Abstract: Due to low effectiveness of both bow and stern planes at low speeds, the near-surface suction force experienced by an underwater vehicle can hardly be counteracted by lifting forces generated by the control surfaces. To improve the performance of depth control system, therefore, a variable water ballast mechanism is introduced and thus leads to an over-actuated characteristic. A composite control scheme is proposed for such an over-actuated control system. The input of the feedforward controller is the disturbance force, which is estimated by the disturbance observer, and the output of the feedforward controller is the mass of ballast water. Besides, the input of the feedback controller is the depth error and the output of the controller is the deflection angle of control surfaces. The results of underwater tests show that the disturbance observer can accurately estimate the unbalanced force on the underwater vehicle, and the self-propelled water tank tests also show that depth control system of underwater vehicle based on composite control strategy has good control performance.

Key words: underwater vehicle; wave-suction force; disturbance observer; depth control; composite control