**2.3.1.1 N-Frame**

150 Autonomous Underwater Vehicles

The control system is responsible for providing the corrective signals to enable the vehicle to follow a desired path. This is achieved by receiving the desired state of the vehicle from the guidance system, and the current state of the vehicle from the navigation system. The control system then calculates and applies a correcting force, through use of the various actuators on the vehicle, to minimise the difference between desired and current states (Fossen, 1994, 2002). This allows the vehicle to track a desired trajectory even in the presence of unknown disturbances. Even though each of the aforementioned systems is responsible for their own task, they must also work collaboratively to fully achieve autonomy within an

Underwater environments can be extremely complex and highly dynamic, making the control of an AUV a highly challenging task. Such disturbances as currents and waves are ever present and must be acknowledged in order for an AUV to traverse such an

Ocean currents, the large scale movement of water, are caused by many sources. One component of the current present in the upper layer of the ocean is due to atmospheric wind conditions at the sea surface. Differing water densities, caused by combining the effect of variation of salinity levels with the exchange of heat that occurs at the ocean surface, cause additional currents known as thermohaline currents, to exist within the ocean. Coriolis forces, forces due to the rotation of the Earth about its axis, also induce ocean currents, while gravitational forces due to other planetary objects, such as the moon and the sun, produce yet another effect on ocean currents (Fossen, 1994, 2002). Combining all of these sources of water current, with the unique geographic topography that are present within isolated coastal regions, leads to highly dynamic and complex currents existing within the world's

There are many factors that lead to the formation of wind generated waves in the ocean. Wind speed, area over which the wind is blowing, duration of wind influencing the ocean surface, and water depth, are just some of the elements that lead to the formation of waves. Due to the oscillatory motion of these waves on the surface, any vehicle on the surface will experience this same oscillatory disturbance. Moreover, an underwater vehicle will experience both translational forces and rotational moments while at or near the surface due

All matter that exists in our universe must adhere to certain differential equations determining its motion. By analysing the physical properties of an AUV, a set of equations can be derived that determine the motion of this vehicle through a fluid, such as water. To assist in reducing the complexities of these equations, certain frames of reference are utilised depending on the properties that each frame of reference possesses. In order to make use of

**2.1.3 Control** 

underwater vehicle setting.

**2.2 Environment** 

environment.

**2.2.1 Currents** 

oceans.

**2.2.2 Wind generated waves** 

to this wave motion.

**2.3 Dynamics** 

The n-frame is a co-ordinate space usually defined as a plane oriented at a tangent to the Earth's surface. The most common of these frames for underwater vehicle control design is the North-East-Down (NED) frame. As its name suggests, the three axes of the translational components of this frame have the *x*-axis pointing towards true North, the *y*-axis pointing towards East, and the *z*-axis in the downward direction perpendicular to the Earth's surface. In general, waypoints are defined with reference to a fixed point on the earth, and therefore it is convenient to conduct guidance and navigation in this frame.
