development of tether mooring type underwater robots
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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: huang@robotics.mes.titech.ac.jp; ueda@robotics.mes.titech.ac.jp;]
ysasaki@robotics.mes.titech.ac.jp;fukushima@mes.titech.ac.jp; hirose@robotics.mes.titech.ac.jp 2 Hitachi Construction Machinery Co., Ltd., 650, Kandatsu-machi, Tsuchiura-shi, Ibaraki-ken 300-0013 Japan
[E-mail: k.itou.xb@hitachi-kenki.com] 3 Hibot Corp, Meguro Hanatani Bldg.801, 2-18-3 Shimo-Meguro, Meguro, Tokyo 153-0064, Japan
[E-mail: debenest@hibot.co.jp]
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
INDIAN J. MAR. SCI., VOL. 40, NO. 2, APRIL 2011
184
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
HUANG et al.: DEVELOPMENT OF TETHER MOORING TYPE UNDERWATER ROBOTS
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
INDIAN J. MAR. SCI., VOL. 40, NO. 2, APRIL 2011
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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
HUANG et al.: DEVELOPMENT OF TETHER MOORING TYPE UNDERWATER ROBOTS
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
INDIAN J. MAR. SCI., VOL. 40, NO. 2, APRIL 2011
188
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
HUANG et al.: DEVELOPMENT OF TETHER MOORING TYPE UNDERWATER ROBOTS
189
Fig. 18 – Search Method without Current
Fig. 19 – Left: Thruster Mode Right: Kite Mode
INDIAN J. MAR. SCI., VOL. 40, NO. 2, APRIL 2011
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
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