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For large civil engineering structures much larger shakers are used, which can weigh 100[[kg]] and above and apply a force of many hundreds of [[newtons]]. Several types of shakers are common: rotating mass shakers, electro-dynamic shakers, and electrohydraulic shakers. For rotating mass shakers the force can be calculated from knowing the mass and the speed of rotation; for the electro-dynamic shaker the force can be obtained through a load cell, or an accelerometer placed on the moving mass of the shaker. Shakers can have an advantage over the impact hammer as they can supply more energy to a structure over a longer period of time. However, problems can also be introduced; shakers can influence the dynamic properties of the structure and can also increase the complexity of analysis due to [[window function|windowing]] errors.
== Types of Modal Testing<ref>{{Cite web|title=What is Modal Analysis: The Ultimate Guide {{!}} Dewesoft|url=https://dewesoft.com/daq/what-is-modal-analysis|access-date=2021-03-25|website=dewesoft.com|language=en}}</ref> ==
Different types of modal testing can be performed.
==== Experimental Modal Analysis (EMA) ====
Experimental Modal Analysis tests can be performed both in the field and in more controlled lab environments. Testing in the lab has the advantage of a higher signal-to-noise ratio (SNR) and the ability to easily change the test setup. When performing EMA testing, objects are excited by artificial forces and both the inputs (excitations) signals and outputs (responses) signals are measured and used to estimate Modal Models.
==== Operating Deflection Shapes (ODS) ====
Operating Deflection Shapes is a simple way to do dynamic analysis and see how a machine or a structure moves by its operational conditions. ODS tests have no applied artificial forces and only response vibration signals are measured.
A Modal Model can not be estimated from ODS measurements but it provides structural deflection shapes which improves the structural analysis of operational DUTs.
ODS is used successfully for machine conditioning monitoring and in civil engineering applications e.g. on bridges, buildings, and other structures that are difficult to excite by applied artificial forces.
==== Operational Modal Analysis (OMA) ====
The test and measurement procedure for Operational Modal Analysis is similar to ODS, but their analysis part is different. Both ODS and OMA do not use external input forces but are based purely on response DOF measurements.
ODS provides basic information about amplitude and phase information of the DOFs on the measured operational DUT and enables geometry animation of the deflection shapes.
OMA estimates a Modal Model (like EMA) with natural frequencies, damping, and mode shapes of the measured operational DUT.
OMA can be used to estimate a modal model in situations where it is difficult to do EMA. Such situations could e.g. be when monitoring running DUTs for health issues, when the size or ___location of the DUT makes it impractical to excite with external force, or when the operational structural conditions of the DUT must be analyzed.
==See also==
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