Manual ultrasonic inspection guided by fiducial positioning

Abstract

A portable, camera-based fiducial positioning approach intended for general contact NDT is presented and experimentally assessed, with phasedarray ultrasonic testing (PAUT) used as the demonstrator application. The positioning method employs inside-out tracking, with a camera rigidly mounted to the probe and a 3D fiducial reference object fixed in the inspection environment. The probe 6-DoF pose is obtained by estimating the camera pose and applying a pivot-based co-calibration to express the phased-array measurement in the fiducial frame. A holistic assessment is performed by (i) quantifying in-plane and out-of-plane translation repeatability and rotational repeatability over relevant working distances and viewing angles, (ii) characterising the camera-to-probe co-calibration accuracy, and (iii) propagating these contributions to an end-toend defect localisation error. Experiments show submillimeter translation repeatability within a practical operating range of 1.5–2.0 m, degrading to a few millimeters at∼3 m and for larger off-axis viewing angles. The pivot co-calibration achieves an RMS error of approximately 1.5 mm relative to a calibration jig. End-to-end validation on a GFRP plate demonstrates millimeter-scale defect localisation and sizing, with sub-millimetre in-plane shift relative to a reference C-scan. These results indicate that fiducial-based positioning provides accuracy sufficient for contact PAUT and constitutes a viable alternative to encoder-based tracking for portable NDT.