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الثلاثاء، 5 أغسطس، 2014

Shape Reconstruction Using Structured Light Techniques

Author         : Dalia Nabil Mousa Mahmoud
 Degree        : M.Sc. Production
Title             : Shape Reconstruction Using Structured Light Techniques
 Abstract:
Shape reconstruction using structured light has become one of the most widely used optical three dimensional scanning methods, especially in the field of reverse engineering, where a three dimensional model of the object is acquired. It has the advantages of being fast, non destructive, and accurate compared to other contact scanning methods. Various complex shaped objects can be reconstructed using structured light, which makes it advantageous in many industrial applications. In this study, a shape reconstruction system using structured light has been developed. The system depends on integrating several open source codes. The purpose of the system is to successfully reconstruct a three dimensional model of the object being measured. The system starts by projecting stepped sinusoidal fringe patterns on the object and acquiring their corresponding images. The phase shift between the reference wavefront and the object wavefront is then extracted. The extracted phase is unwrapped using the flood fill algorithm. Finally the system is calibrated to obtain real world measurements of the object. A single scan longitudinal calibration technique is proposed to calibrate the structured light scanning system. A stepped calibration board is used to calibrate the longitudinal axis instead of the traditional flat calibration board. Each step of the board represents a different position and therefore only a single scan is required to calibrate the measurement system. The calibration scheme then continues as with other traditional methods. The proposed approach neither requires a translating stage nor taking multiple scans of the reference board at various positions. The proposed approach has a significant effect on error reduction, is simpler to implement for in site measurements, and is considerably faster than traditional calibration methods. Experimental results show the reliability and effectiveness of the proposed method, resulting in an error reduction up to 60 when compared to the traditional method. A design of experiments study was performed to test the impact of certain factors on the resolution of the developed system. The considered factors include the distance between projector’s lens center and the camera’s lens center, the normal distance between the camera and reference plane and finally the spatial frequency of the fringe pattern used. The results obtained showed that the developed system longitudinal resolution varies between 0.5mm and 0.2 mm, depending on the values of the tested factors. Further analysis of the results showed that the effect of each factor is different, and that the most significant one is the normal distance between the camera and the reference board. The developed system is used to reconstruct objects with predefined geometrical features such as spheres, and objects with a free form profile. A traditional contact measurement system was used to measure the same test parts and the results, from both systems, were compared. The obtained results show good agreement between both methods, which confirms the accuracy of the developed system. 

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