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On the Use of Modal Parameters as Metrics for Condition-Based Monitoring of Rotating Machinery

[+] Author Affiliations
Jerry H. Ginsberg, Benjamin B. Wagner

Georgia Institute of Technology, Atlanta, GA

Paper No. GT2005-68981, pp. 871-882; 12 pages
doi:10.1115/GT2005-68981
From:
  • ASME Turbo Expo 2005: Power for Land, Sea, and Air
  • Volume 4: Turbo Expo 2005
  • Reno, Nevada, USA, June 6–9, 2005
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 0-7918-4727-6 | eISBN: 0-7918-3754-8
  • Copyright © 2005 by ASME

abstract

Deterioration of a rotordynamic system changes its modal properties. This paper initiates a study of the degree to which such changes can be detected by monitoring modal metrics obtained by a modern technique for experimental modal analysis (EMA). The eigenvalues and residues associated with a complex modal description of the frequency response are identified by processing response data derived from an analytical model. This model, which features an elastic shaft with attached rigid rotor and supported by hydrodynamic bearings, was previously used by Wagner and Ginsberg [Proc. of the 23rd International Modal Analysis Conf. (IMAC), forthcoming, 2005] to explore the merits of using standard or directional frequency response functions to perform EMA. The techniques used there, specfically the original version of the Algorithm of Mode Isolation (AMI) for FRFs and Two-Sided AMI for dFRFs, are used to extract the modal properties from the model’s frequency domain response. The modal eigenvalues and residues are identified for a range of bearing clearances within the limit of acceptable wear. One set of metrics that are considered describes the behavior of the system’s eigenvalues as clearance increases. Another set of metrics describes the modal residue factors, which depend on the drive and response locations. A defect is considered to be detectable if the change in the value of a metric due to deterioration exceeds the uncertainty in that metric’s value associated with the inexact nature of EMA. Although the eigenvalues are identified with great accuracy, they are found to be relatively insensitive to bearing clearance, so that metrics derived from them do not meet the detectability criterion. In contrast, the residue values are identified less accurately, but they are highly sensitivity to the clearance. It is concluded that metrics describing the behavior of the modal residue factors can unambiguously indicate bearing wear that is large, but still acceptable for continued operation. It also is found that it is preferable to monitor the residues obtained by processing standard FRFs using the original AMI version, rather than using Two-Sided AMI to process dFRFs.

Copyright © 2005 by ASME

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