# Study of Motion Control and a Virtual Reality System for Autonomous Underwater Vehicles
## 3 Design of AUV Motion Control Algorithm  
The AUV motion control algorithm is used to autonomously maintain the course of the underwater vehicle. It has periods of course changing and course keeping. During the period of course changing, the motion controller rapidly narrows the course error according to the given course; while during the period of course keeping, the controller must overcome the influences of marine conditions and keep the vehicle steady and sail on a predefined course. Some algorithms for AUV motion control have been studied. PID control strategy [13] is the widely used approach for AUV motion control. However, PID autopilot lacked the adaptability to the change of the working state and environment, so it is hard to achieve optimal control since the parameters have to be set manually. Sliding mode control strategy [14] has been proved to be able to tackle system uncertainties and external disturbances with good robustness. Thus, it is usually used for dynamic tracking control of AUV. However, one major disadvantage is the high frequency of chattering. This high-frequency chattering can cause high heat losses in the system and premature wear in thruster. Self-adaptive control [15] has certain adaptability to the change of the working state and environment of the AUV, and can automatically adjust the control parameters. However, self-adaptive control algorithm is based on the accurate mathematical model, while the actual motion process changes with the working situations and navigational environments. Thus, this is a process of time varying, nonlinearity, and large interference for a model. The control therefore cannot avoid problems, such as difficult adjustment of controller parameters, and robustness. Backstepping control [16] has been used in mobile robot tracking control and is also adapted to AUV. The idea of a backstepping algorithm is to define velocity controller that stabilizes the closed-loop system. It can deal with large initial state errors. However, sharp speed jump occurs with sudden tracking errors. The design of an AUV motion control system implemented from classical and modern control theories usually need to know the linear and nonlinear mathematical models of the controlled object. However, AUV has high nonlinearity and uncertainty, and therefore it may be hard to describe using accurate mathematical models. Thus, the system faces a great difficulty in adopting either the classical or the modern control theory to design an ideal controller. An intelligent control system does not rely on the mathematical model of the controlled object, and has a certain adaptive capacity to the nonlinearity and time variability of the controlled object. Therefore, such an adaptive system can be applied to the nonlinear, time-varying, hysteretic, and uncertain autonomous underwater robotic motion control environment. The latest studies on developing intelligent control systems have improved the development of AUV intellectualization. The fuzzy control approach is employed for controlling uncertain or strongly nonlinear systems without knowing the precise system model. It does not need the computation procedure to be relatively simple. The underwater robots \"Ocean Voyager\" and \"APPA\", developed by the Florida Atlantic University, both adopt fuzzy systems for depth control. For the fuzzy control adopted by the \"Ocean Voyager\", the heading, trim, and depth are well controlled. Each subsystem has  
Figure 2: The path tracking control system for AUV in the horizontal plane. three controllers which are respectively designed for dealing with different velocity ranges. Additionally, the control variable of the rudder angle is a function of the input error and the velocity of error, which have good control effect. The fuzzy control system is easy to design, has good stability, and is also applicable to a nonlinear system for which constructing an accurate mathematical model can be difficult. Therefore, fuzzy control systems are useful for the development of AUV control systems [17; 18; 19; 20].