Spin Swimmer : A Fast, Efficient and Agile Fish-Like Robot

Engineers and scientists designing underwater robots have sought to emulate the speed, efficiency, and agility of fish. Much of the engineering of fish-like robotics reduces to the design of soft or articulated multi-body tails that can oscillate or undulate at frequencies and amplitudes similar to those of the fish they seek to mimic in the hope of achieving their efficiency and speed. Such kinematic approaches do not account for the dynamic interaction between power efficient actuation, response of flexible appendages and hydrodynamic forces. This letter presents a fundamentally novel means of mechanical actuation: a fast spinning unbalanced rotor internal to the body of the robot, that transfers a periodic axial force to an otherwise passive flexible tail. The net result is that the tail acts as a parametric oscillator that undergoes a $2:1$ subharmonic resonance. High tail-beat frequencies are achieved with minimal input power due to this parametric resonance. The resulting robot has the lowest cost of transport amongst free swimming robots while also being fast, extremely agile and gyroscopically roll and pitch stable. The results demonstrate the importance of exploiting parametric resonances in designing efficient fish-like robots.