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DTU AUV Design and Development of the Autonomous Underwater Vehicle 'ZYRA'

Technical Report

DTU AUV Design and Development of the Autonomous Underwater Vehicle 'ZYRA'

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DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 1 | Page
DTU AUV
Design and Development of the Autonomous Underwater
Vehicle ‘ZYRA’
Prof. P. B. Sharma | Prof. R. K. Sinha
Aayush Jha, Faheem Ahmad, Vivek Mishra, Nikhil Singh,
Vatsal Rustagi , Akshay Jain , Aayush Gupta , Raj Kumar Saini, Prateek Murgai ,
Aditya Rastogi , Akshay Uppal , Prithvijit Chattopadhyay , Pronnoy Goswami
Delhi Technological University, India
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 2 | Page
Delhi Technological University
Design and Development of the Autonomous Underwater
Vehicle ‘ZYRA’
Aayush Jha, Faheem Ahmad , Vivek Mishra, Nikhil Singh,, Vatsal Rustagi , Akshay Jain , Aayush Gupta , Raj Kumar
Saini, Prateek Murgai , Aditya Rastogi , Akshay Uppal , Prithvijit Chattopadhyay ,
Pronnoy Goswami
Abstract
-Autonomous Underwater Vehicle are
powerful and complex systems which are capable
of performing underwater ( shallow and deep sea)
tasks like bathymetry calculation, detection of
faults in oil pipelines, collection of deep sea water
samples, counting of fish and even complex tasks
like collecting data which aids in understanding
global warming.
This paper presents the design and development
of a littoral autonomous underwater vehicle
named ‘ZYRA’ (5
th
generation vehicle) which has
been developed by a team of undergraduates of
Delhi Technological University. The design and
development as well as rationale behind the design
of various systems such as the Control systems,
mechanical design, embedded and power systems,
vision and acoustics location estimation system
which form the integral part of ZYRA has been
discussed in the paper.
I. INTRODUCTION
The aim is to design and develop an Autonomous
Underwater Vehicle which will serve as a
technology demonstrator, having following features:
x Highly compact, multirole, customizable
platform which can be used for various
missions with independence of choosing
payload and thus deciding mission backup
time.
x Target localization using image processing
and passive SONAR.
x Dynamic control by achieving co-ordinates
using PID Control algorithms.
x Pilot Software to control an AUV.
x Underwater Communication / retrieval of
data.
x Implementation of grabber, dropper, torpedo
firing and other underwater actuators.
ZYRA is the product of completely redefined
mechanical assembly to embedded and control
systems. This year it has a new custom made LiPo
battery pack, a novel power distribution, redesigned
actuator control board, battery monitor, leak sensor,
an acoustic signal processing module and improved
software.
II. MECHANICAL
DESIGN
Fig1: Assembly of 5TH Generation model
The mechanical model of ZYRA is designed to be
in hydro dynamically stable equilibrium both
below and above water surface. The mechanical
system consist of main pressure hull, front and
back lids, frame, electronics rack. The vehicle will
feature a smooth contoured cylindrical pressure hull.
The fabrication material chosen for pressure hull
is Virgin Cast Acrylic (Clear) because of its
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 3 | Page
excellent workability, strength, shock resistance
and comparatively low density. Simulation is done
on hull at underwater depth of 10 m and Factor of
safety so found out is more than 7.8.
Fig 2: Solid works simulation of main hull
The body shall be propelled by six thrusters and
has net positive buoyancy. The frame is so
designed to ensure easy mounting of thrusters,
sensors, grabbers etc. The profile of the vehicle
makes it highly manoeuvrable.
Fig 3: Exploded view of assembly
The front lid is made of virgin cast acrylic .Its
transparency ensures clear and unobstructed view
for the forward camera that will be mounted inside
the hull. Back-lid will be made of aluminium. Its
smooth surface will ensure easy and efficient
installation of connectors and aluminium being a
good thermal conductor will also help in heat sink.
III.ELECTRONICS HULL
The main hull of ZYRA is of cylindrical shape.
