Artificial Potential Field with Sliding Mode Control for Spacecraft Constrained Attitude Manoeuvres =

This thesis focuses on the development of guidance and control strategies to perform autonomous spacecraft attitude manoeuvres while complying with pointing constraints and actuators limitations. In particular, the objective of the attitude manoeuvre is to achieve the proper spacecraft orientation allowing a sensitive on-board instrument to be pointed towards a scientific target. When performing the attitude manoeuvre, it must also be ensured that sensitive payloads, such as optical instruments, do not point towards bright celestial objects that could damage those instruments. In order to prevent bright objects from entering the sensor field of view (FOV) during spacecraft reorientation, forbidden pointing zones in three-dimensional space, i.e. attitude constraints, are defined. Then, the attitude control system consists of a pyramidal cluster of four reaction wheels (RWs), which impose bounded control torques and angular velocities when performing attitude manoeuvres. The main contribution of this thesis is the development of guidance and control strategies to solve the attitude control problem presented above. The guidance algorithm is based on Artificial Potential Fields (APF) and must generate the reference angular path to direct the boresight vector of the on-board instrument to the inertially-fixed observation target while avoiding the moving forbidden pointing zones. The strategy takes into account the orientation and angular rates of both the spacecraft and the forbidden zones to construct the attractive and repulsive layers of the APF. Furthermore, APF gains are designed to ensure compliance with attitude and angular rate constraints so that the guidance algorithm generates feasible reference angular trajectories. Then, the control laws are defined according to Sliding Mode Control (SMC) theory to track the reference angular path given by the APF. SMC gains are derived to ensure that the control torques allow tracking of the reference angular path given by the APF, while counteracting the orbital disturbances without violating actuation torque constraints. The proposed guidance and control strategies are validated through extended numerical simulations in a three-degree-of-freedom (DOF) orbital simulator, which includes the non-linear models of spacecraft attitude dynamics and kinematics, the model of the actuators, and the guidance and control algorithms. The numerical simulations are performed in Matlab/Simulink environment and the simulation scenario is to reorient the spacecraft to point the on-board sensitive instrument at a specific target while satisfying pointing and actuation constraints. Numerical results shows the effectiveness of the proposed strategy to solve the considered attitude control problem, even when considering particularly challenging situations where the forbidden zones move close to both the spacecraft boresight vector and the observation target. Furthermore, the guidance strategy proposed in this thesis is compared with a second strategy in which the APF does not use the relative angular motions between the spacecraft and the forbidden zones to generate the repulsive layer. The results show that the strategy developed in this work performs better in a more challenging dynamic environment, evading moving obstacles more effectively.