The Research On 3D Optical Measurement Techniques For Complex Shape

Reverse engineering (RE) is a new subject and technology coming up with the development of computer science and data digitization method. It helps to change the design mode from drafts to models in the conventional CAD systems. It is a novel approach for rapid prototype manufacture, and the first step in reverse engineering to acquire physical data of the measured object. With the data acquired from the complex shape of one existent object, the geometrical models can be reconstructed, and then be studied, such as analyzing, modifying, and be manufactured.Our work is a part of the project Optical Three-dimensional (3-D) Sensing and Reconstruction System for Advanced Manufacturing, which is supported by the special-item fund for optical techniques of Shanghai Municipal Commission of Science and Technology. The main purpose of this dissertation is to study the optical measurement technique and its relevant techniques for the complex surface measurement based on fringe projection technique, aiming at developing an efficient and precision method for acquisition of geometric information on shape for RE A novel multi-view measurement system has been constructed based on fringe projection, and a self-made instrument and attached software with intellectual property right have also been developed. A DLP projector is used to project digitally created fringe patterns onto the object; a CCD camera is selected to capture the deformed fringe patterns. In order to realize the 3-D measurement of complex objects, we have paid more attention to the research of relative fields. 1.Phase segmentation technique for reliable phase data in single-view measurement We first provide a review of fringe phase unwrapping algorithms, and then we focus on the temporal phase unwrapping via multi-frequency fringe projection, which is the most efficient solution in the measurement of discontinuous phase distributions, phase with noise, etc. For measuring a surface with step feature, the geometric features of holes, and irregular reflectivity of quasi-specular, noises like local shadow, unlighted background, under-sampling in the fringe patterns, etc., there are some immeasurable parts in just a single-view, i.e. where the phase data obtained are unreliable. In order to get rid of the invalid data, an automatic segmentation technique according to the threshold of the histogram of modulation distribution has been proposed. And an improved maximum entropy algorithm based on the modulation-background co-occurrence matrix for thresholding has been presented too. This method attempts to utilize the information of both modulation and background of the fringe patterns, evaluates two-dimensional entropies based on the co-occurrence matrix to conduct segmentation of phase map, and finally identifies the valid measurement areas of the object. Accordingly the unreliable phase data are cancelled in the single-view measurement. In order to compensate for the lacked 3-D coordinates of the points in the invalid areas, multi-view measurements in different directions combing surface connection are available.2.Calibration technique of fringe projection systemThe old projected fringe system and calibration methods had bad maneuverability, inferior accuracy and are pain-taking, because they must satisfy some requirements. After comprehensive analysis and detailed deduction of the general geometry of measurement system had been performed, a new calibration method for the 3-D measurement system has been presented. By importing the camera lateral calibration technique, in which the neural networks are built to...