A deep and long cruising range AUV “URASHIMA” has been developing by JAMSTEC since 1998. The sea trials had been started on June 2000, and URASHIMA achieved submarine cruise of 3,518m depth in 2001. The test goal is 300km with fuel cell system in cruising range. URASHIMA can cruise long range distance and collect a lot of seawater sample and oceanic data of various depth. Therefore the vehicle has to shift depth during cruising for exploration. The seawater density is also changed, when the depth is changed. It is necessary to control buoyancy for the energy saving. URASHIMA has buoyancy control system like bladder of fish. In the sea trial, we carried out buoyancy adjustment test and got some useful data. We will show experimental results at the sea trial.
INTRODUCTION
Even if the global warming becomes serious problem year by year, we can not elucidate the mechanism of it until now. In order to make clear the mechanism of the global warming, it is necessary to study the global moving process of CO2 in the sea. For this purpose, many geochemical scientist want to gather efficiently a lot of water samples of various sea area and depth all over the world. These missions require the Autonomous Underwater Vehicle (AUV) which cruise long range distance and collects oceanic data and water samples automatically. From this point of view, we started on the research and development project of AUV capable of performing long range
missions since 1998.
A deep and long cruising range AUV (AUV-EX1: development code name) named “URASHIMA” was completed in 2000. The seagoing tests have been started since June 2000. In the sea trials, URASHIMA achieved to dive 3,518m depth and cruise 60km distance by autonomous navigation mode. The buoyancy materials and buoyancy control system are very important for the deep and long cruising range AUV. Because the vehicle changes depth during cruising for exploration, and according to changing depth also water density is change. In the sea trial, we took place buoyancy adjustment tests and got some useful data. We will show experimental results at the sea trial.
THE VEHICLE
The Construct Of The Vehicle
The general arrangement of the vehicle is shown in Figure 1, and specifications are listed in Table 1. URASHIMA has a cylindrical shape for reducing hydrodynamic drag in order to the vehicle can cruise long range distance with limited energy. We plan to use the solid Polymer Electrolyte Fuel Cell (PEFC) system for power source of URASHIMA. Of various fuel cell systems considered, PEFC system seems to be best application for power source of underwater vehicles. Because the PEFC system operates at efficiencies about 50%, at low temperature about 80 degrees centigrade and mechanical noise is very small. The PEFC system of “URASHIMA” has two stacks of generator cells in serial. As each stack generates 2kW electricity, and total output of this fuel cell is 4kW. By using the PEFC system, URASHIMA will be able to cruise about 300km at 3knots for 55 hours. The PEFC system is testing for generation of electricity on the land at present. Until finish this test, we use Li-iron battery for power source, and the goal with this battery is 100km cruising.
The vehicle consists of titanium frames, some pressure hulls for protecting control system and other electrical devices from water pressure of 3,500m depth, and buoyancy materials for additional buoyancy. The body is covered with FRP (Fiberglass Reinforced plastics) covers. URASHIMA has a main thruster (1.5 kW power of a DC brush-less motor) for cruising, and a pair of vertical rudders, a pair of horizontal rudders in order to control depth and heading. The level posture is controlled by trim adjustment system. The mechanism of this system is a device that 100kg weight moves from –1000mm to 1000m.
Observation And Exploration Devices
URASHIMA is equipped with variety of observation instruments such as a color TV camera, a low light level snap shot digital camera, an acoustic image device and a side scan sonar. The acoustic image device has another function as the obstacle avoidance sonar. The digital camera can take seabed photos every 8 seconds from 8 to 40
meters above the bottom.
The vehicle also equips CTDO sensors and an automatic water sampler as exploration devices. The CTDO can measure continuously conductivity of seawater, water temperature, depth and quantity of dissolved oxygen. The automatic water sampler system can collect 200 water samples during one cruising.
Navigation Modes
According to mission type, we can select an operational mode of the vehicle from three modes such as autonomous navigation mode, acoustic remote control mode and UROV mode. In autonomous navigation mode, working scenario is preset on the onboard computer before each mission. The scenario includes the cruising course, procedure of observation and exploration devices. The mother vessel is used only at the launching and pickup, and URASHIMA cruises independently without any communication between mother vessel. In acoustic remote control mode, the mother vessel follows the vehicle at any time, and they communicate each other. Though the onboard computer is preset as same as the autonomous navigation mode, new working scenario can be transmitted from the mother vessel by acoustic telemetry. The images of the acoustic picture device and the TV camera transmitted from the vehicle every a few seconds
interval.
The UROV (Untethered Remotely Operated Vehicle) mode uses a thin optical fiber cable and control and return signals are sent through the fiber cable. The diameter of fiber cable is only 1mm. This mode is used at development stage for watching status of the vehicle.
BUOYANCY MATERIAL AND BUOYANCY CONTROL
A shallow-sea type underwater vehicle can float by buoyancy of pressure hulls, since it has thin lightweight hull. But a deep-sea type underwater vehicle can not float only by buoyancy of pressure hulls. Because of the hulls are very heavy for protecting electrical devices from against pressure of deep seawater. Therefore the deep-sea type underwater vehicle needs supplement buoyancy by another method.
