Underwater Mobile Robotics

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13 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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1



Abstract

In thi
s

paper, we provide some of the objectives in
underwater mobile robots or Auton
om
ous Underwater Vehicles
(AUVs).
We will focus on four aspects power, dynamics,
Navigation and sensors, and pressure hulls.
We
also
define many
advantages and disadvantages in using certain material, design,
and techniques

in these four aspects
.



Index Terms


Underwater mobile robotics
, AUVs,
underwater navigation,


I.

I
NTRODUCTION


lthough an individual may more likely think of robots to
be land type, there i
s a significant amount of water
. Each
type has its own unique design to sufficiently operate in its
environment. Underwater mo
bile robots are used often and
widely into today’s technology. They are designed to
accomplish tasks in all types of water and de
pths. Underwater
Mobile Robot
s are also called Autonomous Underwater
Vehicles (AUV).

AUV is a robotic device which is
maneu
verable in all three dimensions, operated autonomously
by an onboard computer, and
driven through the water by a
propulsion system
.


AUVs are used by the military, scientists,
industries, and inventors.



II.

P
OWER

Power is another strong aspect when creat
ing AUVs. The
growth

of AUV powering systems have been strengthen
ed

due
to the growing involvement of the underwater studying and
engineering. Current AUVs rely on batteries that supply
limited energy. Some of
the common types of batteries are

lead
-
acid
, silver
-
zinc, lithium ion, lithium polymer, and nickel
metal hydride. Silver
-
zinc nearly provides double the energy
density of lead
-
acid batteries. A combination of being low
-
cost and high
-
density is desired to power an AUV. The
downfall of the silver
-
zinc batteries is that they are expensive.
A 325
-
kWh silver
-
zinc battery will roughly cost about
$400,000. Fuel cells or fuel
-
cell
-
like devices which are more
energetic than silver
-
zinc batteries are being considered.
Large AUVs can be powered by aluminu
m based semi
-
fuel
cells. The aluminum based semi
-
fuel cells require high
amounts of maintenance. They require expensive refills and
produce waste produ
cts that are not accepted by

the military.
Lithium ion batteries are highly ideal for underwater missi
ons
such as seabed surveying. From a power standpoint, an


individual can derive the relationship between power, range,
and speed. First we have to assume that power is only devoted
to propulsion and that the drag on the AUV is proportional to
the square
of speed. The equation states



























(1)

with

R= range in meters;

E= energy available in Joules;



= effective drag coefficient in






u= speed in m/s.


III.

DYNAMICS

The dynamics of underwater robotic vehicles are highly
nonlinear, coupled, and time varying. An AUV is a multi
-
body system and modeling can be very complicated.
Hydrodynamics will affect the vehicle motion and it
s

manipulators. This has to be taken in
to

consideration when
modeling the AUV and it
s

manipulato
rs. The designer will
have to perform a series of simulations and calculations to
achieve desired outcomes. 3D graphics and virtual reality
capabilities are useful for developing AUVs. Calculations
using the six degrees
-
of
-
freedom nonlinear equations of

motion of the vehicle are defined with respect to two
coordinate systems. The vehicle coordinate system has six
velocity components of motion that are surge, sway, heave,
roll, pitch, and yaw. The velocity vector in the vehicle
coordinate system is expr
essed as



















.
The global coordinate system O,X,Y, and Z is a fixed
coordinate system. The translational and rotational movement
in the global reference frame is
















.
The
equations of motion for AUVs without manipulators are












,
































,


and





.


J
(
x
)
is a 6x6 velocity transformation matrix that transforms
velocities of the vehicle
-
fixed to the earth
-
fixed reference
frame.
M

is also a 6
x
6 inertia matrix as a sum of the rigid
body inertia matrix and the hydrodynamic virtual inertia.

q)

is a 6
x
6 Corio
lis and centripetal matrix including rigid body.

q)

is a 6
x
6 damping matrix including terms due to drag
Underwater Mobile
Robotics

James J. Holloway,

Member, IEEE

,
and Charles G. Reynolds

A


2

forces. It is also consider
ed to be

real, nonsymmetrical and
strictly positive.
G
(
x
)
is a 6
x
1 vector containing the restoring
terms formed by the vehicle’s buoyancy and gravitational
terms. The gravitational forces will act through the center of
gravity, while the buoyant forces act through the center of
buoyancy.

Ԏ

is a 6
x
1 vector including th
e control forces and
moments.
w
is a 6x1 disturbance vector representing the
environmental forces and moments acting on the AUV.
B
is a
control matrix of appropriate dimensions.
u
is a vector whose
components are thruster forces. The kinetic energy of
the
fluid can be calculated as
















IV.

NAVIGATION

AND

SENSORS

Three groups of the vehicle’s sensors are navigation
sensors, mission sensors, and system sensors. Navigation
sensors are for sensing the motion of the vehicle. This may
include d
oppler, sonar inertial system, and gyroscope.
Mission sensors are for sensing the operating environment.
This may include optical, x
-
ray, acoustic imaging, and laser
scanners. System sensors are for vehicle diagnostics. This
may include sonar, magnetome
ter, laser scanner, magnetic
scanner, chemical scanner, force, tactile, and proximity
sensors. It is difficult in sensing the
x
-
y
position because there
are no internal system sensors for the
x
-
y
vehicle position. So,
AUVs are navigated by using an under
wate
r acoustic
positioning system.
An underwater acoustic positioning
system

uses acoustic distance and/or direction measurements.
It also uses subsequent position triangulation for AUV
navigation as well as vehicle tracking
.

