Asymptotic diving control method for torpedo-type underactuated AUVs

This paper considers diving control problems for underactuated autonomous underwater vehicles (AUVs) with only surge force and pitch moment available for its 3D vertical motion. By introducing certain two polar coordinates transformations, the vehicle´s kinematics and dynamics can be reduced to a certain two-input-two-output (TITO) second-order nonlinear strict-feedback form, upon which the proposed control scheme is derived using general backstepping method. To avoid possible singularity problem in the recursive control design, we introduce an asymptotic modification of orientation concept instead of taking the velocity term as a virtual control input. Proposed diving scheme guarantees asymptotic stability of the position tracking error as well as the velocity and pitch angle tracking errors in the polar frame. And the same tracking property of velocity and pitch angle in the Cartesian frame is also discussed. Numerical simulation studies are also carried out to illustrate the effectiveness of proposed control scheme.