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Modal Analysis and Operating Deflection Shape

Modal Analysis and Operating Deflection Shape

In the process of diagnosing rotating machinery, it may become unavoidable to do some structural vibration analysis as well. Most often this is done by performing ODS (Operating Deflection Shape Analysis) or Modal Analysis.

An operating deflection shape shows the deformation of a structure at a specific frequency or during a time waveform playback. ODS shows the response of a structure to both resonant and forced vibration, and is generally very representative of the real world data. ODS evaluation can be particularly useful in the analysis of rotation machinery equipment where internally generated forces can be high.

Modal analysis is similar to ODS in that deformation of the structure can be viewed, but it is primarily concerned with resonance frequencies (or natural frequencies) of a structure. Modal analysis is typically performed as a vibration exciter by exciting the structure with a known force and measuring the response of the structure at many locations using accelerometers. The measured frequency response functions are analyzed to determine the modal parameters.

In plant environments, users often perform a “bump test” (also called an impact test, hammer test, or resonance test) as the primary excitation for this type of structural vibration analysis. Frequency response functions are evaluated to determine the responses to the vibration excitation, and consequently the modal response at resonance. At times, a better and more repeatable excitation source is needed such as SignalForce Modal and Inertial Shakers.

At its fullest, experimental modal analysis is performed by curve fitting the frequency response functions to determine modal parameters (frequencies, damping factors, modal vectors, and modal scaling) of a structure.

These modal parameters may also be determined by analytical means, such as finite element analysis, and one of the common reasons for experimental modal analysis is the verification or correction of the results of the analytical approach (model updating). When an analytical model does not exist, the modal parameters determined experimentally serve as the model for future evaluations such as structural modification.

Whether performing an operating deflection shape test, a simple resonance search, or modal analysis, the most difficult part of the test to many users is to maintain proper book keeping to assign individual measurements location and direction information to make sure any post processing analysis is meaningful. SignalCalc Dynamic Signal Analyzers handle book keeping and data export extremely well, making them particularly suitable for these types of measurements. Learn more about SignalCalc Dynamic Signal Analyzers.