Analysis of Flexibility Effects on Small Spacecrafts Attitude Dynamics

The complexity of space missions based on small satellite platforms is continuously growing. Consequently, the related technologies used in those spacecrafts are also getting more and more complex and sought-after. Among these, the Attitude and Orbit Control System (AOCS) is one of the most critical subsystems. AOCS is responsible, in part, for achieving mission objectives. For this reason, much attention is paid to its design and improvement. Today, the trend is towards making these systems increasingly reliable and high performing. Achieving this last point requires high fidelity models of the spacecraft dynamics and disturbances, that it will undergo, to be used when synthesizing attitude control algorithms. From this point of view, although most of the literature on AOCS deals with rigid body dynamics, flexibilty effects may have a relevant influence for many spacecraft architectures. Indeed, for platform equipped with large, lightweight and low-stiffness appendages (e.g. deployable solar panels, antennas, booms or robotic manipulators), neglecting the flexible motion in the design phase may lead to degraded control performances, unwanted stresses and deformations or even affect the stability of the control system. This thesis project, developed in collaboration with the Italian space company Argotec, aims to analyse the coupling effects of flexible appendages on the spacecraft attitude dynamics and subsequently develop numerical models to be included in a simulation environment implemented in MATLAB® & SIMULINK®. The comprehension of the flexible terms impact on the dynamics is firstly assessed analysing simplified cases, where flexibility is introduced with lumped parameters (concentrated stiffness, dumping and mass). Then, a more sophisticated model is obtained performing the modal analysis of the solar panels’ structure on the Finite Elements Method (FEM) software MSC®PATRAN/NASTRAN. Moreover, particular attention is focused on the discrete nature of practical digital controllers. To do so, a tool that allows to identify stable and unstable regions as function of the control frequency is implemented. Its results put in evidence how time discretization can strongly affect the interactions between flexibility and the control system. The developed model and tools are finally applied to the HENON spacecraft case study, an ambitious Space Weather mission developed by a consortium led by Argotec, under the support of the European Space Agency (ESA) and the Italian Space Agency (ASI). This model-based approach gives insights on the possible dangerous conditions that its configuration can present, suggesting how design modifications could impact the system performances.