AFFORDABLE DEEP OCEAN EXPLORATION WITH A

HOVERING AUTONOMOUS UNDERWATER VEHICLE

Odyssey IV: a 6000 meter rated, cruising and hovering AUV

V. Polidoro, S. Desset, C. Chryssostomidis, F. Hover, J. Morash, R. Damus

Autonomous Underwater Vehicle Laboratory

Massachusetts Institute of Technology Sea Grant Program

292 main street, E38-300, Cambridge, Massachusetts, 02139, USA

Keywords: AUV, deep ocean exploration, autonomous underwater vehicle, LBL, USBL, benthic, cold water corals.

Abstract: The Autonomous Underwater Vehicle Laboratory (AUV Lab) at The Massachusetts Institute of Technology

(MIT) is currently building and testing a new, general purpose and inexpensive 6000 meter capable

Hovering Autonomous Underwater Vehicle (HAUV), the ‘ODYSSEY IV class’. The vehicle is intended

for rapid deployments, potentially with minimal navigation, thus supporting episodic dives for exploratory

missions. For that, the vehicle is capable of fast dive times, short survey on bottom and simple navigation.

This vehicle has both high speed cruising and zero speed hovering capabilities, enabling it to perform both

broad area search missions and high resolution inspection missions with the same platform.

1 INTRODUCTION

1.1 Motivation

There is a lack of deep submergence vehicles

available to the ocean science community (National

Research Council of the National Academies). The

valuable resources that do exist, such as the manned

submersible Alvin, are under such heavy demand

that the scientists who compete to use the resources

are severely limited in the types of exploratory

missions they are able to pursue. The competition

for these assets limits scientists to working in areas

where there already exists enough knowledge from

previous missions that they are nearly guaranteed to

return with a good data product. As a result it is

difficult to secure funding to investigate new and

completely unexplored areas of the ocean. The

Odyssey IV class autonomous underwater vehicle

has been designed to perform quick and inexpensive

exploratory missions in areas where little or no

prior knowledge exists.

Figure 1: Design evolution towards an underwater

vehicle capable of both high speed cruising and zero

speed hovering. Top left Odyssey II, bottom left

ONR HAUV, right Odyssey IV

This vehicle is intended to be an inexpensive

asset that will be readily available for the deep

ocean science community, and can be replicated

easily. It is not meant to serve as a replacement for

the human occupied vehicles such as Alvin, Mir and

Nautilus submersibles, but rather can be used as an

inexpensive means of performing preliminary

surveys of areas that will potentially be explored by

the larger, more capable assets. The data product

from this platform will be substantially reduced

compared to the more expensive assets, however

the lower operating costs will enable this vehicle to

218

Polidoro V., Desset S., Chryssostomidis C., Hover F., Morash J. and Damus R. (2005).

AFFORDABLE DEEP OCEAN EXPLORATION WITH A HOVERING AUTONOMOUS UNDERWATER VEHICLE - Odyssey IV: a 6000 meter rated,

cruising and hovering AUV.

In Proceedings of the Second International Conference on Informatics in Control, Automation and Robotics - Robotics and Automation, pages 218-223

DOI: 10.5220/0001177202180223

 SciTePress

perform exploratory missions that would not be

possible otherwise. Ultimately, it is our intent that

this vehicle will provide scientists with a less

expensive and more available means of

investigating new and unexplored areas in the deep

ocean, e.g., those with cold corals (Rogers, 2004).

1.2 Concept of Operations

In order to perform these quick exploratory

missions in a wide variety of sites, it is essential that

this platform be highly mobile so that multiple sites

can be investigated in a single day. Traditionally

deep diving vehicles such as Alvin and ABE use

external long baseline (LBL) acoustic networks for

navigation. The LBL systems are very time

consuming to deploy, survey in, and recover. The

ship time dedicated to the LBL system forces

scientists to dedicate at least several days to any site

of interest. The Odyssey IV class AUV is designed

to operate independently of these LBL networks in

order to investigate several different sites in a single

day. The Odyssey IV will be tracked with a ship-

borne ultra-short baseline (USBL) system and

navigation updates will be periodically sent via the

acoustic modem.

