Lift-off Compensation Research Articles

Signal Processing Method to Synthesize Eddy Current Sensors

Abstract This paper focuses on the development of a novel method for synthetizing an eddy current sensor using post-processing techniques based on the linear superposition of multiple parallel acquisition signals. The proposed method will be demonstrated in the context of a cross-wound sensor (CWS) made of a flexible printed circuit board (PCB). Traditionally, the configuration and detection orientation of an eddy current sensor are determined during the fabrication process, which limits the flexibility and versatility of the sensor. However, the method developed in this research enables the post-processing fabrication of the sensor, increasing the customization potential and adaptability. Furthermore, the proposed method also enables the production of a suitable lift-off sensor (LOS) for dynamic lift-off compensation (DLOC). DLOC is a crucial aspect of eddy current sensing as it helps to account for the distance between the sensor and the target surface, which can affect measurement accuracy. The ability to configure the CWS detection orientation with DLOC capabilities in post-processing offers significant advantages in terms of sensor performance and versatility. Experimental results will be presented that demonstrate the effectiveness of the proposed method in enabling post-processing fabrication of an eddy current sensor featuring configurable detection orientation and DLOC capabilities.

Enhancing corrosion detection in pulsed eddy current testing systems through autoencoder-based unsupervised learning

Pulsed Eddy Current Testing (PECT) stands out as an advanced method in Non-Destructive Testing due to its extensive spectrum characteristics in comparison to traditional ECT techniques, making it exceptionally suitable for identifying corrosion. Nonetheless, the analysis of PECT signals for corrosion detection poses a challenge due to the transient nature of these signals and the impact of sensor lift-off effects. As a result, conventional methods are facing hurdles in dealing with corrosion signals of poor quality. In this study, the challenge is addressed by employing unsupervised learning methods utilizing an autoencoder neural network. This autoencoder integrates Long Short-Term Memory and 1D convolutional layers, acquiring the underlying features of normal PECT signals from non-corrosive regions. Significantly, the model is trained exclusively on this normal data, thereby obviating the necessity for pre-existing corrosion information. Through learning the inherent structure of normal signals, the model can detect anomalies in unseen data, potentially indicating corrosion. The unsupervised framework presents several advantages, such as reducing reliance on prior corrosion knowledge, mitigating inherent noise, and addressing sensor lift-off effects. Experimental results were conducted to compare with traditional methods like the lift-off of intersection and lift-off compensation methods. This approach resulted in a significant improvement in SNR, ranging from 100 % to 200 %, thus facilitating more robust NDT applications employing smart PECT sensors empowered by unsupervised learning techniques.

Soft sensor for in-line quality control of turning processes based on non-destructive testing techniques and advanced data fusion

AbstractThis study describes the systematic process of training, testing, and validating a soft sensor designed for quality control of a turning process on components made of AISI 4140 steel. The soft sensor allows product quality to be predicted and unfavorable surface conditions to be identified, in particular the appearance of a phenomenon known as “White Layer”, often characterized in the case of AISI 4140 steel by an ultra-fine-grained microstructure (UFG). Basis of the soft sensor is a data fusion supported by non-destructive testing techniques (NDT), particularly micromagnetic methods (3MA). A critical part of this work is to address challenges such as lift-off compensation and in-process detection using 3MA. The application of machine-learning techniques, including Principal Component Analysis (PCA) and regression analysis, is detailed. These techniques result in robust models capable of detecting the occurrence of the White Layer phenomenon while minimizing the influence of measurement setup variations and process disturbances. In addition, the study demonstrates the integration of NDT into the machining process which drives the soft sensor and allows suitable adjustments of the process parameters. The data-driven soft sensor approach demonstrates a possible In-Line control system and discusses different control theories and their respective advantages and disadvantages. This system can effectively set targeted surface conditions in real time during the turning process.

Eddy Currents Assessment of Rail Cracks Using Artificial Neural Networks in a Laboratory Setup

Although flaws associated with rolling contact fatigue (RCF) and the corresponding traffic induced damage, which are a cause of failure in railways, have been of great concern in railway system maintenance and safety strategies in many countries for at least two decades, this serious problem has not been yet adequately tackled in the Argentine railway system. The present upgrading activities undertaken in the Argentine railway system (in infrastructure and in rolling stock) are prompting the need for R&D in non-destructive testing techniques and procedures, to satisfy requirements of the new rolling stock and to ensure safe and economic operation of passengers and cargo. RCF damage appears as surface and near surface defects and grows into cracks which in time will propagate along the running surface and through the cross-section. Eddy current testing (ET) is a very efficient in-service inspection method for this task, the near field technique being especially recommended for ferromagnetic components.
 In the present paper, an artificial neural network (ANN) method for automatic classification of flaws with lift-off compensation is presented and tested. The tests consist on the ET evaluation of right angle artificial cracks on a rail calibration coupon; the depth of the cracks studied ranged from 1 to 7 mm. The technique permitted to compensate the weakening of the signals caused by the lift-off effect, allowing signal cracks classification with lift-off variations of up to 5.4 mm.
 The effect of crack skewness on the ET signals is also studied. Because the RCF cracks penetrate the rail at oblique angles, (10° to 30° to the rolling surface), an additional uncertainty component is added to the experiments if calibration is made with a piece having perpendicular cracks. In order to estimate this additional uncertainty on the ANN method presented here, further tests were made with a second calibration piece with cracks at 25° to the surface. Comparison of results showed that the peak to peak amplitudes for both types of cracks are not equivalent at all the tested depths.

