Design Of Dimensional Measurement System For Pressure Vessel Head Based On Two-way Structured Light

Pressure vessel head is widely used in petrochemical,pharmaceutical and food industries.As an end cap on pressure vessel,its main function is to seal,and its size will make the sealing degrade if it does not meet the standard.However,the dimensional measurement of pressure vessel head still adopts traditional contact manual measurement method such as the full sample inspection method of head and the coordinate measuring instrument for head inspection.There is no efficient and accurate non-contact measurement method.In this study,a two-way structured light vision dimensional measurement system for head inspection was designed to achieve accurate measurement of depth,diameter,thickness and end roundness for pressure vessel heads.The main contents and conclusions of this study are:（1）A two-way structured light head size inspection system was designed.The two-way structured light head dimensional inspection system designed in this study adopts a face structured light vision measurement method.The system consists of a projector and two industrial cameras,where the pressure vessel head is placed in an attitude with the opening facing the measurement system,and the structured light pattern is projected vertically on it.The head images are collected by the two cameras separately,and the depth and dimensional information is obtained by computer analysis of the collected images and the restoration of its 3D shape with algorithms.The hardware selection of the system was completed,based on which the 0.05mm^-1frequency grating structured light encoding pattern was selected for experiments.（2）Image pre-processing algorithms and system calibration methods were studied.The camera and projector of the measurement system were calibrated,and the internal parameters of the camera and projector and their corresponding rotation and translation matrices were derived.Considering the complex measurement environment on the factory floor and the timeliness of field measurements,the ROI（Region of Interests）of the acquired images was automatically extracted in this study to reduce the pixel points and improve the image processing speed.The denoising ability of three filtering algorithms for the common noise in the field measurement environment was studied.The denoising ability of the three filters for the common Gaussian,pretzel and scattered noise in the field measurement was visually compared by the standard sphere 3D reconstruction simulation experiment,and the filtering method suitable for the field measurement of the head was selected.For the problem of poor quality of field acquisition images,the contrast enhancement methods with gamma correction and histogram equalization to improve the quality of acquisition images were used.（3）The 3D reconstruction and dimensional measurement of the head were realized.The reconstruction accuracy of the standard N-step phase shift method with 8 phase shift steps was compared and the 5-step phase shift method was selected according to the convenience and accuracy requirements of the system.The filtered dual-view point clouds were stitched using the ICP（Iterative Closest Point）algorithm,and the information on the internal and external diameters of the head was obtained by the least-squares fitting of the end face point clouds,verification of the system accuracy using a standard sphere,the data shows that the maximum absolute error of the system is within 0.9324%.The research results show that the dimensional measurement relative error of this system was reduced by 2.7423%compare with that of binocular stereo vision,and the average absolute error of the head dimensional measurement system is within 1.4 mm after repeated experimental measurements.Finally,the system software development was completed,which functions include image pre-processing,3D reconstruction,point cloud screening,point cloud stitching and dimensional calculation.（4）Field measurement experiments were conducted.The results of the field experiments show that the two-way structured optical dimensional measurement system designed for the detection of pressure vessel head dimensions can be applied to the complex environment in the factory floor and has the characteristics of ease of operation and accuracy of the obtained data,which can meet the requirements of national standard size evaluation of pressure vessel head.The measurement system designed by this system collects the information of the object to be measured from dual viewpoints,eliminating the serious influence of shadow noise caused by surface structure light projection on the 3D reconstruction effect of the image of the head,and keeping the absolute error of field measurement within 2.0 mm,which shows that this system has a good application prospect.