Robotic laser sensing and laser mirror excitation for pulse-echo scanning inspection of fixed composite structures with non-planar geometries

Abstract

This paper proposes a system for a laser ultrasonic inspection of fixed non-planar composite structures. A pulseecho configuration is used by which both the sensing and excitation lasers hit the same point on the structure, utilizing bulk waves for a through-the-thickness defect evaluation. A high signal-to-noise ratio is ensured by mounting a laser Doppler vibrometer (LDV), used as the sensing laser, on a 6 degree-of-freedom robot arm, thus allowing the correct distance and angle of the laser incident on the inspected structure to be maintained throughout the scan. The excitation laser is steered separately through a laser mirror scanner (LMS) to keep the robot payload sufficiently low, allowing the use of a lightweight portable robot arm. This robot arm can be mounted on a moveable platform suitable for in-situ inspections. An RGB-D camera is used for imaging the structure in front of the system, and a scan area with the required size can be delimited on the image produced by the camera. A scan path for the robot arm and laser mirror scanner is automatically generated by keeping the shape and distance of the structure within the delimited scan area. A graphical user interface allows the control of all individual blocks and contains the necessary algorithms for image capture of a specimen, geometry calculations, scan-path generation, signal acquisition, signal processing, and display of the results. This is a multithreaded software, ensuring a fast calculation of the structural geometry and scan path prior to the inspection and real-time updating of the results during the inspection. Composite specimens consisting of radial and conic sections with subsurface defects were inspected using the system. The test results show that the system is extremely reliable in detecting defects, particularly in the non-planar parts of the specimen.