Fixed-time observer-based saturated nonsingular terminal sliding mode controller design for an over-actuated ROV with time-varying saturation limits

Input saturation is critical in over-actuated systems when multiple degrees of freedom (DOF) with different levels of disturbance rejection are controlled simultaneously by a set of actuators. The current study introduces a novel saturated non-singular terminal sliding mode controller to address input saturation for an over-actuated remotely-operated vehicle (ROV). The proposed controller consists of a tuning algorithm to ensure that the control commands do not violate the time-varying saturation limits of each DOF. In addition, a fixed-time extended-state observer is designed to estimate the vehicle's velocity along with the lumped unknown dynamics of the system. The observer is also employed as a tool to maintain the orientation of the ROV in extreme environmental conditions. The stability analysis shows that all system's states, except for yaw angle, are globally finite-time stable and the yaw angle is globally asymptotically stable. Several sets of simulations are carried out and the results demonstrate the superiority of the proposed controller in terms of positioning accuracy, saturation compensation and transient behaviour under different environmental conditions.