FEMtools Modal Parameter Extractor
Advanced Modal Parameter Extraction using Global and Local
The FEMtools Modal Parameter Extractor (Ft-MPE) module is a FEMtools add-on tool for extracting modal
parameters (natural frequencies, mode shapes and modal damping) of a structure from a set of Frequency Response Functions (FRFs) or cross-power
spectra (XPS) that are computed from response time series.
Modal parameters can be used for applications in structural dynamics or
to validate and update a finite element model. The extractor can be used
interactively or as a component of an automated process for structural health
monitoring and evaluation.
The Ft-MPE is installed and used as an add-on to the FEMtools Framework or
any other FEMtools configuration.
The extractor offers the following features integrated in a easy-to-use and
- Extraction of modal parameters using a global poly-reference Least
Squares Complex Frequency (pLSCF) method or a local curve-fit method.
- Classical (input-output) or operational (output-only) modal
- Operation using an interactive applet or command-driven.
- Interactive reference and response selection (applet only).
- Frequency-domain operational shapes viewer (applet only).
- Automatic or manual pole selection based on a stabilization chart
(global method only).
- Narrow or wide band extraction (minimum and maximum frequency
- Multiple frequency band selection and combination of poles (global
- Extraction of complex or normal mode shapes.
- Residual modes to compensate out-of-band modes (global method only).
- Validation of the extracted mode shapes using auto-MAC, mode
complexity analysis, FRF re-synthesis and animated mode shape plots.
The MPE add-on also includes a Digital Signal Processsing (DSP) library that
is used for the following tasks:
- Process imported time histories (detrending, filtering,...).
- Automatically select the optimal reference channels to compute the
cross-power spectra for operational modal analysis.
- Computation of cross-power spectra from time series.
- Support of a roving batch of sensors (multiple measurement setups
with the same reference channels) for operational modal analysis.
- Digital filters to remove certain frequency components from time series.
The following types of filter are supported: low-pass, high-pass, band-pass,
- Re-sampling of time series to decimate the number of samples.
- Removing DC components or linear trends from time series.
- Extracting general information on the imported time series.
The functions of FEMtools Framework are used to
- Import and export of FRFs, time series and modal parameters is done using the Universal File format or custom translation scripts.
- Import and display time histories.
- Interactive graphics display of animated mode shapes, FRF curves etc.
- Automated data processing using scripts.
The FEMtools Modal Parameter Extractor (Ft-MPE) module is used to extract the
modal parameter (natural frequencies, mode shapes and modal damping) from
Frequency Response Functions (FRF) for classic input-output modal analysis (EMA) and
from time histories obtained under operational conditions (operational
modal analysis; OMA).
Using third party tools, FRFs can be experimentally obtained using hammer or shaker
excitation and the response signals, usually accelerations, are divided by the
known excitation signal to obtain FRFs. By curve-fitting these FRFs, the modal
parameters can be identified with FEMtools MPE.
Under real operating or ambient conditions, the excitation
signal is unknown and no FRFs can be obtained. In this case, cross correlation
spectral functions are used to extract the modal parameters. FEMtools MPE can
import time histories, compute cross correlation functions and extract the modal
FEMtools MPE can be used as a standalone tool for modal analysis, perhaps
combined with a data acquisition system. As an add-on to any standard FEMtools configurations, modal extraction
can be used in other FEMtools modules for the following purposes:
- Dynamic response analysis - Dynamic analysis relies on modal
superposition of mode
shapes for response analysis in time or frequency domain.
- Structural dynamics modification - Predict the effect
of changes to stiffness, mass and damping in modal space.
coupling - Simulate the dynamic response of assembled structures, using
coupled modal models obtained from test or FE analysis.
- Pretest planning - The modal extraction process can be
evaluated using simulated test data as part of a virtual testing
- Test-Analysis modal correlation - Comparing reference
test modes with predicted modes provides a mean to validate simulation
- FE model updating - Finite element models can be
updated by improving the correlation between reference test modes and
- FRF resynthesis - Test FRFs can be smoothed by
re-synthesis from extracted modal parameters which is recommended if these
FRF will be used for FRF-based updating of FE models.
- Rigid body properties extractor - To obtain a
better mass line for rigid body properties extraction, the first resonance peaks
can be removed from the FRFs. This is done by modal extraction and
re-synthesis without including the first mode shape.
- Material identification - Identification of material
properties using mixed numerical-experimental vibration analysis. Modal
extraction provides a way to identify the mode shapes of a test sample.
- Structural health monitoring - Modal extraction is a component of
a system for automated monitoring of modal
parameters in structures.
- Easy to Use - Only minimal user-interaction
is required and all functions are combined in an interactive applet (see
- Powerfull - Using efficient algorithms and optimized
DSP functions, modal extraction is fast. On 64-bit platforms, it is possible
to use a very high number of channels and a wideefrequency range in a single
pass without running into memory limitations.
- Reliable - Reliable and stable methods are used for
global and local extraction of modal parameters. Out-of-band modes are taken into
account to compensate residual effects and improve extraction accuracy.
- Flexible - Modal extraction can be operated manually
or used as part of an automated process.
Prequisites and Licensing
The FEMtools MPE add-on requires a separate license that is used together with a license for
any FEMtools standard configuration.
(a) FRF selection.
(b) Stabilization chart.
(c) Mode shapes.
Modal analysis of a satellite structure using the global MPE applet. The screen shot of the FEMtools
MPE user interface shows the (a) FRF selection, (b) stabilization chart, and (c) mode shapes, overlay
of original FRF
and re-synthesized FRF and autoMAC plot.
Local curve fit of FRFs with mass loading (local MPE applet).