A comparison shot shows the relative size of the current RoboBee platform with a penny, a previous iteration of the RoboBee, and a crane fly. | Source: Harvard University
Nearly eight years ago, Harvard University researchers unveiled RoboBee, a small, hybrid robot that could fly, dive, and swim. Now, engineers at the Harvard Microrobotics Laboratory have outfitted RoboBee with its most reliable landing gear to date, inspired by the crane fly.
Robert Woodthe Harry Lewis and Marlyn McGrath Professor of Engineering and Applied Sciences in the John A. Paulson School of Engineering and Applied Sciences (SEAS), led the team. The researchers have given their flying robot a set of long, jointed legs that help ease its transition from air to ground.
They also equipped RoboBee with an updated controller that helps it decelerate on approach, resulting in a gentle plop-down.
These improvements are intended to protect the robot’s delicate piezoelectric actuators. These are energy-dense “muscles” deployed for flight that are easily fractured by external forces from rough landings and collisions.
RoboBee gets better at landing
Landing has been problematic for the RoboBee partly because of how small and light it is. The robot weighs just a tenth of a gram and has a wingspan of 3 cm. Previous iterations suffered from significant ground effect, or instability as a result of air vortices from its flapping wings. This is much like the groundward-facing full-force gales generated by helicopter propellers.
“Previously, if we were to go in for a landing, we’d turn off the vehicle a little bit above the ground and just drop it, and pray that it will land upright and safely,” said Christian Chan, co-first author and a graduate student who led the mechanical redesign of the robot.
The team’s paper describes the improvements it made to the robot’s controller, or brain, to adapt to the ground effects as it approaches. This is an effort led by co-first author and former postdoctoral researcher Nak-seung Patrick Hyun. Hyun led controlled landing tests on a leaf, as well as rigid surfaces.
Researchers draw inspiration from nature
“The successful landing of any flying vehicle relies on minimizing the velocity as it approaches the surface before impact and dissipating energy quickly after the impact,” said Hyun, now an assistant professor at Purdue University. “Even with the tiny wing flaps of RoboBee, the ground effect is non-negligible when flying close to the surface, and things can get worse after the impact as it bounces and tumbles.”
The lab looked to nature to inspire mechanical upgrades for skillful flight and graceful landing on a variety of terrains. The scientists chose the crane fly, a relatively slow-moving, harmless insect that emerges from spring to fall and is often mistaken for a giant mosquito.
“The size and scale of our platform’s wingspan and body size was fairly similar to crane flies,” Chan said.
The researchers noted that crane flies’ long, jointed appendages likely give the insects the ability to dampen their landings. Crane flies are further characterized by their short-duration flights. Much of their brief adult lifespan (days to a couple of weeks) is spent landing and taking off.
Considering specimen records from Harvard’s Museum of Comparative Zoology database, the team created prototypes of different leg architectures. It eventually settled on designs similar to a crane fly’s leg segmentation and joint location. The lab used manufacturing methods pioneered in the Harvard Microrobotics Lab for adapting the stiffness and damping of each joint.
Postdoctoral researcher and co-author Alyssa Hernandez brought her biology expertise to the project, having received her Ph.D. from Harvard’s Department of Organismic and Evolutionary Biology, where she studied insect locomotion.
“RoboBee is an excellent platform to explore the interface of biology and robotics,” she said. “Seeking bioinspiration within the amazing diversity of insects offers us countless avenues to continue improving the robot. Reciprocally, we can use these robotic platforms as tools for biological research, producing studies that test biomechanical hypotheses.”
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Researchers look ahead to RoboBee applications
Currently, the RoboBee stays tethered to off-board control systems. The team said it will continue to focus on scaling up the vehicle and incorporating onboard electronics to give the robot sensor, power, and control autonomy. These three technologies will allow the RoboBee platform to truly take off, asserted the researchers.
“The longer-term goal is full autonomy, but in the interim, we have been working through challenges for electrical and mechanical components using tethered devices,” said Wood. “The safety tethers were, unsurprisingly, getting in the way of our experiments, and so safe landing is one critical step to remove those tethers.”
The RoboBee’s diminutive size and insect-like flight prowess offer intriguing possibilities for future applications, said the researchers. This could include environmental monitoring and disaster surveillance.
Among Chan’s favorite potential applications is artificial pollination. This would involve swarms of RoboBees buzzing around vertical farms and gardens of the future.
The National Science Foundation (NSF) Graduate Research Fellowship Program under Grant No. DGE 2140743 supported this research.
A composite image of the RoboBee landing on a leaf. | Source: Harvard University
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