Automatic mass balancing system for the 5-DOF spacecraft simulator
Choi, Su Yeon
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Spacecraft simulators are widely used to validate spacecraft attitude determination and control techniques. The Autonomous Spacecraft Testing of Robotic Operations in Space (ASTROS) facility at the School of Aerospace Engineering at the Georgia Institute of Technology has been developed to create spacelike environment for academic research and education. This facility is composed of two stages: The upper stage has a hemi-spherical air-bearing that allows frictionless rotational motion of the upper stage, and the lower stage has three linear air-bearing pads that make it possible to achieve almost friction-free translational motion of the entire system. Therefore, these two types of air-bearings enable the platform to have five degrees of freedom in order to test both attitude and position controlling problems. In this system, the balancing procedure is a crucial issue that will minimize gravitational torques acting on the spacecraft simulator. This work introduces an automatic mass balancing method and eliminates the center of gravity offset from the center of rotation by actuating three sliding masses. First, PID attitude feedback controller is proposed to balance x and y directions. Second, an Extended Kalman Filter is introduced to estimate the vertical imbalance since the linear actuators cannot control the vertical direction by sliding masses. Finally, the proposed controller and the EKF are tested on the simulation and the ASTROS facility. The quaternions are measured from a dual-axis inclinometer and the angular velocities are calculated from derivatives of the quaternions during the experiments.