The shape of the vehicle has been decided after
calculations, keeping various hydrodynamic
parameters in mind to improve the overall
performance of the robot.
IV. METAL FRAME
Fig 4: Solid works model of main frame
Main frame is designed to ensure easy installation
of thrusters, grabber, hydrophone array etc. The
main consideration while designing it was that
unwanted stresses on hull are uniformly and
efficiently transmitted to whole frame and prevent
cracks. Some of its components are made up of
Stainless Steel GR316 and others are of Aluminium.
The reason for this approach is to have a suitable
combination of weight, buoyancy, resistance to
corrosion along with low maintenance, high
strength and ease of fabrication.
V. VEHICLE DYNAMICS
Six strap-on BTD-150 thrusters from Seabotix Inc.
are used to manoeuvre the vehicle. Two thrusters
facilitate motion in the horizontal direction, while
other two facilitate the vertical motion. Two
additional thrusters are being used to control strafe
and rotations about z-axis i.e. yaw. They are chosen
because of their high thrust to weight ratio and
safeguards for power surges and ground shifts.
These thrusters provide a two blade bollard thrust
of 2.9 kgf and require power in the range of 80-
110 watts depending on conditions outside.
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 4 | Page
Fig 5: O-ring mechanism
VI. ELECTRONICS FRAME
FIG 6: Solidworks model of electronics frame
All electronic components are housed inside the
electronics hull. Components are placed in racks
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 addition of new
shelves later as per requirements and least length of
wires. Battery is placed at lower most shelf to
ensure lower centre of gravity and improve
stability and resist roll.
VII. UNDERWATER CONNECTORS
Connectors from Samtec provide effective leak
proof electrical connections from systems outside to
the main circuitry present inside the hull and are
easy to install and dismantle.
!
VIII. GRABBER MECHANISM
Fig 7: Grabbing mechanism
!
Grabber mechanism uses a servo motor with rack
and pinion mechanism which moves the
polycarbonate jaw and holds the object.
IX. HYDRODYNAMICS
ZYRA is a 0.6m long hydro dynamically stable
AUV, designed and built by DTU AUV.
FIG 8: Flow Simulation of Zyra
TABLE I
CHARACTERISTICS OF ZYRA
Dimensions
60cm x 40cm x 51 cm
Diameter of the hull
30cm
Dry weight
32 kg
Type
Volume Flow Rate
Environment
Pressure
Faces
<0>@tube-1
<1>@tube-1
Value
Volume Flow
Rate: 1.7200 m^3/s
Temperature:
293.20 K
Environment
Pressure:
101325.00 Pa
Temperature:
293.20 K
Result
Max Velocity
0.646 m/sec
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 5 | Page
Propulsion
4 horizontal + 2
vertical thrusters
Horizontal velocity
0.5 m/s max.
X. STABILITY:
FIG 9 Centre of buoyancy and Gravity in same vertical line
The AUV is designed in such a way so that the
Centre of Gravity and Centre of buoyancy lies in the
same vertical line.
FDGBGJKFVFHTRHRHB
XI. DEGREES OF FREEDOM:
The six thrusters are installed in such a way that the
vehicle has 6 Degrees of Freedom including
rotational motions like roll, pitch and yaw.
XII. ROBUSTNESS
FIG 10 depicts the fixture and face of pressure application
FIG 11 Von Mises Stress distribution
Based on the specific parameters the lowest factor of
safety (FOS) is found out to be 5.18 , thus ZYRA
can dive safely till 60 ft.
XIII. EMBEDDED AND POWER SYSTEMS
For this year AUV, “ZYRA” the focus of electronics
department is primarily on implementation on
acoustic positioning module and actuator board for
control of 6 SeaBotix thrusters and weapons system.
A new power distribution board is designed for
voltage regulation and to provide power directly to
all the systems through one particular board which
would eliminate the need of further bucking of
voltage and hence concentrates the heat dissipating
unit in one particular section of hull which can be
placed near the metal ends to allow heat exchange
with the water.
