On the maneuvering of a flexible space crane

The design of an articulated space crane under NASA's 'Pathfinder' program continues to push forward. However, the large size and extreme flexibility of the crane entail major difficulties in precision positioning. To illustrate this and to gain more insight into its structural dynamic behavior, we have built three-dimensional rigid, flexible, homogenized and non-homogenized finite element models (FEM) of the articulated space crane and simulated its three-dimensional maneuvering. We have also begun to develop control strategies for the flexible system using first a simplified finite element model. The model captures much of the system's low frequency characteristics. From the FEM, a reduced-order model (ROM) based controller is designed based on a specific configuration of the crane. The ROM controller rotates the fixed shape crane about the alpha joint and uses three torque wheels at the tip to actuate its control law. Our analysis of the closed-loop system shows the destabilizing effect of control interaction with unmodeled system dynamics (Controller-Structure-Interaction). Making use of a parallel bank of residual-mode filters (RMF), we are able to compensate for CSI and restore system stability and performance. The implementation of the discrete-time control law on the simplified multi-body model is completed using the SAMCEF code.