An overall Pressure Tolerant Underwater Vehicle: DNS Pegel

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An overall Pressure Tolerant Underwater Vehicle: DNS Pegel Carl Thiede, Moritz Buscher, Martin Lück, Heinz Lehr Technical University Berlin Hardenbergstrasse 36 10623 Berlin, Germany thiede@fmt.

1 An overall Pressure Tolerant Underwater Vehicle: DNS Pegel Carl Thiede, Moritz Buscher, Martin Lück, Heinz Lehr Technical University Berlin Hardenbergstrasse Berlin, Germany Gerhard Körner, Jochen Martin, Marion Schlichting ENITECH Energietechnik-Elektronik GmbH Hansestrasse Bentwisch, Germany Siegfried Krüger, Hartmut Huth Leibniz Institut for Baltic Sea Research Warnemünde Seestrasse Rostock, Germany Abstract- This paper will present new approaches regarding component and system design for underwater vehicles. The test vessel DNS Pegel was designed to test pressure tolerant devices for permanent submerged applications. It was built during the project Pressure tolerant systems for permanent submerged applications. The vehicle is equipped with a minimal navigation system and a data logging system for post mission and system examination. The paper will give an overview of the development of pressure tolerant systems and their employment in a test vehicle. I. INTRODUCTION An underwater device is defined as pressure tolerant if no pressure hull is used to protect any part against the hydrostatic pressure. Consequently every component of the device has to withstand the actual hydrostatic pressure, and has to work properly under these conditions. We aim a diving depth of m which corresponds approximately to 600 bar. The limiting factor to dive any depth are mostly the electronic components. Early research was done by [1]. However the fast development of electronic components gives new chances to get such systems out of research laboratories to the market. This paper presents the properties and the layout of various pressure tolerant subsystems as well as experiences gained with their handling. In addition results of different vehicle testings will be presented. Figure 1. Hierarchy of submerged system designs pressure tolerant design is used. Sensitive parts are protected by soft castings. There is a number of unmanned underwater vehicles (UUVs) see [5], [6] available but not one traces the approach of an overall pressure tolerant system. Most of these vehicles use one or more pressure hulls to protect sensitive parts like computers, sensors or electric motors from pressure and water contact. Some use a pressure compensated design to protect e.g. batteries. A. Pressure Tolerant Systems Examples of pressure tolerant devices presented in literature are cameras [2] or batteries [3] as well as illumination [4]. All of these devices are subsystems. Our approach is an overall pressure tolerant system. In case of the DNS Pegel a particular Figure 2. Internal view of the test vessel DNS Pegel This work was supported by Federal Ministry of Economics and Technology (BMWi) under the organisation Project Management Jülich (PTJ). The joint research project Pressure tolerant systems for permanent submerged applications is handled by Enitech GmbH as project leader, Technical University /09/$20.00 Berlin and the Leibniz Institut for Baltic 2009 Sea Research IEEEWarnemünde.

Oil compensated devices are in principle pressure tolerant since they use liquids, e.g. oil, to transmit the ambient pressure on to the relevant components.

