Analysis of a Dual Tendon-Driven Robotic Dolphin Tail: Omnidirectional Motions and Thrust Characteristics

The exceptional swimming agility observed in dolphins is primarily attributed to the flexibility of their tails. Drawing inspiration from this biological phenomenon, this letter introduces a novel dual tendon-driven robotic dolphin tail featuring a passive joint designed to facilitate omnidirectional motion. To accurately capture the motion state of the robotic dolphin tail, a three-dimensional kinematic model is developed based on the constant curvature model. The parameters are identified by collecting data from an inertial measurement unit (IMU) positioned at the end of the caudal peduncle of the robotic tail. A Central Pattern Generator (CPG) controller, incorporating position-based control output, is employed to perform the omnidirectional motion. To investigate the thrust characteristics during fluke motions, advanced six-axis force sensors are utilized to measure propulsion forces and pitch moments under two distinct conditions: with or without the passive joint and a flexible or rigid fluke. Experimental results reveal crucial findings: (1) The presence of passive joints proves indispensable for thrust generation, attributing to the generation of the attack angle. (2) While a flexible fluke reduces pitch moments in comparison to a rigid one, it also decreases thrust. The research can pave the way for the development of dolphin-like omnidirectional motions.