22 June 2022
Inspired by fireflies, researchers have developed new robots that can emit light when they fly.
Image courtesy of MIT researchers
Fireflies that light up dark backyards during warm summer evenings use their luminescence for communication – to attract a friend, ward off predators or attract prey.
These glittering bugs also drew inspiration from researchers at MIT. Using nature, they built electroluminescent soft artificial muscles for insect-scale flying robots. The small artificial muscles that control the robots’ wings emit colored light during the flight.
This electroluminescence could enable the robots to communicate with each other. If it is 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 gives 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 light that they can not carry them sensorsso scientists have to track them down using bulky infrared cameras which does not work well outdoors. Now they have shown that they can track the robots exactly with the help of the light they emit and only three smartphones cameras.
“If you think of large-scale robots, they can communicate with many different people tool Bluetooth, wireless, all that stuff. But for a small, power-limited robot, we have to think of new ways of communication.
“This is a big step toward flying these robots in outdoor environments where we do not have a well-tuned, state-of-the-art motion tracking system,” said Kevin Chen, D.R. Weedon, Jr. Assistant Professor at the Department of Electrical Engineering and Computer Science (EECS), Head of Soft and Micro Robotics Laboratory in the Research Laboratory of Electronics (RLE), and the senior author of the thesis.
He and his co-workers accomplished this by embedding small electroluminescent particles in the artificial muscles. The process adds only 2.5 percent more weight without affecting the robot’s flight performance.
An igniting actuator
These researchers have previously demonstrated a new manufacturing technique for soft building actuators, or artificial muscles, which flutter with the robot’s wings. These are durable actuators manufactured by alternating ultra-thin layers of elastomer and carbon nanotube electrodes in a stack and then rolling it into a squishy cylinder. When a voltage is applied to that cylinder, the electrodes squeeze the elastomer, and the mechanical strain lowers the wing.
To make a glowing 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 did not block light. They built it with very transparent carbon nanotubes, which are only a few nanometers thick and allow light to pass through.
However, the zinc particles only light up in the presence of a very strong and 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 and then drive the robot at high frequency, which makes the particles shine brightly.
“Traditional electroluminescent material is very energetically expensive, but in a way we get that electroluminescence for free because we only use the electric field at the frequency we need to fly. We do not need new actuators, new wiring or anything. It only takes about three percent more energy to shine light, says Kevin Chen.
Image courtesy of MIT researchers
When they created a prototype of the actuator, they found that the addition of zinc particles reduced its quality, which made it easier to break down. To get around this, Kim mixed zinc particles only in the top elastomer layer. He made that layer a few micrometers thicker to take into account any reduction in output power.
Although this made the actuator 2.5 percent heavier, it emitted light without affecting flight performance.
“We take great care to maintain the quality of the elastomer layers between the electrodes. Adding these particles was almost like adding dust to our elastomer layer. It required many different approaches and a lot of testing, but we came up with 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 actuators they built; each actuator lights up in a solid color.
They also adapted the manufacturing process so that actuators can emit multicolored and patterned light. The researchers placed a small mask over the top layer, added zinc particles and then hardened the actuator. They repeated this process three times with different masks and colored particles to create a light pattern that spelled MIT.
Follows the fireflies
Once they had fine-tuned the manufacturing process, they tested the mechanical properties of actuators and used a luminescence meter to measure the intensity of the light.
From there, they ran flight tests using a specially designed motion tracking system. Each electroluminescent actuator acted as an active marker that could be traced with the iPhone cameras. The cameras detect each light color, and a computer program they developed tracks the position and attitude of robots within 2 mm of state-of-the-art infrared motion capture systems.
“We are very proud of how good the tracking results are, compared to the latest. We used cheap hardware, compared to the tens of thousands of dollars that these large motion tracking systems cost, and the tracking results were very close,” says Kevin Chen.
In the future, they plan to improve that motion tracking system so that it can track robots in real time. The team is working to incorporate control signals so that the robots can turn on and off their lights in flight and communicate more like real fireflies. They are also studying how electroluminescence can even improve certain properties of these soft artificial muscles, says Kevin Chen.
This work was supported by the Research Laboratory of Electronics at MIT.
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