Operational Modal Analysis

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Upcoming webinar: Operational Modal Analysis (OMA) using ARTeMIS Modal

March, 6 2025, 15.00 CET (UTC+1)
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Operational Modal Analysis

Operational Modal Analysis (OMA) is the modal testing approach for the vast number of cases where it is impossible to measure the input forces that is acting on a structure during testing.

A description of Operational Modal Analysis

During the last decades OMA has been called many things. Some of the names are Output-Only Modal Analysis, Natural Input Modal Analysis, Ambient Vibration Testing, Ambient Response Testing and In-operation Modal Analysis. No matter what name is used, the algorithms and the way to test a structure are the same.

OMA was developed out from a need to know the modal parameters, i.e. natural frequencies, damping ratios and mode shapes, of structures that cannot be put into a test rig in a modal testing laboratory. This can either be because of the size of the structure or because of the desire to know the modal behaviour in-situ and using the natural source of excitation acting on the structure.

Historically, OMA has been widely used in onshore and offshore civil engineering but over the years its popularity has spread to many other engineering fields such as mechanical-, automotive-, aerospace-, and maritime engineering.

The required measurement setup is very simple as only response measurements are required. Any sensor capable of measuring the dynamic response of a structure can in principle be used. However, most popular sensors are still accelerometers.

For large civil engineering structures, the choice of accelerometers has for years been the Forced Balanced Accelerometer (FBA) capable of measuring both weak and strong motion due to their high dynamic range and low noise level. Lately, mems accelerometers have gained increased interest as well both because of price but also easier handling. Other widely used sensors for OMA are the velocity-based geophones, strain-gauges, laser vibrometers, Fiber Bragg Grating (FBG) and proximity probes.

The common basis for all OMA algorithms is the raw time series response measurements. Based on these measurements a wide range of OMA algorithms have been developed over the last many decades. The common assumption for all algorithms is that there are multiple independent and random input sources acting on the structure during testing. It is assumed that these broad-banded inputs are derived from so-called Gaussian white noise. If there are broad-banded input sources acting on the structure, there can also be other types of external forces like rotating equipment present.

Benefits

ARTeMIS Modal is an open platform allowing response measurements to be uploaded through more than 20 different time series files formats. Response measurements can also be measured using the internal data acquisition module.

ARTeMIS Modal support single and multiple Test Setups testing. Either place all sensors at the structure and perform a single Test Setup or use rowing sensors keeping a few in fixed positions as references to produce multiple Test Setups.

ARTeMIS Modal includes up to three frequency domain operational modal analysis techniques derived from the user-friendly Frequency Domain Decomposition (FDD) technology. FDD utilizes the Singular Value Decomposition of the estimated spectral densities of the measured response. The FDD techniques available are:

  • Frequency Domain Decomposition – FDD.
  • Enhanced Frequency Domain Decomposition – EFDD.
  • Curve-fit Frequency Domain Decomposition – CFDD.

All three techniques are based on peak-picking in the frequency domain using either automatic picking or manual picking using the mouse. Once picked, the mode shapes are ready for immediate animation. The techniques are all specially designed to account for the presence of deterministic signals (harmonics) in case of rotating structural parts.

ARTeMIS Modal also includes up to five time domain modal analysis techniques. They are all of the data driven Stochastic Subspace Identification (SSI) type and are all implementing the powerful Crystal Clear SSI® realization estimator. Crystal Clear SSI® produces extremely clear stabilization diagrams with un-seen accuracy of the physical parameters and nearly no noise modes.  The SSI techniques available are:

  • Extended Unweighted Principal Component – SSI-UPCX.
  • Unweighted Principal Component – SSI-UPC.
  • Principal Component – SSI-PC.
  • Canonical Variate Analysis – SSI-CVA.
  • Unweighted Principal Component Merged Test Setups – SSI-UPC-Merged.

These techniques estimate the modal parameters directly from the raw measured time series. The SSI techniques incorporate effective ways of dealing with noise. As a result, the modal parameter estimations are the most accurate commercially available today. The SSI techniques can work with closely space and repeated modes with light or heavy damping.

Since the SSI techniques are working in time domain there are no leakage bias or lack of frequency resolution. As a result, the modal parameter estimates are asymptotically unbiased. Further, as the SSI techniques are low model order estimators, the statistical errors of the modal parameter estimates are extremely small.

