Crawling Across the Sky

By Jeffrey WintersMay 1, 1997 5:00 AM


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The space station’s new robotic arm certainly bears a resemblance to the one that’s been used on the shuttle for the past 16 years. Each consists of a long boom for the upper arm connected by an elbow joint to another boom for the forearm. But one of the biggest differences between the two will be apparent as soon as the shuttle hauls the arm up to the station in January 1999. After the shuttle’s cargo bay doors open, the new arm will proceed to walk itself like an inchworm onto its perch on the space station’s central truss.

What gives the arm this cartoon mobility are hands at either end, each equally adept at gripping and manipulating objects or anchoring the entire structure to the station. Although the arm will be able to perform most of its chores while riding up and down a rail on the station’s central truss, it will also be able to get at parts of the space station that exceed its 55-foot reach by grabbing hold of one of many handles being placed on every module.

This and other new capabilities make the Space Station Remote Manipulator System (or Télémanipulateur de la station spatiale, in compulsively bilingual Canada) far more complex than the old arm, but they are essential if it is to achieve its objective of reducing the number of astronaut evas to a bare minimum. evas are risky and expensive, says Alain Poirier, head of the Canadian Space Agency’s space station program. Some of the modules are so huge that it would be like manipulating a bus in space. That’s where the robotics comes in--to free human beings from such chores. The arm, in fact, is designed to do much of the same work that a suited astronaut can do--tightening bolts, replacing malfunctioning parts--as well as tasks that humans couldn’t possibly perform, such as grabbing huge modules from the shuttle payload bay and installing them on the ever-expanding station. The arm will even be strong enough to grab and dock the shuttle itself.

At the moment, the arm hangs in pieces in a hangar-size clean room at Spar Aerospace in the Toronto suburb of Brampton. Technicians hover around three barrel-size joints, each of which imparts a degree of movement. The first joint allows the hand to move up and down, the second allows it to move side to side, and the third gives it the ability to turn. The three of them acting in concert give the Canadian space arm great flexibility, much like that of a human wrist.

For its first year in operation, the new arm will be somewhat ham-handed. Each end of the arm will be fitted with a device that can latch onto spikes similar to the ones on the Hubble Space Telescope that the shuttle arm grabs during reservicing missions. But these devices are incapable of doing anything more subtle than grabbing and holding. To do finely detailed maintenance work, an auxiliary unit called the Special Purpose Dexterous Manipulator (spdm) will be delivered to the station in early 2001. This unit will act like a couple of hands on one end of the arm--taken together, the effect will be similar to putting a repairman on the end of a cherry picker. During a typical maintenance job--say, replacing one of the batteries that power the station while it’s passing through Earth’s shadow--a socket head will extend out of one of the hands and unscrew the bolts that keep the battery attached to the station. Then the arm will pull the old battery away, grab a fresh battery from a bin, and bolt the new one in place.

Because the station is a modular design, the spdm will be able to install 70 percent of the replacement parts; astronauts will do the rest. (The arm itself is modular, bristling with replaceable units along its length so that it’s possible for it to replace its own defective parts in many cases.) As time goes on and we understand a bit more about what we can do in orbit, we’ll probably start assigning more and more of the tasks to the robot and fewer and fewer to the astronauts, says Spar engineer Jim Middleton. Of course, the arm will be controlled by an astronaut in the station--at least for the time being. Turning the space arm into a truly autonomous robot, capable of performing tasks without a human hand on the controls, is not out of the question at some future date. The arm is capable of doing many autonomous things, because it has vision systems and computer feelers in its hand, he says. But we’ll have to do some work in orbit to see how best to evolve this. That would be a first: an arm that can think for itself.

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