development of tether mooring type underwater robots...

Indian Journal of Geo-Marine Sciences

Vol. 40(2), April 2011, pp. 181-190

Development of tether mooring type underwater robots: Anchor diver I and II

Ya-Wen Huang1, Koji Ueda

1, Kazuhiro Itoh

2, Yuki Sasaki

1, Paulo Debenest

3, Edwardo F. Fukushima

1 & Shigeo Hirose

1

1 Dept. Mechanical and Aerospace Engineering, Tokyo Institute of Technology

Ookayama Meguro, Tokyo, 152-8552, Japan

[E-mail: [email protected]; [email protected];]

[email protected];[email protected]; [email protected] 2 Hitachi Construction Machinery Co., Ltd., 650, Kandatsu-machi, Tsuchiura-shi, Ibaraki-ken 300-0013 Japan

[E-mail: [email protected]] 3 Hibot Corp, Meguro Hanatani Bldg.801, 2-18-3 Shimo-Meguro, Meguro, Tokyo 153-0064, Japan

[E-mail: [email protected]]

Received 23 March 2011, revised 28 April 2011

Ocean survey is more difficult than land-based investigation, since the underwater vehicles are susceptible to being

swept away by sea currents. Present study proposes a new concept of underwater vehicle, in which the robot is moored by a

tether and utilizes the sea current for movement. Two tether mooring type of underwater vehicles, named “Anchor Diver I”

and “Anchor Diver II”, will be introduced in this paper. Anchor Diver I is an AUV (Autonomous Underwater Vehicles)

developed for long-term ocean survey and Anchor Diver II is a ROV (Remotely Operated Vehicles) which moves with a

principle similar to flying a kite in the sky.

[Keywords: ocean survey, robotics, radio waves, mooring]

Introduction

Nowadays, the technology of localization and

mapping systems has improved a lot. However, when

it comes to underwater fields, it becomes more

difficult to do the same thing compared to on land.

The reasons are described as follows. Firstly, the map

of seabed is not completed yet so it is difficult to

know the position by observing the environment [1].

Secondly, radio waves do not work in the water.

Therefore the GPS will not work without letting the

antenna go higher than the sea surface [2]

. Thirdly, the

underwater vehicles are usually drifting in the water

without a fixed position.

Compared to the vehicles on land, underwater

vehicles tend to drift constantly with the sea current

and they have to consume a lot of power to maintain

their position stably. In this paper we focus on the

third problem and propose a novel tether mooring

method to improve the mobility and stability of

underwater robot. By utilizing this tether mooring

mechanism, “Anchor Diver I” and “Anchor Diver II”

(see Figures 1 and 2) are developed for different

applications. In the following sections the details of

the tether mooring method and the two robots will be

explained.

Materials and Methods

Tether Mooring Type Underwater Robots

One significant feature of conventional AUVs is

that the vehicles are wireless and have a wide range

but are limited by the capacity of the battery. On the

contrary, ROVs move with a tether for receiving

control signals and receiving energy from the mother

ship, but the range of movement is limited by the

length of the tether. The concept of the tether

mooring type underwater robot is to moor the body to

the seabed (for AUVs) or mother ship (for ROVs) by

a tether which is kept tight all the time and to move in

the water by changing the length of the tether using a

reel mechanism, as shown in Figures 3 and 4. The

benefits of this kind of mechanism are described as

follows:

1 The tether, which is attached to the winch, is kept

tight at all times during the operation, allowing

INDIAN J. MAR. SCI., VOL. 40, NO. 2, APRIL 2011

182

the robot to maintain a stable position against the

sea current with no energy consumption. This

kind of characteristic can be effective for both

AUVs and ROVs.

2 By using the sea current with the tether mooring

type underwater robot, it can generate electricity

using a current generator. This feature is

especially valuable for AUVs which need to stay

in the sea for long-term independent observation.

