MIT’s tiny robotic lightning bolts take flight

Inspired by fireflies, MIT scientists have created flexible actuators capable of emitting light in different colors or patterns. Credit: Courtesy of the researchers

Inspired by fireflies, scientists are creating insect-scale robots that can emit light as they fly, enabling movement tracking and communication.

The lightning bugs that light up dark backyards on warm summer evenings use their luminescence for communication – to attract a mate, ward off predators, or attract prey.

These twinkling fireflies have also inspired researchers from[{” attribute=””>MIT. Taking a cue from nature, they constructed electroluminescent soft artificial muscles for flying, insect-scale robots. The tiny artificial muscles that control the robots’ wings emit colored light during flight.

This electroluminescence could enable the robots to communicate with each other. For example, if sent on a search-and-rescue mission into a collapsed building, a robot that finds survivors could use lights to signal others and call for help.

The ability to emit light also brings these microscale robots, which barely weigh more than a paper clip, one step closer to flying on their own outside the lab. These robots are so lightweight that they can’t carry sensors, so researchers must track them using bulky infrared cameras that don’t work well outdoors. Now, they’ve shown that they can track the flying robots precisely using the light they emit and just three smartphone cameras.

MIT Robotic Lightning Bug

These artificial muscles, which control the wings of featherweight flying robots, light up while the robot is in flight, which provides a low-cost way to track the robots and also could enable them to communicate. Credit: Courtesy of the researchers

“If you think of large-scale robots, they can communicate using a lot of different tools — Bluetooth, wireless, all those sorts of things. But for a tiny, power-constrained robot, we are forced to think about new modes of communication. This is a major step toward flying these robots in outdoor environments where we don’t have a well-tuned, state-of-the-art motion tracking system,” says Kevin Chen, who is the D. Reid Weedon, Jr. Assistant Professor in the Department of Electrical Engineering and Computer Science (EECS), the head of the Soft and Micro Robotics Laboratory in the Research Laboratory of Electronics (RLE), and the senior author of the paper.

He and his colleagues accomplished this by embedding minuscule electroluminescent particles into the artificial muscles. This process adds just 2.5 percent more weight without impacting the flight performance of the robot.

The research was published recently in IEEE Robotics and Automation Letters. Joining Chen on the paper are EECS graduate students Suhan Kim, the lead author, and Yi-Hsuan Hsiao; Yu Fan Chen SM ’14, PhD ’17; and Jie Mao, an associate professor at Ningxia University.

A luminous actuator

Previously, these scientists demonstrated a new fabrication technique to build flexible actuators, or artificial muscles, that flap the robot’s wings. These durable actuators are made by alternating ultra-thin layers of elastomer and carbon nanotube electrodes in a stack, then rolling them into a spongy cylinder. When a voltage is applied to this cylinder, the electrodes compress the elastomer and the mechanical stress causes the wing to flap.

To make a glowing actuator, the researchers incorporated light-emitting zinc sulfate particles into the elastomer, but had to overcome several challenges along the way.

First, the team had to create an electrode that wouldn’t block light. They built it using highly transparent carbon nanotubes, which are only a few nanometers thick and let light through.

However, zinc particles ignite only in the presence of a very strong, high-frequency electric field. This electric field excites the electrons in the zinc particles, which then emit subatomic particles of light called photons. Scientists use high voltage to create a strong electric field in the soft actuator, then drive the robot at a high frequency, causing the particles to light up brightly.

“Traditionally, electroluminescent materials are very energy-intensive, but in a sense we get that electroluminescence for free because we’re just using the electric field at the frequency we need to fly. We don’t need new actuation, new wires or anything. It only takes about 3% more energy to make the light shine,” says Kevin Chen.

While prototyping the actuator, they discovered that adding zinc particles reduced its quality, causing it to break down more easily. To circumvent this problem, Kim mixed zinc particles only in the top layer of elastomer. He thickened this layer by a few micrometers to accommodate any reduction in power output.

Although this makes the actuator 2.5% heavier, it emits light without affecting flight performance.

“We pay a lot of attention to maintaining the quality of the elastomer layers between the electrodes. Adding these particles was almost like adding dust to our elastomer layer. It took many different approaches and many tests, but we found a way to ensure the quality of the actuator,” says Kim.

Adjusting the chemical combination of the zinc particles changes the color of the light. The research team made green, orange, and blue particles for the actuators they built; each actuator shines in a solid color.

They also changed the manufacturing process so that the actuators could emit multicolored and patterned light. The researchers placed a tiny mask on the top layer, added zinc particles, then hardened the actuator. They repeated this process three times with different masks and colored particles to create a light pattern that spelled MIT.

In pursuit of fireflies

After refining the manufacturing process, they tested the mechanical properties of the actuators and used a luminescence meter to measure the intensity of the light.

From there, they conducted flight tests using a specially designed motion tracking system. Each light-emitting actuator served as an active marker that could be tracked using iPhone cameras. The cameras detect every color of light, and a computer program they developed tracks the position and attitude of the robots within 2 millimeters of state-of-the-art infrared motion capture systems.

“We are very proud of the quality of the tracking result, compared to the state of the art. We were using inexpensive hardware, compared to the tens of thousands of dollars that these large motion tracking systems cost, and the follow-up results were very close,” says Kevin Chen.

In the future, they plan to improve this motion tracking system so that it can track robots in real time. The group is working to incorporate control signals so the robots can turn their light on and off during flight and communicate more like real fireflies. They are also studying how electroluminescence might even improve certain properties of these soft artificial muscles, says Kevin Chen.

“This work is really exciting because it minimizes overhead (weight and power) for light generation without compromising flight performance,” says Kaushik Jayaram, assistant professor in the Department of Mechanical Engineering at the University of Colorado Boulder, who did not participate in this research. “The wing-beat synchronized flash generation demonstrated in this work will facilitate motion tracking and flight control of multiple microrobots in low-light environments, both indoors and outdoors.”

“While the light production, reminiscence of biological fireflies, and potential use of communication shown in this work are extremely exciting, I think the real impetus is that this latest development could prove to be an important step towards demonstration of these robots outdoors under controlled laboratory conditions,” adds Pakpong Chirarattananon, an associate professor in the Department of Biomedical Engineering at City University of Hong Kong, who was also not involved in this work.

“Illuminated actuators potentially act as active markers for external cameras to provide real-time feedback for flight stabilization to replace the current motion capture system. Electroluminescence would allow the use of less sophisticated equipment and to track the robots remotely, perhaps via another larger mobile robot, for deployment in the real world. That would be a remarkable breakthrough. I’d love to see what the authors accomplish next.

Reference: “FireFly: An Insect-Scale Aerial Robot Powered by Light-Emitting Soft Artificial Muscles” by Suhan Kim, Yi-Hsuan Hsiao, YuFan Chen, Jie Mao, and YuFeng Chen, June 1, 2022, IEEE Letters on Robotics and Automation.
DOI: 10.1109/LRA.2022.3179486

This work was supported by the MIT Electronics Research Laboratory.

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