Exploration of Fin Stiffness for Asymmetric Thrust in a Swimming Robot

Fish commonly combine pectoral fin motion with tail and body motion to create a swimming gait. The rowing pectoral fin motion is characterized by distinct power and recovery strokes where the soft fin bends or rotates significantly to change the hydrodynamic profile of the appendage between the two strokes. In this work, we take inspiration from fish to create an underwater robot that uses a rowing gait to create net forward thrust. We present a mechanical linkage using two cams on a single drive axis to drive both the yawing and rolling rotations of the fin with a single actuated degree of freedom. We also describe the kinematics and geometric constraints of the linkage and how they relate to properties of the cams. We show that a soft fin can create asymmetric thrust in conjunction with the cam-follower transmission and that a fin can produce large spikes in negative force while still creating net forward work throughout the gait cycle.