3 Instead of moving against the sea current, the

tether mooring type underwater robots utilize the

sea current to search from upstream to

downstream. Since this kind of method consumes

less energy and moves stably, it is useful for both

AUVs and ROVs.

4 Since the tether is kept tight at all times, there is a

lower possibility of the tether becoming tangled

with obstacles on the floor. This feature is useful

for ROVs.

5 Since the tether is kept tight at all times, the

position can be recorded by measuring the length

and direction of the tether. This is useful for both

ROVs and AUVs.

In order to be able to keep the tether tensioned all

the time during the operation without breaking, the

strength of the tether is very important. This problem

can be solved by using the High Molecular Weight

Polyethylene tether which is broadly used for fishing

and can stand high tensile force.

Fig. 1 – Tether Mooring Type AUV “Anchor Diver I”

Fig. 2 – Tether Mooring Type ROV “Anchor Diver II”

Fig. 3 – Concept of Tether Mooring Type AUV

Fig. 4 – Concept of Tether Mooring Type ROV

HUANG et al.: DEVELOPMENT OF TETHER MOORING TYPE UNDERWATER ROBOTS

183

Categories of Tether Mooring Type AUVs

Tether mooring type AUVs can be broadly divided

by the number of tethers for mooring: those which are

single tether type and multiple tether type. Single

tether type underwater robots (shown in Fig. 3) are

moored by a single tether which is controlled by

changing the length of the tether and adjusting the

angle of the rudders. On the other hand, an example

of the multiple tether type is the 3 tether type robot

shown in Fig. 5, the balance of the buoyancy and the

tension on the multiple tethers maintains the robot’s

stability. The multiple tether type robot controls its

position by changing the length of each tether,

making 3D movement possible. Furthermore,

according to the type of mooring anchor, tether

mooring type underwater robots can be divided into

three types: fixed anchor, movable anchor, and robot

anchor (Fig. 6).

For fixed anchor, after the anchor is deployed, and

the position of the underwater robot is fixed

throughout the mission. On the contrary, the

adjustable prongs of a movable anchor allow it to be

pulled back and relocated, which expands the range

of activities of the underwater robot (Fig. 7).

However, when movable anchors are combined

with multiple tethers, due to the tether tension applied

to the underwater robot, it is difficult to relocate the

anchors.

As for the robot anchor, the anchor lacks a

swimming function but has the ability to move along

Fig. 5 – 3 Tether Type Underwater Robot

Fig. 6 – Classification of Tether Mooring Type Robot

Fig. 7 – Use of movable anchor


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the sea floor. The robot anchor receives the power

supply from the underwater main body through the

tether. In this case, it is also possible to transfer some

of the functions of the main body to the robot anchor

side. By combining the robot anchor type with the

multiple tether type, the robot can function as

colony robot.

Results and Discussion

Development of Anchor Diver I

For the purpose of detecting CO2 leaks during

ocean CO2 sequestration [3]

, there is a need for

independent underwater robots that can make

observations while maintaining their position over the

seabed against the current for long periods of time. In

order to achieve this objective, “Anchor Diver I [4]

’’

was proposed. The robot is moored and its position is

controlled by changing the length of the tether. In

addition, it can sustain itself with a sea current power

generation system.

Anchor Diver I is developed as a prototype of a

tether mooring type underwater robot. Figure 8 shows

the concept of Anchor Diver. Anchor Diver is

equipped with a screw fan generator. Due to the

design of a movable anchor mechanism, the anchor

can be pulled back and relocated which expands the

range of activities as shown in Fig. 9.

Table 1 describes the specification of Anchor

Diver. It is easy to carry to the experiment spot with

total height within 400 mm and weight within 10 kg.

Fig. 10 shows the image of the turning motion of

Anchor Diver I.

Since the robot has to stay on the seabed for a long

time, it is necessary to attach an auto-cleaner which

can move along the tether to clean the algae

attached on the tether to make the reel mechanism

operate smoothly.

