This paper presents a novel framework for a robust motion control scheme of an eight-thruster underwater vehicle. It combines a model-free approach intertwined with a model-based approach such as sliding mode control (SMC) to counter the unknown disturbances and decouple them to provide better tracking. A proportional-integral (PI) control-like structure is taken as the first sliding surface. A proportional derivative (PD) control-like structure is proposed as the second sliding surface for trajectory tracking. This motion control works for any fully actuated or over-actuated vehicle. Initially, the dynamic model and vehicle configuration are presented. Then the vehicle’s closed-loop behavior is studied in the presence of underwater currents. Later, the study considers external disturbances and compensates them with the help of a nonlinear disturbance observer. Lyapunov’s direct method and Barbalat’s lemma ensure the asymptotic convergence of tracking errors. The proposed controller performance is evaluated using a detailed comparison study with different model-free and model-based controllers from the literature. Later, the control scheme’s effectiveness is demonstrated numerically with the help of computer-based simulations. The robustness against the parameter uncertainties, underwater currents, and unknown disturbances is also presented.

Different thruster configurations (TCs) or arrangements are used in underwater vehicles (UVs) to perform various deep-sea intervention operations. The location and arrangement of thrusters and the number of thrusters can vary the overall vehicle's performance. However, a comparison study of the effect of these different TCs or performances is yet to be available in the literature. This study will provide a base for the vehicle design and choice of thruster along with their arrangements. Therefore, in this paper, computer-based numerical simulations are conducted with these TCs using eight identical thrusters on a common vehicle platform to analyze their relative performances under different operating conditions. The performance parameters studied in this paper are position error, orientation error, and energy consumed. Initially, the maximum allocated force and moments in the vehicle's fixed frame are found by giving a constant input force to the thrusters. Then the TCs are simulated with closed-loop trajectory tracking tasks of following simple to complex profiles to identify the configurations that better perform tracking the given desired trajectories. Several computer-based simulations are conducted in the presence of underwater currents. Vectored horizontal TC (inclined to surge and sway axis) and 3D vectored TCs consume more energy than horizontal and vertical TCs. Energy consumption in horizontal and vertical TC is found to be 30 % and 23 % lesser than 3D vectored TC in the absence and presence of underwater currents, respectively, for the same trajectory tracking simulation.