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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. Introduction Terminology

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status and future outlook of correlation and model updating

STATUS AND FUTURE OUTLOOKOFCORRELATION 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
AGENDA
  • Introduction
  • Terminology
  • Motive, means, and opportunity
  • Current methodology
  • Commercial s/w programs
  • Future possibilities
  • Conclusion
terminology many methods for correlation
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
model correlation is detective work
MODEL CORRELATIONIS DETECTIVE WORK

All knowledge comes usefulto the detective.

Sherlock Holmes

Consulting Detective

221b Baker Street

introduction motive means and opportunity
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?
motive models are not reality
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
motive models need reality check
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!
test is not reality either
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
motive correlation reduces risk

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

MOTIVE:CORRELATION REDUCES RISK
  • Must certify that structure will operate correctly
  • Usually cannot test flight structure to operational loads
  • Use test-correlated FEM to verify correct operation of flight structure to flight loads
opportunity correlation must be fast
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

“There are always some lunatics about. It would be a dull world without them.”

challenge not a unique solution
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

“When you have eliminated the impossible, whatever remains, however improbable, must be

the truth.”

means much research in correlation
MEANS:MUCH RESEARCH IN CORRELATION
  • 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

Were Lestradebut gifted with imagination,he might rise togreat heights.

MEANS:MANY CORRELATION METHODS
  • Trial-and-error
    • Stop doing this! It's almostthe next millenium!
    • Too slow for fast-paced projects
    • Not sufficiently insightful for complex systems
  • FEM matrix updating
  • FEM property updating
  • Error localization
matrix update methods adjust fem k and m elements
MATRIX UPDATE METHODSADJUST 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
matrix update methods have limitations
MATRIX UPDATE METHODSHAVE 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)
property update methods adjust materials and elements
PROPERTY UPDATE METHODSADJUST 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
property update methods better for large models
PROPERTY UPDATE METHODSBETTER 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
means commercial s w for correlation

By Jove, Watson!

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
means commercial s w for correlation1
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)
means commercial s w for correlation2
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!
future outlook for analysis and test

I never guess. It is a shocking habit, destructive to the logical faculty.

FUTURE OUTLOOK FORANALYSIS 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
future outlook for correlation and model updating
FUTURE OUTLOOK FORCORRELATION 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
what about virtual testing
WHAT ABOUTVIRTUAL 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

There is nothing like first-hand evidence.

the road ahead correlation in 2001
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

2

0

0

1

conclusion correlation is an important step in product development
CONCLUSIONCORRELATION 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
make the most of your time at imac xvi
MAKE THE MOST OFYOUR TIME AT IMAC-XVI
  • Put on your detective hats
  • Learn about the latest
    • Test techniques
    • Structural dynamics
    • Correlation methods
  • Become a Sherlock Holmesof Model Updating and Correlation!

Come, Watson, come! The game is afoot!

be a sherlock holmes of correlation and model updating
BE A SHERLOCK HOLMES OFCORRELATION AND MODEL UPDATING

Amazing, Holmes! How did you correlate that model so quickly?

Elementary, My Dear Watson. I went to IMAC-XVI.