Autonomous Underwater Vehicles-Their Design and Functioning

INTRODUCTION

An Autonomous Underwater Vehicle (AUV), also called  unmanned underwater vehicles, are robots that perform underwater survey missions such as detection and mapping of obstructions, rocks, submerged wrecks( like that of ships) without the need for input from an operator.The first ever AUV was the SPURV (Special Purpose Underwater Research Vehicle), developed at the Applied Physics Laboratory at the University of Washington as early as 1957.
The design of these vehicles is influenced by the purpose of their operation. How the individual components function as a whole is also determined largely by the circuital pathways and of course, it is necessary to factor into account, the energy management. AUV's are sold by around 10 major manufacturers on the international market, including prominent ones like Kongsberg Maritime, Bluefin Robotics, and International Submarine Engineering (ISE) Ltd.

Collections of propelled AUVs and gliding AUVs (also called gliders) are now often used for mapping and oceanographic research, for military reconnaissance and harbour protection, or for deep-sea oil-well maintenance and emergency response. Today, fleets of up to 20 such AUVs have been deployed, but in the coming years far larger fleets could come into service.

DESIGN

National Institute of Oceanography (N.I.O), Goa,India, has developed an AUV ‘Maya’ as a tool for scientific and commercial applications. 

Equipment:

AUVs operate independently of the ship and have no connecting cables. AUVs can be equipped with a wide variety of oceanographic sensors or sonar systems. NOAA’s hydrographic survey AUVs are typically equipped with side scan sonar, Conductivity-Temperature-Depth (CTD) sensors, GPS-aided Inertial Navigation Systems (INS), and an Acoustic Doppler Current Profiler (ADCP).Primarily oceanographic tools, AUVs carry sensors to navigate autonomously and map features of the ocean. Typical sensors include compasses, depth sensors, sidescan and other sonars, magnetometers, thermistors and conductivity probes. 

Propulsion:

One of the largest design considerations for autonomous underwater vehicles (AUV’s) that have specific mission scenarios is the propulsive efficiency. The propulsive efficiency affects the amount of power storage required to achieve a specific mission. As the efficiency increases the volume of energy being stored decreases. The decrease in volume allows for a smaller vehicle, which results in a vehicle that requires less thrust to attain a specific speed.

Propeller based thrusters or Kort nozzles are the most common among AUVs.These thrusters are usually powered by electric motors and corrosion of motor internals are avoided. One consideration which impacts this process of waterproofing is the decision to use brushed motors or brush-less motors which also impacts reliability, efficiency, and cost. Propellers are usually designed with a complex geometry that changes along the blades radius. The process of selecting an efficient propulsive system becomes an iterative process between motor, propeller, and battery storage. 

A Kort Nozzle is a propeller fitted with a non-rotating nozzle.

Navigation

AUVs navigate using an underwater acoustic positioning system. When some reference such as a support ship is available, baseline  positioning is used to calculate where the sub sea vehicle is relative to the known (GPS) position of the surface craft by means of acoustic range and bearing measurements which is some sort of a via mechanism.Orientation (including heading) is determined by in an alignment process where the Kalman filter uses gyros and accelerometers to determine local gravity vector and the Earths Rotation which are essential in navigation.Alignment is done both statically and while in motion.

Power

AUVs use many of the existing rechargeable battery systems in existence today, lithium ion(the one in an average mobile phone), lithium polymer, nickel metal hydride among others , and are having some sort of system for battery management. Some vehicles use primary batteries . Some larger vehicles are powered by the extremely efficient semi-fuel cells, but these require proper handling and up keep,also they are not cheap and require disposal of wastes. An emerging trend is to combine different battery and power systems with supercapacitors. High density energy sources are what designers look for today.

The battery compartment showing the 50 kWh Al/HP semi fuel cell. 

AUV BLUEFIN 21 : MH 370

Bluefin-21 AUV is currently completing mission ten in the underwater search area. Bluefin-21 has now completed more than 80 per cent of the focused underwater search area. No contacts of interest have been found to date.

                                                

As per the reports of the indianexpress ,underwater search for the crucial flight recorders of the crashed Malaysian jet can be completed , provided the weather is favorable for the AUV. The focused underwater search area is defined as a circle of 10 km-radius around the second Towed Pinger Locator detection which occurred on April 8.Don't forget to watch the video below.

Finding the black box and the wreckage are crucial to know why the Beijing-bound plane veered off from its route and mysteriously vanished after taking off from Kuala Lumpur. The mystery of the missing plane has continued to baffle aviation and security authorities who have so far failed to trace the aircraft despite deploying hi-tech radar and other gadgets.

THE CHOICE

There comes a choice in the basic design form which influences the hydrodynamic properties of flow around it and which in turn influences the performance and effiecieny. Something which helps in the analysis of the flow are computational flow techniques (CFD), now according to this approach, the design form (B) on the right has a more streamlined form and has better flow charateristics around it, reduces power consumption. 

However the design (A) on the left has better storage and larger equipment carrying capacity. The choice of design is now largely influenced by the needs of the owner or organisation. Both designs have been used, but an AUV equipped with the most routine equipments and basic power storage for a medium endurance mission generally edges in favour of the cylindrical body shape. Having said that, there are underwater semi-submersibles and robots which come in a range of shapes and sizes including the design (B). Factors worth considering before making the choice would be power requirements, equipment capacity, manoeuvring characteristics and cost. Hope this helps in understanding why most AUVs are cylindrical although there are ways to make them more streamlined. LSD

Article By: Sudripto Khasnabis

Author's Note: This article is intended to familiarize the reader about Autonomous Underwater Vehicles in light of the recent MH 370 Flight incident and how AUV's are proving to be useful in this search.The videos and the figure do not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts,do not forget to write to me at learnshipdesign@gmail.com