Thruster fault diagnosis and accommodation for open-frame underwater vehicles

This paper introduces a novel thruster fault diagnosis and accommodation system (FDAS) for open-frame underwater vehicles. Basically, the FDAS is a control allocator, but this primary function is enhanced with the ability of automatic thruster fault detection and accommodation. The proposed FDAS consists of two subsystems: a fault diagnosis subsystem (FDS) and a fault accommodation subsystem (FAS). The FDS uses fault detector units (FDUs), associated with each thruster, to monitor their state. Robust and reliable FDUs are based on integration of self-organising maps and fuzzy logic clustering methods. These units are able to detect internal and external faulty states of thrusters. The FAS uses information provided by the FDS to accommodate faults and perform an appropriate control reallocation. A control energy cost function is used as the optimisation criteria. The FAS uses weighted pseudo-inverse to find the solution of the control allocation problem, which minimise this criteria. Two approximations (truncation or scaling) can be used to ensure feasibility of the solution. The proposed FDS is evaluated with data obtained during test trials. The feasible region concept, related with the problem of thruster velocity saturation, is developed in order to provide geometrical interpretation of the control allocation problem. The proposed FDAS is implemented as a Simulink model (ROV simulator), in order to evaluate its performance in different faulty situations.