Lift-off Point Of Intersection Research Articles

Lift-Off Point of Intersection for Eliminating Lift-Off Noise in Alternating Current Field Measurement

Lift-off is a challenging issue in alternating current field measurement (ACFM) technique. The spurious signals caused by lift-off variations are similar to the response signals of defects, resulting in defect misjudgment. In this paper, a lift-off point of intersection (LOI) feature is discovered for the first time from the raw signals of ACFM for the elimination of spurious signals caused by lift-off variations. A theoretical model is established to analyze the effect of lift-off variations on characteristic signals of ACFM. A 3D finite element model is built to analyze the effect of the excitation frequency on LOI. The probe and testing system are established, and the validation experiments are carried out. The results show that the traditional Bx signal is susceptible to the variation of lift-off, and the higher the excitation frequency, the closer the LOI is to a point. The novel Bx signal plotted by LOI amplitude has no spurious signals caused by lift-off variations and can also accurately evaluate the crack depth as with traditional Bx signals.

Measurement of Lift-Off Distance and Thickness of Nonmagnetic Metallic Plate Using Pulsed Eddy Current Testing

Eddy current testing (ECT) is an efficient method to estimate the thickness of sample but it is affected by other parameters information including electromagnetic properties and lift-off. In the previous researches, the lift-off point of intersection (LOI) feature has been found in pulsed eddy current (PEC), which can overcome the fluctuation of lift-off. In this paper, another feature, i.e. conductivity point of intersection(CPI), is proposed for non-coaxial T-R sensor in PEC. Specifically, the response signals of different sample conductivity almost intersect with each other at the certain time, which can be regarded immune to the conductivity of sample. The analytical solution and numerical solution are conducted, and the results indicate that the intersection point magnitude is mainly related with parameters of sensor and thickness of samples. Considering the elements related with conductivity in the Hessian matrix exists ill-conditioning, it will produce the magnification of measurement error under iterative inversion process. By referring to the CPI feature, the thickness of the sample and lift-off of sensor can be accurately inverted based on the modified Newton–Raphson method. The experiments are carried out to verify the proposed method, and the error is only 2.9%.

Lift-off invariant inductance of steels in multi-frequency eddy-current testing

Eddy current testing can be used to interrogate steels but it is hampered by the lift-off distance of the sensor. Previously, the lift-off point of intersection (LOI) feature has been found for the pulsed eddy current (PEC) testing. In this paper, a lift-off invariant inductance (LII) feature is proposed for the multi-frequency eddy current (MEC) testing, which merely targets the ferromagnetic steels. That is, at a certain working frequency, the measured inductance signal is found nearly immune to the lift-off distance of the sensor. Such working frequency and inductance are termed as the lift-off invariant frequency (LIF) and LII. Through simulations and experimental measurements of different steels under the multi-frequency manner, the LII has been verified to be merely related to the sensor parameters and independent of different steels. By referring to the LIF of the test piece and using an iterative inverse solver, one of the steel properties (either the electrical conductivity or magnetic permeability) can be reconstructed with a high accuracy.

Evaluation of Coating Thickness Using Lift-Off Insensitivity of Eddy Current Sensor.

Defect detection in ferromagnetic substrates is often hampered by nonmagnetic coating thickness variation when using conventional eddy current testing technique. The lift-off distance between the sample and the sensor is one of the main obstacles for the thickness measurement of nonmagnetic coatings on ferromagnetic substrates when using the eddy current testing technique. Based on the eddy current thin-skin effect and the lift-off insensitive inductance (LII), a simplified iterative algorithm is proposed for reducing the lift-off variation effect using a multifrequency sensor. Compared to the previous techniques on compensating the lift-off error (e.g., the lift-off point of intersection) while retrieving the thickness, the simplified inductance algorithms avoid the computation burden of integration, which are used as embedded algorithms for the online retrieval of lift-offs via each frequency channel. The LII is determined by the dimension and geometry of the sensor, thus eliminating the need for empirical calibration. The method is validated by means of experimental measurements of the inductance of coatings with different materials and thicknesses on ferrous substrates (dual-phase alloy). The error of the calculated coating thickness has been controlled to within 3% for an extended lift-off range of up to 10 mm.

