Exoskeletons may one day give U.S. soldiers a crucial advantage as warfare becomes increasingly urban, says Ephrahim Garcia, manager of the new DARPA program. Since troops are less able to use their armored vehicles to fight in confined urban battlefields, military planners want to fasten the armor, heavy weapons, and advanced electronics onto the foot soldiers themselves. Without heavy-duty mechanical support from something like an exoskeleton, however, people would collapse under the load.
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Looking at these and other developments in exoskeleton-related technology, Garcia believes that "all this combined together makes this a good time" to try again for the complete package. "We're going to take some of these technologies that are almost ready . . . and push them over the edge," he says. The result may be some formidable prototype machines. Garcia says his current goal is to equip a soldier with an exoskeleton that will make him or her 3 to 10 times stronger than without it. Fighters would smoothly wield 50-kg weapons while simultaneously wearing 20 kg of armor.
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Compared with a currently equipped U.S. marine, who is required to march 4 kilometers per hour carrying as much as 50 kilograms of equipment, an exoskeleton-equipped marine would be able to move about three times that fast while carrying more than double the load, Garcia predicts. The leatherneck exoskeleton would probably cost no more than the price of a motorcycle, he adds.
Personal flying machines
The leap from today's technology to an exoskeleton meeting Garcia's goals is a huge one. Among the three DARPA contractors working on exoskeletons for ground troopsKazerooni's lab, Jacobson's operation at Sarcos, and Oak Ridge's robotics grouponly Kazerooni's team has actually demonstrated a powered exoskeleton.
Millennium Jet in Sunnyvale, Calif., which is also receiving DARPA funds, is well under way with developing a personal flying machine known as Solo Trek XFV (see below). The vehicle is a one-person device but not a wearable exoskeleton.
To build a system in which a robot shadows every move a person makes is a complex undertaking. After detecting the motion and gauging its speed and force, the robot must translate those readings into a parallel motion by some of its components. All the while, other exoskeleton components have to adjust to maintain the system's balance.
Gravity, friction, thermal effects, sensor errors, and other subtle influences play into the human-robot interactions. Managing it all requires sophisticated mathematical models based on fundamental physics and control theory that builders must program into the machinery, says Oak Ridge's Pin. The researchers at all the DARPA-funded labs are creating these models as they go.
Neither such a computer program nor the motions of an exoskeleton itself have to be off by much to cause the wearer discomfort or fatigue, says Jacobsen. Less than 2 kg of misplaced weight on a person's arm, for instance, can wear a person out in just 10 minutes or so, he says.
Bigger errors may be dangerous. Industrial robots sometimes injure or kill people who stray too close. Powerful exoskeletons will be embracing their wearers when something goes wrong, Pin notes.
Countless challenges to exoskeleton designers involve such details as framework materials, actuators, and sensors, plus the heat, noise, and weight of each of these components. Nothing looms larger, however, than the need for a compact, portable, and ample source of power. Not only do the mechanical motions of the exoskeleton and its various control systems draw a lot of power, but soldiers are increasingly outfitted with computerized communications and information gadgetry that also drinks up energy. Garcia has hired several analysts specifically to investigate this issue.
As a group, the DARPA contractors are pursuing several innovative solutions for powering exoskeletons. These include chemical reactors, a coffee-cup-size turbine that whirls a half-million revolutions per minute, miniaturized internal combustion engines, and fuel cells that feed supercapacitors that can release power in bursts. Each offers its own advantages and disadvantages.
Internal combustion engines and some chemical reactors, for instance, run hot and so will require extra insulation to protect the wearer. Says Garcia, "If you can't do the power, everything else is, in some sense, academic."
Control theories
Using a relatively heavy gasoline engine, as Kazerooni has done with his leggy Lee, is clearly not the way to power an exoskeleton. Equipped with a fuel tank that holds about 1 liter, the engine runs Lee for only about 15 minutes. Then, as the power dies, another flaw of the Berkeley group's first exoskeleton becomes obvious. Unless someone races to scoot a chair under the wearer, the suddenly burdensome load will bring him helplessly to the ground.
Perhaps the worst strike against the prototype is that it "imposes constraints on the person, like a tight shoe or like clothes that aren't comfortable to you," confesses Kazerooni.
Yet making even a crude device that can pull its own weight provides the Berkeley team with an important confirmation. "It verified some of our control theories, which shows we are going in the right direction," Kazerooni says.
Even as the research teams work out the early details of their exoskeleton designs, some of the investigators are looking beyond this round of experimentation. Kazerooni, for one, anticipates that exoskeletons of the future will be "invasive"not just worn but partially implanted within a person's musculature and nervous system. Jacobsen says he's thinking in the opposite directionabout putting more human nature into the machines. His idea is to build an exoskeleton intelligent enough to take care of the soldier wearing it. If the human trooper is badly wounded, the machine would say to itself, in effect, "Take this guy home."