Simple Lock-In Technique for Thickness Measurement of Metallic Plates

Eddy-current techniques are largely employed for measurements of thickness and conductivity of metallic plate, and layers. In these measurements, an alternating voltage is applied to a magnetizing coil placed over the metallic plate and its impedance is measured as a function of the frequency. The eddy currents induced into the plate produce a magnetic field that couples with the coil, and the resulting impedance depends on the metal plate conductance and thickness. It has been found that the phase of the inductance is an especially useful parameter. In this paper, we propose a lock-in method to extract the phase signature and to relate it to the plate conductance and thickness and to the thickness of possible insulating coatings. The equipment needed for the measurement is simple and inexpensive if compared to impedance meters usually employed.