A Comparison between SISO and MIMO Modal Analysis Techniques on a Membrane Mirror Satellite

Abstract

The future of space satellite technology lies in the development of ultra-large, ultra-lightweight space structures, orders of magnitude greater in size than the current satellites. Such large crafts will increase communication and imaging capabilities from orbits. Many such proposed ultra-flexible satellites are inflated structures. To get these ultra-large structures in space, they will have to be stored within the Space Shuttle cargo bay and then inflated on-orbit. However, the highly flexible and pressurized nature of these ultra-large spacecraft poses several daunting vibration and control problems. Disturbances (i.e., on-orbit maneuvering, guidance and attitude control, and the harsh environment of space) wreck havoc with the on-orbit stability, pointing accuracy, and surface resolution capability of the inflated satellite. Fortunately, recent advances in integrated smart material systems promise to provide solutions to these problems.

Recent research into the use of Macro-Fiber Composite (MFC®) devices integrated into the dynamic measurement and vibration control of inflated structures has had promising results (Wilkie et al., 2000). These piezoelectric-based devices possess a superior electro-mechanical coupling coefficient making them superb actuators and decent sensors in dynamic analysis applications. Initially, research was performed on an inflated torus using single-input, single-output (SISO) testing techniques. Since then, steps have been taken to outline a new, multiple-input, multiple-output (MIMO) testing technique for these ultra-large structures. This study applies these results to an inflated torus with bonded membrane mirror to extract modal parameters, such as the damped natural frequencies, associated damping, and mode shapes within the frequency bandwidth of interest for these structures (5–200 Hz). Further, the nonlinear dynamic behavior of the inflated torus and membrane mirror is accentuated through a comparison of SISO and MIMO modal analysis techniques, and a discussion of the nonlinear results follows.

The purpose of this work is to apply the results from prior works to an inflated torus with bonded membrane mirror to accomplish the following three goals: (1) to establish a baseline dynamic characterization of the test structure using SISO modal analysis techniques;(2) to perform a MIMO modal analysis of the test structure to identify natural frequencies, mode shapes, and damping ratios, and compare these MIMO results to the SISO analysis; and(3) to use the discrepancies between the two testing technique results as a platform for discussing the nonlinear nature of the test structure. In the future, the results of this work may form the premise for an autonomous, self-contained system that can both identify the vibratory characteristics of an ultra-large, inflated space craft and apply an appropriate control algorithm to suppress any unwanted vibration – all while on-orbit.

Keywords

inflated satellite modal analysis membrane mirror nonlinearity mode shapes

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