The Robot of Theseus: A modular robotic testbed for legged locomotion

TL;DR

The Robot Of Theseus (TROT) addresses a key gap in locomotion research by providing a low-cost, morphologically modular quadruped designed for biomechanical hypothesis testing. Its telescoping limbs, modular joints, proprioceptive actuators, and open-source hardware/software enable rapid exploration of how morphology and control shape whole-body dynamics, including ground-reaction forces and costs of transport. The study demonstrates onboard torque-based GRF estimation, morphology-driven changes in limb kinematics, and significant COT differences arising from limb inertia, validating the platform’s utility for biology–robotics research and control theory development. By democratizing access to a versatile, open platform, TROT promises to shorten the sim-to-real gap and enable broad empirical investigations across extant, extinct, and theoretical morphologies.

Abstract

Robotic models are useful for independently varying specific features, but most quadrupedal robots differ so greatly from animal morphologies that they have minimal biomechanical relevance. Commercially available quadrupedal robots are also prohibitively expensive for biological research programs and difficult to customize. Here, we present a low-cost quadrupedal robot with modular legs that can match a wide range of animal morphologies for biomechanical hypothesis testing. The Robot Of Theseus (TROT) costs approximately $4000 to build out of 3D printed parts and standard off-the-shelf supplies. Each limb consists of 2 or 3 rigid links; the proximal joint can be rotated to become a knee or elbow. Telescoping mechanisms vary the length of each limb link. The open-source software accommodates user-defined gaits and morphology changes. Effective leg length, or crouch, is determined by the four-bar linkage actuating each joint. The backdrivable motors can vary virtual spring stiffness and range of motion. Full descriptions of the TROT hardware and software are freely available online. We demonstrate the use of TROT to compare locomotion among extant, extinct, and theoretical morphologies. In addition to biomechanical hypothesis testing, we envision a variety of different applications for this low-cost, modular, legged robotic platform, including developing novel control strategies, clearing land mines, or remote exploration. All CAD and code is available for download on the TROT project page.

Figures (4)

• Figure 1: Description of TROT features. A) TROT in a three-link limb configuration with femur and tibia. Link 1 is outlined in red. B) In a four-link limb, the tarsus angle is parallel to the femur angle. Limbs can be oriented with knees backward (as shown) or forward.
• Figure 2: A comparison of possible TROT limb length ratios (purple boxes) to animal limb length ratios across multiple families of quadrupedal mammals Gambaryan. The different shades of purple indicate the possible limb length ratios when constructed with gearboxes of different gear ratios.
• Figure 3: Empirical results for onboard sensor validation for the front right limb during TROT torso rotations in a) pitch, b) roll, and c) yaw. TROT estimates of ground reaction force for one limb from the proprioceptive actuators (green) were compared to data from a force platform (purple) as the ground truth. Six rotations in pitch, yaw, and roll were performed sequentially for each limb.
• Figure 4: Comparisons of TROT (a) hind and (b) fore foot trajectory across different limb length ratios. The zero value for the X-axis was determined by the hip (actuators X2, X3) center of rotation. The zero value for the y-axis was the elevation when the foot was in contact with the substrate. The tracking marker was placed approximately 3.5 cm above the center of the robot foot, along the robot leg, which allows the x-value to fall below zero.