ODS & Modal Testing Using a Transmissibility Chain

Abstract

In this paper, we show how Operating Deflection Shapes (ODS’s) and mode shapes can be obtained experimentally from measurements that are made using only two sensors and two short wires to connect them to a multi-channel acquisition system. This new test procedure is depicted in Fig. 7.1. Not only is the equipment required to do a test much more cost effective, but this method can be used to test any sized test article, especially large ones.

The testing method introduced here involves moving a pair of sensors along together in a prescribed manor, and calculating the Transmissibility between them. The resulting chain of Transmissibility’s is then post-processed to obtain a single reference set of cross-channel measurements, from which ODS’s and mode shapes can be extracted.

A “round trip” example is used to show how an original set of modal parameters can be recovered by curve fitting a single reference set of output-only Cross spectra, and a single reference set of FRFs.