Many future space systems will require the presence of very large flexible appendages. Because of the launcher constraints on payload mass, spacecraft structures are lightweight and highly flexible with densely packed modal frequencies and extremely low damping values. Since these structures are subjected to a variety of dynamic perturbations produced by the coupling with attitude maneuvers, transient thermal states, sloshing phenomena and on-board noisy equipment, the lightly damped nature of these structures can lead to elastic vibrations in a broad frequency spectrum producing unwanted noise to sensitive instrumentations.
Current state-of-the-art for missions with large appendages are solved at AOCS (Attitude Orbit Control System) level by designing controllers with very low control bandwidths or limited input commands in order to minimize the impact of flexible modes on spacecraft dynamics. With the new strict requirements envisioned for pointing and shape accuracy of next generation missions and the novel space architectures in development, the aforementioned problems pose multiple challenges for current control architectures; furthermore, the AOCS design for such complex system encounters an ulterior significant difficulty which is the presence of uncertainties on several vehicle parameters.
In this context, the proposed study investigates the application of advanced control methods for combining satellite AOCS with vibration control of the appendages via a network of smart devices in the unified framework of Linear Fraction Transformation (LFT) and Structured Singular Value (s.s.v). In practice, an uncertain state space plant is extrapolated from the 3D structural model developed in a commercial FEM software together with transducers dynamics. Because of the size of the state space model and the huge number of parametric uncertainties rather novel techniques will be used to generate a suitable LFT model to be handled by robust control optimization algorithms.
The proposed research project has as ultimate goal the definition of an advanced integrated control system (AOCS + vibration control) for large space structure and the construction of a unified framework for the major steps of the control design process (creation of a dynamical model, uncertain quantification and modelling, control synthesis and V&V procedures). This research could be of interest for space agencies and industries which are currently moving towards the implementation of these new design methods and state-of-the-art technological solutions for the control of large systems.
The areas in which the proposed research activity could potentially give a relevant contribution are listed here:
- The development of a smart actuated network to control large flexible appendages in space conditions is a new concept and can present innovative solutions for satellite sensor pointing as well as control actuation.
- The study should give an understanding of how to accommodate opposite requirements concerning the high number of degrees of freedom expected for FEM accurate representation and the far lower suited for typical LFT software and robust control synthesis methodologies.
- The approach will be focused on realizing an advanced integrated robust control of the system, i.e. the orbiting spacecraft will be modelled to simultaneously consider both the attitude control of the rigid platform and the vibration damping of flexible appendages. This will lead to a progress of control design techniques for flexible orbiting satellite with respect to the actual state-of-the-art.
- The application of new optimization strategies for computing lower and upper bound of s.s.v., which, as stated before, manifests serious problems whenever lightly damped system with very large LFT models are considered, will be examined. Since very few dynamical systems present a size of the state-space model comparable with flexible spacecraft this research could constitute a valuable benchmark for the evaluation of the effectiveness of optimization algorithm in determining a good estimate of the structured singular value.
- The development of an in-house open-source code for the implementation of the control design procedures. As the objective of the study is the definition of a unified design framework the researcher will create codes to interface the main in-house software with commercial software (i.e. interface with the commercial Finite Element (FE) software MSC Patran/Nastran for the exportation of the FE structural model and the propagation of uncertainties).