Multi-Reference Modal Testing

A multiple reference modal test is done using either multiple (fixed) exciters with sensors to measure the forces, or multiple (fixed) response sensors.
Each fixed sensor is called a Reference.

Multiple Shaker Test

In a multiple shaker test, two or more (fixed) shakers are used to simultaneously excite the structure.

• The multiple shakers must be driven with uncorrelated broad band signals.

• In a multiple reference test, an FRF between each response and each reference force is calculated

• Also, Multiple & Partial Coherences are calculated

• The FRFs are elements of two or more columns of the FRF matrix in a MIMO model A MIMO model is a Multiple Input Multiple Output frequency domain matrix model containing a vector of Inputs, a matrix of Transfer functions and a vector of Outputs. The MIMO model is an equation where the Transfer function matrix is post-multiplied by the vector of Input DFTs to obtain the vector of Output DFTs. Inputs, Outputs & Transfer functions can be calculated using different forms of the MIMO model equation. of the structure.

Large structures with non-linear dynamic behavior are typically tested using multiple shakers, driven by pure or burst random excitation signals
Random excitation together with spectrum averaging is used to "average out" the non-linear dynamic behavior of the structure from the spectra and hence the FRFs.

Multiple Reference Roving Impact Test

In a multiple reference roving impact test, two or more (fixed) response sensors are used, and the structure is excited one DOF at a time with a roving impactor.

• This test is the same as performing two or more single Reference modal tests, but takes no more time to complete than a single reference roving impact test

• The FRFs are elements of two or more rows of the FRF matrix in a MIMO model of the structure