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Bettiol, Laura (2018) Dynamics and control of highly flexible structures for aerospace applications. [Ph.D. thesis]

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Abstract (english)

Highly flexible space structures such as thin-film solar arrays, deployable antennas, solar concentrators, solar and drag sails, are typically made of very thin tensioned membranes. These deployable structures are considered promising technologies for their lightness, high deployed-to-packed volume ratio, and low cost. These advantages make them an extremely interesting technology for satellites of small classes, such as nanosatellites (including CubeSats) that are becoming more and more common because of their capability to provide many of the services that were performed by larger satellites in the past with a fraction of their cost. However, the size of deployable structures is not proportional to the dimension of the hosting satellite but depends on the specific mission and application. In addition, the miniaturization of satellites comes very often with the reduction of the performance of their attitude control system. Therefore, large appendages mounted on a nanosatellite can be an issue in terms of attitude control, with a possible reduction of the stability and pointing precision, or even the loss of the whole mission. For this reason, control systems must be envisaged in order both to limit or avoid the oscillations of these structures and to reduce the transmission of disturbing forces from the deployable appendage to the central body of the satellite.
This Ph.D. research is focused on the analysis of the dynamic behavior of highly flexible space structures, and the study of the effectiveness of possible methods to control their oscillations, both in terms of damping techniques directly applied to the appendage, and vibrations transmission reduction at the interface between the deployed structure and the main body of the satellite. Two test cases with completely different approaches were identified for this study, both based on solar arrays.
The first test case focuses on a membrane solar array not supported by an external structure. Numerical simulations explore the effectiveness of controlling the oscillations of this membrane, subject to a sinusoidal force and non-flat initial conditions with the use of thin patches of piezoelectric material. This study confirms the possibility to apply the robust Hinf closed-loop control system, in order to correct the initial non-horizontal shape of the membrane to reach a flat deployed configuration.
The second test case studies the dynamics of a solar array structure that is deployed and kept in tension by two external composite tape springs. Bistable boom prototypes have been manufactured and their theoretical dynamic behavior is studied in detail. Numerical simulations and experimental tests focus on the analysis of the dynamics of the system and the exploration of possible control strategies in terms of active oscillations reduction directly applied to the appendage structure and passive vibration isolation between the deployed structure and the main body of the satellite. The main results confirm the concern that uncontrolled deployment of deployable appendages on a small spacecraft with limited attitude control capabilities exploiting only the stored strain energy of the tape springs can generate torques that can lead to uncontrolled rotations and mission failure. Hence, the dynamic behavior of controlled deployments using a small motor is studied and compared to the previously mentioned case. Although mass and complexity are added to the system, this approach brings many benefits such as disturbance reduction and the possibility of retracting the deployed structure whenever needed. Moreover, the effectiveness of two different strategies to control the oscillations of the booms is confirmed, respectively following a numerical and an experimental approach.

Abstract (italian)

Le strutture estremamente flessibili come pannelli solari a film sottile, antenne dispiegabili, concentratori e vele solari, sono tipicamente composte da membrane molto sottili tenute in tensione. Queste strutture dispiegabili sono considerate tecnologie promettenti per la loro leggerezza, elevata efficienza di impacchettamento, e basso costo. Questi vantaggi li rendono una tecnologia estremamente interessante per satelliti di piccole dimensioni, come i nanosatelliti (inclusi i CubeSat) che stanno diventando sempre più comuni grazie alla loro capacità di fornire molti dei servizi che in passato erano effettuati da satelliti più grandi, ad una frazione del loro costo. Le dimensioni delle strutture dispiegabili, però, non sono proporzionali alle dimensioni del satellite che le ospita, ma dipendono dalla missione e dall’applicazione specifica. Inoltre, la miniaturizzazione dei satelliti porta con sé molto spesso la riduzione delle performance del sistema di controllo di assetto. Perciò, appendici di grandi dimensioni montate su un nanosatellite possono essere un problema per quanto concerne il controllo d’assetto, con una possibile riduzione della stabilità e della precisione di puntamento, o persino lo stesso fallimento della missione. Per questo motivo, è necessario prevedere sistemi di controllo in grado di limitare o evitare sia le oscillazioni di queste strutture, sia di ridurre la trasmissione delle forze di disturbo dall’appendice flessibile al corpo centrale del satellite.
Questa ricerca di dottorato si focalizza sull’analisi del comportamento dinamico di strutture spaziali estremamente flessibili e lo studio di fattibilità di metodi possibili per controllare le loro oscillazioni, sia in termini di tecniche di dissipazione direttamente applicate all’appendice, sia di riduzione della trasmissione delle vibrazioni in corrispondenza dell’interfaccia tra la struttura dispiegata e il corpo centrale del satellite. Due diversi “test case” sono stati identificati per questo studio, entrambi basati su pannelli solari.
Il primo “test case” si focalizza su un pannello solare a membrana non supportato da alcuna struttura esterna. Simulazioni numeriche esplorano la fattibilità di controllare le oscillazioni di questa membrana, soggetta a una forza sinusoidale e condizioni iniziali non “piatte”, tramite l’utilizzo di patch sottili di materiale piezoelettrico. Questo studio conferma la possibilità di applicare il sistema di controllo in catena chiusa Hinf, per correggere la posizione non orizzontale della membrana e raggiungere una configurazione dispiegata e piatta.
Il secondo “test case” studia la dinamica di un pannello solare dispiegato e tenuto in tensione da due “tape spring” in materiale composito. Inizialmente sono stati fabbricati prototipi di questi elementi dispiegabili ed è stato studiato in dettaglio il loro comportamento dinamico teorico. Sono state fatte simulazioni numeriche e condotti test sperimentali focalizzati sull’analisi delle dinamiche del sistema e lo studio di possibili strategie di controllo in termini di riduzione delle oscillazioni direttamente applicate alla struttura dispiegabile, e l’isolamento passivo delle vibrazioni tra la struttura dispiegata e il corpo centrale del satellite. I risultati principali confermano il problema che relativo al fatto che un dispiegamento incontrollato delle appendici flessibili su un satellite di piccole dimensioni con capacità di controllo d’assetto limitate che sfruttano solo l’energia immagazzinata nelle “tape spring” può generare coppie che possono portare a rotazioni incontrollate e il fallimento della missione. Perciò, è stato studiato il comportamento dinamico di dispiegamenti controllati che utilizzano un piccolo motore, e sono stati confrontati con il caso precedentemente menzionato. Nonostante vengano necessariamente aumentate la massa e la complessità al sistema, questo approccio porta molti benefici come la riduzione dei disturbi e la possibilità di ritrarre la struttura dispiegata quando necessario. In aggiunta, è stata confermata la fattibilità di due diverse strategie di controllo rispettivamente da un punto di vista numerico e sperimentale.

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EPrint type:Ph.D. thesis
Tutor:Francesconi, Alessandro
Ph.D. course:Ciclo 30 > Corsi 30 > SCIENZE TECNOLOGIE E MISURE SPAZIALI
Data di deposito della tesi:15 January 2018
Anno di Pubblicazione:15 January 2018
Key Words:small satellite, nanosatellite, disturbance reduction, vibration damping, attitude control, solar panel, membrane
Settori scientifico-disciplinari MIUR:Area 09 - Ingegneria industriale e dell'informazione > ING-IND/05 Impianti e sistemi aerospaziali
Struttura di riferimento:Centri > Centro Interdipartimentale di ricerca di Studi e attività  spaziali "G. Colombo" (CISAS)
Codice ID:10960
Depositato il:09 Nov 2018 09:40
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