Final Report - Mechanical Engineering Department
MECH 4010 - Design Project I
Term Report
To: Dr Hubbard
Date: April 9, 2007
Team 3 Members:
Rachel Byers ___________________________
Alex Crandall ___________________________
Valerie Kizell ___________________________
Kelly McGrail ___________________________
Connor O’Shea __________________________
Supervisor
Dr. Jimmy Chuang ______________________
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ABSTRACT
This report outlines the design, construction and testing phases of SWORDFISH, a remotely operated submersible. The goal of this project is to design and build a device capable of inspecting ship hulls.
Currently inspection of ship hulls requires the use of costly commercially available ROVs or divers.
SWORDFISH is an inexpensive, reliable and easy to use device that can quickly and efficiently inspect ship hulls
At the beginning of the design process several design requirements were created. Three design solutions were evaluated and the final design was chosen. The chosen design includes two horizontal thrusters, one vertical thruster and one lateral thruster. Commercially available waterproof components were chosen for the thrusters, cameras and light because they greatly increase reliability. The frame is constructed from PVC because it minimizes drag and provides stability and protection for key components. The final design incorporates a control system which uses feedback from a depth gauge to allow the operator to easily maintain a constant depth. The ROV is easily operated using both a joystick and video screen with two video feeds allowing for inspection of both the sides, and the bottom of a ship. Power is provided to the ROV from a portable power source through a neutrally buoyant tether.
The ROV was constructed according to the schedule outlined in the first semester. Individual components, such as the cameras, thrusters, and ballast system were tested in the tow tank before construction ended. After the ROV was constructed it was tested in the pool tank and tower tank at the
Department of Oceanography. This testing was completed in order to pass the inspection test and to determine if the ROV met the design requirements.
The ROV met the majority of the design requirements. The ROV was not able to meet the speed requirements due to insufficient thrusters. The cameras onboard the ROV were not able to meet the vision requirements. The ROV is still able to effectively inspect ship hulls as demonstrated during testing.
In order to meet the design requirements, more expensive equipment would have to be purchased.
The ROV is easily controlled using the joystick and depth gauge. It is also easily controlled by one person.
The ROV is robust and able to sustain impacts without damage.
The total budget for the ROV is less than the predicted budget. This is due to savings on the tether, and thrusters. This report marks the completion of the MECH 4020 Design Project.
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CONTENTS
iii
iv
LIST OF TABLES
v
LIST OF FIGURES
1
INTRODUCTION
SWORDFISH (Submarine With Onboard Recording Device For Inspecting Ship Hulls) is an ROV (remotely operated vehicle) that was designed and built for the purpose of underwater ship inspection. It is powered by four thrusters: two horizontal thrusters, one vertical thruster and one lateral thruster. The
ROV is operated and controlled by one person on the surface and has the ability to dive down to a depth of 30ft. A tether is used to communicate with the ROV and provide feedback from the depth gauge and the two cameras (one mounted horizontally and one mounted vertically). The horizontal camera is used to navigate the ROV and inspect the side of the ship hull. The second camera, facing upwards, allows for inspection of the bottom of the ship hull from below. The ROV is compact, lightweight and relatively inexpensive. This makes it an ideal tool for underwater ship hull inspections.
ROVs have been used in industry since the 1960s when the first ROV was developed by the US Navy.
Since their development ROVs have been utilized for various purposes include underwater surveillance, object recovery, maintenance to underwater structures, and sea exploration. ROVs are safer to use to use than manned submersibles or divers, however, commercially available ROVs are expensive, ranging from $25,000 to $250,000.
The ROV project was introduced to the MECH Design Project class in 2004 when DRDC was looking for an inexpensive and effective solution for underwater surveillance. The previous ROV was designed with the goal of reaching a speed of 1 knot horizontally and a depth of 10 feet without leaking. It was able to achieve four degrees of freedom with two vertical thrusters and two horizontal thrusters. The ROV was not able to reach the speed and depth requirement without leaking. The design for this project consisted of an external cage, constructed from aluminum, and a main body, constructed from a PVC pipe with a clear acrylic dome, to house the camera, light and motors. The seal between the PVC pipe and the acrylic dome failed and resulted in condensation accumulating in the main body. A catastrophic failure occurred to the prototype at a depth of 19 feet when an internal thruster seal popped and the main body was flooded. A second ROV was constructed with many of the original components with thrusters now mounted externally. This design proved to be more successful than the first.
