## 1 Introduction  
In nature, limbless organisms such as 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 a variety of possible 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 operations, and search-and-rescue activities(SAR) [2; 3]. 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) [4; 5]. The development of POAL is known to be challenging because of the complex interaction between the snake robot and the adjacent cluttered environment [6]. From a control point of view, achieving POAL requires precisely identifying 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. Traditional gear-motor-driven actuators are designed and built for industrial automation. These conventional motors commonly actuate stiff linkages while encoders are employed to make position control possible. The control and overall performance of rigidly actuated robots is heavily impacted by actuator dynamics (i.e., masses, reflected inertias, and stiffness) [7]. Rigid actuators transmit high (virtually infinite) mechanical impedance, thus forcing the robot to resist motion when subject to a force. Rigidly actuated robots are also characterised by having a high bandwidth, which forces them to promptly move to commanded positions regardless of what external forces act on their joints. This characteristic is adequate for industrial automation because it allows robots for tracking trajectories in static or mapped environments, i.e., pick-and-place applications, but it is not suitable for robots that interact with unmapped and dynamic environments or need to navigate terrains cluttered with obstacles, such as snake robots. Indeed, conventional rigid actuators are unable to store and release energy or exploit natural dynamics, making them too inefficient or undesired for mobile locomotion applications. Moreover, when considering POAL, the high reflected inertia of traditional gear-motor-driven actuators can cause potential collisions that may damage both the robot and the environment. 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 progressively prominent over the last years for a variety of robotic applications. Commonly, elastic joints are considered to outperform rigid actuation in terms of peak dynamics, robustness, and energy efficiency [8]. Even though a few examples of elastic snake robots exist [9; 10], they are generally costly to produce and tailored to custom-made hardware/software components that are not openly available off-the-shelf. To give researchers a novel snake robot that is inexpensive to manufacture, 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 'Oq<<) after the homonym constellation of the northern hemisphere.