Towards Reducing Thruster- Flexibility Interactions in Space Robots

Space manipulators mounted on an on-o thruster-controlled base are envisioned to assist in the assembly and maintenance of space structures. When handling large payloads, manipulator joint and link exibility become important, for they can result in payloadattitude controller fuel-replenishing dynamic interactions. In this paper, the dynamic behavior of a exible-joint manipulator on a freeying base is approximated by a singlemode mechanical system, while its parameters are matched with space-manipulator data. Describing functions are used to predict the dynamic performance of three alternative controller-estimator schemes, and to conduct a parametric study on the in uence of key system parameters. Design guidelines and a state-estimator are suggested that can minimize such undesirable dynamic interactions as well as thruster fuel consumption. Introduction Robotic devices in orbit will play an important role in space exploration and exploitation. The mobility of such devices can be enhanced by mounting them on freeying bases, controlled by on-o thrusters. Such robots introduce a host of dynamic and control problems not found in terrestrial applications. When handling large payloads, manipulator joint or structural exibility becomes important and can result in payload-attitude controller fuel-replenishing dynamic interactions. Such interactions may lead to control system instabilities, or manifest themselves as limit cycles. The CANADARM-Space Shuttle system is the only operational space robotic system to date. Its Reaction Control System (RCS), which makes use of on-o thrusters, is designed assuming rigid body motion, and using single-axis, thruster switching logic based on phase-plane techniques. This approach is common in the design of thruster-based control systems. However, the exible modes of this space robotic system have rather low frequencies, which continuously change with manipulator con guration and payload, and can be excited by the RCS activity. The performance degradation of the RCS due to the deployment of a exible payload, with or without the CANADARM has been studied. A new design for the RCS was developed to reduce the impact of large measurement uncertainties in the rate signal during attitude control, thereby increasing signi cantly the performance of the RCS for rigid-body motion. However, the exibility problem was not addressed. Currently, the method for resolving these problems consists of performing extensive simulations. If dynamic interactions occur, corrective actions are taken, which