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Optics Program
Simplifies Analysis and Design
Computer Technology
Originating Technology/NASA Contribution
Future spaceborne astronomy missions will require telescopes
with increasingly greater power, driving the dimensions
of the optics and their housing structures to significantly
greater sizes.
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The
James Webb Space Telescope is a large, infrared-optimized
telescope, scheduled for launch in 2013. It will
find the first galaxies that formed in the early
universe, connecting the Big Bang theory to our own
Milky Way galaxy. |
The increased size of the structures reduces the dynamic
frequencies of the optical system, to the point where disturbance
frequencies and structural modes significantly interact.
At the same time, the requirements on dynamic stability
to achieve the required optical performance are significantly
tighter than for anything that has flown before, and, therefore,
the sensitivity to dynamic effects is correspondingly high.
Finally, the physical size of the optical instruments makes
fully integrated system-level testing extremely problematic.
Not only are the systems too large to test in any existing
environmental chambers, they are susceptible to gravity
loading effects and suspension coupling that will significantly
change the dynamics. Validation of the large designs must
then rest on a combination of analysis and system tests.
Techniques for system tests include using Finite Elements
(FE) and FE model updating tools, system identification,
and using various other tools for performing optical analyses.
These techniques, however, do not provide for analysis
of cross-disciplinary results. Therefore, the conventional
approach is to develop a requirement budget that assigns
error allocations to each of the modeling teams. This approach
is extremely limiting in that it does not allow requirements
to be freely traded among subsystems. NASA, intent on sending
larger, more powerful optics into space, resolved to find
a better way to test them.
Partnership
Engineers at Goddard Space Flight Center partnered with
software experts at Midé Technology
Corporation, of Medford,
Massachusetts, through a Small Business
Innovation Research (SBIR) contract to design a new analysis system.
The result of the two-phase contract was the Disturbance-Optics-Controls-Structures
(DOCS) Toolbox, a software suite for performing integrated
modeling for multidisciplinary analysis and design. The
Toolbox allows the definition of subsystem/component models,
including structural models, control system models, optical
sensitivities, and disturbance models. The component models
are automatically coupled together to create a mathematic
model of a complete physical process, using techniques
that maximize the numerical conditioning, while maintaining
modeling accuracy.
The code has been validated and applied to the following
NASA astronomy projects and facilities: the Terrestrial
Planet Finder Structurally Connected Interferometer Testbed
(TPF-SCIT), the Terrestrial Planet Finder Coronagraph (TPF-C),
the James Webb Space Telescope, and the Solar Dynamics
Observatory.
Product Outcome
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One
of many segments of the mirror assembly being tested
for the James Webb Space Telescope project at the
X-Ray Calibration Facility at Marshall Space Flight
Center. Marshall is supporting Goddard Space Flight
Center in developing the telescope by taking numerous
measurements to predict its future performance. |
The purpose of the DOCS Toolbox is to integrate various
discipline models into a coupled process math model that
can then predict system performance as a function of subsystem
design parameters. The Toolbox accepts as input the discipline
models from a variety of currently available discipline
modeling tools. The Toolbox then connects the discipline
models and applies numerical conditioning algorithms to
improve the numerical accuracy, while still maintaining
model accuracy. It performs the analysis and redesign in
a graphical framework that allows the user to define and
solve the analysis problem, and it then documents results
in a point-and-click environment.
The system can be optimized for performance; design parameters
can be traded; parameter uncertainties can be propagated
through the math model to develop error bounds on system
predictions; and the model can
be updated, based on component, subsystem, or system-level
data.
The Toolbox allows the definition of process parameters
as explicit functions of the coupled model, further enabling
the exact definition of sensitivities. It also includes
a number of functions that analyze the coupled system model
and provide for redesign, including: critical parameter
analysis that formally identifies the design variables
that have the highest influence on system performance,
risk, and cost; optimization of design objective functions
subject to constraints on design variables; formal system
trading using an isoperformance methodology (a design concept
that seeks to create the best-fit concept) that maps out
the non-unique set of design parameters that meet requirements;
uncertainty analysis for computing errors bound to performance
predictions and identifying critical uncertainties; and
model updating to renew component math models using measurement
data.
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A
Marshall Space Flight Center employee is inspecting
one of the mirror assemblies for flaws. |
The software has myriad benefits, including the ability
to automatically couple the discipline models to create
a system math model. This provides a complete description
of the physical process in terms of all component design
variables. The automatic coupling reduces manual effort,
eliminates the chance for user error, and automatically
checks unit compatibility between components.
The software tool provides a set of “canned” routines for
defining parameter dependencies on typical structural parameters,
as well as a set of techniques for identifying critical
design parameters.
The unique formulation of the parameter-dependent math
model enables the designer to formulate many problems as
formal optimization analyses, where other analysis techniques
would have allowed only the evaluation and comparison of
a limited set of design points.
The product is being sold commercially by Nightsky Systems
Inc., of Raleigh, North Carolina, a spinoff company that
was formed by Midé specifically to market the DOCS Toolbox.
Commercial applications include use by any contractors
developing large space-based optical systems, including
Lockheed Martin Corporation, The Boeing Company, and Northrup
Grumman Corporation, as well as companies providing technical
audit services, like General Dynamics Corporation.
DOCS® Toolbox is a registered trademark of Nightsky
Systems Inc.
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