3D-printed sound-powered jet engines propel micro drones — fliers are completely silent; researchers use ultrasonic frequencies to drive 12,000-RPM silent hovering fliers

A research team from Switzerland’s École Polytechnique Fédérale de Lausanne (EPFL) Microbiorobotic Systems Laboratory (MICROBS) has developed a “sound-powered engine” that uses the resonant frequency of a specially shaped 3D-printed cavity to “energize” the air inside it and use it for thrust. The researchers then 3D-printed two small drones that were propelled by sound.

Sound waves at specific frequencies made the air molecules in these objects oscillate much more strongly and are then pushed out a specially designed nozzle as a concentrated jet.

“Instead of pushing devices around with sound waves, we have created acoustic resonators that are tuned to harness sound at specific frequencies to generate directional thrust and controlled motion,” MICROBS head Selman Sakar told the publication. “Our work shows the feasibility of transforming a simple, cleverly designed mechanical piece into robot matter.”

The team has created three practical applications of the “engine.” One is a boat that used three resonators — one placed at the rear of the boat and tuned to 540 Hz for propulsion and two on either side for navigation — while the other two are microfliers or tiny flying machines powered by ultrasonic frequencies.

The first microflier has three cavities, all facing downward, allowing it to lift off the ground like a rocket. The second flier has three blades connected at a central point, like the rotor of a helicopter, with the base of each blade equipped with a resonator pointing backward. When the resonators are hit with the exact frequency that drives them, they deliver thrust that allows them to rotate at over 12,000 rpm and cause the entire thing to hover. Note that ultrasonic frequencies are inaudible to humans, meaning these devices can hover practically noiselessly.

While not exactly the same as xMEMS’ fan-on-a-chip cooling solution, it follows the same principle of using vibrating air molecules to cause airflow. The biggest difference here is that the fan-on-a-chip uses an ultrasonic piezoelectric membrane to excite the air and cause it to move to a desired direction, while the “sound-powered engine” uses a specific cavity shape and resonant frequencies to achieve the same effect.

Even though the team has successfully used it for experiments, the microfliers’ maximum altitude is less than 5mm while the boat weighed next to nothing. Nevertheless, now that they’ve proven that the concept works, the next step is to scale it up to carry payload. If they can make it work, it could potentially be used as propulsion for a micro-drone, like this mosquito-sized unit from China.

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