**2.1 Requirements**

148 Autonomous Underwater Vehicles

performance. The possible range of the vehicle from the base station is also restricted, depending on the length of this tether. These two main benefits of AUVs over ROVs lead, in principle, to autonomous vehicles being selected for survey tasks in complex, dynamic and dangerous underwater environments, and therefore AUVs are the subject of this chapter. Furthermore, combining the desired performance characteristics of AUVs, with the aforementioned complex operation environment, leads to the conclusion that controllers implemented within AUVs must be precise and accurate, as well as robust to disturbances and uncertainties. Hence, the focus of this chapter will primarily be on the precise and

Within the autonomy architecture of AUVs are three main systems. These are:- the *guidance system*, which is responsible for generating the trajectory for the vehicle to follow; the *navigation system*, which produces an estimate of the current state of the vehicle; and the *control system*, which calculates and applies the appropriate forces to manoeuvre the vehicle (Fossen, 2002). This chapter will focus on the control system and its two principal

The chapter will be divided into three main parts with the first part focusing on the design and analysis of the control law, the second looking at control allocation, and the third providing an example of how these two systems combine to form the overall control system. Within the control law design and analysis section, the requirements of how the various systems within an AUV interact will be considered, paying particular attention to how these systems relate to the control system. An overview of the underwater environment will be given, which depicts the complexity of the possible disturbances acting on a vehicle. This will be followed by an analysis of the equations of motion, namely the kinematic and kinetic equations of motion that determine how a rigid body moves through a fluid. A summary of the relevant frames of reference used within the setting of underwater vehicles will be included. A review of the control laws that are typically used within the context of

The second section will look at the role of control allocation in distributing the desired control forces across a vehicle's actuators. An analysis of the principal types of actuators currently available to underwater vehicles will be conducted, outlining their useful properties, as well as their limitations. This section will conclude with an overview of various techniques for performing control allocation, with varying degrees of computational

The third and final section of this chapter will present an example of an overall control system for implementation within the architecture of AUVs. This example will demonstrate how the control law and control allocation subsystems interact to obtain the desired trajectory tracking performance while making use of the various actuators on the vehicle.

Before delving into the laws governing how a particular control system produces a correcting signal, it is necessary to look at the requirements of the various systems within an AUV. This will provide an understanding of how the control system fits in with respect to the overall autonomy architecture of an AUV. The different types of disturbances must be acknowledged such that the effect of these disturbances is minimised, and the equations related to the dynamic motion of the vehicle must be analysed. Only after reviewing these

underwater vehicles will be conducted to conclude this section of the chapter.

subsystems, namely the *control law* and the *control allocation*.

robust control of AUVs.

complexity.

**2. Control law design and analysis** 

factors can the control law be designed and analysed.

As previously stated, the various components that make up the autonomy architecture of AUVs are the guidance system, navigation system, and control system. All three of these systems have their own individual tasks to complete, yet must also work cooperatively in order to reliably allow a vehicle to complete its objectives. Figure 1 shows a block diagram of how these various systems interact.

Fig. 1. Guidance, Navigation and Control Block Diagram.
