Forced Vibration Tests &
Experimental Modal Analysis
by Ardalan Sabamehr, Ph.D.
Experiment Modal Analysis (EMA) has been the conventional modal identification technique used by civil engineering for more than six decades. This approach uses an input excitation of known force levels and frequencies, both controlled by the experimentalist. Forced vibration tests have the privilege of suppressing the effects of extraneous noise in the measured structural response. Devices such as vibrators, vibrator exciters, exciters or shakers have been used for transmitting a vibratory force into the structure for analysis. EMA is a classical technique for deriving the modal properties using the known input load and response of the structure.
(a)
(b)
Servo-hydraulic shakers to excite: (a) bridges (mounted vertically); (b) dams, (mounted laterally) (Source: Cunha, A. and Caetano, E., 2006. Experimental modal analysis of civil engineering structures.)
Types of Exciters
Eccentric rotating mass vibrators
Eccentric rotating mass vibrators are a mechanical vibration machine and are used in order to excite large-scale civil engineering structures. The role of an eccentric mass is to generate a vibratory force using a rotating shaft. The applied magnitude of the force depends on mass, rotational speed and the out of balance displacement produced.
Electrohydraulic Vibrators
Electrohydraulic vibrators are able to generate higher magnitude forces when compared to other types. The high-pressure flow generates the force which is supplied by one or more pump units. The system also includes a servo controller hydraulic exciter. The force magnitude can be changed by varying the weight of the mass. This sort of exciter supplies high vibration movement with accurate excitation at different levels of frequencies in torsion or bending.
Nowadays, due to advancements in vibration sensors and mathematical algorithms, it is possible to derive modal properties without the need for external exciters and can be based solely on measuring the response of the structure in ambient conditions.
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