Rapid slewing of flexible space structures

Abstract

This thesis addresses the problem of computing rapid slew maneuvers for a spacecraft antenna mounted on a double-axis gimbal with elastic joints. The performance of the system can be enhanced by designing antenna maneuvers in which the flexible effects are properly constrained, thus reducing the load on the spacecraft control system. The motion of a mass-spring-damper system is shown to be analogous to a spacecraft antenna slew with linear dynamics. This model is extended to a nonlinear double-gimbal mechanism with flexible joints, which better represents real spacecraft antenna dynamics. Rather than increase maneuver times to control flexible motion, this thesis presents optimal solutions that decrease maneuver times while allowing designers to easily constrain flexibility. Since it is impossible to recast the nonlinear system into a modal representation, an innovative approach is used to map the nonlinear dynamics into a linear system with a fictitious force. The fictitious force captures the effects of the nonlinearities so the vibrational motion can be constrained for a time-optimal slew. It is shown that by constructing an appropriate optimal control problem, the maneuver time for a flexible DGM can be decreased by approximately 42% compared to a conventional computed torque control solution.