Study Of The Key Technology Of Visual Measurement And Its Applications In Automatic Measurement

With the rapid development of computer vision technology and photovoltaic technology, computer vision based precision measurement technology has been well developed and widely applied in various fields. As a non-contact measurement approach, computer vision measurement technique is playing a more and more significant role.The thesis has deeply investigated a series of relevant key topics, including system scheme of precision measurement based on the computer vision, design of software and hardware system, acquisition and processing of the measurement images, sub-pixel positioning, calibration of the measurement system, the key technology of focusing. Furthermore, the built computer vision measurement system is successfully applied in the automatic measurement system for injector base.The injector base, fabricated by metal casting and stamping, is an important part in automobile oil circuits. Such parts are mainly used for welding assembly with cylinder parts. In order to ensure the accuracy of assembly, the inside dimension of the parts are required to be measured accurately. The injector base displays as a bowl shape, and the inner surface is a strong reflecting surface. The edge of the base is connected with the circular bevel, and the bottom surface is linked with the curved face of the base. The projection of the inner surface edge is employed as the measurement object. With the built automatic computer vision precise measurement system, the accuracy and speed of the part measurement are greatly improved.The main contributions of the thesis include:1. An automatic computer-vision based system for the precise measurement of the injector base inner dimension is designed and built. The relevant designs of computer-vision system, software system, controlling and actuating system are completed. The detection of strong reflecting surface is always a challenging topic in the field of computer vision based measurement technique. Through the analysis on the reflecting feature of the object part, we design a LED back lighting source with uniform scattering, hence, the edge in the measurement image is greatly enhanced. Consequently, the vision measurement of the inner surface of the part is completed with a higher accuracy. Additionally, the automatic charging of the parts, automatic positioning, and automatic sorting mechanism are design to realize a genuine automatic measurement and further enhance the efficiency of measurement.2. The imaging processing algorithms for the computer vision based measurement system are studied. The preprocessing and threshold segmentation algorithms are investigated. Through the analysis of common edge detection algorithms and the simple edge, high-contrast characteristics of the measurement image, we propose a novel edge detection algorithm based on the edge segmentation and the chain code tracking. In addition, several sub-pixel algorithms, including the conic curve fitting, polynomial fitting, space mement, grey moment, are studied both theoretically and experimentally.3. The calibration technique for the computer vision based measurement system is studied. The mathematic model of camera imaging is analyzed, and the appropriate plane grid target for the experiment is designed. Furthermore, a rapid adjustment method based on the image measurement plane and the verticality of optical axis is proposed. According to the existing calibration approach for the measurement system, we present a novel method using linear calibration for different zones. The method put the measurement image into several zones so that the linear calibration can work for different zones respectively. As a result, the calibration errors are minimized.4. The focusing technique for the measurement system is experimentally studied. The common evaluation functions for focusing are experimentally compared. And the influences resulting from the focusing region, number of edge point, and the intensity of light source are analyzed. And the influence on the calibration result due to the accuracy of focusing is experimentally analyzed. A focusing evaluation method based on the characteristic edge tracking is propose, with the result that the evaluation function can be correctly focusing and positioning even when the height of the edge in the part measurement image is not coplanar. Hence, the focusing accuracy is improved to a higher degree.5. The characteristic parameters of the curve fitting method are analyzed. And the measurement system is experimentally studied and the relevant errors are investigated.