Transcript PowerPoint (97)

STATUS AND FUTURE OUTLOOK
OF
CORRELATION AND MODEL UPDATING
Christopher C. Flanigan
Quartus Engineering Incorporated
San Diego, California USA
16th International Modal Analysis Conference
Santa Barbara, California USA
February 2-5, 1998
AGENDA
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Introduction
Terminology
Motive, means, and opportunity
Current methodology
Commercial s/w programs
Future possibilities
Conclusion
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TERMINOLOGY:
MANY METHODS FOR CORRELATION
• "Correlation" covers three areas
• Comparison of test and analysis results
– Orthogonality, x-ortho, MAC, COMAC, CORTHOG
• System identification
– Extract K, M, C matrices (“plant”)
• Model updating
– How to update FEM to match test results
• This presentation emphasizes last topic
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MODEL CORRELATION
IS DETECTIVE WORK
All
knowledge
comes useful
to the
detective.
Sherlock Holmes
Consulting Detective
221b Baker Street
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INTRODUCTION:
MOTIVE, MEANS, AND OPPORTUNITY
• Motive
– Why do correlation?
– Why is it important?
• Means
– What methods are used?
– What s/w is available?
• Opportunity
– When is correlation performed?
– How much time is available?
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MOTIVE:
MODELS ARE NOT REALITY!
• Computers models are not perfect (even today!)
• Modeling assumptions
– Joints, fasteners, material properties, effectiveness
– Mass distribution, secondary packages, etc.
• Model fidelity limitations
– Element formulation, FEM size, run times
• Communication
– Model  drawings, structure  drawings
– Pilot error
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MOTIVE:
MODELS NEED “REALITY CHECK”
• While models are typically much better today than ten or
twenty years ago...
– Software capabilities
– Computer speed
– FEM resolution
• Testing still required to provide "reality check”
– Data from actual hardware
– Needed to validate the models
• Unlikely to change for some time!
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TEST IS NOT REALITY EITHER!
• Test article instead of flight article
– Mass simulators, missing items, boundary conditions
• Excitation limitations
– Load level, spectrum (don’t break it!)
– Nonlinearities
• Testing limitations
– Sensor accuracy and calibration
– Data processing
• But it’s the best “reality check” available
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MOTIVE:
CORRELATION REDUCES RISK
• Must certify that structure will operate correctly
• Usually cannot test flight structure to operational loads
Test structure
under
controlled
conditions
Correlate
test FEM
to match
test results
Modify and
extrapolate
Test FEM
to operational
configuration
Predict
response of
oper. FEM to
operating
environment
• Use test-correlated FEM to verify correct operation of
flight structure to flight loads
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OPPORTUNITY:
CORRELATION MUST BE FAST!
• Very limited opportunity to do correlation
– After structural testing and data
processing complete
– Before operational use of model
• First flight of airplane
• Verification load cycle of spacecraft
• Need methods that are fast!
– Maximum insight
– Accurate
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“There are
always
some
lunatics
about. It
would be a
dull world
without
them.”
CHALLENGE:
NOT A UNIQUE SOLUTION
• More “unknowns” than “knowns”
• Knowns
– Test data (FRF, frequencies, shapes
at test DOF)
– Measured global/subsystem weights
• Unknowns
– FEM stiffness and mass (FEM DOF)
• No unique solution
• Seek “best” reasonable solution
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“When you
have
eliminated
the
impossible,
whatever
remains,
however
improbable,
must be
the truth.”
MEANS:
MUCH RESEARCH IN CORRELATION
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Number of Correlation Papers
• Much work done in this field
– Selected topics and authors
– Apologies to authors not
mentioned!
• Recommended reading
– IMAC proceedings (CD-ROM)
– Finite Element Model Updating
in Structural Dynamics,
Friswell and Mottershead,
Kluwer Academic Publishers
MEANS:
MANY CORRELATION METHODS
• Trial-and-error
– Stop doing this! It's almost
the next millenium!
– Too slow for fast-paced projects
– Not sufficiently insightful for
complex systems
• FEM matrix updating
• FEM property updating
• Error localization
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Were Lestrade
but gifted with
imagination,
he might rise to
great heights.
MATRIX UPDATE METHODS
ADJUST FEM K AND M ELEMENTS
• Objective
– Identify changes to FEM K and M
so that analysis matches test
• Baruch and Bar-Itzhack (1978, 1982)
• Berman (1971, 1984)
• Kabe (1985)
• Kammer (1987)
• Smith and Beattie (1991)
• … and many others
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MATRIX UPDATE METHODS
HAVE LIMITATIONS
• Lack of physical insight
– What do changes in K, M coefficients mean?
