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Canadarm2 –
Technical Challenges: Strong as an Ox; Gentle as a Lamb

Designing and building Canadarm2 presented many technical challenges. The most significant was also the most obvious: the arm had to be able to reach all parts of the Space Station. If it were permanently anchored at one end like the Canadarm1, it would have to be enormous—“like the super-giant cranes you see on buildings,” said Sachdev. “That was not very feasible.”

He remembers having this thought back in the early 1980s, while visiting one of the U.S. companies that was bidding to build the Station. The early designs were different from the Space Station taking shape in orbit today; they featured a series of linked modules and didn’t include trusses along which equipment could move. “It occurred to me that the Shuttle arm would only be able to go so far and we wouldn’t be able to construct the Station with just one arm,” said Sachdev. “We would need one on the Station itself. I got the idea that if we had a manipulator that could be moved from one point to another, it could do the job.”

A colleague at Spar, Devendra Gossain, was consulted and together they came up with a concept for a self re-locating manipulator with two identical ends, each capable of grasping objects. With this unique concept in mind, the Spar team met in Middleton’s office in 1983 to hammer out the details of what would become the defining feature of Canadarm2—its ability to “walk” hand-over-hand all over the Station. This became known as the inchworm manoeuvre. Nearly two decades later, in April 2001, Canadarm2 performed this move for the first time when it stepped out of its pallet and onto the Space Station structure.

Sachdev and Gossain ultimately patented the inchworm concept. “It was new and nobody had done it before,” said Sachdev. “We did it to prove the claim that this was a unique technology Canada had invented.”

“As it turns out, the Station does have trusses and the manipulator system now includes a Mobile Remote Servicer Base System (MBS), scheduled for launch in 2002, which will run on tracks along the trusses, carrying Canadarm2 to different locations. However, the inchworm capability remains useful, ” Sachdev says. “We still need the relocating ability of the arm as the Station is getting built; we need to attach modules before the MBS can get up there.” And even with the transporter in place, “the combination of relocation on its own, plus the MBS, gives the system versatility.”

“Although the inchworm capability is simple and elegant in concept, it was one of those devil-is-in-the-details situations,” says Middleton. “When you start to put the meat on the bones of that, you get into the real problems of making this thing walk and hold at both ends. Allowing each end effector to switch suddenly from being a shoulder to a 'hand' requires 'massive' reconfigurations,” he said. It was also a major challenge to design the joints that enable Canadarm2 to do this; unlike the joints on Canadarm1, which are all in line, those on Canadarm2 are side-by-side. “Just doing that was a major problem,” said Middleton.

There’s another difference between the two Canadarms that had major implications for the design team. Unlike the Shuttle arm, which is returned to earth after each mission, Canadarm2 took a one-way trip into space. It’s never coming back. This means it must be maintained and repaired in space. “It was a real challenge figuring out how the arm could be serviced on orbit,” said Middleton.

“This is all the more important since the arm is expected to operate reliably for perhaps as long as 20 or 30 years and must always be available to carry out its critical procedures, ” said Doetsch. This meant the manipulator had to be designed in a modular way, with replaceable components that could be easily swapped in space. This would allow not only the replacement of broken or worn out equipment, but also upgrades when more advanced technologies came along.

Since there was no ground-based version, it was also necessary to develop a “capacity to simulate the system so that new engineers and operators could be trained to operate, maintain and upgrade the system without having a copy on Earth,” Doetsch noted. “Software and simulation of the complex system, as always, provided a major challenge and their development provided for significant new capability in the space industry.”

Another major technical challenge, he added, was to “develop force sensing so that the manipulators did not damage the payloads they were handling, and collision avoidance technologies to avoid inadvertently colliding with other objects on the Station. The system had to be as strong as an ox and as gentle as a lamb.”

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