Science Dimension volume 14 issue 2 1982
GRAPHITE BONES, COPPER NERVES
|From breadboard to on-board: small components were assembled and tested innumerable times before the final product was mated to the shuttle in 1981 (NASA)|
In 1974 Thomas Paine, then Administrator of NASA, visited Canada bearing a proposal: help the United States build its radical new Space Transportation System - the "Space Shuttle." Canada carefully reviewed its capabilities and chose to construct the Shuttle Remote Manipulator System: the RMS, or Canadarm. Canada would pay the cost of developing the first Canadarm flight hardware, destined for the Shuttle orbiter Columbia. In return, NASA contracted to buy at least three more Canadarms from the prime contractor, Spar Aerospace of Toronto. Canadians would be afforded preferential access to Shuttle launches to develop an extensive space program. We could assemble a team of world calibre in one of the future's key technologies, robotics. And we could show the world (including those of our own nation who still do not believe in Canadian excellence) how much Canada can achieve.
The Government of Canada designated the National Research Council as co-signer with NASA of a memorandum of understanding for Canadarm. Since then, specialists from four divisions of NRC have contributed to the Canadarm project, together with their counterparts from industry and other government departments. What's in an arm? Canadarm is a robot analogue of the human arm - its nerves of copper, its bones of graphite-fibre-synthetic tubes, and its muscles of electric motors. Each of these motors is no larger than a telephone handset, and works on direct current. Serviced by gear boxes with gear ratios in the order of 1800:1, the motors lie buried in Canadarm's metal joints. Unloaded, the arm can move its tip at about 70 cm/s. This diminishes to 5 cm/s under its maximum load-carrying capability of almost 30 tonnes.
|The copper wires and motorized muscles of the Canadarm's "wrist" being prepared for flexing in a thermal vacuum chamber (Spar)|
Every part of Canadarm had to be thought through from scratch. NASA's design requirements were severe - stiffness, weight, heat tolerance, toughness, and the like. Often, new techniques had to be devised to produce parts meeting all these demands. For example, a perfect material was found for manufacture of some of the gears - Type 302 stainless steel alloy. But after a toughening heat treatment, the machined gears shrank a few micrometres - enough to throw off tolerances. (The thickness of this sheet of paper is about 100 mu-metres.) Initial heat treatment of the gear blanks (discs from which gear teeth are cut) would make machining impossible; the alloy would be too hard. Accordingly, Spar machined the gears oversize by the shrinkage amount, then heat-treated them until they shrank to proper size.
|Getting a workout in space (NASA)|
Much of Canadarm's initial $100 M cost went for testing. Designed to work in airless, weightless space, the arm could not support even its own weight on earth, so one of Spar's Canadian suppliers built a test rig for use on a very flat floor. Here, cradled in this special rig which glides on air bearing pads, Canadarm can float over a floor as perfectly flat as modern technology can make it. The arm itself could move its shoulder, elbow, and wrist joints all at once, although only in the plane of the floor.
The next time you reach for a cup of coffee, notice how your hand wobbles slightly from your predetermined path, even after repeated practice. Such wobbling is not acceptable in the Shuttle's arm. The orbiter's bay may be the size of a boxcar, with a similar cargo-carrying function, but the cargo could be as delicate as a cabinet full of Dresden china. Also, the cargo-bay doors are not just covers. They also house radiators designed to get rid of the heat generated by Columbia's electrical equipment. These radiators and the cargo bay are crisscrossed with lines carrying high-pressure coolants, electric power, and other services. Inadvertent contact with these vital, fragile components must not occur.
But how can a single astronaut, acting through two hand controllers like those which guide Columbia from her forward control station, possibly keep track of three huge arm segments moving six ways at once? Again the answer came from the way Canadarm mimics our own bodies. When we reach for that cup of coffee, we do not consciously command our wrist to rotate, our elbow to pitch down, our shoulder to yaw left. Our eyes set a goal, and our brain achieves that goal without troubling our active mind. Similarly, the operator of the arm uses hand controllers in the crew compartment to command Canadarm's end effector to move in a desired direction. Shipboard computers then determine what each part of the complex system should do to fulfil that demand in the safest, most effective way.
|Canadarm's hand leaves no finger prints. The end effector uses a unique wire-wrapping mechanism to seize targets (NASA)|
The first space arm was officially signed over to NASA in February 1981, at the Spar plant in Toronto where it had been built. Trucked gingerly to Kennedy Space Center by the same driver who had taken the King Tutankhamon exhibit across North America the previous year, Canadarm was integrated into Columbia in June 1981. At that time, NASA officials praised Canadarm as an exemplary subsystem: dependable, simple to install, and virtually trouble-free. There were, however, a few tense moments still to come. In August 1981, data analyses from the first mission showed that, fractions of a second after the huge solid-fuel boosters had ignited, an airborne shock wave reflected from the launch complex and "twanged" Columbia with a force that, at some frequencies, was many times what had been predicted. NASA immediately began to modify the launch complex, adding massive "water hammocks" underneath the SFB nozzles and increasing the flow rate of the water ducted beneath Columbia to dampen liftoff shock. This did, in fact, virtually eliminate the problems at launch.
Canadarm now takes its place as a vital component of NASA's Space Transportation System. With the success of the third launch, the attention of NASA will move away from the Shuttle system itself and on to the deploying of cargo in space. It is, after all, the Shuttle's raison d'etre.
Reprinted courtesy of the National Research Council