Design Rationale For Leviathan Autonomous Underwater
Kennesaw State University Autonomous Underwater Vehicle Team1 of 7Design Rationale for Leviathan AutonomousUnderwater VehicleA. Cheng, H. Evans, J. Gragg, A. Graves, A. Hagle, C. Meier, J. Nguessan, V. Nguyen, B. SailorAbstract—The Kennesaw State University AutonomousUnderwater Vehicle (AUV) Team designed and built theLeviathan AUV over the course of one year. Designing fromthe ground up, the team’s ambition to complete as many of the2017 RoboSub competition’s tasks as possible manifests itselfin mechanical claw and dropper attachments, a self-propelledtorpedo system, and machine learning based objectrecognition. This machine’s design and manufacture requirecommunication between students of varying disciplines andself directed learning of advanced concepts.I.housing. The realization that the sub lacked a necessarydegree of freedom, pitch, prompted the accommodation oftwo more thrusters in this iteration. This realization andmodification proved to be one of the greater challengeswith its design. The other difficulty in its design was itsweight; the materials chosen, while durable, tend to be quiteheavy, so its weight must decrease both to be within theacceptable weight range and remain positively buoyant.Therefore, weight reduction presents itself as an essentialfuture goal.DESIGN STRATEGYDuring this team’s prior appearance at the 2016 RoboSubcompetition, the former AUV model, Cthulhu, qualified forsemifinals by passing through the start gate. From thisstarting point, the aspirations for the next iteration haveonly increased. The design goals for Leviathan were tocomplete each vision task, cultivate pearls, battle a squid,and collect and classify samples.Achieving these ambitious goals requires the team tomake vast improvements and additions over a short timeframe. Mechanically, Leviathan requires a mechanicalclaw, a system to drop markers, and a torpedo launchmechanism. From a software standpoint, the AUV neededthe necessary vision and motor control requirements toimplement these mechanical systems, as well as afunctioning hydrophone system to detect the pingers’frequencies and navigate the competition properly. Thesechanges enable Leviathan to attempt nearly every challengepresented in the 2017 RoboSub competition.II. VEHICLE DESIGN: MECHANICALA. Outer StructureIn order to progress in the current and futurecompetitions, the outer structure’s redesign must accountfor the following: modularity, durability, and reliablewaterproofing. The eventual design, shown in Fig. 1,features four 1 inch Delrin plastic side plates waterjet toreduce weight, and two 8020 aluminum posts. Thisstructure supports eight BlueRobotics T200 thrusters, eachproducing 20 lbs. of thrust, and a waterproofed acrylicFig.1: Leviathan Initial Design Render.B. HousingThe aforementioned acrylic housing, arguably the mostcrucial piece of structure on the sub, houses the onboardelectronics and protects them from the sub’s aquaticsurroundings. This piece is also a new addition; the fact thatthe prior waterproof housing was rated waterproof only upto three feet in depth prompted this change. The clearacrylic cylinder measures twelve inches in diameter andtwenty four inches in length. Acrylic proves itself to be anideal material for this purpose because it is lightweightenough to minimally counteract its buoyancy, andtransparent enough to contain cameras.In order to sufficiently waterproof the inside, for merelyone drop of water proves disastrous, the team designed twoaluminum flanges each containing two rubber O-rings. Theunremovable back end flange contains fourteen holes cut
Kennesaw State University Autonomous Underwater Vehicle Teamvia waterjet. These holes will contain waterproofconnectors, as detailed under “External Electronics”. Thefront flange, conversely, can be removed to access, modify,or remove the inner electronics. The fact that it alsocontains an acrylic component permits a front facingcamera to view the competition field. The manufacturingfor such important, detailed pieces proves difficult for ourregular resources. However, the Temel Gaskets companyaccepted technical drawings of our design in order tomanufacture them.C. Inner StructureWithin the acrylic housing lies the inner structure. Itsdesign must accommodate all of the sub’s electroniccomponents, allow for ease of access to the electronics andthe onboard computer, act as a sufficient heat sink, keepcomponents