Study On Visual Alignment Technology For On-line Measurement

Visual inspection techniques have many advantages for on-line 3D measurement. For example, they can perform non-contact measurement with fast measuring speed, large measuring range, good flexibility and medium measurement precision, etc. In this thesis, an on-line visual alignment technique is developed to complement 100% on-line and non-contact measurement of the geometrical parameters like straightness and coaxiality of large objects. Some key issues relative to the visual inspection technique are studied in details and a visual alignment measuring system for measuring the central line straightness of seamless steel pipes is developed. Our experimental results illustrate that it is an effective 100% on-line measurement method for large scale objects. In the thesis, the main corresponding research topics can be summarized as follows:(1) The pinhole projective model of cameras in visual sensors is detailed. The lens distortions which affect the image quality taken by cameras and measurement accuracy are analyzed. The mathematic model of the sensor with structured light illumination that is used in the visual alignment technique is given.(2) The computational steps for calculating the spatial coordinates of a measured object by means of the visual alignment measuring system and the evaluation methods of its straightness are summarized.(3) The calibration methods of the intrinsic and extrinsic parameters of a camera are summarized, and the technical merits and demerits of the calibration methods are discussed. Two well-know camera calibration methods, direct linear transformation and radial alignment constraint method, are depicted in details. The calibration target configurations and their characteristics are introduced.(4) The global calibration methods of the alignment measuring system are developed. The technique of establishing a global 3D coordinate system for alignment measurement by means of two thedolites is presented. The coordinate transformation method from sensor coordinate system to the global coordinate system is given.(5) A practical scheme of alignment measurement system for seamless steel pipe is discussed and an experimental setup of the system is developed.