XIV. POWER SYSTEMS
The electrical power system is comprised of lithium
polymer (LiPo) battery, encased within the main
electronics hull. The battery is rated at 18000 mAh
at 18.5V. The battery protection board monitors
voltage of each cell and shuts off the power if the
voltage drops to 16V, also the overcharge and
current drawn is monitored by the same. It is
connected to a Power Distribution Board (PDB)
which is responsible for diverting power to various
modules on the bot. The PDB is equippedwith
fuses in case of battery/circuit failure, multiple
capacitors to smooth out ripples, and eliminates
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 6 | Page
high frequency noise. The PDB divides the power
and provides regulated power supply to sensitive
electronics. It provides Zyra with clean, isolated, and
regulated power at 5V, 12V, and 18.5V.
XV. ACTUATOR CONTROL
The actuator board used for ZYRA is custom
designed. It takes in care the electromagnetic
interference from the thrusters which distort the
motor control signal coming from the
microcontroller unit dsPIC30F2010. This
microcontroller is responsible for taking in signals
from the central processing board through
packetized serial interface. The microcontroller is
capable of processing at 30MIPS.
Fig 12 : Schematic of Actuator Board
The motor controllers from Dimension Engineering
are capable of functioning at ultra-high frequency of
32 KHz thus making inaudible for human ears and
eliminate the irritating humming noise. The
controllers can be configured to use either in tank
style differential drive or analog voltage control. The
microcontroller unit receives signals specifying
direction and speed of the Zyra which in turn actuate
the required thrusters. A hermetically sealed switch
is responsible for killing the power to the propulsion
system. Along with hard kill a soft kill is also
incorporated in the code.
Analog voltage is used to control the speed of
thrusters with another signal to specify the direction
of motion. PWM motor controller of dsPIC30F2010
is used to generate variable duty cycle PWM signal
which is filtered and smoothed into an analog signal
(0V-5V) via a high capacitance, RC filter. The added
advantage of mounting syren-10 on the actuator
board is that any particular controller can be
replaced in case it gets damaged and thus prevailing
re-usability of the board.
Fig 13: Actuator Board.
Learning from past experiences, the propulsion
system of ZYRA have been completely redesigned.
The communication of Master Computer and Micro-
controller have been changed from RS-232 protocol
to TTL. Instead of the fact that RS-232 is noise
tolerant, the communication to UART is done using
USB-TTL chips.
Fig 14 : System Architecture of navigation control
XVI.ELECTRONICS RACK
Modularity of the vehicle has been the priority of the
team. Electronics Rack which holds all the
electronics and can be removed from ZYRA without
disturbing the rest of the system has been developed.
Master
Computer
Microcontroller
Unit
dsPIC30f2010
Actuator
Board
Depth
Thrusters 2
Switch
Torpedo Grabber
Sway/Yaw
Thrusters 1
Surge
Thrusters 2
Surge
Thrusters 1
Sway/Yaw
Thrusters 2
Depth
Thrusters 1
Analog
Analog
Analog
Analog
PWM
Analog
Analog
USB-TTL
PACKETISED SERIAL
DATA
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 7 | Page
Fig 15: Electronics Rack of ZYRA
The level of various racks can be varied according to
payload.
TABLE II COTS Used in ZYRA
Device
Model
Kontron Motherboard
986LCD-M/mITX
Battery
Li-Po 18.5V , 18000mAh
Actuator Controller
dsPIC30F2010 (dsPICDEM 2)
Motor Controller
Syren 10
Camera
Microsoft LifeCam-3000
Logitech HD Pro C920
Propulsion
SeaBotic BTD-150
Pressure Sensor
Applied Measurements Pi9933
IMU
XSENS MTi-28A
Data Acquisition
NI PCI-4462
NI USB-6210
Servo
HiTec HS5646WP
Hydrophones
Reson TC403
XVII. SOFTWARE PLATFORM
The use of Labview, developed by National
Instruments, has been decided to develop our
software framework. The software is designed to run
in decentralized multi-threaded agent architecture,
with the threads handling pressure sensor, acoustics,
cameras, control system, IMU, each performing
input and output operations in continuous loops. It’s
GUI based coding, ease of onsite debugging and
quickly changeable mission strategy is the main
reason for it to be preferred over conventional ‘C’
based coding. Even though vision sub modules have
been tested on OpenCV and acoustics modules on
MATLAB, everything has been finally integrated
into Lab View to keep a uniform software platform.
