Sensitivity of slope measurements on parabolic solar mirrors to positioning and alignment of the laser scanner

ABSTRACT:
A laser scanning system for measuring the geometric accuracy of parabolic solar concentrators in both laboratory and field situations is being developed at Sandia Laboratories. Inaccuracies in positioning and aligning this device with respect to the parabola reference system will introduce

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systematic errors in the measured data. The nature and sensitivity of these measurement errors are analyzed and evaluated by means of a mathematical model and computer simulation techniques. Distinctive patterns of apparent error in the collector surface geometry are introduced by inaccuracies in three apparatus set up parameters: vertical position, angular alignment, and horizontal position. These error patterns scale in proportion to the magnitude oi the relevant set-up inaccuracy. Our studies allowed recognition of the subsequent compensation for the effects introduced by errors in the vertical position and the angular alignment. But, the similarity between patterns generated by errors in the horizontal position and those due to deviations in parabola focal length precluded the use of pattern recognition techniques. This ambiguity between error sources introduces an uncertainty in the best fit focal length estimated from scanning data which is equal to 0.42 times the uncertainty in horizontal positioning. In addition, the practical lower bounds for all three set-up tolerances are dependent on the resolution of the system detector. Analysis of worst case error combinations indicate the need for yet closer positioning and alignment tolerances.

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