Transform | ODSFRFs

Calculates a set of ODSFRFs from operational (or output-only) data.  

What is an ODSFRF?

A Typical ODSFRF.

Advantages of ODSFRFs

  1. The ODSFRF provides the true response (in displacement, velocity or acceleration units) at each DOF of a machine or structure, together with its phase relative to a Reference response.

  2. An ODSFRF contains peaks at resonant frequencies.

  3. ODS's cab be displayed  in animation from set of ODSFRFs

  4. Operating mode shapes. can be extracted by curve fitting a set of ODSFRFs

ODS's from a Set of ODSFRFs

If shape data from the cursor position in two or more ODSFRFs is displayed in animation on a model of a test structure, the resulting ODS is the true overall response of the structure at each DOF, with the correct phase relative to all other DOF.

What is Transmissibility?

Transmissibility is defined as the spectrum of a Roving response divided by the spectrum of a Reference response.  It is the traditional measurement that is calculated when no excitation forces are measured.  Transmissibility is calculated in the same way as an FRF, but a (fixed) Reference response is used instead of the unmeasured force.

An advantage of the Transmissibility is that if the excitation force level varies from one Measurement Set to the next during data acquisition, its effect on both the Roving and Reference responses is assumed to be the same, so its effect will be "canceled out" in the Transmissibility calculation.

Difficulty with Transmissibility's

A Transmissibility has a ”flat spot” instead of a peak in the vicinity of each resonant frequency, as shown below.

Roving Auto Spectrum & Transmissibility.

Scaled Cross Spectra

Scaled Cross Spectrum = Transmissibility x Reference Auto Spectrum

Advantages of Scaled Cross Spectra

Operating Mode Shapes

Setting Up Your Data

The Transform | ODSFRFs command can process multiple Measurement Sets of three different types of Source data;

To calculate ODSFRFs from time waveforms,

To calculate ODSFRFs from Auto & Cross spectra,

To calculate Scaled Cross Spectra from multiple Measurement Sets of Transmissibility data,

When this command is executed, the following dialog box is opened.

NOTE:  The Source measurements can be stored either in the same Data Block, or in two different Data Blocks.

Measurement Sets