Mode Localization Experiments on a Ribbed Antenna

Abstract

In this paper, the mode localization (ML) phenomenon is investigated experimentally and analytically to determine the influence of its parameters. For this purpose, a full-scale 12-rib loosely coupled antenna testbed with small imperfections is dynamically tested for various levels of inter-rib coupling stiffness and excitation force. The experimental results are described herein. Using a simplified numerical model of the structure, a sensitivity analysis of the modal behavior is also performed. The numerical and experimental results are shown to agree remarkably well, thereby providing conclusive validation of the ML phenomenon on a testbed having the dynamic characteristics of space structures. ODE localization (ML) is a dynamic phenomenon associated with weakly coupled periodic structures, and resulting from small imperfections (< 5%) which perturb the periodicity. Such imperfections typically result from random manufacturing or assembly imprecisions. Numerous numerical and analytical investigations have shown that such structures can be susceptible to displaying modal deformations which are quite different from those expected if the periodic structure had no imperfections . In extreme cases of ML, the modal energy and deformations are confined to a single component, instead of having the periodic distribution predicted for a perfect structure. If not adequately incorporated into the analysis model, ignoring ML could have adverse effects on the design of active control systems. It could compromise system identification schemes based on periodicity, or could lead to erroneous results if accurate shape control is required. Also, the confinement of the vibrational energy to particular components could yield dangerously high stress levels. Conversely, ML could be advantageous in reducing vibrations in specific regions of a structure. However, the existence of ML or the influence of its parameters have not yet been demonstrated on an actual space-like structure. This provided the motivation for the research described herein, with the objective of demonstrating analytically and experimentally the degree to which actual structures—having the complexity and dynamic characteristics of space structures—are susceptible to mode localization.1 For this purpose, the antenna testbed shown in Fig. 1 was used to investigate and ascertain the existence of ML. After a brief overview of the theory of ML, the testbed facility, the data acquisition system, and the data-reduction software are described, as well as the identified and measured small structural imperfections. A preliminary analysis of the sensitivity of the modal properties to variations in the inter-rib stiffness was conducted. A description of the model with the actual imperfections and conclusions derived from the numerical modal analysis are presented. Based on these conclusions, certain modes were targeted for the experiments and were excited at several forcing levels for various amounts of interrib stiffness. The impact of nonlinear dynamic response and