Some Experience with Identification of the Caltech Experimental Space Structure

Future space missions will rely on structures with many closely spaced and lightly damped modes. To meet the alignment performance requirements of the missions, these structures will need to incorporate active control. Control which does not destabilize the system requires very accurate models of all modes present in a certain bandwidth of the structure´s response. Development of the accurate models through system identification is significantly complicated by modes with almost equal frequencies and dampings which appear as single peaks in the structure´s frequency response function. A structure was built at Caltech which exhibits many of the features of large space structures, and a frequency domain modal parameter transfer function identification experiment was carried out. Through use of several single input multiple output experiments assembled as a multiple input multiple output (MIMO) data set, modes with nearly equal frequencies and dampings were characterized distinctly through use of the differences in mode shapes. Results of one set of experiments and estimates from finite element models are presented.