Nonsingular terminal sliding mode control technique for attitude tracking problem of a small satellite with combined energy and attitude control system (CEACS)

Attitude control and power storage subsystems are two of the essential utilities provided on a spacecraft. As they comprise a significant fraction of a spacecraft mass, a synergism concept that integrates these two into one subsystem can reduce the mass and volume of a spacecraft. The reduction will decrease the total cost of development and deployment of a satellite. A combined energy and attitude control system (CEACS) utilizes flywheels as a means of power storage and simultaneously as actuators. A series of works on CEACS have proposed solutions for attitude control problem of pitch axis. However, their analyses disregarded the high non-linearity involved in the attitude control of a spacecraft. In addition, the proposed controllers' feasibility in the presence of unknown perturbations and uncertainties were not examined. This study proposes a Nonsingular Terminal Sliding Mode (NTSM) control scheme for the attitude tracking control of roll, pitch and yaw axes of a small satellite with CEACS. The nonlinear system is subjected to unknown but bounded disturbances and uncertainties. The Lyapunov stability theorem is used to prove finite-time convergence in both reaching and sliding phases. This proposed method avoids inherited singularity of conventional terminal sliding mode. The numerical analysis provides proof of the controller robustness in rejecting unknown disturbances and keeping the attitude errors within limits under the influence of uncertainties. Results provided by NTSM control method demonstrate the superiority of this sliding mode scheme compared to the previous proposed techniques for attitude control of the CEACS.