HOUSTON - The Canadian-bulit Dextre robot that the shuttle Endeavour's crew delivered to the international space station looks for all the world like an extra for one of the "Terminator" movies — except for the kid's beanie attached to its head.
That giggle-provoking hat may spoil the fearsome effect of Dextre's muscular shoulders and multijointed arms and hips, bristling with gadgets and canisters. But it's actually a cool piece of space hardware that gives the robot special powers. The "beanie" is the attachment point between Dextre and the muscular robot arm now attached to the space station.
Astronauts started putting Dextre together Thursday night during the Endeavour mission's first spacewalk, and the assembly work will continue over two more spacewalks. Robotics experts still have to resolve a problem with Dextre's power system — but the result will be well worth the trouble. Once Dextre is on the job, the international space station's crew will have an upgraded robotic assistant that is far more valuable than the sum of its parts.
Even before Dextre's arrival, the space station's arm has served as much more than just an arm. It's really a robotic biped with arms at both ends, each topped with a grapple fixture. The robot performs its duties by "walking" across the outer surface of the station, grabbing onto handles one after the other. Each grapple fixture contains plugs and sockets for power, data, command and video links between the control post and the free-roving robot.
But the twin arm's powers are limited: The two grapple fixtures are fairly clumsy mechanisms, not much more flexible than an old Hollywood pirate’s hand-replacing hook. They can attach to prepared anchor points, or to movable modules (even very big ones) and astronaut work platforms (some quite small). But they can’t “pick up” anything else.
The 2,860-pound (1,300-kilogram) Dextre will change all that. It will revolutionize space station external maintenance and servicing operations by becoming the "business end" of the station's robotic arm — and much more.
Dextre will provide precise manipulation capabilities for external work on the station — without the presence of spacewalking astronauts. Its vaguely humanoid architecture has two seven-jointed arms plus "hands," outfitted with a rudimentary nervous system. These grapplers do not have "fingers" as fine as a human's, but the grippers can hold a wide array of tools and can take hold of small objects such as cables, levers or thermal blankets.
Each of Dextre's arms extends from a "shoulder," and has a chain of positional joints: shoulder roll, shoulder yaw, shoulder pitch, elbow pitch, wrist pitch, wrist yaw, and wrist roll. The string of rotating joints can manipulate the far end with millimeter-level accuracy.
Each wrist has lights and a high-resolution black-and-white TV camera. And at the very end is a completely new device, a force sensor that measures just how much push the arm is imparting to the object of its attention. This will allow operators inside the station (or on the ground) to tighten bolts "just enough," or to tug hard enough but not too hard, when manipulating the arms.
The "hand" is known technically as the Orbital Replacement Unit/Tool Changeout Mechanism (OTCM). It consists of a set of parallel retractable jaws, which serve to grip payloads and tools. Each OTCM is also equipped with a retractable motorized socket wrench to attach or detach mechanisms in orbit.
Below the shoulder/torso are the hips, with a "body roll" joint between them. On each hip, and very much like any human handyman, are tool caddies for each arm to reach into, as well as an extendable temporary work platform. Another set of high-resolution TV cameras is installed in this area. Below the hips is another latching mechanism that allows Dextre to be placed on any of a dozen external worksites, including the cart that moves along the station's truss-mounted railway.
All the things that Dextre can do
Dextre is designed to relieve astronauts of many of the tasks that now require arduous, time-consuming and hazardous spacewalks. The exact portion of astronaut labor that Dextre can perform is hard to judge in advance, but NASA experts believe it’s between a quarter and half. This includes standard services such as battery replacement, attaching or detaching instrument covers, plugging or unplugging cables, emplacement and retrieval of exposure samples, adjustment of thermal and debris impact shielding, and inspection of the exterior for damage.
Items that need to be sent out from inside the station, or brought back in, can pass through a garbage-can-sized airlock on the Japanese science lab, which is within Dextre’s grasp when mounted on the end of the manipulator arm. That lab will also be installed in a few months.
Dextre's operational value goes beyond mere workload relief. Because spacewalks take so long to prepare and have such a high overhead, minor inspection and maintenance tasks are strung together for each walk — but they may have to wait months before enough work has piled up to justify the activity. The robot arm and its new appendage could be moved into position for a desired task in a matter of days from the time the activity was approved.
Dextre can also be parked atop the station’s mobile work platform, using the grapple fixture at its base. While in that position, the big arm could bring Dextre objects that require robotic handwork, and could hold the objects while Dextre worked on them. In practice, Dextre will grab the object it’s working on with one hand and manipulate with the other — the arms are operated alternately to keep simultaneous motion from leading to a collision.
Fixing the robot
During the years that Dextre will be in space, it will need preventive maintenance as well as breakdown servicing. One arm can detach the other, if needed, and pass it into the larger Quest airlock normally used for astronauts. There it can be repaired with the best manipulators in orbit: human fingers.
At other times, spacewalking astronauts and Dextre will operate side by side, with the robot passing tools and hardware as needed, and helping push the "cheater bar" when more force is required on a sticky nut. The design will evolve through experience, and new specialized tools can be sent up in the future to load into Dextre’s tool caddies.
In a massive system as complex as the space station, unexpected and unpredictable needs are sure to arise that could be addressed by Dextre's close examination, manipulative dexterity and sometimes a hard smack. The most important accomplishments of Dextre are inherently hard to predict in advance. But I will make one prediction: Someday, NASA will look back and be glad it brought Dextre on board.
A shorter version of this report appeared in Robot magazine.