Modal-Space Control of Large Flexible Spacecraft Possessing Ignorable Coordinates

One problem peculiar to spacecraft in free space is that of ignorable coordinates, which are present when the spacecraft has no natural ability of preventing rigid-body motions. Ignorable coordinates can be eliminated from the problem formulation by introducing a reduced state vector. However, this results in having the ignorable coordinates uncontrolled. This paper solves this problem by introducing a dual-level control. The firstlevel controls are designed to control the ignorable coordinates and they lead to a positive definite "augmented system" and the second-level controls are designed for the final control of the complete state vector, including the ignorable coordinates. The second-level controls are designed in modal space and can be based on several types of control laws. In particular, proportional control providing artificial viscous damping, proportional optimal control, and nonlinear on-off control are discussed. A numerical example, illustrating all of these control laws, is presented.

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