Delhi Technological University: Designand Development of the Littoral AUV

Zyra

Aayush Gupta,Vatsal Rustagi,Raj Kumar Saini,Akshay Jain,Prateek Murgai,Aditya RastogiPronnoy Goswami,Ayush Tomar,Abhijit Ray,Bhavy Dikshit,Prithvijit Chattopadhyay

Shubham Raina,Mayank Gupta,Vikas Kumar Singh,Shubham Jain,Chirag Gupta

Abstract—The Delhi Technological University Au-tonomous Underwater Vehicle project team’s main objec-tive is to design and develop an autonomous underwatervehicle for the AUVSI and ONR International Robosub-2014 competititon.The competition is held at the TRANS-DEC facility,part of SPAWAR Systems Center Pacific inSan Diego,California.The paper presents the design anddevelopment of a modular littoral autonomous underwatervehicle called ’ZYRA’ having six degrees of freedom forperforming the following tasks underwater target localiza-tion and homing, buoy detection,path following,obstacledetection and obstacle manipulation tasks.The develop-ment of the AUV has been divided into namely five depart-ments mechanical design and fabrication, embedded andpower systems, control and software , image processing, underwater acoustics.A fully functional AUV has beentested in a self created arena with different tasks spreadout in a shallow water environment.

I. Introduction

Autonomous Underwater Vehicle are powerfuland complex systems which are capable of per-forming underwater (shallow and deep sea) tasks.The range encompasses bathymetry calculation, de-tection of faults in oil pipelines, collection ofdeep sea water samples, collection of marine data,strategic and commercial applications like underwa-ter surveillance, recovery and monitoring of sub-merged installations and even complex tasks likecollecting data which aids in understanding globalwarming.The aim is to design and develop an Au-tonomous Underwater Vehicle which will serve asa technology demonstrator which is highly com-

pact,multirole and can be used for various missionswith the independence of chosing payload. ZYRAis the product of completely refined mechanical as-sembly, control systems, embedded systems, visionsystem and acoustics module.The mechanical modelof ZYRA is designed to be in hydrodynamicallystable equilibrium both below and above water sur-face.The mechanical system consist of main pres-sure hull, front and back lids, frame, electronicsrack.The focus of embedded and the power sys-tems department is mainly on the implementationof acoustic positioning module, actuator board andthe designing of a power distribution board fordiverting power to various modules on the AUV[1]. Dynamic control is achieved by retrieval ofcoordinates using various PID control algorithms [2],[3].The software is designed to run in decentralizedmulti-threaded agent architecture, with the threadshandling pressure sensor, acoustics, cameras, con-trol system, IMU, each performing input and outputoperations in continuous loops.The mission planmodel is also taken care by the control departmentas it is responsible for the artificial intelligence ofthe vehicle [4],[5] .The vision system is designed toperform tasks like buoy detecion, bin detection, pathdetection, entry gate detection.The image process-ing software consists of various color filters, edgeand line detection algorithms.For the target(soundsource or pinger) localization the AUV employs athree dimensional array of hydrophones forming apassive SONAR system.The data acquired through

hydrophones is passed through a amplifier, filter anddetector circuit before it is converted into its digitalform for further processing.The localization task isa sequential task in which first we calculate a setof values of TDOA( Time Difference of Arrival)using the hydrophone signals and using these valueswe estimate the bearing and the position vector ofthe sound source with respect to the AUV.For theexperimental testing of the AUV an arena is createdin a shallow water area and the tasks of entry gatedetection, buoy detection, path detection and soundsource are spread throughout the arena.

II. Mechanical Design

Fig. 1. Solidworks Assembly of ZYRA

ZYRA is the product of indigeniously designedmechanical assembly to embedded and control sys-tems.It has a custom made LiPo battery pack, anovel power distribution, redesigned actuator con-trol board, battery monitor, leak sensor, an acous-tic signal processing module and improved soft-ware.The fabrication material chosen for pressurehull is Virgin Cast Acrylic (Clear) because of itsexcellent workability, strength, shock resistance andcomparatively low density. Simulation is done onhull at underwater depth of 10 m and Factor ofsafety is found out to be more than 5.1.

The body shall be propelled by six thrustersand has a net positive buoyancy. The frame isso designed to ensure easy mounting of thrusters,

Fig. 2. Solidworks simulation of the main hull-Von Mises Stressdistribution

sensors, grabbers etc. The profile of the vehiclemakes it highly manoeuvrable. The front lid is made

Fig. 3. Exploded view of assembly

of virgin cast acrylic .Its transparency ensures clearand unobstructed view for the forward camera thatis mounted inside the hull.Back-lid will be made ofaluminium.Its smooth surface will ensure easy andefficient installation of connectors and aluminiumbeing a good thermal conductor will also help as aheat sink.

