in the pick-up signal, which in turn masks the important information produced by real sub-surface defects. Unfortunately, these distorted PEC response signals are generally what is measured in the real world and analyzed to interpret the true defect signal response. There is no doubt, this kind of distortion will produce unnoticed misleading information about a signal s interpretation and identification in later processing. How to remove lift-off so that the signal variation of interest can be truly observed is a very challenging and crucial task. The Automatic Differential Lift-Off Compensation (AD-LOC) Method is described in detail in section 2 in this paper. Experiments have been carried out and the corresponding lift-off compensation results are shown in section 3, in which several amounts of lift-off were introduced during PEC C-Scan inspection. The automatic variable lift-off compensation of C-Scan PEC imaging is also shown. The conclusions are given in section Automatic Differential Lift-Off Compensation (AD-LOC) Method 2.1 PEC Signal Analysis and Lift-Off Characterization In order to overcome the difficulties caused by lift-off, several compensation methods have been proposed by researchers and engineers. For example, the popular LOI-C Scan method proposed by B.A.Lepine[2], which is based on finding the lift-off intersection point; The effective empirical function g(t) compensation method proposed by G.R.Hugo[3] defines the time independence of the scaling and is determined by fitting experimental data for each probe. In this paper, we look at how the lift-off response is affected by the amount of material lost (defect size). Figure-1 shows the experimental arrangement. Generally the lift-off signal is much bigger than the signal due to the defect. Probe LiftOff Figure-1 Experimental arrangement

3 The effect of different amounts of Lift-off with different sizes of defect is shown in Figure-2. Three sizes of defect were introduced representing 5% 10% and 20% of the specimen thickness respectively. For each size of defect the PEC signal response was measured with lift-off distances of 0.05mm 0.25mm and 0.5mm. The dark blue curves show the response for each of the defects without lift-off. The green, red and light blue curves show the response for the same defects but with vary amounts of lift-off P u ls e d E d d y C u rr e n t S ig n a l B lu e : M a t L o s s 5 % 1 0 % 2 0 % w it h o u t L ift O ff e ffe c t G re e n : M a t L o s s 5 % 1 0 % 2 0 % w it h m m L ift O ff e ffe c t R e d : M a t L o s s 5 % 1 0 % 2 0 % w it h m m L ift O ff e ffe c t L ig h t G re e n : M a t L o s s 5 % 1 0 % 2 0 % w it h 0. 5 m m L ift O ff e ffe c t Figure-2 PEC response for three sizes of defect each measured with three levels of lift-off. The derivative of the responses with and without lift-off were calculated and plotted in Figure-3. From which we discovered that the peak of the first order differential of the PEC response signal is mainly determined by lift-off distance, once lift-off is present. The PEC signals with different defect sizes but same lift-off distance were clustered into the same group. Since the real defect signals are far less strong than lift-off disturbances, they can be ignored during the lift-off analysis. As expected, from the differential PEC signal analysis, the signals produced by different defect sizes (with different material loss) but same lift-off distances are bundled together. Three groups of signals in green, red and light blue represent three amounts of lift-off. The dark blue group is a differential PEC signal without lift-off P u l s e d E d d y C u r r e n t D o ffe r e n t i a l B l u e : M a t L o s s 5 % 1 0 % 2 0 % w i t h o u t L i ft O ff e ffe c t G r e e n : M a t L o s s 5 % 1 0 % 2 0 % w i t h m m L i ft O ff e ffe c t R e d : M a t L o s s 5 % 1 0 % 2 0 % w i t h m m L i ft O ff e ffe c t L i g h t G r e e n : M a t L o s s 5 % 1 0 % 2 0 % w i t h 0. 5 m m L i ft O ff e ffe c t Figure-3 The first order differential of the PEC sensed signals Based on above data analysis, the conclusion was reached that the peak of the differential of the PEC sensed signal could be defined as a key indicator of the lift-off distance.