from shifting with movement, and modularenough to change with any future upgrades. Partiallymanufactured from ABS 3D printed plastic and water-jet6062-T6 aluminum sheet, it balances heat sink capabilityand durability with a slightly decreased weight. The biggestchallenge presented itself as the cylindrical shape, as seenin Fig. 2, of the sub, which is notably more difficult todesign for than our prior rectangular housing.2 of 7When creating this subsystem, the team sought toovercome several challenges. The claw was to operate onone servo with a limited rotation range, but to maximize themovement range of the fingers in conjunction with saidservo is essential to its task. To combat this, the gear systemwas designed base on S rΘ, so that the 130 degree rotationof the servo would translate to at least two full rotations ofthe central gear. Using one servo had both advantages anddisadvantages, however it allows a decrease of the weightof the claw, which led to a final weight of just under 2pounds. In the future, there are several obviousimprovements that will be made, from the movement of thegrabber relative to the sub, to the precision of the grabbersgrip range. To allow for expanded axes of mobility,creating an “arm” that can move in at least relative axeswill prove itself necessary. To make the gripper moreprecise, other design options will need to be explored, toinclude modifying or creating a servo that does not have alimited rotation rate, or using more than one servo.Fig. 2: Leviathan’s inner structure with its electronicsD. Mechanical ClawThe mechanical claw, displayed in Fig. 3, will bemounted with the fingers parallel to the forward direction ofthe craft, just within the vision of the ZED. Mechanically, ituses one Savox servo to open and close its fingers aroundobject, ensnaring them in such a way that precision is notrequired. The servo, generating 22 Kg*cm of torque, shouldbe able to exert enough force for the grip strength of thefingers to not fail when picking up larger objects. The vastmajority of the grabber will be 3D printed of ABS plastic,with crucial parts (geometry willing), being milled out ofhigh strength acrylic via a computer numerically controlled(CNC) mill. The parts to be manufactured via CNC consistof the both gears in the image, and the fingers themselves.Fig. 3: Leviathan’s mechanical claw attachment.E. DropperIn order to complete the “cultivate pearls” task, the teamdesigned a dropper to deliver the markers, a payload of four1 inch diameter ball bearings. The bulk of the structureconsists of a large cylinder containing eight smallercylinders, much like a revolving door, as shown in Fig. 4.Each chamber can carry up to one marker. Driven by aHitec HS-5646WP servo capable of turning 180 degrees,the 8 mm aluminum rod attaches to a circular plate with 45degrees cut out. As the servo rotates the plate, the markersfall from the housing. The team manufactured the housing,plate, and all connecting pieces with ABS plastic via 3D
Kennesaw State University Autonomous Underwater Vehicle Team3 of 7printing. This system may improve in the future by achange in the gearing to accommodate a full 360 degreerange of motion, or a modification of the housing to nothave unusable chambers.Fig. 4: Leviathan’s Dropper.F. Torpedo StructureIn order to implement the torpedoes, the AUV requires acontainment structure for the missile components. Thisstructure consists of two 3-inch inner diameter acrylic pipes24 inches in length. Because the propelling system of thetorpedoes has more of a basis in electronics more thanmechanics, we filed the details for their construction under“Vehicle Design: Electrical”.III. VEHICLE DESIGN: ELECTRICALA. External ElectronicsThe sub makes use of BlueRobotics cable penetratorconnectors and one MacArtney Subconn Ethernet connectorto facilitate waterproofed connections from the externalelectronics to the non-waterproof inner electronics, thecommunication between which appears in Fig. 5. Rated at300 V from 5 to 10 amps with a pressure rating of 700 bar,the MacArtney Subconn Ethernet connector is used forcommunication to and from the sub. The wet mateableconnector saves time when uploading new code. Leviathanutilizes eight BlueRobotics thrusters, brushless DC motorsencased in ABS plastic housings, for maneuverability.These thrusters produce a peak forward thrust of 5.1 Kg*fat 16 V and a peak reverse thrust of 4.1 Kg*f at 16 V. Eightelectronic speed controllers (ESC) control and regulate thespeed of the thrusters. The ESCs receive instructions bypulse width modulation from the PixHawk and give usersthe ability to control the rotational speed and direction ofthe thrust.Fig. 5: Electrical Wiring DiagramB. Torpedo SystemThe goal of the torpedo system is to have a completelyisolated system by which we could fire the torpedoes.Separating the torpedoes from the sub by way of lightsignals achieves this goal. The torpedo system consists offour hollow 3D printed semi-modular torpedoes mountedwith an external photoresistor and an Arduinomicrocontroller inside. A brushless motor attached to apropeller on the back of each torpedo launches the torpedo.To combat wild rotation from one spinning propeller, thetorpedoes have been printed with twisted fins to provide acounter rotation as the torpedo moves through the water.Details about the torpedoes’ design can be found in Fig. 6.When Leviathan receives a signal to fire a torpedo, itflashes high power LEDs at a specific interval from inside
Kennesaw State University Autonomous Underwater Vehicle Teamthe main sub housing. The photo resistors in the torpedoread that pattern into the Arduino and fire accordingly. Onechallenge in detecting the correct pattern of light was theconstant changing of ambient light from sun hitting thewater. Constantly adjusting the photoresistors’ receivedbaseline voltage resolves this issue.Fig. 6: Leviathan’s Torpedo Render.C. AcousticsLeviathan has an array of three Aquarian Hydrophones todetermine the bearing of the pingers. The hydrophonesreside on the front and bottom of the sub. The hydrophonessignals pass through a U-Phoria UMC404 four input USBaudio interface where they can be digitized and timestamped.D. Battery MonitorA minimum of four MultiStar lithium polymer (LiPo)batteries at 10000mAh and 14.8V can power the sub, whichallows the sub to operate for many continuous hours oftesting. HobbyKing LiPo voltage checkers monitor thebatteries. They provide a digital voltage indicator and analarm. The monitors connect to an Arduino. If the alarm onthe monitors are triggered, the Arduino deactivates themotor and the AUV will rise to the surface.E. Printed Circuit BoardsFor the 2017 sub, the electrical division decided to designcustom printed circuit boards (PCBs) in order to improveand simplify the circuitry. The plan to use EagleCad toproduce our PCBs gave way to a decision to use CircuitMaker due to a superior parts library. The first challenge toovercome was the team’s lack of prior Circuit Makerexperience; the team had to start from scratch on learninghow to design and export the files needed to build thePCBs. The PCB design proved to take the longest due tounfamiliarity with the software. Once each PCB wasdesigned, the necessary files were submitted for a company4 of 7produce the boards.The team designed the main connector board as the hubfor all the connections for each motor and servo. Thisdesign allows for ease of access to fix any technical issues.The main board contains three different types of connectorsadded onto it. The first and most complex type is the motorboard. It uses two of the leads out of the three that eachmotor connects with. Its design provides that eachconnection is spaced out in a way to allow for heatdissipation. The second and third type of board correspondto servos necessary for the robotic arm, dropper, andcamera. One version uses just eight connections as theminimum that the servo uses. The other version has fourconnections for servos that require more current. Eachconnection type has a different wiring to each pad forfourteen gauge servo wire, twelve gauge motor wire, andsixteen gauge wire for other connections.F. SensorsThe sub has an full sensor suite, which corresponds to theevents in whichthe team chose to compete. It has twocameras: one facing forward to locate and work through thechallenges, and one facing the floor to handle line followingfor the movement to each successive challenge. One of thecameras is a Point Grey Chameleon CMLN-13S2C and theother is a ZED 2K Stereoscopic Camera. The pressuresensor that we chose was the Measurement SpecialtiesMS5837-30BA, which can measure up to 30 bar with adepth resolution of 2mm. The pressure sensor keeps the subwithin the proper range of the pool floor, ensuring the subdoes not breach the surface unexpectedly. The InertialMeasuring Unit (IMU) detects changes in the vehicle’sorientation in three major axes: