Research On Technologies For Accuracy Improvement Of Light Pen Coordinate Measuring System With Single Camera

The light pen portable coordinate measuring system with single CCD camerabased on vision measurement is combined with probe detection technology intraditional coordinate measurement, which makes the focus of visual inspection fromobject surface to measuring instruments. It is an important representation of thedevelopment of coordinate measuring technology and has a very broad applicationprospects. Through processing the image of control points to determine the3Dcoordinate of detected point, the system has a set of advantages such as light andportable, large measuring scope, simple operation and establishment, and can measurelarge workpiece with medium accuracy in situ. However, the overall accuracy of oursystem is still inferior to the international advanced level. In this thesis, improvedalgorithms of three important techniques are described in detail.The main topics in this thesis are as follows:1. Research and developing status of portable coordinate measuring systemsespecially those with hand-hold probe are introduced.2. Structure and measuring principle of portable light pen coordinate measuringsystem are described. System error is analyzed for a3LED light pen visioncoordinate measuring system.3. Center extraction algorithm of control points is improved withmulti-threshold segmentation and average theory to higher the centerpositioning accuracy of control points. In addition, an adaptive medianfiltering method is proposed according to the actual situation.4. Research on sub-regional camera calibration technique with moving lighttarget is studied. The image plane of camera is divided into several regionsand improved two-stage camera calibration algorithm by Tsai is adopted toachieve better calibration accuracy. A round light target moves with theCoordinate Measuring Machine (CMM) to form a set of accurate spacialpoints.5. Based on the previous research, a new object function is establishedaccording to the actual situation to estimating the center position of probe tip. We optimize the measurement process combined average theory to make theconvergence speed faster.6. The measuring precision of light pen portable coordinate measuring systemwith single camera has been compared with the Hexagon Global CMM.Experiments show that in a range of2m, the stability precision can reach0.005mm,0.005mm,0.039mm in x, y, z axis respectively, and the maximumof distance measuring errors in the direction of x, y, z axis are0.039mm,0.011mm and0.080mm. Both are increased by an order of magnitude with agreat improvement. The sources of the measuring error are discussed afterthis improvement and effective solutions are suggested in two aspects ofhardware and algorithm.