Vision-based Research On Diameter Measurement Of Spiral Welded Pipe

Spiral welded pipes have the advantages of lower production cost, higher strength and safety. They are the main pipes used in oil and gas pipeline in China. The steel tube production standards have stipulated the quality of steel pipe, which mainly stipulate geometric parameters including the permissible deflection of outer diameter of steel pipes, ovality, etc. Measuring system based on visual inspection has high precision, high efficiency and is easy to realize automation. It is benefit to improve the production quality significantly.In this paper, a visual inspection system with two CMOS industrial cameras and a linear laser is developed to measure the diameter of spiral welded pipes. Camera 1 is perpendicular to the laser plane and used to capture images of laser arc on the spiral welded pipe. It is calibrated by placing the calibration target overlaping with laser plane. Meanwhile, camera 2 captures images of the edge of spiral welded pipe and the laser line from top side.By analyzing the grayscale of these images, reasonable image processing algorithm is designed. The angle between spiral welded pipe and laser plane and actual size of the laser arc are obtained. Based on the angle, the laser arc is projected to the right plane, and the diameter of the pipe in this position can be obtained by least-squares circle fit.As the pipe rotates, diameters in different positions can be measured. The outer diameter of spiral welded pipes can be measured precisely. Meanwhile, based on the data of different positions, ovality and weld deformation of the pipe can be obtained.Using the software MATLAB, a mathematical model is established to analyze possible factors which affect measurement accuracy. According to the analysis, calibrated error and angle error have the most important effect upon the measurment. Measures are proposed to eliminate or reduce the error in the measurement and to realize the precise measurement of spiral welded pipe’s diameter.