Three broad classes of
underw
ater acoustic positioning systems are Long Baseline
Acoustic Positioning System (LBL), Ultra Short Baseline
System (USBL), and Short Baseline System (SBL). LBL
systems use networks of sea
-
floor mounted baseline
transponders as reference points for navigat
ions.
It
s technique
produces excellent

positioning accu
racy and position stability
which

is

also independent

of water depth.
The LBL systems
are more frequently used for precision underwater survey than
the other two classes.
The Ultra Short Baseline Sy
stem
consist
s

of a transceiver,

transponder
/responder, and towfish.
The transceiver

is mounted

on a pole under the ship, and the

transpon
der/responder on the seafloor, the towfish is

a
remotely operated underwater vehicle (ROV).
The transceiver
communicates to a computer, or “topside unit” and this unit
calculates position based on ranges and bearings incoming
from the transceiver.
The Short Baseline Acoustic Positioning
System (SBL) is used for tracking underwater targets from
sh
ips that are anchored. The SBL positioning accuracy
improves with transducer spacing. When operating from a
large dock, the SBL system can create a precision and position
robustness that is like the LBL system. The smaller the dock,
the less precision t
he SBL will exhibit.

V.

PRESSURE

HULLS

When considering the creation of an AUV, an individual has
to understand the pressure it is subjected to as the depth
increases. A depth of 10 meters will double the normal
atmosphere pressure of 203 kPa. The depth rat
ing will
influence the AUV’s size and range. To achieve high depths,
the pressure hull must be larger, thicker, and heavier. Pressure
Hulls are most commonly made out of aluminum and
titanium. Most AUVs are designed by having torpedo
-
shaped
airings that

include a few pressure hulls for on
-
board
electronics and batteries. Common vehicle shapes include
single sphere, cylinder, saucer, and egg. A single sphere has a
low weight to volume ratio and is excellent for deep diving.
The disadvantage is that it
will have a low optimum vehicle to
diameter ratio. A Cylinder is easily fabricated and has a high
optimum vehicle to diameter ratio. The disadvantage is that it
has a high weight to volume ratio. A saucer will have an
improved hydrodynamics in horizonta
l plane and the ability to
easily hover in currents. The disadvantage is an inefficient
structure, low controllability, and only limited to shallow
depths. An egg has good hydrodynamics and weight to
volume ratio. It is also difficult to design and fabr
icate.

R
EFERENCES

[1]


J. Yoh,

Design and Control of Autonomous Underwater
Robots: A Survey
.


Honolulu, Hawaii: University of Hawaii,
2000.



[2]

Shoults, G.A. 1996.

Dynamics and control of an
underwater robotic vehicle with an N
-
axis manipulator
. Ph.
D.
Thesis, Washington University.



[3]


T.H. Koh, M.W.S. Lau, G. Seet, and E. Low, "A Control
Module Scheme for an Underactuated Underwater Robotic
Vehicle" J Intell Robot Syst 2006, ch. 46: pp. 43
-
58



[4]


Albert M. Bradley, Michael D. Feezor, Hanumant
Singh,
and F. Yates Sorrell. "Power Systems of Autonomous
Underwater Vehicles," IEEE Journal of Oceaic Engineering.,
vol. 26, No. 4, Oct. 2001 pp.526
-
538



[5]

Adakawa, K. 1995. Development of AUV: Aqua explorer
1000. In

Underwater Robotic Vehicles: Desig
n and Control
, J.
Yuh (Ed.), TSI: Albuquerque.



[6]

Cox, R. and Wei, S. 1994. Advances in the state of the art
for AUV

inertial sensors and navigation systems. In
pr
oceedings on IEEE AUV Technology
, pp. 360

369.



[7]

A. Bradley, “Low power navigation a
nd control for long
range autonomous underwater vehicles,” in

Proc. 2nd Int.
Offshore and Polar Engineering Conf.
, June 1992.



[8]


John J. Leonard, Andrew A. Bennett, Christopher M.
Smith, and

Hans Jacob S. Feder, "Autonomous Underwater
Vehicle Navigatio
n," Cambridge, MA: Massachusetts Institute
of Technology 1998



3






James J. Holloway
(M’10)

was born in
K.I. Sawyer,
Michigan on April 8, 1986. He

will graduate with a Bachelor
of Science degree in electrical engineering from Auburn
University, Auburn, A
L, United States in December of 2011.


He worked as an intern for
Rockwell Automation in the
summer of 2011.



Charles G. Reynolds

was born in Ashland AL on March
25, 1988
. Mr. Reynolds will graduate with a Bachelor of
Science degree in electrical
engineering from Auburn
University, Auburn, AL, United States in December of 2011.


He worked as an intern for Southern Company in the
summer of 2011. He is currently working for the Auburn Fire
Department in Auburn AL.