Directly-digitized pulsed eddy current based quantification of aluminum sheet thickness

Direct interfacing technique (DIT) eradicates additional circuit requirements for sensor-embedded system interface and digitization of analog signals. This technique provides advantages in designing an efficient, portable, and low-cost sensor system. Pulsed eddy current testing (PECT) systems are used for thickness and defect measurements of conductive materials. Circuitous sensor-interfacing methods and tedious data interpretation processes make PECT systems inapt for miniaturization and portable applications. In this work, DIT is used in conjunction with PECT for thickness estimation of conductive material. Change in the de-energizing time of a single coil probe with sample thickness, with respect to air, is used as a signal. The curve fitting method yields a maximum relative error of ≈2% in the thickness estimation. Effects of temperature and liftoff on system accuracy are also investigated. A liftoff compensation method using a 3-signal data group is proposed. It is shown that for thicknesses in the range of 0.508 mm–3.175 mm and liftoffs up to 3.000 mm (step size: 0.500 mm), the proposed scheme produces a maximum relative error of 5.2%. The DIT and PECT combination can be applied for different structural eddy current testing in the future.

Enhancement of Defect Characterization With AC Magnetic Flux Leakage: Far-Side Defect Shape Estimation and Sensor Lift-Off Compensation

One of the most common methods for performing non-destructive testing (NDT) of the steel tank floors in aboveground storage tanks is dc magnetic flux leakage (MFL). This test method gives an estimate of the defect depth and width based on the MFL signal strength and its peak location, respectively. A key limitation is that the signal strength depends on not only the defect depth and width but also various other factors including a defect’s wall profile and sensor lift-off. Moreover, in a practical MFL test, the sensor lift-off changes due to surface roughness and uneven steel plate surface in tank floors. We present an ac MFL system to increase the accuracy of defect characterization by: 1) distinguishing between two common defect shapes located on the far side of the steel plate used in a typical above storage tank (AST) floor: lake-shaped and rectangular defects and 2) developing a sensor lift-off compensation scheme based on ac signal phase. Simulation results show that skewness of the ac MFL can be used to distinguish between far-side lake-shaped and rectangular defects. AC MFL signal phase is shown to be a suitable compensator for the dependence of signal strength on unintended variations in sensor lift-off. The simulation results have been validated through experiments.

A Novel Conductivity Measurement Method for Non-Magnetic Materials Based on Sweep-Frequency Eddy Current Method

Metallic plates are widely used in a variety of industrial applications, from which conductivity is a key parameter related to the quality of metals. In order to accurately measure the conductivity, this paper proposes a novel eddy current method based on sweep-frequency strategy. First of all, a simplified theoretical model for conductivity measurement is derived from the classical Dodd and Deeds analytical solution. Based on the analytical solution, the real and imaginary parts of the mutual inductance signal have an intersection in the frequency domain. According to mathematical derivation, the corresponding frequency (crossover-frequency) of the intersection is inversely proportional to the conductivity of the sample. Moreover, the lift-off compensation method is proposed to reduce the effect of lift-off variation. Experiments have been carried out for samples with different conductivities, ranging from 0.5MS/m to 58MS/m, to verify the proposed method. The results show the average error of the conductivity estimation with the proposed approach can be controlled within 1.1%, and the lift-off compensation method can significantly reduce the conductivity estimation error.

Measurement of Permeability for Ferrous Metallic Plates Using a Novel Lift-Off Compensation Technique on Phase Signature

Lift-off of sensor affects the prediction of electromagnetic properties for both ferrous and non-ferrous steel plates. In this paper, we developed a strategy to address this issue for ferrous plates. With increased lift-off, the phase of the measured impedance for steel plates reduces. Meanwhile, the magnitude of the impedance signal decreases. Based on these facts, a phase compensation algorithm is developed which corrects the phase change due to lift-off considering the magnitude of the impedance signal. Further, a new magnetic permeability prediction technique is presented, which has been validated by analytical and measured results. With this new technique, the error in permeability prediction is less than 2% within the range of lift-offs tested.