2 Oil compensated devices are in principle pressure tolerant since they use liquids, e.g. oil, to transmit the ambient pressure on to the relevant components. See Figure 1 for hierarchy of submerged system designs. The devices are located in liquid filled housings. However, compensators are required - e.g. elastic bellows - due to the compressibility of oil to adjust the volume change when diving depth is altered. The approach presented in this paper uses soft compounds, like polyurethane or silicon to transmit the hydrostatic pressure to every housed component. Further, a pressure tolerant system with elastic coating is leak proof and free from any complex fill or refill procedures unlike liquid compensated systems. Pressure tolerant devices can operate in almost any water depths. For further information about pressure tolerant systems refer to [7]. The test vessel DNS Pegel can be seen as the first approach of an overall pressure tolerant autonomous submerged vehicle. II. PRESSURE TOLERANT TEST VESSEL DNS PEGEL To prove the approach of an overall pressure tolerant submerged vehicle the test platform DNS Pegel was built to achieve the basic design needs. A. DNS Pegel Specifications Technology: overall pressure tolerant Weight: ~ 300 kg Overall Length: 3.1m Overall Diameter: 0.55 m Survey Speed: 2 m / s Maximum Speed: 4 m /s, projected Main Thrust: 400 N Depth Rating: > m Battery Technology: lithium polymer Onboard Energy: 5.2 kwh Payload: profiler system Figure 3. Internal structure view of the DNS Pegel B. Structure The following section describes the structure of the test vehicle DNS Pegel, see Figure 3. The inner space frame is made of titanium. For modular design, the vehicle is separated into six sections, including payload. To minimize flow resistance while cruising, the vehicle has a hydrodynamic efficient streamlined form. Every section consists of a box and a front and rear bolted flange. The boxes are made of two millimeter thick titanium sheets bend to its specific form. To reduce the construction weight lightening holes are placed into the frame sheets, with marginal sacrifice of stiffness. For an easy handling and flexibility of the vehicle configuration, all space frame joints are screwed. Neutral buoyancy is achieved by a m depth rated syntactic foam which is placed between the space frame and the outer streamline shell. To form a light, robust and hydrodynamically efficient vehicle hull several thermoplastic polymer shells are mounted onto the titanium frame. Because of the pressure tolerant design there are no centered gas filled pressure hulls to support any additional lift on certain locations in water. Due to this fact a careful location of all possible weights and displacements of all internal devices is required for an accurate buoyancy trim. For the development of DNS Pegel a CAD program was used to define the weights and displacements of all internal devices, see Figure 2. C. Payload The vehicle center comprises a cylindrical flooded payload section variable in length. Currently the payload is 0.6 m in length and contains either a profiler winch system for long term tests or an ORE acoustic transponder with Ultra - Short Baseline (USBL) function for tracking purposes during dive tests. For further wet payload items like CTD sensors or side scan sonars additional space is available within this section. Because of the overall pressure tolerant design the payload can be changed without any respect of pressure hulls which makes the vehicle highly flexible in use. D. Propulsion and Maneuvring The test vehicle is driven by an eight-bladed propeller powered by a pressure tolerant seawater flooded outrunning brushless DC ring motor. With this propulsion the vehicle can reach cruising speeds between m / s (0.4-6 knots). To increase the efficiency of the main propulsion system as well as to protect the propeller blades against damage for e.g. while the vehicle is launched or recovered the propeller is ducted. The maneuverability of the test vehicle is ensured by four control planes. To get the best steering effect they are located after four fixed fins behind the main propulsion. The two interconnected rudders and the two separately controllable elevators are actuated by three pressure tolerant steering gear modules, see figure 4. Every module works with a pressure tolerant brushless DC motor in combination with a spindletype gear in a servo arrangement.

Figure 4. Pressure tolerant control plane actuators Every steering gear can shift its control plane to a maximum deflection of 30 even on higher cruising speeds.

Energy System As electrical power source we use lithium polymer batterys. Lithium polymer batteries today have the best energy to weight ratio.

The primary energy source is a 110 V setup of pressure tolerant lithium graphite battery modules. Each module contains seven cells within a customized cast, see Figure 5.

3 Figure 4. Pressure tolerant control plane actuators Every steering gear can shift its control plane to a maximum deflection of 30 even on higher cruising speeds. To detect the orientation of each control plane in combination with the steering modules we developed a new pressure tolerant magneto-resistive angle sensor. E. Energy System As electrical power source we use lithium polymer batterys. Lithium polymer batteries today have the best energy to weight ratio. The assembly of the batteries in elastic bags makes them optimal in use for the pressure tolerant technology. The primary energy source is a 110 V setup of pressure tolerant lithium graphite battery modules. Each module contains seven cells within a customized cast, see Figure 5. Further, each battery module has a safety mechanism against overheat and is equipped with an electronic cell monitoring system. A 24 V lithium titanat battery works as a backup and emergency energy source. This backup module features very low selfdischarge and self-level abilities To match different voltage levels a pressure tolerant energy converter transforms the bus voltages between 110 V and 24 V DC. The energy converter is characterized by 95 percent efficiency at a nominal rating of 750 W. F. Control Since the goal of this project is to test pressure tolerant components and assemblies, the focus was not on implementing and programming complex navigation and guidance hard- and software. The vehicle is controlled by a pressure tolerant programmable logic controller (PLC), see Figure 6. Figure 5. Pressure tolerant 5.2 kwh battery setup The employed PLC enables to program simple basic diving missions so that it is possible to control the vehicle with the aid of pressure and pitch sensors for diving depth and a compass for heading. Two computers on the supporting surface ship provide simple mission planning, and monitoring abilities. All data from the onboard instruments, like actuators and sensors can be sent to the surface and is received via a suitable communication channel by one of the surface computers as well as displayed on a custom designed graphic user interface (GUI), see Figure 7. Figure 6. Pressure tolerant programmable logic controller with capacity to work in m depth

uploading missions, retrieving data after a mission, as well as to make settings to the different controllers a 100 megabit Ethernet link is used.

