A new study details a hybrid robot that combines the wind-driven mobility of tumbleweeds with active quadcopter control, offering a new paradigm for energy-efficient terrestrial exploration. This innovation addresses the current lack of wind-exploiting systems in terrestrial exploration and the complexity of large, drag-driven robots. The study was published in *Nature Communications*.
The inspiration for the Hybrid Energy-efficient Rover Mechanism for Exploration Systems, or HERMES, came to Sanjay Manoharan, a Ph.D. researcher at EPFL's Laboratory for Advanced Fabrication Technologies and first author of the study. Manoharan observed kite surfers harnessing wind along the shores of Lake Neuchâtel, which led him to consider nature's own wind-dispersed designs.
He then focused on tumbleweeds, known for traversing vast desert distances using only ambient wind, while also spreading seeds. Researchers noted that these chaotic balls of twigs generate more drag than solid spheres, posing a scientific puzzle.
To understand this, the researchers used computational fluid dynamics and wind tunnel experiments, discovering a previously unreported structural feature. Tumbleweeds possess a vertical porosity gradient, with approximately 60% porosity at the top and 40% at the bottom. This asymmetry fundamentally alters wake dynamics and dramatically enhances pressure drag.
"In the upright position, the upper half, being more porous, allowed airflow to pass through freely. In contrast, the lower half was denser and thus offered greater resistance," the researchers explained in their paper. When inverted, the denser region forces air around its perimeter like a solid sphere. The porous lower portion then creates dual-lobed wake patterns.
At 12 metres per second winds, tumbleweeds generated 50% more drag than solid spheres, despite their porosity. The team also discovered orientation-dependent lift forces that produce the tumbleweed's characteristic rolling behaviour, including somersaults at low winds and hops at high speeds.
For robotics applications, the researchers designed lightweight spherical shells with engineered porosity gradients, based on these insights, using selective laser sintering. The bio-inspired sphere outperformed both natural tumbleweeds and solid spheres. It rolled effortlessly at just 1 metre per second wind speed while generating substantially higher drag forces.
"The structure had to be strong enough to survive rolling and impacts, porous enough to generate the drag for passive motion and still roomy enough to hold sensors or a propulsion system," Manoharan explained. Achieving this balance required custom computational modelling.
Loaded with payloads several times its own weight, the sphere climbed steep slopes and formed GPS-equipped mesh networks in field tests. It autonomously dispersed across terrain while transmitting geotagged environmental data over long ranges.
While passive wind-driven motion offers unmatched energy efficiency, it faces a critical limitation: stagnation. "When the wind drops or the terrain gets complicated, they get stuck," Manoharan noted.
To address this, the team embedded a lightweight quadcopter within the porous sphere. The system operates in four modes: tumbling for terrain reorientation, spinning for directional changes, gliding for ground-level movement, and aerial mode for clearing obstacles.
"The guiding philosophy is beautifully simple and energy-aware," Manoharan explained. "If the wind is blowing and the robot is rolling, it remains perfectly passive, spending zero energy." "If motion stops for a set period, it attempts a low-energy nudge—a quick motor pulse to reposition. Flight is always the last resort."
Laboratory tests demonstrated remarkable efficiency. In maze navigation experiments, HERMES consumed 48% less energy than the active-only control, using 26 mWh versus 50 mWh. It completed the course 37% faster, taking 105 seconds versus 166 seconds.
Brief motor bursts of just 0.25 to 0.5 seconds enabled 25 to 50 degree course corrections with 90 to 95% energy savings compared to continuous actuation.
Researchers envision HERMES enabling planetary exploration, post-disaster mapping, and landmine detection. On Mars, wind-driven swarms could perform wide-area biomarker sweeps, trading pre-planned routes for decentralised coverage and serendipitous discovery, according to Manoharan.
"Current missions rely heavily on careful planning—it's impractical for a single rover to explore vast areas," Manoharan said.
On Earth, the robots could map radiation or toxic plumes in disaster zones without risking lives. They could also drift over minefields in regions like Ukraine, Afghanistan, and Yemen to flag hazards safely. Field tests showed them handling rocks and roots surprisingly well with wind alone.
These robots formed GPS mesh networks to relay geotagged data. Tall grass proved a stubborn limit, however. Lab surprises included brief 0.25-second thrusts reorienting the robot for wind to take over.
Future work targets autonomy via IMU-based finite state machines, solar energy harvesting, swarm coordination, and adaptive shells that tune porosity on the fly. Current two-minute hovers limit sustained flight, but selective actuation already slashes energy use. Manoharan reflected on his initial insight, "What if we stopped fighting and started sailing?"
A new hybrid robot, HERMES, combines wind-driven mobility with quadcopter control for energy-efficient exploration.
Inspired by tumbleweed aerodynamics, the robot uses a unique vertical porosity gradient for enhanced drag and rolling.
HERMES consumes significantly less energy and completes tasks faster than active-only systems due to its hybrid design.
Source: TECHXPLORE