ZYRA has interactive Graphical User Interfaces
(GUIs) developed for acquiring data from all sensors,
adjusting control parameters, implementing
individual codes, as well as for the mission control
using Lab View. This helps quick onsite editing of
codes / mission parameters, payload & target
selection even by a person with very minimal
knowledge of the coding languages. This complies
with the basic idea a highly modular, flexible and
multiple application AUV platform.
.
Fig16 : Operator status screen during AUV mission run .
Fig 17 : Orientation data received from IMU used as an input in
Control system module.
XVIII. MISSION PLAN MODULE
Mission plan module of ZYRA is responsible for
the artificial intelligence of the vehicle. It is at the
highest level in the software hierarchy, coordinating
the global state of the AUV and the state of each
subsystem. It makes calls to sensor modules like
vision, sound etc. determine the position and
orientation of the AUV and to identify targets in the
arena. The mission plan module coordinates the state
of the AUV as it goes through the entire mission
arena. It has a scheduler/ timing module which times
each operation and is capable of making smart
decisions of leaving a task and moving to the next
one based on mission time elapsed and pre-written
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 8 | Page
contingency plans. Once the AUV determines what
type of action is to be performed, it calls the control
module which commands the actuators to function
precisely.
Fig 18 : GUI based PID loop coding in Labview.
Fig 19 : Various software modules working in tandem to achieve
mission control.
XIX. CONTROL MODULE
Control module is called by the mission plan module
as and when required to change the orientation and
position and orientation of the AUV based on the
operation being pe8rformed and input from vision,
acoustic, depth and other sensor modules. Control
module maintains the orientation of AUV using
continuous PID loops running simultaneously. It
relies on the mechanical stabilization for both roll
and pitch movement, and thus, only the yaw, depth
and horizontal movement of AUV is controlled by
this module. The PID control algorithm has been
coded in Lab View. This method has proven to be
more efficient, less processor intensive and easily
implementable. The system attempts to maintain its
state using dynamic feedback from the IMU,
pressure sensor and the acoustic and vision modules.
User interfaces have been specifically developed to
tune and adjust the PID parameters easily.
XX. MISSION CONTROL APPROACH
TOWARDS THE ROBOSUB PROBLEM
STATEMENT
Fig 20 : Arena for ROBOSUB Competition.
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 9 | Page
Fig 21: Flowchart showing ROB OSUB mission controller’s approach on how
to complete the whole track of the ROBOSUB competition’s problem
statement .
XXI. IMAGE PROCESSING
The Computer Vision module was developed using
the “NI Vision” library in NI LabView. The high
Parallelism during execution of programs on multi-
core CPUs in LabView gives the vision module the
required real-time computational power. The module
incorporates concepts involving image processing,
particle analysis, image segmentation, binary
morphology and machine vision. The major change
this year is that the navigation system works on
absolute yaw (angle) control. The previous
generation vehicles had a less accurate navigation
system partly based on heuristics.
XXII. GATE VALIDATION
!The forward facing camera is used in this task. The
image is segmented for the specific color. An edge
filter is then applied to the binary image thus
formed. The Center of Symmetry of vertical edges
thus gives the correct heading (in degrees) to the
vehicle.
Fig 22: Original Image Fig 23: Image after edge detection
Fig 24: Generalized flow chart for target detection using Image
Processing.
XXIII. PATH DETECTION
For path detection our vision algorithm receives
video feed from the downward facing camera and
outputs a heading relative to the AUV. To
accomplish this we employed “colour based
segmentation” along with “blob analysis”.
Fig 25: Original Image Fig 26: Processed Image
XXIV. TRAFFIC LIGHT DETECTION
The next task we have to do is “Traffic light
detection”. Algorithm for traffic light detection is
similar to that of path detection. This algorithm
receives video feed from the forward facing camera.
