Evolution of Fin Undulation on a Physical Knifefish-inspired Soft Robot

Soft robotics is a growing field of research and one of its challenges is how to efficiently design a controller for a soft morphology.
This paper presents a marine soft robot inspired by the ghost knifefish that swims on the water surface by using an undulating fin underneath its body.
We investigate how propagating wave functions can be evolved and how these affect the swimming performance of the robot.
The fin and body of the robot are constructed from silicone and six wooden fin rays actuated by servo motors.
In order to bypass the reality gap, which would necessitate a complex simulation of the fish, we implemented a Covariance Matrix Adaptation Evolution Strategy (CMA-ES) directly on the physical robot to optimize its controller for travel speed.
Our results show that evolving a simple sine wave or a Fourier series can generate controllers that outperform a hand programmed controller.
The results additionally demonstrate that the best evolved controllers share similarities with the undulation patterns of actual knifefish.
Based on these results we suggest that evolution on physical robots is promising for future application in optimizing behaviors of soft robots.