Analyze Data

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ObserVIEW is largely dedicated to analyzing time waveform data; that is, separating data for evaluation. Vibration data are recorded over time, but it is often advantageous to transform the data into a different domain for analysis. Patterns such as resonant frequencies that are not apparent in the time domain become readily apparent in others, such as frequency.

Fast Fourier Transform

The frequency domain is a standard domain for analysis in vibration test engineering. ObserVIEW uses the fast Fourier transform (FFT) to project continuous time data into the frequency domain.

For a complex signal, the FFT helps to determine excitation frequencies and the amplitude at each frequency. Additionally, it highlights changes in frequency and amplitude and the harmonic excitation in a frequency range.

To add an FFT graph in ObserVIEW, select Add Graph > FFT.

The FFT properties include analysis lines, window function, and data bias range. The FFT assumes data to be continuous, and a window function is applied to minimize aliasing.

Power Spectral Density

The power spectral density (PSD) is another powerful analysis tool for understanding and characterizing random vibration. It estimates the distribution of a signal’s strength across a frequency spectrum. Vibration test engineers can leverage the PSD in various ways to gain insight into the dynamic behavior of the test data. Use examples include:

• Estimating stress levels and assessing fatigue life of critical components
• Comparing a test run to a test standard or multiple test runs
• Troubleshooting problems like component failure and assessing root causes
• Identify contributors to vibration energy and their frequency ranges

Analyze Data

ObserVIEW includes several options for analyzing data in the frequency domain in addition to the FFT and PSD.

Analyzer Traces

Analyze time waveform data in the frequency domain.

• Fast Fourier Transform (FFT): transforms time-domain data into the frequency domain
• Power Spectral Density (PSD): displays the amount of power in a band of frequencies
• Transfer Function: identifies a resonance or antiresonance in a system
• Coherence: indicates how closely a pair of signals are statistically related
• Cross Spectral Density: displays the amount of power in the frequency band of a pair of signals that have been multiplied in the frequency domain
• Transmissibility: identifies a resonance or antiresonance in a system
• Spectrogram: displays the amplitude of multiple FFTs

Analysis Options

The analyzer trace settings are located in the Analyzer Trace Settings section of the Home ribbon menu and the Properties pane.

ANALYSIS LINES

Determines the number of samples the software uses when analyzing the waveform. Along with the sample rate, analysis lines determine the frequency resolution of the analyzer.

ANALYSIS FRAMES AND AVERAGING DOF

The analysis frames value is the number of FFT windows the software uses to compute traces based on Welch estimators. The value can be entered manually entered and locked so the number of frames is not affected by zooming in and out on the time waveform graph.

The averaging degrees-of-freedom (DOF) value displays the statistical degrees of freedom associated with the PSD. After entering an averaging DOF value, the software automatically selects the analysis frames value to meet the target averaging DOF. Averaging DOF can also be locked.

WINDOW FUNCTIONS

Windowing provides a weighted selection of a portion of a time waveform for the purpose of FFT analysis. It is accomplished by multiplying the original time waveform by a window function of some selected width. The ObserVIEW Help file includes a comprehensive table of window functions for assistance with selection.