A. Electronics Hull

The main hull of ZYRA is of cylindrical shape.The shape of the vehicle has been decided aftercalculations, keeping various hydrodynamic param-eters in mind to improve the overall performanceof the robot. Main frame is designed to ensureeasy installation of thrusters, grabber, hydrophonearray etc. The main consideration while designingit was that unwanted stresses on hull are uniformly

Delhi Technological University Autonomous Underwater Vehicle

Fig. 4. Solidworks model of main frame

and efficiently transmitted to the whole frame andprevent cracks. Some of its components are madeup of Stainless Steel GR316 and others are ofAluminium. The reason for this approach is tohave a suitable combination of weight, buoyancy,resistance to corrosion along with low maintenance,high strength and ease of fabrication. The hull iswaterproofed using Toggle Clamps instead of Rodsfor faster opening and closing of the rear lid.

B. Vehicle DynamicsThrusters: Six strap-on BTD-150 thrusters from

Seabotix Inc. are used to manoeuvre the vehicle.Two thrusters facilitate motion in the horizontaldirection, while other two facilitate the verticalmotion. Two additional thrusters are being used tocontrol strafe and rotations about z-axis i.e. yaw.They are chosen because of their high thrust toweight ratio and safeguards for power surges andground shifts. These thrusters provide a two bladebollard thrust of 2.9 kgf and require power in therange of 80-110 watts depending on the conditionsoutside. All electronic components are housed in-

Fig. 5. Solidworks model of electronics frame

side the electronics hull. Components are placed inracks inside the main hull so as to facilitate easy

removal of component in case of a technical fault.The rack is so designed to ensure easy additionof new shelves later as per requirements and leastlength of wires. Battery is placed at lower mostshelf to ensure lower centre of gravity and improvestability and resist roll.

C. Degrees OF Freedom

Fig. 6. ZYRA’s Degrees of Freedeom

The six thrusters are installed in such a way thatthe vehicle has six degrees of freedom includingrotational motions like roll, pitch and yaw.

D. StabilityThe AUV is designed in such a way so that

the centre of gravity and centre of buoyancy lie inthe same vertical line.Fig.8 and Fig.9 display thestability parameters.

E. RobustnessFig.2 displaying Von Mises Stress Distribution

and Fig.8 showcase the robustness of the mechanicaldesign.

III. Embedded and Power SystemsAs discussed before for the AUV ZYRA the focus

of electronics department is primarily on implemen-tation of SONAR and signal processing module, ac-tuator board for control of six SeaBotix thrusters.Anew power distribution board is designed for voltageregulation and to provide power directly to all thesystems through one particular board which wouldeliminate the need of further bucking of voltageand hence concentrates the heat dissipating unit inone particular section of hull which can be placednear the metal ends to allow heat exchange with thewater.

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TABLE ICharacterisitics of ZYRA

Property ValueDimensions 60cm x 40cm x 51 cm

Diameter of Hull 30cmDry Weight 32Kg (dsPICDEM 2)Propulsion 4 horizontal + 2 vertical thrusters

Velocity 0-1.5m/s

Fig. 7. Centre of buoyancy and gravity in same vertical line

A. Power SystemsThe electrical power system is comprised of

lithium polymer (LiPo) battery, encased within themain electronics hull. The battery is rated at 18000mAh at 18.5V. The battery protection board mon-itors voltage of each cell and shuts off the powerif the voltage drops to 16V, also the overchargeand current drawn is monitored by the same. Itis connected to a Power Distribution Board (PDB)which is responsible for diverting power to variousmodules on the bot. The PDB is equipped with fusesin case of battery/circuit failure, multiple capacitorsto smooth out ripples, and eliminates high frequencynoise. The PDB divides the power and providesregulated power supply to sensitive electronics. It

Fig. 8. Depicts the fixture and face of pressure application

provides ZYRA with clean, isolated, and regulatedpower at 5V, 12V, and 18.5V.

B. Actuator Control

The actuator board used for ZYRA is customdesigned. It takes in care the electromagnetic inter-ference from the thrusters which distorts the motorcontrol signal coming from the microcontroller unitdsPIC30F2010. This microcontroller is responsiblefor taking in signals from the central processingboard through packetized serial interface. The mi-crocontroller is capable of processing at 30MIPS.

