Page 1 of 33 UNIVERSITY OF MAINE MECHANICAL ENGINEERING ROTurtle Biomimetic Remotely Operated Turtle Christopher Freeman, Zachary Gregory, Erik Hill, Brian Nielsen, Terrence O’Brien, Perry Powell Submitted To: Murray Callaway 5/4/2012 ABSTRACT This technical paper describes the process of designing and fabricating an underwater remotely operated vehicle that maintains the characteristics of a turtle, including the usage of a two-degree of freedom flapping foil propulsion system, for the purpose of non-intrusive aquatic exploration. The ROV was intended to prove the effectiveness of the flapping foil propulsion system in an underwater application. Key components to the ROV consists of the propulsion system that utilizes two servos to create a roll and pitch motion to produce thrust, wireless connectivity, electronic waterproofing, and a handcrafted fiberglass shell resembling that of a turtle. The ROV culminated to a final testing period where the ROV swam through an Olympic sized swimming pool successfully.
Transcript
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UNIVERSITY OF MAINE MECHANICAL ENGINEERING
ROTurtle
Biomimetic Remotely Operated Turtle
Christopher Freeman, Zachary Gregory, Erik Hill, Brian Nielsen, Terrence O’Brien, Perry Powell
Submitted To: Murray Callaway
5/4/2012
ABSTRACT
This technical paper describes the process of designing and fabricating an underwater remotely operated vehicle that
maintains the characteristics of a turtle, including the usage of a two-degree of freedom flapping foil propulsion system,
for the purpose of non-intrusive aquatic exploration. The ROV was intended to prove the effectiveness of the flapping foil
propulsion system in an underwater application. Key components to the ROV consists of the propulsion system that
utilizes two servos to create a roll and pitch motion to produce thrust, wireless connectivity, electronic waterproofing, and
a handcrafted fiberglass shell resembling that of a turtle. The ROV culminated to a final testing period where the ROV
swam through an Olympic sized swimming pool successfully.
Table of Contents .................................................................................................................................................................... 2
Table of Figures ...................................................................................................................................................................... 3
Concept Design Process and Fabrication ................................................................................................................................ 6
Structures and Materials ...................................................................................................................................................... 7
Control Systems ................................................................................................................................................................ 11
Propulsion Control Scheme ........................................................................................................................................... 15
Wireless Communication ............................................................................................................................................... 16
How Flapping Foils Work ............................................................................................................................................. 19
Rear Fins ........................................................................................................................................................................ 22
Conclusions and Recommendations ..................................................................................................................................... 23
Strengths and Weaknesses ................................................................................................................................................. 23
Works Cited .......................................................................................................................................................................... 24
Appendix D: COntrol Scheme Virtual interfaces .............................................................................................................. 30
TABLE OF FIGURES
Figure 1: SolidWorks Model of Propulsion System ............................................................................................................... 6 Figure 2: SolidWorks Model of Top and Bottom Shells and Top Shell after Fabrication...................................................... 7 Figure 3: Rotation Point of Frame .......................................................................................................................................... 7 Figure 4: Roll Case Waterproofing Locations ........................................................................................................................ 8 Figure 6: Cover for Torxis Servo Case (left) and Torxis Servo Case (right) .......................................................................... 9 Figure 5: Torxis Servo after Internal Waterproofing .............................................................................................................. 9 Figure 7: Main Electronics Case and Cover ......................................................................................................................... 10 Figure 4: 1 24V 10AH V2.5 LiFePO4 Battery Pack ........................................................................................................... 11 Figure 5: Power Indicator (Left), Main Power Switch (Right), Fused Power Distribution (Top) ........................................ 12 Figure 6: 12VDC/5VDC Converter (left), Power Indicators (Right).................................................................................... 12 Figure 7: Netgear Wireless Router ........................................................................................................................................ 13 Figure 8: NI cRIO-9022, Compact Real Time Controller .................................................................................................... 13 Figure 9: 9401 Breakout Board ............................................................................................................................................. 14 Figure 10: Video Display Server .......................................................................................................................................... 14 Figure 11: Cameras ............................................................................................................................................................... 14 Figure 12: Invenscience i00600 torxis servo ........................................................................................................................ 15 Figure 13: 4 HS-5645MG digital high torque servo motor ................................................................................................... 15 Figure 18: Unified Power and Circuit Diagram .................................................................................................................... 16 Figure 19: Wireless Router Underwater Testing .................................................................................................................. 18 Figure 20: The Kármán Vortex Street Behind a Circular Cylinder [1] ................................................................................. 19 Figure 21: Von Kármán Vortex Street and Reverse Von Kármán Street Behind a Flapping Foil [2] .................................. 19 Figure 22: Gearing and Servo Out of PVC Roll Case .......................................................................................................... 20 Figure 23: NACA 0012 Foil Profile and Dimensions ........................................................................................................... 20 Figure 24: Fabricated Hydrofoil ........................................................................................................................................... 21 Figure 26: NACA 0012 Data Compared to Turtle Fin Data ................................................................................................. 21 Figure 25: Umaine Tow Tank ............................................................................................................................................... 21 Figure 27: Rear Steering Fins ............................................................................................................................................... 22 Figure 28: ROV During Operational Observation in Pool .................................................................................................... 22 Figure 29: Image Taken from ROV camera ......................................................................................................................... 23 Figure 30: Chart of Finance Distribution .............................................................................................................................. 26 Figure 31: Chart of Team Expenses ...................................................................................................................................... 26 Figure 32: Controller Labview Console ................................................................................................................................ 30 Figure 33: Controller VI ....................................................................................................................................................... 31 Figure 34: Motor Control and Sensor Package Labview Console ........................................................................................ 32 Figure 35: Labview Virtual Interface of Motor Control and Sensor Package ...................................................................... 33
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CONTRIBUTIONS
Christopher Freeman
Project manager, responsible for organizing tasks and meetings, keeping track of group deadlines and work
towards completing objectives and building group communication
Organize paper, writing sections includes waterproofing, frame, shell, design evaluation and conclusion
Zachary Gregory
Control systems team member, responsible for electronics, waterproofing and wireless connectivity.
