Underwater Marking AUV using Paraffin Wax

Transcription

1 SP6 Underwater Marking AUV using Paraffin Wax Seokyong Song and Son-Cheol Yu Dept. Of Creative IT Engineering Pohang University of Science and Technology Pohang, South Korea {einst1879, Abstract It is still hard mission for human divers or robotic systems to investigate complex underwater environments, composed of several turning points and rooms. This paper explains a design of the Autonomous Underwater Vehicle (AUV) for marking physical and visible path logs in water. Then, it will make the repeated exploration easier. For the solution of underwater marking, we chose to use paraffin wax which can be melted easily in the heated body of machine and also be hardened easily in water. This vehicle has a container for storing the bulk of paraffin wax filament, nozzle part for ejecting the filament, and cameras for detecting filament mark. The marking method is similar with Fused Filament Fabrication (FFF) method of 3D printers. Nozzle motors extrude filament into nozzle, heating core melts it, and it comes out from the nozzle in water. As well as leaving a path mark, this AUV finds interesting places with realtime topic modeling algorithm and draws a circle-shaped mark, can be detected the next time on the point. Keywords AUV; underwater marking; underwater 3D printing; FFF method; paraffin wax; AUV Hansel I. INTRODUCTION Exploring complicated underwater environments, such as shipwrecks and caves, is dangerous and difficult for human divers. Thus, underwater robotic systems have been considered to solve those underwater mysteries. Especially, since Autonomous Underwater vehicles (AUVs) have better mobility than Remotely Operated Vehicles (ROVs) among underwater structures, several AUVs for investigation were developed and tested before. As more sophisticated AUVs have been developed, the capability of underwater inspection has grown a lot. For example, the vehicle DEPTHX [1] explored Zacatón cenote, 110 m wide, and achieved 3D point cloud of the environment. However, it is still hard mission to investigate complex environments composed of several turning points and rooms. In order to achieve enough data when exploring unmapped shipwrecks or caves, AUV has to investigate the environments multiple times in most cases. However, it is confusing to distinguish whether the robot has gone by the path, due to absence of any absolute path logs in water or in sensor data before making 3D map of environments. As AUV has to inspect more complicated structures, these problems become more serious. Therefore, we devised a new method of marking physical and visible path logs in water. This paper suggests the design of AUV, which can leave underwater trace on the path the vehicle has gone. In the next investigation, detecting the mark, this AUV can follow it and revisit the valuable sites, or choose new path where no trace exists. This concept is described on Fig. 1. By using this autonomous robot system, exploration of complex environments will be accomplished optimally. Fig. 1. Key concept of the AUV /16/$ IEEE 33

2 This scenario is quite similar with the scene in well-known fairy tale Hansel and Gretel, where Hansel laid white pebbles on his trails. Thus, we named this AUV as Hansel. II. KEY CONCEPT One of the solution that we chose for underwater marking is using paraffin wax. Paraffin wax has a melting point between 47 and 64, and a density of approximately 0.9 g/cm3. That is, paraffin wax can be melted easily in the heated body of machine, and can be set in water. The AUV of this project has a refillable paraffin wax case inside, so it stores the bulk of filament - made of solid paraffin wax. In the similar way with 3D printers using Fused Filament Fabrication (FFF) method, the filament of paraffin wax is guided into nozzle by the nozzle motors. Nozzle has a heating core with temperature higher than 100, therefore the filament gets melted in nozzle. While exploring underwater, this AUV ejects the melted paraffin wax from nozzle, then it is cooled down by seawater and solidified in water. Ejecting paraffin wax continuously, the underwater trace is formed on the path the AUV has swum. Because the density of paraffin wax is usually lower than seawater, the printed path made of paraffin wax tends to float on the surface of water and will be broken. To solve this problem, we need materials with higher density than water. Therefore, for making the printed path heavier, dusts of iron or rubber are added when making bulk of paraffin wax filament at first. In this way, front camera of AUV can find paraffin wax mark at the front in the water, or bottom camera can find it on the seafloor in next mission. When the vehicle detects the paraffin wax mark with its optical cameras or sonar, it updates its next navigation as we commanded; follows the trail or visits new path. Also, the second path marking is still ongoing until second exploration is finished. In addition to marking a curved line on the way the robot passed, this AUV can mark different shapes, such as circle and rectangle, on the interesting or weird point of environments. Those marks with special shapes are more easy to detect and will be recognized as places to be investigated in detail. III. VEHICLE DESIGN A. General Specifications AUV Hansel is a turtle-shaped hovering type vehicle. Its hull is made of aluminum alloy and has several aluminum frames which enable the vehicle to sit on flat ground and hold TABLE I. SPECIFICATIONS OF THE AUV Body Size 115 (W) x 90 (H) x 120 (D) cm3 Hull Material Aluminum alloy Air Weight 300 kg Max. Depth 1,000 m Batteries 2 kwh Lithium-ion x 2 Sensors Imaging sonar, HD cameras, DVL, Accelerometers, Depth gauge, IMU Fig. 2. Features of the AUV thrusters or buoyant materials. The vehicle has an around 115 cm 72 cm 120 cm size for robot frames, thrusters, camera, lights, computers, and batteries. We also considered a margin from volume of paraffin wax case inside. Including all of them, this vehicle weighs approximately 300 kg. As shown in Fig. 2, the vehicle has filament case, computers, batteries, and some proprioceptive sensors, including accelerometers, depth gauge, and IMU, inside its aluminum body. On the surface, there are four holes for equipping water-proof and pressure-resistant cylinders, containing optical cameras and LED lights. Also, aluminum frames make it convenient to attach sonar camera, thrusters, Doppler Velocity Log (DVL), some materials for buoyancy, and some other underwater sensors. The four thrusters are used for horizontal movement, and two are for vertical /16/$ IEEE 34

