Bandwidth-limited robust nonlinear spacecraft control

Robust large-angle spacecraft pointing and tracking by Variable Structure Control (VSC) are developed. The control of multiaxial attitude dynamics is a highly nonlinear problem, which can be resolved by exact feedback linearization with respect to attitude variables. This requires exact measurements and perfect model parameters. However, spacecraft maneuvers in uncertain circumstances. Thus robust control is required.; It is shown that asymptotic stability can be achieved by VSC, provided that the parasitics are assumed be bounded. Chattering from high gains is undesirable due to actuator limit and excitation of unmodeled high frequency dynamics.; Allowance of tracking errors (boundary layer around the sliding surface) with guaranteed bounds around attractive sliding surfaces eliminates the discontinuity caused by chattering. It is found that the trade-off study can be easily performed with actuator band-width parameters and sliding surface characteristics. Numerical simulations are conducted to validate the results.; Coupled with the elastic dynamics of the structure, the angular acceleration of spacecraft excites the unmodeled elastic dynamics, which also interacts with the dynamics of the whole spacecraft. While the trajectory of the controlled variable (attitude angle) is near or at the sliding surface, the differential equations of flexible generalized amplitudes and sliding surface dynamics are derived in the state space form within the boundary layer so that the robust stability constraint can be obtained from analyzing the system matrix. It is shown that the stability of the system is determined by the following factors: modeling error, control band-width, desired target history and flexible structure configuration and its material characteristics. Numerical simulations are performed to show that the control band-width and the sliding surface characteristic are the main factors for steady state error and stability of flexible modes.; To extend the stability range and vibration suppression capability, the active damping design method with a variational principle for distributed and discrete actuators is introduced by the undesired energy dissipation in the flexible structures.