Abstract-This paper presents a control system that enables an underwater snake robot to converge towards and follow a straight path in the presence of constant irrotational ocean currents. The robot is assumed to be neutrally buoyant, fully submerged and moving in a virtual plane with a sinusoidal gait and limited link angles. The proposed control approach uses a heading controller that exponentially stabilises the heading of the robot towards the desired heading, which is obtained by an integral line-of-sight guidance law. Uniform semi-global exponential stability of the control system is formally proved using cascaded systems and Lyapunov theory. Simulations are presented that illustrate and validate the theoretical results.

Abstract-This paper presents an analysis of planar underwater snake robot locomotion in the presence of ocean currents. The robot is assumed to be neutrally buoyant and move fully submerged with a planar sinusoidal gait and limited link angles. As a basis for the analysis, an existing, controloriented model is further simplified and extended to general sinusoidal gaits. Averaging theory is then employed to derive the averaged velocity dynamics of the underwater snake robot from that model. It is proven that the averaged velocity converges exponentially to an equilibrium, and an analytical expression for calculating the forward velocity of the robot in steady state is derived. A simulation study that validates both the proposed modelling approach and the theoretical results is presented.

This paper investigates the problem of planar maneuvering control for bio-inspired underwater snake robots that are exposed to unknown ocean currents. The control objective is to make a neutrally buoyant snake robot which is subject to hydrodynamic forces and ocean currents converge to a desired planar path and traverse the path with a desired velocity. The proposed feedback control strategy enforces virtual constraints which encode biologically inspired gaits on the snake robot configuration. The virtual constraints, parametrized by states of dynamic compensators, are used to regulate the orientation and forward speed of the snake robot. A two-state ocean current observer based on relative velocity sensors is proposed. It enables the robot to follow the path in the presence of unknown constant ocean currents. The efficacy of the proposed control algorithm for several biologically inspired gaits is verified both in simulations for different path geometries and in experiments.

Abstract-This paper presents a control-oriented model of a neutrally buoyant underwater snake robot that is exposed to a constant irrotational current. The robot is assumed to move in a horizontal, fully submerged plane with a sinusoidal gait pattern and limited link angles. The intention behind the proposed model is to describe the qualitative behaviour of the robot by a simplified kinematic approach, thus neglecting some of the non-linear effects that do not significantly contribute to the overall behaviour. This results in a model with significantly less complex dynamic equations than existing models, which makes the new model well-fitted for control design and analysis. An existing, more complex model and a class of sinusoidal gait patterns are analysed, leading to several properties that serve as a basis for the simplified model. Some of the revealed properties are also valid for ground robots. Simulations that qualitatively validate the theoretical results are presented.

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 pathfollowing 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 eellike 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.