GUI for navigation, mission planning and monitoring G.

required. The communication between the test vessel and the surface station can be established by three different ways: 1.

The acoustic modem is able to send telemetric data like battery status or drawn current to the surface station.

4 uploading missions, retrieving data after a mission, as well as to make settings to the different controllers a 100 megabit Ethernet link is used. This type of communication is limited by the cable length and can only be used on deck or if the vehicle is close to the surface ship under very calm weather conditions. Figure 7. GUI for navigation, mission planning and monitoring G. Communication Systems Although the test vehicle DNS Pegel is used for testing pressure tolerant components in an overall system to enable the contact with the remote station on the surface is required. The communication between the test vessel and the surface station can be established by three different ways: 1. Underwater Acoustic Data Modem To maintain a communication link under submerged conditions an acoustic modem link is used. The acoustic modem is able to send telemetric data like battery status or drawn current to the surface station. With this type of communication the user also has the opportunity to stop a running mission and force the vehicle to emerge. During the first sea trails a m rated LinkQuest acoustic modem was mounted behind the main communication mast. The LinkQuest modem did not show the designated performance in the baltic sea due to often appearing multipath propagation. Hence an Evologics S2C R 48/78 High Speed Modem which works with a different acoustic data transfer technology [8] was mounted at a further forward position with the result of much better performance. H. Navigation Because the DNS Pegel should act as a test platform for pressure tolerant equipment the navigation system contains no additional instruments to increase its autonomous performance. A reduced navigation system for shallow water navigation including a pressure sensor in combination with a 3-axis tilt compensated compass module OS5000-S from OceanServer was used. For vehicle velocity measurement a simple speed log was implemented. The navigation sensor data was processed by the PLC used for controlling all the actuators in the vehicle. III. TESTS AND RESULTS Experiments were performed to prove the usability of an overall pressure tolerant vehicle concept. There were three goals: verification of pressure tolerance of each component in the submerged system and compressive strength up to 600 bar, long time submerged functionality and evaluation of vehicle drive and dive abilities. The sea trails reported here were performed at various places in the Baltic Sea. A. Lab Pressure Test To simulate the pressure in a depth of m for the compressive strength verification every subsystem of the DNS Pegel e.g. electronics, electric motors, sensors and batteries were tested in a pressure tank, see Figure 8 below. Pressure tests were performed before a subsystem was assembled to the test platform DNS Pegel to guarantee its function under the ambient pressure of 600 bar. 2. Wireless Radio Data Modem The wireless radio communication link is mainly for remote control of the vehicle during habitation on surface or in a launch and recovery scenario. Only in this case communication via radio link is possible. To suffice the special requirements of pressure tolerant systems a customized antenna was used. After launch the vehicle DNS Pegel can be remote controlled via the radio link. An autonomous preprogrammed mission can also be started via the radio link. 3. Hardwire Ethernet Cable Currently the wireless radio communication link can only be used for remote-controlling, telemetry and misson starting. For Figure 8. Pressure tank for lab testing up to 600 bar

For this test a winch was installed within the payload. With the actual winch setup the vehicle could move 50 m between surface and the anchor on the sea bottom, see Figure 9.

