Modal analysis

Modal analysis is a method to describe a vibrating structure in terms of its natural characteristics which are the frequency, damping and mode shapes – its dynamic properties. A standard setup for experimental modal testing requires sensor technology (force transducers, accelerometers, cameras or non-contact laser vibrometers), data acquisition and a computer for monitoring and analyzing the measurement data (DAQ). Without using a rigorous mathematical treatment, the linked whitepaper introduces some basic concepts about structural vibration and mathematical approaches for solving structural dynamics issues. Please sign up for downloading the free whitepaper.

In this application note, learn how Eigenfrequencies and vibration modes of components can be determined quickly, precisely, and without contact using state-of-the-art laser Doppler vibrometry and suitable excitation methods. Automatic modal hammer or shaker are discussed for enabling reliable analysis of the dynamic properties.

During an experimental modal test, first the vibration response of a structure is measured over frequency. The excitation should be spectrally broad to excite all relevant natural frequencies. Typically, the excitation spectrum is acquired as well, so that transfer function's (FRFs) response-input force can be recorded. The setup should be well defined to avoid any unwanted influences from the environment or the excitation process itself. A typical setup could be attachment of the structure to some soft rubber strings or putting it onto soft foam to decouple it from the environment and to excite it with e.g. a modal hammer.

For vibration measurement and experimental modal testing, the underlying test structure needs to vibrate. Some test structures vibrate by themselves (motors or fans) and others need external excitation. For the first group operational modal analysis can be used to some extent, the second group is the typical subject of experimental modal testing. For such induced excitation in experimental modal analysis, there are different means. Typical excitation techniques in modal testing include excitation by shakers, broadband noise excitation by a loudspeaker or dedicated manual or automatic modal hammers. 

As a result of the experimental modal test the operational deflection shapes as answers to this special modal test excitation are available. To compare these results with the calculated results from a numeric modal analysis based on a FE model a second step called curve fitting is required. In the pure measurement results, the modes are potentially still coupled. The dynamic behavior of a mechanical system can be described as the superposition of the Eigen-modes, one mode being considered as a single degree of freedom (SDOF). In curve fitting the SDOF results are extracted with various methods, typically based on single value decomposition (SVD).

Experimental modal analysis software packages like PolyWave allow for curve fitting and the comparison of the EMA test results with the FEA results in MAC analysis. The findings like damping values, Eigen-frequencies and Eigen-vectors are fed back into the model to update the FE model parameters.