Fig. 9. Schematic for Actautor board

Delhi Technological University Autonomous Underwater Vehicle

The motor controllers from Dimension Engineer-ing are capable of functioning at ultra-high fre-quency of 32 KHz thus making it inaudible forhuman ears and eliminate the irritating hummingnoise. The controllers can be configured to use ei-ther in tank style differential drive or analog voltagecontrol. The microcontroller unit receives signalsspecifying direction and speed of the ZYRA whichin turn actuate the required thrusters. A hermeticallysealed switch is responsible for killing the powerto the propulsion system. Along with hard kill asoft kill is also incorporated in the code.Analogvoltage is used to control the speed of thrusters withanother signal to specify the direction of motion.PWM motor controller of dsPIC30F2010 is used togenerate variable duty cycle PWM signal which isfiltered and smoothed into an analog signal (0V-5V)via a high capacitance, RC filter. The added advan-tage of mounting syren-10 on the actuator boardis that any particular controller can be replaced incase it gets damaged and thus prevailing re-usabilityof the board. Learning from past experiences, the

Fig. 10. Actuator Board

propulsion system of ZYRA have been completelyredesigned. The communication of Master Com-puter and Micro-controller have been changed fromRS-232 protocol to TTL. Instead of the fact that RS-232 is noise tolerant, the communication to UARTis done using USB-TTL chips.

C. Electronics Rack

Modularity of the vehicle has been the prioritywhile developing the vehicle. Electronics Rack isdesigned such that it holds all the electronics and

can be removed from ZYRA without disturbing therest of the system

Fig. 11. Electronics rack for ZYRA

The level of various racks can be varied accordingto payload.

Fig. 12. Orientation data received from IMU used as an input inControl system module.

IV. MissionModuleMission plan module of ZYRA is responsible for

the artificial intelligence of the vehicle. It is at thehighest level in the software hierarchy, coordinat-ing the global state of the AUV and the state ofeach subsystem. It makes calls to sensor moduleslike vision, sound etc. determine the position andorientation of the AUV and to identify targets in

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TABLE IICommericial Off the Shelf(COTS) products used in ZYRA

Devices ModelKontron Motherboard 986LCD-M/mITX

Battery Li-Po 18.5V , 18000mAhActuator Controller dsPIC30F2010 (dsPICDEM 2)Motor Controller Syren 10

Camera Microsoft LifeCam-3000 and Logitech HD Pro C920Propulsion SeaBotic BTD-150

Pressure Sensor Applied Measurements Pi9933IMU XSENS MTi-28A

Data Acquisition NI PCI-4462 and NI USB-6210Servo HiTec HS5646WPHydrophones Reson TC-4013

the arena. The mission plan module coordinates thestate of the AUV as it goes through the entire mis-sion arena. It has a scheduler/ timing module whichtimes each operation and is capable of making smartdecisions of leaving a task and moving to the nextone based on mission time elapsed and pre-writtencontingency plans. Once the AUV determines whattype of action is to be performed, it calls the controlmodule which commands the actuators to functionprecisely.

V. ControlModuleControl module is called by the mission plan

module as and when required to change the orienta-tion and position and orientation of the AUV basedon the operation being performed and input fromvision, acoustic, depth and other sensor modules.Control module maintains the orientation of AUVusing continuous PID loops running simultaneously.It relies on the mechanical stabilization for both rolland pitch movement, and thus, only the yaw, depthand horizontal movement of AUV is controlled bythis module. The PID control algorithm has beencoded in Qt Creator 3.0. This method has proven tobe more efficient, less processor intensive and easilyimplementable. The system attempts to maintainits state using dynamic feedback from the IMU,pressure sensor and the acoustic and vision modules.User interfaces have been specifically developed totune and adjust the PID parameters easily.

VI. Image ProcessingThis year we have shifted all our codes to open

source softwares. We have integrated all the codes

Fig. 13. Various software modules working in tandem to achievemission control

in Qt SDK 5.2 using Qt Creator 3.0. The ComputerVision module is developed using OpenCV libraryof C++.

The Computer Vision module was developed us-ing the OpenCV library of C++ integrated with QtSDK using Ubuntu 12.04. The high parallelism dur-

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(a)Fig. 14. ROBOSUB Mission Controller’s Approach .

