Machine Design Magazine
Cable Drives Make Arm Fast and Stiff
A robot arm that was originally designed at MIT's AI Lab is now being developed for NASA's use by Barrett Technology Inc., Cambridge, Mass. NASA is testing the Whole-Arm Manipulator (WAM) as part of the proposed EVA (extra vehicular activity) Retriever, an autonomous robot that will recover crew or tools outside the Space Station. The arm can reach around objects and clasp them, much like a person holding a stack of firewood between their fore and upper arm. It has no appendages, protruding linkages, or motors that could get caught on a space suit. The transmission is small and housed inside the arm.
Conventional robot arms use hand-like end effectors instead of the whole arm the way a human does, and so are limited to moving light things compared with their mass. The whole-arm manipulator, on the other hand, is stronger and steadier.
The WAM is noteworthy because it does not use any gears for manipulating the joints, but rather a cable drive, so there are no backlash problems and it is both fast and stiff. Cable drives permit low friction and ripple-free torque transmission from the actuator to the joints. Two-stage speed reducers, which maximize transmission stiffness and minimize cable loading, are placed at the joint they power.
The arm operates with four degrees of freedom that match those of the human shoulder and elbow, but with a greater range of motion at each joint. Position, velocity, and torque sensors are used at each of the four axes. The WAM is "backdriveable," meaning that any force at the tip is immediately felt at the motors, so the machine reacts rapidly, drawing back from an object as a person stubbing a toe would do.
If NASA approves the WAM, it will be used on the Space Station Freedom by 1995. Barrett Technology is currently updating the exterior appearance of the WAM and researching a "wrist" for the arm to increase its usefulness. WAM applications on earth include environmental cleanup, fruit harvesting, and manufacturing.
Publish Date: June 25, 1992
Reprinted with permission from Machine Design Magazine © 1992, Penton Publishing Inc.