Estimation of Parameters and Design of a Path Following Controller for a Prototype AUV

Abstract

In order to improve the performance of autonomous underwater vehicles (AUVs) deployed in different applications such as oceanographic survey, search and detection tasks in a given area necessitates the development of an appropriate path following controller which offers a precise and rapid control of the AUVs’ control surfaces and propeller system. In order to design such a vehicle control system, there is a need for good approximation of the vehicles static and dynamic model. Based on a combination of theoretical and empirical data, it can provide a good starting point for vehicle control system development as well as an alternative to the typical trial-and-error methods used for controller design and tuning. As there are no standard procedure for AUV modeling, the simulation of each autonomous underwater vehicle (AUV) represents a new challenge. This thesis describes the development of a six degree of freedom, non-linear simulation model for the prototype AUV. In this model, all the forces which strongly affect the dynamic performance of an AUV such as the external forces and moments resulting from hydrostatics, hydrodynamics, lift and drag, added mass, and the control inputs of the AUV propeller and fins are all defined in terms of vehicle coefficients. Computational Fluid Dynamics along with empirical formulas have been applied to determine the hydrodynamic coefficients of the AUV. In order to model the behavior of the AUV as closely to the real-world system as possible, the equations used for determining the coefficients, as well as those describing the AUVs’ motions were left in non-linear form. Simulation of the AUV motion was achieved using numerical integration techniques of the equations of motion based on the derived coefficients. From the simulation, of the AUV model, results observed led to the development of a controller for the prototype AUV. Sliding Mode Controller was chosen as the desired controller because of its definitive advantages over the PID controller, some of which are the straightforward firmware implementation, use of discrete decision rules which allows the controller to function in hybrid feedback configuration and the fact that it does not suffer from issues related with the drift in controller signal output with time, i.e. latency issues for real time applications. The developed model of the prototype AUV was decoupled into two separate parts namely Heading control and Depth control. State Space Model for each part was derived and a Sliding Mode controller was developed based on the required dynamics of each part. Simulations of the AUV model integrated with Sliding Mode Controller (SMC) was carried out to determine whether the controller was able to direct the motion of the prototype AUV along the desired path, i.e. the level of accuracy of the prototype AUV in path following task.