Single Reference Modal Test

The most common type of Experimental Modal Analysis (EMA ) is done using a single reference.  A single reference EMA can be performed in two ways;

MIMO Model

A MIMO model of a structure completely represents the dynamic properties of the structure between all pairs of Input (excitation) and Output (response) DOFs for which it is defined.  A FRF matrix of a MIMO model contains rows & columns of FRFs.  Each FRF defines the Output (response) of the structure at a DOF due to the force Input (excitation) at another DOF.

Roving Response Test

In a roving response test, a single fixed (reference) exciter is used to excite the structure, and a roving response transducer is used to measure its response.  Each FRF measurement is calculated between the excitation force signal applied to the structure and the response signal from a response transducer.  The FRFs correspond to measuring elements from a single column of the MIMO model for the structure. (See the Multi-Input Multi-Output Modeling & Simulation chapter for details.)  The excitation force can be provided in two ways,

  1. Attaching a shaker to the structure and driving it with a broad band signal.

  2. Impacting the structure at the same DOF (point & direction) for each measurement.

Roving Impact Test

In a roving impact test, a single fixed (reference) response transducer is used, and the structure is excited using a roving impacter.  Each FRF is calculated between the response signal from the same DOF and the excitation signal from a different roving DOF.  The FRFs correspond to measuring elements from a single row of the MIMO matrix for the structure.  The impact hammer applies a broad band impulsive force to the structure, which excites many resonances at a time.  The excitation force is measured with a load cell attached to the head of the hammer.

Maxwell's Reciprocity

Maxwell’s Reciprocity: The structural response at DOF A due to the excitation force at DOF B is the same as the structural response at DOF B due to the excitation force at DOF A. In modal testing, Maxwell’s Reciprocity is usually assumed to be valid.

When Maxwell’s Reciprocity is valid,

When Maxwell’s Reciprocity is valid,