Technical Paper
An Exploration of Power Spectral Density (PSD) Estimation, with an Introduction to iDOF™ Instant Degrees of Freedom
2015-09-15
2015-01-2620
Random vibration control systems produce a PSD plot by averaging FFTs. Modern controllers can set the degrees of freedom (DOF), which is a measure of the amount of averaging to use to estimate the PSD. The PSD is a way to present a random signal-which by nature “bounces” about the mean, at times making high excursions from the mean-in a format that makes it easy to determine the validity of a test. This process takes time as many frames of data are collected in order to generate the PSD estimate and a test can appear to be out of tolerance until the controller has enough data to estimate the PSD with a sufficient level of confidence. Something is awry with a PSD estimate that achieves total in-tolerance immediately after the test begins or immediately after a change in level, and this can hide dangerous over or under test conditions within specific frequency bands, and should be avoided.