X-ray computed tomography (CT)—more commonly known as CAT scan—has recently evolved from the world of medical imaging and nondestructive evaluation to the field of dimensional metrology; the CT technique can now be used to measure a specimen’s geometrical dimensions (of both internal and external features). As a result, CT presently contributes to the areas of dimensional inspection and geometric analysis for technology companies that produce manufactured parts for a variety of industries such as automotive, aerospace, medical devices, electronics, metalworking, injection molding plastics, composite materials, ceramics, and 3D printing or additive manufacturing. While dimensional accuracy is not crucial for medical diagnoses or other qualitative analyses, accurate dimensional quantification is the essence of X-ray CT metrology. Despite increasing advances in this technology, the current state of the art of CT metrology still confronts challenges when trying to estimate measurement uncertainties, mainly due to the plethora of influencing factors contributing to the CT measurement process. Gradual progress has occurred over the last decade toward a better understanding of some of these influencing factors that were illuminated by a series of collaborative research initiatives between a collective of several universities and institutions (predominantly located in the European Union) committed to the advancement and development of industrial CT scanning as a measuring technology. In an effort to further understand phenomenologically the role of variables affecting the precision and accuracy of CT dimensional measurements, this dissertation presents a series of experimental studies that evaluate the performance of cone-beam CT measurements, and their uncertainty estimates, in comparison with reference measurements generally obtained from tactile coordinate measurement machines (CMMs). In some cases, the results are contrasted against simulations performed in Matlab software (to compute fan-beam projection data) and an additional simulation tool called “Dreamcaster” (for ray casting and Radon-space analysis). The main CT variables investigated were: temperature in the X-ray CT enclosure, number of projections for a CT scan, workpiece tilt orientation, sample magnification, material thickness influences, software post-filtration, threshold determination, and measurement strategies. For dimensions of geometric features ranging from 0.5 mm to 65 mm, a comparison between dimensional CT and CMM measurements, performed at optimized conditions, typically resulted in differences of approximately 5 µm or less for data associated with dimensional lengths (length, width, height, and diameters) and around 5 to 50 µm for data associated with measurements of form, while expanded uncertainties computed for the CT measurements ranged from 1 to over 50 µm. Methods for estimating measurement uncertainty of CT scanning are also assessed in this work. Special attention is paid to the current state of measurement comparisons (in the field of dimensional X-ray CT) by presenting a comprehensive study of metrics used for proficiency testing, including rigorous tests of statistical consistency (null-hypothesis testing) performed with Monte Carlo simulation, and particularly applied to results from two recent CT interlaboratory comparisons. This latter study contributes to the knowledge of methods for performance assessment in measurement comparisons. In particular, it is shown that the use of the En-metric in the current state of CT interlaboratory comparisons could be difficult to interpret when used to evaluate performance and/or statistical consistency of CT measurement sets.
|Advisor:||Smith, Stuart T.|
|Commitee:||Davies, Angela D., Evans, Christopher J., Morse, Edward P., Steck, Todd R.|
|School:||The University of North Carolina at Charlotte|
|Department:||Optical Science & Engineering|
|School Location:||United States -- North Carolina|
|Source:||DAI-B 80/01(E), Dissertation Abstracts International|
|Subjects:||Mechanical engineering, Physics, Optics|
|Keywords:||Dimensional metrology, X-ray computed tomography|
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