Showing papers on "Modal testing published in 1978"

Modal confidence factor in vibration testing

Abstract: The modal confidence factor (MCF) is a number calculated for every identified mode for a structure under test. The MCF varies from 0.00 for a distorted nonlinear, or noise mode to 100.0 for a pure structural mode. The theory of the MCF is based on the correlation that exists between the modal deflection at a certain station and the modal deflection at the same station delayed in time. The theory and application of the MCF are illustrated by two experiments. The first experiment deals with simulated responses from a two-degree-of-freedom system with 20%, 40%, and 100% noise added. The second experiment was run on a generalized payload model. The free decay response from the payload model contained 22% noise.

Modal analysis of turborotors using planar modes—theory

Abstract: The generalized dynamic equations of motion have been obtained by the direct stiffness method for multimass flexible rotor bearing systems including the effects of gyroscopic moments, disk skew, and rotor acceleration. A set of undamped critical speed mode shapes calculated from the average horizontal and vertical bearing stiffness is used to transform the equations of motion into a set of coupled modal equations of motion. The modal equations are coupled by the generalized bearing coefficients and the gyroscopic moments. An analysis using only undamped critical speeds or decoupled modal analysis assuming proportional damping may lead to erroneous results. This paper presents a rapid method of calculating rotor resonance speeds with their corresponding amplification factors, stability and unbalance response of turborotors. Examples of the application of this modal approach are presented and results are compared to those of other methods such as matrix transfer analysis.

Comparison of Modal Test Methods on the Voyager Payload

Abstract: A comparison of the performance of modern modal data analysis methods on test data from the Voyager Jupiter/Saturn payload is presented. Four different test/data-analysis combinations are compared - multiple-point sine excitation tests, single-point random-excitation tests using two different techniques of manipulating Fourier transform data, and a time-domain method for analyzing random data. Results indicate that all four methods can give comparable results. Of the four, the time-domain approach detects more modes in the test data and, at the same time, shows the greatest promise for reducing the time and cost of modal testing.

Electrical and Photon Tests of a Resonant Satellite Shape

Abstract: A resonant body has been tested and analyzed to determine its SGEMP response. Both electrical and photon excitation were used and their results compare favorably with predictions. A modal analysis of the experimental data using Prony's algorithm yielded the dominant frequencies, damping and excitation amplitudes. Within the accuracy of the measurements, the presence of the photon-induced electron cloud had no effect on the period or damping times of the RESMOD modes. The proximity of an electrical pulser and its 200-Ohm terminating resistor markedly decreased the damping time during electrical excitation.

Residual-Flexibility Corrections for Transient Modal Rotordynamic Models

Abstract: A modal residual-flexibility approach due to Schwendler and MacNeal (1962) is adapted to account for the 'static' contribution of higher-frequency modes without requiring their integration It is assumed that each rotor mode acts as a lightly damped second-order system The additional accuracy provided by residual-flexibility corrections becomes progressively more important as a modal model's actual boundary conditions are forced to deviate from the boundary conditions used to define the rotor's original structural model (stiffness matrix) and its associated eigendata input to the transient modal model An analysis of the high-pressure-oxygen turbopump of the Space Shuttle main engine shows that the residual-flexibility approach ensures a substantial improvement in accuracy for a relatively moderate increase in computer-time requirements

A Study on Higher Mode Vibration of Ships (1st report)

Abstract: A method for the calculation of the coupled vibration has been derived using the concept of modal synthesis. The modal parameters such as frequency, modal mass and modal damping and the vibration mode of the coupled vibration can be calculated by the present method using the modal parameters, vibration mode and mass distribution of uncoupled vibration. The applicability of the method is studied to the higher mode vibration of ship structure which is the complicated coupled vibration of the substructures and the main hull girder. The isolated calculation of individual local structure, such as superstructure, double bottom in engine room etc can be combined to yield the final results coupled with main hull vibration. By the present method, it is easy to distinguish what is the main part of coupled vibration. This is very useful from the point of view of the structural design. The calculation and the experiment should be compared in order to confirm the validity of the method in the future works.

Determination of Modal Parameters from Experimental Frequency Response Data.

Abstract: : In recent years, a number of advances have been made in determining the dynamic properties of a structure. These advances have opened the field of modal analysis, the term used to describe the use of experimental frequency response data to determine the modal properties of natural frequencies, damping ratios, and mode shapes. One of the main problems in the area of modal analysis is the lack of complete documentation for the techniques in use. The purpose of this thesis is to investigate some of the modal analysis techniques in use, and to document, fully, the assumptions and the equations and computer algorithms associated with each method. Three curve fitting techniques are presented along with some example problems which demonstrate the limitations of each method. (Author)

Vibration of a Shaft Passing through Several Critical Speeds

Abstract: The transient vibration of a shaft passing through its several critical speeds at a uniform acceleration rate was investigated. The shaft is assumed to have a uniform section and be supported at both ends by springs and dampers. First, the free vibration of the system was analyzed to get the relationship between the damping given at the ends of the shaft and the modal damping ratio. Next, after evaluating the modal eccentricities and the modal damping ratios of the system, we calculated the maximum amplitudes of the shaft passing through the first and the second critical speeds at various acceleration rates using the approximate equations, which has been obtained from the analysis of the transient vibration of a system with one degree of freedom. Further, these calculated results were compared with the experimental ones.