ArticleModal Testing. Part 2: Experimental Modal Analysis
by Robert J. Sayer, P.E.
This second of three papers on the topic of modal analysis describes experimental modal analysis, what it is, why you would do it and the basics on how to do it. The differences between experimental modal analysis (EMA) and finite element analysis (FEA) are described. How to validate the results of a modal test using the modal assurance criterion (MAC) is discussed.
“Experimental modal analysis (EMA) is a method used to determine the modal parameters (natural frequency, damping, and mode shapes) of a mechanical system. Software packages are available to simplify the curve-fitting of modal test data and provide animations of the resulting mode shapes.
Resonance problems can be solved by either changing the natural frequency that is being excited or removing the dynamic force that excites the resonance. In many cases, the dynamic force is developed as part of the normal operation of the equipment and cannot be removed. Thus, the natural frequency must be changed.
Knowledge of the mode shape associated with each natural frequency of concern is necessary to evaluate structural and/or mechanical modifications that need to be made to a vibrating mechanical system. The mode shape provides information about the location (position and orientation) of a modification within the mechanical system for the greatest beneficial effect. This information can be used to optimize the design of modifications made for the purpose of altering the modal characteristics of a machine or structure.
EMA requires impact response data for many locations and directions, referred to as degrees of freedom. The number of degrees of freedom of the test must be sufficient to accurately define all mode shapes of interest. The relative magnitude and phase information at all degrees of freedom are plotted to provide an animation of the mode shapes. If the number of degrees of freedom is not sufficient, a visual aliasing of the mode shape can occur. This is analogous to the aliasing errors experienced in digital signal analysis of data with insufficient sampling.
EMA can be performed using either of two impact test methods − fixed excitation and fixed response. In the fixed excitation test, the impact location and direction remain stationary for the entire test. The degree of freedom associated with the impact force is referred to as the reference or driving point. The response transducer is moved to obtain data at all of the degree-of-freedom locations. This is referred to as roving with the response transducer. The fixed response method requires that a response transducer remains stationary at a single data location while the impact hammer is moved to excite the mechanical system at each degree of freedom. This is referred to as roving with the impact hammer. The location of the fixed response transducer is the reference or driving point.”
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