The fireflies 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 insects also sparked inspiration from MIT scientists. Inspired by nature, they built light-emitting flexible artificial muscles for insect-scale flying robots. The tiny artificial muscles that control the robots’ wings emit colored light during flight.
This electroluminescence could allow robots to communicate with each other. If sent on a search and rescue mission in a collapsed building, for example, a robot that finds survivors could use lights to signal others and call for help.
The ability to emit light also brings these microscopic robots, which weigh little more than a paperclip, one step closer to autonomous flight outside the lab. These robots are so light that they can’t carry sensors, so researchers have to track them using bulky infrared cameras that don’t work well outdoors. Now they’ve shown they can track the robots precisely using the light they emit and just three smartphone cameras.
“If you think of large-scale robots, they can communicate using many different tools – Bluetooth, wireless, all those sorts of things. But for a tiny robot with limited power, we have to think about new modes of communication. This is a major step towards piloting these robots in outdoor environments where we don’t have a state-of-the-art motion tracking system,” says Kevin Chen, who is the assistant to D. Reid Weedon, Jr. Professor in the Department of Electrical Engineering and Computer Science (EECS), Head of the Soft and Micro Robotics Lab at the Research Laboratory of Electronics (RLE), and senior author of the paper.
He and his collaborators achieved this by incorporating tiny light-emitting particles into the artificial muscles. The process adds only 2.5% more weight without affecting the robot’s flight performance.
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, associate professor at Ningxia University. The research was published this month in IEEE Letters on Robotics and Automation.
A luminous actuator
These researchers previously demonstrated a new fabrication technique for building 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 electrode 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 team incorporated light-emitting zinc sulfate particles into the elastomer, but had to overcome several challenges along the way.
First, the researchers 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 allow light to pass 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. The researchers 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 found 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 researchers 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 team 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 the 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 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 less sophisticated equipment to be used and robots to be tracked remotely, perhaps via another larger mobile robot, for real-world deployment. It would be a remarkable breakthrough. I’d love to see what the authors do next.
This work was supported by the MIT Electronics Research Laboratory.