A Study on 3D Scan-Based Robot Arm Control Approach for Pulse-Echo Laser Ultrasonic Testing of Curved Composite Structures

Abstract Recent aircraft designs involve increasing use of composite materials in aircraft structures, requiring efficient non-destructive testing (NDT) to ensure structural integrity. Pulse-echo (PE) laser ultrasonic testing (LUT) is an NDT technique for inspecting subsurface defects in various structures. A Q-switched laser generates the excitation laser pulses, while a laser Doppler vibrometer (LDV) functions as the sensing device. Internal conditions across the specimen wall thickness are assessed by the propagation of direct waves and back-wall echo signals. Maintaining a constant stand-off distance and a perpendicular incident angle between the sensing laser beam and the inspection surface is essential for achieving a high signal-to-noise ratio (SNR). To achieve these requirements, a 6-axis robot arm carrying the specimen is controlled to move along the positions and rotations derived from the inspection surface shape. In this study, a handheld 3D scanner is employed to capture the inspection surface and produce a mesh model. Geometric data from the mesh is then used to generate scan grids in a raster pattern, and the surface normal vectors at the grid points are employed to determine the rotations. The robot arm follows a raster scan path generated from 3D scan data, emitting laser trigger signals that enable the execution of PE LUT. This 3D scan-based approach for path generation enhances the applicability of robotic PE LUT in inspecting curved composite structures.