David J. Ewins

Measurement of Translational and Angular Vibration Using a Scanning Laser Doppler Vibrometer

An experimental procedure for obtaining angular and translational vibration in one measurement, using a continuously scanning laser Doppler vibrometer, is described. Sinusoidal scanning, in a straight line, enables one angular vibration component to be measured, but by circular scanning, two principal angular vibrations and their directions can be derived directly from the frequency response sidebands. Examples of measurements on a rigid cube are given. Processes of narrow-band random excitation and modal analysis are illustrated with reference to measurements on a freely suspended beam. Sideband frequency response references are obtained by using multiplied excitation force and mirror-drive signals.

Structural modification analysis using Rayleigh quotient iteration

This paper highlights the application to structural dynamics of the sensitivity analysis methods developed by numerical analysts and presents a historical development of first- and higher-order eigenvalue and eigenvector sensitivities. Different formulae for an eigenvalue sensitivity are presented and it is shown that all of these are implicitly the same. A condition number is presented to give the limited bound of application for the first-order eigenvalue and eigenvector sensitivities. An alternative structural modification method based on Rayleigh Quotient Iteration is presented. A lumped spring-mass system with 7 degrees-of-freedom (7DoF) is used to show the applicability of the Rayleigh quotient iteration method.

Compatibility of Measured and Predicted Vibration Modes in Model Improvement Studies

Basically, there are two approaches to resolve this incompatibility: condensing the analytical model to those coordinates that are or that can be identified experimentally, and expanding the measured vibration modes to the full set of coordinates using the existing analytical model.

Measurement of translational and angular vibration using a scanning laser Doppler vibrometer

An experimental procedure for measuring angular rotation vibration as well as translation, using a continuously-scanning Laser Doppler Vibrometer, is described. Sinusoidal scanning enables one angular vibration component to be measured but, by circular scanning, two principal angular vibrations and their direction can be derived directly from the frequency response sidebands. Examples of sine excitation measurements on a rigid cube are given. Processes of narrow-band random excitation and modal analysis are illustrated with reference to measurements on a freely-suspended beam. The laser beam was scanned by using x-y axis deflection mirrors; if this facility was not available, circular scanning could probably be achieved by using a rotating tilted mirror. Sideband frequency response references were obtained by using multiplied excitation force and mirror-drive signals.