Depth control of autonomous underwater vehicles using indirect robust control method

In this paper we propose a new robust depth control scheme for autonomous underwater vehicles (AUV) in the presence of hydrodynamic parameter uncertainties and disturbances. The controller is designed via an indirect robust control method that handles the uncertainties by formulating the uncertainty bounds into the cost function and then transforming the robust control problem into an equivalent optimal control problem. Robust asymptotic stability and optimality can be both achieved. The θ-D method is utilized to solve the resultant nonlinear optimal control problem with an approximate closed-form feedback solution and thus is easy to implement onboard without intensive computation load. Simulation results demonstrate that robust depth control is accomplished under the system parameter uncertainties and disturbances with small control fin deflection requirement.