Practical Aspects of On-Orbit Modal Identification Using Free-Decay Data.

Abstract

Future large space structures such as Space Station Freedom cannot be tested as a total structure on earth. Size, zero-g design, and the existing atmosphere conflict with traditional structural dynamic testing. On-orbit modal identification will become necessary. This paper discusses practical aspects of performing such tests using time-domain analysis of free-decay data. The effects of environmental constraints, structural characteristics, and excitation and sensing are reviewed. A recent laboratory application is also discussed. In this test, an on-orbit experiment is simulated using free-decay data and a limited number of excitationand measurement points. The test article is dynamically similar to the truss/solar arrays of Space Station. The identified modal parameters from this simulation correlate well, though not uniquely, with those obtained in a complete modal survey. Practical difficulties in performing the correlation are illustrated. Modal parameters are identified with the Eigensystem Realization Algorithm.