Artur Lopes Ribeiro

Inductive Probe for Flaw Detection in non-Magnetic Metallic Plates Using Eddy Currents

This paper describes the design of an inductive sensor and its application to detect the presence of structural flaws inside electrically conducting nonmagnetic plates. The principle of operation is based on the measurement of the induced voltages in two sensing coils. These voltages are related to the eddy currents that are generated inside the plates with perturbations due to the electrical conductivity inhomogeneities. The modeling work aims at determining the optimum operating frequencies and the resulting signal magnitudes.

A Four-Terminal Water-Quality-Monitoring Conductivity Sensor

In this paper, a new four-electrode sensor for water conductivity measurements is presented. In addition to the sensor itself, all signal conditioning is implemented together with signal processing of the sensor outputs to determine the water conductivity. The sensor is designed for conductivity measurements in the range from 50 mS/m up to 5 S/m through the correct placement of the four electrodes inside the tube where the water flows. The implemented prototype is capable of supplying the sensor with the necessary current at the measurement frequency, acquiring the sine signals across the voltage electrodes of the sensor and across a sampling impedance to determine the current. A temperature sensor is also included in the system to measure the water temperature and, thus, compensate the water-conductivity temperature dependence. The main advantages of the proposed conductivity sensor include a wide measurement range, an intrinsic capability to minimize errors caused by fouling and polarization effects, and an automatic compensation of conductivity measurements caused by temperature variations.

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

This paper uses a linear transformer model to investigate the effect of the lift-off on the results of the thickness measurement of nonferromagnetic metallic plates. The transformer model previews that the time derivative of the magnetization curves obtained for different gaps between the excitation coil and the plate should intercept in a single point when low magnetic coupling factors are considered. To assess the validity of the model, results are compared with experimental data obtained with a giant magnetoresistive sensor probe. For comparison, the sensor output voltage time derivative must be performed as well. The similarity of the theoretical model results and those obtain experimentally with pulsed excitation, confirms the correctness of the transformer approach.

NDT on Aluminum Aircraft Plates based on Eddy Current Sensing and Image Processing

Non-destructive testing (NDT) applied to aluminum aircraft plates refers to testing the plates without impairing its worth or functional properties. In the work a NDT system architecture based on the use of fully inductive probes with excitation and sensing coils and hybrid probe with an excitation coil and a magneto-resistance as sensing element is presented. Appropriate signal conditioning and signal acquisition modules are included in the system. Using an X-Y plotter the probe is moved over the aluminum plate under test and a set of eddy current images is obtained. Image conversions, image filtering, image segmentation with feature extraction and geometrical characteristics of the detected cracks determines the capability of the designed and implemented software for non destructive testing. A practical approach concerning the optimal filtering of the image for a better segmentation and feature extraction was included in the paper. Several results concerning the aluminum plate crack detection and geometrical characterization are included.

A novel uniform eddy current probe with GMR for non destructive testing applications

Defect detection in conductive plates represents an important issue. The work presents the usage of a novel uniform eddy current probe (UECP) architecture that includes a magnetometer sensor based on a set of giant magnetoresistances as part of a non-destructive testing (NDT) system developed using a virtual instrument system technology. A practical approach concerning the UECP design and implementation and experimental results obtained on cracks and non-uniformities of welding lines performed in aluminum plates are presented. A comparison between the proposed novel eddy current probe architecture and the GMR pancake eddy current probe is included in the paper.

A low-cost temperature controlled system to test and characterize sensors

Almost all parameters (physical and chemical) used for in-situ water quality monitoring of rivers and seas, are measured using sensors that have characteristics highly sensitive to temperature. The objective of this paper is to present the project, design and implementation of a low cost testing bath with automated temperature control for environmental monitoring sensors characterization. Besides the description of the overall testing system, the paper details the cooling and heating thermoelectric pump based on Peltier modules implemented for the temperature control of the liquid, the tests that were conducted in order to obtain its optimal performance, the set of sensors and their signal conditioning to measure the temperature and the digital proportional-integral-derivative (PID) algorithm implemented in LabVIEW to control the temperature of the bath. The developed testing apparatus proved to be an efficient tool to test and characterize sensors for in-situ water quality monitoring

Induction defectoscope based on uniform eddy current probe with GMRs

Defect detection in conductive plates represents an important issue. The present work proposes an induction defectoscope that includes a uniform eddy current probe with a rectangular excitation coil and a set of giant magnetoresistance sensors (GMR). The excitation current, the acquisition of the voltages delivered by the GMR and the signal processing of the acquired signal are performed by a real-time control and processing unit based on a TMS320C6713 digital signal processor (DSP). Different tests were carried out regarding the excitation coil position versus crack orientation and also regarding the GMR position inside the coil and the best response concerning the crack detection for a given aluminum plate specimen. Embedded software was developed using a NI LabVIEW DSP module including sinusoidal signal generation, amplitude and phase extraction using a sine-fitting algorithm and GUI for the induction defectoscope. Experimental results with probe characterization and detection of defects were included in the paper.

Eddy Current Testing of Conductive Materials

The objective of our work is to design and implement an automatic recognition system of mechanical wear out in a conductive non magnetic material. Especially in the flight industry, this is a very important issue as recognizing wear out in a material can avoid material failures. Nowadays this exploration needs highly skilled operators that using hand-operated eddy current testing (ECT) are able to recognize the flaws type and shape. With the introduction of a microprocessor based system the performance is enhanced by increasing the test speed while avoiding errors due to human failure like inexperience and inconsistency. In the present paper a computer based eddy current testing system is proposed. A description of the implemented prototype including some details on the digital processing techniques used and the measurements obtained for different artificial defects in an aluminum plate are presented.

Lift-Off Point of Intersection Feature in Transient Eddy-Current Oscillations Method to Detect Thickness Variation in Stainless Steel

Detection of corrosion and measurement of the thickness of stainless steel in industrial structures, such as pipelines, has been an important issue to avoid unwanted leakages. A recently proposed transient eddy-current oscillations (TECO) method has been used in this paper to measure thickness in non-magnetic stainless steel. The TECO method works based on the electromagnetic induction principle, as the conventional eddy-current testing method. An inductor (excitation coil) connected in parallel with a capacitor to generate damped oscillations is used to induce eddy currents into the test material beneath the coil. A Hall sensor, which is fixed at the bottom side and at the axial center of the excitation coil, detects the resultant magnetic field due to the induced eddy currents. Experiments were performed on the stainless steel specimens, having different thicknesses to test the potential of the TECO method. The results were analyzed using the fast Fourier transform. This paper proposes a lift-off point of intersection in the frequency domain that can be used to eliminate the false indications due to the lift-off variations during the thickness measurement of the test material. The results reveal that the proposed method can be applied to detect the thickness variations in metal structures covered with non-conductive paints or insulation coatings.

GMR versus differential coils in velocity induced eddy current testing

This paper presents a development on a new nondestructive testing method using eddy currents induced by velocity. This new method uses a constant magnetic field that attached to a moving media induces eddy currents in the conductive material to be tested. By measuring the opposing magnetic field generated by the eddy currents it is possible to obtain information regarding the presence of defects. Two different magnetic field sensors, GMR and differential pick-up coils, were used and compared in the detection of perpendicular components of the magnetic field created by disrupted eddy currents due to linear defects machined on an aluminum plate.