Modal MAC and COMAC

Modal validation, MAC & COMAC

Modal Assurance Criterion (MAC) values can be used to compare two arbitrary complex vectors. The MAC value between two vectors who have linear relationship is near to one. The MAC value between two linearly independent vectors will be near zero. MAC calculation has two applications in the modal analysis.

First, it can be used to compare two mode shapes obtained from two different modal parameter estimation processes on the same test data. Two similar mode shapes have a high MAC value, and two same mode shapes have a MAC value of 1.

Second, it can be used to check the orthogonality of mode shapes when weighted by the mass matrix. Even when no exact mass matrix is available, the orthogonality of mode shapes is approximately satisfied. We can use it to validate the modal result.

The MAC value table can also be showed by the right click popup menu.

Coordinate Modal Assurance Criterion (COMAC) is an extension of Modal Assurance Criterion (MAC). COMAC attempts to identify which measurement degrees of freedom contribute negatively to a low value of MAC. COMAC is calculated over a set of mode pairs, analytical versus analytical, experimental versus experimental or experimental versus analytical. In Modal, COMAC between modes from different identification algorithms or imported modes can be calculated.

To calculate COMAC, the first step is to set the two sets of modes you want to be compared.

In the second step, the mode pairs should be determined. The process of modes paring will be done by Modal automatically, according to the MAC values between each modes in the two sets. You are also allowed to select part of the mode pairs.

Then the COMAC values table will be displayed.