Methods Study And Experimental Device Design For Measuring The 3-D Profile Of A Moving Object

Projection grating profilometry has been widely used in experimental mechanics, 3-D sensing, machine vision, robot simulation, industry monitoring, biomedicine, etc. because of the advantages of full field measurement, high speed measurement and high resolution. In this work, we use a frame CCD camera and a line-scan CCD camera to construct image detection systems to detect the 3-D profile and surface defect of a moving object, respectively. In addition, the line-scan image system is used to detect the dynamic deformation.3-D profile measurement of a moving object using a novel phase-shifting technique is introduced. Digital gratings with two steps phase-shifting are projected periodically onto a measured object surface. The deformed fringe patterns are captured by a frame CCD camera within a short exposure time. By synchronizing the projector and the CCD camera accurately, there is an overlapping part which is the same part of the object among three neighbouring frames. Hence the intensity values at the same surface point modulated by three neighbouring gratings can be obtained, and its phase value can be computed by an improved phase-extracting algorithm. The profile of a specimen is detected by the proposed method. Experimental results demonstrate that this method is effective for the profile measurement of a moving object with larger surface slope coefficient.The phase errors caused by intensity error, such as quantization error, nonlinear response error and saturation error, in projection grating phase-shifting profilometry were discussed through simulation and experiment. (1)The formula of the standard of phase error caused by quantization is deduced, and discrete digital sinusoidal series are used to simulate the phase error. Simulative results indicate that the phase error will be decreased with the increase of the phase-shift steps, the intensity quantization level, the effective quantization coefficient and the modulation depth of the sinusoidal fringe patterns. (2)The phase errors and their periodic wave behavior caused by the nonlinearity of the system are studied. The theoretic formula of the phase error is deduced, from which it is found that the phase error caused by the nonlinearity of the imaging and projected system can be effectively decreased by using the four-step algorithm instead of the three-step algorithm. The experimental results are fit well with the theoretic analysis. (3)The intensity saturation of fringe patterns can also induce the fluctuating phase error. To decrease the phase error introduced by saturation, a novel phase recovering algorithm is proposed and further studied. The real phase can be recovered by the unsaturated intensity values. Simulative results indicate that the phase error caused by the intensity saturation can be effectively decreased by the phase-recovering algorithm when the saturation coefficient of fringe patterns is within the applicability range of the corresponding phase-recovering algorithm. Furthermore, the applicability range of the phase-recovering algorithm will be extended with the increasing of phase-shifting steps.A line-scan image system is constructed to measure the surface profile of a moving object with a variable speed. By using the speed coder, the relative moving speed is obtained and the pulse signal is fed back to the computer for synchronizing the moving object and the line-scan CCD camera. So that, undistorted fringe patterns can be captured by the line-scan camera. The classical fringe skeletonizing method and Fourier transform method are used to evaluate the fringe deformation. Comparing the results obtained by the two different methods, it is found that the measuring accuracy of Fourier transform method is higher than that of fringe skeletonizing method. However, fringe skeletonizing method has the advantage in measuring the surface with higher slope coefficient. Furthermore, the projection dual-frequency composite grating technique is used to solve 2Ï€phase ambiguity problem because of some bigger surface steps. Experimental results are presented to prove the feasibility of the inspection system. Dual-frequency grating inspection can increase the detection accuracy and overcome the 2Ï€phase ambiguity at the same time. The line-scan CCD camera is used to measure vibration characteristics of a string. The CCD line array sensor is set parallel to the vibration direction of a tensile string. There is a distinct track at the center of the captured image because of the different reflectivity between the string and the background. The fundamental frequency of the vibrating string can be obtained from the acquired images by Fourier transform technique. In addition, the line-scan image system is also used to detect the dynamic profile of human arm caused by muscle deformation. One line of the detected object surface is captured along a period of time. Fourier transform method is used to get the profile sequence of the same surface line in different time.