Eleni Kelasidi

Modeling of underwater snake robots

Increasing efficiency by improving the locomotion methods is a key issue for underwater robots. Hence, an accurate dynamic model is important for both controller design and efficient locomotion methods. This paper presents a model of the kinematics and dynamics of a planar, underwater snake robot aimed at control design. Fluid contact forces and torques are modeled using analytical fluid dynamics. The model is derived in a closed form and can be utilized in modern model-based control schemes. The proposed model is easily implemented and simulated, regardless of the number of robot links. Simulation results with a ten link robotic system are presented.

Innovation in Underwater Robots: Biologically Inspired Swimming Snake Robots

Increasing efficiency by improving locomotion methods is a key issue for underwater robots. Moreover, a number of different control design challenges must be solved to realize operational swimming robots for underwater tasks. This article proposes and experimentally validates a straightline-path-following controller for biologically inspired swimming snake robots. In particular, a line-of-sight (LOS) guidance law is presented, which is combined with a sinusoidal gait pattern and a directional controller that steers the robot toward and along the desired path. The performance of the path-following controller is investigated through experiments with a physical underwater snake robot for both lateral undulation and eel-like motion. In addition, fluid parameter identification is performed, and simulation results based on the identified fluid coefficients are presented to obtain a back-to-back comparison with the motion of the physical robot during the experiments. The experimental results show that the proposed control strategy successfully steers the robot toward and along the desired path for both lateral undulation and eel-like motion patterns.

Integral line-of-sight for path following of underwater snake robots

This paper considers straight line path following control of underwater snake robots in the presence of constant irrotational currents. An integral line-of-sight (LOS) guidance law is proposed, which is combined with a sinusoidal gait pattern and a directional controller that steers the robot towards and along the desired path. Integral action is introduced in the guidance law to compensate for the ocean current effect. The stability of the proposed control scheme in the presence of ocean currents is investigated. In particular, using Poincaré map analysis, we prove that the state variables of an underwater snake robot trace out an exponentially stable periodic orbit when the integral LOS path following controller is applied. Simulation results are presented to illustrate the performance of the proposed path following controller for both lateral undulation and eel-like motion.

Integral Line-of-Sight Guidance for Path Following Control of Underwater Snake Robots: Theory and Experiments

This paper proposes and experimentally validates a straight line path following controller for underwater snake robots in the presence of constant irrotational currents of unknown direction and magnitude. An integral line-of-sight guidance law is presented, which is combined with a sinusoidal gait pattern and a directional controller that steers the robot toward and along the desired path. The stability of the proposed control scheme in the presence of ocean currents is investigated by using Poincaré map analysis. Simulation results are presented to illustrate the performance of the proposed path following controller for both lateral undulation and eel-like motion. In addition, the performance of the path following controller is investigated through experiments with a physical underwater snake robot. The experimental results show that the proposed control strategy successfully steers the robot toward and along the desired path in the presence of an unknown constant irrotational current in the inertial frame.

Experimental investigation of efficient locomotion of underwater snake robots for lateral undulation and eel-like motion patterns

AbstractUnderwater snake robots offer many interesting capabilities for underwater operations. The long and slender structure of such robots provide superior capabilities for access through narrow openings and within confined areas. This is interesting for inspection and monitoring operations, for instance within the subsea oil and gas industry and within marine archeology. In addition, underwater snake robots can provide both inspection and intervention capabilities and are thus interesting candidates for the next generation inspection and intervention AUVs. Furthermore, bioinspired locomotion through oscillatory gaits, like lateral undulation and eel-like motion, is interesting from an energy efficiency point of view. Increasing the motion efficiency in terms of the achieved forward speed by improving the method of propulsion is a key issue for underwater robots. Moreover, energy efficiency is one of the main challenges for long-term autonomy of these systems. In this study, we will consider both these two aspects of efficiency. This paper considers the energy efficiency of swimming snake robots by presenting and experimentally investigating fundamental properties of the velocity and the power consumption of an underwater snake robot for both lateral undulation and eel-like motion patterns. In particular, we investigate the relationship between the parameters of the gait patterns, the forward velocity and the energy consumption for different motion patterns. The simulation and experimental results are seen to support the theoretical findings.

Path following control of planar snake robots using virtual holonomic constraints: theory and experiments.

This paper considers path following control of planar snake robots using virtual holonomic constraints. In order to present a model-based path following control design for the snake robot, we first derive the Euler-Lagrange equations of motion of the system. Subsequently, we define geometric relations among the generalized coordinates of the system, using the method of virtual holonomic constraints. These appropriately defined constraints shape the geometry of a constraint manifold for the system, which is a submanifold of the configuration space of the robot. Furthermore, we show that the constraint manifold can be made invariant by a suitable choice of feedback. In particular, we analytically design a smooth feedback control law to exponentially stabilize the constraint manifold. We show that enforcing the appropriately defined virtual holonomic constraints for the configuration variables implies that the robot converges to and follows a desired geometric path. Numerical simulations and experimental results are presented to validate the theoretical approach.

Stability analysis of underwater snake robot locomotion based on averaging theory

This paper presents an averaged model of the velocity dynamics of an underwater snake robot, suited for stability analysis and motion planning purposes for general sinusoidal motion gait patterns. Averaging theory is applied in order to derive a model of the average velocity for a control-oriented model of an underwater snake robot that is influenced by added mass effects (reactive fluid forces) and linear drag forces (resistive fluid forces). Based on this model we show that the average velocity of an underwater snake robot during sinusoidal motion patterns converges exponentially to a steady-state velocity. An explicit analytical relation is given between the steady state velocity and the amplitude, the frequency, the phase shift and the offset of the joint motion for the case of a sinusoidal gait pattern. The results of the paper are general and constitute a powerful tool for achieving faster forward motion by selecting the most appropriate motion pattern and the best combination of the gait parameters. Simulation results are presented both for lateral undulation and eel-like motion.

A control-oriented model of underwater snake robots

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.

Planar Path Following of Underwater Snake Robots in the Presence of Ocean Currents

This letter 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 semiglobal 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.

A waypoint guidance strategy for underwater snake robots

In this paper, a waypoint guidance strategy is proposed for an underwater snake robot. The robot is directed to follow a path which is derived using path planning techniques. A first version of the path is derived by the path planner by using the artificial potential field method for obstacle avoidance. Afterwards, by subsampling the derived path, a set of waypoints are chosen along the path. The path is then defined by interconnecting these waypoints by straight lines. Secondly, a straight line path following controller is proposed, to make the underwater snake robot follow the desired path. Simulation results are presented, illustrating the performance of the proposed guidance control strategy for both lateral undulation and eel-like motion.