MAC and POC

The Modal Assurance Criteria (MAC) was initially developed as a tool for the test engineer to distinguish between different mode shapes from different experimental modal tests. This was later extended to identify the similarity between analytical and experimental vectors. The MAC is an extremely powerful tool used in the correlation of vectors.

One of it's main advantages (as well as a main drawback) is that the mass of the system is not included in this check. This lack of mass scaling can lead to some confusing results at times. The MAC is most sensitive to the largest degrees of freedom in the modal vector and does not give a good indication to the smaller degrees of freedom in the vector. The MAC is also very useful for identifying missing modes of mode switching that can occur.

The Pseudo Orthogonality Check (POC) is actually nothing more that a mass scaled MAC. Of course, we recognize this as an orthogonality check - it's called a Pseudo Orthogonality Check because it really isn't an orthogonality check because we are mixing analytical and experimental matrices to determine how similar the experimental vectors are related to the analytical matrices.

On the surface this check looks very simple and harmless. But due to the extremely large mismatch between the analytical degrees of freedom and the measured experimental degrees of freedom, either the analytical model must be reduced to the set of tested degrees of freedom or the experimental mode shape must be expanded to the finite element set of degrees of freedom. This reduction or expansion can have a significant effect on the resulting correlation study depending on various items. Several reduction schemes have been developed to minimize some of the distortion resulting from the reduction process.

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