After reviewing the previous design project, other hobby and school ROVs and commercially available
ROVs, it was evident that improvements can be made for a more efficient and reliable ROV to be created.
This report summarizes the progress made by the design team over two semesters. It outlines the design requirements for the project, the designs considered to fulfill these requirements, and a description of the selected design. The report also describes work done during the second semester, when construction of the ROV was completed and the ROV was tested to determine its capabilities.
Future recommendations are provided, as well as an analysis of the design’s strengths and weaknesses.
A complete budget for the project and a schedule summarizing work done during the winter term is also included.
DESIGN REQUIREMENTS
The objective of this project is to design and build a device to inspect ship hulls while ships are underwater. In order to accomplish this task, specific design requirements were created. Requirements are divided into 5 categories; geometry; quality and durability; maneuverability; documentation; and timing and deliverables.
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GEOMETRY:
Small and light enough for one person to lift (max 18 ft 3 , 40 lbs)
50 ft tether to provide power to the ROV, to control the motors and to provide feedback from the camera
Highly visible tether and ROV in order to be visible from the bottom of a standard sized pool
QUALITY AND DURABILITY:
Colour video with image quality sufficient to read the 10/25 line on an eye chart
Lighting sufficient to read the 15/35 line on an eye chart in dark conditions
No leaks to key components, electrical equipment and wires at depth of 30 ft
Stable under water in order for ROV to sit level in optimal conditions
External cage to protect key components
Built out of corrosion resistant materials suitable for operation in saltwater environment
Ability to crash into a wall at full speed and not sustain critical damage
Ability to reach a minimum depth of 30 ft
Ability to operate for a minimum of three continuous hours
MANEUVERABILITY:
3 or more degrees of freedom
Failsafe to float to surface in event of power failure
Ability to reach a minimum forward speed of 1 knot and minimum up and down speed of
0.5 knots
Ability to turn within a 1 m radius
Ability to be controlled by one person
DOCUMENTATION:
User’s manual including setup and operating instructions, hazards, and troubleshooting guide
Build instructions with pictures and technical drawings
TIMING AND DELIVERABLES:
As per MECH 4010 guidelines
One submersible will be built
Prototype to be built for less than $3500
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DESIGN SELECTION
In order to ensure the best design was chosen the design selection choices were divided into five different categories. Different options were brainstormed and researched within each category to determine as many design solutions as possible. Weighted design selection charts were assembled with all important considerations to determine the best choice. The categories were: maneuverability; propulsion; material and frame; cameras and light; ballast; power supply and tether; and control system.
A detailed description of the design selection process can be found in the December report.
SELECTED DESIGN
Figure 1 - Final Design
The final design of the ROV is divided into seven systems: maneuverability, propulsion, material and frame, camera and light, ballast, power supply and tether, and control system.
MANEUVERABILITY:
The final design has four thrusters; two horizontal, one vertical, and one lateral. The vertical thruster allows the ROV to dive straight down (heave) and return without tilting or rolling. This is the easiest way to reach the required depth without any horizontal movement. The vertical thruster is mounted in the middle of the ROV to keep the ROV balanced while heaving. The two horizontal thrusters are mounted approximately at the middle of each side of the ROV. They are mounted so that the propellers do not extend past the frame, thus protecting them from damage. The horizontal thrusters provide power to reach the required horizontal speed; and when controlled individually they allow the ROV to yaw.
Mounting the horizontal thrusters in the middle of the frame keeps the ROV balanced and prevents it
from pitching or rolling. The lateral thruster is also mounted in the middle of the ROV so that the center of effort of the thruster acts along the same line as the center of resistance of the ROV. This prevents the ROV from yawing unnecessarily during sway movement.
