Control of spacecraft with flexible structures using pulse-modulated thrusters

A methodology for the design of robust control systems for flexible spacecraft undergoing a large angle maneuver using pulse-modulated thrusters is proposed. The need for developing such a methodology stems from the need to reduce the weight of the modern spacecraft and to satisfy strict performance specifications resulting in vibrational modes within the control system bandwidth. Moreover, the discontinuous operation of pulse-modulation may excite large flexible motion that can lead to deterioration of performance, limit cycles, and even instability. Consequently, the current practice of designing the control system using a rigid body model and evaluating performance by simulation using a flexible model is inadequate for the modern spacecraft. In the proposed methodology structural flexibility is included in the control design model thus resulting in control systems with robust stability and improved performance.;This dissertation has two main parts: formulation of flexible spacecraft dynamics performing large angle motion and control system design. The formulation of equations of motion is performed using quasi-coordinates that allow for parameterization of the attitude in terms of Euler-Rodrigues parameters that avoid singularities. Moreover, the formulation is in terms of variables measurable by the control system, such as angular velocities and is in a form that is convenient for establishing closed-loop robust stability. The control law structure proposed here results in guaranteed closed-loop performance in the presence of parameter uncertainties, measurement noise, and command implementation errors. A theorem that guarantees the bounds of trajectory following errors of a spacecraft performing a general translational and rotational motion in three-dimensional space is formulated and proven using Lyapunov stability theory. Finally, a model of a flexible spacecraft is used to demonstrate the performance of a control system developed using the proposed methodology are presented. A number of different maneuvers are studied by simulation showing satisfactory performance in the presence of uncertainties and implementation errors.