Liftoff Tolerant Pancake Eddy-Current Sensor for the Thickness and Spacing Measurement of Nonmagnetic Plates

The liftoff spacing distance between the eddy current (EC) sensor and the test piece will influence the detected signals and accuracy of the measurement. Various techniques including novel sensor designs, features (liftoff point of intersection, liftoff invariance phenomenon), and algorithms have been proposed for the compensation of error caused by the liftoff effect using the EC sensor. However, few of these have directly measured the liftoff spacing distance, particularly for the distance up to 15 mm. In this article, a liftoff tolerant pancake sensor has been designed. By analyzing the sensitive region of the magnetic vector potential change (due to the test piece), the receiver of the sensor is designed as a circular spiral pancake coil with a large mean radius and span length (the difference between inner and outer radius). Experiments on the inductance measurement of three different nonmagnetic samples have been carried out using both the designed pancake sensor and the previous triple-helix sensor. From the experiment results, the detected signal of the designed sensor has been proven much larger than that of the triple-helix sensor. Besides, simplified algorithms have been proposed for the measurement of the liftoff spacing and thickness of non-magnetic samples when using the proposed pancake sensor. Results show that the liftoff spacing and thickness can be measured with a small error of 0.14 mm (absolute error under 209.66 kHz), and 1.35% (relative error, under low working frequencies of 142, 238, and 338 Hz) for the liftoff spacing from 1 to 15 mm.

Measurement of coating thickness using lift-off point of intersection features from pulsed eddy current signals

In industries, the thickness measurement of non-ferromagnetic coatings on a ferromagnetic substrate is a difficult problem. In this paper, we proposed a simple but liftoff insensitive method for measuring the thickness of non-ferrous coatings using lift-off point of intersection (LOI) extracted from pulsed eddy current (PEC) signals. Firstly, an analytical model was formulated to calculate PEC signals using Fourier transform. Then, the simulations were carried out to investigate the characteristics of LOI points and the applicability of the presented method for measurement of coating thickness. Subsequently, the critical factors on coating thickness measurement were discussed. The results show that the developed new method is feasible for measurement of nonferrous coating thickness. The rising time of pulse excitations could balance the sensitivity and accuracy. The results demonstrate that the relative error for measurement of coating thickness is 4.9% when the rising time increases to 90 μs

Features extraction and discussion in a novel frequency-band-selecting pulsed eddy current testing method for the detection of a certain depth range of defects

Local wall thinning defects are unavoidable defects in actual engineering structures and in some cases it occurs in a certain depth range of the object structures. In this study, a novel frequency-band-selecting pulsed eddy current testing (FSPECT) has been proposed for the detection of local wall thinning defects in a certain depth range. Feature of peak value has been extracted and analyzed. The results demonstrate that the FSPECT possess the comparable performances in terms of detection sensitivity over the traditional square wave pulsed eddy current testing (PECT). In addition, other fruitful detailed features extraction in the numerical calculation and experimental signals of FSPECT are deeply studied. The features obtained by simulation and experiment mainly include lift-off point of intersection (LOI) and zero-crossing time. Furthermore, the influences of the depth of local wall thinning defect on features of LOI and zero-crossing time have been explored, which enhance the accuracy and reliability of FSPECT method for the evaluation of local wall thinning defects.

A Twice Subtraction Method for Obtaining LOI in Pulsed Eddy Current Signals of Ferromagnetic Samples

Lift-off point of intersection (LOI), as a desirable signal feature independently of the lift-off effect, is expected to be applied in pulsed eddy current (PEC) testing of ferromagnetic samples. The time-domain LOI in PEC signals is easily recorded and easier to be extracted for evaluating ferromagnetic samples, compared with the frequency domain LOI in spectral PEC signals. However, the time-domain LOI cannot be obtained in the PEC signals of ferromagnetic samples. In this article, a twice subtraction method was proposed to obtain an LOI feature in PEC signals of ferromagnetic samples for evaluating the sample thickness, which is verified through simulations and experiments. In the twice subtraction method, the first subtraction is performed between PEC signals from the sample and air to obtain a difference in the PEC signal. The secondary subtraction is implemented between the difference in the PEC signal and its standard deviation. As a result, an LOI can be extracted from the twice subtraction signals when only lift-off distances are introduced. The results demonstrate that the twice subtraction method is a feasible program to obtain an LOI in twice subtraction signals of ferromagnetic samples, and the obtained LOI has the potential to be used to measure the thickness of ferromagnetic samples without lift-off effect.

Adjusting LOI for Enhancement of Pulsed Eddy Current Thickness Measurement

The liftoff point of intersection (LOI) is a desirable signal feature in the pulsed eddy current (PEC) testing. It is immune to liftoff effect caused by the change of liftoff distance between a probe and a sample. However, adjusting LOI remains an open problem for enhancement of PEC evaluation or extends the application of LOI to thicker samples. In this paper, LOI point could be adjusted by varying the rising time of pulse excitation to extend the measurement range or increase sensitivity for enhancement of the PEC evaluation. The simulations and experiments demonstrate that the difference PEC signal increases and decreases significantly slowly when the rising time of pulse excitation increases. When the rising time of pulse excitation increases, the measurement range of LOI amplitude increases, and the measurement range of LOI time decreases, whereas the sensitivity of LOI amplitude decreases, and the sensitivity of LOI time increases. This investigation helps us to optimize the rising time for the improvement of specific PEC evaluation.

