Modeling and configuration-independent control of a self-mobile space manipulator

TheSelf-Mobile Space Manipulator (SM2) is a 5-DOF, 1/3-scale, laboratory version of a robot designed to walk on the trusswork and other exterior surfaces of Space Station Freedom. It will be capable of routine tasks such as inspection, parts transportation and simple maintenance and repair. We have designed and built the robot and gravity compensation system to permit simulated zero-gravity experiments. The control ofSM2 is challenging because of significant structural flexibility, relatively high friction at the joints, positioning error amplified from joint errors due to the long reach, and high performance requirements for general 3-D locomotion. In this paper, we focus on the modeling of the robot system and the design of the control system based on the model. We address the kinematics and dynamic modeling of the 3-D motion ofSM2 and demonstrate the simulation and experimental modal analysis results. The robot dynamic characteristics vary significantly when the robot configuration changes. To consider this effect, we develop a control system that is composed of two basic parts, the model-based part and the servo part. The model-based loop can be updated based on the off-line dynamic model, and the servo control loop is updated by a gain schedule according to the off-line relationship between the closed-loop frequency and the modal frequency estimated from the off-line dynamic model. By taking dynamic variation into account in the controller, the control system is independent of the robot configuration, and the motion performance ofSM2 is greatly enhanced in implementation.