The James Webb Space Telescope (JWST) is the next big telescope at NASA. More ambitious than its predecessor, the Hubble Space Telescope, NASA will place the JWST at a stable Lagrange point approximately 1 million miles from the earth. This telescope is the next stepping stone toward understanding the universe and studying the Big Bang theory at NASA.

The near infrared spectrometer (NIRSpec), developed by the European Space Agency (ESA) with major NASA contributions, is the primary instrument on the telescope. It observes thousands of distant galaxies to probe the epoch of initial galaxy formations in the universe. To measure numerous faint objects, the instrument must simultaneously observe a large number of objects in previously unknown positions.

To observe objects at these positions, NASA developed the microshutter array, a 171 by 365 matrix of 100 by 200 µm shutters that can open under random access control. Four microshutter arrays in a 2 by 2 matrix create a programmable transmission mask of about 250,000 shutters so that the NIRSpec can simultaneously target more than 100 faint objects, proportionally improving the efficiency of this major scientific facility. This system is essential to the development of the microshutter array, and it will be critical for the array’s flight qualification in this major international mission.

What Is a Microshutter?

A microshutter is a 100 by 200 µm rectangular door that opens and closes to block light or let it pass through. The shutters pivot on a silicon nitride flexure, actuate magnetically with the help of magnetic coating, and latch electrostatically through electrical connections.

When we began working on this project, manufacturing shutter arrays was a new and complex process that was still under development. NASA manufactures the shutters in arrays with 365 columns and 171 rows for a total of more than 62,000 shutters per array. We mounted the shutters on a substrate and wired the array in a grid so that we can assert its rows and columns to address each shutter. To open a shutter, we passed a magnet across the front of the array while applying high voltage to the row and column of each shutter. The magnetic field opened the shutter, and the static charge at the intersection of the row and column held it open.

We fabricated each shutter array to test some aspect of the overall design. Tests in this facility inform the further definition of the fabrication process. Using the NI PCI-7344 four-axis stepper motor controllers and the NI MID-7604 power motor drivers, we developed the software that controls the vacuum chamber, shutter control instrumentation, cameras, and other apparatuses to evaluate array performance.

Testing with this system revealed that uncontrolled shutter release limits shutter performance. In this uncontrolled approach, one closed a shutter by turning off the power to the row and column of the shutter. With each approach, the shutter impacts its light baffle in a way that significantly limits its lifetime.

The development team decided that we should release the shutters in synchronization with a passing magnet so that the magnetic field cushions the impact as the shutter closes. A test chamber completed in 2005 includes this new synchronized latching-and-release capability.

Microshutter Control System

The microshutters must function reliably for up to 100,000 cycles on different shutter designs. Instead of testing for years, the new test chamber must cycle the shutters rapidly. The motor rotates at up to 240 rpm; thus, the sled, connected to the motor with off-center cables, crosses back and forth in front of the shutter array four times per second. The control system needs to latch or release each of the 365 columns of the shutter array exactly as the magnet passes. To get an idea of the precision and speed required, imagine that each column of the shutter array is a slat 1 in. wide in a picket fence that is 30 ft long. The magnet would be like a jet plane moving past it at more than 700 mph and only 3 ft away.

To control the shutters, we have to communicate with the control electronics and custom high-voltage shift registers. The new system also needs to rapidly communicate and provide utilities to test and verify many operations of the 584 chips. The system must meet all of these control requirements and be fail-safe. The tests run for days at a time, opening and closing all 62,000 shutters 240 times per minute. If the system loses synchronization, the loss can damage the shutters in just a few minutes.

In order to meet these requirements, we had to either design and manufacture a custom chip or use the LabVIEW FPGA Module. We selected a PXI chassis and controller containing a PXI-7813R reconfigurable I/O module and used the LabVIEW FPGA Module to perform shutter control.

The Control Design

The entire system contains a host computer that controls the test chamber, a field-programmable gate array (FPGA) host program that runs on the PXI controller, and FPGA software that runs on the PXI-7813R. With the FPGA host interface, engineers can calibrate the system and perform manual control functions, create and download bitmaps to write to the arrays, and run self-test diagnostics on the other functions of the 584 chips.

The FPGA software reads the position of the magnet from a quadrature encoder or an absolute encoder. We placed the encoder-decoding algorithm in a single-cycle loop running at 40 MHz to ensure it does not miss any steps. After some filtering to remove jitter, we placed the position value in a first-in-first-out memory buffer (FIFO). Another loop on the FPGA reads the FIFO and determines what to do with the shutters based on the current location of the magnet. This state machine communicates with the 584 chips using the protocol to turn the appropriate rows and columns on or off.

If the FIFO overflows, the state machine controlling the shutters is not going fast enough. The software indicates a synchronization error to the host computer so the system can shut down.

This algorithm works very well and has become the foundation for control experimentation on the shutter arrays. As engineers develop new ideas to improve shutter operation, we can easily add or change algorithms in the state machine block.

The LabVIEW FPGA Module and PXI-7813R saved us hundreds of man-hours and thousands of dollars over developing a custom chip. In addition to saving costs, the control algorithm is also inexpensively modified to improve testing, explore shutter issues, and further the development of the NASA microshutter arrays.