Our fly has all the same primary mechanical flight components of an actual fly: an airframe (exoskeleton), actuators (flight muscles), a transmission (thorax), and airfoils (wings) [see diagram, ”Anatomy of a Robotic Fly”]. The function of each is simple. The airframe must provide a solid mechanical ground for the actuators and transmission. The actuators power the thorax at mechanical resonance. The transmission maps actuator movements to the desired wing motions. Finally, the airfoils must remain sufficiently rigid to maintain their shape in a number of radically different aerodynamic conditions.
Our design takes due note of the physics of the minuscule. As the parts of a robotic device shrink, surface forces begin to dominate the dynamics of motion. Bearings become less efficient because a decrease in size means an increase in the surface-area-to-volume ratio and thus in friction. However, just because we designed the robot didn't mean we knew how to make it, and mechanical components with features of one micrometer are well below the resolution of standard manufacturing techniques. Nor could we turn to processes employing microelectromechanical systems, or MEMS, which use materials that are too fragile for the forces a robotic fly must withstand. What is more, it takes a lot of time to turn out a prototype with MEMS, and our design strategy entails building a lot of prototypes.
Over hundreds of iterations, our robotic fly has followed its own evolutionary path to more closely resemble the shape of a real fly
My approach has been to develop a process based instead on laser micromachining and thin materials, usually carbon-fiber-reinforced composites, laminated to have precisely tailored stiffness and compliance. Using these fairly simple techniques, we can make a fly prototype in less than a week.
To build a joint, we make gaps in two thin, rigid sheets of carbon fiber. We sandwich between them a thin-film polymer, which can bend repeatedly without losing its ability to flex. Four such joints, connected in series by flat, rigid carbon-fiber links of various lengths, make a microscale transmission. With a proper choice of link lengths, the transmission can amplify the small angular motions of one link into larger movements of the opposite link.
To make actuators that mimic real flight muscles, we add to the carbon-fiber-based composite a few layers of an electroactive material, which changes its shape when an electric field is applied. Designing these actuators to be as small and light as possible, while keeping them strong enough to deliver sufficient power, was our first key accomplishment. The power density of our robot's actuators comes to more than 400 watts per kilogram, some four times that of an ordinary fly's wing muscles. Our second breakthrough came when we successfully converted the actuator movements to biomimetic wing motions, using a four-bar linkage. Only after we made the transmission did we discover, to our great satisfaction, that its mechanism is remarkably similar to a dipteran fly's thorax driving its wing movements.
Our latest version weighs 60 milligrams, about the same as certain dipteran flies, ï»¿and can generate nearly twice its weight in thrust. That's almost on par with a real fly, which typically can attain lift forces three to five times its weight. Our immediate goal is to get the fly to hover, which is key for maneuvering in constricted environments. A hovering vehicle can turn itself in place and does not require forward motion to remain aloft.
To achieve stable, untethered flight, we will need to miniaturize and install three more things: sensors, controls, and a power source. A number of laboratories and companies are developing a promising suite of sensors, inspired by biological sensory systems, to enable the robot to stabilize its own flight and to control simple behaviors. My former advisor Ron Fearing's work at the Biomimetic Millisystems Lab, at Berkeley, has demonstrated bio-inspired gyroscopes and sensors capable of detecting the horizon. Centeye, in Washington, D.C., has built vision sensors weighing less than a gram to help flying robots navigate.
Control remains a challenge. A real fly can make rapid turns, called saccades, because it has a specialized neural system that allows for speedy responses. In a fly, neural impulses from internal feedback sensors directly modulate the flight muscles—without processing from the central nervous system—to counter disturbances. We are studying practical ways to emulate this system by using inputs from a number of attitude sensors that figure out the orientation of the fly and directly manipulate the actuators.
Then there's the question of getting a power source onto the fly. A battery small enough to fit aboard a robotic fly will have a much higher surface-area-to-volume ratio than its macroscale counterpart, so a greater percentage of its mass will be the packaging. We expect that scaled-down versions of today's best lithium-polymer batteries will weigh about 50 mg, accounting for half the fly's weight, and will provide 5 to 10 minutes of flight. For more flight time we will have to increase the battery's energy density, make the propulsion more efficient, or develop energy-harvesting techniques, perhaps by mounting tiny solar panels on the insect's back or converting the fly's vibrations into electric current.
We're now turning our attention to the robot's low-power, decentralized control algorithms. Again, we begin with nature. Social insects use simple local rules and minimal direct communication, yet they achieve tasks of astounding complexity. For example, termites can produce a structure millions of times their own size, even though no one termite has a blueprint for it. We believe that our robots can eventually be used as tools to study such insect behaviors; what we learn could then help us to design algorithms to enable swarms of simple robots to accomplish complex tasks.
Even with basic control algorithms, however, we expect microrobots to be able to perform useful roles as ad hoc mobile sensor networks. Search-and-rescue operations, hazardous environment exploration and monitoring, planetary exploration, and building inspections are just a few of the potential applications for highly agile, insect-scale rescue robots. Smart sensors on wings are not a distant dream: we predict that a fully autonomous robotic insect will be flying in laboratory conditions within five years. Five years beyond that, we could begin seeing these devices in our daily lives.
About the Author
ROBERT WOOD, author of ”Fly, Robot Fly,” is an assistant professor of electrical engineering at Harvard and principal investigator at the Harvard Microrobotics Laboratory. When not tinkering with robot flies, his group investigates the development of other ”species” of arthropod-inspired robots. Look for Wood's robotic flies in New York's Museum of Modern Art this spring.