Similarly to path, the goal of this algorithm is to
output a target pixel, or a target point. Color based
segmentation is used for this purpose. The HSV
colour space is used for all segmentation based
operations. The algorithm segments out the red
colour, giving a binary image. Then the binary
image is passed through filter to remove noise .The
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 10 | Page
particles in the binary image are then analysed and
the centre of the largest connected particle gives the
current heading. The vehicle tries to approach the
flare keeping the centre at 0 degrees.
Fig 27 .Original Image Fig 28. Processed Image
XXV. BIN DETECTION
The downward facing camera is used for bin
identification. The algorithm implements concepts of
pattern recognition and is able to differentiate
between the given geometrical patterns. The centre
of the bins is found by calculating the centre of
symmetry of the edges found in the captured image
of the bin. (The white-black edges are only
considered). The marker is then dropped after the
vehicle aligns according to the centre given by the
algorithm.
Fig 29: Original Image Fig 30: Processed Image
XXVI. UNDERWATER SOUND SOURCE
LOCALISATION
ZYRA has an acoustic navigation system which
employs a Square/Tetrahedron array of
hydrophones for data acquisition to calculate
azimuthal & compass bearing and estimate
hyperbolic location of the sound source in far-
field approximation using Time difference of
arrivals (TDOAs).
Fig 31 Tetrahedral array of hydrophone rendered in Solid Works
Above figure shows a tetrahedral array of
hydrophones which have been used for the
localization of the sound source. A symmetric array
such as the above helps in eases calculation while
making the time difference of arrival measurements.
Hydrophones are basically piezoelectric material
which responds to any physical disturbances in water.
The acquisition of signals is done through NI Data
Acquisition Card which pre-amplifies the received
weak and feeble signals and then Analog to Digital
conversion.
The Generalized cross-correlation technique using
phase transform (GCC-PHAT) is employed to
calculate the time difference of arrival corresponding
to the correlation peak. Estimated TDOA’s give
bearing estimation for far field approximation. In
environments of high levels of reverberation GCC-
PHAT helps to improve robustness and accuracy in
calculating the time difference of arrival [3], [4]. We
can see from the above MATLAB simulation that
GCC-PHAT enhances the peak and whitens the
region around it, whereas in the Cross Correlation
simulation the region around the peak displays some
ripples, also the peak is not that well defined as it is
in the GCC-PHAT. Both the methods show similar
performance under noise, GCC-PHAT is better in
case of added reverberations.
Estimation of range is done by solving the
hyperbolic TDOA equations using ‘Chan and Ho’
method which is non-iterative and gives an explicit
solution. It is an approximate realization of
maximum likelihood estimator and is shown to attain
Fig 32 Acoustic System Flowchart
the ‘cramer-rao’ lower bound near small error region
DTU AUV ‘ZYRA’ – AUVSI and ONR’s 16th Robosub Competition Journal Paper 11 | Page
It is clear that a pair of hydrophones yields one
equation so to localize a source in 3-D we need three
equations i.e. a minimum of 4 hydrophones are
required to successfully locate the source in far field
Fig 33. Typical Cross Correlation function simulated in MATLAB
Fig 34. Typical GCC-PHAT simulation in MATLAB
XXVII. ACKNOWLEDGMENT
Team DTU AUV wishes to extend its thanks to past
and present sponsors, namely ONGC, Yamaha,
Samtec, NIOT, Seabotix and National Instruments.
Further thanks to our faculty advisors P.B.Sharma
and Prof. R.K.Sinha. We are grateful to Mr. Anil for
providing his pool for testing. Without these
individuals’ support, DTU AUV’s work would not
have been possible.
XXVIII. VEHICLE STATUS
ZYRA is right now in testing phase. Testing phase
was started late in June 2013 only after the novel
actuator board started working error free and
mechanical systems were totally leak proof . Team is
spending at least 3 hours in pool daily and is
working on schedule to participate in Robosub 2013.
Fig 35. ZYRA inside pool for testing.
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