Abstract: This paper presents a locomotion control implementation of a robotic system mimicking the undulating fins of fish. To mimic the actual flexible fin of a real fish, we created a ribbon fin type actuation device with a series of connecting linkages and attached it to the robotic fish. By virtue of a specially designed strip with a slider, each link is able to turn and slide with respect to the adjacent link. The driving linkages are used to form a mechanical fin consisting of several fin segments, which are able to produce undulations, similar to those produced by actual fins. By virtue of the modular and reconfigurable fin mechanisms, two robotic fish with different fin layouts have been designed and constructed: The first prototype is a robotic stingray, swimming by undulations of a pair of lateral fins the second is a robotic knifefish, swimming by undulations of a long anal fin. The locomotion scheme and mechatronics implementation of the robotic stingray are presented and discussed with a parametric study of the slider's workspace and joint trajectory. Some experimental observations of the robotic knifefish are also shown and discussed. The results demonstrate that the designed fin mechanisms are able to undulate sections of pectoral and anal fins, with different amplitudes and phases, along the length of the fin in the direction of motion. This may lead to further developments that better mimic the fin locomotion capability of real fish.

Key words: Biomimetic, robotic fish, modular and re-configurable layouts, undulating fin, motion planning

1. INTRODUCTION

One of the biggest changes to have occurred in the world's oceans in the last century has been the introduction of propeller driven ships. Despite its versatility, the propeller has created new dangers for marine life: It produces a greater amount of marine debris, increased mortality of marine creatures, such as manatees, as a result of propeller strikes, and disturbance of shallow water ecosystems.

Researchers and scientists have recently been more vocal than in the past in promoting an awareness of the need to preserve the environment. One of their main concerns is the sustainability of the underwater ecology, especially in marine environments, which is deteriorating due to extensive use of propellers and which has recently gained public and government attention.

1.1. Fish Swimming Modes

Each fish species has its own unique way of interacting with different environments, which then dictates the species' body shape and body size, as well as the way it propels itself, through a process of natural selection. Therefore, fish provide useful illustrations of propulsor design, swimming modes, and body shape (morphology). Sfakiotakis et al. (1999) provide an excellent review of these factors, including the classification scheme illustrated in Figures 1 and 2. Their classification is based on two main factors: (1) The extent to which propulsion is based on undulatory motion versus oscillatory motion and (2) the body structures or fins that are most active in generating thrust.

Breder (1926) proposed two swimming modes for fish, based on the propulsive structure used: Body...