Three-dimensional Shape Measurement System For Highly Reflective Objects Based On Fringe Projection

Fringe projection measurement technology has the advantages of non-tactile,fast speed,high accuracy,large range and strong robustness.It has been has been established in a wide range of applications,such as industrial manufacturing,manipulator operation,medical health and Digital protection of cultural relics.However,the image saturation is inevitable when obtaining three dimensional(3D)shape measurements of shiny surfaces.And the fringe patterns are overwhelmed by the image saturation.Therefore,the phase retrieval cannot be implemented in the saturate regions,which leads to errors in the 3D reconstruction.The purpose of this paper is to develop accurate 3D measurement system for highly reflective objects based on fringe projection.Two key research contents are explored in this paper,projector calibration method and measurement method of high reflective objects.The main contents are as follows.(1)By analyzing the source of coordinate mapping errors,an accurate projector calibration method based on gray mapping is proposed.The sub-pixel coordinate mapping is implemented with corresponding gray distribution from CCD to DMD is investigated.The mapped sub-pixel DMD coordinates and their gray values are treated as a set of 3D points.The set of 3D points is fitted to a plane based on PCA.The 3D points above the plane are segmented and then projected onto the plane to generate a set of 2D points.The elliptic contours on DMD are extracted by searching the convex hull of the 2D points.Then ellipse fitting is performed on the convex hull points using the least squares method to gather the centers at sub-pixel.Therefore,a set of corresponding points is also obtained on the DMD to calibrate the projector.Additionally,the space reprojection error is utilized for evaluating the accuracy of the projector calibration.Comparative experiments are carried out to verify the feasibility and effectiveness of the proposed method.(2)According to the imaging characteristics of the fringe projection,a blocksmooth adaptive fringe projection method is presented to achieve a complete and reliable measurement of highly reflective surfaces.The method projects a small number of images to calculate the initial projection intensity of saturated region by using the block strategy.Then the precision of coordinate mapping can be improved by phase expansion and fusion in saturate regions.Finally,the polynomial function is is fitted to the initial mapping projection intensity.The function is utilized to construct the smooth projection intensity curve.The optimal projection intensity of each saturated pixel is refined.And the mapping holes caused by the inconsistent resolution of the camera and projector are filled simultaneously.The adaptive fringe patterns are eventually generated.(3)The 3D measurement system for highly reflective objects based on fringe projection was set up.The practicability of the system was verified by measuring highly reflective objects with different reflectance and curvature.In addition,the effectiveness of the proposed method was verified by comparing the experimental results with the existing methods.The experimental results proved that the proposed projector calibration method reduced the mean reprojection errors by 76.4%,45.3%and 28.1% by comparing with other three methods respectively.The measurement accuracy was 0.059 mm.In addition,the proposed block smooth adaptive fringe projection method was proved to be capable of measuring the 3D topography of highly reflective objects only by projecting a few patterns.The measurement errors were reduced by 40% and 28.6% respectively.Moreover,the proposed method had better reconstruction effects on the saturated regions.