Pipe eccentricity measurement using laser triangulation

A pipe eccentricity measurement system based on laser triangulation is presented. The custom measurement head implements the Scheimpflug condition to ensure maximum working depth and integrates an FPGA to perform real time image processing at a rate of 200 frames per second. The eccentricity of the pipe is estimated by approximating the position of the upper tangent point of the pipe and used as a measure of its straightness. The correlation coefficients for the parameters r, x(0) and y(0) estimating from a least square approximation of a circle via the algebraic distance are theoretically derived, demonstrated using Monte-Carlo simulation and experimentally verified. It is shown that perturbation of the measurement, leads to less uncertainty in the estimated position of the tangent point than for the centre point. Harmonic filtering based on Elliptical Fourier Descriptors is used to filter the measurement data. Laboratory measurements show that the repeatability is in the order of 1 mu m. The accuracy of industrial measurements exhibits a systematic bias of -4 mu m and has a standard deviation of 2.6 mu m. Selected measurements from the production plant are presented to show that the deviation of the cross-section of the pipe from a perfect circle can be determined from the measured radius of curvature. (C) 2006 Elsevier B.V. All rights reserved.