Writing sections include electronic equipment and wireless connectivity
Erik Hill
Structures team member, responsible for 3D computer modeling and aesthetics
Writing sections include shell
Brian Nielsen
Propulsion team member, responsible for hydrofoil construction and testing, propulsion analysis, and
waterproofing
Writing sections include hydrofoil, and flapping foil sections
Terrence O’Brien
Control systems team member, responsible for control scheme, electronic organization, and buoyancy control
Writing sections include propulsion control scheme, circuit diagrams, VI images
Perry Powell
Propulsion team member, responsible for propulsion system design and construction
Writing sections include flapping foil and rear fin sections section
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GLOSSARY
ROV Underwater Remotely Operated
Vehicle
Heave Motion rotating about the y-axis
(nautical)
Roll Motion rotating about the x-axis
Hydrofoil An underwater operation foil
NACA National Advisory Committee for
Aeronautics
Fiberglass Fiber reinforced polymer
Aerodynamic
Center
Point on a foil where the pitching
moment does not vary with the life
coefficient
Neutrally
Buoyant
Physical body’s mass equals mass it
displaces in water or other
surrounding medium
INTRODUCTION
Remotely Operated Vehicles (ROVs) can be large and invasive to the surrounding environment, limiting their ability to
observe wildlife in its natural state. The desire is to make a ROV that can enter marine habitats without disturbing the wild
life. With this, we may be able to obtain more accurate data of marine animal behaviors. The best way to accomplish this
goal may be to simulate an already existing creature that lives amongst them. It was decided that the final design should
be modeled around a sea turtle because of its acceptance among other marine creatures and because while robotic fish
have become fairly common place, no attempts have yet been made to mimic the motions and behaviors of a sea turtle.
The objectives of the project were defined by the group to make the ROV as turtle-like as possible. More specifically the
project objectives are:
• Design a flapping hydrofoil propulsion system (fins)
• Fabricate a life-like outer shell
• Build a wireless control system
• Test and install a propulsion system
• Have the entire ROV operational underwater
The finished product should be able to move from one side of an Olympic sized (50 meters) swimming pool, turn around
and move back.
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DESIGN DESCRIPTION
The overall design of the ROTurtle underwater ROV consists of a dual two degree of freedom flapping foil propulsion
system (shown in Figure 1) designed to propel the ROV forward. The propulsions system is operated by four servos, two
for each foil. The propulsion system consists of a hydrofoil with an aluminum rod through it geared to a HiTec 4645MG
servo for motion about the x-axis (known as roll motion) inside a PVC case that rotates on small aluminum rods at both
ends. The PVC case is geared at one end to an Invenscience i00600 torxis servo that produces motion about the y-axis
(pitch motion). The ROV propulsion is designed to reach minimal speeds, but create small amounts of disturbance while
underwater.
The frame is constructed primarily almost exclusively out of 6105-T5 T-slotted aluminum extrusions. The total length of
the frame is approximately 48 inches long and 18 inches wide. The back of the frame holds the electronics case which
consists of a 12 volt 9.9 AH battery for power, a National instruments data acquisition computer (DAQ) for distributing
power and separating signals, and a router for wireless connectivity. The case is equipped with sensors monitoring the
pressure, temperature and humidity level.
The ROV frame is covered by a fiberglass shell shaped to the appearance of a sea turtle. The shell is approximately 52
inches long. The bottom shell is covered in another fiberglass shell about 30 inches in length.
The ROV is designed to explore underwater and observe its environment through an underwater video camera. The ROV
moves at approximately 0.5 ft/s without the shells attached and produces small amounts of water disturbance during
operation.
Figure 1: SolidWorks Model of Propulsion System
CONCEPT DESIGN PROCESS AND FABRICATION
KEY GOALS
The target of the ROV was to obtain movement underwater via wireless control from one end of a 50 meter pool to
another and turn 180° and return. We will also have live video streaming from the front of the ROV as it moves through
the water. The project is split into three groups with different design targets: structures and materials, control system, and
propulsion. Each group’s responsibilities and process are described in detail below.
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STRUCTURES AND MATERIALS
Objectives
The responsibilities of the structures and materials group included the design of the exterior of the ROV, the propulsion
frame, and the waterproofing of the electronics. The objective was to make the ROV look as much like a turtle as possible
while also keeping its functionality.
