**1. Introduction**

34 Will-be-set-by-IN-TECH

132 Autonomous Underwater Vehicles

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Autonomous Underwater Vehicles (AUV) speed and position control systems are subjected to an increased focus with respect to performance and safety due to their increased number of commercial and military application as well as research challenges in past decades, including underwater resources exploration, oceanographic mapping, undersea wreckage salvage, cable laying, geographical survey, coastal and offshore structure inspection, harbor security inspection, mining and mining countermeasures (Fossen, 2002). It is obvious that all kinds of ocean activities will be greatly enhanced by the development of an intelligent underwater work system, which imposes stricter requirements on the control system of underwater vehicles. The control needs to be intelligent enough to gather information from the environment and to develop its own control strategies without human intervention (Yuh, 1990; Venugopal and Sudhakar, 1992).

However, underwater vehicle dynamics is strongly coupled and highly nonlinear due to added hydrodynamic mass, lift and drag forces acting on the vehicle. And engineering problems associated with the high density, non-uniform and unstructured seawater environment, and the nonlinear response of vehicles make a high degree of autonomy difficult to achieve. Hence six degree of freedom vehicle modeling and simulation are quite important and useful in the development of undersea vehicle control systems (Yuh, 1990; Fossen 1991, Li et al., 2005). Used in a highly hazardous and unknown environment, the autonomy of AUV is the key to work assignments. As one of the most important subsystems of underwater vehicles, motion control architecture is a framework that manages both the sensorial and actuator systems (Gan et al., 2006), thus enabling the robot to undertake a user-specified mission.

In this chapter, a general form of mathematical model for describing the nonlinear vehicle systems is derived, which is powerful enough to be applied to a large number of underwater vehicles according to the physical properties of vehicle itself to simplify the model. Based on this model, a simulation platform "AUV-XX" is established to test motion characteristics of the vehicle. The motion control system including position, speed and depth control was investigated for different task assignments of vehicles. An improved Ssurface control based on capacitor model was developed, which can provide flexible gain selections with clear physical meaning. Results of motion control on simulation platform "AUV-XX" are described.

Modeling and Motion Control Strategy for AUV 135

translational velocities associated with surge, sway and heave to ocean current in the bodyfixed frame, here assuming the sea current to be constant with orientation in yaw only,

forces *X* ,*Y* , *Z* , *K* , *M* , *N* includes positive buoyant *BW P* − = Δ ( since it is convenient to design underwater vehicles with positive buoyant such that the vehicle will surface automatically in the case of an emergency), hydrodynamic forces *XYZK MN HH H H H H* ,,, , ,

The modeling of thruster is usually done in terms of advance ratio <sup>0</sup>*J* , thrust coefficients*KT* and torque coefficient *KQ* . By carrying out an open water test or a towing tank test, a unique curve where 0*J* is plotted against *KT* and *KQ* can be obtained for each propeller to depict its performance. And the relationship of the measured thrust force versus propeller revolutions for different speeds of advance is usually least-squares fitting to a

Here we introduce a second experimental method to modeling thruster dynamics. Fig.2 shows experimental results of thrusters from an open water test in the towing tank of the Key Lab of Autonomous Underwater Vehicles in Harbin Engineering University. The results are not presented in the conventional way with the thrust coefficient *KT* plotted versus the open water advance coefficient <sup>0</sup>*J* , for which the measured thrust is plotted as a

The thrust force of the specified speed of vehicle under a certain voltage can be finally approximated by Atiken interpolation twice. In the first interpolation, for a certain voltage, the thrust forces with different speeds of the vehicle (e.g. 0m/s, 0.5m/s, 1.0m/s, 1.5m/s) can be interpolated from Fig.1, and plot it versus different speeds under a certain voltage. Then based on the results of the first interpolation, for the second Atiken interpolation we can

Fig. 2. Measured thrust force as a function of propeller driving voltage for different speeds

α

. The resultant

which can be described by the vector <sup>T</sup> [ , , ,0,0, ] *Uc cc c c* = *uvw*

function of different speeds of vehicle and voltages of the propellers.

find the thrust force for the specified speed of the vehicle.

and thruster forces.

quadratic model.

of vehicle

**2.2 Thrust hydrodynamics modeling** 