4 2.2 The Automatic Differential lift-off Compensation Scheme Based on the above observations, lift-off can be recognized, assessed, classified and finally compensated [1]. The implementation scheme is mainly described by the following steps: Step1: Obtaining calibrated lift-off response references. After balancing the probe, the defect free lift-off PEC reference signals are obtained by placing plastic shims of different thicknesses between the probe and the surface of the specimen. The lift-off distance is determined by the thickness of the plastic shim. The responses are differentiated and the peak amplitude values are obtained and stored with the corresponding lift-off reference signals. Step2: Testing and compensating for lift-off Differentiate the testing signal. Find the peak in the differential response and compare with the stored differential peak values. Pick out the closest match and its corresponding lift-off reference signal. The compensation ratio is defined as the current differential peak value divided by the selected reference differential peak value. The compensated test signal is the difference between the measured test signal and the product of the compensation ratio with the selected lift-off reference signal. 3. Real Inspection Experiments with AD-LOC Method In order to evaluate the algorithms performance, two experiments have been processed on the real inspection specimen with a single Coil and a GMR array. 2 0 % M a t e r i a l L o s s D e fe c t w i t h o r w i t h o u t L i ft -O ff P E C S i g n a ls a n d L i ft O ff C o m p e n s a t e d P E C S ig n a l s D e fe c t w i t h 0. 5 m m m m a n d m m l i ft o ff C o m p e n s a t e d P E C S i g n a l s Voltage D e fe c t w it h o u t L ift O ff P E C S ig n a l D e fe c t w i t h m m L ift O ff P E C S i g n a l D e fe c t w i t h m m L i ft O ff P E C S i g n a l D e fe c t w i t h 0. 5 m m L i ft O ff P E C S i g n a l T i m e Figure-4 single channel AD-LOC signal restoration using a single coil probe The original defect signal with 20% material loss of the specimen thickness is shown in figure-4 (red curve). The corresponding 0.05mm, 0.25mm and 0.5mm lift-off distortion signals for the same defect are also shown (black curves). After AD-LOC method was applied, the three lift-off masked signals were compensated and displayed in the Green curves. As shown, all compensated green curves mostly overlap the original

5 lift-off free defect signal. Despite different lift-off distances, all lift-off distortion signals are automatically recovered. 10 mils 20 mils 15 mils 10 mils 20 mils 5 mils 15 mils 15 mils 2 Figure-5 C-Scan Variable lift-off patch occurrence plot. The AD-LOC method was tested on PEC C-scan inspection with fixed lift-off patches (0.127mm, 0.254mm, 0.381mm and 0.508mm), see Figure 5. The contrasting PEC images, with the AD-LOC method switched on and off are shown in Figure-6. This shows the powerful capability of the method for restoring masked defect signals. When the AD-LOC compensation method is switched off, the lift-off effect dominates the PEC image and real defects are hidden due to their signals being much weaker than the lift-off signal. Once AD-LOC method was switched on, the different lift-off masks were automatically and correctly recognized and classified. Therefore the optimal lift-off compensations could be properly applied before PEC Image processing proceeds. Based on this compensation data, the signals from the patches were restored, and the patch patterns were removed for subsequent PEC imaging processing. The background of the PEC image is then restored almost free of the disruption caused by lift-off and the defects revealed. C - S c a n P E C I m a g i n g w i t h o u t l i f t - o f f c o m p e n s a t i o n C - S c a n P E C Im a g i n g w i t h l i f t - o f f c o m p e n s a t i o n Figure-6 C-Scan PEC Imaging using a GMR array with AD-LOC off (Top) and on (Bottom)

6 4. Conclusions The innovative PEC Automatic Differential Lift-Off Compensation (AD-LOC) technology proposed here is simple and easy to be implemented. It is able to provide full recovery of the defect signal in the presence of reasonable amounts of lift-off.. There is also no requirement for empirical compensation based on data fitting. By learning the response for a small, fixed number of lift-off distances and setting these as a reference, the AD-LOC method is capable of automatically recognizing the occurrence of lift-off and removing its effects from the measured response. The AD-LOC method provides satisfactory lift-off compensation results regardless of the complexity of the sample. Acknowledgements The authors would like to thank Dr Yuri Plotnikov at the GE GRC for providing the GMR data used in this paper. References [1]. Joanna.X.Qiao & John P. Hansen, AUTOMATIC LIFT-OFF COMPENSATION FOR PULSED EDDY CURRENT INSPECTION, invention application No /12394, 2007 [2] B.A.Lepine, J.S.R.Giguere, etc, Appling Pulsed Eddy Current NDT to the Aircraft Hidden Corrosion Problem, 2001 [3] G.R.Hugo, Characterization of Hidden Corrosion in MultiLayer Aircraft Structures Using Transient Eddy-Current NDE Review of progress in Quantitative Nondestructive Evaluation, Vol