Hop Hop Hopbots! Designers of small, mobile robots take cues

from grasshoppers and frogs By PETER WEISS

ush Robinett got the idea for his lab’s new robot while out on his R father’s New Mexico farm. He was

catching grasshoppers for trout bait and noticed that when he reached for the grasshoppers, they seemed to spring in random directions. They fell on their sides as they landed and then struggled back onto their feet before springing again.

A robot designer at Sandia National Laboratories in Albuquerque, Robinett realized that this kind of grasshopper mobility might be just the thing for developing new kinds of small, mobile robots. Program man- agers at the Defense Advanced Research Projects Agency (DARPA), in Arlington Va., had been interested in creating wide ranging minirobots for years.

As both Robinett and his mili- tary sponsors knew only too well, small robots have a big problem.

Consider Sojourner, NASA’s celebrated hassock-size rover that poked around a tiny patch of Mars as part of the 1997 Pathfinder mission. According to the dictionary, to sojourn im- plies to stop and stay in one place for a spell. It seems that the small, wheeled robot did just that nearly every time a rock as big as itself blocked its way. Some times, it got hung up for days on end.

Not exactly the kind of get-upand-go you want from a robot sent millions of kilometers to explore exotic worlds.

Agencies like NASA and the Depart- ment of Defense have been championing the idea of little robots because they can be cheap and portable but can still carry a payload-cameras, scientific instru- ments, or perhaps bombs. However, “as you get smaller, obstacles the size of yourself come along much more fre- quently,” explains Sandia’s Barry Splet- zer. “Ifyou’re an Abrams tank, there are not many obstacles your size, but if you’re an ant, every blade of grass is bigger than you are.”

While Sojourner was getting stuck on Martian rocks, two independent groups-the Sandia re-

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searchers and a joint team from NASA’s Jet Propulsion Laboratory (JPL) and the California Institute of Technology (Cal- tech), both in Pasadena-independently hit upon the same new concept for dealing with the problem of obstacles.

From the robot’s perspective, it goes something like this: Throw yourself as high and as far as you can in the general direc- tion you want to go. After the crash land- ing, right and reorient yourself. Then, do it again until you reach your goal. In the past few months, each group has unveiled a

prototype of what it intends to develop in- to a new generation of hopping robots.

he Sandia robot looks nothing like a grasshopper, but it sure can jump. T “They’ve put an emphasis on really

good thrusting. It’s amazing,” comments Joel Burdick, one of the designers of the rival JPL/Caltech machine.

Dubbed by its inventors as “the knight” -after the chess piece that leaps over others-the robot is basically a piston- driving combustion chamber mounted

inside a spherical, plastic shell about the size of a grapefruit. To propel each of the half-kilogram robot’s leaps, a small charge of liquid propane or other fuel ignites within the chamber. This slams the piston against the ground. Typically, that push-off hurls the device about 1 m e ter into the air and a couple of meters away from its starting point.

The New Mexico team is also working on a powerful, 2.5.kg version of the robot, which has leaped 4 m high and 5 m away in preliminary tests, says Sandia engineer

Gary J. Fischer. When the hopper lands, the

imbalance between its relatively lightweight, spherical top and rel- atively heavy, coneshaped base causes it to automatically right it- self. Then, a small, battery-pow- ered motor rotates an internal, offcenter weight to pivot the d e vice around and realign it accord- ing to its built-in compass.

Like a tacking sailboat, the robot can head more-or-less in a chosen compass direction. “Even though individual hops are fairly inaccurate, over the long term you get where you’re going,” Spletzer says. The Sandia team figured that more sophisti- vigation would be unnecessary

for the uses their sponsors had in mind for the robot and would only add to the complexity, weight, and cost.

Initially, DARPA officials envisioned a small, rugged, camera-equipped robot that soldiers or a SWAT team could toss into a building to snoop around. Then, another DARPA program took over fund- ing for the robot’s development. This p re gram’s goal is to create mobile antitank mines that can shuffle around the mine field and close up any gaps that enemy mineclearing actions might have opened.

The energy-efficient knight can hop thousands of times on a 2Ogram fuel tank,

enabling it to wander for at least several kilometers be- fore running out of gas. With that kind of range, the device might serve other applications as well. “My favorite is for planetary ex- ploration,” Spletzer says.

A fallen frogbot pushes with plastic levers to right itself after a hop. Continued on p. 90

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lands, frogbot is equipped 8

he robot designers at JPL and Cal- tech found their inspiration in T another hopper in the animal king-

dom. They call their electrically powered invention “frogbot.”

By appearances alone, the 40centime- ter-tall aluminum and plastic gizmo seems more like the skeleton of some big-beaked bird than a frog.

Watch it move, however, and the rea- son for the frog moniker jumps out: A pair of hinged shafts, bent like a pair of squat- ting frog’s legs and tethered together at the knees by a strong spring, suddenly straighten. The action resembles a frog’s learn exceDt the legs share a single foot.

