Reaction-mass devices are to be used in a dual role: first for the pur
pose of system identification, and second for the purpose of vibration
suppression. Unlike ground-based shakers, reaction-mass devices have
a position constraint which must be maintained. A two-part system iden
tification procedure is described which maintains this constraint. In
the first part, a new algorithm is used to obtain the impulse response
from input-output data. The second part uses the eigensystem realizat
ion algorithm to determine linear matrix models of the structure and o
f the reaction-mass device. This procedure is demonstrated on a small
experimental structure. The usefulness of the model for the design of
a vibration suppression controller is also demonstrated.