Attitude Determination and Control of CubeSat using Magnetic Torquer and LQR Controller

Abstract

In this thesis, attitude determination and control of CubeSat using magnetic torquer and LQR controller were performed. The requirement of attitude control performance that needs for CubeSat mission was set, and proper ADCS (Attitude Determination and Control System) satisfying the requirement was designed. In simulation, using this ADCS, the requirement of attitude control performance could be met. The CubeSat is very small satellite, and it has weight less than 2 kg. However, compared with ordinary satellites, it can achieve most mission that ordinary satellites performs. The merits of CubeSat are shorter development period, less cost, and more simple system than ordinary satellites. However, CubeSat has more restriction to weight, size than ordinary satellites. In the same manner, the ADCS design have some restriction and it is very important to select proper actuator and controller. The main mission of the CubeSat in consideration in this thesis is testing of GNSS (Global Navigation Satellite System) receiver developed in GNSS Laboratory at Seoul National University, South Korea. For this mission, it is expected to consume considerable power, so the ADCS must operates in efficient way that save power. For this reason, the actuator and controller should be selected in same manner. The most general actuators for CubeSat attitude control are magnetic torquer and reaction wheel. The reaction wheel can generate relatively high torque output than other many actuators, but it is heavy, consume much power, and have relatively high possibility to malfunction. In contrast with reaction wheel, the magnetic torquer generates relatively small torque output, but it is light, consume low power, and have relatively low possibility to malfunction. On account of reliability and many merits, the magnetic torquer suits for the CubeSat in consideration. Also, in this thesis, LQR (Linear Quadratic Regulator) control is being applied to attitude control of CubeSat. The LQR control is one of the optimal control method that minimizing cost (energy). Therefore, the LQR control is suitable for mission of the CubeSat in consideration with respect to energy saving standpoint. In this thesis, dynamics and kinematics are derived using mathematic modeling, and they are linearized for applying to LQR controller. Also, EKF (Extended Kalman Filter) is used for attitude estimation. Overall attitude control process consists of detumbling mode and attitude maintenance (Earth pointing) mode in two steps. In simulation, the detumbling mode was found to be possible to attenuate rapid rotation of CubeSat from initial tip-off rotation velocity, then attitude maintenance mode for mission performing, next step, could be operated. The attitude maintenance mode was found to be possible to maintain Earth point attitude in accuracy which goal.