Miniature flying robots

An insect-like robot, no bigger than a fly, takes to the air


SOME people are convinced they are already out there: swarms of tiny
flying drones discreetly surveying the world on behalf of their
shadowy masters. In 2007 anti-war protesters in America claimed they
were being watched by small hovering craft that looked like
dragonflies. Officials maintained they really were dragonflies.
Whatever the truth, robotic flies actually are now getting airborne.

This week the successful flight of what are probably the smallest
hovering robots yet was reported in Science by Robert Wood and his
colleagues at the Wyss Institute for Biologically Inspired Engineering
at Harvard. These robots (pictured above) are the size of crane flies.
Most small flying robots are helicopters—kept aloft by one or more
rotating wings. These, though, are ornithopters, meaning their wings
flap. Wingtip to wingtip they measure 3cm and they weigh just 80
milligrams. Like true flies (those known to entomologists as Diptera),
and unlike dragonflies or butterflies, they have but a single pair of
wings.

Dr Wood, as he is quick to point out, is not trying to build a
military drone. Rather, it is the basic science behind flying insects
that he and his team are interested in. No doubt the armed forces are
taking a keen interest in this sort of work. But civilian applications
such as search and rescue, he thinks, are likely to be as important as
military and security ones. Indeed, the idea that inspired the study
was that of using swarms of robotic flies to pollinate crops.

Flies, as anyone who has tried to swat one knows, are the most agile
of flying creatures. Dr Wood and his colleagues considered it
impossible, even with the best miniaturised mechanical and electrical
parts currently available, to build an artificial version of one that
would show anything like that level of aerial prowess. They therefore
had to come up with a new form of manufacturing, which they call smart
composite microstructures (SCM), to do the job. SCM employs lasers to
cut shapes from extremely thin sheets of material and then bonds them
together and folds them to make components. The materials' properties
come from their layered structures.

Getting into a flap

The robot's wings, for example, are powered by artificial muscles.
These are made from layers of a piezoelectric material—one that
deforms when an electric current is applied to it. Correct alignment
of these layers creates a structure analogous to an insect's flight
muscles, which it contracts and relaxes in order to flap its wings.

Dr Wood's robots are modelled on a hoverfly called Eristalis. They
have a long way to go before they can mimic the precision of such a
creature's flight. They can, nevertheless, hover. They can also carry
out simple manoeuvres. These include turning by flapping one wing
harder than the other.

These acrobatics are possible because of the flight-control system Dr
Wood has designed. Like jet fighters, flying insects are inherently
unstable. And so are Dr Wood's robots. Insects have nervous systems to
deal with this. Fighters have computers. Dr Wood's flies are similarly
computer-controlled—and this, for the moment, is where the illusion
breaks down, because the computer is on a desktop and is connected to
the robot by a thin copper wire.

That could be fixed with a suitable chip. But the wire also carries
electric power: 19 milliwatts, which is equivalent to the power
consumed by a flying insect of the same size. Batteries light enough
to fly with do exist. But they would keep the robot going for only a
few minutes.
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Dr Wood's robot is not the only experimental tiny flying machine
around. The others, though, are bigger and heavier than most insects.
Some, such as the DelFly Micro, a robot with a 10cm wingspan build by
Delft University of Technology in the Netherlands, are also
ornithopters. Others are helicopters. Researchers at the University of
Pennsylvania have demonstrated how a swarm of palm-sized devices with
a rotor on each corner can fly together in formation. And Seiko Epson,
a Japanese firm, has built an 8cm-tall robot that uses contra-rotating
blades mounted on the same shaft to achieve stability.

What is really needed is a breakthrough in battery technology. In the
meantime, though, Dr Wood says there is plenty of research to get on
with, in order to improve the flying abilities of his new robots and
the way they are made. And eventually, like real insects, they will
have to fly outdoors. Buzzing around a cosy laboratory is one thing.
Coping with rain, gusts of wind and even predators that cannot tell
the difference between a robot and the real thing is quite another.