Development of Anchor Diver II

Most of the present ROVs equip more than one

thruster and operate by subtle thrust modulation to

enable steering and to maintain its position in the

environment against current. However, the multiple

thruster arrangement means these ROVs often have

difficulty in maintaining their position against the

current. In addition, position identification sensors

using an on-board ultrasonic transmitter are

commonly used. However, according to divers’

experiences, it is clear that they often cannot be used

in real situations due to secondary reflections of

ultrasonic waves. Additionally, the tether is generally

required to have some slack to enable free movement

and this length can often cause accidents by tangling

with obstacles on the floor or buoys on the surface.

Anchor Diver II is a tethered underwater robot

with kite-style steering developed to solve this kind

of problem. The concepts of Anchor Diver II are

as follows.

Overview of the System

As Figure 11 shows, the mother-ship which

measures its own position by GPS has an on-board

winch W, with connected tether. The wire needs to be

under tension while the robot is operating.

As Figure 12 shows, Anchor Diver II is equipped

with an actuated 2-DOF arm and one thruster, and

measures its position and orientation relative to the

point P, where the arm is attached to the tether. When

the mother ship is moored and stationary, or when

there is no sea current, the thruster will be actuated to

make Anchor Diver II move away from the winch to

keep the tether under tension. The 2-DOF arm is

actuated to change the orientation of the robot and

hence change the direction of thrust.

A surveillance camera and short-range sonar is

attached to the bottom of the body of the robot. The

Fig. 8 – Concept of Anchor Diver

Table 1—Mechatronics characteristics

Measure

(Length×Width×Height)

830mm×484mm×392mm

Displacement 10.6kg

Wight in the air 10.3kg

Diameter of screw fan 196mm

Horizontal rudder angle range ±90deg

Vertical rudder angle range ±45deg


185

camera is utilized when the underwater environment

is clear and sonar is utilized when the water is cloudy

due to sediment, etc. The observation data obtained

will be sent to the mother-ship by the tether and it

will be possible to calculate the real-time terrestrial

position coordinates using the GPS data and the

length and direction of the tether.

The hull structure shown in Figure 12 has a flat

panel shape with top and bottom cylinders for

buoyancy and ballast. The center of buoyancy is

located at the top of the body. When there is current

or the mother ship moves, the wire is kept tight to

ensure that the flat side of the body faces the current

which is in the same manner as a kite opposes the

wind. The orientation of the robot relative to point

P, where the tether is fixed, can be controlled

by adjusting the angular positions of two actuators

on the arm.

Movement of Anchor Diver II

The movement of Anchor Diver II can be divided

into two modes: thruster mode and kite-style mode.

When there is no current, the thruster is actuated to

make the tether tight and to move to the search area.

Fig. 9 – Lock-release Mechanism of Movable Anchor


186

The steering control by thruster is shown in

Figure 13. When there is current, the robot can move

to the search area without power. After the robot

reaches the survey area, the robot moves from one

side to another, then the winch actuates to gradually

roll back the wire. The steering movement of Anchor

Diver II shown in Figure 14 uses the same principle

as a kite.

Main Body Design

Anchor Diver II is developed as a prototype of a

tether type underwater robot. Figure 15 shows the

appearance of Anchor Diver II. Anchor Diver II is

equipped with a 2-DOF robot arm. Due to the power

supply coming from the mother-ship, Anchor Diver II

can operate for a long time which increases the

mobility.

The ship has been developed before the

development of a reel mechanism. The thruster can

generate continual bollard thrust of 2.2 kgf.

Figure 16 shows the mechanism of the joint of the

robot arm. Each joint is driven by a 60 W DC motor

and the rotation speed is reduced by spur gears and a

flat-hollow type harmonic drive in which all the wires

can go through the center hole. The torque of each

joint is 21.36 Nm. Each joint of the axis of gyration is

Fig. 10 – Circuitous Cruising


187

water-proofed by oil seal. Other parts of the robot are

water-proofed by O-rings and adhesion bond. The robot

is designed with positive buoyancy. Table 2 describes the

specification of Anchor Diver II.

