Robot With Bimetal Feet Can Walk in a Frying Pan Forever

With no sensors, motors, or actuators, hot feet are all this robot needs to walk

2 min read
Robot With Bimetal Feet Can Walk in a Frying Pan Forever

A bimetal consists of two different kinds of metal stuck together. Bimetals are different than alloys, where the two metals are blended; in a bimetal, the two metals are just layered on top of each other. The reason to do this is to take advantage of the different characteristics of different metals when they’re heated: they expand at different rates, causing your piece of bimetal to deform until it cools off again. Essentially, a bimetal is a way to convert heat directly into mechanical energy, and researchers at the University of Tokyo have come up with a way to leverage this to get a robot to walk. The robot has no sensors and no actuators, and as long as it’s got a hot surface to walk on, it can keep going pretty much forever.

The trick to this is a combination of two things. First, Thermobot’s feet are heavily weighted at the backs, meaning that when the bimetal on one foot deforms and the robot rocks sideways, the foot swings forward. At the same time, the weights on Thermobot’s legs control the distance that the robot tips laterally. If everything is calibrated properly, Thermobot will tip back onto the swinging foot when it’s ahead of the body, and the robot has taken a step.


It’s important to note that this technique won’t work in an environment that’s hot overall: the surface has to be significantly hotter (50 degrees Celsius, ideally) than the rest of the environment in order to give the bimetal a chance to cool, so the cycle can repeat itself. In this particular case, Thermobot is walking on a hot plate that’s 170 degrees C in a 26 degree room. However, it doesn’t matter that much what the absolute temperatures are, just that there’s a significant relative difference between them, meaning that Thermobot is quite happy to walk in 300 degrees C temperatures as long as the surface is hotter than ambient.

This is certainly one of the more novel walking techniques we’ve ever seen, especially since it’s completely passive, able to walk on a level surface by harvesting heat. We have to admit, though, that no practical applications immediately leap to mind. The researchers say that they’re working on combinations of metals that will allow Thermobot to operate at much lower temperatures, perhaps even safe temperatures, and that it might make for a pretty cool toy. They’re also going to be working on “other locomotion mechanisms that can be realized using the self-oscillation of bimetal sheets,” meaning that we could see more creative robots making use of this tech in the near future.

“Thermobot: A Bipedal Walker Driven by Constant Heating,” by Takeru Nemoto and Akio Yamamoto from The University of Tokyo, was presented this week at IROS 2015 in Hamburg, Germany.

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Robot with threads near a fallen branch

RoMan, the Army Research Laboratory's robotic manipulator, considers the best way to grasp and move a tree branch at the Adelphi Laboratory Center, in Maryland.

Evan Ackerman

This article is part of our special report on AI, “The Great AI Reckoning.

"I should probably not be standing this close," I think to myself, as the robot slowly approaches a large tree branch on the floor in front of me. It's not the size of the branch that makes me nervous—it's that the robot is operating autonomously, and that while I know what it's supposed to do, I'm not entirely sure what it will do. If everything works the way the roboticists at the U.S. Army Research Laboratory (ARL) in Adelphi, Md., expect, the robot will identify the branch, grasp it, and drag it out of the way. These folks know what they're doing, but I've spent enough time around robots that I take a small step backwards anyway.

The robot, named RoMan, for Robotic Manipulator, is about the size of a large lawn mower, with a tracked base that helps it handle most kinds of terrain. At the front, it has a squat torso equipped with cameras and depth sensors, as well as a pair of arms that were harvested from a prototype disaster-response robot originally developed at NASA's Jet Propulsion Laboratory for a DARPA robotics competition. RoMan's job today is roadway clearing, a multistep task that ARL wants the robot to complete as autonomously as possible. Instead of instructing the robot to grasp specific objects in specific ways and move them to specific places, the operators tell RoMan to "go clear a path." It's then up to the robot to make all the decisions necessary to achieve that objective.

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