Contents I. Introduction
Due to the diminution of land-based natural resources, exploration and acquisition of the same in the oceans have gained significant interest and importance over the past few decades. Unmanned Underwater Vehicles (UUV) are being extensively used for conduct of a variety of underwater operations such as inspection and monitoring [1], oceanography, seabed mapping [2], payload delivery, Search and Rescue (SAR) operations, and bathymetric data acquisition [3] [4], etc. In order to accomplish the underwater missions, there are two conventional techniques have been implemented to improve the performance of UUV missions. Firstly, design a robust control for UUVs that are able to steer precisely and accurately in the presence of model perturbations and uncertainties. Secondly, design actuating parts such as thrusters and rudders etc. For this purpose, the second technique take account into consideration. A variety of rudders have been implemented to UUV to improve the maneuverability issues, such as X-rudder [5], H shape rudder, and cross rudder [6] etc. X-rudder has much greater flexibility and better performace as compared to other rudders [7]. Because in X-shape configuration, all four rudders control surfaces can be operated individually [8]. Moreover, the controller design of the X-rudder is more complex as compare to traditional cross shape rudder configuration in that the computed control torque has to be distributed to all the four rudder fins simultaneously. This essentially requires two subsystems namely, the dynamic controller and rudder angle allocator to work seamlessly. The dynamic controller produces the pitch and steering inputs respectively, for the vertical and horizontal motion of the vehicle. These are accordingly converted into four rudder angles via rudder allocator control system. The X-rudder AUVs are pre-programmed, self-governed vehicles with onboard sensor suit [9]. There is no connection between operator and the vehicle. The important aim in developing the vehicle is to achieve an autonomous vehicle. All the tasks and missions are burned to the vehicle's computer in form of instructions. The structure of X-rudder AUV is designed to make its smooth operation under sea environment. The X-rudder AUV consist of propulsion module, power module, hull module, submerging unit. Propulsion module consists of a single thruster and X form rudder that are responsible for the motion control of the X-rudder AUV. A single thruster is able to provide thrust in forward direction and X-rudder is able to perform roll, pitch and yaw. Generally, stepper motors are used for rotation of the X-rudder. The hull module is a water tight designed to keep the sensors and other sensitive components to water. The hull module must be corrosion-less and cylindrical shape. It consists of power distribution and generation module. Mostly lithium-ion batteries are used for providing the power then power is distributed to sensor suits based on the requirements.
