The effect of velocity and orientation on the simulation and experimental characterization of an AUV tunnel thruster
Date
2002
Authors
Saunders, Aaron
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Abstract
Streamlined Autonomous Underwater Vehicles (AUV's) have traditionally been used in high-speed missions that require the vehicle to traverse long distances. Missions such as profiling of salinity and temperature of large-scale water phenomena like the Gulf Stream required measurements to be taken over many hundreds of kilometers. Some new applications require an AUV capable of completing missions including both high-speed straight-line runs and slow maneuvers or station keeping tasks. These missions could include oceanographic sampling, environmental monitoring, iceberg profiling, and pipeline inspection. At low, or zero, forward speeds the AUV's control surfaces become ineffective, and this presents difficulties in accommodating these types of missions. To enable or improve an AUV's low speed maneuverability, while maintaining a low drag profile, through-body tunnel thrusters have become a popular addition to modern AUV systems.
Small underwater vehicles react quickly to changes in the magnitude and direction of thrust. This causes the overall system dynamics during low speed maneuvering to be strongly affected by the dynamics of the thrusters themselves. Improvements in the mathematical models used to describe the dynamics of underwater thrusters have led to improved control systems, capable of precise low speed maneuvering. Presently there are several dynamic models that have been developed to represent the performance of tunnel thrusters. However, these models do not include the effects of forward vehicle motion or yawed orientation on 'through-body' thrust performance.
In order to adapt the current tunnel thruster models to include the effects of vehicle motion, an experimental system was designed to characterize the effects of forward vehicle speed and yaw angle on tunnel thruster performance. This full scale system includes a transverse tunnel thruster mounted in a streamlined AUV. A 6-axis load cell balance mounted internally was used to measure the thrust directly. The AUV was attached to a yaw plate assembly in Memorial University of Newfoundland's tow tank, and several tests were run to characterize the effect of vehicle motion on the transient and steady state thruster performance. This thesis will present and discuss the experimental setup as well as the results obtained from the tow tank tests. The trends shown by the experimental results are highlighted and the relevant parameters which affect thruster performance are identified. Finally, a conventional thruster model is modified to include the effect of these parameters.