• Lack of physical plausibility
– Baruch/Berman method doesn't enforce connectivity
• Limitations for large problems
– Great for small “demo” models, but ...
– “Smearing" caused by Guyan reduction/expansion
• Lack of usability if test article different than flight vehicle
• Requires very precise mode shapes (unrealistic)
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PROPERTY UPDATE METHODS
ADJUST MATERIALS AND ELEMENTS
• Objective
– Identify changes to element and
material properties so that FEM
matches test
• Hasselman (1974)
• Chen (1980)
• Flanigan (1987, 1991)
• Blelloch (1992)
• Smith (1995)
• … and many others
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PROPERTY UPDATE METHODS
BETTER FOR LARGE MODELS
• Design sensitivity methods
– Computationally efficient
– Can examine many parameters
• Modal approach
– Match frequencies, shapes, x-ortho
– Better if modes available
• FRF approach
– Match resonances, anti-resonances
– Better if mode extraction difficult
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MEANS:
COMMERCIAL S/W FOR CORRELATION
• Correlation not a large market
• Commercial s/w slow to develop
– Main reason for trial-and-error
• Significant improvement recently
– Design sensitivity and optimization
• Major vendors
– SDRC, LMS, MSC, UAI
• Specialty companies
– DDS
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By Jove, Watson!
MEANS:
COMMERCIAL S/W FOR CORRELATION
• SDRC
– I-DEAS Correlation (MAC, ortho, x-ortho, mapping)
– CORDS (property adjustment using modes and DSA)
– FRFCORR (property adjustment using FRF)
• LMS
– CADA LINK (parameter updating, Bayesian estimation)
– Close ties with MSC/NASTRAN
• MSC
– SOL 200 design optimization (modes, FRF)
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MEANS:
COMMERCIAL S/W FOR CORRELATION
• UAI/NASTRAN
– Analysis-Test Correlation (ATC) in next release
– Sensor placement, error localization, parameter
updating, mode tracking
• Dynamic Design Solutions (DDS)
– FEMtools (follow-on to Systune)
– Wide range of pretest, correlation, and model
updating features
• Others (Sandia/ACTA SSID, etc.)
• Happy to see you at vendor exhibit!
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FUTURE OUTLOOK FOR
ANALYSIS AND TEST
• Major analysis trends
– Larger models (105-107 DOF)
– Faster computers
– More accuracy? More insight?
• Major test trends
– More accels (102-103 DOF)
– More modes
• More reliance on analysis
– Fewer major tests
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I never guess. It is
a shocking habit,
destructive to the
logical faculty.
FUTURE OUTLOOK FOR
CORRELATION AND MODEL UPDATING
• Many opportunities for further development
– Genetic algorithms
• Integration
– Modal and FRF methods
– Frequencies, shapes, ortho, x-ortho
• Process improvements
– Analysis - test - analysis
• Commercial software
– Critical for widespread use of advanced correlation
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WHAT ABOUT
VIRTUAL TESTING?
• Virtual testing
– FEM represents the hardware
– Simulated noise, nonlinearities
• Helpful to optimize a test
– Training and shakedown
– Sensor and exciter placement
• Does not replace a test
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There is nothing
like first-hand
evidence.
THE ROAD AHEAD:
CORRELATION IN 2001
• Analysis
– FEM with 10,000,000 DOF
– COTS s/w to assist test planning
• Accelerometer and exciter placement
• Test
– 1,000 accelerometers, 100 target modes
• Correlation
– Seamless integration of analysis and test
– COTS s/w for property updates
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CONCLUSION
CORRELATION IS AN IMPORTANT
STEP IN PRODUCT DEVELOPMENT
• Correlation helps analysis match reality
– Within limits of test hardware and test accuracy limits
• Correlation can be done quickly with correct tools
– Identify “best” changes to model to match test results
• Correlation reduces risk
– Use verified model to predict operating responses
• Correlation can be a powerful step that maximizes the
combined benefits of analysis and testing
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MAKE THE MOST OF
YOUR TIME AT IMAC-XVI
• Put on your detective hats
• Learn about the latest
– Test techniques
– Structural dynamics
– Correlation methods
• Become a Sherlock Holmes
of Model Updating and
Correlation!
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Come, Watson,
come! The game
is afoot!
BE A SHERLOCK HOLMES OF
CORRELATION AND MODEL UPDATING
Amazing, Holmes!
How did you
correlate that
model so quickly?
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Elementary, My
Dear Watson. I
went to IMAC-XVI.