Fault tolerant control system for an ROV based on a novel integral sliding mode control and a state and fault observer in the presence of thruster limitations

This paper presents a novel fault tolerant control system for remotely operated vehicles in the presence of unknown system dynamics and unknown thruster faults and failures while accounting for magnitude and rate limits of the thrusters. The proposed control system consists of a novel integral sliding mode control (ISMC) and a new state and fault observer. The proposed ISMC utilizes a novel asymptotic sliding surface to enhance the performance of the control system during both initial transients and steady-state conditions. The observer takes thrusters system dynamics into account and provides estimation of faults and failures. Control allocation is utilized for fault accommodation and to separate the high-level control system and the low-level thruster controllers. Stability analyses are carried out and it is shown that the state observer is guaranteed to be fixed-time convergent while the fault observation error is input-to-state stable. It is also shown that the ISMC-based closed-loop control system is uniformly-ultimately bounded with practically fixed-time reaching phase. Simulations are carried out and comparisons are made with several fixed-time and finite-time control systems. The results outline the superior performance of the proposed control system in terms of positioning accuracy and fault tolerance during initial transients and steady-state conditions.