AbstractThis PhD project is a collaboration between Smart Light Devices, Ltd. in Aberdeen and Aberystwyth University on the development of such 3D laser scanners with an ultimate aim to inspect the underwater oil and gas pipes or structure. At the end of this project, a workable and full functional 3D laser scanner is to be developed. This PhD project puts a particular emphasis on the engineering and implementation of the scanner according to real applications’ requirements. Our 3D laser scanner is based on the principle of triangulation and its high accuracy over a short range scanning. Accurate 3D data can be obtained from a triangle between the scanner, camera lens, laser source, and the object being scanned. Once the distance between the scanner camera lens and
laser source (stereo baseline) is known and the laser projection angle can be measured by the goniometer, all the X, Y,Z coordinates of the object surface can be obtained through trigonometry.
This 3D laser scanner development involves a lot of issues and tasks including image noise removal, laser peak detection, corner detection, camera calibration and 3D reconstruction. These issues and tasks have been addressed, analysed and improved during the PhD period. Firstly, the Sparse Code Shrinkage (SCS) image de-noise is implemented, since it is one of the most suitable de-noising methods for our laser images with dark
background and white laser stripe. Secondly, there are already plenty of methods for corner and laser peak detection, it is necessary to compare and evaluate which is the most suitable for our 3D laser scanner. Thus, comparative studies are carried out and their results are presented in this thesis. Thirdly, our scanner is based on laser triangulation, in this case, laser projection angle α and baseline distance D from the centre of the camera lens to laser source plays a crucial role in 3D reconstruction. However, these two parameters are hard to measure directly, and there are no particular tools designed for this purpose. Thus, a new approach is proposed in this thesis to estimate them which combines camera calibration results with the precise linear stage. Fourthly, it is very expensive to customize an accurate positional pattern for camera calibration, due to budget limit, this pattern is printed by a printer or even painted on a paper or white board which is inaccurate and contains errors in absolute distance and location.
An iterative camera calibration method is proposed. It can compensate up to 10% error and the calibration parameters remain stable. Finally, in the underwater applications, the light travel angle is changed from water to air which makes the normal calibration method less accurate. Hence, a new approach is proposed to compensate between the estimate and real distance in 3D reconstruction with normal calibration parameters. Experimental
results show the proposed methods reduce the distance error in 3D down to
±0.2mm underwater. Overall, the developed scanning systems have been successfully applied in several real scanning and 3D modelling projects such as mooring chain, underwater pipeline surface and reducer. Positive feedback has been received from these projects, the scanning results satisfy the resolution and accuracy requirements.
|Date of Award
|Yonghuai Liu (Supervisor)