5 Figure 9. Test vessel in profiler configuration reaches the sea bottom at 25 m. The picture was shot by a diver in Baltic Sea. B. Long Time Submerged Test To evaluate the long time submerged functionality as well as the possibility of DNS Pegel to act as a profiler system a six weeks open sea test was performed. For this test a winch was installed within the payload. With the actual winch setup the vehicle could move 50 m between surface and the anchor on the sea bottom, see Figure 9. An acoustic releaser system on the anchor acted as a rescue and recovery system. In this setup the vehicle was able to descent and to resurface with the aid of the winch by sending a command via acoustic data modem from the support vessel. For six weeks the test vehicle was lowered to the sea bottom. Afterwards the test platform was recovered by sending the resurfacing command. All electronic pressure tolerant systems especially the batteries worked absolutely reliable. The inner titanium space frame showed no corrosion as expected. Some stainless steel parts in combination with plastic material showed crevice corrosion. Tested plastic fittings and tubings from Swagelok will be used in further work as well as the plastic farings, indicated both no noticeable problems at all. C. Sea Trails Verifying the handling, maneuverability and diving behavior of the vehicle was the third task. There was no goal to verify autonomous underwater vehicle (AUV) -like characteristics rather to examine the system interaction of an overall pressure tolerant system. Figure 10 shows DNS Pegel at surface in calm sea. For the first trails deep water > 100 m was not necessary cause every system e.g. electronics or motors was tested up to a pressure representing of m in the pressure tank at the lab. Testing in shallow water also minimizes the costs of the experiments and the risk of loosing the whole system. Besides the PLC for control the test vehicle is additionally equipped with an embedded PC within a pressure hull. The purpose of this computer is only to simplify data logging as well as sensor or rather actuator monitoring during test dives. Since its design is not pressure tolerant it is removed for deep sea dives. Launch and recoveries were possible at weather conditions up to wind speeds of five bft with corresponding sea conditions in the Baltic Sea. All data collected during controller adjustments or execution of a given mission were performed by the embedded logging PC. The sea trail was conducted in two sections described below. 1. Controller Adjustments A requirement for mission testing are efficient and well adjusted course and depth controllers. First of all the course controller was tested and adjusted. A specific course was transmitted via remote control from the supporting vessel. In order to avoid the effects of any surface disturbances all diving maneuvers were performed at a depth of more than two meters. After adjusting and optimizing the course controller the adjustments of the depth controller were conducted. The DNS Pegel was programmed to follow a round course in various depths. Because the vessel was slightly positive buoyant the depth controlling behavior was influenced by the speed, but the depth controller was able to compensate the speed influence very well. 2. Mission Following In summer of 2008 first preprogrammed rectangle missions were performed in various depths. The mission plans were programmed with the GUI and transfered to the vessel PLC by the hardwire Ethernet cable. After the mission command the test vessel acts as an autonomous vehicle following the mission preset, see Figure 12. First experiences show adequate mission following characteristics. At the moment there is no possibility to transfer the mission data over a wireless communication link. So it was always necessary to connect the Ethernet cable manually. The data exchange with the logging PC also required a hardwire connection. Figure 10. Pressure tolerant test vessel DNS Pegel at surface during a sea trail in calm weather conditions

6 IV. FUTURE WORK In the year 2009 further sea trails in the Baltic Sea are planned to improve the controller settings as well as to arrange a second long time submerged test. A 600 bar pressure test with the complete vehicle is intended. Further new maneuver instruments like an active trim system as well as a variable buoyancy system will be attached to the vessel. Current development efforts focus on a reliable wireless communication link for mission transfer and data exchange which will be tested also in the year [4] L. R. McBride, J. T. Scholfield, Solid-State Pressure-Tolerant Illumination for MBARI s Underwater Low-Light Imaging System Jornal Of Display Technology, vol. 3, No. 2, June 2007, pp [5] R. Damus, I. Manley, S. Desset, J. Morash, C. Chrys, Design of an Inspection Class Autonomous Underwater Vehicle, Oceans 02 MTS/IEEE October 2002, pp [6] T. Austin, Autonomous Underwater Vehicles New Autonomous Underwater Vehicle technology development at WHOI to support the growing needs of scien tific, com-mercial and military undersea search and survey operation, Presentation, WHOI akukulya/2006/3/remus_show(short)_8763.pdf, March 2009 [7] G. Körner Pressure tolerant systems for permanent submerged applications, BMWi conference, Rostock, December 2008 [8] K. G. Kebkal and R. Bannasch, Sweep-spread carrier for underwater communication over acoustic channels with strong multipath propagation, J. Acoust. Soc. Am. 112 (4), October Y [m] X [m] Figure 11. Rectangular mission plot during course controller adjustment V. CONCLUSIONS The sea trails in 2008 were successfully accomplished. Open water dive tests and long time tests in the Baltic Sea proved the usability of the concept of an overall pressure tolerant vehicle. For all subsystems pressure tolerant solutions that passed the pressure tests were developed. The pressure tolerant concept showed up to form a successful basis which presents the opportunity to build up extremely reliable underwater vehicles. Predominant features are low construction costs, high flexibility to change mission goals in a short time and, in addition, to achieve diving depths which, till now, may only possible with very expensive pressure hulls. REFERENCES [1] H. E. Barnes; J. J. Gennari, A Review of Pressure-Tolerant Electronics (PTE), NAVAL RESEARCH LAB WASHINGTON DC, Final report A769720, Washington DC, June 1976 [2] J. E. Holzschuh, A Pressure Tolerant TV Camera, IEEE Journale of Oceanic engineering, vol. 3, No.1, January 1978, pp [3] Willcox S, Streitlien S. Pressure-Tolerant Batteries for Autonomous Undersea Applications. ONR report under contract Number: N C-0205

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