The off-road robo-roach
By Andrea Baird

Anyone who has ever chased a cockroach through the kitchen with a rolled-up magazine has seen what incredible movers they are. A cockroach can dash across the linoleum, up the cabinet door, over this morning’s butter knife and breakfast plate, down the wall, and under the refrigerator before a thrown shoe ever hits the counter. Few people appreciate such speed and agility in their household pests, but robotics engineers know a good design when they see one. “If you had a robot that could do half the things a cockroach can do,” says Roy Ritzmann, a biology professor at Case Western Reserve University, “you’d have the best legged robot in the world.”

Traditionally, if engineers have wanted a robot to move they’ve used that quintessential human invention: the wheel. On a flat surface, (such as that other great human invention: the road) wheels work wonderfully. To make robots that can go off road, however, engineers are turning to the leg, nature’s invention for moving animals in any environment. In the near future, engineers hope legged robots will explore places too bumpy and obstructed for wheels and too dangerous for people—like the rubble of fallen buildings, unmapped minefields or (way off-road) the boulder-strewn surface of Mars. As the model for this new frontline of robotic rescue workers, soldiers and astronauts, many engineers have chosen that nimble legged runner, the cockroach.

To make a robot as agile as a roach, scientists first have to figure out why roaches are so nimble. To unlock this secret, biologists put cockroaches through rigorous road testing. In his University of California, Berkeley lab, one such scientist, Robert Full, videotapes the insects running on roach-sized treadmills and over uneven ground to see how their legs move. He glues little jetpacks to their backs to discover what they do when a blast of air gives them a push in mid-stride. (One of the advantages of working with roaches is that no one really cares what you do to them, says Roger Quinn, professor of mechanical engineering at Case Western Reserve University: “PETA has not been banging down the door.”) Video of roaches racing across blocks of Jell-O reveals how much force each foot exerts on the ground as the animals run. (A tip for the at-home biologist: Full quickly discovered that it’s best to use unflavored Jell-O to study running rather than eating behavior).

Through these cockroach calisthenics and some more invasive physiological studies, Full and other roach biologists have discovered that cockroaches can run just as easily over uneven ground as they can on a kitchen floor. This is in part due to the inherent stability of having six legs to stand on (a big advantage over a two-legged or four-wheeled design if one of your legs or wheels is damaged). A roach runs using what scientists call an alternating tripod gait, explains Fred Delcomyn, an entomology professor at the University of Illinois. The front and back legs on one side of the body and the middle leg on the other move together. So with each three-legged step, the cockroach is poised on top of a triangle of supports like a tiny footstool. Compare this to a human step, in which only one leg on one side of the body is touching the ground at a given time, and the implications for stability are clear.

The other key to cockroach stability is their natural springiness. Their flexible muscles, joints and exoskeleton act like the shock absorbers in a tiny, perfectly tuned car suspension system. According to Full, if a running roach is pushed or steps in a hole or on a pebble, its spring-like legs will give, then push back against the ground, restoring the insect’s equilibrium before it loses its balance. The adjustment is a completely automatic structural act; it happens so fast (10 milliseconds) that the cockroach’s nervous system never even has time to get involved.

Using this growing knowledge of the mechanics of cockroach motion, several groups of engineers across the country are designing and building off-road roach robots. Early in the design process, each group faced a fundamental decision: how closely should their robot’s design mimic the body of an actual cockroach?

Not very closely at all, according to Daniel Koditschek, an electrical engineering professor at the University of Michigan and the head of that school’s robo-roach project, which began in 1998. Cockroaches, after all, have structures involved in reproduction and feeding (like arms jointed to bring food to the mouth) that a robot would never need. So Koditschek and his collaborators reduced the cockroach to its most fundamental locomotory principles to build a simple, but effective, robot.

Koditschek, working with Full, found that when a cockroach runs, its legs, though jointed, act like pogo-sticks. So, the team built their robot with six unjointed, C-shaped, flexible plastic rods for legs, which act as the machine’s shock absorbers. Each leg is powered by a motor, which spins the leg alongside the body like a one-spoked wheel. The legs hit the ground in the classic cockroach tripod gait and then swing over the shoulder to come down again. The resulting robot looks like a four-year-old’s drawing of a cockroach – a rectangle two-feet long with six sticks coming off it. Using a cockroach-inspired walk and suspension system, it runs stably over rocky ground, through vegetation, and can climb stairs.

Engineers at Stanford chose a design that takes the cockroach model a little more literally. Though the robot’s legs are, like the Michigan lab’s model, simple jointless pogo-sticks, its swinging stride and elastic hip joints mimic those of an actual cockroach.

Each of the robot’s legs is a pneumatic piston (a rod that moves within a nearly air-tight tube, like the motion damper on the top of many school and church doors), which, when filled with air, pushes its inner rod down and back, moving the robot forward. The legs themselves aren’t flexible, but a constant air pressure maintained in the hollow piston tube absorbs shocks and gives the legs their springiness.

The robot has another shock absorber in its hip, which Mark Cutkosky, the mechanical engineering professor who runs Stanford’s cockroach robot lab, and his team designed about four years ago. Roach hips contain resilin, the highly elastic material that launches fleas on their epic jumps, explains cockroach biologist Ritzmann. The resilin stretches during the backswing of the step and springs the leg forward again when the roach lifts its foot. Cutkosky’s team’s robot has a soft polyurethane hip joint that works the same way, automatically springing the legs back into stepping position after each tripod step. The hip-spring saves energy and is tuned to give the six-inch long robot, which can climb over obstacles as tall as its hip and run up to 2.5 body lengths per second, an extra degree of stabilizing suspension.

In the past, Roger Quinn and his robotics team at Case Western Reserve University have also worked with stylized cockroach robots like the Stanford and Michigan machines. For their newest project, however, they decided to take a new tack and mimic a cockroach’s structure as closely as possible.

Their robot, like the real thing, has jointed legs that are small and nimble in the front and long and powerful in the back. The legs, though 20 times bigger than a live roach’s and attached to a simple metal rod rather than a cockroach-like body, look disturbingly realistic and ready to run. Their appearance is misleading, however. The robot can’t move. The jointed legs are so complicated that the engineers are still designing their controls. But Quinn says that once the robot is mobile its joints will allow it to do things that simpler robots can’t, like stepping sideways to avoid an obstacle or change where it places its feet to stepping across a hole or over a slatted surface.

Though the robot can’t walk, it is mechanically stable. It can step in place in the tripod gate, and, if pushed, bounces back into place. The artificial air-controlled muscles that power the robot act as shock absorbers at every joint in the leg. The muscles consist of rubber tubing inside a woven metal tube that works like a stretched Chinese finger trap. When the rubber tube is filled with air, the metal tube widens and consequently shortens. By filling both halves of an opposing pair with a little air, each joint of the leg becomes a tuned spring working to balance the robot.

With these three robots, engineers have demonstrated that a legged robot can run stably—even over bumpy terrain—using basic, mechanical cockroach motion. But a robot that can run over rough ground in a blind, unending straight line is still not very useful (except, perhaps, in the minefield scenario). The next step for engineers is to give their robots sense organs like antennae, eyes, ears, and noses so they can avoid obstacles, and, most importantly, a brain that will allow them to complete their missions without a human at the helm.

Though the obstacles engineers face are formidable, Quinn projects that a fully autonomous off-road robot will be developed in the next five years. Five years beyond that, robotic cockroaches may be finding survivors in disaster wreckage, exploring the surfaces of distant planets, navigating minefields, and fetching our glasses and slippers for us. With offspring like these, the cockroach may soon graduate from household horror to hero.