Harvard roboticists have demonstrated flying microrobots, nicknamed RoboBees, that can perch during flight to save energy—like bats, birds or butterflies.

“Many applications for small drones require them to stay in the air for extended periods,” says Moritz Graule—first author of a paper published in Science describing the technology—who conducted the research as a student at Harvard's John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering. “Unfortunately, smaller drones run out of energy quickly. We want to keep them aloft longer without requiring too much additional energy.”

“A lot of different animals use perching to conserve energy,” says Kevin Ma, a postdoc at SEAS and the Wyss Institute and the paper's co-author. “But the methods they use to perch, like sticky adhesives or latching with talons, are inappropriate for a paper clip-size microrobot, as they either require intricate systems with moving parts or high forces for detachment.”

Instead, the team turned to electrostatic adhesion—the same basic science that causes a static-charged sock to cling to a pants leg or a balloon to stick to a wall.

RoboBees use static electricity to stick to surfaces and conserve energy. Image credit: SEAS.When you rub a balloon on a wool sweater, the balloon becomes negatively charged. If the charged balloon is brought close to a wall, that negative charge forces some of the wall’s electrons away, leaving the surface positively charged. The attraction between opposite charges then causes the balloon to stick to the wall.

“In the case of the balloon, however, the charges dissipate over time, and the balloon will eventually fall down,” says Graule. “In our system, a small amount of energy is constantly supplied to maintain the attraction.”

The RoboBee uses an electrode patch and a foam mount that absorbs shock. The entire mechanism weighs 13.4 mg, bringing the total weight of the robot to about 100 mg—similar to that of a real bee. The robot takes off and flies normally. When the electrode patch is supplied with a charge, it can stick to almost any surface, from glass to wood to a leaf. To detach, the power supply is simply switched off.

The patch requires about 1,000 times less power to perch than it does to hover, potentially extending the operational life of the robot dramatically. Reducing the robot’s power requirements is critical for the researchers as they work to integrate onboard batteries on untethered RoboBees.

“The use of adhesives that are controllable without complex physical mechanisms, are low power and can adhere to a large array of surfaces is perfect for robots that are agile yet have limited payload—like the RoboBee,” notes Robert Wood, professor of engineering and applied sciences at SEAS and the Wyss Institute and senior author of the study.

Right now, the RoboBee can only perch under overhangs and on ceilings, as the electrostatic patch is attached to the top of the vehicle. Next, the team hopes to change the mechanical design so that the robot can perch on any surface.

“There are more challenges to making a robust, robotic landing system, but this experimental result demonstrates a very versatile solution to the problem of keeping flying microrobots operating longer without quickly draining power,” says Ma.