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Proceedings Paper

Control system for spincasting 8-m borosilicate honeycomb mirrors
Author(s): John M. Hill; Mark R. Hunten; Kendahl J. Johnson; Dana D. Mitchell; Skip Schaller; R. S. Esterline
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Paper Abstract

The process for spincasting 8 meter borosilicate honeycomb mirrors requires us to heat 14 tons of glass in a complex mold to 1170 °C while spinning the entire furnace at 6.8 rpm. After casting, the honeycomb blank must be cooled through the annealing temperature range at 0.2 °C per hour. The glass will be in the furnace for an eight week period. We describe here the computer control system to read the 600 N-type thermocouples and control the 270 8-kilowatt heaters used in the spincasting furnace. The control system uses a proportional — integral — derivative (PID) algorithm to regulate the furnace temperature to a few degrees over the entire casting cycle. Considerable design effort has gone into assuring that a component failure or a control system error does not turn an 8 meter nilrror into an expensive patio ornament. Errors are avoided by four strategic steps: fault avoidance, fault detection, fault containment and fault recovery. Examples are provided in each of these categories. System redundancy begins with three on-board 68000-family VME-bus computers which control overlapping areas of the furnace. Redundancy extends down through the temperature measurement and power control systems with many modular, interleaved, and optically isolated subsystems. Data logging and system monitoring are achieved with a Sun 3/280 workstation running IRAF in the control room. Rotation of the furnace is controlled by two 40 HP DC servomotors with speed regulation to 0.1%. The oven control system contains over 300 circuit cards, dozens of subracks and power supplies, more than 3000 connectors of at least 10 different types. there are more than 20 miles of wire and cable, most with multiple conductors, ranging from fiber optics thinner than a hair to power cables more than an inch thick. Results are presented from a subset of this control system which has been used to cast three 3.5 meter honeycomb mirrors in 1.988 and 1989.

Paper Details

Date Published: 1 July 1990
PDF: 17 pages
Proc. SPIE 1235, Instrumentation in Astronomy VII, (1 July 1990); doi: 10.1117/12.19113
Show Author Affiliations
John M. Hill, Steward Observatory/Univ. of Arizona (United States)
Mark R. Hunten, Steward Observatory/Univ. of Arizona (United States)
Kendahl J. Johnson, Steward Observatory/Univ. of Arizona (United States)
Dana D. Mitchell, Steward Observatory/Univ. of Arizona (United States)
Skip Schaller, Steward Observatory/Univ. of Arizona (United States)
R. S. Esterline, Steward Observatory/Univ. of Arizona (United States)


Published in SPIE Proceedings Vol. 1235:
Instrumentation in Astronomy VII
David L. Crawford, Editor(s)

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