A novel approach for the optimal control of autonomous underwater vehicles

The SDRE (State-Dependent Riccati Equation) is a technique recently proposed as a nonlinear control method. Despite the benefits due to its flexibility, the SDRE places high demand on the computational load of real-time applications, which is one of its most significant drawbacks. This paper discusses a new nonlinear feedback controller for autonomous underwater vehicles (AUVs), which eventually converges to a conventional SDRE-based optimal controller. The proposed controller is derived by direct forward integration of an SDRE. This enables fast computation, and so is applicable to real-time applications. For a state-dependent system, the proposed controller may be an alternative candidate to a conventional SDRE-based optimal controller if the system is slow-varying to different states. To cope with fast-varying systems, we introduced a deviation index, which indicates the extent of deviation of the proposed controller from the solution of a conventional SDRE-based one. Whenever the index exceeds a designated bound, the controller is initialized to the conventional SDRE optimal value. Using the deviation index, a designer can achieve a compromise between computation time and optimality. We applied the proposed controller to a numerical model of an AUV called ODIN (Choi et al., 1995), a well-known nonlinear, relatively higher order, and slow-varying system. The global position/attitude regulation, tracking problems, and fault tolerance properties were examined in the simulation to show the effectiveness of the proposed controller.