Desing of Sliding Mode Techniques for a CMG-based Testbed Attitude Control System

The observation of the universe, the Earth monitoring, the docking of a capsule on the ISS, all these things require a certain attitude to carry out their tasks, so a robust attitude controller should be designed. The following thesis describes the design of a Sliding Mode Control (SMC) for a CMG (Control Moment Gyro) based testbed. A testbed is a platform for performing rigorous and replicable testing of scientific theories, new technologies, calculation tools and many control law is being developed on it. The testbed is driven by four CMGs which are torque generators used to perform attitude manoeuvre thanks to angular momentum conservation. The main reason to choose the Sliding mode control (SMC), which is a well-established method for control of nonlinear systems, is its robustness versus imprecise knowledge of the plant to control, because there will be always a discrepancy between the real plant and its mathematical model used for controller design. Two sliding mode techniques have been developed: a Super twisting and an adaptive continuous twisting. The second one has the ability to change its gains in real time to track controlled variables even if system parameters are unknown or if they change over time, this ability increases the robustness of the control system, since it is able to handle disturbances and adjust the gains. In the second part of this thesis, a mathematical spacecraft model has been developed taking into account the effects deriving from appendages flexibility. The attitude maneuvering or the effect of external disturbances can produce continuous vibration of flexible appendages that interferes with the attitude control. The attitude dynamics of satellite considered is driven by the same CMGs actuators used on the testbed, while a cluster of thrusters manage the position dynamics by an Hinf control law. In particular, the study is applied to a rendezvous maneuver, in which a moving satellite (Chaser) is trying to reach a second satellite (Target). In this case, a fixed Target is considered, furthermore both position and attitude dynamics is modeled for the Chaser including orbital disturbances. Both scenarios are simulated by Matlab and Simulink software, in which the dynamics, guidance and control algorithms are implemented. Lastly, the testbed control algorithm has been converted in C++ programming language and used to perform experiments on the testbed located at the Yamada’s laboratory at Osaka University.