Martian-Inspired Tripod Walking Robot Generates Its Own Gaits

Why don't animals have three legs? This robot is trying to figure it out

3 min read
This Osaka University three-legged robot is called Martian Petit
Osaka University researchers hope their robot will provide some answers about how animals have evolved to move the way they do, and perhaps why none of them have three legs.
Photo: Osaka University

When Yoichi Masuda set out to design a new legged robot, he found inspiration in the Martian Tripods from the classic sci-fi novel “The War of the Worlds” by H.G. Wells. A three-legged configuration seems to offer some advantages when it comes to walking and balancing, and Masuda became curious about the absence of three-legged animals in nature. Are there evolutionary factors that explain why we haven’t seen any? And if three-legged creatures existed, could there be a universal principle of walking locomotion, common for bipeds, tripeds, and quadrupeds? To explore those questions, Masuda and his colleagues at Osaka University built a three-legged robot named Martian.

This Osaka University three-legged robot is called Martian Petit The Martian Petit robot built by Osaka University researchers. Photo: Osaka University

Why exactly there aren’t any tripedal creatures is an interesting question. There are a few animals (like the kangaroo and the tripod fish, which totally exists) that are sort of tripedal, in that they use two limbs and a tail for mobility. However, both of these animals exhibit bilateral symmetry, meaning that their left half is a mirror image of their right half. The vast majority of animals are symmetrical like this, although there are interesting (and very minor and specific) exceptions including crossbills, owls, some crabs, a few sorts of whales, and of course the mighty honey badger, which has one extra tooth on the left side. There are also organisms with radial and spherical symmetry.

Were the Martians and their Tripod machines in “War of the Worlds” on to something, or is there some fundamental reason why tripedal locomotion isn’t as efficient or useful as bipedal or quadrupedal locomotion?

But all this doesn’t really answer the question of what’s wrong with trilateral symmetry: Were the Martians and their Tripod machines in “War of the Worlds” on to something, or is there some fundamental reason why tripedal locomotion isn’t as efficient or useful as bipedal or quadrupedal locomotion?

A decade ago, Dennis Hong and his team at RoMeLa developed a tripedal robot called STriDER, the Self-excited Tripedal Dynamic Experimental Robot, which they used to test a variety of tripedal gaits. The experiments showed that STriDER was able to walk, but like most tripedal robots that we’ve seen, it was more like an intermittent biped, primarily using two of its three limbs at a time.

Yoichi Masuda’s tripedal design is different in that regard. His robot has not only trilateral symmetry but a trilateral gait as well. In other words, the three legs are each moving in a similar way, except they are out of phase:

This robot, Martian Petit, has three highly compliant leg modules, and is able to move in circles as well as in a line without any one leg knowing what the other legs are doing, as the researchers explain in a recent paper:

The control scheme . . . consists of a phase oscillator and sensory feedback of reaction force from the ground, where the control law for each leg is decoupled from the others (i.e., it has no explicit feedback of the other legs’ information). We show that rotary and forwarding locomotion successfully emerge using the control method, depending on the choice of frequency ratio of the oscillators.

The kind of decentralized approach described here has already been successfully applied to other kinds of biomimetic robots, and different kinds of stable gaits can emerge autonomously. It’s no different with the tripod robot: Depending on what oscillation frequency is selected, the robot can be made to either rotate or move in a single direction, without having that specific gait programmed into it. The researchers are hoping that as they continue working on their robot, it’ll provide some answers about how animals have evolved to move the way they do, and perhaps why none of them have three legs.

Osaka University tripod robot

Photo: Osaka University
Tripod robot with 3-DoF legs.

While this is the third generation of Martian robot the Osaka University researchers have built, it was intentionally designed with very simple dynamics to make it easier to approach the gait generation problem. The researchers, led by Masato Ishikawa, a professor of mechanical engineering at Osaka University, are already working on a new version with a few extra degrees of freedom (and consequently more complex dynamics) in the legs.

“On Decentralized Control of Tripedal Walking Robot Using Reaction Force Feedback,” by Masato Ishikawa, Naoto Yasutani, and Ryoichi Kuratani from Osaka University, will be presented at CLAWAR 2017. 

“Simplified Triped Robot for Analysis of Three-Dimensional Gait Generation,” by Yoichi Masuda and Masato Ishikawa from Osaka University, appears in the Journal of Robotics and Mechatronics.

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How the U.S. Army Is Turning Robots Into Team Players

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11 min read
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

“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.

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

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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