What aspects of a cheetah’s anatomy allow it to sprint at incredible speeds? What features give a wolf its remarkable stamina? While animal studies can provide some insights, many of the factors involved are difficult to separate. Researchers at the University of Michigan have introduced TROT, a versatile open-source robotic platform.
A Versatile, Low-Cost Robot Inspired by Evolution
Named after the “Ship of Theseus,” the robot uses off-the-shelf motors and 3D-printed parts for versatile designs. The platform aids animal researchers studying biomechanics and roboticists needing adaptable models. With access to a 3D printer, the total cost of parts and materials is under $4,000.
“In paleontology, bones tell us little about how limb changes affected movement,” said Talia Moore, assistant professor of robotics and the study’s corresponding author. “Some robots have provided great insights by precisely replicating extinct species, but each one took years to design and build.
“I wanted a robot that could quickly mimic multiple extinct animals to study how limb changes affected locomotion.” With TROT, 60 million years of evolutionary adaptation can be explored in just 20 minutes.”
Modular, Flexible TROT
The modular robot’s plans and assembly guides provide three key advantages. First, they can be used by individuals without formal robotics training, relying on equipment commonly available at many universities. As Moore noted, robotics can shed light on biological questions, yet few evolutionary biology labs have in-house robotics expertise.
Second, the robot’s form is highly adaptable. Although the study highlights four-legged configurations, users can modify nearly any body segment—adding or removing parts, adjusting the range of motion, and more. This flexibility allows TROT to model most mammals and facilitates direct comparisons between structural variations, such as those between closely related living and extinct species. Researchers can also test theoretical designs to see whether they are biomechanically unviable or simply unexplored by evolution.
Third, the robot replicates the springiness and stiffness of muscles without using physical springs or elastic components, which can interfere with measurements. Instead, TROT uses backdrivable motors to simulate biological energy storage and return, recovering energy as the motors are driven in reverse.
TROT Designed for Easy, Rapid Assembly
“Traditional robots are usually built for industrial purposes and are costly to produce. “TROT, by contrast, was designed for easy fabrication,” said Karthik Urs, the study’s first author.
“The robot keeps the total number of parts low, and most components only fit together one way. This allows scientists to produce the parts in-house with standard 3D printers, assemble them quickly, and start experiments sooner. It also speeds up the iteration process, which is crucial for exploring both robot design and experimental setups.”
Moore was inspired to create TROT after reading a 1974 study on running cheetahs and goats. Because a leg swings from the hip like a pendulum, physics dictates that legs with more mass farther from the hip—a higher moment of inertia—require more energy to swing than legs of the same weight concentrated near the hip. This principle has helped researchers interpret evolutionary changes in limb design, suggesting that increasingly tapered legs likely improve running efficiency.
TROT Isolates the Impact of Limb Weight on Energy Use
The 1974 study showed that, despite a cheetah’s more favorable limb moment of inertia, its running required nearly the same energy as a goat’s. Moore explained that because so many other factors differed between the animals, the advantage of a lower moment of inertia was essentially undetectable. By contrast, her team used TROT to vary only the limb weight distribution, allowing them to precisely measure the energetic cost or benefit of that single change.
TROT is intended for research and teaching rather than practical robot applications. While some 3D-printed parts are fragile, they are simple to repair or replace. Nonetheless, insights from studies using this robot could inform commercial designs. Most existing quadrupedal robots have fore and hind legs of identical length and design, but TROT could help determine optimal leg configurations for specific tasks or terrains and evaluate whether the performance gains justify additional manufacturing costs.
Researchers and hobbyists can download the robot’s plans from the University of Michigan. Most parts are designed for standard fused deposition modeling (FDM) 3D printers, with only a few components requiring a stereolithography (SLA) printer.
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