Fully Coupled 6 Degree-of-Freedom Control of an Over-Actuated Autonomous Underwater Vehicle

Abstract

Unmanned underwater vehicles (UUVs) are increasingly being used by civilian and defence operators for ever more complex and dangerous missions. This is due to the underlying characteristics of safety and cost effectiveness when compared to manned vehicles. UUVs require no human operator be subject to the conditions and dangers inherent in the underwater environment that the vehicle is exposed to, and therefore the risk to human life is greatly minimised or even removed. Cost effectiveness, in both time and financial respects, comes from a much smaller vehicle not containing the various subsystems required to sustain life whilst underwater, as well as smaller, less powerful actuators not placing the same levels of stress and strain on the vehicles as compared to a manned vehicle. This leads to a much smaller team required to undertake the regular maintenance needed to keep a vehicle operational. Taking these two main factors into account, the progression from manned vehicles to unmanned vehicles is a logical step within the oceanographic industry. Within the broad class of UUVs are the remotely operated vehicles (ROVs) and the autonomous underwater vehicles (AUVs). Both of these types of vehicles have been successfully used in industry, and their fundamental differences determine which type of vehicle is suited to a particular mission. The key difference between the two is that an ROV requires a tether of some description back to a base station, whereas an AUV does not. This tether connects the ROV to a human operator who can observe the current state of the vehicle and therefore provide the control for the vehicle while it executes its mission. This tether, depending on the configuration of the vehicle, can also provide the electricity to power the vehicles actuators, sensors and various internal electronic systems. AUVs have an advantage over ROVs of not requiring this tether, which leads to two main benefits. Firstly, an AUV requires little or no human interaction while the vehicle is executing its mission. The vehicle is pre-programmed with the desired mission objectives and, upon launch, attempts to complete these objectives without intervention from personnel located at the base station. This minimises the effect of human error while the vehicle is operational. The second benefit is the increased manoeuvrability that is possible without a cable continuously attached to the AUV. This tether has the potential to become caught on underwater structures, which could limit the possible working environments of an ROV, as well as cause drag on the motion of the vehicle, thus affecting its manoeuvring