Development of laser pulse echo ultrasonic inspection system based on a robot arm mounted LDV and an angular scan Q-switched laser for 3D objects

Abstract

This thesis details the development of a robotic laser pulse-echo ultrasonic system for non-contact inspection of 3-dimensional structures. The sub-surface defects are detected using the Bulk waves that travel through the thickness of the specimen. These waves are generated by a Q-switched laser and are sensed through a Laser Doppler Vibrometer (LDV). Both the lasers impinge the specimen at the same point on its surface allowing the system to inspect in a pulse-echo fashion. In this system, the inspected specimen remains stationary during the inspection and scanning is done by mechanical movement of the LDV and optical steering of Q-switched laser. The main objective of the system is to ensure optimal signal quality from the LDV even while inspecting a non-planar surface. In order to achieve this the LDV is mounted on a 6 degree of freedom robot arm. This robot arm moves the LDV in front of the structure in a raster scan pattern according to the shape of the specimen. The raster scan path remains in a plane parallel to the surface of structure if the inspected structure is planar

otherwise, the robot moves the LDV to adjust its distance and angle according to the orientation and distance of the structure. Thus, during the scan, distance between the LDV and point of measurement on the structure always remains constant and the angle of incidence of the laser from the LDV remains parallel to the normal of the inspected surface. During this scan, the system makes a grid of ultrasonic measurements to investigate sub-surface conditions of the structure. A Q-Switch laser pulse creates an ultrasonic signal at each of these measurements. This excitation laser however is not mounted on the robot arm, as there are no constraints for it to be at a certain distance or normal to the surface. Instead, the excitation laser is optically steered through a laser mirror scanner. This keeps the robot payload low thus making itself lighter with fewer constraints on its installation and power supply. This in turn allows installation on a moveable platform making the system capable of in-situ inspections. A 633nm Helium-Neon continuous wave laser with 2mW power is used by the LDV and a 532nm Q-switched Nd-YAG laser is used for excitation. A lightweight and portable industrial robot has been selected for the system. This industrial robot arm is designed for repetitive tasks that are programmed once at the start of commissioning in a production line-up. Certainly, it is not practical to program the scan path each time for a new inspection. Using the available programming modes and communication capabilities of the robot, the programming is done in a way to allow for on-spot scan-path updates without the need for any coding. The robot also does not support synchronization signal frequencies in access of 100Hz and further interpolation of the synchronization signal is necessary to achieve higher resolutions. An RGB-D range sensor is used to image the specimen in 3 - dimensions from its front side. This provides a quick, relatively accurate, cheap and lightweight solution for 3-d imaging of the specimen. The scan area delimitations are done on the colour image produced by the sensor. The scan-path for the robot arm is generated to cover the delimited area. The coordinate base for the generated scan-path is converted from sensor coordinate system to the robot base coordinate system. This scan-path is uploaded to the robot controller using a self-designed command interface utilizing TCP/IP communications. This scan path is stored in the robot controller memory and there is no need for re-programming of the controller for new scan-paths or inspections. The ultrasonic measurements are recorded at regular interval during the scan. This interval defines the resolution of the resulting scan grid and it can be chosen from a set of values prior to the start of inspection. This grid of measurement is stored in a 3 dimensional buffer in the PC and can be viewed in a frame –wise fashion or in a point –wise fashion. The system offers scan area selection of any shape allowing for minimum inspection times and resource usage. The system also supports automatic scan area detection based on specimen boundaries, which can be useful for automation of the scanning process. The system supports a scan area within the bounding box of 400mm x 400mm with an interval of 0.125 mm and a speed of 200mm/s.