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5
PROPULSION:
Figure 2 - Thruster
The thrusters are modified bilge pump replacement motor cartridges (Figure 2). They are the least
expensive way to provide power to the ROV while minimizing leak potential. The impeller was removed and a fabricated shaft adapter was used to couple the motor shaft to a propeller. The motor lead wires were attached to the tether using waterproof connectors. The thrusters require 3 Amps of 12 volt DC current and are capable of operating at the maximum depth of our ROV (30ft). They are approximately 4
½ inches long and 2¼ inches in diameter.
The thrusters are mounted to the frame using a PVC T with the side cut to match the profile of the thruster. Hose clamps secure the thruster in place. The mounting T is attached to an oversized PVC T joint which can slide along the frame allowing the position of the thruster to be adjustable.
The frame was designed such that the position of the thrusters is adjustable making the ROV adaptable for future additions. A force analysis was performed to determine the optimal positions of the horizontal thrusters. Fine tuning of the positions was performed during the final stages of the design project.
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MATERIAL AND FRAME:
Figure 3 - Frame
key components from hazards, and provides numerous mounting locations for thrusters, cameras, lights, meters and gauges. PVC is inexpensive, lightweight, easy to work with, and can be purchased at a local hardware store. PVC pipes are connected together using T and 45° joints. The PVC pipe is glued and fitted into the joints.
CAMERA AND LIGHT:
Figure 4 - Camera
The ROV does not have the ability to pitch, so two cameras are necessary to see in two directions. One camera is mounted at the front of the ROV along the centerline facing forward. The second camera is mounted at the top of the ROV in the centre of the horizontal plane and is facing up in order to see the bottom of the ship from below. Both cameras are waterproof to prevent leaks and increase reliability.
located inside the frame to protect them from underwater hazards. Other components are mounted as to not obstruct their view. The cameras are equipped with LED lights which provide sufficient light for maneuvering and inspecting in the forward direction.
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BALLAST:
Figure 5 - Ballast
The ballast system is comprised of buoyant materials at the top of the ROV and heavy materials at the bottom of the ROV to provide stability in the roll and pitch directions. Positive buoyancy allows the ROV to float to the surface in the event of a power failure or other emergency.
POWER SUPPLY AND TETHER:
The power supply is a 115Amp-h battery located at the surface. The battery chosen is a deep cycle 12V battery. It is able to be discharged and recharged numerous times and provides enough power for 50 hours of continuous operation. An automatic battery charger was also purchased to recharge the battery, allowing the ROV to complete multiple missions.
Power is provided to the ROV by means of a neutrally buoyant tether. This tether supplies power and control to the 4 thrusters, provides power to the cameras, returns the video feedback from the cameras, and returns feedback from the depth gauge. The tether was assembled from multiple wires held together and waterproofed with 3M shrink wrap. Light weight material was added to the tether to make it neutrally buoyant.
CONTROL SYSTEM:
The purpose of the SWORDFISH control system is to allow a user to move the ROV through a few intuitive commands. The control system consists of five different electronic components; a 3-axis
joystick with slider, a Basic Stamp 2sx, an LCD screen, four HB-25 H-bridge motor controllers, and a depth gauge. These components are arranged to serve three distinct functions: user input, motor control and depth feedback.
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USER INPUT
The Joystick is connected to the Basic Stamp 2sx to provide user input to the control system. The joystick’s main stick can move in three directions (forward/back, left/right and clockwise/ counterclockwise twist). On all three axes there are potentiometers that have been changed into variable resistors. There is a fourth variable resistor mounted to the slider switch that moves in a circle around the base of the control stick. All variable resistors are linked to the ground rail of the Basic Stamp control board, and each variable resistor is connected to its own RC circuit on the same board. The RC circuit is used by the Basic Stamp in conjunction with the “RCTime” command to read the time that the
RC circuit takes to drop from five volts to two volts. The decay time is linearly related to the resistance of the variable resistor attached to that RC circuit. The Basic Stamp uses the four RC circuits to read the four variable resistors in the joystick and stores these values to be used for motor control.
