Research On Calibration Method Of Handheld Target Coordinate Measurement System

The target imaging coordinate measuring method is a good solution to the application limitation introduced by the complex shape or the surface material property of the measured object.The target imaging coordinate measuring system has the advantages of good flexibility,small size,simple operation and other characteristics.So it has been widely used in the measurement field.The system mainly consists of a high-resolution camera(long focal length),hand-held plane coding target and computer system with corresponding software.Face to the high measurement accuracy and fast measurement requirement under variable on-site measurement environment,there are some important aspects:how to calibrate the camera internal parameters efficiently and accurately,how to calibrate hand-held target probe on real-time and how to extract the center coordinates of target point rapidly and accurately.This article has done the following work:(1)The development status of the hand-held coordinate measuring system at home and abroad is introduced,and the current technology related to system calibration and its problems is analyzed.Based on this,the system calibration scheme of this paper is designed.(2)The camera parameter calibration method of a narrow field of view is researched.This paper analyzes the camera imaging model and adopts a camera calibration method based on virtual stereo target.The idea of reasonably designing the number and position distribution of the target points before camera calibration is proposed for resolving the problems of inefficiency and heavy workload.Based on this idea,the camera calibration is realized with efficiently and accurately in the practical experiment.This method has practical application value and is applicable to camera calibration without target stitching.(3)On-site calibration of the probe tip center of hand-held planar target is researched.The coordinate correction model of the hand-held target probe tip center in the target coordinate system is established.A two-step calibration method for the probe tip center of the planar target is proposed.The probe tip calibration can be realized effectively with the help of two groups of different position of target image information and a reference conical hole,and the feasibility of the method is validated by simulation and practical experiment.(4)In view of the distinctive feature points of the target image,a center location algorithm with sub-pixel edge detection by the gray moment is proposed to locate the image target spot.The region of interest is segmented by the Otsu,and the region of interest is expanded by the size of the convolution template used in the sub-pixel edge detection by the gray moment,and the center coordinates of target points can be calculated by the least squares ellipse fitting with the edge of the obtained sub-pixel points.The method can improve the operational efficiency of the algorithm while ensure the center location accuracy.(5)The calibration software for hand-held target coordinate measuring system is designed.An interactive human-machine interface is provided and the functions of each module completed.The image processing,the calibration of camera parameters and the calibration of target probe tip are encapsulated to facilitate the subsequent calibration and measurement.