EARTHQUAKE survivors trapped in rubble could one day be saved by an unlikely
rescuer: a robotic caterpillar that burrows its way through debris. Just a few
centimetres wide, the robot relies on magnetic fields to propel it through the
kind of tiny crevices that would foil the wheeled or tracked search robots
currently used to locate people trapped in collapsed buildings.
The caterpillar鈥檚 inventor, Norihiko Saga of Akita Prefectural University in
Japan, will demonstrate his new method of locomotion at a conference on magnetic
materials in Seattle later this week. In addition to lights and cameras, a
search caterpillar could be equipped with an array of sensors to measure other
factors鈥攕uch as radioactivity or oxygen levels鈥攖hat could tell human
rescuers if an area is safe to enter.
The magnetic caterpillar is amazingly simple. It moves by a process similar
to peristalsis, the rhythmic contraction that moves food down your intestine.
Saga made the caterpillar from a series of rubber capsules filled with a
magnetic fluid consisting of iron particles, water, and a detergent-like
surfactant, which reduces the surface tension of the fluid. Each capsule is
linked to the next by a pair of rubber rods. The caterpillar鈥檚 guts are wrapped
in a clear, flexible polymer tube that protects it from the environment.
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To make the caterpillar move forwards, Saga moves a magnetic field backwards
along the caterpillar. Inside the caterpillar鈥檚 鈥渉ead鈥� capsule, magnetic fluid
surges towards the attractive magnetic field, causing the capsule to bulge out
to the sides and draw its front and rear portions up. As the magnetic field
passes to the next capsule, the first breaks free and springs forward and the
next capsule bunches up. In this way, the caterpillar can reach speeds of 4
centimetres per second as it crawls along.
Moving the magnetic field faster can make it traverse the caterpillar before
all the capsules have sprung back to their original shape
(see Graphic). The
segments then all spring back, almost but not quite simultaneously.
Saga plans to automate the movement of the caterpillar by placing
electromagnets at regular intervals along the inside of its polymer tube. By
phasing the current flow to the electromagnets, he鈥檒l be able to control it
wirelessly via remote control. He also needs to find a new type of rubber for
the magnetic capsules, because the one he鈥檚 using at the minute eventually
begins to leak.
But crawling is not the most efficient form of locomotion for robots, says
Robert Full of the University of California at Berkeley, an expert in animal
motion who occasionally advises robotics designers. 鈥淚f you look at the
energetic cost of crawling, compared to walking, swimming or flying, crawling is
very expensive,鈥� he says. Walking, on the other hand, is very efficient because
with every step, energy is conserved in the foot and then released to help the
foot spring up.
Saga acknowledges this inefficiency but says his caterpillar is far more
stable than one that walks, rolls on wheels or flies. It has no moving parts
save for a few fluid-filled rubber capsules. Biped robots and wheeled robots
require a smooth surface and are difficult to miniaturise, and flying robots
have too many moving parts. 鈥淢y peristaltic crawling robot is simple鈥攁nd
it works,鈥� he says.