FEMtools Modal Parameter Extractor
Modal Parameter Extraction using an Advanced Polyreference
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 that are computed from response time series (Operational Modal
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 poly-reference Least Squares
Complex Frequency (pLSCF) method adapted for classical or operational modal
- Operation using a UI 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.
- Narrow or wide band extraction (minimum and maximum frequency
- Multiple frequency band selection and combination of poles (applet
- Extraction of complex or normal mode shapes.
- Residual modes to compensate out-of-band modes.
- Support of a roving batch of sensors (multiple measurement setups
with the same reference channels) for operational modal analysis.
- Validation of the extracted mode shapes using auto-MAC, mode
complexity analysis, FRF re-synthesis and animated mode shape plots.
- Automated data processing using scripts.
- Interactive graphics display of animated mode shapes, FRF curves etc.
Import and export of FRFs, time series and modal parameters is done using the
universal file format or custom translation scripts.
For operational modal analysis, DSP commands are used to
- 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 for roving sensors (multiple sets of time series with common
- 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.
- Displaying the time series.
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 and
directly from time histories obtained under operational conditions (operational
FRFs can be experimentally obtained using hammer or shaker
excitation. The response signals, usually accelerations, are divided by the
known excitation signal to obtain FRFs. By curve-fitting the FRFs, the modal
parameters can be identified.
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 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:
- Structural dynamics - 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.
- Powerful - Efficient algorithms that make the best use
of your computing environment. On 64-bit platforms, it is possible to use a
very high number of channels and a wide frequency range in a single pass
without running into memory limitations.
- Easy to Use - Only minimal user-interaction interaction
is required. Poles can be automatically selected.
- Efficient - Very clear stabilization
charts are produced making them suitable for automatic identification of stable poles.
- Reliable - A powerful polynomial curve
fitting method is used that has proven to provide reliable estimates of natural
frequencies, damping and mode shapes. 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. 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.