This work is dedicated to the theoretical and experimental study of the utilization of the eddy current transducer with orthogonal coils [1] inflaw reconstruction by means of the multifrequency procedure.

The analytical solutions are presented for the operation of the eddy current transducer in which the two coils -that parallel to the inspected surface and that perpendicular on it-are connected in an impedance bridge [2]. Given the particular characteristics of this type of transducer, despite its large area resulting in the increase of the penetration depth, it has a good resolution in defect identification.

By using hierarchical finite element method for every frequency we used, the prediction of the control installation response is done for different type of discontinuities, solving thus the forward problem. Then the inverse problem is solved after the linearization on every frequency, by using the Levenberg-Marquardt procedure. This method gives the discontinuity map in different planes parallel with the surface, their position depending on the frequency. The precision in establishing the flaw position in plane depends on the transducer performances and the precision in knowing the transducer position during the sweeping, while that in the vertical direction is given by the ability in choosing the working frequencies.

In order to validate the reconstruction method, over the transducer signal a Gaussian noise with different amplitudes and dispersions is superposed.

The calculated data are compared with the experimental results referring to shape reconstruction for flaws machined in aluminium blocks. The geometrical shape of these volumic discontinuities is parallelepipedic.

A correct reconstruction of the machined discontinuities is noticed. REFERENCES

1. A. Savin, E. Radu, O. Mihalache, B. Bradu, A. Andreescu, R. Grimberg, "Transducer with Orthogonal Coils Used to Control Flat Products", The International Symposium on Nonlinear Electromagnetics Systems, Cardiff, Wales, UK, 17-20 September 1995.
   R. Grimberg, E. Radu, O. Mihalache, A. Savin, "The Response of the Eddy Current Transducer with Orthogonal Coils to Different Material Discontinuities", The International Symposium on Nonlinear Electromagnetics Systems, Cardiff, Wales, UK 17-20 September 1995