Three-dimensional spiral tracking control for gliding robotic fish

Three-dimensional curve tracking is an important and challenging research topic for autonomous underwater vehicles and robots. In this paper we consider the tracking control of gliding robotic fish, a new type of energy-efficient and highly maneuverable robots representing a hybrid of underwater gliders and robotic fish; in particular, buoyancy-driven gliding with a deflected tail results in efficient spiral maneuvers. We propose to decompose a space curve into continuously evolving spirals from the viewpoint of differential geometry, and then utilize the spiral trajectory characteristics for three-dimensional curve tracking. Due to the nonlinear dynamics and strong coupling among multiple control inputs, the design of controller is highly demanding. A novel two degree-of-freedom (DOF) tracking control strategy is proposed, which consists of a feedforward inverse controller and a robust H_∞ controller designed based on linearized dynamics. Simulation results are presented to demonstrate the effectiveness of the proposed two-DOF control scheme, in comparison with a PI controller and a pure feedforward inverse controller.