Modal Test Data Research Articles

Mount Stiffnesses and Inertia Properties from Modal Test Data

A new method for determining an analytical model of a rigid body on mounts is presented. The method uses experimental data from a modal test as input. The output is rigid body mass properties and the stiffnesses of the mounts. The theory behind the method is discussed in detail and a case history is documented.

Propellant interaction with the payload control system of dual-spin spacecraft

Significant interaction between the payload control system and the liquid propellant on dual-spin spacecraft is shown to exist for a new class of vehicles with large liquid mass fractions. Experience during the transfer orbit operations of the first two LEASAT synchronous orbit communications satellites has demonstrated that lateral sloshing modes within the propellant can become unstable when coupled to the payload control system. Scaled-model single-axis transfer function testing is shown to define critical propellant modal parameters accurately. Analytical modeling techniques using this propellant modal test data are presented that can be used to predict both the strength and basic stability of the interaction. Design guidelines are presented for use in selecting system parameters to ensure stability. On-orbit verification of the interaction hypotheses, validating the test data and analytical modeling, is demonstrated with results of experiments conducted on the third LEASAT spacecraft.

Direct update of dynamic mathematical models from modal test data

Two methods for the direct updating of mathematical models based on modal test data are described. A set of minimum required constraints derived from eigendynamic and force equilibrium conditions are presented. Both methods generate mass and stiffness matrices fitted exactly to the given modal test data. One method changes all coefficients of the mass respective to the stiffness matrix and lessens the solution effort by the algebraical elimination of the Lagrangian multipliers. The other method changes only those selected matrix coefficients requiring additional solutions for the numerical calculation of the Lagrangian multipliers. Examples and comments on the applicability of the methods are given.

Discrete Model Improvement by Eigenvector Updating

An eigenvector updating method is proposed to fit modal test data. The calculated eigenvectors are updated by coinciding some of them with the corresponding measured eigenvectors as a result of an orthogonal transformation. The advantage of the method is the applicability to large complex structures without necessity of recomputation of the eigendata.

Seismic Qualification of Large Equipment with Test-Verified Analytical Techniques

The seismic qualification of large structural items by analytical methods gains considerable credibility if the analytical techniques are verified by test-obtained modal data. Analytical seismic qualification of the McGuire Nuclear Station main control boards for Duke Power Company provide an example of the types of analysis procedures which are well suited to large structural pieces of equipment, and illustrate the enhancement of analytical results with modal test data. This approach resulted in an analytical seismic qualification which was suitable from both the standpoint of structural integrity assessments and defining required response spectra for component qualification.