The main purpose of this work is to design a robust and reliable algorithm to control a satellite attitude exploiting thrusters’ off-modulation logic. Low Earth Orbit (LEO) satellites are often required to perform frequent manoeuvres for orbit maintenance. Depending on the altitude, the duration of these manoeuvres can last up to 300 seconds. During the firing, displacement of the centre of gravity, uncertainties in the thrusters’ accuracy and misalignments will generate parasitic torques. In addition, high precision requirements on pointing do not allow the employment of magnetic torquers, therefore the attitude is generally undertaken with reaction wheels only. To overcome this problem, part of the angular momentum for the attitude control can be provided by the off-modulated thrusters. This strategy consists of generating angular momentum by means of an appropriate thrusters switching off during the firing phase. An innovative feedback-based approach is developed in the following paper and, afterwards, it is tested on a high-fidelity simulator (the MatLab Simulink OHB Attitude and Orbit Control System (AOCS) simulator) to prove its robustness and efficiency. Thanks to the presence of two adjustable thresholds, the algorithm can be easily tailored to whichever mission requirement. Results are shown in the last chapter, where a Monte Carlo analysis is carried out to validate the algorithm in almost all the possible scenarios. Eventually, it will be shown that this new off-modulation approach brings remarkable improvements characterizing itself as one of the best solutions to be adopted by the Attitude and Orbit Control System (AOCS) system for the attitude control during thrusters firing.
Questo lavoro si propone di sviluppare un algoritmo robusto e affidabile per il controllo dell’assetto di un satellite tramite off-modulation dei propulsori. I satelliti in orbita terrestre bassa devono spesso eseguire manovre per il mantenimento orbitale che, a seconda dell’altitudine, possono durare fino a 300 secondi. Durante la fase di sparo, lo spostamento del centro di massa insieme alle incertezze e ai disallineamenti dei propulsori possono generare coppie parassite. Inoltre, se il puntamento richiede elevati requisiti di precisione, non è possibile utilizzare bobine magnetiche e, di conseguenza, l’assetto è controllato unicamente dalle ruote di reazione. Per superare questo problema, parte del momento angolare per il controllo dell’assetto può essere fornito da propulsori in off-modulation. Questa strategia prevede la generazione di momento angolare tramite un opportuno spegnimento dei propulsori durante la fase di sparo. Nel seguente documento, un’innovativo metodo di off-modulation basato sulla retroazione viene sviluppato e, successivamente, testato su un simulatore ad alta fedeltà (il simulatore per il controllo di assetto MatLab Simulink di OHB) per verificarne la robustezza. Grazie alla presenza di due soglie regolabili, l’algoritmo può essere facilmente adattato a qualsiasi missione. I risultati sono presentati null’ultimo capitolo dove viene eseguita un’analisi Monte Carlo per validare l’algoritmo in quasi tutti i possibili scenari. Infine, verrà mostrato che questo nuovo metodo di off-modulation porta grandi benefici caratterizzandosi come una delle migliori soluzioni per il controllo d’assetto durante la fase di sparo.
Thruster's off-modulation for attitude control during orbital manoeuvres
CEDERNA, LORENZO
2019/2020
Abstract
The main purpose of this work is to design a robust and reliable algorithm to control a satellite attitude exploiting thrusters’ off-modulation logic. Low Earth Orbit (LEO) satellites are often required to perform frequent manoeuvres for orbit maintenance. Depending on the altitude, the duration of these manoeuvres can last up to 300 seconds. During the firing, displacement of the centre of gravity, uncertainties in the thrusters’ accuracy and misalignments will generate parasitic torques. In addition, high precision requirements on pointing do not allow the employment of magnetic torquers, therefore the attitude is generally undertaken with reaction wheels only. To overcome this problem, part of the angular momentum for the attitude control can be provided by the off-modulated thrusters. This strategy consists of generating angular momentum by means of an appropriate thrusters switching off during the firing phase. An innovative feedback-based approach is developed in the following paper and, afterwards, it is tested on a high-fidelity simulator (the MatLab Simulink OHB Attitude and Orbit Control System (AOCS) simulator) to prove its robustness and efficiency. Thanks to the presence of two adjustable thresholds, the algorithm can be easily tailored to whichever mission requirement. Results are shown in the last chapter, where a Monte Carlo analysis is carried out to validate the algorithm in almost all the possible scenarios. Eventually, it will be shown that this new off-modulation approach brings remarkable improvements characterizing itself as one of the best solutions to be adopted by the Attitude and Orbit Control System (AOCS) system for the attitude control during thrusters firing.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/167045