3 Moreover, on the AUV s upper part, it has a cover which can be opened or closed for convenience of refilling filament and checking inside components. Attaching a triangular frame on upper side will be also useful when crane s lifting hook is hung on. The following parts of this paper will explain each significant part of the AUV; filament part for storing enough amount of paraffin wax, nozzle part for melting and ejecting it, sensor systems for detection, and control system related to navigation update. B. Filament Part If the diameter of paraffin wax filament is assumed to be 1 cm and aimed exploration distance is 1000 m, the required volume for filament is approximately L. This amount of filament can be stored in the container with 50 cm 50 cm 50 cm size. In same assumption for amount for paraffin wax, the bulk of filament takes up about 71 kg. Of course, it should be tested several times in advance whether the thickness of filament is enough for paraffin wax mark to be sustained in water or not. If filament should be thicker, we have to set the shorter exploration distance or make the size of the AUV bigger than first one. As explained above, during the filament manufacturing specific amount of iron or rubber powder should be added on the bulk of paraffin wax due to density. According to [2] and [3], we know that the density of seawater is about 1.03 g/cm3 and one of pure iron is nearly 7.86 g/cm3. Therefore, it is possible to calculate the approximate density of iron-mixed paraffin wax and control it as we add measured weight of iron powder. The goal density of paraffin wax filament depends on environments to explore. If the investigating site is quite shallow sea and has fast current speed, using much heavier filament and detecting it with bottom camera will be better strategy. On the contrary, in the deep seawater with slow current we do not need to worry about paraffin wax mark being carried away by current. In this situation, it will be good to choose filament with a little heavier density than 1.03 g/cm3. Additionally, too heavy filament can lead to being broken, so it is important to adjust the density and diameter of filament. Therefore, inevitably many times of experiments are needed to solve this problem. It is fundamental to check that the paraffin wax mark can be detected by optical and sonar cameras. Apart from improving computer vision, mixing fluorescent materials in the bulk of filament can be simple solution for optical detection. Further, in order to find mark successfully with sonar camera in turbid water, thickness of the filament and features of the materials have to be considered. C. Nozzle Part The ideas about nozzle structure of AUV Hansel came from 3D printers using FFF method. FFF is also known as Filament Deposition Modeling, one of the most used rapid prototyping technologies. As explained in [4], this method uses two rollers Fig. 3. Components of nozzle part, using FFF method to extrude filament, a temperature controller and heating core to melt filament, and nozzle tip where melted filament comes out. With the platform below the nozzle tip moving and filament layer being stacked up, 3D objects are created. In this AUV, two gear-shaped nozzle motors rotate and extrude paraffin wax filament from case into nozzle. Heating core is heated by temperature controller around it, so it makes filament semi-liquid state. Then, the melted filament can be ejected from the nozzle tip. It will be helpful for ejecting paraffin wax against water pressure that additional piston inside nozzle part pushes melted filament outward. The fused filament, continuously ejected from nozzle tip, should become a connected mark. Thus, controlling swimming speed of the AUV and temperature of nozzle tip is very significant. Because paraffin wax on nozzle tip contacts directly with cold seawater, the heating core needs to have high temperature enough for filament not to set on nozzle tip. At the same time, hot core and tip must not affect the function of other components and environments. D. Sensor Systems The AUV Hansel uses optical cameras and acoustic sensor for mark detection and obstacle recognition. It is equipped with two HD optical cameras; one is for front and another is for bottom. Front camera is usually used to detect hovering paraffin wax marks and obstacles in front of the AUV. The marks on the seabed can be discovered with bottom one. With each of the LED light, the camera can see objects in deep and dark seawater. For acoustic sensing, a forward looking imaging sonar is attached on a frame of the vehicle. Thus, the AUV also can find the paraffin wax marks in turbid seawater and avoid obstacles with estimating the distance with them. Since this imaging sonar is able to pan and tilt, we can choose effective /16/$ IEEE 35

