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Experimental Modal Analysis (EMA) and Output Only Modal Analysis (OMA)

Modal analysis via measurement

The modal analysis via measurement is a sophisticated analytical procedure in the field of structural dynamics. The goal here is to define the decisive vibration characteristics of a machine or component, i.e. its:

  •  Natural frequencies
  • Mode shapes
  • Degrees of modal damping

Generally, it can be differentiated between two procedures; we implement both on a regular basis and tailored to the individual problems:

  • Experimental Modal Analysis (EMA), or
  • Output Only Modal Analysis / Operational Modal Analysis (OMA)
Laser scanner: Modal analysis of a plate

One of the distinguishing features of the OMA is that no defined external excitation is applied to such a measurement. The vibration excitation is triggered by the environment - ideally broadband. The OMA procedure was originally developed in the field of building dynamics, where e. g. a broadband excitation by the wind can take place. When analyzing machine building structures, an additional broadband excitation by a shaker can be generated artificially. Here, too, a special software is used which extracts the modal parameters.

The results of both analyses can directly be used for the analysis of a problem due to vibration, or, for example, to update the FE model (model update).

Measurement result: Eigenform plate

During the classic EMA, the vibrational behavior of the test object is analyzed after installation or under “free free” conditions. For this procedure the test object is sufficiently equipped with acceleration sensors – or, alternatively, we can also employ a Scanning-Laser-Doppler-Vibrometer (SLDV). During the test, the test object is usually excited by a (modal) hammer or a shaker. The excitation signal (force) is measured by a force sensor. Specialized software is used to determine the modal parameters using the measurements of the excitation force and the vibration response of the test object.

Transfer function at plate