To further reduce the ship time required to

investigate any given site, the Odyssey IV is

designed to rapidly dive to depths as great as 6000

meters using a large external drop weight. This

descent weight and the highly streamlined body

enable the Odyssey IV to achieve vertical descent

speeds as high as 3.5 m/s, compared to the

maximum descent speed of 0.5 m/s for Alvin. The

round trip travel time to descend to a depth of 3000

meters would require approximately 30 minutes for

the Odyssey IV or at least 3 hours for Alvin. In

order to achieve these large descent speeds, it is

crucial that the vehicle be passively stable in tow.

Simply put, the body should travel smooth and

straight when being pulled to the bottom with a

large weight, without the use of any actuators.

Relying on an active control system to reject any

wild oscillatory motions on the descent would place

a high requirement on the bandwidth of the control

system, and would also waste energy which could

otherwise be spent collecting useful data at the

bottom. To achieve passive stability in tow, a

significant portion of the design effort was focused

on the hydrodynamics. The results of our

hydrodynamic experiments and simulations will be

presented later in this paper.

2 DESIGN

2.1 Lessons from Previous Vehicles

The initial design criteria for this vehicle was based

on the knowledge gained from previous experiences

in designing, building, and operating AUVs in the

field. This vehicle is designed to be manufactured

and operated at a low cost. By relaxing the packing

efficiency constraints, this design will save time and

cost by eliminating the need for highly customized

components that are required for extremely compact

designs.

ODYSSEY IV Specifications

Weight ~350 kg

Overall Length 2.6 m

Overall height 1.3 m

Overall width 1.3 m

Thrusters 4 x 1hp

Deep Sea Systems

Max Thrust per axis 400N

Surge velocity 3.5m/s

Heave velocity 1.0m/s

Sway velocity 0.5m/s

Yaw velocity

(hovering mode)

20 degrees per second

Dive speed

(with 10 kg weight)

>200m/min

Reserve buoyancy

(for payload)

30kg

Depth rating 6000 meters

Budgeted price

(excluding labor)

$170,000

Battery technology lithium ion

Onboard Energy

stored

4500 Wh

Power available 6000 W

Controlled DOF 4 (Surge, Heave, Sway,

Yaw)

Righting moment 120Nm at 45 degree

pitch

Drop weight 1 external for descent, 1

internal for ascent

The size of the vehicle is driven primarily by the

desired payload capacity and the use of commercial

off the shelf components. The size of the AUV is

limited by the typical deck size and crane capacity

for vessels of opportunity (Damus, 2004 - a). The

size of the vehicle adds to the cost of shipping,

which accounts for a substantial portion of the

operating budget.

AFFORDABLE DEEP OCEAN EXPLORATION WITH A HOVERING AUTONOMOUS UNDERWATER VEHICLE -

Odyssey IV: a 6000 meter rated, cruising and hovering AUV

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A good AUV design needs to be very robust to

endure the damage that routinely occurs during the

launch and recovery process. The frame design

needs to be rigid, yet sparse to allow easy access to

the housings to perform regular maintenance. The

frame and fairing should be made of mostly plastic

to minimize the machining costs and corrosion

problems.

Good AUV designs are quiet electrically and

acoustically, in order to minimize the interference

with sensitive payload instruments such as sonars.

The AUV should contain enough battery power to

support a full day of operation with large payloads.

2.2 Cruising and Hovering

Cruising AUVs are generally torpedo shaped, and

are optimized for efficient surveys and long range

transit, see Figure 1. Cruising AUVs have very

limited maneuverability and therefore lack the

ability to make sharp turns, to stop for inspection, or

to travel at low speeds to closely follow a rough sea

floor. Obstacle avoidance becomes a major

problem for high speed cruising vehicles operating

near the sea floor (Damus, 2004 - b).

Hovering AUVs are highly maneuverable,

which makes them ideally suited for short range,

high resolution inspection tasks such as ship hull

inspections. Most hovering AUVs are not

streamlined, and therefore have low top speeds and

limited range. The limited range and speed of

hovering vehicles makes them ill suited for

searching wide areas.

The Odyssey IV was designed for both high

speed cruising and zero speed hovering. A highly

streamlined and maneuverable vehicle has a much

broader range of mission applications. The ability

to both cruise and hover makes the Odyssey IV a

well adapted platform for wide area searching and

for close area inspection, or possibly physical

sampling and manipulation tasks in the future.