Extraction of LOI Features From Spectral Pulsed Eddy Current Signals for Evaluation of Ferromagnetic Samples

Different applications of pulsed eddy current (PEC) testing have been reported in safety detection due to its characteristics of high sensitivity, non-contact, and rich information. In the determination of metal thickness by PEC testing, the liftoff effect commonly gives rise to inaccurate measurement. As a desirable signal feature immune to liftoff effect, liftoff point of intersection (LOI) is a common point where PEC signals intersect when only the liftoff distance changes. However, for ferromagnetic samples, a LOI point could not be obtained directly from the PEC signals when the liftoff distance changes. This paper investigated spectral PEC signals of ferromagnetic metal plate by numerical simulations and experiments. Excitingly, a frequency LOI was first observed in spectral PEC signals for ferromagnetic samples, and the behavior of LOI was further analyzed when sample conductivity, permeability, and thickness change. This paper concludes that the LOI in spectral PEC signals is affected by sample conductivity, permeability, and thickness. In addition, the frequency LOI allows for accurate evaluation of ferromagnetic samples with the variation of liftoff distance.

Pulsed Eddy Current Data Analysis for the Characterization of the Second-Layer Discontinuities

Pulsed eddy current (PEC) technique has been applied as a viable method to detect hidden discontinuities in metallic structures. Conventionally, selected time-domain features are employed to characterize the PEC data, such as peak value, lift-off point of intersection, rising point, crossing time, and differential time to peak. The research presented in this paper continues the effort in a previous study on detecting the radial cracks starting from the fastener hole in second layer of a two-layer mock-up aircraft structure. A large diameter excitation coil with ferrite core is used to induce a strong pulse, and the magnetic field generated by eddy current is detected by Hall sensors. Instead of analyzing the limited time-domain features, we propose using machine learning methods to interpret the raw data without feature extraction. Thus, the second-layer discontinuities can be characterized presumably with all the information contained in a waveform. An automated detection framework is proposed in this paper and the experimental results demonstrate the effectiveness of the proposed method.

Lift-Off Point of Intersection in Spectral Pulsed Eddy Current Signals for Thickness Measurement

Lift-off invariant strategies play an important role in pulsed eddy current (PEC) testing. As an effective signal feature immune to lift-off effect, lift-off point of intersection (LOI) has been applied to determine thickness or evaluate defects. Typically, an LOI feature is extracted from time domain PEC signals. In this article, we observed a novel LOI feature in real and imaginary part of frequency domain PEC responses via simulation and experiment. An analytical model is formulated to investigate the novel LOI feature. It is found that the proposed LOI feature varies with sample thickness increasing, which indicates that the LOI feature from spectral PEC signals could be used for thickness measurement.

Thickness measurement using liftoff point of intersection in pulsed eddy current responses for elimination of liftoff effect

Pulsed eddy current (PEC) responses would intersect at a point for a nonferrous plate when only liftoff distance is varied. In this paper, the behaviors of liftoff point of intersection (LOI) points due to a plate with varying conductivity and thickness were investigated and interpreted in physics by means of simulations and experiments. LOI points result from the invariance of the phase spectrum of PEC responses to liftoff effect, and the time of LOI points would increase with an increase of phase spectrum. Subsequently, the signatures of LOI points were compared with the peak heights and time to peaks of PEC signals and their derivatives. The results demonstrate that the signatures of LOI points possess the comparable performances in terms of sensitivity, linearity and measurement range. Moreover, the signatures of LOI points are immune to liftoff variations in nature, which shall enhance the accuracy and reliability of PEC evaluation.

Signal characteristics of differential-pulsed eddy current sensors in the evaluation of plate thickness

Differential-pulsed eddy current (PEC) signals and their characteristics are investigated as tools for the evaluation of plate thickness by using the reflection-type PEC probe, which consists of an exciter coil and two sensor coils in a differential arrangement. Numerical and experimental signals are first compared to validate numerical simulation results, and good agreement between them is achieved. Investigation of differential PEC signals and their characteristics against thickness and conductivity variations shows that time-related features, such as the time-to-peak and the zero-crossing time, correspond well to plate thickness and conductivity. However, the feature quality of peak value turns out to be very poor. To improve this, the effects of coil characteristics on the input pulse, and consequently on differential PEC signals, are investigated. Results show that the feature quality of peak value may be improved by reducing the time constant, but doing so would worsen the quality of the other two features. Lift-off signals obtained by this probe show that the lift-off point of intersection also appears in the differential reflection-type PEC signals.

Transient and harmonic eddy currents: Lift-off point of intersection

It has been previously determined experimentally that the transient responses of pulsed eddy current excitation of a conductive semi-infinite plane pass through a common point, regardless of the lift-off value. The origin of this intersection point is investigated and related to the harmonic components of the Fourier series representation of the pulse. Discrete sinusoidal signals, when individually sent through the same inspection system, also exhibit this common point feature when only the lift-off is varied. Moreover, the transient responses can be reconstructed from the harmonic outputs by using truncated Fourier series.