Autonomous NDT – Mitigating Misalignment Effect of Inductively Coupled Ultrasonic Transducer Systems

Abstract

In the realm of non-destructive evaluation and structural health monitoring, an inductively coupled ultrasonic
transducer system (ICTS) is a permanently-installed measurement sensor system that benefits from its
lightweight, wireless, battery free, and low-profile design. A common application is measuring remaining wall
thickness. This system mainly consists of a probing unit and permanently attached sensors, a setup which is
particularly suitable for robotic data collection to reduce cost and time of inspection, especially for large
process plants with millions of thickness measurement locations.
A slight variation in measured thickness has been observed that is dependent on the misalignment between
the probing unit and the sensor. This is not ideal and increases the positioning requirement for robots to
collect repeatable thickness data. This research aims to mitigate the misalignment effect and improve the
measurement performance of an ICTS to detect ever smaller changes in absolute thickness of structures
consistently, without requiring perfect alignment of the system. Successful development of this study will
ease manual inspection and enable robotic interrogation with lower cost and complexity as the requirement
for accurate positioning is less onerous.
A thickness measurement correction algorithm is first developed by eliminating a system delay from the
measured first arrival time based on the measured cross-talk bandwidth through a correction curve, to allow
a more accurate thickness measurement to be calculated within the region of up to 20 mm lateral
misalignment and 10 mm vertical misalignment. This correction algorithm 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 equal to the true thickness to ±0.01 mm. It can
also unambiguously detect a thickness loss of 0.09 mm; without correction, an ICTS could fail to detect any
loss in this scenario.
Subsequently, it is realised that the correction curve is sensor-specific: each installed sensor requires a unique
correction curve, causing inconvenience for the thickness measurement correction algorithm to be applied
in the field. Through simulations and microscopy of the bonded piezoelectric transducer, it is confirmed that
varying adhesive couplant thicknesses lead to the change of correction curve between sensors. Adhesive
couplant thickness cannot be easily controlled in practice, hence a new mitigation method is required.
Finally, a geometric coil optimisation approach is proposed to achieve a misalignment-insensitive inductive
coupling. This is achieved by designing the outer diameter of the probing coil as smaller as possible than that
of the inner diameter of the sensor coil. Preliminary experimental results show that this approach has huge
potential to address the misalignment effect on an ICTS.

Date of Award	10 Dec 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Anthony J Croxford (Supervisor) & Paul D Wilcox (Supervisor)