FPGA Software Tests Telescope Microshutters

The James Webb Space Telescope (JWST), the next big telescope at NASA, will be placed 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 a microshutter array, a 171 × 365 matrix of 100 × 200 μm shutters that can open under random access control. Four microshutter arrays in a 2 × 2 matrix create a programmable transmission mask of about 250,000 shutters. This enables the NIRSpec to 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 × 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 the project began, manufacturing shutter arrays was a 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. The shutters were mounted on a substrate, and the array was wired in a grid to assert its rows and columns to address each shutter. To open a shutter, a magnet was passed 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.

Each shutter array was fabricated to test some aspect of the overall design. Using a four-axis stepper motor controller and power motor drivers, software was developed 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, impacting its light baffle in a way that significantly limits its lifetime.

The development team decided to release the shutters in synchronization with a passing magnet so that the magnetic field cushions the impact as the shutter closes. A test chamber 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" wide in a picket fence that is 30' long. The magnet would be like a jet plane moving past it at more than 700 mph and only 3' away.

To control the shutters, it is necessary 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 failsafe. 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 selected a PXI chassis and controller containing a reconfigurable I/O module, and used the LabVIEW FPGA Module from National Instruments 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 I/O module. 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. The encoder-decoding algorithm was placed in a single-cycle loop running at 40 MHz to ensure it did not miss any steps. After some filtering to remove jitter, the position value was placed 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.

The algorithm has become the foundation for control experimentation on the shutter arrays. As engineers develop new ideas to improve shutter operation, algorithms can easily be added or changed in the state machine block.

This article was written by Eric Lyness of Mink Hollow Systems (Ashton, MD), and David Rapchun and Knute Ray of NASA Goddard Space Flight Center (Greenbelt, MD). For more information, Click Here .