104 5. EXPERIMENTAL MODAL ANALYSIS
ISPE Test Modal Orthogonality
(NASA STD-5002 Method)
ISPE Test Modal Orthogonality
(SFD-2018 Complex Modes)
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Figure 5.15: ISPE test mode orthogonality estimates.
TAM mass matrix. Further discussion on the merits associated with SFD-2018 is included in
Chapter 6.
5.2.7 CLOSURE
Experimental modal analysis is a mature discipline in the structural dynamics community, which
is as much an “art” as it is a “science.” Modern procedures for estimation of modal parameters
from measured data are highly automated; however, applications involving complicated struc-
tural systems and/or systems with closely spaced, parametrically sensitive modes require the test
engineer’s experience and judgment (“art”) to discern the difference between authentic and spu-
rious (“junk” or “noise”) system modes. A prevailing metric for experimental modal data valida-
tion is the orthogonality check, which relies on a model-based (TAM) mass matrix. In addition,
reconstructive synthesis of measured FRF data is another widely used strategy for experimental
mode validation. e present EMA study employs mathematical operations aimed at isolating
individual candidate experimental modes without reliance on a TAM mass matrix.
e key to mathematical and visual isolation of individual modes from measured data is
the left-hand eigenvector. e most effective approach to determination of left-hand eigenvec-
tors stems from employment of techniques that estimate the measured system’s plant or effective
dynamic system matrix. Since a complete set of (authentic and “noise”) system modes are es-
timated for the plant, left-hand eigenvectors are determined from the inverse of the complete
right-hand eigenvector set.
e following metrics provide a systematic basis for EMA.
1. e estimated SDOF modal FRF, formed by the product of a single estimated left-hand
eigenvector and FRF matrix, is plotted in terms of real and imaginary components vs.
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