Shell
The most identifiable part of a turtle is its shell. The carapace of a turtle consists of layers of bone plates below a layer of
keratin. They are naturally extremely hard. The focus of our shell was to model the appearance of the turtle’s shape and
pattern while keeping a strong casing.
First we had to decide how we wanted to make the shell. After making a 3-D model in SolidWorks (Figure 2) we thought
about using a CNC machine or injection molding to make it. However, these two options were too costly due to the large
size of the shell. The final material used was fiberglass due to the low weight (80-110 lbs./ft.3), low cost, and ease in
molding.
The plug was shaped from four layers of polystyrene foam using various saws to get a rough idea, and then a surform tool
and sand paper were used to obtain the final shape. It took about 3 weeks to carve the desired shape. The plug was
covered with two part epoxy resin to create a smooth surface before covering plug with paraffin wax for releasing. Five
layers of fiberglass and epoxy were applied to each shell for structural rigidity. The epoxy is an exothermic reaction,
which melts the wax and creates a space between the shell and plug.
The two shells were attached using Velcro between the shell and frame. The shell was based on pictures of green sea
turtles which inhabit the most oceans. The coloring schemes and patterns were hand painted onto the shell.
Figure 2: SolidWorks Model of Top and Bottom Shells and Top Shell after Fabrication
Propulsion Frame
The frame gives a solid structure in the ROV for mounting the shell and motors. The original design followed the contours
of the shell, allowing for easy connection between the two. The design became an issue when trying to place the
propulsion system, however, and the frame had to be redesigned.
The new design focused on giving the necessary space between the servo
operating the roll case and the roll case itself as well as allowing free range
of motion. The roll case needed to rotate about 60 degrees and have a range
Figure 3: Rotation Point of Frame
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of motion, which in this case would be the distance between the lowest points of motion to the highest of about six inches.
The sacrifice of focusing the frame on setting the propulsion system is the ability to fit the shell comfortably.
The material for the frame is 6105-T5 t-slotted aluminum alloy with a 1inch by 1inch profile. T-slotted aluminum was
selected because it is designed for easy mounting and reconfiguring if necessary. The locations for the propulsion “roll
cases” (referred to in propulsion section) are fitted with a quick release flange for easy removal (Figure 3), and allow for
rotation while keeping it rigid.
Waterproofing
An ROV by definition is an underwater vehicle. Water that has not been purified contains positively and negatively
charged ions making it conductive. With this constraint, it is necessary to keep all electronics for ROV operation free of
conductive liquids. The areas of concern are the heave operation servos, roll operation servos, and the key power and
communication components. The roll servos are located in PVC pipes on the right and left side of the ROV. It was
determined in order to successfully waterproof the case, both of the ends would have to be covered with two part GC
Potting Epoxy which hardens to create a waterproof seal.
At the same time, the location where the shaft exits the case also needs to be sealed while allowing it to rotate 180°. Originally a plastic with significant stretch capabilities was obtained to serve that purpose as a shaft seal. It was later
determined that the seal surrounding the shaft would be too weak with the adhesives we were using and would wear down
quickly. A simpler solution that arose was to internally waterproof the individual components in the servos.
Figure 4: Roll Case Waterproofing Locations
The roll servos (HI-TEC 4565MG) are split into two sections, the gears and electronics. The electronic circuit board is
covered in 100% architecture silicone, which is non-conductive, protecting the servo from short-circuiting.
The next step is to use mineral oil to flood the gear housing. Mineral oil serves multiple purposes at this juncture:
Prevent the gears from inevitably rusting
Fill any cracks in the seal
Apply counter pressure for added water pressure at deeper surfaces
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The servo exit shaft is equipped with a small plastic o-ring to prevent any fluid from escaping or entering
The next area of concern was to waterproof the heave servos. The
heave servos (Invenscience i00600 Torxis Servo) are larger and
have more areas to allow for water entry. Cases were made out of
two layers of fiberglass and slow hardening epoxy and were
bolted to the frame. The cases were then covered in GC potting
epoxy and cured in an oven at 200°C to quicken the hardening
process. The case was covered with a plexiglass sheet with a
milled track for a custom quarter inch diameter rubber o-ring. To
improve the seal, the o-rings and surrounding areas are covered
with Calcium-Lithium marine grease. The marine grease is does
not dissolve in water making it an effective sealant. While the
case proved to be waterproof, the shaft area was again going to
prove to be an issue as seen with the roll cases. To fix this issue,
the servo received an experimental waterproofing technique. The
seal near the top and bottom of the servo (the area where the red
tin meets the plastic) is sealed with the potting epoxy mentioned earlier. Inside, the servo is flooded with mineral oil and
the circuits are immersed in silicon. This technique proved to have issues of its own when the servo’s amp draw started
increasing dramatically with a decrease in power. This is possibly due to potting epoxy drying in the shaft region, adding
friction. Figure 4 displays the servo after the attempted waterproofing.
We chose to go back to the first method of waterproofing the cases and closing the shaft seal with the stretching plastic.
The plastic is attached to the cover using potting epoxy and glued to the shaft with Loctite 404 industrial adhesive.
