[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
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
tension on the multiple tethers maintains the robot’s stability. The multiple tether type robot controls its
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
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
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
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
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
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
Fig. 18 – Search Method without Current
Fig. 19 – Left: Thruster Mode Right: Kite Mode
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