_ I N n>., oc, I. 20.2 I I l l . . L a . . l 1

notes, models ultimately sent to space will rely on solar power, which will enable them to roam indefinitely.

In 5 to 10 years, NASA could be sending hoppers into space by the hundreds, pre- dicts Neville I. Marzwell, manager of JPL‘s advanced concepts and technology inno- vation office. His office and the National Science Foundation are funding the JPL Caltech hopper’s development.

Spacecraft would drop groups of h o p pers onto planets, asteroids, or other bodies. The robots would then fan out to explore the surface. Compared with wheeled rovers, hoppers would have a better chance of climbing mountains or descending into craters and canyons, Marzwell asserts.

Mission-read y hoppers in the coming years would probably be smaller and lighter than today’s ex- perimental models. They would also be equipped with cameras and one or two scientific instru- ments, each no larger than a sugar cube. “Like a colony,” Marzwell says, all the individual units would keep in touch with a “mother brain” on the orbiter or on one large rover on the surface.

If hoppers stay simple and cheap, “the return on investment for science will be 1,OOO times what it is today,” Marzwell pre- dicts. More than $100 million was spent on the lander and rover in the Pathfinder mission, he explains, whereas hop-

roughly as far a s the Sandia robot. But in the low gravity of Mars, it would go three times as far in a single bound.

So far, the frogbot prototype hops only a halfdozen times before it must recharge its batteries, says Fiorini. However, he

pers dropped from an orbiter would probably cost considerably less than $100,000 apiece and carry out more ex- tensive scientific investigations.

What’s more, Marzwell argues, the penalties-financial and scientific-from

losing a few of the robots would be mini- mal. “If I lose one or two, it’s not a big deal. The mission can go on,” he says.

Hoppers may keep minefields lethal

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hile both of the critter-inspired robots’ designs leap beyond W what roboticists have done

before, they’re not the first efforts to make machines hop.

The record for the largest, heaviest hopper ever conceived may go to a leap ing battle tank patented in 1945 by Henry W. Wallace. Although there’s no record that Wallace ever built the machine, he described it in detail in his patent applica- tion. He proposed a multiton metal canis- ter bristling with cannons and bounding around on a single, telescoping leg. A driv- er would steer the leg, powered by exple sives and diesel fuel, as it hurled the tank into the air with each thrust.

The idea of hoppers reappeared in a dif- ferent guise when the space race began. Translated from German, a 1959 book called The Moon Car (Hermann Oberth, Harper) featured a hopping lunar vehicle.

In the 1960s, H.S. Seifert of Stanford University set to work on a hopping car that he intended to transport astronauts around the moon’s surface. He built a prototype and gathered evidence that hoppers are a particularly efficient mode of transportation in low gravity.

Robotic hoppers first appeared in the late 1970s. They were built by scientists interested in both robotics and new ways to study how people and animals walk and run. Those researchers built ma- chines with sophisticated mechanisms and controllers that mimicked the ways animals balance and propel themselves.

Until this work, self-propelled robots were “like tables with moving legs,” says Marc Raibert, a pioneer of hopping and running robots who now heads a company called Boston Dynamics in Cam- bridge, Mass. During the 1980s and 1990s, Raibert and other scientists at Carnegie Mellon University in Pittsburgh, the

Antivehicle mines scattered from air- craft form an effective barrier against tanks and other military vehicles. How- ever, if enemy troops blow up just one lane of the devices, a dangerous force can rumble through the breach.

Clearing a lane would become much harder if the remaining mines immediate ly flowed into the gap, much as water on a table. if you draw your finger through it, says condensed-matter physicht Thomas W. Altshuler of the Defense Advanced Research Projects Agency PARPA) in Arlington, Va. He heads the agency’s p m gram to develop a “selChdng minefield.“

The program is exploring the possi- bility of mounting 2Wlogram mlnes-

shaped like half a coffee can--on the hopping robots invented by Sandia Na- tional Laboratories of Albuquerque. Al- ternatively, tiny rocket thrusters built in- to the mines’ rims might flip neighboring mines into the breach ‘like tiddlywinks.” Altshuler says. Whichever propulsion method is eventually chosen, the mobile mlnes will also require rudimentary intel- ligence and the ability to communicate with each other.

By fall 2002, DARPA aims to field-test systems of 50 s ich mines. Scientists are developing algorithms for determining how mines can efficiently move to fill gaps. ‘Those are very hard questions” that become even more challenging in

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actual minefields with hundreds or thou- sands of mines, Altshuler says.

Current U.S. military tactics call for sprinkling antipersonnel mines among antivehicle mines to deter enemy troops from clearing the antivehicle weapons. Because of those tactics, the United States has not signed the 1997 Ottawa Convention banning mines designed specifically to kill or maim people.

Following a directive from then- President Clinton, DARPA is develop ing the fluidlike minefield plan and other technologies to ”obviate the need for the antipersonnel mine” and clear the way for U.S. endorsement of the treaty, Althshuler says. 4 w

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