Ground and Space-Based Telescopes

Correcting encoder interpolation error on the Green Bank Telescope using an iterative model based identification algorithm

[+] Author Affiliations
Timothy Franke, Mario Garcia-Sanz

Case Western Reserve University, Electrical Engineering and Computer Science Department, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States

Tim Weadon, John Ford

National Radio Astronomy Observatory, P.O. Box 2, Green Bank, West Virginia 24944, United States

J. Astron. Telesc. Instrum. Syst. 1(4), 044005 (Dec 09, 2015). doi:10.1117/1.JATIS.1.4.044005
History: Received May 11, 2015; Accepted October 28, 2015
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Abstract.  Various forms of measurement errors limit telescope tracking performance in practice. A new method for identifying the correcting coefficients for encoder interpolation error is developed. The algorithm corrects the encoder measurement by identifying a harmonic model of the system and using that model to compute the necessary correction parameters. The approach improves upon others by explicitly modeling the unknown dynamics of the structure and controller and by not requiring a separate system identification to be performed. Experience gained from pin-pointing the source of encoder error on the Green Bank Radio Telescope (GBT) is presented. Several tell-tale indicators of encoder error are discussed. Experimental data from the telescope, tested with two different encoders, are presented. Demonstration of the identification methodology on the GBT as well as details of its implementation are discussed. A root mean square tracking error reduction from 0.68 arc seconds to 0.21 arc sec was achieved by changing encoders and was further reduced to 0.10 arc sec with the calibration algorithm. In particular, the ubiquity of this error source is shown and how, by careful correction, it is possible to go beyond the advertised accuracy of an encoder.

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© 2015 Society of Photo-Optical Instrumentation Engineers

Citation

Timothy Franke ; Tim Weadon ; John Ford and Mario Garcia-Sanz
"Correcting encoder interpolation error on the Green Bank Telescope using an iterative model based identification algorithm", J. Astron. Telesc. Instrum. Syst. 1(4), 044005 (Dec 09, 2015). ; http://dx.doi.org/10.1117/1.JATIS.1.4.044005


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