Attitude Control of a flexible spacecraft with Sliding Mode Super Twisting Control

The increasing complexity and articulation of modern aerospace sector lead to the necessity towards more advanced and robust control strategies in satellite attitude regulation. Satellites equipped with flexible appendages, such as solar panels or antennas, enlighten additional challenges due to the coupled rigid-flexible dynamics that bring out unwanted vibrations and performance degradation. Moreover, real actuators (such as Control Moment Gyroscopes (CMGs) and/or Reaction Control Systems thrusters) inherently bring in the closed loop system certain dynamics, making it crucial a design strategy that can effectively cope with bandwidth limitations, saturation and other nonlinearities. In this context, sliding mode control emerges as a promising nonlinear alternative thanks to its intrinsic robustness against uncertainties and external disturbances. This thesis aims to design and validate a Super-Twisting Sliding Mode Control (STA/SMC) strategy tailored for satellite attitude regulation in the presence of both rigid-body dynamics and structural flexibility. The goal is to achieve high-performance stabilization while guaranteeing robustness against uncertainties, bounded control effort and minimal chattering. The parameters of the sliding surface are defined using considering the dominant flexible mode frequency, thus seeking for effective vibration suppression. The gains of the STA algorithm are selected via a cost-function minimization procedure inspired by the Describing Function (DF) and Harmonic Balance (HB) techniques to ensure accurate chattering mitigation. In addition, the control law is designed to ensure that the torque limits imposed by the actuator are not exceeded, thus ensuring the actual implementation. Moreover, the stability of the closed-loop system is validated through a Lyapunov-based approach. Monte Carlo simulations are conducted to assess robustness across a large set of randomized parametric uncertainties, while an Extended Kalman Filter (EKF) is implemented to improve state estimation accuracy. The proposed STA/SMC control architecture achieves accurate attitude tracking with high robustness and significant reduction of chattering effects compared to traditional high-gain strategies. The bounded torque strategy successfully keeps the control effort within actuator limits without compromising performance, aspect that has been particularly problematic for the instant torque peaks that both actuators showed. Frequency domain analyses via Fast Fourier Transform (FFT) confirm the effectiveness of chattering suppression. Statistical evaluation through Monte Carlo simulations demonstrates limited performance degradation under varying conditions. Overall, this thesis presents a coherent and innovative nonlinear control design for satellite applications, offering a credible alternative to conventional methods, especially under uncertain and constrained operational conditions