Thickness measurement optimisation for permanently installed inductively coupled ultrasonic transducer systems

An inductively coupled ultrasonic transducer system (ICTS) is a lightweight, wireless, battery-free, and low-profile (0.5 mm thick) permanently installed sensor system for measuring remaining wall thickness at fixed locations for structural health monitoring. In an ICTS, a slight variation in measured thickness has been observed that is dependent on the misalignment between the handheld probing unit and the permanently attached sensor. A correction algorithm has been developed to improve the ultimate thickness measurement performance of ICTS even with misalignment. The algorithm eliminates a system delay from the measured first arrival time based on the measured bandwidth of the cross-talk signal. Crucially, the algorithm does not require identification of the second back-wall echo as this is not always possible if the back surface of the structure is badly corroded. The correction algorithm has successfully improved the thickness measurement precision from 0.19 mm to 0.07 mm. A higher accuracy of thickness measurement is also achieved, with the mean of the corrected thicknesses equals to the true thickness to ±0.01 mm. The algorithm also enabled the detection of a thickness loss of 0.09 mm. The improvement on thickness measurement performance is important for manual inspection, and future robotic interrogation which will become more viable as up to 20 mm lateral misalignment and 10 mm vertical misalignment can be accommodated.