| Vision-based portable 3D coordinate measuring system has been an emerging hot topic in the measuring research area in recent years. Compared with traditional 3D coordinate measuring machines (CMM), vision-based coordinate measuring system has many advantages, such as light weight, good portability, flexible construction, simplicity of measurement and large measuring space. It is very suitable for on-site measurement and large-scale measurement in industry, and can be widely used in automotive, aviation, aerospace and other large equipment manufacturing industry. Some commercial products of vision-based CMM have emerged abroad. However, the core techniques are monopolized. Therefore, it's of great significance to do research in this field and it will have broad application prospects.A concise portable coordinate measuring system based on binocular stereo vision is presented in this thesis. The system is mainly composed of two CCD cameras (i.e. the binaocular stereo camera), a computer, and a hand-held measuring pen with circular features which are easy to recognize. The CCD cameras supervise the circular features while the measuring ball of the measuring pen touches the target point to be measured. The 3D coordinates of the measuring ball, and consequently that of the target point, are calculated by analyzing the two images of the measuring pen. The structural parameter of the measuring pen, and consequently the coordinate of the measuring ball in the measuring pen coordinate system, is an important factor that affects the accuracy of this system. This parameter will be changed when the probe is replaced.Therefore,the calibration of the measuring pen is a key issue in the vision-based 3D coordinate measuring system.Due to the limitation of cameras'field of view, for large size objects,all the target points can't be measured while the binocular stereo camera is put only in one position, therefore, measurement registration is essential for the vision-based coordinate measuring system.This thesis mainly focuses on the calibration of the measuring pen and the method of vision-based coordinate measurement. The main contributions of this thesis are summarized as follows:Firstly, a simple and effective method for the measuring pen calibaration is proposed. The calibration can be achieved with a conical bore.Put the measuring ball of the measuring pen into the conical bore and rotate the measuring pen, take stereo images of the measuring pen in a number of different locations, then the coordinates of the circular features and the measuring ball's center in the measuring pen coordinate system can be obtained according to the observation that the center location of the measuring ball keeps unchanged in the process.Secondly, the principle and method of single-position measurement is discussed, in which the relative position of the binocular stereo camera and the object remains unchanged. To overcome the limitation of the cameras'field of view, the binocular stereo camera is allowed to be put in different positions, and the measurement registration is achieved by sticking some markers, called global points, on the measured object.With the global points, the system can be used together with the 3D surface measuring system to take a comprehensive measurement of curves and surfaces.Thirdly, the methods for fitting basic geometric elements, including lines, planes, 3D circles, spheres and cylinders, have been researched and implemented in the developed system, so, this system has the functions of measuring the form and site tolerance.Based on the copyright reserved binocular stereo system Recreator, the software of vision-based coordinate measuring system is developed. In addition, the wireless measuring control is achieved by making use of a remote controller, so, one single operator can do the measurement and the measurement efficiency is improved. A series of experiments have been done with the developed system, the results of which show that the measurement accuracy and efficiency are good. |
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