RESEARCH PAPERS: Mechanical Signature Analysis

The Reduction of Bias Error in Transfer Function Estimates Using FFT-Based Analyzers

[+] Author and Article Information
P. Cawley

Department of Mechanical Engineering, Imperial College of Science and Technology, London, England

J. Vib., Acoust., Stress, and Reliab 106(1), 29-35 (Jan 01, 1984) (7 pages) doi:10.1115/1.3269148 History: Received July 13, 1983; Online November 23, 2009


The susceptibility to bias error of two methods for computing transfer (frequency response) functions from spectra produced by FFT-based analyzers using random excitation has been investigated. Results from tests with an FFT analyzer on a single degree-of-freedom system set up on an analogue computer show good agreement with the theoretical predictions. It has been shown that, around resonance, the bias error in the transfer function estimate H2 (Syy /Sxy *) is considerably less than that in the more commonly used estimate, H1 (Sxy /Sxx ). The record length, and hence the testing time, required for a given accuracy is reduced by over 50 percent if the H2 calculation procedure is used. The analysis has also shown that if shaker excitation is used on lightly damped structures with low modal mass, it is important to minimize the mass of the force gage and the moving element of the shaker.

Copyright © 1984 by ASME
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