Abstract
Micro-vibration is a low level disturbance, which cannot be controlled or reduced by the Attitude and Orbit Control System of a spacecraft. It can emanate from various sources on a typical spacecraft, notably subsystems with moving parts such as reaction wheels or cooler mechanisms. Micro-vibration can also result from thermo-elastic effects due to stick-slip from differential expansion of parts. It causes problems for sensitive payloads especially high resolution cameras where the demand for higher resolution (which drives stability requirements), has made analysis and control of microvibrations relevant for a larger number of satellites. The availability of mathematical models to represent the disturbance sources in a format that can themselves be coupled to the mathematical model of the structure to perform end-to-end analysis to obtain predictions of the stability level at the receiver is crucial. In the simplest form the sources can be represented with forces and moments of appropriate characteristics, adding also some inertia at the source location. But even for this simplistic implementation it is necessary to have available the details of the forces and moment produced by a source, and these can be either calculated from schematic models of the functioning of the device, or experimentally determined, or a mixture of the two. Typical test techniques applied to micro-vibration measurement /characterization will be described, highlighting advantages and drawbacks of the various methods. A simple experimental apparatus for the measurement of the micro-vibrations emitted by a reaction wheel is presented. A mathematical model of the reaction wheel disturbances is also presented together with its coupling to a typical spacecraft structural model.