Microdynamic Characterization of Modal Parameters for a Deployable Space Structure

Results are presented from an experimental investigation of the microdynamics of deployable space structures. The dynamic response of a representative deployable truss at submicrostrain levels of vibration was characterized in terms of modal parameters. The test article was subjected to stepped-sine sweeps through its fundamental flexible modes over a range of excitation amplitudes. High-sensitivity piezoceramic strain sensors were used in conjunction with a lock-in amplifier to measure the truss response from tens of microstrain down to one nanostrain. The results show that the values of modal frequency and damping ratio are strain dependent at high response amplitudes and strain independent at low amplitudes. It is inferred that, at microdynamic levels of excitation, the internal loads needed to overcome the joint friction are not attained. The nonlinear mechanisms in the deployable structure are, thus, not activated, resulting in a linear truss response.