Features

Test Geometry:

  • Can be prepared through the Prepare Geometry Task
  • Can be imported from Universal File Format (UFF/UNV), SVS Configuration File Format (CFG), AutoCAD DXF/DWG, Stereolithography File Format (STL)

Internal Data Acquisition Module

  • Require a data acquisition plugin license for one of the supported data acquisition systems
  • Support data acquisition systems are:
    • National Instrument modules that can be controlled using NI-DAQmx driver
    • SINUS Messtechnik Apollo acquisition modules
    • HGL Dragonfly acquisition modules
    • Hottinger Baldwin Messtechnik (HBM) MX1601, MX840 and MGCplus devices
  • Support IEPE sensors with or without TEDS
  • Direct streaming of measurements to ARTeMIS Modal test setups
  • View acquired time series real-time of user selectable channels
  • On-the-fly calculation of spectral densities and singular value decomposition (SVD) of spectral densities
  • Multiple recording modes: Manual or triggered start/stop, predefined interval of time, continuous recording mode with optional definition of a ring buffer period in days
  • Seamless integration with all other tasks in ARTeMIS Modal

Frequency Domain Decompostion Techniques:

  • Intuitive modal parameter estimation based on peak-picking in frequency domain
  • Immediate results even in case of hundreds of measurement channels and modes
  • Extremely robust being based on the Singular Value Decomposition of spectral densities
  • Handles single or multiple Test Setups in exact same way
  • Using the powerful harmonic detection and reduction tools, modal parameters can be extracted in the presence of harmonic components from e.g. rotating machinery
  • Peak-picking can be made manually or automatic
  • EFDD and CFDD methods provide estimates of damping ratios as well as improved estimates of natural frequencies and mode shapes
Operational Modal Analysis

Stochastic Subspace Identification Techniques:

  • Modal parameter estimation based on estimation of state space models in time domain
  • State space model estimation can be done on even large channel counts using Projection Channels
  • Projection Channels can be found manually, semi-automatic or fully automatic
  • Very fast estimation of even hundreds of models of increasing dimension
  • Limited user interaction required. Can be fully automated. By default, the techniques always start with a fully automatic estimation of the global modes
  • Unbiased modal parameter estimation using standard least-squares or the Crystal Clear SSI® realization estimator
  • Estimation of uncertainties of extracted modal parameters (SSI-UPCX)
  • Handles single or multiple Test Setups
  • Using the powerful harmonic reduction tools, modal parameters can be extracted in the presence of harmonic components from e.g. rotating machinery
  • Extraction of global mode estimates can be made manually or automatic
  • Estimated state space models can be validated against data in frequency domain. Both spectral response and prediction errors can be validated
  • Estimated state space models as well as the entire stabilization diagram can be exported in ASCII file format for use outside ARTeMIS Modal
Operational Modal Analysis

Mode Validation:

  • Auto- and Cross Modal Assurance Criterion of estimated mode shapes
  • Mode shape complexity diagram
  • Overlaid- and difference animation of mode shapes
  • Side-by-Side and Top-Bottom animations of mode shapes
  • Import and validation of modes from third party software through Universal File Format (UFF/UNV)

 

License Options

ARTeMIS Modal OMA is available as a perpetual license or a subscription-based license and comes in three versions: Basic, Standard, and Pro. You can further customize your setup with a wide range of data acquisition plug-in modules to suit your needs. Whether you need more or fewer features, we’re here to help you find the version that fits your requirements best.

Which Solution is Right for You?

Choosing the right licensing option depends on your organization's needs, budget, and long-term plans. Below are the advantages of each type to help you decide:

Perpetual License

  • One-Time Investment: Pay once and own the license for a lifetime.
  • Cost-Effective Over Time: Ideal for long-term use without recurring subscription fees.
  • Optional Maintenance Plan: An annual maintenance fee ensures continued access to technical support and software updates.
  • Full Ownership: Retain access to the software indefinitely, even without maintenance (though updates and support require an active plan).
  • Best for Stable, Long-Term Monitoring Projects.

Subscription License

  • Lower Initial Cost: Start with a smaller investment compared to a perpetual license.
  • Always Up to Date: Includes access to the latest software updates and features.
  • Flexible Commitment: Adapt to changing project needs with an annual payment model.
  • Best for Short-Term Projects or Flexible Budgets.

Feel free to contact us for guidance or a tailored recommendation.

Contact us at sales@svibs.com for more information or assistance.

Download OMA Product Data below.







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    Operational Modal Analysis using ARTeMIS Modal
    September 2023

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