NVH toolkit – Bringing Test & Simulation Together

Import test data in Universal File Format (UNV)

There are many ways to carry out an EMA - from pulse hammers and accelerometers to excitation via shakers or loudspeakers and measuring the response with 3D laser vibrometers. The manufacturers of measuring equipment often use proprietary file formats that cannot be read without the associated software. This is why the Laboratory for Structural Dynamics at the University of Cincinnati defined the Universal File Format (*.unv or *.uff). This standard describes a pure text file and can be exported from almost any measurement software. The file contains text blocks that are referred to as formats and, depending on their content, adhere to a predefined structure (measurement point coordinates, eigenmodes) [1].

In this article, we look at a simple steel beam for which a measurement is available. This contains a total of ten different measuring points and the first six eigenmodes. In Ansys Mechanical, all we have to do is enter the path to the corresponding measurement file in the details window. The information read from the UNV file is then automatically displayed in the graphics window. We can see the measurement grid used in red so that we can directly recognize the extent to which the orientation matches our simulation model. If necessary, we can also correct the orientation or scaling directly in the MAC Calculator.

Visual comparison of the mode shapes

After an update of the results, the view shows a matrix display with coloured cells that represent the simulated modes row by row and those from the UNV file column by column - a comparison of the two worlds. To visualise two modes side by side for comparison, click in the corresponding field of the matrix. The FE result is then displayed in the graphics window on the left and the measured mode on the measuring grid on the right. A side-by-side animation of the paired modes is also possible. This allows us to make an initial subjective assessment of what belongs together. Does this mean that we now have to play "mode memory" and the path to automation is blocked by all the looking?

The Modal Assurance Criterion (MAC)

Let's take another look at this MAC matrix. Behind it is the Modal Assurance Criterion (MAC), which is a correlation criterion for eigenmodes. It provides information on how "related" two mode shapes are to each other. For all indices i and j, the possible mode combinations from FEM and EMA are calculated according to

Calibration of dynamic simulations with tests

But what should be done if mode pairs are found, but large deviations, e.g. in the frequency, are diagnosed? The MAC provides us with an objective quality criterion for the correspondence of eigenmodes as a numerical value, which also allows automatic mode pairing. An objective function for optimization can be derived from this. The deviation between simulation and test is minimized by varying uncertain simulation parameters (material parameters, physical contact parameters, etc.) with the aim of identifying these. This workflow can be set up with Ansys optiSLang in the Workbench. We will see how this works in practice in the linked CADFEM training!