On warm summer evenings, fireworks that illuminate the yard at dusk use their luminescence to communicate – to attract a couple, repel predators or lure prey.
These glittering mistakes have also inspired MIT researchers. Following the example of nature, they developed electroluminescent soft artificial muscles for flying, insect-sized robots. The tiny artificial muscles that control the wings of the robots emit colored light during flight.
This electroluminescence would allow robots to communicate with each other. For example, if a robot is 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 propagate light also brings these microscopic robots, which weigh just over a paper clip, one step closer to self-flight outside the laboratory. These robots are so light that they can’t carry sensors, so researchers have to track them using large infrared cameras that don’t work well outdoors. Now they have shown that they can accurately track robots using the light they emit and only three smartphone cameras.
“If you’re thinking of large-scale robots, they can communicate with many different tools – Bluetooth, wireless, and more. But because of the tiny, limited-power robot, we’re forced to think of new ways to communicate. “It’s a big step to fly these robots in an outdoor environment where we don’t have a well-tuned, state-of-the-art motion tracking system,” said Kevin Chen, Reid Weedon, Jr.’s assistant. Professor at the Department of Electrical Engineering and Informatics (EECS), Head of the Laboratory of Soft and Microrobotics at the Electronics Research Laboratory (RLE) and Senior Author.
He and his co-workers did this by inserting small electroluminescent particles into the artificial muscles. This process adds only 2.5 percent more weight without affecting the robot’s flight performance.
Chen in this paper is joined by EECS graduates Suhan Kim, lead author, and Yi-Hsuan Hsiao; Yu Fan Chen SM ’14, PhD ’17; and Jie Mao, associate professor at Ningxia University. The study was published this month IEEE Robotics and Automation Letters.
These researchers have previously demonstrated a new manufacturing technique for soft drives or artificial muscles that reveal robot wings. These durable actuators are fabricated by alternating the ultra-thin layers of elastomer and carbon nanotube electrode in a pile and then twisting them into a thin cylinder. When a voltage is applied to that cylinder, the electrodes expel the elastomer and the mechanical stress counteracts the wing.
To make the glow drive, the team incorporated electroluminescent zinc sulfate particles into the elastomer, but had to overcome several challenges.
The researchers first had to create an electrode that would not block light. They built it using highly transparent carbon nanotubes that are only a few nanometers thick and allow light to pass through them.
However, zinc particles ignite only in the presence of a very strong and high-frequency electric field. This electric field excites electrons in the zinc particles, which then emit subatomic light particles known as photons. The researchers use a high voltage to create a strong electric field in the soft drive, and then drive the robot at a high frequency, allowing the particles to light up brightly.
“Traditionally, electroluminescent materials are very expensive in terms of energy, but in a sense we get that electroluminescence for free because we just use the electric field at the frequency we need to fly. We don’t need a new launch, new wires or anything. Light requires only about 3 percent more energy, ”says Kevin Chen.
When developing a prototype drive, they found that the addition of zinc particles reduced its quality, making it easier to break down. To avoid this, Kim mixed the zinc particles only into the top layer of the elastomer. It made that layer a few micrometers thicker to reduce the output power.
Although this made the engine 2.5 percent heavier, it shed light without affecting the flight results.
“We pay a lot of attention to maintaining the quality of the elastomer layers between the electrodes. The addition of these particles was similar to the addition of our elastomer layer to the dust. It took a lot of different approaches and a lot of testing, but we came up with a way to ensure the quality of the drive, ”says Kim.
By controlling the chemical combination of zinc particles, the color of the light changes. The researchers produced green, orange, and blue particles for their manufactured gears; each gear glows in one solid color.
They also adjusted the manufacturing process so that the drives could emit multicolored and patterned light. The researchers put a small mask on the top layer, added zinc particles, and then hardened the gear. They repeated this process three times with different masks and colored particles to create a pattern of light in which MIT was written.
Following the firefighters
After fine-tuning the manufacturing process, they tested the mechanical properties of the gears and used a luminescence meter to measure the light intensity.
From there, they conducted flight tests using a specially designed motion tracking system. Each electroluminescent drive served as an active marker that could be tracked using iPhone cameras. The cameras detect every color of light, and a computer program they create tracks the position and attitude of the robots with an accuracy of 2 millimeters from the latest infrared motion capture systems.
“We are very proud of the good tracking results compared to the latest ones. We used low-cost hardware compared to tens of thousands of dollars for this large motion tracking system, and the tracking 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 include control signals so the robots can turn the lights on and off during the flight and communicate more like real fireworks. They are also exploring how electroluminescence could even improve some of the properties of these soft artificial muscles, says Kevin Chen.
“This work is really exciting because it reduces the cost of generating light (weight and power) without compromising flight performance,” says Kaushik Jayaram, an associate professor in the Department of Mechanical Engineering at the University of Colorado, who was not involved in the study. “The generation of synchronized flash wings shown in this work will facilitate the tracking of multiple microrobot movements and flight control in low light conditions both indoors and outdoors.
“While this work presents the production of light, the memories of biological firefighters, and the potential use of communication, I think the real moment is that this latest development could be an important step toward demonstrating these robots in the field. controlled laboratory conditions, ”adds Pakpong Chirarattananon, an associate professor in the Department of Biomedical Engineering at Hong Kong City University, who was also not involved. “Illuminated drives can act as active external camera markers to provide real-time feedback to stabilize the flight and replace the current motion capture system. Electroluminescence would allow the use of less sophisticated equipment and track robots from a distance, perhaps through another larger mobile robot, to be used in the real world. That would be an extraordinary breakthrough. I would love to see what the authors achieve next.
This work was supported by the MIT Electronics Research Laboratory.
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