The two-armed robot (or telemanipulator) Dextre, also known as the Special Purpose Dexterous Manipulator (SPDM), is the latest component of the Mobile Servicing System on the International Space Station (ISS). It is specifically designed to replace some of the activities that would otherwise have required dangerous spacewalks.

Dextre (shown in Fig.1) which is part of Canada’s contibution to the International Space Station (ISS) is the 3RD & final stage of the Mobile Servicing System. Dextre works in conjunction with the Canadarm2 which was installed on the ISS in 2001 & the mobile base which was installed in 2002. It is sometimes referred to as the Canada hand by analogy with the Canadarm and Canadarm2.

Figure 1: Dextre the Canadian Space Agency’s Special Purpose Dexterous Manipulator

Installation

The Canadian Space Agency’s Special Purpose Dexterous Manipulator, aka “Dextre,” was successfully launched into orbit on March 11, 2008 aboard the space shuttle Endeavour (mission STS-123). Figure 2 shows Dextre being prepared for launch while Figure 3 shows Dextre all packed up and ready to head off into space aboard the space shuttle Endeavour.

Figure 2: Dextre is being folded up in preparation for launch

Figure 3: Dextre is loaded and ready to go.

Installing Dextre to the ISS required Endeavour astronauts Rick Linnehan and Robert L. Behnken to make three lengthy space walks. Firstly, the astronauts deployed the body section of Dextre on March 14, 2008. Then they connected Dextre to the space station’s Canadarm2 (see Fig 4), which supplied the necessary power for Dextre to be “woken up”.

Figure 4: Dextre fully assembled and attached to Canadarm2

Dextre immediately activated the heaters so vital for keeping its joints and electronics warm in the cold environment of space.

One end of Dextre’s torso has a special grapple feature that the International Space Station’s (ISS) larger Canadarm2 can grasp in order to position Dextre at various work sites around the ISS.

The other end of Dextre’s torso has an end effector like that of Canadarm2, which allows Dextre to be stored on the Space Station’s special external grapple fixtures or for attachment to the Canadarm2 (Fig.5) to act as a high precision extension to the larger arm.

Figure 5: Graphical depiction of Dextre attached to Canadarm2

This combination of the Canadarm2 and Dextre allows these two robotic systems to reach virtually any site of the ISS desired. It also gives the ISS the ability to perform work upon other vehicles at a greater and safer distance from itself than before.

Bearing in mind that objects in orbit are travelling in excess of 25,000 miles/hour and will cause a lot of damage should they accidentally collide. The additional features, capabilities and added safety that Dextre delivers is essential if we humans are to successfully conquer space.

It was during the second space walk of the mission on March 16, 2008 that astronauts Rick Linnehan and Robert L. Behnken attached Dextre’s two 3.35 meters (11 ft) long arms. Then they reattached Dextre to Canadarm2 in order for it to keep warm and allow NASA to conduct a series of thoroughly rigorous tests to ensure that all of Dextre’s electronics and moving parts.

After NASA was satisfied, the crew of the Endeavour then conducted further in situ testing of all of Dextre’s joints and braking systems. Final outfitting of Dextre the space maintenance robot took place during a third space walk on St Patrick’s Day, March 17, 2008.

Design and Delivery

MDA Space Missions designed and built Dextre under contract from the Canadian Space Agency. The Canadian Space Agency bears the ongoing responsibility for its future operations and any training that may be necessary for future space station and ground crews alike. Do not forget that Dextre has the capacity for remote control from earthbound facilities.

With all testing complete Dextre arrived at the Kennedy Space Center in Florida mid-June 2007. There it underwent final flight verification testing and space shuttle integration (packaging and loading) in preparation for launch (Fig.2 and Fig. 3).

Features

Dextre’s design plan (see Figure 6) incorporates the following features:

Two identical detachable highly dexterous 3.25 meters (11 ft) arms each of which has 7 joints allowing the following range of movements: Shoulder Roll, Shoulder Yaw, Shoulder Pitch, Elbow Pitch, Wrist Pitch, Wrist Yaw & Wrist Roll

Pivoting Body and Body Roll Joint – The nimble Dextre can pivot at the waist.

Torso mounted lights & video equipment. Dextre’s five cameras allow its operators to view its work from practically every angle.

Remote Control via a Power Data Grapple Fixture – Dextre is controllable from a workstation aboard the ISS or by flight controllers on the ground. This is one of its most promoted safety features. In the event of a calamity, earthbound operators are able to conduct procedures via Dextre thereby freeing the ISS crew to attend to more pressing matters.

Ground based operators can also benefit from specialised high quality optics aboard the ISS, other space vehicles and even ground-based ones in conjunction with the pictures from Dextre’s own video system. If Dextre’s onboard systems fail these remote viewing and remote control features could be the difference between life and death in space aboard the ISS.

This also allows them to use Dextre in the event of onboard video becoming unavailable for whatever reason; a luxury the ISS crew does not have.

Other proposals suggest that Dextre might be of use in making running repairs on the space shuttle (e.g. fixing the ceramic thermal tiles) but whether or not this eventuates is something that only time will tell.

Orbital Replacement Unit (ORU)/Tool Changeout Mechanism (OTCM). At the end of each of Dextre’s arms is a system called the Orbital Replacement Unit/Tool Changeout Mechanism (OTCM). It has built-in grasping jaws, a retractable socket drive, a monochrome TV camera, lights, and an umbilical connector that can provide power, data, and video to/from a payload.

Figure 6: Dextre; the Special Purpose Dexterous Manipulator, design plan

Dextre’s lower body has a Camera Light & Pan/Tilt Unit (CLPA) consisting of pair of operator orientable colour TV cameras with lights, as well as a platform for stowing Orbital Replacement Units (ORU), and a tool holster. The tool holster is equipped with three different tools used to perform various tasks around the Space Station.

The jaws at the end of each arm will allow Dextre to grasp and hold objects while the retractable motorised socket wrench allows Dextre to connect or detach objects. The tool change mechanism and tool holder are critical components since everything in orbit is travelling very fast.

Any objects that are “free” to move will tend to do so since this is the normal behaviour for all objects in free fall inlcuding those in orbit. If accidently nudged, free to move objects, will cause incredibly large impacts resulting in considerable damage.

The latching end effector is as already mentioned a vital component of Dextre’s attachment systems. Another system that helps place Dextre on a pedastal of greater precision than the two Canadarms are an assortment of sensors such as force moment sensors that give Dextre the capacity to be “gentle” when required.

Preventing Accidental Collisions

Dextre can only move one arm at a time. This is a very deliberately designed safety feature as it helps prevent the likelihood of Dextre’s operator(s) from unwittingly causing both of Dextre’s arms to collide. Note that I did say operator/operators as Dextre will at most times be under the control of a team of operators. Although when a crew member of the ISS is in charge they will perform the majority of commands.

Bearing in mind that Dextre’s arms are 3.25 meters (11 ft) long and that we humans are unused to operating appendages of this length with any dexterity the in built safety guard of Dextre only being able to move one arm at a time assumes greater importance than would seem to be the case at first sight.

Another reason for building this feature into Dextre is that it helps to maintain stability and stability is critical whenever one is attempting a delicate procedure.

Dextre can also be attached to the space station Canadarm’s railway which is shown in the graphic of Figure 5.

Figure 7: Dextre at work

Until next time, enjoy.