A cost-effective, customizable quadruped could help researchers discover the particular advantages related to the length and segmentation of animal limbs
Study: The Robot of Theseus: A modular robotic testbed for legged locomotion (DOI: 10.1088/1748-3190/ae3ec1)
What is it about a cheetah's build that enables it to run so fast? What gives the wolf its exceptional endurance?
While these questions can be partly answered through animal experiments, many contributing factors can't be isolated from one another. Now, a new tool has arrived: a highly customizable, open-source robot design called The Robot of Theseus, or TROT, developed at the University of Michigan.
Named in homage to Greek philosophy's "Ship of Theseus," the robot is composed of commercially available motors and 3D-printed parts, which can be rearranged to take on a broad array of designs. The plans address several pain points for animal researchers who might be able to harness robotics for biomechanical experiments, as well as for roboticists seeking more task-specific designs. Assuming access to 3D printers, the cost in parts and materials is under $4,000.
"In paleontology, we can go back and look at bones, but it's really difficult to understand how these changes in limb proportion, or in range of motion, may have affected the way an animal can move. There have been some really great insights on this question from robots that each mimic one extinct animal very precisely," said Talia Moore, assistant professor of robotics with a background in evolutionary biology and corresponding author of the study in Bionspiration and Biomimetics. "But each robot took years to design and construct.
"I wanted to make a robot that could easily shapeshift into several different extinct species proportions, so that we could compare them, and see how the evolution of those limb lengths and other features would affect their locomotion. With TROT, 60 million years of evolutionary changes in body size can happen in 20 minutes."
Usable, customizable and easy to measure
The modular robot plans and assembly guides offer three major benefits. First, they are usable by people without robotics degrees, with help from equipment that is available at many universities. As Moore pointed out, robotics offers insights into biological questions, but not many evolutionary labs have the benefit of robotics expertise.
Second, the robot's shape is highly customizable. While the published study focuses on four-legged designs, experimenters can change nearly any body segment-adding and removing parts, changing the range of motion and more. This means TROT can model most mammals and enable direct comparisons of variations on the same structure-for instance, between closely related extant and extinct species. And they can try out theoretical designs to determine whether they are biomechanically unfavorable or just untried by evolution.
Third, researchers mimicked the springiness and stiffness of muscular structures without actual springs or elastics, which can muddy measurements. TROT simulates this biological energy storage and return mechanism with backdrivable motors, which recover energy as they are driven backwards.
"Traditional robots are designed with an emphasis on industrial applications and are expensive to make. TROT was designed with ease of fabrication in mind," said Karthik Urs, a recent master's graduate in robotics and first author of the study.
"The overall part count is kept low, and most of the parts only fit together one way. That means that scientists can make most of the robot parts in-house with commodity 3D printers, assemble them and get to experimenting faster. It also makes the iteration process quick-key to enabling exploration in both robot and experimental design."
Isolating biomechanical factors that are tough to measure in animals
Moore was first inspired to make this robot when reading a 1974 experiment on running cheetahs and goats. Because the leg swings from the hip like a pendulum, physics holds that legs with more mass away from the hip, known as a greater moment of inertia, require more energy to redirect than legs that weigh the same but have most of the mass near the hip. This concept has informed the interpretation of evolutionary changes in legs-increasingly tapered limbs are likely associated with more efficient running.
However, the 1974 experiment showed that although a cheetah has a more favorable moment of inertia in its limbs, running costs nearly the same amount of energy as it does for a goat. Because so much else was different between these animals, Moore explained, the benefit from a lower moment of inertia was basically unmeasurable. In contrast, Moore's group varied only the weight distribution in their robot's limbs and was able to isolate the exact amount of energetic cost or benefit associated with that change.
TROT is designed for research and teaching rather than for operational robot work-while some 3D-printed parts break easily, they are also easy to repair and replace. Still, the results of future studies with this robot could inform commercial designs. At present, most commercial quadrupeds have fore and hind legs of the same length and style, but this test robot could reveal how to optimize the legs for the robot's intended purposes and terrains, and quantify whether the gains are worth the increase in manufacturing costs.
Researchers and enthusiasts can download the plans for the robot from U-M. The printing instructions for the parts are largely written for typical resin 3D printers, known as fused deposition modeling printers, with a stereolithography printer needed for a couple of components.
Urs is now the lead spacecraft engineer at Argo Space.
