# Measurement of Large-Sized-Pipe Diameter Based on Stereo Vision
###### Abstract  
To address the lack of fast and high-precision in situ measurement of large-sized pipes in current industrial applications, a pipe-diameter-measurement method based on stereo vision is designed in this paper. By using multiple sets of binocular cameras to perform 3D reconstruction and integration of multiple laser markers projected on measured cross sections of pipes, the pipe diameter can be estimated. In this method, a measurement algorithm is adopted to enable automatic matching of feature points through affine distance transformation, and an optimized point-cloud-registration algorithm with normal-vector constraints is used to ensure measurement robustness. To verify the feasibility of the method, an experimental system was built under laboratory conditions, and three types of pipes with outer diameters from 285 mm to 325 mm were measured. The experimental results show that the relative error is within \(\pm\)0.570% and the maximum repeatability standard deviation is 0.551 mm. The experimental results basically meet industrial standards, and the proposed method therefore has good application prospects. large-sized pipe; 3D reconstruction; affine distance transformation; matching of feature points; point-cloud registration +
Footnote †: journal: Applied  
moving platform that moves the part in the measurement plane. For a larger measurement range with fewer sensors, some researchers tend to use a fixed laser rangefinder to measure roundness, straightness, or concentricity of rotating pipes [7; 8; 9; 10] or a rotary laser rangefinder to measure fixed pipes [11; 12]. Although a laser rangefinder has high precision, it is expensive and complicated in maintenance. Such shortcomings can be overcome with the most advanced vision technology. Scholars have proposed various solutions based on monocular vision for quickly measuring straightness or roundness of rotationally symmetrical workpieces. In these solutions, cameras are first used to acquire continuous images of a rotating workpiece. Next, image processing and modeling analysis are performed on the acquired images. Finally, a measurement is obtained [13; 14]. Based on a 3D model constructed from boundary curves or beelines of forgings captured in two images, [PERSON] [15] and [PERSON] [16] presented a similar measurement system with two high-resolution single-lens reflex cameras to compute the straightness of rotationally symmetric forgings. However, due to the weak surface texture of pipes, it is often hard to achieve the desired feature point matching. Some scholars have therefore applied the structured-light technique to pipe measurement [17; 18; 19]. Based on the homographic relationship between a projector and a camera used for measurement, the specific images projected on the surface of an entire pipe can be converted into three-dimensional spatial information. To obtain a larger measurement range, researchers have devised new schemes, such as mounting the line-structured light with a CCD (charge coupled device) on a servo motor and allowing movement and measurement along the axis of the pipe [20; 21], using an omni-directional laser and an omni-directional camera for measurement [22] and merging the pipe profile information obtained with multiple groups of structured lights [23; 24; 25]. There are also some schemes that combine laser markers with stereo vision. For example, [PERSON] [26] used intersections of laser stripes as feature points and extracted laser stripes using stereo vision. By contrast, more researchers tend to reconstruct laser stripes in a direct way [27; 28] and derive the outer diameters of pipes directly from the length of laser stripes, which has much room for improvement in measurement accuracy. In this paper, a large-sized-pipe-diameter-measurement method is designed based on stereo vision.