Robustness of Fuzzy Logic based Controller for Unmanned Autonomous Underwater Vehicle

The underwater vehicle is six degrees of freedom model. The execution of spatial maneuvers are determined mainly by the dynamic properties of underwater vehicle particularly controllability and stability. The control surfaces are situated at the rear end of the underwater vehicle which moves either vertically or horizontally (pitch, yaw, roll, pitch-rate, yaw-rate etc.) used to steer the vehicle to run according to preprogrammed course as per logic till such a time the target is acquired. The underwater vehicle response is slow compared to air scenario due to constraints like higher density of water; the resistance motion is many hundred times greater than air. In this paper a rule-based fuzzy logic controller is designed for yaw control, which is used for the rudder movement of an underwater vehicle. A plant model is extracted using the input and output behavior and is assumed to the either linear time invariant first order or second order. Since the plant models obtained by the above process are very approximate, 50% variations are given on damping (xi) and natural frequency (omegan) of second order plant and similarly the gain and time constant are varied for the first order plant. The performance evaluation of fuzzy logic controller under the above varying plant conditions is done and the results have been presented and analyzed.