Control system for spincasting 8-m borosilicate honeycomb mirrors

John M. Hill; Mark R. Hunten; Kendahl J. Johnson; Dana D. Mitchell; Skip Schaller; R. S. Esterline

doi:10.1117/12.19113

1 July 1990 Control system for spincasting 8-m borosilicate honeycomb mirrors

John M. Hill, Mark R. Hunten, Kendahl J. Johnson, Dana D. Mitchell, Skip Schaller, R. S. Esterline

Proceedings Volume 1235, Instrumentation in Astronomy VII; (1990) https://doi.org/10.1117/12.19113
Event: SPIE Astronomical Telescopes and Instrumentation for the 21st Century, 1990, Tucson, AZ, United States

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.

Citation Download Citation

John M. Hill, Mark R. Hunten, Kendahl J. Johnson, Dana D. Mitchell, Skip Schaller, and R. S. Esterline "Control system for spincasting 8-m borosilicate honeycomb mirrors", Proc. SPIE 1235, Instrumentation in Astronomy VII, (1 July 1990); https://doi.org/10.1117/12.19113

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