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Identification of Vibrational Characteristic of System under Colored Excitation base on the Transmissibility Functions

Afshar, Mehrnoosh | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48851 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Behzad, Mehdi
  7. Abstract:
  8. Operational modal analysis (OMA) techniques are proper methods to identify the modal parameters of the structures such as natural frequencies, damping ratios and mode shapes based on the system responses. First research in this area are stared from civil engineering. Difference between civil structure and industrial machines are in the type of excitation.in most industries such as aerospace or mechanical, colored or harmonic forces due to rotary machines, excited the system. Under this condition, traditional methods in OMA are not capable any more. Many works has been done to modify traditional methods. Most of these work limited to situations that forces are harmonic and harmonic frequencies have far difference from natural frequency.in last decade a new method called power density transmissibility function(PSDT) have been developed that are independent of stimulation’s spectrum in only special condition.
    In this research for improving identification method and distinction real poles from artificial poles (Emanates from many factors such as colored excitation) a new algorithm named multi screening algorithm based on PSDT functions are developed.in recommended algorithm base idea for separation real poles of artificial ones is that system’s poles become most repeated of other identified poles. For reach this goal, two new criterion, the Phase angle criterion and the coherence criterion are introduced. The mathematical poles from real ones are distinct by using the coherence criterion. The Phase angle criterion reduce the number of stable nonphysical poles so can be expressed that physical poles of the system are most repeated ones.In continuation, with the introduction of a new index named colored excitation coefficient , effect of diffract factors of stimulation such as power, bandwidth and dominated frequency on the transmissibility functions and identified modal parameters are statistically discussed.as a result is indicated that band width is the most effective factor that causes the error in identification.To comparing accuracy of the PSDT method with the power spectral density (PSD) method, a case study base on four degree of freedom system under eight different loading condition is done. The result show that the PSDT method are more accurate and less sensitive to excitation than the PSD methods.For validation the recommended identification method in real situation, this method is used for identifying an experimental set up that subjected to harmonic forces and result indicate that this method is capable in real situation and have good accuracy in identification
  9. Keywords:
  10. Operational Modal Analysis ; Power Spectral Density (PSD)Analysis ; Least Squares Method ; Transfer Function ; Colored Noises ; Multi Screening Algorithm

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