System and periodic disturbance identification for feedforward-feedback control of flexible spacecraft

This paper presents a method to identify the control-to-output dynamics of a flexible spacecraft when it is disturbed single or multiple periodic disturbances. The disturbance profiles are unknown, but their periods are assumed to be known. Available for identification are the excitation signals at the control inputs and the resultant output responses, which include the effect of the unknown, and possibly dominating disturbances. The paper offers a formulation that uniquely separates the effect of the control excitation from that of the unknown periodic disturbances so that both the control-to-output dynamics and the disturbance effect are identified correctly. This information can then be used to design both feedback and feedforward controllers. The formulation introduces a special mechanism that makes any periodic disturbance input automatically appear periodic in the output after a prespecified, generally 'small' number of time steps. This will occur regardless of the system dynamics which can be highly damped, lightly damped, or even unstable. This critical feature allows transient data to be used, eliminating the need to wait for steady-state, which is unsatisfactory for lightly-damped flexible structures. The method is illustrated by several examples, including vibration control of a communications satellite. (Author)

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