How Dextre moves

Dextre on the end of Canadarm2
(Credit: NASA)

Since Dextre will take over many of the routine chores previously done by human spacewalkers, the robot was designed to look and move much like an astronaut, but with some important differences.

Dextre has two arms measuring 3,35 metres in length (almost as long as the robot is tall). Each arm has seven joints that are able to move up and down, side to side, and are also able to rotate, or roll. This gives the space handyman a much wider range of motion than the human arm (your wrist can move in three directions, as can your shoulder, but your elbow can only move in one direction, whereas Dextre can bend backwards at the elbow to touch the back of its arm). Whereas Canadarm2 is able to move equipment as large as a school bus, Dextre can handle gear as small as a phone book or as large as a phone booth.

Dextre's hands are actually grippers that work much like a Swiss Army knife: each hand has a retractable motorized socket wrench to turn bolts and connect or detach hardware; a camera and lights for close-up viewing; and a retractable umbilical connector (like a plug-in) to provide electricity, data and video connections when the robot handles sensitive electronic equipment and to support experimental scientific experiments. Dextre's grippers also have sensors that provide the robot with a human-like sense of touch, which enables it to apply just the right amount of force in the precise direction, and is accurate within millimetres of its target. Those targets are mostly fixtures on boxes that Dextre handles and inserts into place around the International Space Station (ISS). To grasp the fixture, Dextre uses the cameras he has located at the grippers to line him up with the fixture target which helps him get that millimetre accuracy.

Dextre is able to pivot at the waist, allowing it to turn on the spot without having to move (useful for picking up tools or putting down hardware). And, like astronauts, Dextre was designed to ride on the end of Canadarm2 or the mobile base to move from one worksite to another. However, while astronauts stand on the end of the robotic arm by stepping into foot restraints, Canadarm2 picks Dextre up by grasping the fixture on Dextre's head.