3D Measurement Technology Based on Conoscopic Holography

Title 3D Measurement Technology Based on Conoscopic Holography

As a main subject in 3D measurement, Optical non-contact measurement can improve detection velocity, reduce loss and intensify adaptability to environment. Among all kinds of the optical non-contact measurement methods, Conoscopic Holography measurement is more suited to on-line detection because of its large measurement scope, high accuracy and adaptability to environment. But Conoscopic Holography measurement has not been applied in industry for its limitation on research theory. In this paper, theories about Conoscopic Holography method from foreign countries and factors restraining its on-line application have been analyzed, and new ways to improve antijamming ability and measurement resolution have been proposed. Modularization operation system with commonality and flexibility was built, and non-linear calibration for Conoscopic Holography system was performed. Research goal is developing strong adaptabilty and on-line detectin system in industry, which is important for optical non-contact measurement.After analysis on overall scheme and subsystem schemes, Conoscopic Holography measurement system adapted to measure distance and 3D shape was designed, and experiment flatform built was used to adjust parameters and debug prototype. At the same time, shortages of the Conoscopic Holography measurement system were analyzed and resolved based on theories about geometrical optics, crystal optics and polarization optics. Compared to the problems of light intensity on hologram, which is easy to be disturbed by light source fluctuation and noise, measured information can be obtained from phase of fringe on hologram easily. And this method also utilized translation charactor of conscopic light in the crystal fully. Based on these analysis, measurement expression based on phase of Conoscopic Holography measurement system was proposed, which reduces the system demand on stability of light source.Technology on noise suppression was researched. According to orientation characteristics of the fringe image, an improved MCD filter was proposed based on existing orientation filter ways, which accelerates the algorithm. From the crystal optics transmission model and research on fringe detection, the method of fringe scanning in a small area with integral frequency and decimal phase was used to obtain the measured information. So the two ways ware connected together. One is the method of testing fringe frequency which can recognize a pixel, and another is the method of testing fringe phase which can fix sub-pixel position. This also saved times on disposing and calculation, and accelerated measurement velocity without precision loss.To ensure Conoscopic Holography system’s high precision in large measurement range, two methods were adopted to enlarge phase difference. One is adding crystal phase of rotation and another is adding optical lens, which improves test precision of the system and reduces calibration difficulty. Research on transmition characteristic and non-linear calibration technology of the system, and theories of incremental measurement and relative phase measurement, which make the Conoscopic Holography measurement system realize incremental and relative phase measurement, which avoids difficulty in locating absolute zero point. IT avoids joggle operation of image because of image’s area surpassing CCD image surface. Characteristic parameters of the system are optimized by genetic algorithm(GA). Artificial neural network(ANN) has been used to realize non-linear calibration of incremental measurement system, so non-linear error was eliminated and the difficulty of obtaining antitromic model was avoided.Analyze all the influence factors of measurement system, and compensate system error from optic elements. The method of self-adaption with PID was adopted to restrain temperature variation which can cause shifting of laser wavelength and variation of refracting index of the crystal. This way also reduces demands of the system to environment and improves environment adaptive ability of the Conoscopic Holography system.

Category Internet
Keywords artificial neural network, birefringence, conoscopic holography, incremental and relative phase measurement, non-linear calibration,
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Pages 108
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