Review on Design of Control Systems for Tethered UUVs using PID Controllers

Unmanned Underwater Vehicles have gained popularity for the last decades, especially for the purpose of not risking human life in dangerous operations. On the other hand, underwater environment introduces numerous challenges in control, navigation and communication of such vehicles.Certainly, this fact makes the development of these vehicles more interesting and engineering-wise more attractive. Conventional Proportional-Integral-Derivative (PID) controllers exhibit moderately good performance once the PID gains are properly tuned. However, when the dynamic characteristics of the system are time dependent or the operating conditions of the system vary, it is necessary to retune the gains to obtain desired performance. Self-tuning of PID controllers has emerged as a new and active area of research with the advent and easy availability of algorithms and computers. The proposed study operated and autonomous underwater vehicles. The difference between an autonomous underwater vehicle, or AUV, and a ROV


I.INTRODUCTION
The term UUV is a generic expression to describe both remotely operated and autonomous underwater vehicles. between an autonomous underwater vehicle, or AUV is that the ROV is connected to a command platform (for instance a ship) with a tethered cable or an acoustic link [3]. The tethered cable ensures energy supply and information signals, in this way an operator is able to constantly monitor and control the vehicle. The AUV however, is equipped with a battery pack and sonar to fulfill its mission avoiding the use of an operator, which introduces conflicting control requirements. It has to be simple enough to ensure an online implementation of control techniques but at the same time has to cope with a time-varying vehicle/environment interaction.  In the proposed study, an experimental remotely   operated vehicle as a test platform for experiments and analysis is considered [2]. This is a torpedoshaped under actuated ROV, without any side thrusters to control the sway direction (this is not implemented because of economical and weight considerations) (Fig. 1).
There are only two stern propellers which are offering control inputs as the force in the surge direction and the control torque in yaw direction in the horizontal plane. Commands as well as the tracking error are used in arriving at the optimal gains.

III. Problem definition
In order to achieve a high degree of autonomy, a suitable controlling method of underwater vehicles is very challenging due to the nature of underwater dynamics and parameter uncertainties (disturbances, wind , waves, ocean-currents etc.).In this proposed work, among the mentioned problems, control of underwater vehicles, particularly the motion control will be focused upon.

IV. Methodology
This section describes the methodology used in the proposed work. The problem specification will be identified first to have a clear view in order to overcome the limitations, weakness and to improve the systems. For the Unmanned Underwater Vehicle, the most crucial issue is the control system needed by the ROV to perform the underwater applications and tasks.

V. Control mechanism of UUV
The control scheme will be tested in simulation studies using a six degrees of freedom UUV model which contains both kinematic and dynamic elements.

VI. Motion control
The motion of UUVs can be described in 6 degreesof freedom (DOF), since 6 independent coordinates are necessary to determine the position and orientation of a rigid body. The 6 different motion components are defined as 'surge','sway', 'heave', 'roll', 'pitch' and 'yaw', as shown in

IX. Stability of Underwater Vehicles
Stability of an underwater vehicle can be defined as "the ability of returning to an equilibrium state of motion after a disturbance without any corrective action, such as use of thrusters power or control surfaces" [1]. Hence, maneuverability can be defined as the capability of the vehicle to carry out specific manoeuvres.
At this point, the following issue about the stability shall be emphasized. Excessive stability implies very high control effort; whereas it would be easy to control a marginally stable vehicle.
Consequently, there exists a compromise between

IX. Stability of Underwater Vehicles
Stability of an underwater vehicle can be defined as "the ability of returning to an equilibrium state of motion after a disturbance without any corrective action, such as use of thrusters power or control surfaces" [1]. Hence, maneuverability can be defined as the capability of the vehicle to carry out specific manoeuvres.
At this point, the following issue about the stability shall be emphasized. Excessive stability implies very high control effort; whereas it would be easy to control a marginally stable vehicle.
Consequently, there exists a compromise between stability and maneuverability.Furthermore, it makes sense to distinguish between controls-fixed (open-loop) and controls-free (closed-loop) stability. The essential difference between these terms can be stated as follows: -Open-loop stability implies investigating the vehicle's stability when the control surfaces are fixed, and when the thrust from all the thrusters is constant.
-Closed-loop stability refers to the case when both the control surfaces and the thruster power are allowed to vary. This implies that the dynamics of the control system must also be considered in the stability analysis.

X. Communication
The ROV system is unmanned, highly maneuverable and it is operated by a person on board of a vessel. It will be linked to the ship by a tether (optical fibre cables).
As the sophistication of acoustic sensor and communication systems related to unmanned underwater vehicles (UUV) has increased, the requirement for greater volume and higher speed data transfers has emerged. Fiber optic technology provides an effective means for high bandwidth communications with a UUV while minimizing weight and space criteria aboard the UUV [3].
Increase in data transmission speed has permitted real time processing of data on the launch platform when using large high powered computing systems.
Maximum system reliability at advanced performance levels can also be realized.

CONCLUSION
The considered experimental ROV will be hosted in the MATLAB/Simulink environment.
Simulation results show the viability and attractiveness of the approach adopted.
Each underwater vehicle should have a motion control system specific to its characteristics and needs.
Although, numerous control strategies which were successfully applied for the motion control problem of underwater vehicles exist and are literally accurate, it is hard to determine which approach is the most suitable and furthermore applicable to our cases. Not only for the motion, but also for the mission and formation control, the most optimal algorithms should be adopted and strategies should be carefully chosen in order to acquire robust underwater vehicles that will perform critical applications.
Regarding motion control of underwater vehicles, utilization of more advanced system control systems is inevitable in order to design more intelligent, adaptive, and robust controllers that provide optimal control solution in terms of non-linearity handling, and cost minimization.