On the other hand, the buoyancy changes in proportion to the specific gravity of seawater. It is very important to control the buoyancy for the underwater vehicle changes depth over and over. And it is necessary to match the gravity to buoyancy for saving energy.
Buoyancy Material
We use syntactic foam for URASHIMA as buoyancy material. It is composed of glass micro balloon solidified in the epoxy resin (Figure 2). The specific gravity is 0.5. The syntactic foam can easily form in various figures and arrange in any clearance of frames and pressure hulls like a 3-D puzzle (Figure 3).
Buoyancy Control
URASHIMA is an observation vehicle. One of its missions is to collect a lot of oceanic data and seawater samples in various sea depths. For this purpose, it is required to change working depth over and over during observation cruising. Therefore it needs to change its buoyancy in proportion to density of seawater. URASHIMA has buoyancy control system like a bladder of fish. This system consists of oil tank contained in pressure hull (VBT: Variable Ballast Tank) and oil bladder shown in figure 4. 
The dimension of VBT is 440mm diameter and 750mm length. VBT is made of the aluminum by rust prevention processing. The maximum oil capacity is 90 liters and transferable volume of oil is 50 liters. The oil bladder is made of rubber. The maximum capacity of bladder is 50 liters. The maximum capacity of volume change of oil is determined enough to compensate for buoyancy amount of change at the 3,500m depth and disproportion of buoyancy near the surface. URASHIMA can change its volume by shifting oil between tank and bladder. With this system, URASHIMA can adjust optimal buoyancy of present depth from surface to 3,500m.
The concept of buoyancy control system is shown in figure 5. The buoyancy increases by transferring oil in VBT to bladder by the hydraulic pump. The system transfers oil by hydraulic pump under 1,200m depth, and transfers oil through the booster in below 1,200m depth. The buoyancy decreases by transferring oil in bladder to VBT. The system transfers oil by hydraulic pump under 150m depth, and transfers oil by the water pressure in below 150m depth.
SEA TRIAL
The development project of URASHIMA has been started on 1998. The construction of the body is completed in March 2000. We carried out 4 times of sea trials in 2000. In these sea trials, we tested control program system, function of observation and exploration devices and motions of the vehicle. And we carried out 4 times sea trial in 2001 too. In these sea trials, URASHIMA achieved to dive over 3,500m depth and cruised 60km distance by autonomous navigation mode. Figure 6 shows the result of trim adjustment at 3,518m diving and the track of the vehicle. The diving point was the Naze basin in
Amami-Oshima eastward offshore. The track was measured by inertial navigation system installed in the vehicle. The trim depth is 500m, 1,000m, 2,000m, 3,000m, 3,500m. The trim adjustment was manual operation and command for buoyancy control system was sent by acoustic telemetry. The vehicle was quiescent condition when the trim was adjusted. The volume of URASHIMA decreases in proportion to the increasing of the seawater density.
The results of trim adjustment test and the measuring result of seawater density from surface to 1,000m depth at Suruga Bay are shown in figure 7. The diving area is in Suruga Bay. The test took place 300m to 1,500m depth, twice diving and one ascent. The trim adjustment was manual operation and command for buoyancy control system was sent by acoustic telemetry. At the first step indicated by No.1 to 2, URASHIMA needs to decrease his volume in large quantities for diving. And next step No.2 to 5, where a little oil was transferred in spite of diving into deep sea. At the first step, buoyancy materials expanded since the body was still warm. Then according to diving into deep, the buoyancy materials got cold and contracted and decreased buoyancy automatically, so there was no need to work the buoyancy control system actively. The points of No.5 to 9 are ascent action. URASHIMA changed his volume by buoyancy control system because of the buoyancy materials contracted enough. In ascent process the volume increase in proportion to rising up to shallow. The last steps indicated by No.9 to 11 are second diving. The buoyancy adjustment is good agreement with each depth. It is proven that buoyancy changes in proportion to the shift of the sea water density. It
is considered that it does not need to use the extra energy for suppressing ascent and sink motion by adjusting buoyancy at the present depth.
JAMSTEC has planned to build deep and long range AUV since 1998. The AUV named URASHIMA was completed in March 2000. We have been carried out 8 times sea trial until now. We got some successful results shown in section 4. JAMSTEC is interested in the exploration and study of the Arctic Circle, in order to the meteorological prediction on a global scale, and wants to apply autonomous underwater vehicle to an arctic observation under ice plate. The AUV needs to overcome this severe sea condition area and huge range cruising. URASHIMA is the first step of this aim. We need increase accuracy of positioning, long range capacity, automatic buoyancy control system and improve service to scientists to
achieve goals.
REFERENCES
Aoki, T. et al. (1998). “Development of a Fuel Cell Power Source for Long Range AUV,” Underwater Intervention.
Tamura, K. et al. (2000). “The Development of The AUV - URASHIMA-,” OCEANS MTS / IEEE.
Aoki, T. et al. (2001). “Deep and Long Cruising AUV Hybrid-powered with Li-ion Battery and Fuel Cell,” Underwater Intervention.
Aoki, T. et al. (2001). “Deep and Long Range AUV ”URASHIMA”,” Proc 11th Int offshore and Polar Eng Con
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