Effect of shielding of eddy current probes on the sensitivity for sweep frequency measurements

Abstract

Eddy current measurements are prone to edge effects when measurements are carried out near a sample boundary. In order to reduce these edge effects, typically ferrite shields are used which constrain the magnetic field within the shield volume. However, their effects on sensitivity towards changesin conductivity for Sweep Frequency Eddy Current (SFEC) measurements have not been investigated. A 2D axisymmetric Finite Element Model (FEM) has been developed to predict the frequency response of impedance of SFEC probes. The model predicted that the addition of shields minimized edge effect but with reduced sensitivity. A simulation study is carried out to optimize the probe design to increase the sensitivity of shielded ferrite probes. Based on these results, a probe is constructed with a custom ferrite shield and its shielding efficiency and probe sensitivity are compared experimentally with those for the probe with conventional ferrite shields.