pitch, roll, and yaw. ThreeAquarian Audio hydrophone equip for the acousticchallenge.G. Power Conversion and DistributionLithium polymer batteries power the sub. Each packprovides 14.8 Volts, 10000 mAh, a peak discharge rating of20 c for 10 seconds, and a continuous discharge of 10 c.The sub power systems divide into two primary categories:computer and task, and propulsion. Separating thesesystems into two categories simplifies power distributionand reduces noise and crosstalk for electrical components.There are three primary voltage rails: the 12 V rail whichpowers the LED to instruct the torpedo to fire, the 19.5 Vrail that will charge the laptop, and the 5 V rail whichpowers sensors and controllers.Two buck converters, which step-down voltage whilestepping up current from its input to the load, handle the 12
Kennesaw State University Autonomous Underwater Vehicle TeamV and 5 V conversions. The buck converters were chosenbecause they are 80% to 90% efficient at stepping downvoltage. The 19.5 V conversion is handled by a DCDCUSB-200 intelligent buck-boost DC-DC converter withUSB interface. This converter allows us to monitor the LiPopack voltage and vary the output voltage and current fromare custom script that is program on the board.IV. VEHICLE DESIGN: SOFTWAREA. Operating SystemThe software architecture of the sub, as explained in Fig.7, is based on Robot Operating System (ROS) kinetic. Itallows access to precompiled and developed packages aswell as a foundation to provide feedback to the ROScommunity [1]. The software division has created separatepackages which encompass several tasks. Some notableinclude: Sub Ai State Manager, and YOLO ObjectiveExecuter, PI Loop. Community source open sourcedprojects handle a number of major functions; they includebut are not limited to: SMACH, MavROS, rosserial,zed ros wrapper.Fig. 7: Leviathan’s software architecture.B. AutonomyAutonomy occurs through a high level decision tree tocontrol the state of the machine and the functions it willexecute. State Machine (SMACH) is a package in ROS thatprovides these functions. Each state can be user defined forgreater control [2]. Each state triggers a series of functionswhich work together to complete the competition’s tasks.SMACH contains a built in graphical interface to easily5 of 7debug and view the current state the program is in.C. VisionVideo input is handled by a new hardware that is beingtesting this year. The ZED Camera is a stereoscopic camerawhich can output independent left/right camera feeds, adepth map, and a point cloud map [3]. This year is the firstin which the software team has access to point cloud dataand can test the functionality of the camera for underwateruse. Because underwater environments tend to befeatureless, the hypothesis was that may be difficult toimplement. The intention was to localize Leviathan to otherobjects that are in the pool. Currently, the camera willcollect data to determine if localization with SLAM isachievable via zed ros wrapper, which collects all of thedata outputted by the camera into a format that can berecorded [4].D. Machine LearningMuch of the RoboSub 2017 competition relies on areliable method to track various objects underwater. Due tothe noise and variability of the environment, the lightscattering effects of water, the lack of information abouthow new objects will appear in this environment and timeconstraints, our software division deemed the use ofmachine-learning based object detection more economicalthan the creation of traditional hand-crafted detectionalgorithms. The recent success of YOLOv2, aConvolutional Neural Network designed for objectdetection and localization, at various datasets such asMicrosoft COCO: Common Objects in Context andPASCAL Visual Object Classes persuaded the softwaredivision use a network with a similar architecture [5].A dataset of the objects in the Robosub 2017 competitionmust be created in order to train YOLOv2 to recognizeobjects. The creation of such a dataset entails labeling thelocation of objects in images from competition runs.Because YOLO is a large network, it is very prone tooverfitting if trained from random initialization on a limitedamount of data. To reduce this risk and allow for learningfrom a smaller dataset, the method of fine-tuning a pretrained model is employed. Tests using data collected byour team indicate YOLOv2 is sufficient to perform objectdetection for this competition, as demonstrated in Fig. 8.
