Model-based fin ray joint tracking control for undulatory fin mechanisms

Undulatory fin propulsion, inspired by the swimming of knifefish and rays, holds considerable potential for enhancing the maneuvering and station-keeping capabilities of underwater vehicles. Most robotic fins are comprised by a series of individually actuated “fin rays”, interconnected via a flexible “membrane” sheet. Propulsive forces are generated by the propagation of a traveling wave along the fin, obtained via appropriately timed ray oscillations. In prior work, we have presented a dynamic model for an elementary two-ray fin system, analyzed as a standard robot mechanism with additional contributions arising from the elastic deformation of the flexible membrane and from the hydrodynamic forces. This paper extends the model to also consider the dynamics of the fin ray actuators, and introduces a combined feedforward-feedback scheme for tracking of the rays´ reference trajectories, in order to accurately implement a desired undulation pattern. Experiments conducted with a two-ray fin prototype confirm the efficacy of the proposed controller, and demonstrate its improved performance over non model-based control schemes.