Fish present a natural source of inspiration for the design of high-performance under-water robots. Conventionally, fish-like robotic systems consist of a chain of rigid links connected by a series of rigid actuators. Devices of this nature have demonstrated impressive speeds and maneuverability, but from a practical perspective, their mechanical complexity can make them expensive to build and prone to failure. One possible solution is to replace the mechanical body of the swimmer with a passive elastic element. In this scenario, the robot uses a single actuator, housed within a rigid forebody, to generate a fish-like propagating wave along a flexible trailing tail. A number of groups have explored this approach, but so far, these devices have demonstrated relatively limited performance. Here, we study the kinematics and dynamics of elastic swimmers and apply the results of this process to guide the design and testing of a high-performance passive robotic swimmer. We begin the investigation with a first-principles approach. We use analytical models of fish hydrodynamics to characterize the kinematics of efficient propulsion in swimming animals. Armed with the insight developed through this process, we construct a numerical model of a passive elastic swimming sheet. Through the application of optimization methods, we demonstrate that the sheet can achieve 70-80% of the efficiency of an equivalent swimmer with actuators along its entire body. Based on this, we design, build, and test a passive elastic swimming robot which uses a novel inertia-based actuation system. Experiments with the robot show that it can achieve a top speed of 1m/s (3.17 body lengths/s) and a peak turning rate of 515 deg/s, among the highest reported to date, while swimming at efficiencies comparable to those of fully actuated systems.;
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