The vibration parameters have been estimated in a spacecraft, where active vibration damping is apart of the attitude control. A mathematical model of a simple spacecraft is modeled in MATLAB and suitable controllers are developed in order to perform both effective vibration damping and attitude control. The spacecraft is a 6 degree of freedom model and normally controlled by at least one force and one torque acting along each of the x, y and z axis of the spacecraft. The model was reduced to include only one external torque acting about the z axis. Identification of open loop vibration parameters during active vibration damping may be performed using direct identification, indirect identification or joint input-output identification. Direct identification, as the easiest method, is considered. ARX (Auto-Regressive with eXternal input) and ARMAX (Auto-Regressive Moving Average with eXternal input) models are applied to identify the open loop vibration frequencies. Simulations show that high order feedback does not work properly if the modal vibrations are rapidly damped. Thus, accurate identification can be achieved only as long as the active damping is not made too effective, i.e. the vibrations must have sufficient energy and duration to be identified. Nonlinear feedback and time varying feedback, although not shown, give approximately the same results. A time-varying perturbation signal may amplify certain frequencies and degenerate the pointing accuracy. ARMAX models, using a Gauss-Newton numerical search method, give slightly more accurate frequency estimates than ARX models, using a least squares method. Closed loop parameters may be identified using both AR (Auto-Regressive), ARX, ARMA (auto-Regressive Moving Average), ARMAX methods and subspace methods. Normally open loop vibration parameters are the goal, but if closed loop parameters differ little from open loop parameters, closed loop parameters may be a good choice. Simulations show that the subspace method called ESPRIT gives more accurate parameter estimates than AR, ARX, ARMA, ARMAX methods, when dealing with small SNRs (Signal to Noise Ratio) measurements.


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    Title :

    Vibration parameters identification in a spacecraft subjected to active vibration damping


    Additional title:

    Parameterschätzung zum Entwurf einer aktiven Schwingungsdämpfungsregelung in einem Raumfahrzeug


    Contributors:

    Published in:

    Mechatronics ; 8 , 6 ; 691-705


    Publication date :

    1998


    Size :

    15 Seiten, 8 Bilder, 3 Tabellen, 12 Quellen




    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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