Self-Mobile Space Manipulators
Astronaut extra-vehicular activity (EVA) at a space station is costly, potentially dangerous, and requires extensive preparation. Some EVA tasks, such as unplanned repairs, may require the versatility, skill, and on-site judgment of astronauts. Many other tasks, particularly routine inspection, maintenance and light assembly, can be done more safely and cost effectively by robots.
We are developing a relatively simple, modular, low mass, low cost robot for space station EVA that is large enough to be independently mobile on the station exterior, yet versatile enough to accomplish many vital tasks. Because our design is for a robot that is independently mobile, yet capable of conventional manipulation tasks, we call it the Self-Mobile Space Manipulator or (SM)2. The robot can perform useful tasks such as visual inspection, material transport, and light assembly. It will be able to work independently or in cooperation with astronauts, and other robots. The research accomplishment we have achieved so far include:
The zero-gravity environment at an orbiting space station has significant impact on the design and performance of a robot. The absence of gravitational forces permits a long, spindly robot to move relatively large masses with small forces and power consumption. In order to perform realistic experiments on earth, we have developed a gravity compensation system that balances the more significant gravitational effects on the robot so it behaves as if weightless. The gravity compensation system includes a passive, vertical counterweight system, and an actively controlled horizontal system. We have currently two GC systems developed in the lab, the x-y gantry system for the large, global locomotion experiments and the polar frame system for the fine, local motion experiments.
The first version of SM2 robot was designed to have the minimum size and complexity needed for walking on the space-station trusswork. The basic walker includes five rotational joints and two slender links. Grippers at each end of the robot enable it to attach itself to threaded holes in the truss nodes or other regular structure. Walking is accomplished by alternate grasping and releasing of the nodes by the grippers, and swinging of the feet from one node to the next. During each walking step, one end of the robot releases from a node, swings 90 or 180 degrees to a desired node location, and reattaches to that node. SM2 moves along the trusswork using such steps with alternate feet.
The second version of (SM)2 robot was based on the first version as a main frame, and extended the body to enable manipulation capability. Two additional joints were connected to each one of the node grippers, and a gripper that we designed and built was attached to these joints to enable the robot to grasp and carry objects. Two such 2-DOF arm-hand devices assembled at both ends of 5-DOF manipulator provide a 9-DOF manipulator configuration, while the node gripper remains available for walking. In addition to its own drive motor, the gripper contains a motorized hex driver between its jaws to drive the hold-down screw of the ORU mockup.
The third version of (SM)2 was based on the new design of the international space station alpha whose trusswork is preintegrated, I-beam shape flanges. The robot now has seven joints: one at the elbow and three at each end to allow the out-of-plane motions needed for stepping from one truss face to another, while preserving the robot's symmetry to simplify control. Joints are self-contained and modular, so a minimum inventory of parts is needed for joint repair or replacement. The robot links are long enough to permit climbing between adjacent longerons. At each end of the robot is a three-fingered gripper for grasping the truss I-beam flanges. Capacitive proximity sensors at the base of the fingers are used to sense beam proximity, and can be used for aligning the gripper with the beam.
The space station structural design has evolved from the original strut-and-node design to the current preintegrated truss (PIT) design, utilizing aluminum I-beam members. For laboratory experiments, we built a truss mockup which is a full-scale representation of a small portion of a truss segment. It includes two longerons, two bulkheads and a diagonal beam, and is constructed of wood with sheet aluminum laminated to the beam flange faces to provide a realistic appearance for video images. Having four faces for robot walking and beam widths of 4.0 and 5.8 inches, the mockup allows a variety of stepping and grasping motions to verify (SM)2's general motion capability.
We have developed a teleoperation control station including a 6-DOF free-floating, hand-controller, master-slave type walking demonstrator, data-glove for gesture control, real-time graphics interface for both display and control, and graphical simulation system for off-line motion previewing.
The control is executed through Chimera real-time operating system that is developed in the lab. A variety of low-level controllers have been developed, including acceleration feedback control, adaptive control, and neural network learning control. Multiple phase control strategies are executed during different phases of motion and operations. An intelligent control scheme is implemented for reliable walking using both vision and proximity information for the robot. Hidden Markov model is used for learning teleoperation for defined tasks and is implemented for facilitating ORU exchanging tasks.
For more information, please contact Yangsheng Xu (xu+@cs.cmu.edu) or Ben Brown (hbb@cs.cmu.edu).