Development of attitude and trajectory control strategies for small satellites involved in rendez-vous and docking operations

Rendez-vous and docking (RVD) is an orbital manoeuvre whose aim consists of allowing an active spacecraft, the so-called chaser, to approach a passive one, the target. The outcome of the said procedure, if successful, is the structural connection between the two vehicles. This manoeuvre, in the framework of small satellites, has never been performed before, hence this problem opens the field to a broad set of possible solutions. The focus of this work is on the final phases of the RVD manoeuvre, as these constitute the most critical part, from a control perspective, because of the extremely high accuracy expected on position, velocity and attitude accuracy. Additionally to a demanding set of requirements, a significant amount of disturbances, both external, due to the surrounding environment, and internal, caused by intrinsic characteristic of the spacecraft, must be taken into account. As much as concerns attitude, Sliding Mode Control is selected for its effectiveness, versatility, given by the chance to refine the control law, and tuning simplicity. For trajectory a Model Predictive Control is employed; even though such solution is not frequently encountered in literature for this kind of applications, its real-time working principle, predictive logic and large amount of tunable settings are proven to be features very well suited for the investigated problem. The closed loop control system is modelled, according to a work logic based on performing simulations, in a MATLAB and Simulink environment, with an increasing amount of details and progressive reduction of idealities. The controllers are firstly designed in a strongly ideal setup and then applied to a gradually more complex and refined scenario, including more and more detailed system’s plant, actuators and disturbances modelling. Each design step requires a further tuning and adaptive adjustment of the controllers, thus getting to a final version of the closed loop control system. The effectiveness of the ultimate model is verified by means of a robustness analysis performed with Montecarlo simulations, setting strong uncertainties on the most critical parameters. In the last part of the work, the study of an off-nominal condition is performed in a single-failure framework involving the modelled propulsion system: the obtained results suggest that a severe breakdown of the latter, in most of the simulated scenarios, can heavily affect the outcome of the manoeuvre.