Figure 6: Cover for Torxis Servo Case (left) and Torxis Servo Case (right)
The last area of concern is the main electronics waterproofing. Through measurements of the necessary components
(mentioned later on in the paper) we discovered that the necessary space would be approximately 7 in long by 6 in wide
by 9 in tall. Originally, we planned on building a case out of PVC pipe, but due to its circular cross section it would end
up being too inefficient use of space. Instead a case was made out of fiberglass like the one for the heave servos at the
correct size needed.
Figure 5: Torxis Servo after Internal Waterproofing
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Figure 7: Main Electronics Case and Cover
The cover is made of plexiglass the same as the heave servo cases with a slot for a custom fit o-ring.
Buoyancy
In order for the ROV to be able to control its depth, the buoyancy had to be taken into account. With the propulsion
system described later in the report the ROV can travel vertically in the water on its own, so the ROV just needed to be
neutrally buoyant. Appendix C is a spreadsheet that shows the calculations for overall buoyancy of the ROV.
The center of mass was found using the equations:
𝑥 𝑚 = 𝑚𝑖 ∙ 𝑥𝑖𝑖
𝑚𝑖𝑖 𝑦 𝑚 =
𝑚𝑖 ∙ 𝑦𝑖𝑖
𝑚𝑖𝑖 𝑧 𝑚 =
𝑚𝑖 ∙ 𝑧𝑖𝑖
𝑚𝑖𝑖 Eqns. B1, B2, B3
The over-bar values are the center of mass coordinates. The Numerators are the sum of the mass of each item times the
location on the respective axis, divided by the total mass.
The center of buoyancy was using these equations:
𝑥 𝑏 = 𝑉𝑖 ∙ 𝜌ℎ2𝑜 ∙ 𝑥𝑖𝑖
𝑉𝑖 ∙ 𝜌ℎ2𝑜𝑖 𝑦 𝑏 =
𝑉𝑖 ∙ 𝜌ℎ2𝑜 ∙ 𝑦𝑖𝑖
𝑉𝑖 ∙ 𝜌ℎ2𝑜𝑖 𝑧 𝑏 =
𝑉𝑖 ∙ 𝜌ℎ2𝑜 ∙ 𝑧𝑖𝑖
𝑉𝑖 ∙ 𝜌ℎ2𝑜𝑖 Eqns. B4, B5, B6
Here the over-bar values are the coordinates of the center of buoyancy. The Numerators are the sum of the buoyancy
(volume time the density of water) of each item times the location on the respective axis, divided by the total buoyancy.
The overall values can be found using these same equations on the cumulative values found for each of the different
groups of items in the turtle.
Based on the analysis, the ROV had approximately 3 lb of positive buoyant force. In order to counteract the force a steel
keel is needed to increase the weight.
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CONTROL SYSTEMS
Objectives
The objectives of the Control Systems team were to create a LabVIEW system through which a pilot could take direct
remote control of the Turtle ROV. It would consist of several algorithms to simplify the control of the flopping foil
system, as well as algorithms which allow the pilot control of the secondary control surfaces. The Control system team
was also in charge of implementing several sensor systems onboard the ROV. These sensors allow the pilot to monitor the
conditions inside the ROV’s onboard electronics case as well as visualize the seascape before the turtle. More generally
the Control Systems team was responsible for all the wiring used in the turtle.
Electronic Equipment
Power Supply and Distribution: Everything onboard the ROTurtle is being powered either from 12 volts or 5 volts. The direct power supply is a 12V
10AH V2.5 LiFePO4 Battery Pack. For the electronics that needed a 5-volt supply (Servos, Video Server, and analog
sensors), two OKR-T/10-W12-C DC/DC converters were used. All of the sensors and the video server run off of one of
the converters and the four, 5-volt servos run off of the other converter. Both of these DC to DC converters have a 10 amp
max.
Figure 8: 1 24V 10AH V2.5 LiFePO4 Battery Pack
After the Power Comes in through the main battery, it can be switched on using the main power switch, seen in Figure 5
below. This will turn on the main power indication. After this, the power is separated and fused appropriately into seven
different sections. The sections include NI cRIO, Servo Side A, Servo Side B, 5-volt Distribution, Wireless & Video
Server, Cameras, and last Analog inputs. Each one of these sections has its own indicator light as seen in Figure 6.
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Figure 9: Power Indicator (Left), Main Power Switch (Right), Fused Power Distribution (Top)
Figure 10: 12VDC/5VDC Converter (left), Power Indicators (Right)
Wireless Connectivity: Instead of having the ROT tethered to a surface watercraft, like most ROVs, the decision was made to go wireless. There
are several issues with underwater wireless connectivity that normally prevent underwater wireless from being attempted,
which will be discussed later. So the ROV turtle can achieve a realistic appearance and still be able to function
completely, a wireless router was added onboard so that the only “tethered“ part of the turtle would be an antenna going to
the surface. The NetGear router was used because of its 12-volt power input, which prevented us from needing to do any
conversions. (See Figure 7).
