ABSTRACT:

Application of robust design techniques in the design of controllers for Autonomous Underwater Vehicles (AUVs) is discussed in this paper. Proper tuning of controllers is essential in achieving the desired positional and tracking accuracy of AUVs in underwater missions. A new scheme of gain tuning using Taguchi method is introduced here for the proportional-integral-derivative (PID) control of an under-actuated AUV with less controllable degrees of freedom than the total number of degrees of freedom. With four controllable states, this vehicle has twelve controller gains to be tuned. Through the help of design of experiments (DOE) and robust design techniques, as applied in the Taguchi optimization method, an optimal and robust set of PID controller gains have been obtained. A simulation study of the pitch control of an AUV using the above method is presented in the paper. The simulation results are compared with the other tuning methods like Ziegler - Nichols, Auto tune, Tyreus - Luyben and Cohen-Coon method. The results show that the index of aggregate absolute error has reduced to 78.73% and 24.34% from the Ziegler - Nichols method 1 and Tyreus-Luyben method respectively.

INTRODUCTION

Considering the importance of underwater technology in exploring the untapped resources beneath the sea, there seems to be a renewed interest in underwater robotics research, especially in developing advanced control strategies for autonomous vehicles. Recent developments in this area are well summarized in [Fossen, 1994 and Yuh, 2000]. Dynamics and control of AUV in a constrained environment poses great challenges to designers. This, coupled with the uncertainty of hydrodynamic parameters, make the controller design an extremely tough task. Many researchers have approached this problem and many solutions have been proposed in the literature, with varying degrees of success, which are summarized in [Antonelli, 2001, 2007 and Choi, 1996].

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