Research Papers

Uncertainty Analysis and Characterization of the SOFAST Mirror Facet Characterization System

[+] Author and Article Information
Charles E. Andraka

e-mail: ceandra@sandia.gov
Sandia National Laboratories,
P.O. Box 5800, MS 1127,
Albuquerque, NM 87185-1127

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received January 2, 2013; final manuscript received April 5, 2013; published online July 2, 2013. Assoc. Editor: Akiba Segal.

J. Sol. Energy Eng 136(1), 011003 (Jul 02, 2013) (7 pages) Paper No: SOL-13-1002; doi: 10.1115/1.4024251 History: Received January 02, 2013; Revised April 05, 2013

Sandia Optical Fringe Analysis Slope Tool (SOFAST) is a mirror facet characterization system based on fringe reflection technology that has been applied to dish and heliostat mirror facet development at Sandia National Laboratories and development partner sites. The tool provides a detailed map of mirror facet surface normals as compared to design and fitted surfaces. In addition, the surface fitting process provides insights into systematic facet slope characterization, such as focal lengths, tilts, and twist of the facet. In this paper, an analysis of the sensitivities of the facet characterization outputs to variations of SOFAST input parameters is presented. The results of the sensitivity analysis provided the basis for a linear uncertainty analysis, which is also included here. Input parameters included hardware parameters and SOFAST setup variables. Output parameters included the fitted shape parameters (focal lengths and twist) and the residuals (typically called slope error). The study utilized empirical propagation of input parameter errors through facet characterization calculations to the output parameters, based on the measurement of an Advanced Dish Development System (ADDS) structural gore point-focus facet. Thus, this study is limited to the characterization of sensitivities of the SOFAST embodiment intended for dish facet characterization, using an LCD screen as a target panel. With reasonably careful setup, SOFAST is demonstrated to provide facet focal length characterization within 0.5% of actual. Facet twist is accurate within ±0.03 mrad/m. The local slope deviation measurement is accurate within ±0.05 mrad, while the global slope residual is accurate within ±0.005 mrad. All uncertainties are quoted with 95% confidence.

Copyright © 2014 by ASME
Topics: Errors , Mirrors , Uncertainty
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Fig. 1

Illustration of SOFAST facet characterization system setup. Note the sinusoidal fringe pattern displayed on the target.

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Fig. 2

The ADDS facet used in this study. 24 ADDS facets are used to comprise a full ADDS dish system. Each facet utilizes eight individual mirrors.

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Fig. 3

Sensitivity of facet focal length, x-direction to input parameter variation at measurement location 2f

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Fig. 4

Sensitivity of facet twist, x-direction to input parameter variation at measurement location 2f

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Fig. 5

Sensitivity of facet global slope error to input parameter variation at measurement location 2f

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Fig. 6

Points from the ADDS facet where local slope error sensitivities were assessed

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Fig. 7

Sensitivity of local slope error, x-direction at facet point 1 to input parameter variation at measurement location 2f

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Fig. 8

Sensitivity of local slope error, y-direction at facet point 1 to input parameter variation at measurement location 2f

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Fig. 9

Sensitivity of local slope error, root sum square, at facet point 1 to input parameter variation at measurement location 2f



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