Special Purpose Dexterous Manipulator

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ISS Canada "hand" (NASA)

The Special Purpose Dexterous Manipulator (SPDM) is a robotic arm or telemanipulator, or "waldo" which is part of the Mobile Servicing System. It is one of the many segments of Canada's contribution to the International Space Station, scheduled to be transported to the station on STS-123, no earlier than 2007. Its creation is managed by the Canadian Space Agency, which will oversee its future operations and the necessary training of station crews.

The Manipulator has been named Dextre, to represent its dexterous nature. It is also known as the Canada hand in contrast to the Canadarm and Canadarm2.

The "Canada hand" resembles a headless torso fitted with two extremely agile, ~11ft long arms and several smaller appendages. The 12 ft long "torso" pivots at the "waist". The "torso" has a grapple fixture at one end that can be grasped by the larger Space Station Arm, Canadarm2 so that the SPDM can be positioned at the various ORU worksites along the Space Station trusses. The other end of the "torso" has a grasp fixture virtually identical to that of Canadarm2, so that the SPDM can be stored on Space Station grapple fixtures or can be used as an extension to the larger arm.

The two SPDM arms each have seven, specially offset joints giving a greater freedom of movement in a smaller package than previous systems. At the end of each of these arms is a system called the Orbital Replacement Unit/Tool Changeout Mechanism (OTCM). It has built-in grasping jaws, a retractable socket drive, monochrome TV cameras, lights, and an umbilical connector that can provide power and data to a payload. The lower "torso" of Dextre has a pair of orientable colour TV cameras with lights, an ORU platform, and tool holders.

Designed primarily for servicing tasks on the exterior of the space station, it is flexible and strong enough to accomplish many other kinds of tasks in space. It will typically be used to perform such tasks as replacing a 100 kg (220 lb) battery. There is a range of sizes among its set of compatible ORUs (Orbital Replacement Units), the smallest of which is about shoe box sized. Each side of the ORU/Worksite interface must both meet geometric and functional requirements before SPDM can be used. The gripper jaw interface cannot merely grasp anything it comes across. The ORU must have one of four types of grasp interfaces that the OTCM can access either directly or using one of three tools stored on lower the "torso".

Many Americans who have been working with the ISS program for years refer to SPDM by pronouncing the acronym in one drawn out syllable /spĭdem/. Whereas most Canadians would have pronounced each letter separately S-P-D-M, and they may not approve of the Americanism. However very few in the industry would use the recent appellation "Dexter" as it is widely regarded as a marketing and public relations gimmick.

Dextre has completed its trials in the thermal and vacuum space simulation chambers of the Canadian Space Agency's David Florida Laboratory in Ottawa. It has also been subjected to various electrical and shaking tests in this same facility, to simulate extreme launch conditions.

In early August 2004 NASA declared its intention to use Dextre (or a close copy of it) as the robotic component for the Hubble Space Telescope rescue mission. Technically, NASA intends to have a sole bidder for the construction of the manipulator for the Hubble mission: The robotics division of MacDonald Dettwiler and Associates, which is known as MD robotics. This Canadian engineering firm has already built Dextre for the Canadian Space Agency, and has previous experience in building Canadarm and Canadarm2 and participating in the training of US astronauts and payload specialists in the use of these devices.