Experimental investigation of locomotion efficiency and path-following for underwater snake robots with and without a caudal fin

Over the last few decades, the robotics community has shown increasing interest in developing bioinspired swimming robots, driven by the need for more economical, more efficient, autonomous, highly flexible and maneuverable robotic systems for underwater operations. In this paper, we present a bioinspired underwater snake robot (USR) equipped with a passive caudal (tail) fin. In particular, a highly flexible USR configuration is presented that is capable of locomotion both on the ground and underwater due to its robust mechanical and modular design, which allows additional effectors to be attached to different modules of the robot depending on the requirements of the application. This provides flexibility to the operator, who can thus choose the proper configuration depending on the task to be performed in various uncertain environments on the ground and underwater. Experimental results on locomotion efficiency and path-following control are obtained for a physical USR to enable a comparison of the USR motion with and without the passive caudal fin, for both lateral undulation and eel-like motion patterns. Results comparing the locomotion efficiency in both simulations and experiments are presented in order to validate the proposed models for USRs. By means of fluid parameter identification, both a qualitative and a quantitative comparison between the simulated and experimental results are performed regarding the achieved forward velocity. Furthermore, the experimental results show that a path-following control approach that has previously been proposed for USRs without a caudal fin can be directly applied to solve the path-following control problem for this bioinspired USR with a passive caudal fin. In particular, it is shown that this path-following control approach successfully steers the robot toward and along the desired path, and furthermore, the results show that it is possible to almost double the forward velocity of the robot by using a passive caudal fin