Look Up Table Based Fuzzy Logic 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 be a linear time invariant second order. For on line implementation a decision table is stored in underwater vehicle computer memory in the form of Lookup table. For each combination of Inputs the required search will be done in the table and the appropriate value will be picked up. Using this technique the control algorithm becomes shorter and runs faster than those that reinterpret the rules at each control cycle of the system. This Lookup Table is used in the simulation of Yaw control of a Six Degrees of Freedom Model. The plant responses are compared for both conventional controller and fuzzy logic controller with regard to time of response, overshoot and steady state error.