2.3 Symmetry and Stability

line of s

mmetr

center of buoyancy

and center of gravity

Figure 2: Odyssey IV has highly symmetric thruster

placement and a large hydrostatic righting moment

Asymmetries in the body shape and thruster

placement cause large torques on the body which

significantly increase the bandwidth and power

required by the control system. For example, a

strong coupling between surge and pitch in the

hydrodynamics wastes energy because pitch

actuation is required to achieve pure surge. Large

disturbances in the dynamics cause navigation

errors and discontinuities in the payload data.

Our previous hovering AUV could

independently control all six degrees of freedom in

the body motion, however the current and

foreseeable applications have only required the roll

and pitch to be maintained at zero (level flight).

The relatively fast dynamics on the rotational axes

places high demands on the control bandwidth and

authority. Achieving level flight on the roll and

pitch axes using passive hydrostatic stability

eliminates the need for actuators dedicated to those

axes. Reducing the number of actuators has

obvious benefits for the overall cost, size, and

power budget. A large hydrostatic righting moment

causes a high degree of passive stability. A large

hydrostatic righting moment is achieved by

maximizing the vertical separation between the

center of volume and center of mass, which was the

major design reason for the elongated vertical

shape.

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220

Figure 3: Simplified view illustrating the major

components of the Odyssey IV design

Four degree of freedom hovering capability is

enabled by passive stability and symmetric thruster

placement. Passive roll and pitch stability are

enabled by a large hydrostatic righting moment

(121Nm at 45 degree) and fixed lifting surfaces.

Symmetry in thruster placements reduces required

control bandwidth and decreases energy required

for station keeping. The rotating thruster unit

reduces the total number of thrusters, and therefore

enables more payload space and a more streamlined

shape. The location of the rotating thruster pair was

chosen to minimize the heave pitch coupling in the

hydrodynamics. The center of gravity has been

brought as close as possible to the line of symmetry

in the vertical direction, in order to minimize the

cross coupling between the body axes in the

hydrodynamics.

3 HYDRODYNAMICS

3.1 Range and Speed

The maximum surge velocity of the Odyssey IV is

3.5 m/s. The Odyssey IV has a range of 100 km at

1.1m/s for a hotel load of 100 Watts. This is an

estimation that accounts for the varying efficiency

of the thrusters at different thrust values and also for

the varying coefficient of drag of the streamlined

body as a function of Reynolds number. The

coefficient of drag for the streamlined body was

experimentally measured to be 0.1 at a Reynolds

number of 450,000. The hydrodynamic tests were

performed at the MIT Towing Tank facility using a

30% scale model.

main electronics housin

four battery

spheres

horizontal and

vertica

l stabilizers

rotatin

thruster

cross bod

tunnel thrusters

olished sphere with

stereographic cameras

Figure 5: Yaw moment as a function of yaw angle for a

series of tail sizes. These results indicate that the tails

used for these test have insufficient area to make the

body passively stable. In order to achieve passive

stability, the rear vertical stabilizer will need to have at

least 400% more area. A new model tail is being

constructed, and another series of tests will be

performed to verify that the increased tail area will

enable the body to be passively stable in tow

Figure 4: Descent speed and pitch angle as a function

of drop weight mass. The weight is placed at the nose

to cause the body to pitch down to present a more

streamlined axis to the flow. The lift on the rear

stabilizers cause the body to pitch further nose down,

increasin

the s

eed and stabilit

of the dive.

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4 NAVIGATION

ODYSSEY IV has a Doppler Velocity Log (DVL)

and an Attitude and Heading Reference System

(AHRS) to allow precise dead-reckoned navigation

(Whitcomb et al.). During normal cruising, vehicle

position will be estimated using an Extended

Kalman Filter, with accuracy slowly degrading at a

rate of 1-2 percent of distance travelled. However,

during deep dives, the DVL will be unable to

acquire bottom lock. The high pitch angle of the

body will point the transducer head away from the

sea floor, and the great depth of the water column

will often be beyond the instrument's maximum

sensing range. No DVL velocimetry means no

position estimates, so the vehicle will have to

suspend estimator operation and dive "lost".

Once ODYSSEY IV reaches its target altitude

above the sea floor, a subsequent pause to hover in

place will enable the science team at the surface to

precisely locate the survey start point. The vehicle

may be tracked using ship-borne USBL, or GPS-

enabled LBL buoys (Desset et al., 2003).

