Fireflies that light up dark backyards on hot summer nights use their luminescence for communication – to attract a mate, ward off predators or attract prey.
These shimmering insects also sparked the inspiration of MIT scientists. Following a suggestion from nature, they built electroluminescent soft 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 to a collapsed building, for example, a robot that finds survivors can use lights to signal others and call for help.
The ability to emit light also brings these microscale robots, which weigh little more than a paperclip, one step closer to flying on their own outside the lab. These robots are so light they can’t carry sensors, so researchers must track them using bulky infrared cameras that don’t work well outdoors. Now, they’ve shown they can track the robots accurately using the light they emit and just three smartphone cameras.
“If you think about large-scale robots, they can communicate using a lot of different tools – Bluetooth, wireless, all that sort of thing. But for a tiny, energy-constrained robot, we are forced to think of new modes of communication. This is an important step in piloting 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 D. Reid Weedon, Jr. Assistant Professor in the Department of Electrical Engineering and Computer Science (EECS), head of the Soft and Micro Robotics Laboratory at the Electronics Research Laboratory (RLE) and senior author of the article.
He and his collaborators achieved this by embedding tiny electroluminescent particles in artificial muscles. The process adds just 2.5% more weight without affecting the robot’s flight performance.
Joining Chen in the article 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. The survey was published this month in IEEE Robotics and Automation Charts.
a light actuator
These researchers have previously demonstrated a new fabrication technique for building soft actuators, or artificial muscles, that flap the robot’s wings. These durable actuators are made by alternating ultra-thin layers of elastomer electrode and carbon nanotube in a pile and then rolling it into a soft cylinder. When tension is applied to this cylinder, the electrodes compress the elastomer and the mechanical tension strikes the wing.
To manufacture a brilliant actuator, the team incorporated electroluminescent zinc sulfate particles into the elastomer, but had to overcome several challenges along the way.
First, the researchers had to create an electrode that didn’t block light. They built it using highly transparent carbon nanotubes, which are just a few nanometers thick and allow light to pass through.
However, zinc particles only light up 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 known as photons. The researchers use high voltage to create a strong electric field in the soft actuator, and then drive the robot at a high frequency, which allows the particles to illuminate intensely.
“Traditionally, electroluminescent materials are energetically very expensive, but in a way, we get this electroluminescence for free because we use the electric field at the frequency needed to fly. We don’t need new acting, new wires or anything. It only takes about 3% more energy to emit light,” says Kevin Chen.
When prototyping the actuator, they found that adding zinc particles reduced its quality, causing it to break more easily. To get around this, Kim mixed zinc particles only into the top layer of elastomer. He made this layer a few micrometers thicker to accommodate any reduction in output power.
Although this made the actuator 2.5% heavier, it did emit light without affecting flight performance.
“We are very careful to maintain the quality of the elastomer layers between the electrodes. Adding these particles was almost like adding dust to our elastomer layer. It took a lot of different approaches and a lot of testing, but we found a way to ensure the quality of the actuator,” says Kim.
Adjusting the chemical combination of zinc particles changes the color of the light. The researchers made green, orange and blue particles for the actuators they built; each actuator glows a solid color.
They also tweaked the manufacturing process so the actuators could emit multicolored, patterned light. The researchers placed a small mask over the top layer, added zinc particles, and then cured the actuator. They repeated this process three times with different masks and colored particles to create a pattern of light that spelled out MIT.
following the fireflies
After fine-tuning the manufacturing process, they tested the mechanical properties of the actuators and used a luminescence meter to measure light intensity.
From there, they performed flight tests using a specially designed motion tracking system. Each electroluminescent 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 develop tracks the position and attitude of robots down to 2 millimeters of state-of-the-art infrared motion capture systems.
“We are very proud of how good the tracking result is compared to the state of the art. We were using cheap hardware, compared to the tens of thousands of dollars these big motion tracking systems cost, and the tracking results were pretty 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 lights on and off during flight and communicate more like real fireflies. They are also studying how electroluminescence might even improve some properties of these soft artificial muscles, says Kevin Chen.
“This work is really interesting because it minimizes the overhead (weight and power) for light generation without compromising flight performance,” says Kaushik Jayaram, an assistant professor in the Department of Mechanical Engineering at the University of Colorado at Boulder, who was not involved with this research. “The wingbeat 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 the potential use of communication presented in this work are extremely interesting, I believe the real impetus is that this latest development could become a milestone for demonstrating these robots outside controlled conditions. laboratory,” adds Pakpong Chirarattananon, an associate professor in the Department of Biomedical Engineering at the 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 for the use of less sophisticated equipment and tracking of robots from a distance, perhaps via another larger mobile robot, for real-world deployment. That would be a remarkable advance. I would be thrilled to see what the authors accomplish next.”
This work was supported by the MIT Electronics Research Laboratory.