Serpens, a low-cost snake robot with series elastic torque-controlled actuators and a screw-less assembly mechanism ⇤ Filippo Sanfilippo 1 , Erlend Helgerud 1 , Per Anders Stadheim 1 and Sondre Lieblein Aronsen 1 Abstract—Even though a few examples of elastic snake robots exist, they are generally expensive and tailored to custom-made hardware/software components that are not openly available. In this work, Serpens, a newly-designed low-cost, open-source and highly-compliant modular snake robot with series elastic actuator (SEA) is presented. Serpens features precision torque control and stereoscopic vision. Only low-cost commercial-off-the-shelf (COTS) components are adopted. The robot modules can be 3D- printed by using Fused Deposition Modelling (FDM) manufac- turing technology, thus making the rapid-prototyping process very economical and fast. A screw-less assembly mechanism allows for connecting the modules and reconfigure the robot in reliable and robust manner. By using a low-cost sensing approach, functions for torque sensing at the joint level, sensitive collision detection and joint compliant control are possible. The concept of modularity is also applied to the system architecture on both the software and hardware sides. The software architecture is based on the Robot Operating System (ROS). This paper describes the design of Serpens and presents preliminary simulation and experimental results which illustrate its potential. Index Terms—snake robot, series elastic actuator, SEA, ROS. I. I NTRODUCTION In nature, limbless organisms like snakes may exploit rocks, stones, branches, obstacles, or other irregularities in the terrain as a means of propulsion to achieve locomotion [1]. This remarkable ability allows biological snakes to be exceptionally adaptable to various types of environments. Snake robots that can replicate this range of behaviour could enable different applications for use in challenging real-life operations and hazardous or confined areas that conventional robots (i.e. wheeled, tracked and legged) and humans are unable to access, such as explorations of earthquake-hit areas, pipe inspections for the oil and gas industry, fire-fighting opera- tions, and search-and-rescue activities (SAR) [2]. Snake robot locomotion in a cluttered environment where the snake robot utilises a sensory-perceptual system to exploit the surrounding operational space and identifies walls, obstacles, or other external objects for means of propulsion can be defined as perception-driven obstacle-aided locomotion (POAL) [3], [4]. The development of POAL is known to be challenging because of the complex interaction between the snake robot and the adjacent cluttered environment. From a control point of view, *This work is supported by the Dept. of Science and Industry systems, University of South-Eastern Norway (USN), project title “Secure Multi-sensor Autonomous RoboTs and surveillance operations for Search And Rescue (SMART-SAR) operations in smart buildings”. 1 Filippo Sanfilippo, Erlend Helgerud, Per Anders Stadheim and Sondre Lieblein Aronsen are with the Dept. of Science and Industry systems, University of South-Eastern Norway (USN), Post box 235, 3603 Kongsberg, Norway. filippo.sanfilippo@usn.no Fig. 1: A real snake locomoting in a cluttered environment (top); Serpens, the proposed low-cost ROS-based snake robot with series elastic actuator (SEA), precision torque control and a screw-less assembly mechanism (bottom). achieving POAL requires to precisely identify potential push- points and to accurately determine achievable contact reaction forces. Accomplishing this with traditional rigidly-actuated robots is extremely demanding because of the absence of compliance. To facilitate the control complexity for robots that interact with unmapped and dynamic environments or need to navigate rough terrains cluttered with obstacles, compliant motion and fine torque control on each joint is desirable. Consequently, intrinsically elastic joints have become pro- gressively prominent over the last years. Commonly, elastic joints are considered to outperform rigid actuation in terms of peak dynamics, robustness, and energy efficiency [5]. Even though a few examples of elastic snake robots exist [6], they are generally costly to produce and tailored to custom-made hardware/software components that are not openly available off-the-shelf. In order to give researchers a novel snake robot that is inexpensive, easily customisable, and fast to fabricate, a newly-designed low-cost, open-source, and highly-compliant multi-purpose modular snake robot with series elastic actuators (SEA) is introduced in this work. The presented snake robot is named Serpens (“the Serpent”, Greek 'Ofic) after the homonym constellation of the northern hemisphere. Serpens is shown in Fig. 1. Serpens features compliant torque-controlled actuators and stereoscopic vision. Only low-cost commercial- off-the-shelf (COTS) components are adopted to achieve a