ODYSSEY IV is compatible with a fixed LBL net,

but we do not foresee frequent use of this mode of

navigation, due to the time and expense of

deployment. In quick inspection dives, the vehicle

need not perform continuous Earth-referenced

navigation, but may simply follow a pre-planned

dead-reckoned survey path relative to its start point.

In post-processing, the estimated vehicle path may

be overlaid on a map relative to the georeferenced

start point, or the path may be plotted directly from

surface tracking data. Experiments are planned to

test the feasibility of updating the AUV's self-

position estimate with surface tracking data via

acoustic modem, such that the on-board estimator

can work with Earth-referenced coordinates at all

times.

The possibility exists for rapid deployment of

multiple vehicles from a single vessel. Each vehicle

would be tracked during its dive and its survey start

point carefully noted, then each recovered after its

mission was completed.

5 PAYLOAD

The Odyssey IV is designed to be a flexible

instrument platform, with dedicated space to

support a variety of science payloads throughout the

lifetime of the vehicle. The high maneuverability,

substantial depth rating, low cost and easy

deployment of this AUV will make it a good choice

for many different scientific inquiries. Odyssey IV

has a generous 100 liters of dedicated payload

space, and has sufficient reserve buoyancy to carry

30 kg (wet) of additional instrumentation. The main

electronics housing has three identical payload

ports, each able to deliver up to 2kW peak power

from the main battery bus (accounting first for

thrust demands). Each payload port can be wired

internally for 10/100 Ethernet, RS-232/422/485

serial, and/or general purpose analog and digital

I/O, with optically isolated connections to the

PC/104-based main vehicle computer.

The first payload planned for Odyssey IV

integration is a stereographic digital camera system.

A pair of six-megapixel color cameras will share a

polished optical viewport in spherical glass pressure

housing. The remaining space inside the camera

sphere will be occupied by the lighting electronics.

These will support one or more strobes (roughly

200 J each), for high-quality still.

Stereo imagery from this camera, displayed

through an appropriate stereoscopic device, will

enable scientific users to feel as though they are

flying over the seafloor along the track of the AUV,

with sharp full-color images to examine. After

careful calibration of the camera, the raw data may

be post-processed into a three-dimensional

photomosaic, allowing precise measurements to be

made of high-relief targets (typically distorted in 2-

D images) (Pizarro et al., 2004).

The other payload sensor that will likely be

included in the first generation Odyssey IV is the

C3D Sonar Imaging System from Benthos. The

C3D system functions both as a sidescan sonar and

also as a high resolution bathymetric system. The

C3D would be ideally suited for systematically

searching relatively wide areas of the sea floor.

Targets identified with the C3D sonar could be

inspected more closely with the sterographic

cameras.

As for more complex instruments, past

experience has shown that the 'smart sensor' is a

very effective approach to AUV payload

development. Despite the additional engineering

required, a 'smart sensor' design allows independent

construction and testing of the complete subsystem

on the bench (and even in limited field

deployments) prior to installation in the AUV. The

on-board computer is typically responsible for data

collection (triggering sensor sampling) and data

storage; some devices even perform real-time

interpretation (e.g., online CAD/CAC in MCM

sidescan sonar applications). Examples of smart

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sensor subsystems integrated in MIT Sea Grant

AUVs include a synthetic aperture sonar (Edwards

et al., 2001), mass spectrometer (R. Camilli), high

resolution digital still camera and high-frequency

sidescan sonar (J. Morash).

Future payloads for Odyssey IV may include

solid or water sampling devices. The logical goal of

a hovering vehicle is the ability to hold perfectly

still - scientists might take advantage of this ability

by requesting a returned sample from the seafloor.

Simple corers, sediment traps, and biological "slurp

guns" have been tested successfully on other

vehicles, most notably ROVs (Paull et al.,

2001)(Salamy et al., 2001). The high power density

available to Odyssey IV payloads may enable more

powerful samplers such as chipping hammers for

sampling rock or coral.

ACKNOWLEDGMENTS

The authors would like to acknowledge the support

of their colleagues at the MIT Sea Grant Program,

the MIT Ocean Engineering Department, the MIT

Towing Tank, and the MIT Propeller Tunnel. The

MIT Sea Grant Program supported this work under

grant number NA-16RG2255.

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