Combustion-Powered, Bug-Sized Robots can Leap, Lift, and Race

Andrew Corselli

As anyone who has witnessed ants at a picnic knows, insects are far stronger than their size suggests. However, robots at that scale have yet to reach their full potential. One of the challenges is “motors and engines and pumps don’t really work when you shrink them down to this size,” said Lead Author Cameron Aubin, so researchers have tried to compensate by creating bespoke mechanisms to perform such functions. So far, the majority of these robots have been tethered to their power sources — which usually means electricity.

“We thought using a high-energy-density chemical fuel, just like we would put in an automobile, would be one way that we could increase the onboard power and performance of these robots,” he said. “We’re not necessarily advocating for the return of fossil fuels on a large scale, obviously. But in this case, with these tiny, tiny robots, where a milliliter of fuel could lead to an hour of operation, instead of a battery that is too heavy for the robot to even lift, that’s kind of a no brainer.”

While the team has yet to create a fully untethered model — Aubin says they are halfway there — the current iteration “absolutely throttles the competition, in terms of their force output.”

The four-legged robot, which is just over an inch long and weighs the equivalent of one and a half paperclips, is 3D-printed with a flame-resistant resin. The body contains a pair of separated combustion chambers that lead to the four actuators, which serve as the feet. Each actuator/foot is a hollow cylinder capped with a piece of silicone rubber, like a drum skin, on the bottom. When offboard electronics are used to create a spark in the combustion chambers, premixed methane and oxygen are ignited, the combustion reaction inflates the drum skin, and the robot pops up into the air.

The robot’s actuators are capable of reaching 9.5 newtons of force, compared to approximately 0.2 newtons for those of other similarly sized robots. It also operates at frequencies greater than 100 hertz, achieves displacements of 140 percent, and can lift 22 times its body weight.

The work was sponsored by the Air Force, Navy, and NSF.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Aubin.

Tech Briefs: I’m sure there were too many to count, but what was the biggest technical challenge you faced while developing these robots?

Aubin: This might seem kind of obvious, but the biggest technical challenge was building an insect-sized device that could properly harness the large chemical potential of hydrocarbon fuels, without being damaged or destroyed. Although the volumes are really small (we’re talking fractions of a milliliter here), we are still dealing with combustibles. Each movement of the robot is triggered by a tiny, rapid explosion — it more or less has a miniature built-in engine. We had to do a lot of testing to sort of wrap our heads around what is happening at that small scale and bring the physics/chemistry under our control. Everything from the weight of the robot to the volume of the combustion chambers, to the size of the ignition spark, to the fuel-oxygen mixture flowing into the robot had to be finely tuned in all of our early testing.

Tech Briefs: Can you explain in simple terms how they work?

Aubin: The robots are small, 4-footed devices. They weigh 1.6 grams, which is about one and a half paper clips in mass, (or slightly lighter than your average gummy bear). They function by having a fuel (methane) and oxygen mixture being constantly pumped into their bodies, which are hollow. The robot actually has two hollow cavities, a left half and a right half. Each cavity has a pair of thin wires going into it. We can create a spark across those wires using offboard high-voltage electronics, and that spark ignites the gas and causes a miniature explosion. This leads to an expansion of hot gases. Each hollow chamber is capped by a pair of thin, flexible membranes, which attach to the feet. The expanding gases cause these membranes to rapidly inflate, kind of like little balloons, and this presses against the feet and makes the robot jump.

We can control a lot of parameters in real time, like the sparking frequency or the fuel chemistry, to make bigger, smaller, or more frequent explosions. This allows us to control the robot’s gait. It can crawl, hop, or make great leaps. We can also steer the robot by performing combustion in just one of the robot's two halves. This turns the robot in place.

Tech Briefs: Any future research/work/etc. on the horizon? In other words, what are your next steps?

Aubin: There are lots of smaller projects that sort of branch from this one project. The main thing we are working toward, though, is untethering these devices. That's sort of the hill (or maybe mountain is more appropriate) that all insect-scale robots and actuators need to climb if they are to be truly useful in the field. Part of what’s attractive about using chemical fuels is that they have a much higher energy density than batteries, so a system at this scale running on a small amount of self-contained fuel could potentially possess much greater operating times than a battery-powered one.

We want to demonstrate that in practice. What our current work does is show the performance benefits (high force, relatively large deformation, high power density) of these types of actuators. Taking it to the next level means untethering everything. That means taking all of our offboard electronics and miniaturizing them to fit on the robot. It also requires us to think about new types of fuels that can be packaged onboard.

Tech Briefs: You’re quoted as saying, “Everybody points to these insect-scale robots as being things that could be used for search and rescue, exploration, environmental monitoring, surveillance, navigation in austere environments. We think that the performance increases that we’ve given this robot using these fuels bring us closer to reality where that’s actually possible.” How far away from that being reality do you estimate we are?

Aubin: It’s tough to say exactly. Something like 10 years could be a reality if some major breakthroughs are made. Or, more likely, it could be much longer. The fields of microrobotics and insect-scale robotics are still quite young. There are still many questions that need to be answered.

Tech Briefs: Do you have any advice for engineers/researchers aiming to bring their ideas to fruition?

Aubin: I always tell those that are new to research to "get comfortable being uncomfortable." Research (and particularly experimentalist research like in our robotics work) is an incremental, often slow process. The answers you are seeking are not readily apparent, because the questions you are asking may have never been asked/answered before. It makes it tough to measure progress or appreciate what you are doing sometimes. It's also easy to get caught in a rut. I try to make sure my newer researchers understand this paradigm, so that they don't get weighed down by the pace of things. It's also important to celebrate accomplishments, however small they may be.

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