| Surface structured light three-dimensional scanning measurement technology is currently a more widely used active measurement method in industry,with non-contact,practical and high detection accuracy,but this method is limited to three-dimensional morphology measurement of diffuse reflective objects.For strongly reflective objects,the specular reflected light on their surfaces can easily lead to uneven brightness and pixel oversaturation in the images collected by the camera,which affects the subsequent image processing and three-dimensional measurement,due to the insufficient dynamic range of the camera.The dynamic range of ordinary image sensors is usually between 60 and 70 d B,while the brightness range of nature or some specific fields require the dynamic range of the sensor to reach more than 100 d B,so the dynamic range of the imaging system needs to be extended.A digital micromirror(DMD)is an optical modulator that can modulate the light intensity entering the camera by controlling the deflection of each mirror element,resulting in a reduced grayscale of the acquired image.Thesis proposes a high dynamic range imaging method for measuring the surface of strongly reflective objects by integrating digital micromirrors with surface structured light three-dimensional measurement techniques to achieve high dynamic imaging of strongly reflective metals.Thesis mainly includes the following research elements:(1)According to the structural characteristics and imaging principle of digital micromirror,a digital micromirror-based high dynamic range measurement system based on the surface structure optical measurement technology is designed.After the reasonable selection of each device in the system,the experimental platform is built and the preliminary imaging verification is carried out according to the designed optical circuit diagram.(2)The geometric deviations between the CCD camera image element and the digital micromirror mirror element are matched and calibrated by using the Mohr streak.Secondly,the Zhang’s calibration method is chosen to calibrate the parameters of the camera and the projector in the system,and the internal and external parameter matrices and the projection errors of both are solved.Then the phase information of the projection streak is solved by the method of phase shift plus Gray code decoding,and the method is experimentally verified to be able to realize the phase unfolding on the surface of the object under test.(3)In order to make the digital micromirror array and CCD camera pixel array correspond one to one,thesis uses the algorithm of white whale optimized BP neural network to establish the coordinate mapping relationship between digital micromirror and CCD camera,uses the checkerboard pattern to evaluate the accuracy of the mapping algorithm.Based on the principle of PID controller,the parameter optimization method based on the mean square error of the image is designed in combination with the measurement system to take the original overexposed image as the input and the corresponding DMD mask as the output,so as to realize the controlled flip of the DMD micromirror.(4)Experiments on structured light-based high dynamic range scanning measurement of metal workpieces with strongly reflective surfaces using the studied DMD mask generation method are conducted to reduce the gray value of overexposed areas in the images,thus obtain highly dynamic images of the strongly reflective surfaces and improve the dynamic range of the system.The proposed high dynamic range measurement method is verified to be beneficial to the 3D scanning measurements of strongly reflective surfaces by performing three-dimensional point cloud recovery on the surface of the object under test. |
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