Home - EPFL Engineers Use Sound Resonance to Power Microscale Robots Research EPFL Engineers Use Sound Resonance to Power Microscale Robots Aura Moreno 2 days ago 0 Fitting a motor and battery into a robot shrinks fast as the robot gets smaller, and at microgram scales it becomes close to impossible. Researchers at the Swiss Federal Institute of Technology in Lausanne (EPFL) have proposed a way around that limit: powering small robots with sound instead of onboard mechanical components, opening a path to machines at sizes where conventional propulsion simply does not fit. How Acoustic Resonance Generates Thrust The approach relies on Helmholtz resonance, the effect that produces a tone when air is blown across the neck of a bottle. The team 3D printed hollow cavities with narrow necks, tuned so that sound at a specific frequency causes the air inside to oscillate strongly enough to generate a directional jet, and with it, usable thrust. Resonators were fabricated from centimeter to micrometer scale, operating between 200 Hz and 40 kHz, with measured resonance frequencies landing within 5 percent of theoretical predictions. Testing resonators of different sizes showed thrust increasing linearly with cavity volume, adding roughly 44 micronewtons of force per additional cubic centimeter. Cavity shape made little difference to output once a resonator was small relative to the sound’s wavelength, but neck geometry proved decisive: resonators without a neck produced substantially less thrust, straight necks outperformed converging or diverging shapes, and thinner walls increased output by about 35 percent over thicker ones. Glass, PLA, and soft elastomer resonators all produced comparable thrust despite sharply different stiffness. Steerable Boats Powered by Sound Alone The team validated the concept first with miniature boats. A single resonator produced forward motion; a boat fitted with three resonators tuned to different frequencies, one for propulsion and two for steering, was piloted along a predefined infinity-shaped path and maneuvered around obstacles, with a mean tracking error of 5.7 millimeters. The researchers then moved the sound source onto the robot itself, attaching a small transducer directly to the resonator instead of relying on an external speaker. That allowed them to build a fully untethered boat carrying its own battery, control electronics, and two vibration actuators, communicating over Bluetooth with about 30 milliseconds of latency. The boat traced the letters “EPFL,” navigated around obstacles under manual and pre-programmed control, and recovered its course after being disturbed by a physical push or a blast of air. Helmholtz resonators as wireless actuators in air. Image via Hwang et al., Science Advances. Two Designs for Microscale Flight At microscale, the team built two flier designs using two-photon polymerization (2PP), a high-resolution 3D printing method.