Luis S. Rosado

Defect Characterization With Eddy Current Testing Using Nonlinear-Regression Feature Extraction and Artificial Neural Networks

The estimation of the parameters of defects from eddy current nondestructive testing data is an important tool to evaluate the structural integrity of critical metallic parts. In recent years, several works have reported the use of artificial neural networks (ANNs) to deal with the complex relation between the testing data and the defect properties. To extract relevant features used by the ANN, principal component analysis, wavelet decomposition, and the discrete Fourier transform have been proposed. In this paper, a method to estimate dimensional parameters from eddy current testing data is reported. Feature extraction is based on the modeling of the testing data by a template of additive Gaussian functions and nonlinear regressions to estimate their parameters. An ANN was trained using features extracted from a synthetic data set obtained with finite-element modeling of the eddy current probe. The proposed method was applied to both simulated and measured data, providing good estimates.

A reconfigurable digital signal processing system for eddy currents non-destructive testing

This paper presents a digital signal processing system specially designed for eddy currents non-destructive testing. This new system has a field programmable gate array based processing core, communication interfaces, data conversion and analog devices to interface the probes. Communication with personal computers is ensured by Ethernet 10/100 and universal serial bus 2.0 high speed interfaces. The proposed architecture enables to set several combinations of peripherals cards to generate or acquire probe signals. Also, the new system allows the digital generation and analysis of the probe signals through multiple digital signal processing algorithms. Two different peripheral cards have been developed to meet the needs for the new IOnic concept of eddy currents probe. The IOnic acquisition card is composed by a programmable gain amplifier and a high speed analog to digital converter. The current stimulus generation is achieved with a digital to analog converter and a high output current transconductance amplifier. Together, the two peripherals are able to operate the probe from 10 kHz up to 10 MHz. An additional peripheral card to interface stepper motors was designed for sensor positioning.

Magnetic tunnel junction based eddy current testing probe for detection of surface defects

In recent years, magnetoresistive sensors have been applied to a large spectrum of applications from biomedical devices to industrial devices. Their high sensitivity and high spatial resolution are of special interest for eddy current based non-destructive testing. In this particular application, giant magnetoresistive sensors have been recently used for detecting surface and buried defects. Nevertheless, although very promising, magnetic tunnel junctions (MTJs) are still barely used in this application. In this work, two sensors with 6 and 10 MTJs in series were successfully fabricated, characterized, and tested on an aluminum mock-up including defects 100 μm wide and with a depth ranging from 0.2 to 1 mm. The sensors including 6 MTJ in series showed sensitivities of 50.8 mV/mT, while the sensor with 10 MTJ in series showed a sensitivity of 84.5 mV/mT. Due to its high sensitivity the latter was able to detect the smallest defect with a signal to noise ratio of 50, which seems promising for more challengi...

Real-Time Processing of Multifrequency Eddy Currents Testing Signals: Design, Implementation, and Evaluation

The design, implementation, and evaluation of a real-time digital signal processing architecture to generate and process multifrequency signals for eddy currents testing is described in this paper. This architecture is implemented on a dedicated instrument whose processing core is a field-programmable gate array. Stimulus generation is achieved using direct digital synthesis with some important improvements to reduce spurious frequency components. An in-phase and in-quadrature demodulation scheme is implemented to estimate the real and imaginary parts of the probe output signal. A cascaded integrator comb decimator is used to lower the sampling frequency and then allow narrowband filtering with low resources. The proposed architecture is able to generate and process the stimulus and input data at 125 MSamples/s and to estimate the input data components at 1.25 MSamples/s rate for eddy-currents with multiple simultaneous testing frequencies between 2.5 kHz and 10 MHz. Experimental validation is performed using a set of synthetic defects and two stimulus with different spectral composition.

Improved Magnetic Tunnel Junctions Design for the Detection of Superficial Defects by Eddy Currents Testing

In the last decade, magnetoresistive sensors attracted great interest for integration in eddy current-based non-destructive testing due to their high sensitivity and signal to noise ratio in a large range of frequencies (from dc to hundreds of megahertz). In this paper, a sensor composed of several magnetic tunnel junction (MTJ) elements in series is optimized and included in a custom probe configuration for the detection of superficial defects. Since the signal magnetic fields are very low, a finite element modeling simulation was supporting the sensor design optimization. The MTJ chips were microfabricated, assembled with the excitation coils and used in experimental measurements of defects 400 μm wide and 500 μm deep. The experimental results obtained showed very good agreement with the simulations.

Real-time processing of multi-frequency eddy currents testing signals

In this paper, a real-time Digital Signal Processing (DSP) architecture is proposed to generate and process multi-frequency signals for eddy currents testing. This architecture was implemented on a dedicated instrument whose processing core is a Field-Programmable Gate Array (FPGA) for DSP purposes. Stimulus generation is achieved using Direct Digital Synthesis (DDS) with some improvements to remove spurious frequency components. An In-phase and Quadrature (IQ) demodulation scheme is implemented to estimate the real and imaginary part of the probes output signals. A Cascaded Integrator Comb (CIC) decimator is used to lower the sampling frequency allowing narrowband IIR filtering using low resources. The proposed architecture is able to generate and process the stimulus and input data at 125 MSamples/s and to estimate the input data components at 1.25 MSamples/s rate for frequencies between 50 kHz and 10 MHz.

Eddy currents testing defect characterization based on non-linear regressions and artificial neural networks

Feature extraction and defect parameters estimation from eddy current testing data has received special attention in the last years. Principal component analysis, wavelet decomposition and Fourier descriptors are some of the tools used for feature extraction. Particular interest is devoted to using artificial neural networks to perform parameters estimation and profile reconstruction of defects. This work reports the use of non-linear regressions for feature extraction based on the modeling of the measured response by a set of additive Gaussians and artificial neural networks to estimate the width and depth of defects.

INNOVATIVE EDDY CURRENT PROBE FOR MICRO DEFECTS

This paper reports the development of an innovative eddy current (EC) probe, and its application to micro‐defects on the root of the Friction Stir Welding (FSW). The new EC probe presents innovative concept issues, allowing 3D induced current in the material, and a lift‐off independence. Validation experiments were performed on aluminium alloys processed by FSW. The results clearly show that the new EC probe is able to detect and sizing surface defects about 60 microns depth.

Embedded measurement system for non-destructive testing using new eddy currents planar array probe

This paper describes the development, implementation and characterization of an embedded measurement system for non-destructive testing using a new eddy currents planar probe array. The probe has multiple, independent excitation driver traces as well as multiple sensing coils placed between the excitation traces in a matrix like configuration. The measurement system is based on a FPGA that controls which excitation traces are used and which sensitive coils are measured either in a differential or absolute configuration. FEM simulations validate the probe configuration and measurement results demonstrate the system operation.

Portable instrument for eddy currents Non-Destructive Testing based on heterodyning techniques

In this work an eddy current testing instrument employing heterodyning based measurements is presented. The instrument is composed of a portable computer and a dedicated electronic measurement circuitry to operate a specific type of probe. A DDS waveform generator and a transconductance amplifier are used to drive the probe with high amplitude currents and frequency up to 10 MHz. The probe output voltage is amplified up to 60 dB and down-converted to a lower intermediary frequency. The signal is converted into the digital domain and processed to compute the real and imaginary parts of the probe output voltage. The developed dedicated electronics were installed inside a rugged laptop computer to form a standalone instrument with post-processing and visualization capabilities. Electrical characterization and application results using the instrument and a particular eddy current probe are presented.