Design of a Robust Attitude Determination Control System for LICIACube mission

CubeSats, as a compact and miniaturized version of traditional Satellites, are nowadays a trend thanks to the numerous goals reached by the space agencies missions. Among the various companies involved in the space field, Argotec is participating to NASA Double Asteroid Redirection Test (DART) mission with its CubeSat Light Italian CubeSat for Imaging of Asteroids (LICIACube). Since the goal of the mission is to acquire many pictures during a high-speed fly-by of an asteroid, the Attitude Determination Control System (ADCS) is a critical part of LICIACube. The design of an attitude control system for small satellites is a challenging task, especially when high velocity - high precision is asked and to include the adaptability to different satellites. Different approaches are possible, each with its own pros/cons, strictly depending on situations. The work done in this thesis explores the possibilities of realizing an ADCS for small satellites as modular and flexible as possible, using LICIACube as a reference model. The goal is to implement and test such solution on the next platform generation. To successfully achieve mission requirements, strict performances for the control system are to be considered; moreover, the spacecraft shall have different operating modes, all of them using Reaction Wheels as main actuators and thrusters as support. After considering the worst possible set of initial conditions to work with, the model of LICIACube has been developed on Simulink. Different operative modes are considered. Firstly, Fully Operational Mode, where LICIACube works in nominal conditions, is shown. A nonlinear robust controller based on the Sliding Mode theory is applied for attitude tracking problem. The simulated sensors noise is handled by an Extended Kalman Filter. The second operating mode is the Detumbling Mode. A set of Proportional Integrative Derivative (PID) controllers is used to reduce the microsatellite residual angular velocity caused by the detachment from DART. The last operating mode is the Desaturation Mode. The Reaction Wheels are discharged of their accumulated momentum by directly sending commands to the actuators through a custom logic. The resulting high angular velocity of the satellite is then reduced with thrusters, whose modelling involves the implementation of a switching management logic for fuel consumption minimization. To simulate the correct behavior of the ADCS, a finite-state structure on Stateflow makes possible to let all control systems communicate, while guaranteeing a certain level of abstraction and a good degree of flexibility, allowing the customization of all control parameters at will. The structure allows the ADCS, through the analysis of the sensors data, to switch independently in the correct operating mode. The results show that the control systems are compatible and the performances are in line with the mission requirements. In conclusion, although DART mission has strict mission requirements, the developed ADCS seems adequate for the task; moreover, the Stateflow structure provides an abstraction layer that makes viable the concept of adaptability for similar kind of CubeSats