Magnetic Yaw-Roll Control and Momentum Dumping for SROSS
SROSS, a 3-axis spacecraft is a low earth satellite intended to carry an earth pointing payload. The low altitude (400 Kms) orbit is always associated with high aerodynamics and magnetic disturbance torques that disturb the satellite attitude. The magnetic control scheme envisaged not only corrects for these disturbances but also steers the pitch axis along the orbit normal which is continuously changing by approximately five degrees per day. In addition, the scheme also dumps the momentum accumulated in the momentum wheel which is employed to correct the pitch error. The control torques required for the dual functions of yaw-roll control and momentum dumping were envisaged to be generated by a single torquer mounted (at 45 degrees from the yaw axis) in the yaw-roll plane. While, each function is found to be independently controllable, simulation studies have clearly shown that it is not possible to carry out both the functions (YRC and MD)by a single torquer. This was subsequently corroborated analytically. It was observed that the average yaw-roll disturbance torque vector in the orbital plane due to magnetic momentum dumping has a fixed direction and and magnitude for a given pitch disturbance. The direction of the aver-average yaw-roll ‘control torque’ by the same torquer is orthogonal to the direction of the above ‘disturbance’ torque. Hence, the yaw-roll errors introduced by magnetic momentum dumping can not be controlled in the average sense by the same torquer. This has clearly brought out the inadequacy of a single torquer to control yaw/roll errors and pitch wheel momentum. Hence, an additional torquer along pitch axis was introduced for yaw-roll control while the original torquer in the roll/ yaw plane was retained for momentum dumping. As the additional torquer can not produce a pitch torque, the wheel speed is unaffected by the yaw-roll control. The paper presents the detailed analysis carried out for this study and is supported by extensive computer simulation results.
KeywordsWheel Speed Switching Scheme Attitude Control System Disturbance Torque Pitch Axis
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