Work-class ROVs are always subject to external disturbances and system uncertainties. The sliding mode control is an effective method to overcome the disturbances, but its chattering phenomenon restricts its practical application on ROVs. In this paper, we propose a model-free discrete double-loop sliding mode controller, the feasibility and robustness of the controller are illustrated by quasi sliding mode theory. To further reduce chattering, an adaptive reaching law is proposed while ensuring robustness to conquer the poor steady-state error. The experiment results show that the proposed method has better robustness than PID, and the adaptive reaching law can lightly reduce chattering.
Remotely operated vehicles (ROVs) have been extensively applied in deep-sea exploration, submarine pipeline maintenance, deep-sea mining and underwater search and rescue, becoming an indispensable tool for exploration, development and protection of the oceans. The ROVs for deep sea applications are generally manually operated, with the accompanying drawback of inefficiency and high costs. To exploit the potential benefits provided by ROVs, automatic control capability improvement has become a crucial issue (Zereik et al., 2018). Therefore, how to realize control algorithm upgrade of ROVs is urgent to be solved.
The precision control of underwater vehicle is never an easy task, especially for the ROVs. The complex underwater environment and strong nonlinearity make it a challenge to deriving controller, let alone the influence of cable and model uncertainty. To address the above problems, especially to achieve the accurate trajectory tracking, a kind of double-loop control strategy is commonly utilized to design the motion controller. The principle of this strategy is to divide the control system of the carrier into two subsystems, kinematics and dynamics, and to design separate feedback controllers for the two systems. The inner loop is the dynamics system, which mainly considers the handling of external disturbances and emphasizes the robustness of the controller. The outer loop is the kinematic system, which achieves accurate tracking of the given trajectory, requiring accuracy and avoiding overshoot as much as possible. The advantage of this control strategy is that multiple control methods can be integrated into the controller design of the inner and outer loops to obtain better control performance.