There is growing interest in applying “intelligent” technologies to aerospace propulsion systems to reap expected benefits in cost, performance, and environmental compliance. Cost benefits span the engine life cycle from development, operations, and maintenance. Performance gains are anticipated in reduced fuel consumption, increased thrust-to-weight ratios, and operability. Environmental benefits include generating fewer pollutants and less noise. Critical enabling technologies to realize these potential benefits include sensors, actuators, logic, electronics, materials and structures. For propulsion applications, the challenge is to increase the robustness of these technologies so that they can withstand harsh temperatures, vibrations, and grime while providing extremely reliable performance. This paper addresses the role that optical metrology is playing in providing solutions to these challenges. Optics for ground-based testing (development cycle), flight sensing (operations), and inspection (maintenance) are described. Opportunities for future work are presented.
We present a complete system for building complex 3D models using multiple range views measured from a fiber-optic projected-fringe interferometry system, and from a structured light range scanner. For the interferometry images, the BLS phase unwrapping algorithm was used to obtain true depth values. A robust registration algorithm is proposed that uses randomly selected control points, and the interval least medium squares error estimator. It is not necessary to triangulate the raw data before the algorithm is applied. Also, a 'holes-and-gaps' filling algorithm is described to improve the display of the integrated 3D models. The experimental results show that the system robustly integrates different views into a single 3D model, and that the holes-and-gaps algorithm improves the visual results.
A new instrument, the liquid point diffraction interferometer (LCPDI), has been developed for the measurement of phase objects. This instrument maintains the compact, robust design of Linnik's point diffraction interferometer and adds to it phase stepping capability for quantitative interferogram analysis. The result is a compact, simple to align, environmentally insensitive interferometer capable of accurately measuring optical wavefronts with high data density and with automated data reduction. The design of the LCPDI is briefly discussed. An algorithm is presented for eliminating phase measurement error caused by object beam intensity variation from frame-to-frame. The LCPDI is demonstrated by measuring the temperature distribution across a heated chamber filled with silicone oil. The measured results are compared to independently measured results and show excellent agreement with them. It is expected that this instrument will have application in the fluid sciences as a diagnostic tool, particularly is space based applications where autonomy, robustness, and compactness are desirable qualities. It should also be useful for the testing of optical elements, provided a master is available for comparison.
A point diffraction interferometer with a liquid crystal filter is used to measure a phase object. A reference beam is locally generated by a microsphere embedded within the liquid crystal layer. Phase shifts between the object and reference beams are introduced by varying a voltage across the birefringent nematic liquid crystal layer. Periodic phase measurement errors caused by modulations in the average intensity distributions are discussed, and a normalization method is described to reduce the errors. Experimental results are compared to a computer simulation to verify that the periodic errors are caused by intensity variations in the object beam.
Three dimensional surface measurements are required in a number of industrial processes. These measurements have commonly been made using contact probes, but optical sensors are now available that allow fast, non-contact measurements. A common characteristic of optical surface profilers is the trade-off between measurement accuracy and field of view. In order to measure large objects with high resolution, multiple views are required. An accurate transformation between the different views is needed to reconstruct the entire surface. We demonstrate a method of obtaining the transformation by choosing a small number of control points that lie in the overlapping region between two views. The selection of the control points is independent of the object geometry, and only an approximate knowledge of the overlapping region is required.
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