H. Geirinhas Ramos

Detection and characterization of defects using GMR probes and artificial neural networks

This work presents an eddy-current testing system based on a giant magnetoresistive (GMR) sensing device. Non-destructive tests in aluminum plates are applied in order to extract information about possible defects: cracks, holes and other mechanical damages. Eddy-current testing (ECT) presents major benefits such as low cost, high checking speed, robustness and high sensitivity to large classes of defects. Coil based architecture probes or coil-magnetoresistive probes are usually used in ECT. In our application the GMR sensor is used to detect a magnetic field component parallel to a plate surface, when an excitation field perpendicular to the plate is imposed. A neural network processing architecture, including a multilayer perceptron and a competitive neural network, is used to classify defects using the output amplitude of the eddy-current probe (ECP) and its operation frequency. The crack detection, classification and estimation of the geometrical characteristics, for different classes of defects, are described in the paper.

Velocity induced eddy currents technique to inspect cracks in moving conducting media

This paper presents a new nondestructive testing technique based on velocity induced eddy currents. When a time constant magnetic field moves along a conductive material velocity induced eddy currents are generated in the media. The technique developed within this work uses these eddy currents in the same way as any eddy current testing method where the flow pattern disturbed by the existence of defects creates variations of the secondary magnetic field that can be assessed by a magnetic field sensor. This magnetic field provides the information about the defects. Within this paper the magnetic field parameters are measured using “giant” magnetoresistors (GMR). As far as the authors know this is an original method, that has not yet been reported in the literature, and presents undoubted advantages when the material to be tested is in motion relative to the test sensor (like when inspecting a rail track with a bogie) because the sensitivity of the method increases with speed. To validate the results simulations of the actual testing conditions are also presented using a commercial finite element model.

Image analysis for crack detection

Evaluation of cracks using eddy currents (ECT) is one of the most used methods for non destructive testing (NDT). However, this technique still needs to be improved. One of the problems is related to the fact that eddy currents tests are so sensitive that a reproducible background noise signal is always present mitigating the informative data. This paper presents new developments in the area of signal processing that promise significant improvements to an accurate data interpretation. The steps carried out in the processing algorithm are illustrated using the experimental data acquired in the detection of a crack in an aluminum plate.

Software environments for the implementation of virtual instrumentation

In this paper the authors present and discuss in detail the several software platforms that are nowadays used to implement virtual instrumentation. It is aimed at the systematization of the various classes of software in order to propose some guidelines for the selection of the right packages for the application in mind. Besides, as the performance of any PC based instrument depends not only on the hardware and software used but also, on their level of compatibility, some special attention is paid to some non-trivial aspects related to the choice of hardware/software.In this paper the authors present and discuss in detail the several software platforms that are nowadays used to implement virtual instrumentation. It is aimed at the systematization of the various classes of software in order to propose some guidelines for the selection of the right packages for the application in mind. Besides, as the performance of any PC based instrument depends not only on the hardware and software used but also, on their level of compatibility, some special attention is paid to some non-trivial aspects related to the choice of hardware/software.

Impact of virtual instrumentation in teaching automated measurement systems

This paper is a contribution from the authors to the academic formation in automated measuring systems of postgraduate electrical engineering students. A methodology for teaching virtual instrumentation in the context of a course on automated measurement systems is proposed.This paper is a contribution from the authors to the academic formation in automated measuring systems of postgraduate electrical engineering students. A methodology for teaching virtual instrumentation in the context of a course on automated measurement systems is proposed.

Current around a crack in an aluminum plate under nondestructive evaluation inspection

This paper presents an inversion problem algorithm applied in eddy current inspection using a sinusoidal magnetic field with fixed-amplitude, in a metallic plate sample with machined crack defects. The uniform magnetic field is obtained in a wide area around the probe due to the building technique of the excitation coil. The magnetic sensor in use is a giant magnetoresistance (GMR) with high sensibility. It measures the amplitude and phase of the magnetic field originated from eddy currents. A perturbation occurs in the measured magnetic field in the proximity of a crack defect. The current density is then determined around the crack defect through an inversion problem algorithm. The applied method consists in determining the transformation kernel and applying a Tikhonov regularization algorithm. With this data it is possible to obtain information about the defect geometrical characteristics.

A two-dimensional vector model of ferromagnetic hysteresis

Abstract A new two-dimensional vector model of ferromagnetic hysteresis, which we have designated the UB model, is presented in this paper. Two types of entities constitute the model: u niaxial particles, having an energy per unit volume according to the Stoner-Wohlfarth (SW) theory, and b iaxial particles that differ from the former in the magnetocrystalline energy term. The equilibrium distribution of magnetisation corresponds to an energy minimum for all particles that constitute the model. A description of the model identification is also reported. The process, although essentially different, bears some resemblance to the one used to identify the simplified Preisach model [1] (all the Preisach surface contents concentrated along the two axes of the plane) and requires knowledge of the major hysteresis loops of the specimen under study along the x and y directions. The relationship between experimental values and model parameters is obtained taking into consideration the link that is established between an SW-type particle and Preisach entities. The paper ends with examples where UB model performances are displayed.

A rotating field automated measurement system for the characterization of ferromagnetic materials

This paper presents an automated measurement system which uses general purpose IEEE‐488 controlled devices for the characterization, according to a Preisach‐type model, of ferromagnetic materials subjected to magnetic fields of variable amplitude and direction. For that purpose, the measurement of component values in two orthogonal directions of both magnetic field and magnetic induction are required. The magnetic induction in the sample is obtained by using analog integration of the resulting induced electromotive forces in two test coils when the magnetizing current is changed. The magnetic field inside the sample is obtained by measuring the electromotive forces induced in two air core sensing coils placed on both surfaces of the sample. Besides hardware and software details, this paper presents results of tests conducted on ferromagnetic materials commonly used in electrical apparatus.

Low-cost stand-alone system for eddy current testing of metallic non-ferromagnetic plates

This paper describes a nondestructive testing equipment designed with the purpose of being low-cost and able to work in a stand-alone way. The main feature is related to the integration of an eddy-current probe using a giant magnetoresistance (GMR) sensor with a computer mouse to be used as a positioning system. The system was constructed and tested. The results that were obtained allow the graphical representation of the acquired signals as a function of position, which is considered an improvement in relation to the systems that only represent the defect signatures.

Taxonomic problems on ADC characterisation

In this paper the authors emphasise the need for the development of a new standard with parameter definition and measurement methods for ADC characterisation. This new standard should establish a complete taxonomy. Experimental and numerical simulation results of ADC dynamic characterisation are presented. They show that different values of the integral nonlinearity vector are obtained for the same converter by using different definitions currently in use for this parameter. Regarding the effective number of bits, different results obtained by using the histogram and FFT methods are presented. In both cases results as a function of the frequency show that the domain of validity of the parameters must be specified.In this paper the authors emphasise the need for the development of a new standard with parameter definition and measurement methods for ADC characterisation. This new standard should establish a complete taxonomy. Experimental and numerical simulation results of ADC dynamic characterisation are presented. They show that different values of the integral nonlinearity vector are obtained for the same converter by using different definitions currently in use for this parameter. Regarding the effective number of bits, different results obtained by using the histogram and FFT methods are presented. In both cases results as a function of the frequency show that the domain of validity of the parameters must be specified.