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Cornell researchers have combined soft microactuators with high-energy-density chemical fuel to create an insect-sized quadruped robot powered by combustion. These tiny robots can outrace, outfit, outflex, and outleap its electric-driven competitors.

The project was led by Rob Shepherd, an associate professor of mechanical and aerospace engineering at Cornell Engineering. Shepherd’s Organic Robotics Lab has previously used combustion to create a braille display for electronics. The lead author on the paper, which was published in Science, is postdoctoral researcher Cameron Aubin, PhD ’23.

The research team set out to create a small robot that more closely mirrored the capabilities of insects, which can often lift heavy loads despite their size. Ants, for example, can carry 10-50 times their weight. Robots of this size, however, have yet to reach their full potential.

One of the things holding small robots back, according to Aubin, is the fact that motors, engines, and pumps don’t work as well when you shrink them down to size. So, the research team compensated for these drawbacks by creating bespoke mechanisms to perform these functions.

Typical tiny robots are tethered to their power sources, which is usually a battery transmitting electricity. While the team hasn’t created an untethered model yet, according to Aubin the researchers are about halfway there, the current iteration of the team’s robot has a strong force output.

These four-legged robots are just over an inch long, and weigh the same as one and a half paperclips. The robots are 3D-printed with a flame-resistant resin, and their bodies contain a pair of separated combustion chambers that lead to four actuators that serve as feet.

Each actuator is a hollow cylinder capped with a piece of silicone rubber, like a drum skin, on the bottom. These actuators are capable of reaching 9.5 newtons of force, compared to approximately 0.2 newtons for those of similar-sized robots.

The robot uses offboard electronics to create a spark in the combustion chambers to ignite premixed methane and oxygen. The resulting combustion reaction inflates the drum skin on the actuators and the robot pops into the air.

The actuators operate at frequencies greater than 100 hertz, achieve displacements of 140%, and allow the robot to lift 22 times its body weight. The design of the robot enables a high degree of control. By just turning a knob and changing the fuel input, the operator can adjust the speed and frequency of sparking, or carry the fuel feed in real-time, triggering a dynamic range of responses.

With just a little fuel and some high-frequency sparking, the robot will skitter across the ground. With a bit more fuel and less sparking, the robot will slow down and hop. When the fuel is turned all the way up and the robot is given one big spark, it will leap around 23.6 in (60 cm) in the air, roughly 20 times its body length.

“Being powered by combustion allows them to do a lot of things that robots at this scale haven’t been able to do at this point,” Aubin said. “They can navigate really difficult terrains and clear obstacles. It’s an incredible jumper for its size. It’s also really fast on the ground. All of that is due to the force density and the power density of these fuel-driven actuators.”

In the future, the researchers plan to string together more actuators in parallel arrays so they can produce very fine and very forceful articulations on the macro scale. The researchers also plan to continue working on creating an untethered version of the robot. This goal will require a shift from a gaseous fuel to a liquid fuel that the robot can carry onboard, along with smaller electronics. 

Co-authors on the paper include E. Farrell Helbling, assistant professor of electrical and computer engineering; Sadaf Sobhani, assistant professor of mechanical and aerospace engineering; Ronald H. Heisser, Ph.D. ’23; postdoctoral researcher Ofek Peretz; Julia Timko ’21 and Kiki Lo ’22; and Amir Gat of Technion-Israel Institute of Technology.