Design Of Three-Dimensional Shape Measurement System For Highly Reflective Metal Workpiece Based On Full-Field Fringe Projection

The 3D shape measurement of metal workpiece can be used to detect the design error and size of the workpiece on the production line to ensure the production quality and improve the production efficiency.However,there are often highly reflective areas in metal workpiece,which will lead to the three-dimensional topography of the cavity,can not be effectively measured,its quality cannot be guaranteed,affecting the subsequent processing and use of the workpiece.At present,the measurement methods of high reflective metal workpiece 3D topography mainly include manual measurement,3D coordinate machine method,ultrasonic technology and so on.However,manual measurement is time-consuming and laborious;Three-dimensional coordinate machine and ultrasonic detection cost high,low efficiency.In order to avoid the above problems,the full-field fringe projection technique is used in this thesis to process the highly reflective fringe pattern of the metal workpiece to complete the 3D topography measurement quickly and efficiently.The specific work content of this thesis is as follows:(1)Based on full-field fringe projection and image processing technology,the software and hardware system of 3D topography measurement of highly reflective workpiece is designed.Firstly,sinusoidal fringe sequence is generated by computer,which is projected to the surface of metal workpiece by projector,and then CCD camera at another angle is used to collect fringe image,and the 3D topography of metal workpiece is measured by the pre-processing of highlight image.(2)Camera calibration and system calibration.In this thesis,black and white checkerboard calibration board and Matlab camera calibration toolbox are used to complete the calibration of internal and external parameters of the camera.A ring calibration plate was used to select several positions in the range of depth of field measurement,and the relative depth of each calibration plate was determined by combining camera calibration parameters and pixel coordinates.The polynomial relationship between absolute phase and depth data was established to complete the system calibration.By comparing the known depth with the actual measured depth,the maximum absolute error and the maximum standard deviation of the system are about0.069 mm and 0.058 mm respectively,which verifies that the system has met the requirements of 3D topography measurement after calibration.(3)Design of adaptive Gamma corrected 3D morphology measurement method for metal workpiece.Firstly,the reason of the cavity in the 3D morphology is analyzed by the highly reflective surface model.After that,color space conversion,normalization,adaptive Gamma correction and other preprocessing were carried out to improve the entropy of image information and reduce the invalid pixel points in the highlight region.Finally,the 3D topography of the highly reflective workpiece is obtained by using the system calibration parameters and the phase and depth information.The image information entropy and the number of highly reflective pixels are used to quantitatively evaluate the actual workpiece measurement.The experimental results show that the proposed method can increase the image information entropy of highly reflective metal workpiece to about 2 times and reduce the number of invalid pixels in the highly reflective area by about 30 times without requiring complex prior knowledge.In order to verify the effectiveness of the proposed method in reducing the nonlinear error,the phase error before and after correction is reduced from 0.18 rad to 0.012 rad by simulation experiments,which proves that the proposed method can effectively reduce the phase error and obtain the 3D topography of metal workpiece with higher quality.