1 Introduction
Considerable effort has focused on two control problems of underactuated underwater vehicle (UUV), i.e., path following problem and coordination control problem. For path following problem, controllers are designed to drive each UUV to a desired path. Numerous approaches to the path following control of UUV have been reported in the literature, such as sliding mode control[1], adaptive control[2], and predictive controll[3], to name a few. There also exist some other approaches, such as virtual sonar based methods, and PID control methods.