Attitude control of an underactuated LEO CubeSat in presence of uncertainties with state estimation
Najafizadeh Sari, Naeimeh
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This thesis addresses the nonlinear attitude control of an underactuated satellite equipped with only three magnetorquers. The problem involves stabilizing the sun-pointing 3U CubeSat in the presence of environmental perturbations and sensor noises from launch separation and deployment until the nominal mode. The sensors include a three-axis gyroscope, a three-axis magnetometer and a two-axis sun sensor. A detumbling and a proportional-derivative(PD) controller are applied to the underactuated dynamic system. Since a spin-stabilized configuration is naturally stable, a spin-stabilized mode is selected given the spin-axis is a major axis of inertia. Further, a power management strategy has been applied, which requires a small amount of control effort. The proposed linear control law achieves attitude convergence for three-axis stability within the 0.18 degrees attitude error and 1 deg/s spin rate norm. The design has been implemented using a high fidelity real-time simulation in Simulink/MATLAB while considering the environmental effects to study the in-orbit behaviour and the effectiveness of the system's stability in overcoming the disturbance torques, including gravity gradient torque, residual dipole torque, dynamic uncertainties, and sensors noise. Since sun sensors do not provide any measurements in the eclipse, an Extended Kalman Filter (EKF) is embedded in the simulation to give estimates of the attitude angles in the eclipse based on gyroscope measurements. The estimator benefits from a dimension reduction technique to increase the estimation reliability and reduce the onboard computational and power usage. Numerical simulations successfully corroborate the effectiveness of the controller and the estimator.
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