## 6 Conclusions and Future Work  
_Serpens_, a low-cost snake robot with elastic joints, torque-controlled actuators and a screw-less assembly mechanism, is presented in this paper based on a modular design and the use of the Robot Operating System (ROS) [12]. The design of the robot relies exclusively on low-cost commercial-off-the-shelf (COTS) components. Fused Deposition Modelling (FDM) manufacturing technology is adopted for 3D-printing the robot modules with _polymctic acid_ (PLA), thus making the rapid-prototyping process very fast and economical. A screw-less assembly mechanism makes it possible to assemble the modules and reconfigure the robot in a very reliable, fast and robust manner. A low-cost sensing approach is adopted to allow for torque sensing at the joint level, sensitive collision detection and joint compliant control. These characteristics make _Serpens_ very suitable for the interaction with unmapped and dynamic environments or for traversing terrains cluttered with obstacles. The system architecture also follows the concept of modularity on both the software and hardware sides. Each module is independent, being controlled by a self-reliant controller board. The choice of ROS for the implementation of the control framework enables researchers to develop different control algorithms for perception-driven obstacle-aided locomotion (POAL) in a simulated environment with Gazebo. This integration makes the development of control algorithms safe, rapid and efficient. Experimental and simulation results are presented to illustrate the potential of the proposed design.  
As future work, the design of reliable low-level control algorithms for the proposed elastic joints will be investigated. Indeed, the design of robust and effective low-level control approaches is essential to enable the achievement of POAL for real-world applications. To achieve this, the current low-level software architecture of _Serpens_ must be complemented with a hierarchical organisation by considering the standard functions and capabilities of guidance, navigation, and control (GNC) [37]. Regarding the mechanical design of _Serpens_, the possibility of testing polymers or elastomers and comparing the compliance with our current design based on the use of mechanical springs will also be considered in the future. Moreover, the possibility for _Serpens_ to locomote in applications where the gap is narrower than the width of the robot body, i.e., within narrow vertical pipes or walls, will also be explored. More intensive heat dissipation tests are required for practical applications.  
Conceptualization, F.S.; Methodology, F.S.; Software, F.S. and E.H.; Validation, F.S., E.H. and S.L.A.; Formal Analysis, F.S.; Investigation, F.S., E.H., P.A.S., S.L.A.; Resources, F.S.; Data Curation, F.S., E.H., P.A.S., S.L.A.; Writing--Original Draft Preparation, F.S., E.H., P.A.S., S.L.A.; Writing--Review & Editing, F.S.; Visualization, F.S., E.H., P.A.S., S.L.A.; Supervision, F.S.; Project Administration, F.S.; Funding Acquisition, F.S.  
Figure 17: The deviation over time between the motor gear position and the spring reference position.  
**Acknowledgments:** The authors gratefully acknowledge the contribution of [PERSON] in the 3D-printing prototyping process of this work. The authors also thank [PERSON], [PERSON], [PERSON] and [PERSON].  
**Funding:** This work was 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\".  
**Conflicts of Interest:** The authors declare no conflict of interest.