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Inspired by fireflies, which use their glow to attract mates, ward off predators, and lure prey, the robots were developed by engineers at the Massachusetts Institute of Technology (MIT).
The light-up actuators also enable extremely low-weight tracking capabilities, expanding their potential uses without the addition of heavy sensors.
“If you think of large-scale robots, they can communicate using a lot of different tools – Bluetooth, wireless, all those sorts of things. But for a tiny, power-constrained robot, we are forced to think about new modes of communication.
“This is a major 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, head of the Soft and Micro Robotics Laboratory in the Research Laboratory of Electronics (RLE) and senior author of a paper on the work.
The electroluminescence could enable signalling between the machines during search-and-rescue missions, for example. The ability to emit light also means they can be precisely tracked using smartphone cameras, instead of the bulky infrared cameras that would otherwise be required.
The artificial muscles that flap the wings of the robot were made by alternating ultrathin layers of elastomer and carbon nanotube electrode in a stack, and then rolling it into a ‘squishy cylinder’. When a voltage is applied, the electrodes squeeze the elastomer and the mechanical strain flaps the wing.
To make the actuators glow, the team incorporated electroluminescent zinc sulphate particles into the elastomer. The researchers used high voltage to create a strong electric field in the soft actuator and drive the robot at a high frequency, which enabled the particles to light up brightly.
“Traditionally, electroluminescent materials are very energetically costly, but in a sense, we get that electroluminescence for free because we just use the electric field at the frequency we need for flying. We don’t need new actuation, new wires, or anything. It only takes about 3% more energy to shine out light,” said Chen.
As they prototyped the actuator, the researchers found that adding zinc particles reduced its quality, causing it to break down more easily. To get around this, lead author Suhan Kim mixed zinc particles into the top elastomer layer only. He made that layer a few micrometres thicker to accommodate for any reduction in output power. This made the actuator 2.5% heavier, but it emitted light without impacting flight performance.
The team ran 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 detected each light colour, and a computer program tracked the position and attitude of the robots to within 2mm 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 large motion-tracking systems cost, and the tracking results were very close,” said Chen.
In the future, the researchers plan to enhance the motion tracking system so it can track robots in real-time. They are also working to incorporate control signals so the robots can turn their light on and off during flight, communicating more like real fireflies. Electroluminescence might even improve some properties of the soft artificial muscles, Chen said.
The research was published in IEEE Robotics and Automation Letters.
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