Research On3-D Profilometry Technology Based On Digital Grating Projection

The3-D profilometry technology has a plenty of practical value in scientific research and industrial manufacture, which was widely used in different fields such as reverse engineering, biomedicine, machine vision, quality control, model design and criminal detection etc. Under the circumstances it was becoming a hot research topic in the domestic and overseas. The projected grating method was extremely studied by researchers for its characteristic:full-field measurement, real-time, high degree of accuracy.Phase distribution map was calculated by inverse trigonometric functions, whether using the phase-shifting profilometry or Fourier transform profilometry based on the projected grating method, and the wrapped phase distribution can only be ranging from-n to%by the phase extraction method of inverse trigonometric functions. Phase-unwrapping is necessary to recover the object shape. On a wrapped phase map acquired from the experimental system, there are normally noises, shadows and discontinuities, which can often make spatial phase unwrapping based on spatial coordinate fail. Obviously, the difficulty in phase unwrapping has become the obstruction that prevents the projected grating method from automation. However, the basic idea of temporal phase unwrapping is that the pitch of projected grating can be changed over time. The temporal phase unwrapping can unwrap each pixel independently of all the pixels in the image compared with the traditional spatial phase unwrapping. The advantage of the method is that it can measure the shape of discontinuous object.A complete set of3-D profilometry measurement system using projected grating phase method was constructed in this dissertation, including projection of the grating patterns, collection of the deformed grating patterns, phase unwrapping and reduction of the object profilometry. The main work of this dissertation was focused on phase unwrapping. The main research results of this paper are listed below: (1) Phase-unwrapping method based on the multi-frequency grating was studied. The high-frequency wrapped phase can be unwrapped by low-frequency unwrapped phase, and the process of phase-unwrapping was realized automatically. Simultaneously, detail information of object can be obtained from high-frequency phase distribution. The experimental results prove that the proposed method is effective and feasible.(2) Phase-unwrapping method based on the multi-frequency heterodyne principle was studied. The superposition of high-frequency wrapped phase is taken as the low-frequency unwrapped phase. The high-frequency wrapped phase can be unwrapped, which was guided by the low-frequency unwrapped phase. And the high accuracy feature of phase-shifting method was remained.(3) A novel absolute phase recovery technique based on phase coding and2+1phase-shifting was presented. Unlike the conventional gray-coding method, the codeword is embedded into the phase, and then used to determine the fringe order for absolute phase retrieval. This technique is robust because it uses phase instead of intensity to determine codeword. The fringe order for absolute phase retrieval was determined independently of the wrapped phase pattern.(4) Experiments were presented to verify the performance of the proposed phase unwrapping method in the paper, consist of the simulated experiments under MATLAB R2008a programming environment and the actual3-D profilometry measurement.