Nonlinear Orientation Controller for a Compliant Robotic Fish Based on Asymmetric Actuation

Abstract

Compliant fish-like robots are being developed as efficient and dependable underwater observation platforms with low impact on the observed environment. Orientation control is an essential building block to achieve autonomy for those vehicles. So far, the major focus has been on rigid tails or on flexible tails with a high degree of actuation. We present a novel control strategy for an underactuated robotic fish with a flexible tail optimized for cruising. The basis for our approach is the generation of asymmetric velocity profiles of the robot's tail beats. To achieve such velocity profiles, the usual sinusoidal tail actuation is replaced with skewed triangle waves. We provide a simple formulation for such waves, where their skew is dependent on only one variable which we define as skew factor. Furthermore, a nonlinear control law is derived to achieve the desired turning motions. We implement the controller on a compliant fish-like robot with a simple actuation mechanism. The control scheme is experimentally validated, and its robustness is tested in field trials.