Attitude Control of Flexible Spacecraft

Abstract

During the last decades, large deployable structures are starting to be seen as a plausible configuration to multiple space missions, such as solar sailing, LEO (Low Earth Orbit) deorbiting missions or solar power generation. Technological advances in key areas, such as thin film solar cells or new deployment methods, as well as the miniaturization of satellites and their components, have considerably increased the usefulness of this design. The research presented in this document aims to contribute to these technological developments, helping to unfold the whole potential of this structural solution. This research comprises the MSc thesis of the author, in partial fulfillment of the MSc in Aerospace Engineering by the Technical University of Delft. The research was conducted during an internship at the department of Guidance Navigation and Control (GNC) Systems, Institute of Space Systems, German Aerospace Center (DLR). The research focuses in the study of the behavior of large thin flexible structures (LTFSs) in space from the perspective of the attitude determination and control system (ADCS). This work was performed in close relation to the DLR’s mission GoSolAR (Gossamer Solar Array). The contributions of the research can be classified in two areas, related to the modeling of the structure and to the design of the ADCS. In relation to the first one, a methodology to integrate the flexibility of a structure into its equations of motion is explained from a theoretical perspective and implemented over a flexible spacecraft. This methodology is based in Lagrange’s equations and can be applied to diverse structures, allowing modeling accurately the dynamics of the system while maintaining the model comprehensive and of relatively low order. The implementation of this approach leads to equations of motion containing both rigid and flexible motion, enabling to conduct an evaluation of the impact of the flexibility of a structure on its dynamics in a space environment. The second major area in which this thesis aims to contribute to the body of knowledge is related to the design and evaluation of attitude controllers targeting flexible satellites. Different controller’s designs are proposed, based in: 1) a linear-quadratic regulator (LQR) approach, 2) robust control theory, particularly in the minimization of HInf and H2 norms, and 3) a control approach based in analytical dynamics, known as the Udwadia–Kalaba approach. The performance of these designs was evaluated not only in relation to control instructions related to the rigid motion of the satellite, e.g. angular rate, but also to the capability of each controller of damping the system, reducing the vibrations that appear due to its flexibility. The particularities that need to be considered when implementing each controller are also studied, in order to give a clear idea of when would each controller be an adequate control solution. The result is a set of different control designs, including for each of these designs: 1) the methodology to derive the controller, 2) the performance of the controller and 3) the particularities and limitations of its implementation.