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Figure 11: Netgear Wireless Router
Inputs and Programming Platform: The electronics for the turtle consist of whatever we could salvage from the prior year’s Remotely Operated Vehicle
(ROV) project and new electronics that needed to be purchased. The main electronics that were used were a DAQ cRIO-
9022, made by National Instruments. This was configured with a 4 cartridge chassis. Three NI 9401 PWM and one NI
9205 boards were used for all of the sensory input that was involved. The three NI 9401 allowed for PWM outputs and
inputs. Our turtle uses this as a technique to send commands to the servos powering both the front propulsion and the rear
propulsion systems. The NI 9205 is used for all the analog sensory input that is used on the turtle, including temperature,
humidity and pressure sensors. The DAQ cRIO-9022 was able to be used wirelessly by connecting it to the router seen
above. The power usage of the DAQ cRIO-9022 was 12 VDC (See Figure 8).
Figure 12: NI cRIO-9022, Compact Real Time Controller
All of the digital inputs and outputs are separated on a break out board, which provides extra protection by fusing each of
the individual inputs and outputs. One of these breakout boards is seen in figure below.
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Figure 13: 9401 Breakout Board
One of the inputs that needed a bit more work than the others is the underwater camera system. In order to enable remote
operation of the turtle, the camera system is vital and interference and delay was not an option. A video server seen in
figure 8 was used to interpret the video signal in parallel with the wireless router to send all of the pictures to a port that
we could access from a browser connected to the turtle control router.
Figure 14: Video Display Server
The Cameras that were used were black and white underwater cameras that were salvaged from the previous year’s ROV.
These cameras have low power consumption and run off of the 12 volt source preventing any conversion complications.
The figure 6 below shows the cameras.
Figure 15: Cameras
Propulsion Systems The propulsion system electronics are divided into two sections, front and rear propulsion. The front is mainly used for
thrusting and generating forward power, with the ability to make large turning corrections. The rear propulsion is mainly
used for ruddering, but has the ability to help with forward and turning propulsion.
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The forward propulsion system consists of four servos. Two larger servos are capable of outputting several pounds of
force used for the vertical flapping motion. These servos are seen in Figure 12 below. These servos are each paired with
two HiTec 5645 servos, which have a much smaller force output of a max 6.8 ounce per inch. Figure 13 below shows
these HiTec Servos.
Figure 16: Invenscience i00600 torxis servo
The rear propulsion system is comprised of two of these HiTec servos. Larger servos were discussed but not used, due to
the smaller corrective movements of their servos. Quick Movements will not need to be made using the rear propulsion. It
just needs to be able to adjust slowly and hold a position.
Figure 17: 4 HS-5645MG digital high torque servo motor
Propulsion Control Scheme
The Propulsion Control System (PCS) consists of the NI cRIO-9022 connected wirelessly to the ROV team laptop
running a suite of LabVIEW Virtual Interface (VI). The suite consists of a primary VI which interacts directly with the
cRIO and a secondary VI which takes in input from the human interface, an Xbox 360 controller, and translates it into
global variables for use in the primary VI. Simultaneously, an Aviosys IP9000 video server wirelessly connected to the
team laptop allows the human pilot to observe the progress of the turtle using the server’s onboard website. Figure 34 in
appendix D shows the LabVIEW VI used for the Main propulsion system. Figure 32 shows the LabView VI which allows
the input of control data from the pilot through the controller.
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The primary VI consists of four sine waves. The heave sine waves are 180 degrees out of phase, so that the two rise and
fall in unison. The roll sine waves operate 90 degrees out of phase with their respective heave wave to produce the
optimal heave-roll dynamic.
Control inputs to the VI allow the direct manipulation of the flapping frequency, position of the midpoint of the heave
motion, and position of the rudders. Control of the frequency translates into a speed control while the turtle is in water.
The heave midpoint control translates into a change in lift force produced by the fins allowing vertical control of the
turtle. The rudders allow a dynamic stabilization of the turtle in water, permitting minor adjustments to heading; the
rudders also act as a direct yaw control. The controls also allow the manipulation of the heave amplitudes to produce a
turning action via the propulsion fins.
The Sensor Package is visualized in the same VI as the main propulsion system
The unified power and control circuit diagram can be seen below in Figure 18. Most of the individual electronics
components require the 12V which the battery provides. However the small servos, the camera server, and most of the
sensors require a 5V input. This is achieved using the DC-DC converter.
Figure 18: Unified Power and Circuit Diagram
Wireless Communication
Overview Control of a biomimetic organism wirelessly is always desirable. Large power, pneumatic, and control signal wires usual
takes away from a lifelike organism. Having onboard power takes care of the large power cables, an intricate control
system and a well balanced ROV takes care of the pneumatic cable lines. The last issue to overcome is the control signal
wires. Several Techniques were initially discussed and will be overviewed in this section. The decision was made to use a
wireless router with Wi-Fi and have a small line tethering its antenna to the surface.
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Possible Solutions: VLF Transmitters
VLF transmitters, also known as Very Low Frequency transmitters, refer to radio frequencies are in the 3 to 30 kHz range
and 10 to 100 kilometers in wavelengths. The main benefit of using this is that the signal can penetrate up to 40 meters
into saltwater. It is often used to transmit radio navigation controls and is sometimes used for secure military
communications. They have very low bandwidth and would make live video streaming impossible. Also, VLFs are
reserved for military applications.
