The Lift-Off Effect in Eddy Currents on Thickness Modeling and Measurement

Král, Jakub; Šmı́d, Radislav; Ramos, Helena G.; Ribeiro, A. Lopes

doi:10.1109/tim.2013.2247713

Cited by 77 publications

(32 citation statements)

References 15 publications

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“…However, complicated and advanced signal processing and interpretation are required to abstract the features of the PEC signal and link these features explicitly to the sample conditions. In addition, both PEC and SEC techniques are very sensitive to distance (lift-off) variation between the sensor and the target, because the strength of the eddy current induced in the target rapidly decreases with increased lift-off [8]. Therefore, lift-off effect is the main obstacle for thickness measurement using ECS.…”

mentioning

confidence: 99%

Noncontact Thickness Measurement of Metal Films Using Eddy-Current Sensors Immune to Distance Variation

Wang

Feng

2015

IEEE Trans. Instrum. Meas.

107

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A simple method for measuring the thickness of metal films based on eddy-current sensors (ECSs) immune to distance variation is proposed. The slope of the lift-off curve (LOC) in the RL impedance plane is a good feature for characterizing target thickness independent of lift-off distance variation. A simple equivalent model was built to deal with the ECS problem, and the essential relationship between the slope of LOC (SLOC) and target properties was obtained. Full finite element analysis was conducted to analyze the relationship between SLOC feature and target thickness, and the results matched the modeling results very well. A sensor coil probe was then manufactured and used to measure the thickness of copper films with high performance, and the capability of this technique for online noncontact thickness measurement was verified. The basic characteristics and performances of this thickness measurement technique were tested and discussed. The SLOC feature for thickness measurement had significant advantages, such as simplicity, reliability, immunity to the lift-off effect (most important), high speed, simple signal processing, and negligible design limitation of the sensor probe. The results of this paper revealed that online thickness measurement systems could be developed for various advanced industrial applications.Index Terms-Eddy-current sensor (ECS), electromagnetic measurement, impedance measurement, industrial applications, lift-off curve (LOC), metal films, sensor system, slope of LOC (SLOC), thickness measurement.

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mentioning

confidence: 99%

Noncontact Thickness Measurement of Metal Films Using Eddy-Current Sensors Immune to Distance Variation

Wang

Feng

2015

IEEE Trans. Instrum. Meas.

107

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“…It is found that the LOI phenomenon is a general characteristic of the eddy currents, not only to transient eddy currents. Kral, et al [74] presented a linear transformer model to investigate the origin of LOI phenomenon, and obtained LOI points from the time derivatives of the output of a GMR-based probe. Interestingly, a LOI point is recently observed in the spectral PEC signals [67].…”

Section: Lift-off Problemmentioning

confidence: 99%

Pulsed Eddy Current Non-destructive Testing and Evaluation: A Review

Sophian

Tian

Fan

2017

Chin. J. Mech. Eng.

198

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Pulsed eddy current (PEC) non-destructive testing and evaluation (NDT&E) has been around for some time and it is still attracting extensive attention from researchers around the globe, which can be witnessed through the reports reviewed in this paper. Thanks to its richness of spectral components, various applications of this technique have been proposed and reported in the literature covering both structural integrity inspection and material characterization in various industrial sectors. To support its development and for better understanding of the phenomena around the transient induced eddy currents, attempts for its modelling both analytically and numerically have been made by researchers around the world. This review is an attempt to capture the state-of-the-art development and applications of PEC, especially in the last 15 years and it is not intended to be exhaustive. Future challenges and opportunities for PEC NDT&E are also presented.

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“…The singlefrequency eddy current technique, 9,12,[14][15][16] where the coil is excited with a sinusoidal signal and the eddy currents are distributed in a fixed depth, is the most traditional method and a) Electronic mail: fff@ustc.edu.cn is the basis of the other methods. The PEC technique, 11,17 as a time-domain method, contains more frequency-rich information than the conventional single-frequency excitation. Three signal features, namely, the peak height, the time of occurrence of the first peak, and a characteristic zero-crossing time, were found to characterize the measurands, that is, thickness, conductivity, defect, and so on.…”

Section: Introductionmentioning

confidence: 99%

“…10,13 For eddy current methods, an important factor limiting the response interpretation is the lift-off effect on the signals because the lift-off variation changes the mutual inductance between the excitation coil and the target. Reduction of the liftoff effect has been attempted with some successes by using the lift-off point of intersection (LOI), 16,17,20 using multi-sensor coil, 2,21 and optimization of the sensor structure. LOI is an experimentally observed fixed point where the time-domain transient eddy current signals intersect as the lift-off solely changes.…”

Section: Introductionmentioning

confidence: 99%

Non-contact online thickness measurement system for metal films based on eddy current sensing with distance tracking technique

Wang

Feng

2016

Review of Scientific Instruments

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This paper proposes an online, non-contact metal film thickness measurement system based on eddy current sensing. The slope of the lift-off curve (LOC) is used for characterizing target thickness. Theoretical derivation was conducted to prove that the slope is independent of the lift-off variation. In practice, the measurement has some immunity to the lift-off, but not perfect. The slope of LOC is still affected at some extent by the lift-off. Hence, a height tracking system was also proposed, which could stabilize the distance between the sensor and the target and significantly reduce the lift-off effect. The height tracking system contains a specially designed probe, which could vibrate rapidly to obtain a fast measurement speed, and its height can be adjusted up and down continuously to stabilize the lift-off. The sensor coil in the thickness measurement system was also used as the height sensor in the height tracking system. Several experiments were conducted to test the system performances under static and dynamic conditions. This measurement system demonstrated significant advantages, such as simple and clear conversion between the slope of LOC and target thickness, high resolution and stability, and minimized effect of lift-off variation. Published by AIP Publishing. [http://dx

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The Lift-Off Effect in Eddy Currents on Thickness Modeling and Measurement

Cited by 77 publications

References 15 publications

Noncontact Thickness Measurement of Metal Films Using Eddy-Current Sensors Immune to Distance Variation

Noncontact Thickness Measurement of Metal Films Using Eddy-Current Sensors Immune to Distance Variation

Pulsed Eddy Current Non-destructive Testing and Evaluation: A Review

Non-contact online thickness measurement system for metal films based on eddy current sensing with distance tracking technique

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