FEMtools Modal Parameter Extractor

Modal Parameter Extraction using an Advanced Polyreference Estimation Method

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 Analysis).

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 evaluating monitoring.

The Ft-MPE is installed and used as an add-on to the FEMtools Framework or any other FEMtools configuration.

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 adapted for classical or operational modal analysis.
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 specification).
Multiple frequency band selection and combination of poles (applet only)
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 reference channels).
Digital filters to remove certain frequency components from time series. The following types of filter are supported: low-pass, high-pass, band-pass, band-stop.
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.

Applications

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 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.

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.
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 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.

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 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.

MPE FRF Selection Tab

(a) FRF selection

MPE Pole Extraction Tab

(b) stabilization chart

MPE Modes Extraction Tab

(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.