ATTITUDE CONTROL TECHNOLOGY FOR MASS MOMENT NANO-SATELLITE IN LOW EARTH ORBIT

Due to the high aero to inertia ratio and the presence of strong aerodynamic forces, the low Earth orbit nanosatellites are not very appropriate to depend on a set of momentum wheels for attitude controlling. A method of utilizing aerodynamic disturbance torque as control input based on mass moment technology is innovatively proposed for the Nano-satellite in the low Earth orbit to solve the problem of the external aerodynamic force. The exclusive use of moving mass actuator would lead to an underactuated as the aerodynamic torque was perpendicular to the relative flow vector. To achieve full three-axis stabilization, a three-axis magnetorquer is used to complement the moving mass system to generate a torque along the orbital velocity. The whole dynamic equations are derived, which describes the system with two actuators, the movable mass and the magnetorquer, actuating simultaneously. According to the influence of disturbance items, the equations are simplified. Considering the uncertainty of the aerodynamic forces, the error of system parameters, and unknown environmental disturbance, a sliding mode control scheme based on disturbance observer is designed for ideal control input. An optimal torque allocation strategy is designed in order to generate the torque determined by the aforementioned nonlinear control law by moving the masses and commanding the magnetotorquer, and therefore combining the subspace of two actuators. Finally, a semi-physical simulation platform was built for two actuators and the results indicate that, additional inertia torque, related to the mass acceleration, is the main disturbance torque during the attitude maneuver and can be significantly reduced by optimal torque decomposition strategy. Meanwhile, during the attitude maintenance, the disturbance observer can effectively observe the system disturbances and improve the attitude control accuracy. The error of attitude angle is less than \pm 0.1^\circ. The results verify the feasibility of the use of the moving mass actuator to actively control the aerodynamic torque.