Fig. 15. Generalized flow chart for target detection using ImageProcessing

ing execution of programs on multi-core processorsin Ubuntu (Linux) gives the vision module the re-quired real-time computational power. The moduleincorporates concepts involving the various imageprocessing algorithms for particle analysis, imagesegmentation, binary morphology, Hough transfor-mation and Machine Vision. The major change this

year is that the navigation system works on absoluteyaw (angle) control and we have used the UserDatagram Protocol (UDP) for communication ofthe AUV. The previous generation vehicles had aless accurate navigation system partly based onheuristics.

We have used an Android phone as our IMU(Inertial Measurement Unit) using an application forAndroid operating System to get the orientation ofthe AUV as an alternative of our IMU. The Androidphone uses the UDP communication for sendingand receiving data packets between the Androidphone and the single board computer. The AUV iscapable of doing the tasks described below.Fig.15gives the generalized flow chart for target detectionusing vision module.

A. Gate Validation

Starting with the Gate Detection, our vision al-gorithm receives video feed from the front facingcamera and gives the heading relative to the AUV asthe output. To accomplish this the captured image isconverted to HSV and segmented for specific color.Edge Detection is used to seprate desired edges. Anedge filter is then applied to the binary image thusformed. The Center of Symmetry of vertical edges

Delhi Technological University Autonomous Underwater Vehicle

Fig. 16. Original Image and Image after Edge Detection

Fig. 17. Sample flowchart used in vision system

thus gives the correct heading (in degrees) relativeto the vehicle.

B. Path Detection

For Path Detection our vision algorithm receivesvideo feed from the downward facing camera andgives the heading relative to the AUV as the out-put.To accomplish this we employed color basedsegmentation along with blob analysis. After con-verting the video feed to hue, saturation and value,color segmentation is used to separate out the spe-cific color(orange) and convert to binary . We use afiltering technique to isolate only the pixels we want

Fig. 18. Original Image and Processed Image for Path Detection

to see, i.e, to remove any type of noise. After thisprocess is applied we are left with a single black andwhite image. Blob analysis is then used to identifythe centroid of the white colored blob left in theimage. This information is then used to determinethe heading of the blob relative to the vehicle.

C. Buoy Detection

Fig. 19. Original Image and Processed Image for Buoy Detection

The next task we have to do is buoy detection.Algorithm for buoy detection is similar to that ofpath detection. This algorithm receives video feedfrom the forward facing camera. Similar to the pathdetection, the goal of this algorithm is to output atarget pixel, or a target point. Color based segmenta-tion is used for this purpose. The HSV color space isused for all segmentation based operations. Originalimage is converted to HSV.The algorithm segmentsout the red color, giving a binary image.The binaryimage is then eroded and dilated suitably to removeany errors present. Then the binary image is passedthrough filter to remove noise .The particles in thebinary image are then analyzed and the center of thelargest connected particle gives the current heading.The vehicle tries to approach the flare keeping thecenter at 0 degrees.

Delhi Technological University Autonomous Underwater Vehicle

VII. UnderWater AcousticsZYRA has an acoustic navigation system which

employs a three dimensional array of hydrophonesfor data acquisition to calculate azimuthal and com-pass bearing and estimate hyperbolic location ofthe sound source in far-field approximation usingTime difference of arrivals (TDOAs). Fig.20 shows

Fig. 20. 3-D hydrophone array

a tetrahedral array of hydrophones which has beenused for the localization of the sound source. Sucha symmetric array eases calculation while makingtime difference of arrival(TDOA) calculations andhelps in reducing symmetric noise at different hy-drophones.

The localization task has been broken down intotwo parts as follows :

A) TDOA( Time Difference of Arrival Estimation)

B) Position Estimation

A. TDOA EstimationThe Generalized cross-correlation technique us-

ing phase transform (GCC-PHAT) is employed tocalculate the time difference of arrival correspond-ing to the correlation peak.Estimated TDOAs givebearing estimation for far field approximation.Themathematics of the method is discussed as belows:Let S 1(t) and S 2(t) be two signals at two differenthydrophones and ∆ be the Fast Fourier transform ofthe signals , then

X1( f ) = ∆(S 1(t))

X2( f ) = ∆(S 2(t))

GCC-PHAT is defined as:

GPHAT =X1( f )[X2( f )]∗

|X1( f )[X2( f )]∗|where * is the complex conjugate of a function

T DOA = argmax(∆−1(GPHAT ))

In environments of high levels of reverberationGCC-PHAT helps to improve robustness and accu-racy in calculating the time difference of arrival aswe can see from Fig.21 that GCC-PHAT enhancesthe peak and whitens the region around it.

