Real-Time Thickness Measurement Using Resonant Eddy-Current Sensor

In this work, we propose a real-time noncontact thickness measurement system for conductive materials based on an inductor-capacitor (LC) resonant eddy-current sensor. For the aluminum, copper, stainless steel 304, and carbon steel samples of various thicknesses, the in-phase and quadrature components of the sensor output taken during liftoff scans are found to be a series of quadratic-like curves on the phase versus logarithmic amplitude diagram. These curves can be used to generate the standard phase-amplitude relations for thickness conversion. The phase-amplitude relations of the thickness between the values of standard samples are calculated by cubic spline interpolation. With the standard database of phase-amplitude relations, the noncontact thickness measurement was implemented in real time for the copper foil sample with an error of +/- 3% when the liftoff distance is less than 5 mm. The liftoff tolerance is as large as 100% of the probe diameter, indicating that the proposed method is useful for online quality inspection of conductor films on flexible substrates.