A method for conductivity measurement through eddy current testing and closed-loop feedforward control

Metallic plates are widely used in aerospace, transportation, and the chemical industry. Conductivity is a significant physical property of metal materials, and material aging and corrosion can be detected in timely to ensure the safe and stable operation of industrial equipment and systems through monitoring variation of conductivity. In order to achieve high-precision conductivity measurement, this paper innovatively investigated an eddy current testing (ECT) method incorporating a feedforward Proportion-Integration-Differentiation (PID) controller. Specifically, a closed-loop PID controller is introduced to act on classical Dodd-Deeds analytical model. Conductivity can be accurately inverted by continuously reducing the deviation between the output of the analytical model and actual measurements. Considering the lift-off fluctuation affects the conductivity measurement accuracy, a feedforward controller is designed to reduce the lift-off distance variation, and the transfer function of the feedforward controller can be obtained by theoretical deduction. Experiments are also conducted to verify the proposed method. Results indicate the maximum relative error of conductivity measurement remains merely 1.64 % across the 3 mm lift-off range, demonstrating the efficiency and reliability of the proposed method.