Underwater Acoustics
Underwater acoustics are a proven method for data transmission underwater. It uses the propagation of sound in water and
the interaction of mechanical transducers to receive or send these waves. The technique is commonly used in oceans,
lakes and tanks. Several frequencies are commonly used starting at 10Hz and going up to 1Mhz. 10Hz has the potential to
travel very far and can even penetrate deeply into the seabed, while on the order of 1MHz the frequency is generally
absorbed by water quite quickly. There are several reasons acoustics would not work for our purposes:
- Potentially disruptive to other organisms in the proximity of the ROV
- Data rates are not good enough, maxes out in the 100kbs range even on experimental models
- Would require a large amount of work to send and receive signals
Underwater IR Communication System
Underwater communications system comprising first and second communications modules which transmit and receive
data utilizing infrared radiation were also looked at. Each module has a transmitter/receiver which converts each received
data byte to RS-232 formatted data for transmission to a computer. Each communication module also includes a timer for
providing a 40 kHz pulsed signal. The issues with IR are similar to acoustics. It is potentially dangerous to aquatic life in
the proximity to the ROV, it has a higher data transfer rate (maxing at around 350 kbs), and requires a large amount of
work to send and receive signals. Underwater IR is still experimental and has a lot of future potential for wireless
underwater robotics.
Wireless Fidelity (Wi-Fi)
Wi-Fi has some very desirable properties including:
- Fast data rates, not limited to 54mbs.
- Commonly used, making it easy to modify something that is already in existence. So whatever hardware we need can be
easily obtained.
- Non disruptive to other animals in the surroundings
Wi-Fi is not without some problems. Outside the visible spectrum, electromagnetic waves travel slower in water, but that
doesn't affect the data rate. It will just add maybe a microsecond to packet response times. The real problem is absorption.
Water is a strong absorber of electromagnetic waves outside a narrow band in the visible spectrum. For frequencies in the
1-10 GHz range, where most fast Wi-Fi resides, the absorption constant is greater than 10 cm^-1, which means that a
signal loses half its strength in (1 / (10 cm^-1)) = 0.1 cm or less. The absorption gets smaller at lower frequencies, but
doesn't do so quickly. We tested signal strength of a Netgear wireless N router at the bottom of a 15 foot diameter circular
tank (Figure 13) with mostly positive results. With the antenna above the surface signal strength remained the same as if
the router was not underwater, confirming that Wi-Fi was a viable option.
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Final decision The final decision was made to have a wireless receiver onboard the turtle. We would also need to have a tethered antenna
which would allow us to bypass water absorption issues.
Figure 19: Wireless Router Underwater Testing
PROPULSION
Objectives
The Propulsion Team was tasked to create a propulsion system that imitates that of a Sea Turtle. This was chosen over
other methods of movement (propellers, pump jets, etc.) because of its superior energy efficiency (amount of movement
vs. energy required) and its superior maneuverability. Sea Turtles swim underwater similarly to how birds fly and the
physics behind both is also similar (summarized below). This type of motion is referred to as a Flapping Foil. Flapping,
like how a bird flies, and Foil refers to the shape of the cross-section of the wing or fin. The foil cross-section is the same
as the cross section of a normal airplane wing (seen in Hydrofoil section below). If we can begin to observe and
understand this type of motion we are one step closer to making vehicles that can actually move like birds, fish and turtles
granting us their efficiency and superior maneuverability.
Flapping Foil Design
To begin the flapping foil design, a few hours were spent watching turtles swimming, more specifically paying attention
to how they moved their fins when they swam. After determining how the fins moved, the propulsion team met to
determine how many degrees of freedom were necessary to recreate the same motion. After a few weeks of going back
and forth with designs Mick suggested we speak to Dr. Douglas Read, Professor of Naval Architecture over at Maine
Maritime Academy. He gave us specific direction in all of our flapping foil trouble areas.
Using the knowledge gained from this meeting we figured out the fin dimensions (span and chord length) and a basic
usable control scheme (maximum angle of attack (AoA), heave amplitude and phase angle between heave amplitude and
AoA). The calculations are shown in appendix B.
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Figure 20: The Kármán Vortex Street Behind a Circular Cylinder [1]
How Flapping Foils Work
The basic mechanism behind flapping foil propulsion is a fluid mechanics phenomenon called vortex shedding. Normally
when there is fluid flow around an object, say a cylinder or hydrofoil, the flow “separates” from the object and vortices
are shed. Normally the vortices spin “inward” causing a net velocity deficit, or drag, but under the proper conditions a
flapping foil will cause the shed vortices to spin in the opposite direction, pushing the fluid faster and causing a net
velocity excess producing thrust (See Figure 21 below). The rows of vortices trailing behind the objects are called Von
Karman Vortex Streets.
