Robotic lightning bugs take flight

Fireflies that illuminate dim backyards on warm summer evenings use their luminescence for communication — to attract a mate, ward off predators, or entice prey.

These shiny bugs also inspired scientists at MIT. Drawing on 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 would allow the robots to communicate with each other. For example, when sent on a search and rescue mission to a collapsed building, a robot that finds survivors can use lights to signal others and call for help.

The ability to emit light also brings these micro-scale robots, which weigh barely more than a paper clip, one step closer to flying independently outside the lab. These robots are so light they can’t carry sensors, so researchers have to track them using bulky infrared cameras that don’t work well outdoors. Now they have shown that they can accurately track the robots using the light they emit and just three smartphone cameras.

“If you think of large-scale robots that can communicate with many different tools – Bluetooth, wireless, all that sort of thing. But for a small robot with limited power, we are forced to think about new ways of communicating. This is a big step toward flying these robots in outdoor environments where we don’t have a well-tuned, state-of-the-art motion tracking system,” said Kevin Chen, D. Reid Weedon Jr.’s 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 collaborators achieved this by embedding tiny electroluminescent particles into the artificial muscles. The process adds just 2.5 percent 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, an associate professor at Ningxia University. The research was published this month in IEEE Robotics and Automation Letters.

A light up actuator

These researchers have previously shown that new manufacturing technique to build soft 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 and then rolling them into a soft cylinder. When a voltage is applied to that cylinder, the electrodes squeeze the elastomer and the mechanical stress snaps against the wing.

To fabricate 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 wouldn’t block the light. They built it using highly 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 known as photons. The researchers create a strong electric field in the soft actuator with high voltage and then drive the robot at a high frequency, causing the particles to glow brightly.

“Traditionally, electroluminescent materials are very energetically expensive, but in a way we get that electroluminescence for free because we just use the electric field at the frequency we need to fly. We don’t need new controls, new wires or anything. It only takes about 3 percent more energy to shine light out,” says Kevin Chen.

When they prototyped the actuator, they found that adding zinc particles reduced its quality, making it easier to break. To get around this, Kim only mixed zinc particles into the top elastomer layer. He made that layer a few micrometers thicker to accommodate 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 took many different approaches and a lot of testing, but we found a way to ensure the quality of the actuator,” Kim says.

By adjusting the chemical combination of the zinc particles, the light color changes. The researchers made green, orange and blue particles for the actuators they built; each actuator shines one solid color.

They also modified the manufacturing process so that the actuators could 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 using different masks and colored particles to create a light pattern that spelled out MIT.

Follow the fireflies

After fine-tuning 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 electroluminescent actuator served as an active marker that could be tracked by iPhone cameras. The cameras detect every light color and a computer program they have developed tracks the position and attitude of the robots to within 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 used cheap hardware, compared to the tens of thousands of dollars that these large motion tracking systems cost, and the tracking results came very close,” said 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 on incorporating control signals so the robots can turn their lights on and off during flight and communicate more like real fireflies. They’re also investigating how electroluminescence might actually enhance some properties of these soft artificial muscles, Kevin Chen says.

“This work is really interesting because it minimizes the overhead (weight and power) of light generation without compromising flight performance,” said Kaushik Jayaram, an assistant professor in the Department of Mechanical Engineering at the University of Colorado at Boulder, who were not involved in this study. “The wingbeat-synchronized flash generation demonstrated in this work makes it easier for motion tracking and flight control of multiple microrobots in low-light environments, both indoors and outdoors.”

“While the light production, the memory of biological fireflies and the possible uses of communication in this work are extremely interesting, I believe the real momentum is that this latest development could prove to be a milestone towards the demonstration of these robots outside. controlled laboratory conditions,” 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. “The illuminated actuators may act as active markers for external cameras to provide real-time feedback. for flight stabilization to replace the current motion capture system. The electroluminescence would make it possible to use less sophisticated equipment and track the robots remotely, perhaps via another larger mobile robot, for real-world deployment. That would be a remarkable breakthrough. I would be very happy to see what the authors achieve next.”

This work was supported by the Research Laboratory of Electronics at MIT.

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