Performance-Oriented Understanding and Design of a Robotic Tadpole: Lower Energy Cost, Higher Speed

A compliant plate driven by an active joint is frequently employed as a fin to improve swimming efficiency due to its continuous and compliant kinematics. However, very few studies have focused on the performance-oriented design of multijoint mechanisms enhanced with flexible fins, particularly regarding critical design factors such as the active-joint ratio and dimension-related stiffness distribution of the fin. To this aim, we developed a robotic tadpole by integrating a multijoint mechanism with a flexible fin and conduct a comprehensive investigation of its swimming performance with different tail configurations. A dynamic model with identified hydrodynamic parameters was established to predict propulsive performance. Numerous simulations and experiments were conducted to explore the impact of the active-joint ratio and the dimension-related stiffness distribution of the fin. The results reveal that (a) tails with different active-joint ratios achieve their best performance at a small phase difference, while tails with a larger active-joint ratio tend to perform worse than those with a smaller active-joint ratio when a larger phase difference is used; (b) the optimal active-joint ratio enables the robot to achieve superior performance in terms of swimming velocity and energy efficiency; and (c) with the same surface area, a longer fin with a wide leading edge and a narrow trailing edge can achieve higher swimming speeds with lower energy consumption. This work presents novel and in-depth insights into the design of bio-inspired underwater robots with compliant propulsion mechanisms.