FEMtools Modal Parameter Extractor

Extract Modal Parameters from Frequency Response Functions or Cross Power Spectra

The FEMtools Modal Parameter Extractor (Ft-MPE) module is a FEMtools add-on module for extracting modal parameters from a set of Frequency Response Functions (FRFs) or cross-power spectra.

Overview

The FEMtools Modal Parameter Extractor (Ft-MPE) module is a tool for extracting modal parameters (natural frequencies, mode shapes and modal damping) from a set of Frequency Response Functions (FRFs) or cross-power spectra. The Ft-MPE is installed and used as an add-on to the FEMtools Framework or any other FEMtools configuration. For working with time series, it can be used together with the FEMtools DSP add-on module to compute cross power spectra.

Key Features

The extractor offers the following features integrated in a easy-to-use and interactive environment:

Extraction of modal parameters using a poly-reference Least Squares Complex Frequency (pLSCF) method.
Automatic or manual pole selection based on a stabilization chart.
Narrow or wide band extraction (minimum and maximum frequency specification).
Residual modes to compensate out-of-band modes.
Extraction of complex or normal mode shapes.
Validation of the extracted mode shapes using auto-MAC, mode complexity analysis, FRF re-synthesis and animated mode shape plots.
Operation using a GUI wizard or command-driven.
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 script.

For operational modal analysis, the FEMtools DSP module is 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.

Applications

The FEMtools Modal Parameter Extractor uses Frequency Response Functions (FRF) for classic input-output modal analysis. If excitations are not known, then time series of responses obtained under operational conditions are used (operational modal analysis).

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.

The modal parameters obtained by modal analysis 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.
Modal 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 process.
Test-Analysis modal correlation - Comparing reference test modes with predicted modes provides a mean to validate simulation models.
FE model updating - Finite element models can be updated by improving the correlation between reference test modes and predicted modes.
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 included 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.

Combined with a data acquisition system, Ft-MPE becomes a standalone tool for complete modal analysis.

Benefits

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 wide frequency range in a single pass and 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 MPE and DSP tools require a separate common license that is used together with a license for any FEMtools standard configuration.

FEMtools MPE User Interface

Modal analysis of a satellite structure. The screen shot of the FEMtools MPE user interface shows a stabilization chart, mode shape, overlay original FRF and re-synthesized FRF and autoMAC plot.