Meet Canada's Special Purpose Dexterous Manipulator or Dextre for Short

Call it an orbiting Edward Scissorhands. Although Dextre’s first major robotic work on the International Space Station this month was delayed due to a snagged truss, both the Canadian Space Agency and NASA anticipate it will replace spacewalkers for minor outside tasks during and after the station’s construction.
Launched in 2008 aboard STS-123, the Canadian robotic hand – built by MacDonald, Dettwiler and Associates in Brampton, Ont. – was initially designed for astronauts to operate from inside the station.

If a payload required heavy lifting, or a task was simple enough to program robotically, the idea was for astronauts to do the work on the station using the three-ton robot rather than suit up and take on the risk themselves.
However, the hand was barely installed on the station when NASA took a harder look at crew workload and asked the CSA if they could reprogram the robot to accept tasks from Mission Control.
The two agencies have worked together for two years to fix up the software, and one last milestone looms before Dextre will be ready to work on the station: swapping out an ailing Remote Power Control Module from one part of the station and replacing it with another.
However, an initial test July 20 revealed that it would take more force than anticipated to pull out the nine-pound unit because of a faulty spring in the module. NASA is currently testing the module to make sure it can withstand the estimated 34 pounds of force Dextre will need to apply to pull it out, pushing the first checkout test into at least September.
Arm-Wrestling Robotics
On the Canadian Space Agency’s website, a tongue-in-cheek biography cheerfully calls Dextre both a guinea pig and a handyman.
Dextre’s chief pet peeve is “Waiting for Mission Control to say, ‘Dextre, we have a problem…’, and its chief goal is supposed to be “Arm wrestle with a spacewalking astronaut! (Just kidding!)”
More seriously, the CSA says it hopes Dextre will be a model for future space robots.
“Dextre in general represents sort of the ability of Canada to produce robotics and to do space robotics, and this mission in particular is sort of the culmination of many years of hard work for the whole team and everyone involved,” said Leslie Sponder, an operations engineer and mission planner.

Dextre is almost as tall as it is wide, with two arms stretching the system out to 3.35 metres long. Much like the Canadarm and Canadarm2, which are also maintained by MDA, these arms have joints that can rotate or roll to get a grip on payloads. But what sets Dextre apart is what’s at the end of those arms. The Canadarms are limited in what kind of payloads they can grab because the end of those arms end in “end effectors”, a modified ring that can only spin onto a piece of space hardware that has a matching ring on its end.
By contrast, Dextre has grippers on each of its hands. It can turn bolts and move a camera close up to where it’s working.
The hands are also equipped with sensors so astronauts or mission controllers can sense how hard or soft Dextre’s touch is, which lets it manipulate sensitive hardware that could be damaged by a more brute-force approach.
Compared with the Canadarms, which can move school-bus sized stuff, Dextre is limited to payloads about the size of a phone booth. Where it will be more useful is in moving and manipulating smaller items; the design constraints say it can handle something as small as a phone book.
Japanese Rendezvous
Once Dextre is able to pull its weight on the station, the CSA is looking ahead to the first major maneuver involving Canadarm2, the mobile base the arm rests on, and Dextre.
Slated for January 2011, the Japanese HTV-2 cargo spacecraft will make a supply run to ISS. Once it arrives, a robotic ballet dance will ensure between the Canadian instruments.
Canadarm2 will first snag HTV-2 and take out a hardware rack – called the Exposed Pallet – to hand over to a Japanese robotic arm aboard ISS on the Kibo module. Next, the arm will be moved over to its mobile base to move Dextre to the pallet. Dextre will take out two payloads from the rack, then hitch a ride on Canadarm2 to the opposite side of the station to install them.
When the installation is complete, Canadarm2 will drop Dextre off in the middle of the station and move back towards the Kibo module, where it will pick up the empty rack and stow it on HTV-2.
Around 40 days after HTV-2 arrives at the station, Canadarm2 will release it. Both the Japanese spacecraft and the rack will take a controlled plunge into Earth’s atmosphere, burning up upon re-entry.
As for what the robot will do next, a lot of that will have to be determined on the fly. “With Dextre and the maintenance of the space station, we don’t really know what the next problem is going to be,” said Ken Podwalski, manager of Dextre’s mission operations, in another CSA video interview.
“So it requires us to be ready at a much larger level, a much broader scope of readiness, and I think that’s going to be a very unique challenge for anybody that’s participated in this program. As much as engineers would look at a problem and want to know how they have to be prepared, we don’t have that luxury.”

About Elizabeth Howell

Is SpaceQ's Associate Editor as well as a business and science reporter, researcher and consultant. She recently received her Ph.D. from the University of North Dakota and is communications Instructor instructor at Algonquin College.

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