Deployment of contact-based ultrasonic thickness measurements using over-actuated UAVs

Watson, Robert J. and Pierce, S. Gareth and Kamel, Mina and Zhang, Dayi and MacLeod, Charles N. and Dobie, Gordon and Bolton, Gary and Dawood, Tariq and Nieto, Juan; Rizzo, Piervincenzo and Milazzo, Alberto, eds. (2021) Deployment of contact-based ultrasonic thickness measurements using over-actuated UAVs. In: European Workshop on Structural Health Monitoring. EWSHM 2020. Lecture Notes in Civil Engineering . Springer International Publishing AG, ITA, pp. 683-694. ISBN 978-3-030-64594-6 (

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Unmanned Aerial Vehicles (UAVs) are increasingly being utilized for the structural health assessment of on and off-shore structures. Visual inspection is the usual methodology for acquiring data from these structures, but there is often a need for contact based structural measurements, for example to assess local thickness on corroding structures. Conventional UAV platform dynamics have not traditionally allowed for such contact measurements. The limited dynamic control afforded by fixed plane rotor UAVs means that forward thrust (to apply contact forces for surface transduction) is only possible by tilting the whole platform, thus taking the UAV into a non-stationary state and limiting positional accuracy. An over-actuated UAV platform (with fully vectored thrust capability) may provide the required contact force for such thickness measurements whilst maintaining stable hovering next to the structure. The authors herein present a contact based ultrasonic thickness measurement technique, whereby an ultrasonic wheel probe deployed from a UAV was used to make single point and scanned measurements across a surface to provide a set of local thickness measurements. A 5 MHz, dry-coupled, dual-element, ultrasonic wheel probe is used to measure the thickness of an aluminum sample plate with thicknesses of 8.2 mm, 4.5 mm and 3.2 mm, and a precision stepped calibration block with size from 31.5 mm to 17.5 mm in steps of 1 mm, then steps of 0.1 mm down to 16.5 mm over a total length of 500 mm. The thickness resolution obtainable from the ultrasonic wheel probe was typically 0.1 mm, and the positional accuracy attained from the over-actuated deployment platform was 16.6 mm when performing single point measurements.