Search Method of Anchor Diver II

When there is current in the river or sea, Anchor Diver

II is moored to the mother ship which is anchored and

still. As Figure 17 shows Anchor Diver II should be

placed on the downstream side and then search from side

to side by changing only the angle of the arm without

applying electrical power. Adjusting the length of the

cable can enlarge the searching area. When there is no

current, the mother ship should drag the robot and move

to generate relative speed as shown in Figure 18. Then

the robot can search the area by changing the angle of the

robot arm.

Operation Check of Anchor Diver II

In order to simulate the situation while the robot is

under hydraulic pressure, the water-resistance test was to

put a certain amount of dry ice into the body then place

the robot into the pool. Here we simulated the depth of

water of 5m and 10m. During the test no bubbles leaked

out. Therefore water-resistance was confirmed.

The ability to recover from non-upright positions was

confirmed by putting the robot upside-down in the water

and it moved back to the original state. Kite-style mode

and thruster mode can be implemented by operating the

2-DOF arm and the thruster. Figure 19 shows the image

of the thruster mode and kite-style mode.

The experiment was held in Hawaii Undersea

Research Laboratory. Figure 20 shows the force acting

on the end of the arm when the mother ship is under sea

Fig. 11 – Overview of Anchor Diver II

Fig. 12 – Components of Anchor Diver II

Fig. 13 – Steering Control by Thruster

Fig. 14 – Kite-style Steering Control


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current. From the result of the test it showed that the

force acting on the robot is not completely steady.

The reasons can be considered as follow:

1 Direction of the sea current was not stable.

2 While the mother ship was anchored, it was still

floating and changing the orientation by current.

More experiments need to be carried out in order to

find the optimal angle of the robot arm to move

efficiently in different speeds of sea current.

Fig. 17 – Search Method with Current

Fig. 15 – Appearance of Anchor Diver

Fig. 16 – Section of the Joint 1 of the Robot Arm

Table 2—Mechatronics characteristics

Measure

(Length×Width×Height)

1036mm×781mm×155mm

Displacement 31.5kg

Wight in the air 31kg

Performance of the thruster 2.2kgf

1st joint angle range ±540deg

2nd joint angle range ±540deg

Forward Speed 0.21m/s

Backward Speed 0.16m/s


189

Fig. 18 – Search Method without Current

Fig. 19 – Left: Thruster Mode Right: Kite Mode


190

Conclusions

The concept of tether type underwater robot “Anchor

Diver I” and “Anchor Diver II” were proposed, and the

first two models have been built. The mobility of

Anchor Diver I and II were evaluated. The next step will

be the development of the reel mechanism. Additionally,

it is necessary to attach a high resolution sonar for

underwater search in cloudy water.

Acknowledgement

The authors would like to thank Prof. Reza

Ghorbani and the Hawaii Undersea Research

Laboratory for lending their facility and helping with

the experiments.

References 1 Toshihiro Maki, Hayato Kondo, and Tamaki Ura,

Underwater Visual Mapping by an Autonomous Underwater

Vehicle, Journal of the Society of Instrument and Control

Engineers, Volume 47 October 2008 Number 10, P.810-816

2 Tamaki Ura, System Integration for Ocean Engineering,

Journal of the Society of Instrument and Control Engineers,

Volume 47 October 2008 Number 10, P.787-790

3 Kenkichi Tamura, CO2 Storage on Deep Seabed, Journal of

the Society of Instrument and Control Engineers, Volume 47

October 2008 Number 10, P.803-809

4 Ya-Wen Huang, Koji Ueda, Kazuhiro Itoh, Edwardo F.

Fukushima, Shigeo Hirose, Development of Tether Mooring

Type Underwater Robot, The 2009 IEEE/RSJ International

Conference on Intelligent Robots and Systems, P.267-272

Fig. 20 – Force Acting on the End of the Arm

development of tether mooring type underwater robots...

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