Counter-propagating waves enhance maneuverability and stability: A bio-inspired strategy for robotic ribbon-fin propulsion

Weakly electric knifefish, Eigenmannia, are highly maneuverable swimmers. The animals rely on a long, undulating ribbon fin to generate propulsive force. During closed-loop control of hovering and station keeping, knifefish partition their fin to produce two inward counter-propagating waves, enabling them to hover and rapidly change direction. In response to moving objects or changes in ambient flow speed, the fish can actively modulate the nodal point where the two waves meet. During hovering, this nodal point is somewhere in the middle, but it can be moved forward or backward changing the relative force generated by the front and back portions of the fin. Although this strategy for thrust generation may be energetically inefficient, we show here that it enables rapid switching of swimming direction and produces a linear drag-like force that confers passive stability. Robotic results and simple computational simulations reveal that the net force generated by counter-propagating waves changes linearly with respect to the nodal position. Another strategy for reversing swim direction would be to completely reverse the direction of a single traveling wave. We show why full wave reversal (and similar strategies) may be ineffective for low-speed swimming - a regime where counter-propagating waves may simplify control.