Underwater robotic vehicles (URVs) are widely used for iection, long range survey and oceanographic mapping, data collection, surveillance, drill support and industrial endeavours that are often too dangerous or tedious for human divers. Automatic control of URVs presents several difficulties due to the highly nonlinearity of the dynamics, the high dergree of model uncertainty resulting from poor knowledge of the hydrodynamic coefficientes, and the effect of external, unmeasurable disturbances such as underwater currents. This thesis describes the development and detailed analysis of a six degrees of freedom, nonlinear model for the underwater hexapod robot vehicle . In this research, we extract kinematic and dynamic models of the vehicle that include calculation of gravity, buoyancy, hydrodynamic and propulsion forces and torques. Finally, simulation of the underwater hexapod robot motion is achieved through numeric integration of the nonlinear motion equations in the earth fixed reference frame. In this thesis, two different approaches have been used to control the underwater robot states. The first approach is a new metheod based on robust nonlinear time delay control for underwater robots. Simulation and implementation of a robust nonlinear control procedure which uses time delay control (TDC) for the nonlinear plant dynamic are discussed. Robust control law using time delay control has a very simple and efficient straucture and does not require the nonlinear plant dynamics model in it. The proposed control algorithm uses the direct estimation of some plant parameters to cancel the unknown nonlinear dynamics and unexpected disturbances using observed information them with the time-delayed values of control inputs and acceleration at the previous time step. It is work to mention that the sufficient conditions of the overall stability requires knowledge of the inertial matrix of the underwater robot. The second approach uses the linear equations of motion, and deals with the uncertainties and unknown dynamics. The design of controller is formulated as the problem of minimizing the mixed sensitivity function, and then solving it numerically by the LMI-based method. The behaviour of the controllers is assessed by simulations with a full nonlinear model of the vehicle. Finally, simulation results show that both proposed controller can effectively track given trajectories in the presence of uncertainty in hydrodynamic parameters. Keywords: Undewater Hexapod Robot, Rigid Paddle, Robust Control, Time Delay Control (TDC), Control.
