Characterisation of the propulsion systems of autonomous underwater vehicles

Abstract

The process of engineering involves devising machines capable of performing new functions, developing them and standardising them. Machines to perform higher order or multiple functions are created by integrating sets of standard components into systems. As the number of components integrated increases the systems become more complex, both internally and in terms of their interaction with the rest of the world. During development, simplifying assumptions are necessarily made in order to predict the performance of the overall system. Trials in an idealised environment are performed. Once the system is considered to have demonstrated the minimum performance requirement it is put into production. However, although the system now meets the minimum performance requirement in the environment in which it was tested, if it is at all complex it is most unlikely that it will have been tested under all of the conditions it will experience during its in-service life; it will not have been fully characterised in terms of defining its performance under all conditions; and its overall performance is unlikely to have been globally optimised. This Thesis describes a means of improving the performance of in-service complex systems by more fully characterising them. The method addresses the full complexity of the system. This necessarily involves the measurement of its response to a large number of parameters. Taguchi experimental methods are used to make this feasible. Complementary sets of measurements under controlled conditions on a physical model in the laboratory and measurements on the full-scale system in service are devised. The method is developed in the context of the propulsion system of a specific Autonomous Underwater vehicle (AUV), AUTOSUB. The generic properties of AUV systems and their propulsion requirements are described, together with that of the subject vehicle. A Systems Engineering approach is taken to describe the system and determine its response to its principal characteristics. The issue of complexity is discussed and the case made for the propulsion system of a multi-function AUV being considered as a complex system. The requirement for laboratory experiments and full- scale trials is derived and their design developed. A description of the conduct of these and the results of the experiments and trials are provided, together with the method for analysing the results. Finally conclusions are drawn, both in generic terms and in terms of the vehicle under discussion.

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