Thickness Measurement of Titanium-Alloy Sheets Based on the Resistance-Frequency Eddy Current Method

Eddy current (EC) testing is a common method of thickness measurement because of its noncontact, high-efficiency, and low-cost advantages. The impedance linear calibration method is a conventional EC method frequently employed for measuring the thickness of copper sheets. It utilizes the approximate linear relationship between the ratio of the imaginary-to-real parts of the impedance change and the thickness. However, the conductivity of titanium (Ti)-alloy sheets is significantly lower than that of copper with a range of only 0.5-2.5 MS/m. Impedance linear calibration methods exhibit reduced linearity when applied to materials with low conductivity, leading to larger measurement errors. In this article, a resistance-frequency EC method for the thickness measurement of Ti-alloy sheets is proposed and a corresponding EC measurement system is constructed. The proposed method uses an LC resonator as a measurement probe, where the ratio of the change in resonant frequency to that of the resonant resistance exhibits a good linear relationship with the thickness of the Ti-alloy sheet. The linearity of the proposed method is verified through theoretical calculations and experiments, showing significant improvements over the impedance linear calibration method. The experimental results indicate that the proposed method offers a higher measurement accuracy, with the relative error remaining within 2%, which is well below the permissible error of the current industry standard. In addition, the proposed method involves a simpler signal extraction and circuit design, making it easy to implement in online applications. In short, this study presents a fast and accurate method for measuring the thickness of Ti-alloy sheets, which can be of great value in practical applications.