360°Profilometry By Multi-aperture Overlap-scanning Technique

In this dissertation, 360 3-D measurement and its relative techniques have been studied.We have developed a multi-view measurement system. An LCD projector is used as grating projector. Grating projection technique is employed to capture the profile of the tested object. In order to accurately measure and reconstruct 360 3-D shape of an object, we have paid more attention to the relative research fields, and proposed novel techniques as follows:(1) Fast calibration technique of grating projection system When the objective reference plane is shifted, its phases havebeen measured and registered. According to the shifted distance and the phase increment, the mapping relationship between phase difference and depth of object can be determined by a least squares method. This calibration method is characterized by simple principle, high accuracy, easy to handle and efficient. Importantly, it is also benefit to break through the accuracy bottleneck of the grating projection technique, and to promote its development.(2) Reference plane idealization techniqueIn this technique, idealized reference phase distribution is obtained by least squares fitting of the measured phase distribution of an objective reference plane according to its theoretical distribution. The advantages of both theoretical reference andobjective reference method can be combined in this technique, by which, the undesirable influence of local deformation of the objective reference is repressed, and the noise and quantization error of the fringe pattern is eliminated. In addition, large-scale parts can be measured by utilizing a relatively small reference. By this technique, one of the main error-inducing factors in grating projection technique can be cancelled with low cost.(3) Iterative phase-shifting algorithm immune to tilt phase-shifting errorThe tilt phase-shifting error induced by the orientation error of phase shifter, as an important potion of phase shifting error, will depress the accuracy of the reconstructed phase. To overcome this limitation, a novel algorithm is proposed in this dissertation, by iterative operation of computing phase distribution and phase shift plane coefficients, which can eliminate both translational and tilt phase shifting error and get accurate phase distribution. This method gives a further development to the least squares phase-shifting algorithm and its iterative algorithm. It is also the first time to study the tilt error in phase-shifting technique. It reveals potential practicability in phase-shifting interferometry and grating projection technique to loosen the required accuracy of phase shifter.(4) Iterative algorithm of multi-aperture overlap-scanning technique in cylindrical coordinatesThe principle of the multi-aperture overlap-scanning technique (MAOST) is to make the adjacent sub-apertures partially overlapped, and then their relative location and orientation can be obtainedthrough the overlapped area. Therefore, the sub-apertures can be transformed to a global coordinate system. A novel algorithm aiming at 3-D measurement in cylindrical coordinates is suggested in this dissertation. To improve the computability, tangly coordinate transform equations in cylindrical coordinates are linearized. In order to guarantee the multi-aperture transformation and connection, two criteria are introduced. From the criteria, an iterative method is deduced, that is, the error movement calculationand coordinates transformation are iterated. This technique provides a low-cost software-based global calibration approach to 360° 3-D measurement.The above novel techniques being applied in 360° 3-D measurement system can improve the measurement accuracy, and save the measurement cost and time. The experimental result shows that the 360° 3-D shapes can be measured and reconstructed successfully with high accuracy and repeatability, and only about 20 minutes are needed to fulfil one measurement.