Research On Key Technologies Of 3D Shape Measurement Based On Digital Fringe Projection

The objective world is of the form of three dimensions.In the process of observing and understanding the world,the eyes and brain present the images in three-dimensional form in our cognition.With the development of science and technology,human beings are no longer satisfied with obtaining two-dimensional information of objects through sensors.How to obtain three-dimensional information of objects has become a research hotspot.Among the non-contact three-dimensional imaging and measurement technologies,three-dimensional measurement technology based on digital fringe projection is the most representative.Because of its high accuracy,high resolution,low cost,and fast speed,it has become the most widely used three-dimensional measurement technology.However,this technology still has the problems of limited system calibration accuracy,non-linear effects of the projector and limited measurement speed.Focusing on these issues mainly,the model and calibration method of the projector are studied in this paper.An improved method of projector calibration based on coordinate mapping and two improved methods of projector calibration based on coordinate transformation are proposed.The principle of binary fringe defocusing technology and the performance of different binary patterns are investigated,and a novel method of binary fringe defocusing combining with multi-frequency phase unwrapping is proposed.The main content of this paper includes three parts:1)The projector calibration method based on coordinate mapping is studied,and the limited accuracy and the calibration errors are analyzed.A sub-pixel-level,instead of pixel-level,contour mapping fitting method is proposed to improve the calibration accuracy.The experimental results of the center point direct mapping,pixel-level contour mapping fitting and the proposed sub-pixel-level contour mapping fitting methods are compared,which proves that the sub-pixel-level contour mapping fitting method possesses the highest calibration accuracy.The experiments of mapping and fitting the circular marks with different radius in concentric circles are carried out,and the results prove that the circular marks with larger radius have higher calibration accuracy.The fusions of the concentric centers of the big and small circles with different weights after the mapping and fittings are conducted.The investigation shows that when the weights of the big and small circles are 4/5 and 1/5,respectively,the best calibration result can be obtained.In this case the maximum reprojection error is 0.1756 pixels,and the standard deviations are(0.0539,0.0504)pixels.2)The projector calibration method based on coordinate transformation is studied,and the limited accuracy is analyzed.In the subsequent experiments,the interference between the calibration plate pattern and the projection pattern is avoided by using the secondary projection technology.The calibration technology of moving projector is studied,and the error sources are investigated.It is proposed to use local homography in this technique to improve the calibration accuracy.The proposed method is compared with the method requiring camera parameters and the global homography method experimentally,which shows that by the local homography,the standard deviations of the reprojection error are respectively reduced from(0.2275,0.2264)pixels and(0.1237,0.1098)pixels to(0.0535,0.0468)pixels.The invariance of the cross ratio in the projective transformation is applied to the calibration of the projector,and a related model of accurately extracting the center of the projected feature point is proposed.Compared with the method requiring camera parameters and the global homography method,the standard deviations of the reprojection error are respectively reduced from(0.2275,0.2264)pixels and(0.1397,0.0997)pixels to(0.0645,0.0601)pixels by the proposed method.3)The principle and error characteristics of binary fringe defocusing technology are studied,and different ways of generating binary patterns are investigated.The multi-frequency phase unwrapping technique is investigated which is then applied in binary fringe defocusing model.According to the characteristics of binary patterns,one-dimensional binary patterns are used for high-frequency fringe generations,and two-dimensional binary patterns are used for low-frequency fringe generations.Simulations under low defocus condition show that the optimized SPWM method is the best in high-frequency fringes,while the SD method is the best in low-frequency fringes.The experiments verify the correctness of the simulation results.The 3D shape measurements of the whiteboard,mask,David model and separated objects are conducted by the proposed mixed multi-frequency binary defocusing and by the normal multi-frequency binary defocusing.The experimental results prove the effectiveness of the proposed method.