System and disturbance identification for feedforward and feedback control applications

This paper examines the problem of system identification in the presence of unknown disturbances. Specifically, we consider the case where the disturbance effect on the output is modeled explicitly and various ways to separate it from the system disturbance-free dynamics,Available for identification are the excitation signals and the resultant responses, which may be corrupted by unknown and possibly dominating disturbances. We assume that no actual measurement of the disturbances themselves is available for identification. The case where the disturbance profile is unknown and mag. be quite complicated, but its period is known, is considered first, Next, we consider the case where the disturbance frequencies are known only approximately, and a procedure is applied to iteratively refine the estimation of the disturbance frequencies fur successful system identification. Finally, we examine the situation where the unknown disturbance is nonperiodic. The identification produces results that can be used far the synthesis of feedforward and feedback control to cancel the disturbance effect. Both simulation and experimental results will be used for illustration. The simulation is carried out with a model of a communications satellite, and the experimental results are obtained from a flexible truss structure. A companion paper addresses the system identification problem where the disturbance effect is modeled implicitly.