Optimal Control of Underactuated Underwater Vehicles with Single Actuator

The research on underwater systems has gained an immense interest during the last decades with applications taken place in many fields such as exploration, investigation, repair, construction, etc. Hereby, control of underwater systems has emerged as a growing field of research. Underwater vehicles, in fact, accounted for 21% of the total number of service robots by the end of 2004, and are the most expensive class of service robots (UNECE/IFR, 2005). Typically, underwater vehicles can be divided into three underwater systems, namely, the manned submersibles, remotely operated vehicles (ROV) and autonomous underwater vehicles (AUV). ROVs and AUVs are mostly utilized in the oil and gas industries, and for scientific and military applications. AUVs, especially are of great importance due to their ability to navigate in abyssal zones without necessitating a tether that limits the range and maneuverability of the vehicle. However, their autonomy property directly affects the design of the control system. This requires advanced controllers and specific control schemes for given tasks. Almost all AUVs are six degrees-of-freedom (DOF) systems, and various types of actuator configurations are available in the industry for the vehicles ranging from fully-actuated vehicles to underactuated ones. The vehicle of interest here falls into the class of underactuated AUVs. Any mechanical system having fewer actuators than its degrees of freedom is defined as an underactuated system. Some examples of underactuated systems include manipulators; (Arai et al., 1998), (Oriolo & Nakamura, 1991), (Yabuno et al., 2003), marine vehicles; (Reyhanoglu, 1997), (Pettersen & Egeland, 1996), space robots; (Tsiotras & Luo, 1997), and the examples given in (Fantoni & Lozano, 2002). Controlling all of the DOF of underactuated mechanical systems is an arduous task compared to the fully actuated systems since the mathematical analysis of the system renders it difficult. Determining whether an underactuated system is controllable is one of these difficulties encountered. Control synthesis is also another challenge in this field and is still accepted as an open problem. The techniques used for fully actuated systems cannot be used directly for underactuated systems. However, there are some potential benefits over fully actuated systems depending on the efficiency of control and the task. In case of actuator failures, a fully actuated mechanical system falls into the class of underactuated systems and might still be controlled if a successful control scheme can be designed. Besides that, reduction of the weight and cost, and the increase of reliability can be considered as O pe n A cc es s D at ab as e w w w .in te ch w eb .o rg