Robotic Arms Help Upgrade International Space Station

Article initialement publié sur IEEE Spectrum / Automaton blog

CANADIAN MUSCLE: The Space Shuttle Endeavour’s robotic manipulator Canadarm 1 hands off a payload to the International Space Station (out of frame), which is also equipped with a mechanical arm, the Canadarm 2. A new ISS arm, Dextre, will become operational later this year. Photo: NASA (19 July 2009)

At a moment when the world is celebrating the historic lunar landing of Apollo 11, even the crew on the International Space Station found time to pay homage. But then, it’s back to work, as the astronauts continue with their busy schedule of construction and scientific work.

For the current mission, the astronauts count on a tireless partner: the robotic manipulator Canadarm 2. With space exploration at the center of earthlings’ attention, I thought it’d be great to learn more about this amazing space telemanipulator. And I couldn’t find a better person to talk to than Alexandre Grégoire-Rousseau, a mission planning engineer at the Canadian Space Agency (CSA).

The goal of the current mission is, among other things, to install two Japanese modules to the ISS. This process will take 16 days and involve three robots: the Canadarm 2 on the ISS, its older brother Canadarm 1 on the Space Shuttle, and an arm on Kibo, one of the Japanese modules. A true robotic ballet in outer space!

Those who have programmed robotic manipulators would have their brains tingling while hearing Grégoire-Rousseau describe how the arms work and all the challenges involved. Each mission requires that the planners prepare new control algorithms and trajectories for the different payloads.

“Every two-week mission takes two to three years to plan,” he says. “Fifty percent of our time is to define the actual mission scenario; the other 50 percent is to think about the what if’s.”

One interesting challenge is that, whereas industrial manipulators are bolted to factory floors that remain stationary, the Canadarm 2 is attached to the ISS, which floats in space. That means the dynamics of the arm is coupled with the ISS’s dynamics. Move the arm and conservation of momentum may cause the space station to move too. Grégoire-Rousseau says they haven’t had problems lately, because the ISS is getting heavier.

“Still,” he adds, “it is sometimes necessary to activate the gyros or in rarer cases the thrusters of the Station to maintain its attitude while the arm is moving around.”

The arm was originally designed for assembly tasks. But since it became operational, mission planners have come up other uses, such as positioning cameras for inspection or moving the astronauts around when they are out of the ISS. In fact, during the current mission, the Canadarm 2 will also be used to move an astronaut who has to replace electronic components on the ISS as part of an extra-vehicular walk.

Direct mechanical interactions between robots and humans is an area that receives a lot of attention from Grégoire-Rousseau’s team and other research labs on Earth. It’s a hot topic in robotics and presents many challenges related to control and safety. Just think that the ISS crew is maneuvering a 17.6 meter long manipulator … in free space. No need to say that a failure could have dramatic consequences up there.

To ensure flawlessness, astronauts plan a lot, train a lot, and operate very slowly to make sure that they have time to react. Many layers of safety wrap the operations. The arm can detect faults and stop its own operation, execute pre-planned parameterized trajectories, or follow joystick commands from the astronaut watching it on video displays from inside the ISS. Teams in Houston and at CSA’s headquarters near Montréal continuously monitor the telemetry information sent back to Earth.

But it seems that the best is still to come in terms of robotics and the ISS. Dextre, whose official name is the Special Purpose Dexterous Manipulator, is a torso with two smaller arms that will be added at the end of the Canadarm 2. Dextre was assembled on site last spring, and it’s docked and tested, ready to come into action around October of this year. The main innovation in Dextre? It comes from the use of force-torque sensors, which will enable fine manipulation.

“Dextre could insert a DVD into a player”, Grégoire-Rousseau says. “Its capabilities will significantly reduce the number of necessary extra-vehicular astronaut walks.”