Proceedings of the RAAD 2013 22nd International Workshop on Robotics in Alpe-Adria-Danube Region September 11-13, 2013, Portoroˇ z, Slovenia Development and Experimental Evaluation of an Undulatory Fin Prototype Michael Sfakiotakis a , Manolis Arapis a , Nektarios Spyridakis a,b and John Fasoulas c a Dept. of Electrical Engineering, Technological Educational Institute of Crete, Heraklion, Greece, E-mail: msfak@staff.teicrete.gr b Institute of Oceanography, Hellenic Centre for Marine Research, Heraklion, Greece, E-mail: nektar@hcmr.gr c Dept. of Mechanical Engineering, Technological Educational Institute of Crete, Heraklion, Greece, E-mail: jfasoulas@staff.teicrete.gr Abstract. Bio-inspired thruster designs encompass significant potential for developing a new generation of underwater vehicles with enhanced propulsion and maneuvering abilities, to address the needs of a growing number of underwater applications. Undulatory fin propulsion, inspired by the locomotion of cuttlefish and of certain electric eel species, is one such approach currently under investigation. Within this framework, we present the design and experimental evaluation of an undulatory fin prototype, comprised of eight actively-controlled fin rays, which are interconnected by a flexible membrane. A control architecture, based on an artificial Central Pattern Generator (CPG), is used to produce the rays’ motion pattern associated with the undulatory movement of the fin. Experimental results from a parametric study indicate that the prototype can achieve speeds up to 1.45 fin lengths per second, and highlight the effect of the various kinematic parameters on the attained velocity and wave efficiency. Keywords. Undulatory fin propulsion, Bio-inspired robotic locomotion, Central Pattern Generators. 1. Introduction In recent years, the development of unmanned un- derwater vehicles (UUVs) has received considerable attention by the research community, driven by an ever-increasing number of applications related to the scientific study, the protection and the sustainable exploitation of the oceans’ natural resources. Current UUVs, both autonomous and remotely op- erated, rely for maneuvering on either lift-based con- trol surfaces, or on multi-thruster configurations. The performance of both systems declines considerably when low-speed maneuvering or station-keeping in the presence of drifting currents are required. Such tasks, along with operation through cluttered environments, are becoming increasingly important in the context of perspective missions related to, e.g., ship wreck ex- ploration, pipeline inspection and oil rig maintenance. In order to address such aspects of these increasingly demanding applications, research has turned to the study of the remarkable propulsion mechanisms of fish and sea mammals, which enable them to attain high swimming speeds, and/or to maneuver with great agility underwater (Webb, 2004). As these capabilities far exceed those of conventional man-made propul- sive mechanisms, it is expected that the study of the biomechanics of fish locomotion will lend itself to the development of a new generation of high-performance and energy-efficient UUVs. In this framework, a number of research efforts have proposed propulsion mechanisms which are based on the undulating fin locomotion of animals such as electric eels, rays and cuttlefish (Sfakiotakis et al., 2001; Willy and Low, 2005; Epstein et al., 2006; Zhang et al., 2008). Two of the underlying motivations are the thrust vectoring capability of undulating fins, and the fact that they are mainly encountered in fish that retain a rigid bodyform while swimming. There- fore, the prospect of developing artificial counterparts for integration to suitably designed (yet still rigid- hulled) underwater vehicles, becomes one of signifi- cant interest. Other postulated advantages of undula- tory fin propulsion include increased energy efficiency, reduced sediment disruption and stealth operation.