4 heading for investigation in advance and this vehicle can adjust the angle during exploring in water. The vehicle has several sensors for navigation. DVL on the bottom side, accelerometer, IMU, and depth gauge inside the vehicle enable it to localize itself, estimate its relative heading and depth, and follow the path as it planned to go. E. System Flow and Algorithm The flow chart of Fig. 4 describes the operation flow of the vehicle, and one of Fig. 5 shows the mission flow during exploration. AUV Hansel starts the mission from the launch location, such as the point above entrance of shipwrecks or caves. At the same time when starting underwater exploration, heating core is heated by temperature controller, and nozzle starts to eject paraffin wax filament. Continuously printing the path in water, main computer inside the robot interprets and stores the image frames through front and bottom camera. The vehicle basically moves forward, but it can avoid obstacles like rocks, wall, and seaweed with swaying or turning when it detects them. In that way, the AUV swims deep inside the environments and inspects the several Fig. 5. Flow chart of the AUV mission Fig. 4. Flow chart of the AUV Operation districts without any collision. Hansel can also discriminate paraffin wax mark in the water, so it can follow the path or explore another site by setting we commanded before launch. When the vehicle finds a surprising site which is worth more inspection, it rotates around the point and draws a circle mark with filament. The next time Hansel discovers the mark, it will investigate the place in more detail. After enough exploration, the nozzle stops heating and ejecting, and the vehicle returns to the starting point through navigation logs. In this way, leaving the underwater mark during the first mission helps to find interesting places or look into new places, so we can conduct next mission efficiently and successfully. Computer vision algorithm is used fundamentally for paraffin wax mark detection. The main computer extracts local features in an image using Scale Invariant Feature Transform (SIFT) [5] or Speeded Up Robust Features (SURF) [6], then matches with the database model of marks. As we mentioned above, adding fluorescent materials will be helpful for realtime mark detection. Additionally, in order to distinguish if the place is surprising or not, and to write navigation summaries of a number of video frames, we use Real-time Online Spatiotemporal Topic modeling (ROST) in [7]. Through this method we find visual words from observation of the cameras and represent each of the image frames by spatiotemporal distribution of topics, such as sand, rocks, seaweeds, and a cave. Then, we measure the score which express how interesting it is among navigation summaries until that time, and update them with distinctive image frames. In this way, the vehicle can /16/$ IEEE 36

5 recognize whether the site is surprising or not, so it can leave circle mark around it. IV. CONCLUSION This paper has described a design of the AUV Hansel for leaving visible marks on its path and interesting places in water. This AUV uses paraffin wax filament stored in container and FFF method of rapid prototyping technologies. With two optical cameras and imaging sonar, it can avoid obstacles, find surprising sites, and detect the mark. In this method, underwater exploration for caves or shipwrecks, which needs to be repeated several times, will be much easier. ACKNOWLEDGMENT We would like to thank AUV2016 award committee for their generous travel support for the student poster competition. This research was supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the ICT Consilience Creative Program (IITP-R ) supervised by the IITP (Institute for Information & communications Technology Promotion) and the project titled Gyeongbuk Sea Grant Program funded by the Ministry of Oceans and Fisheries, Korea and Civil Military Technology Cooperation Center, Korea. REFERENCES [1] Nathaniel Fairfield, George Kantor, and David Wettergreen Towards Particle Filter SLAM with Three Dimensional Evidence Grids in a Flooded Subterranean Environment, IEEE International Conference on Robotics and Automation, 2006 [2] Beicher and Robert J Physics for Scientists and Engineers, 3 rd ed. Orlando: Sauders College, [3] CRC Handbook of Chemistry and Physics, 75 th ed. Florida: Chemical Rubber Co, [4] Iwan Zein, Dietmar W. Hutmacher, Kim Cheng Tan, and Swee Hin Teoh, Fused deposition modeling of novel scaffold architectures for tissue engineering applications, Biomaterials, vol. 23, issue 4, pp , 15 February 2002 [5] David G. Lowe, Distinctive Image Features from Scale-Invariant Keypoints, The International Journal of Computer Vision, vol. 60, issue 2, pp , November 2004 [6] Herbert Bay, Tinne Tuytelaars, Luc Van Gool SURF: Speeded Up Robust Features, European Conference on Computer Vision, vol. 3951, pp [7] Yogesh Girdhar, Philippe Giguére, and Gregory Dudek Autonomous adaptive exploration using realtime online spatiotemporal topic modeling, The International Journal of Robotics Research, 13 November /16/$ IEEE 37