"FPGA Control System for the Automated Test of Microshutters"

Eric Lyness1, David Rapchun2, and Knute Ray3

1Mink Hollow Systems, Inc.
2Global Science and Technology
3Swales Aerospace

Abstract 

The James Webb Space Telescope, scheduled to replace the Hubble in 2013, must simultaneously observe hundreds of faint galaxies.  This requirement has led to the development of a programmable transmission mask which can be adapted to admit light from an arbitrary pattern of galaxies into its spectrograph.  This programmable mask will contain a large array of micro-electromecanical (MEMs) devices called MicroShutters.  These microscopic shutters physically open and close like the shutter on a camera, except each shutter is microscopic in size and an array 365 by 171 is used to select the objects under spectroscopic observation at a given time, and to block the unwanted background light from other areas.  NASA developed and is currently refining the exceptionally difficult process of manufacturing these shutters.   This paper describes how the authors used an FPGA and a reconfigurable I/O board to control the shutters in a test chamber and how the flexibility of the system allows us to continue to modify the control algorithms as NASA optimizes the performance of the remarkable MicroShutter arrays. 

The shutters pivot on a silicon nitride flexure, and a magnetic coating allows them to be actuated magnetically, while electrical connections allow electrostatic latching. The shutters are mounted on a substrate wired in a grid such that each shutter can be address by asserting its row and column. Shutters are opened by passing a magnet across the front of the array while applying a high voltage to the row and column of each shutter to be opened.  The magnetic field opens the shutter and the static charge at the intersection of the row and column holds the shutter open.  It is critical for the safe operation of the shutters that the electrical activation and deactivation occur in tight synchronization with the movement of the magnet. 

For life tests, all 62,000 shutters must be cycled 4 times per second for days at a time.  Reconfigurable software running on an FPGA reads the position of the magnet and activates the shutters in tight synchronization and can detect when synchronization is lost.  The control algorithm is in constant adjustment as more is learned about how the shutters operate.  Using an FPGA allows us to quickly adjust the program as necessary.

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