XVII IMEKO World Congress Metrology in the 3rd Millennium June 22−27, 2003, Dubrovnik, Croatia A MEASUREMENT SYSTEM FOR INDUCTANCE TOMOGRAPHY ON CONDUCTIVE MATERIALS Andrea Bernieri, Luigi Ferrigno, Marco Laracca and Antonello Tamburrino D.A.E.I.M.I, University of Cassino, Cassino, Italy Abstract − The paper describes the realization of a measurement system for executing Eddy Current Non Destructive Testing (EC-NDT) to detect defects on conductive materials. The measurement method is based on the adoption of a useful parameter, the mutual impedance matrix, obtained using a suitable probe and analyzed by means of a novel low cost non-iterative inversion algorithm. In particular, the probe is constituted by a set of coils arranged in a two-dimensional array: during a measurement session, the mutual impedance matrix is obtained exciting one coil at time and capturing the voltage at the terminals of the other coils on the probe, at different excitation current frequencies. In the paper, the probe design, the inversion algorithm, the architecture of a useful measurement station, and the preliminary results obtained carrying out measurement sessions on a specimen with known defects, are reported. Keywords: non-destructive testing, eddy current testing, multi-sensor probe, digital signal processing, automatic measurement systems, inverse problems. 1. INTRODUCTION The online quality control of conductive material is a very important task in many industrial processes. This is particularly true in the case of manufacturing of materials for nuclear, aerospace, and similar application. In fact, the presence of a defect on the final product can occur even if the industrial process was correctly designed and controlled, thus giving rise to unacceptable risks and costs. Therefore, a complete non-destructive inspection on the entire production during the manufacturing process is strongly required. These reasons have lead to a rapid development of Non- Destructive Testing (NDT) techniques based on a number of different measurement principles, such as eddy current testing, magnetic particle testing, ultrasounds, thermo- graphy, radiography [1, 2]. Among these, electromagnetic techniques (E-NDT), and in particular Eddy Current (EC) based techniques, are still less widespread in industry, but are receiving a growing attention by the international scientific community thanks to their low cost. EC-NDT techniques are based on the analysis of the magnetic field generated by eddy currents induced in the material under test, and perturbed by the presence of defects. The authors are involved in this research field, realizing algorithms, measurement methods, probes and measurement systems [3-11]. In this paper, the design of a EC-NDT method using a two-dimensional probe and a new low-cost non-iterative algorithm is presented. In particular, the proposed method is based on the retrieval of the resistivity variations of the conductive material under test due to the presence of defects. To this aim, a probe constituted by a two-dimensional matrix of coils is realized to induce EC in the material under test and to pick-up the voltage signals due to the reaction field. Exciting one coil at time with a suitable excitation current and measuring the voltage signals at the terminal of other coils, and repeating this procedure for all the coils on the probe with different excitation current frequency values, it is possible to determine a particular parameter, the mutual impedance matrix [8,9], related to the resistivity spatial distribution on the material under test. A suitable algorithm is then adopted in order to reconstruct the defect characteristics starting from the mutual impedance matrix values measured during an investigation session. In the following, the principles of the proposed method are discussed, together with considerations carried out during the probe design phase. A preliminary sensitivity analysis, carried out in order to determine the probe characteristics able to assure a suitable defect identification also in presence of noise, is also detailed. Finally, with reference to a probe built-up on the basis of the achieved results, the architecture of a suitable measurement station is also reported. 2. THE MUTUAL IMPEDANCE MATRIX METHOD The EC techniques are based on the generation of eddy currents in the material under test by an external magnetic field, generated by a suitable excitation coil. A reaction field is produced by EC (see Fig. 1), whose characteristics depend on the specimen material (i.e. spatial values of the resistivity and magnetic permeability), and modifications are related to the presence, position and geometry of defects (cracks). Suitable sensors and measurement methods are then required to measure the reaction field modifications and then to determine the defect characteristics. In particular, many solutions are based on the use of suitable pick-up coils capable of sense reaction field variations due to spatial resistivity modifications. In this way, the impedance value: excitation up pick I V Z − = & Proceedings, XVII IMEKO World Congress, June 22 – 27, 2003, Dubrovnik, Croatia TC4