Experimental modal parameters are normally estimated by curve fitting a set of Frequency Response Functions (FRFs). To calculate an FRF, all excitation forces must be simultaneously acquired with each structural response caused by the excitation forces.
Structural responses can always be acquired, even when the excitation forces are not acquired.
Fourier spectra, Cross spectra & ODS FRFs can be calculated from Output only, response only, or operating data
Operating Modal Analysis (OMA) is the process of extracting modal parameters (called operating mode shapes) from a set of output-only measurements.
FRF-based curve fitting is applied mainly around the resonance peaks in a Fourier spectrum, Cross spectrum, or ODS FRF. If the following assumption is met, then peaks in these output-only measurements are assumed to be caused by the excitation of resonances, or modes of vibration.
ASSUMPTION: If the frequency spectrum of the un-measured excitation forces is assumed to be "relatively flat", then operating mode shapes can be extracted from output-only measurements using FRF-based curve fitting methods
A Fourier spectrum is the FFT of single channel time waveform.
If all response time waveforms are simultaneously acquired, operating mode shapes can be extracted from a set of Fourier spectra using FRF-based curve fitting methods
A Cross spectrum is a cross channel measurement that is calculated between two channels of response data. The correct relative magnitude & phase between all Roving responses is preserved if all Cross spectra are calculated between a Roving response and the same (fixed) Reference response.
Multiple Measurement Sets of Cross spectra can be calculated from one or more simultaneously acquired Roving responses and the same (fixed) Reference response.
After a DeConvolution window has been applied to a set of Cross spectra, Operating mode shapes can be extracted from them using FRF-based curve fitting methods
An ODS FRF is a "hybrid" cross channel measurement that is calculated between two channels of response data. The magnitude of the ODS FRF is the Auto spectrum of a Roving response and the phase is the phase of the Cross spectrum between the Roving and a (fixed) Reference response. The correct relative magnitude & phase between all Roving responses is preserved if all ODS FRFs are calculated between a Roving response and the same (fixed) Reference response.
Multiple Measurement Sets of ODS FRFs can be calculated from one or more simultaneously acquired Roving responses and the same (fixed) Reference response.
After a DeConvolution window has been applied to a set of ODS FRFs, Operating mode shapes can be extracted from then using FRF-based curve fitting methods