The Robot Of Theseus: a modular robotic testbed for legged locomotion.

Robotic models of biological systems are useful for independently varying specific features to determine their contribution to whole-system behavior, 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 3D printable, 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 ≈$4000 to build out of 3D printed parts and standard off-the-shelf supplies. There are three main mechanisms to enhance morphological modularity: (1) each limb can consist of 3 or 4 rigid links, (2) the direction of the femur-tibia joint can be easily switched to mimic a knee or elbow, and (3) telescoping mechanisms allow users to 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 with assembly and user guides. We demonstrate the use of TROT to compare locomotion among extant, extinct, and theoretical morphologies. We found that a 29% percent increase in leg moment of inertia resulted in a 28.3% increase in cost of transport. 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 atwww.embirlab.com/trot.