The term Multi-Reference Modal Analysis means that FRFs are acquired using two or more references (fixed input or output sensors) and are curve fit using Multi-Reference curve fitting methods.
Multi-Reference Modal Analysis is required when a structure has resonances that occur under one of the following conditions;
Closely Coupled modes: One resonance peak represents two or more modes.
Repeated Roots: Two or more modes having the same natural frequency but different mode shapes.
Local modes: Different resonance peaks occur in FRFs from different references.
In each of the above cases, multiple reference curve fitting is required in order to properly extract all modal parameters from a set of FRFs. Multiple reference FRFs correspond to multiple rows or columns of the FRF matrix in the MIMO model .
A single reference set of FRFs is,
The minimum requirement for extracting experimental modal parameters using FRF-based curve fitting
Obtained by exciting the structure with a single (fixed) exciter or using a single (fixed) response sensor
Is a single row or column of elements in the FRF matrix of a MIMO model of the structure
Not sufficient for extracting closely coupled modes, repeated roots, or local modes of a structure
A multi-reference set of FRFs is,
Required for extracting closely coupled modes, repeated roots, or local modes of a structure
Obtained by exciting the structure with multiple (fixed) exciters or using multiple (fixed) response sensors
Consists of multiple rows or columns of elements in the FRF matrix of a MIMO model of the structure
Useful for extracting modes when a structure has high modal density (many resonance peaks in small frequency bands)