Figure 21: Von Kármán Vortex Street and Reverse Von Kármán Street Behind a Flapping Foil [2]
Fabrication of the flapping foil mechanism started off with a 3 inch PVC pipe. PVC was chosen because it’s easy to work
with, lightweight, and inexpensive. Cut into 8 inch pieces, these house the two smaller servos that rotate the flippers. The
small servos are mounted inside the pipe by use of a plexiglass shelf. Also on this shelf is a hole for the flipper rod to pass
Page 20 of 33
through. Chained gears connect the servo to the rod (Figure 22) which allows the flipper to roll in either direction, while a
mounted bearing allows for smooth rotation. This small casing will be rotating with the flipper, and thusly is mounted on
two bearings. The large servos rotate the cases by means of another set of chained gears, creating the flapping motion for
the fins.
Figure 22: Gearing and Servo Out of PVC Roll Case
Hydrofoil
The chosen hydrofoil shape for the flapping fins is the NACA 0012 (see Figure 23). This particular profile was chosen for
its simplicity and level of existing research, which includes an extensive flapping foil analysis [3]. The amount of existing
research allows us to compare our design to the results other researchers have obtained.
Figure 23: NACA 0012 Foil Profile and Dimensions
Because our “wing” shapes attempts to follow the shape of a turtle’s fin instead of a normal wing shape, we were forced
to use experimentation to find the aerodynamic center (AC) in order to obtain usable drag and thrust data. Once the AC
was found our drag and thrust data could be compared to existing NACA 0012 experimental data. This will allowed us to
validate our data and find the magnitudes of the motion parameters that allow for greatest efficiency in movement.
Page 21 of 33
The fabrication of the fins and foils started with an aluminum rod bolted to a rectangular plate. The plate was covered
with a hand mouldable thermoplastic that was shaped similarly to a turtle’s fin. The plastic was covered with insulating
foam and then was shaped to fit the NACA 0012 profile for the fin’s cross-section.
Figure 24: Fabricated Hydrofoil
Drag Test Unfortunately because of some equipment failure it appears that all data
collected during the drag test is completely inaccurate and unrealistic.
The 500lb load cell we used did not have the proper resolution for use in this
experiment and all of the output numbers were identical.
The strain gage data looked more promising at first but in the end the data
reduction showed drag coefficients way too low compared to the NACA 0012
data. The graph comparison of the two is shown in Figure 26.
Figure 26: NACA 0012 Data Compared to Turtle Fin Data
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
-20 -10 0 10 20
Comparison Of Drag Coefficients
Turtle Fin Drag Coefficient
NACA Series Drag Coefficient
Figure 25: Umaine Tow Tank
Page 22 of 33
Rear Fins
The rear fins, which are to be used as a rudder-like system, were originally
designed with two degrees of movement, heave and roll. After re-
evaluating the purpose of these fins, it was determined that the same
output could be achieved with only one degree of movement, if the fins
were angled 90 degrees perpendicular to each other. The rear fins did not
need to be in the shape of a hydrofoil, they just need to be able to deflect
water in any of 4 directions, and a V-shaped rudder system allows that.
The fabrication of the rear fins began with the sizing of the fins. From
there rods were cut for the fins to be rotated. Differing from the front
flippers, the rear fins are directly connected to the servos that will be
rotating them, and mounted bearings on the other end. The framing to
support the rear fins consists of 8020 Aluminum, arranged in a W-shape
for minimum height, and connected at the center point of the assembly to
the rest of the frame. This is because the majority of the rear frame is
angled at 45 degrees from the rest of the frame.
DESIGN EVALUATION
As stated at the beginning of the document, the objects of the ROV are to operate remotely underwater while swimming
from one end of an Olympic sized swimming pool and back around to the other.
We wanted to observe how the ROV behaved in water as well as:
The overall buoyancy of the ROV
The average speed of the ROV
The effectiveness of the wireless antenna
The flapping foil system ability to propel and turn the ROV
Obtain live video from the onboard camera
At the pool, the ROV skeleton (frame, fins and head) was
placed on the surface of the water. The hydrofoils built for the
ROV proved to be too buoyant, and it is possible that the
servos would not be able to force the fins underwater. The
other possibility would be that the fins would push the ROV
above the water as it tried to swim. Instead, temporary
rectangular aluminum fins were used. The ROV floated on the
water’s surface, and operation was conducted using the
flapping foil system.
The ROV swam straight across the pool, maintaining a
constant pressure, temperature, and humidity in the electronics
case. The ROV traveled approximately 0.5 ft/s, a value based
on inspection from the video that was taken of the test.
The ROV turned around at the end of the pool and traveled
back in the opposite direction.
Figure 27: Rear Steering Fins
Figure 28: ROV During Operational Observation in Pool
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To test the antenna, the fins were removed and the ROV was pushed underwater. The computer lost connection with the
router in the ROV, indicating an issue with the antenna’s connection to the router.
While no quantitative results were obtained for the testing, the qualitative operational results of the ROV proved the
effectiveness of the flapping foil propulsion system. It moved through the water smoothly and turned without issue.
The camera was able to receive live video of objects in front of it
while underwater. Unfortunately the data cannot be saved except for
screen captures such as Figure 23.
CONCLUSIONS AND RECOMMENDATIONS
STRENGTHS AND WEAKNESSES
Strengths:
The propulsion system is effective in its main purpose of producing forward thrust and turning the ROV. It should be
more ostensible than propellers and more maneuverable. It is also less dangerous for other wildlife.