Kennesaw State University Autonomous Underwater Vehicle Team6 of 7H. Arduino Auxiliary ControlThe functions required for control of the dropper,mechanical claw, and torpedo mechanisms require anexternal interface. An Arduino easily connects via serial tothese objects to communicate on a low level system; inthese situations, to execute a higher level communicationmay be difficult. The Arduino has large support forintegration into ROS, and can run independently from anyof the higher level functions because it has its ownmicrocontroller [10].Fig. 8: YOLOv2 Network in Action.E. MavROSMavROS serves as an all-in-one package to controlmovement of the submarine [6]. Virtual RC values arepublished to the /rc/override channel as well as the FlightMode for the IMU: a px2 Pixhawk. We used a flightcontroller for drones because there is an open sourcecommunity for AUVs and remotely operated vehicles(ROVs) from BlueRobotics [7]. They created the Ardusubproject fork from ArduPilot, and their firmware easilywraps into MavROS and MavLink [8].F. Hydrophone AcousticsA new addition to the sub is a hydrophone array. Theinput is recorded via a Behringer U-Phoria UMC404HDDAC on the host machine to 88.2kHz sound files. Ananalysis on the files finds the four loudest frequencies atany give time between (25 - 40) kHz with a Fast FourierTransform (FFT) at each interval. It records the loudestfrequency as the closest pinger and assigns a timestamp.The sub uses the timestamps in conjunction with anequilateral triangle array to determine a heading which isoutput into a vector. The vector is converted into/rc/override for the MavROS package [9].G. Navigation and PI ControlThe PI controller is a variation on the PID controller. Byomitting the derivative, quick implementation of amovement package and reduction of the amount of tuningnecessary for our controller become possible. It takes twopoints from the field of view: one provided by YOLO andone provided by the center of the camera. The program usesboth the differences in the x and y directions to calculatethe distance between the two points and obtain the error. Itthen processes the error through the control loop andoutputs a data point that is converted into an RC valuepublished to MavROS.V. EXPERIMENTAL RESULTSThe three occasions on which the Kennesaw StateUniversity Marietta campus pool was reserved to test andimprove our systems provided the team with importantdata. Each instance was dedicated to gathering videofootage of the start gate and the red and green buoys. Giventhat lighting at the competition will not be consistent, thelighting in which we tested was variable so that Leviathanmight somewhat consistently recognize these obstacles.Labeling the data gained and training the recognitionsoftware accordingly affords a consistent computer visionsolution.VI. AcknowledgmentsThe team would like to thank Temel Gaskets for theircrucial support in manufacturing our O-rings and end caps.The team would like to thank the KSU Alumni Associationand the KSU Student Activities Board and Council, fromwhom this project derives the bulk of our budget. The teamalso would like to thank our faculty advisor Dr. KevinMcFall, as well as all of the Kennesaw State Universityprofessors who have taught the team’s members muchabout engineering and technology. Lastly, the team wouldlike to thank SolidWorks for their support with our studentlicenses. Without each of these parties’ contributions, thisproject would not reach the point that it has.VII. APPENDIX—OUTREACH ACTIVITIESIn an effort to reach out to the metropolitan Atlantacommunity, this team appeared at Maker Faire Atlanta inOctober of 2016. The team brought the prior model,Cthulhu, a remotely operated mini-sub in a water-filledtank, and prototypes of some of the mechanical sub-systemsfor Leviathan. This event provided the opportunity to teachattendees, a large number of whom are young children,about robotics, mechanical design, and the engineeringprocess. The team hopes to have inspired visitors to learnmore about technology-related fields and robotics.
Kennesaw State University Autonomous Underwater Vehicle TeamVIII.APPENDIX--REFERENCES[1]"kinetic - ROS Wiki", wiki.ros.org, 2016. [Online]. Available:http://wiki.ros.org/kinetic. [Accessed: 19- Jun- 2017][2] "smach - ROS Wiki", Wiki.ros.org, 2017. [Online]. Available:http://wiki.ros.org/smach. [Accessed: 19- Jun- 2017][3]"ZED - Depth Sensing and Camera Tracking", Sereolabs, 2017,[Online]. Available:https://www.stereolabs.com/zed/specs/. [Accessed: 19- Jun- 2017]"zed-ros-wrapper - ROS Wiki", Wiki.ros.org, 2017. pper. [Accessed: 19- Jun- 2017][5] "You Only Look Once: Unified, Real-Time Object Detection",University of Washington, 2017. [Online]. yolo.pdf. [Accessed: 19- Jun2017][6] "mavros - ROS Wiki", Wiki.ros.org, 2017. [Online]. Avaiabile:http://wiki.ros.org/mavros. [Accessed: 19- Jun- 2017][7] "Software and Hardware - Blue Robotics", BlueRobotics, 2017.[Online]. -hardware/. [Accessed: 19Jun- 2017][8] "Overview · ArduSub GitBook", BlueRobotics, 2017. [Online].Available:https://www.ardusub.com/. [Accessed: 19- Jun- 2017][9] “Rolando Panez thesis.pdf”, University of Florida, 2004. ions/thes diss/Rolando Panez thesis.pdf [Accessed: 19- Jun- 2017][10] "rosserial - ROS Wiki", Wiki.ros.org, 2017. [Online]. Available:http://wiki.ros.org/rosserial arduino. [Accessed: 19- Jun- 2017][4]7 of 7
The wet mateable connector saves time when uploading new code. Leviathan utilizes eight BlueRobotics thrusters, brushless DC motors encased in ABS plastic housings, for maneuverability. These
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