Review on system development in eddy current testing and technique for defect classification and characterization

Eddy current testing (ECT) is one of the non-destructive evaluation techniques widely used, especially in oil and gas industries. It characterized noisy data to the less-than-perfect detection and as an indication of serious false alarm problem. However, not many researchers have described in detail the intelligent ECT crack detection system. This paper introduces a review of ECT technique and factors that affect the signal fundamental according to the hardware and software development. First, describe the magnetic excitation resources including the sinusoidal and pulse exciting signal. Second, outlines explanation about the ECT probe. The explanations are more about the probes development for air core probe and giant magnetoresistance probe. Third, there is discussion on ECT circuit that used including ECT system, ECT rotating magnetic field and application measurement for optimal control parameters. The defect in characterizations and measurement are discussed on the fourth part of this paper. The fourth part discusses the ECT lift-off compensation including the lift-off and application of intelligent technique in ECT. The limitations of lift-off for coil probe and compensation techniques also discussed in this part. Finally, a comprehensive review of previous studies on the application of intelligent ECT crack detection in nondestructive ECT is presented.

Modelling and characteristic analysis of a displacement sensor with liftoff compensation

PurposeThe purpose of this paper is to present a magnetic contactless displacement sensor with liftoff compensation. The sensor consists of two‐axis Hall sensor, moving permanent magnet (PM) and ferromagnetic substrate which is considered as a magnetic flux concentrator.Design/methodology/approachThe two Hall‐effect sensors are used to detect the BX and BY induced by the moving PM. The BX and BY curves reflect the nature of the liftoff and the displacement of the inducing PM, respectively. Then the model of the displacement sensor, based on the facing current method, is constructed. Finally, the finite element method is used to compute the characteristics of BX and BY.FindingsThe BY curve can be corrected according to the liftoff distance of Hall‐effect sensor distinguished by the BX trough value. Therefore, the influence of the liftoff on the output signal can be eliminated when BY curve is corrected.Originality/valueThe paper focused on the design of a contactless displacement sensor with compensated liftoff performance for the control measurement technology in general and for automotive engineering in particular.

Simulation and Analysis of 3-D Magnetic Flux Leakage

In this paper, we present simulation results and analysis of 3-D magnetic flux leakage (MFL) signals due to the occurrence of a surface-breaking defect in a ferromagnetic specimen. The simulations and analysis are based on a magnetic dipole-based analytical model, presented in a previous paper. We exploit the tractability of the model and its amenability to simulation to analyze properties of the model as well as of the MFL fields it predicts, such as scale-invariance, effect of lift-off and defect shape, the utility of the tangential MFL component, and the sensitivity of MFL fields to parameters. The simulations and analysis show that the tangential MFL component is indeed a potentially critical part of MFL testing. It is also shown that the MFL field of a defect varies drastically with lift-off. We also exploit the model to develop a lift-off compensation technique which enables the prediction of the size of the defect for a range of lift-off values.

Lift-off compensation for improved accuracy in ultrasonic lamb wave velocity measurements using electromagnetic acoustic transducers (EMATs)

The crystalline texture of a sheet metal strongly affects its formability, so having knowledge of this texture is of great industrial relevance. The texture of rolled sheet metals, such as aluminium and steel, may be determined by ultrasonic measurement of the velocity of the zero order symmetric ( S 0) Lamb wave as a function of angle to the rolling direction. Electromagnetic acoustic transducers (EMATs) may perform this measurement without contacting the sample, therefore reducing perturbation to the plate wave system, as they are electromagnetically coupled to the sheet. The EMAT system measurements are non-destructive and may be made in real time, therefore offering advantages over the conventional techniques such as X-ray and neutron diffraction. It has been noticed that in the two EMAT pitch–catch system, the apparent arrival times of the ultrasonic waves change with variation in lift-off (distance between sample and transducer) due to impedance and aperture effects. For precise and accurate texture parameters to be obtained, accurate absolute ultrasonic velocity measurement is required and hence lift-off must be compensated for. This is of particular importance to online inspection systems where constant lift-off may be difficult to maintain. The impedance behaviour of various coil geometries has been investigated as a function of lift-off and frequency and compared to the received ultrasonic signal and the drive current pulse profile. Theoretical models have been used to explain the observed behaviour, and hence a scheme has been proposed for the compensation of lift-off effects in real time.

Lift-off Compensation for Vibrating Amplitude Meter Using a Capacitive Transducer

Vibrating amplitude measurment using a condenser sensor has a weak point that measuring sensitivity varies by the distance between the probe and the vibrating body (lift-off).This paper describes an instrumental technique suitable for industrial-measurement of the vibrating amplitude in the range 1-100, μm.In order to achieve constant sensitivity against lift-off changes, the two kinds of power sources are supplied in series with the vibrating body and the probe. One is a DC source and the other is an AC (high frequency) source. The resulting composite signal are separated into two signals by a low and a high pass filter. Low frequency component (eOL) is proportional to the vibrating amplitude and inversely proportional to the lift-off squared. High frecancey component (eOH) is inversely proportional to the lift-off. Then lift-off compensation is achieved by performing an operation (eOL/e2OH) on an electronic circuit.When the efective area of the probe is 4.11mm2, the lift-off compensation covers from 0.2mm to 2.0mm within the error of 5 %.