Author: Daniel Ackerman
If you’ve ever dropped a mosquito out of your face, just to make it come back again (and again and again), you know that insects can be remarkably acrobatic and flexible during a flight. These features help them navigate an air world with all its gusts of wind, obstacles, and general uncertainty. Such features are also difficult to build in flying robots, but an MIT assistant professor Kevin Yufeng Chen has built a system that approaches the agility of insects.
Chen, member Department of Electrical Engineering and Computer Science and Electronics Research Laboratory, has developed insect – sized drones with unprecedented dexterity and flexibility. Air robots are powered by a new class of soft actuator that allows them to withstand the physical journeys of a real flight. Chen hopes that one day robots could help people by pollinating plants or performing machine inspections in cramped spaces.
Chen’s work will appear in the magazine this month IEEE transactions in robotics. His authors include MIT doctoral student Zhijian Ren, Harvard University doctoral student Siyi Xu, and Hong Kong City University robot Pakpong Chirarattanano.
Typically, drones require large open spaces because they are not agile enough to navigate enclosed spaces and not durable enough to withstand collisions in a crowd. “If we look at most drones today, they’re usually pretty big,” Chen says. “Most of their applications are related to flying outdoors. The question is: Can you create insect-scale robots that can move in very complex, confusing spaces? “
According to Chen, “the challenge of building small air robots is huge.” Surface-sized drones require a fundamentally different structure than larger ones. Large drones usually run on motors, but motors lose efficiency when you reduce them. So, Chen says, for insect-like robots, “you have to look for alternatives.”
The most important option so far has been a small, rigid actuator constructed of piezoelectric ceramic materials. Piezoelectric ceramics enabled the first generation of small robots take a flight, they are quite fragile. And that’s the problem when you’re building a robot to mimic insects – food slugs can withstand a collision about once a second.
Chen designed a more flexible small drone device using soft actuators instead of hard, sensitive ones. The soft actuators are made of a thin rubber cylinder coated with carbon nanotubes. When voltage is applied to the carbon nanotubes, they produce an electrostatic force that compresses and lengthens the rubber cylinder. Repeated elongation and contraction causes the drone’s wings to strike – quickly.
Chen actuators can blink nearly 500 times per second, giving the drone insect-like flexibility. “You can hit it when it flies, and it can recover,” Chen says. “It can also make aggressive movements like convexities in the air.” And it weighs just 0.6 grams, roughly the mass of a large bumblebee. The drone looks a bit like a small cassette tape on the wings, though Chen is working on a new dragonfly-shaped prototype.
“Achieving a flight with a centimeter-scale robot is always an impressive achievement,” says Farrell Helbling, an assistant professor of electrical and information technology at Cornell University who was not involved in the study. “Due to the inherent adherence to soft actuators, the robot can safely collide with obstacles without greatly obstructing flight. This feature is well suited for flying in cluttered, dynamic environments and can be very useful for any real-world application.”
Helbling adds that a key step towards these applications is the disconnection of robots from the wired power supply currently required by the high operating voltage of the actuators. “I’m excited about how the authors are reducing the operating voltage so that one day they can achieve unconnected flight in real environments.”
Building insect-like robots can provide a window into the biology and physics of insect flight, a long-term way for researchers to research. Chen works on these issues through a kind of reverse planning. “If you want to learn how to fly insects, it’s very instructive to build a scale robot model,” he says. “You can disturb a few things and see how it affects the kinematics or how the fluid forces change. It will help you understand how those things fly. “But Chen is striving to do more than increase the number of entomology textbooks. His drones may also be useful in industry and agriculture.
Chen says his small pilots could move on complex planes to ensure safety and functionality. “Think of a turbine engine inspection. You want the drone to move [an enclosed space] with a small camera to check for cracks in the turbine plates. “
Other possible applications include artificial pollination of crops or the conduct of search and rescue operations after a disaster. “All of these things can be very challenging for existing large robots,” Chen says. Sometimes bigger is not better.