Researchers from Cornell have tackled the small-scale actuation problem with what is essentially a very tiny, very soft internal-combustion engine. Methane vapor and oxygen are injected into a soft combustion chamber, where an itty-bitty li’l spark ignites the mixture. In half a millisecond, the top of the chamber balloons upward like a piston, generating forces of 9.5 newtons through a cycle that can repeat 100 times every second. Put two of these actuators together and you’ve got an exceptionally powerful soft quadruped robot. Each of the two actuators powering this robot weighs just 325 milligrams and is about a quarter of the size of a U.S. penny. Part of the reason that they can be so small is that most of the associated components are off board, including the fuel itself, the system that mixes and delivers the fuel, and the electrical source for the spark generator.
But even without all that stuff, the actuator has a bunch going on that enables it to operate continuously at high cycle frequencies without melting. The biggest issue may be that this actuator has to handle actual explosions, meaning that careful design is required to make sure that it doesn’t torch itself every time it goes off. The small combustion volume helps with this, as does the flame-resistant elastomer material and the integrated flame arrestor. Despite the violence inherent to how this actuator works, it’s actually very durable, and the researchers estimate that it can operate continuously for more than 750,000 cycles (8.5 hours at 50 hertz) without any drop in performance. The robot is 29 millimeters long and weighs just 1.6 grams, but it can jump a staggering 59 centimeters straight up and walk while carrying 22 times its own weight.
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