Another positive the overall design can be adapted for multiple applications other than simple exploration. Other
applications include collecting animal DNA, painting the sides of sea vessels, detecting large deposits of underwater
nuclear waste, or plugging up oil spills.
Weaknesses:
The design lacks a reliable depth control system. One way to do this would be to have adjustable ballasts with an air
supply through a compressor. That would eliminate the desire to go without a tether for the ROV, which it currently does
not have.
There are obvious weaknesses in the waterproofing that would need to be addressed. The waterproofing is heavily reliant
on fiberglass, which in itself is hard to notice imperfection and can take damage. The ones used in the ROV needed to be
cover with GC potting compound because the cases themselves had holes that could not be found. The fiberglass case
cannot withstand the kind of pressure that will inevitably be applied at lower depths.
The internal waterproofing of HiTec servos also proved to be unreliable. Using the method of mineral oil and silicon
presented positive results but in half the cases the servos stopped working. The same results occurred with the servos that
were plastic dipped.
With the current frame design there is no reliable way to attach the shells.
There are concerns with the reliability of the wireless connection due to line loss from the length of wire. The electronic
design also did not have enough ground wires, which cause the propulsion to move slowly and sporadically.
Figure 29: Image Taken from ROV camera
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IMPROVEMENTS
The issues mentioned above can be fixed with some small changes. Already waterproofed servos with the specifications
as the ones in the PVC cases and the rear fins can be purchased. This would eliminate the need to waterproof the cases,
which are unreliable. A different material for the waterproof cases would prove to be more effective at lower depths.
Plexiglass would have less unreliable points in the walls, and would not require any potting compound on the outside.
Improvements for the wireless connection include having a powered antenna to increase signal strength. Also, using a
thicker diameter wire increases the amount of data that travels through, while keeping the line loss the same.
REFLECTION
This project has taught us a lot about prioritizing tasks and ordering things correctly. By fixing the frame around the
propulsion system from the beginning, we could have saved time and money. This project also showed the necessity of
effective communication amongst group members, especially when they are working on different facets of the project. It
is important to have everyone’s ideas and opinions on where they believe the project is going heard to avoid errors. These
are skills just as important as any technical skill an engineer learns.
It is important to branch out to experts early on to make sure that you are on the right track with your ideas. There is
always someone that is more familiar with what you are working on than you, and they can most likely point you in the
right direction in an hour of talking that would have taken you weeks if you ever did find out.
Overall the senior capstone experience focused on most of the challenges of designing, building and selling a product to a
variety of individuals. The skills learned from the capstone will be used in future endeavors.
WORKS CITED
[1] Dr. Douglas Read, "Ocsillating Foils for Propulsion and Maneuvering of Ships and Underwater Vehicles," Cambridge,
MA, 1997.
[2] Melissa D. Flores, "Flapping Motion of a Three-Dimensional Foil for Propulsion and Maneuvering of Underwater
Vechicles," Massachusetts Institute of Technology, Cambridge, MA, 2003.
[3] Jamie M. Anderson, "Vorticity Control for Efficient Propulsion," Massachusetts Institute of Technology, Cambridge,
MA, 1996.
ACKNOWLEDGMENTS
We would like to thank the following individuals for all their help with our project this year:
Dr. Mohsen Shahinpoor, Dr. Michael Peterson, Dr. Douglas Read, Professor Murray Callaway Colleen Swanger, Diane
Maguire, Karen Fogarty, and the MIT “Finnegan” Program
Page 25 of 33
APPENDICES
APPENDIX A: FIN LENGTH ANALYSIS
Fin Length Calculation (length in meters)
Drag Coefficient (Estimated from teardrop shape of turtle shell)
Estimated Front facing surface area of turtle shell(m^2)
Freestream velocity (m/s)
Density of room temperature, sea level water (kg/m^3)
Strouhal Number (Set from experimental data collected in [1])
Thrust coefficient (Set from experimental data collected in [1])
Frequency (Hz) (Set from estimate of turtle flapping speed)
Dynamic Pressure
Drag Force
Equal to chord length
Wing Area Wing Length
Cd 0.4
AT 0.41264
U 0.447
998
St 0.4
Ct 0.52
f 0.5
q .5 U2
Drag q Cd AT 16.457
Heave
ho
St U 2 f( )
0.179
S
Drag
2
0.5 Ct U2
0.159S
ho
0.888
Page 26 of 33
APPENDIX B: COSTS
The following chart is a representation of finance distribution on the project. All amounts are in US dollars. The total for
the project is $ 3,446.
Figure 30: Chart of Finance Distribution
Figure 31: Chart of Team Expenses
435.51131.31
274.68
252.79
1316.37
438.3
474.01
123.48
Finance Distribution
Framing
Shell
Waterproof
Hydrofoils
Servos
Propulsion
Electronics
Testing
2007.46474.01
841.5
Team Breakdown
Propulsion
Electronics
Structures
Page 27 of 33
APPENDIX C: BUOYANCY CALCULATIONS
Dens Al: 0.0975 CSA: 0.425 Dens H2O: 0.036123
lengths in inches, weights in lbf.
Frame:
CENTER OF MASS CALCS
MEMBER X Y Z Length V W Buoyancy (X+30)*W (Y+15)*W (Z+1)*W
