Techniques for improved performance in CMG based atitude control systems

Control moment gyroscopes (CMG) are desired as actuators for spacecraft attitude control due to certain properties which include rapid retargeting, precision pointing, and low specific mass (mass per unit torque output). Ambiguity in some of the on-orbit parameters such as actuator alignment and mechanism imperfections such as imbalance in flywheels lead to deteriorated performance of the attitude control system. Novel techniques are developed to address these issues. Imprecise knowledge of the gimbal orientation of a CMG can lead to spacecraft pointing errors. Knowledge of spacecraft angular acceleration is shown to be beneficial in the design of techniques to estimate on-orbit parameters. A method to estimate the spacecraft angular acceleration using linear acceleration measurements is developed. The method uses six uniaxial accelerometers and a Kalman filter to obtain bias-free estimates of the angular acceleration along with smoothed angular velocity estimates. Using the estimates of angular acceleration, a technique to estimate the on-orbit gimbal orientation is developed. Measurements of spacecraft angular velocity and angular acceleration along with measurements of angular velocity and acceleration of the CMG flywheel are used in a linear least squares construct to estimate the unknown orientation. Three least squares solution variants are discussed, and their performances are compared. High-fidelity simulations utilizing data from commercially available hardware are presented. Imbalance in the flywheel of a CMG leads to high frequency attitude disturbance called jitter. A three-flywheel system is developed to reduce the magnitude of jitter emitted by the CMG. The dynamics of jitter due to rotor imbalance is investigated and a modification to the CMG flywheel system which involves the replacement of the single flywheel by a three-flywheel system is proposed. The method results in a system that produces lower jitter compared to the single-flywheel system, provides limited redundancy against flywheel failure, and constitutes a long term jitter management solution. The dynamics of the three-flywheel system are developed and elaborate simulations are performed to verify the validity of the method. The effect of single/multiple flywheel failure in the three-flywheel system is investigated. The power and mass characteristics are analyzed and compared with those of the single-flywheel system.