Fig. 21. Typical GCC-PHAT function

B. Position Estimation

Fig. 22. Geometrical model of the localization system

We have to note a few terminologies here. X, Yand Z are my sensor positions respectively on thex,y and z Cartesian axes. The point S represents mysound source. Since TDOA measurements require

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2 hydrophones , we will be dealing with sourcehydrophone triangles such as .∠(ZOS ) = b is my bearing , we have defined∠(ZS N) = a and ∠(ZNS ) = c.ZN has been drawnsuch that ZS = NS .

The path difference x = cτ where c is thespeed of sound and τ is the TDOA. We defineOZ=OX=OY=d.

We define (vectorially) , OS = rs and ZS = rz andin a similar fashion we define rx and ry.

The basic TDOA equations are:-

||ri| − |r j|| = Xi j

Where i and j run over x ,y and z and x is mypath difference. Note that Xi j is symmetric due tothe modulus on the left hand side.

A little manipulation with the parallelogram lawof addition and TDOA equations would show us thatfor that triangle OZS we can derive the quadratic in|rs| as :

A = 4sin2(a/2)

B = ±4sin2(a/2)Xzs

C = (Xzs)2−d2

In A2x + Bx + C = 0 , x is the dummyvaraible(different from Xi j) , where

x = |rs|Now clearly ON is the path difference that is

ON = Xzs.Determine ZN = f by cosine rule and finally using

sine rule in triangles OZN and NZS.

sin(a/2) =√

1− df sin2(b)

The discriminant of the quadratic gives us thecondition that :

cos(a/2)ϵ− dXzs, d

Xzsor in a better sense d

Xzs< 1.

The idea is to find |rs| from the quadratic andcalculate p = |rs|sin(b) and z- coordinate of sourceusing z(s) = |rs|cos(b).

And finally for Xxs or for Xys using the fact that,

|ry| =√|rs|2−2dy(s)+d2

And that y(s) = psinψ , ψ being measured in theXY plane from X axis we find out by using this inthe TDOA equation in triangle OYS.

sinψ = d2−Xys2±|rs|Xys

2dp

The beauty of this method lies in the fact that wehaven’t used any approximations for calculating theexact positions .

Although there is the possibility of multiple rootsfrom the quadratic yet only one of the solutionswill be feasible satisfying all other conditions . Anyerror if encountered will be on part of the TDOAanalysis method , it will only be multiplied by ascalar constant while calculating the overall error.

VIII. Conclusion and Current ProgressIn this paper the development of the AUV ZYRA

is presented to perform various shallow water tasks.The idea and logic behind the mechanical design,embedded and power systems design, control algo-rithms and software platforms , image processingtechniques and the geometric mathematics involvedin the localization process of the pinger have beenexplained in detail.Zyra is now in the testing phase.Prior to vehicle assembly, the mechanical system-swere thoroughly leak tested, and the electricalsystems were bench tested.

AcknowledgmentThe authors would like to thank the sponsors,

namely ONGC, Yamaha, Samtec, NIOT, Seabotixand National Instruments for their financial andtechnical support. Further thanks to our facultyadvisors P.B.Sharma and Prof. R.K.Sinha. We aregrateful to Mr. Anil for providing his pool fortesting. Without these individuals support,the devel-opment of the AUV would not have been possible.

References[1] Sinha, R.K. Delhi Technological. University and Ahmed,S. ,

Bahuguna,P. , Kumra,R. , Agarwal,V., Saxena,V. , Mittal,V. ;Gupta,P. , Tutwani,M. , Vehicle for Automation Research andUnderwater Navigation, 3rd ed.

[2] Ming Chen , Qiang Zhan and Sanlong Cai Control SystemDesign of an Autonomous Underwater Vehicle, 3rd ed. IEEEConference on Robotics, Automation and Mechatronics, Dec.2006.

[3] Daniel Moussette , Ashish Palooparambil and Jarred RaymondOptimization and Control Design of an Autonomous UnderwaterVehicle, 3rd ed. WORCESTER POLYTECHNIC INSTITUTEThursday, April 29, 2010.

[4] Ann Marie Polsenberg, MIT , Drew Gashler Developing anAUV Manual Remote Control System, 3rd ed. WORCESTERPOLYTECHNIC INSTITUTE Thursday, April 29, 2010.

[5] Robert Pavish Dynamic control capabilities developments of theBluefin Robotics AUV fleet, 3rd ed. Cambridge 2009

Delhi Technological University Autonomous Underwater Vehicle