MOTOR CONTROL
Once the Basic Stamp has timed the RC circuit discharge, each circuit timing is converted into a sixteen bit variable within the Stamp. The code stored in the stamp changes these raw readings into pulses that can be used to control the H-Bridge motor controllers. The HB-25 motor controllers are attached to the
Basic Stamp through a 5 volt connection. The HB-25 motor controllers are also attached to a 12V battery kept at the surface, and a thruster mounted on the ROV. The HB-25 receives a pulse from the Basic
Stamp, and interprets the pulse to send power to the thruster. The HB-25 can power the thruster from full forward to full reverse. The HB-25 interprets a pulse of 0.8 milliseconds as full reverse, 1.5 milliseconds as switch off, 2.2 milliseconds as full forward, and any pulse length between as a fraction of full forward or full reverse. Any pulse that falls outside of this range is ignored. The HB-25 will also ignore any pulse sent within one millisecond after the end of the previous pulse. If a second HB-25 is connected to the first in series, then the first HB-25 will send the signal received shortly after the first on to the second HB-25. Each HB-25 uses a five Amp fuse to protect the thruster attached to it. The variable resistor reading for the joystick slider is compared to the reading from the depth gauge. The comparison between the depth gauge signal and the slider signal dictates the pulse sent to the vertical thruster.
DEPTH FEEDBACK
The ROV control system uses a depth gauge to provide feedback from the ROV and allow the user to maintain a set depth while carrying out inspections. The depth gauge outputs a current between four and twenty milliamps. This current is sent across a resistor, and the voltage across the resistor is measured by an Analog to Digital converter. The Basic Stamp polls the AD converter and the value received from the converter chip is used in conjunction with the joystick reading to trigger the vertical
thruster to move the ROV up or down until the readings match. An LCD screen attached to the Basic
Stamp reports both the depth set by the joystick and reported by the depth gauge, in feet.
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DESIGN ANALYSIS
To ensure the chosen design meets the design requirements several analyses were performed on the model. The drag forces and buoyancy forces on the ROV were analyzed to determine the total drag while driving forward in the water. A finite element analysis of the ROV was performed while in the water and while being lifted from the water.
The drag analysis indicated that the thrusters would provide sufficient force to meet our speed design requirements; however, the tether was not taken into account in this analysis. It was therefore expected that the actual drag forces would be higher than the calculated drag forces.
A buoyancy analysis was completed to determine the center of mass as well as the center of buoyancy about the central axis of the ROV. Results of this analysis were used to determine the location of the vertical and lateral thrusters. It was also determined that more positive buoyancy would need to be added to make the ROV slightly positively buoyant.
In order to validate the choice of 1” PVC pipe as structural material, a finite element analysis was performed on the frame of the ROV. Results showed that the ROV would pass an impact test.
DESIGN ITERATIONS
After completing these analyses a few design changes were made. The 1” PVC pipe was replaced with
¾” PVC pipe. This is based on the findings of the finite element analysis. This pipe size reduction reduces drag on the ROV and made it easier to find the proper components.
The flashlight on the initial design was removed as the LEDs in the cameras are sufficient lighting. This reduced the weight and cost of the ROV.
It expected that the ballast system would be designed during the second semester during testing. This is due to the fact that empirical data is more reliable than calculated data. The ballast system required initial testing of the finished product before it could be finalized. The ballast system was built using PVC air tanks for light ballast and lead balls and rebar for heavy ballast.
The propellers were chosen after the thrusters arrived. This is due to the fact that no information was provided with the motors so the optimal propeller needed to be determined experimentally.
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CONSTRUCTION
Construction on the ROV was completed during the winter semester. The major systems that the team constructed are: frame, mounts, thrusters, control system, tether, and ballast system. Detailed drawings of these systems can be found in Appendix
FRAME
The frame is used to protect the internal components of the ROV including the thrusters, cameras, depth gauge, and compass. The frame provides structural support and mounting locations for these devices, and encloses the heavy ballast and traps air for light ballast. The frame was constructed from
PVC pipes connected together using “T” and 45° joints. PVC pipe with diameter ¾” was chosen because it meets the stress requirements as determined from a FEM analysis. ¾” PVC is small, inexpensive, and minimizes drag. The frame was glued and sealed using PVC joint cement. Certain frame members are adjustable. This was accomplished by boring out the PVC “T” joints to an inner diameter of 57/64”, which is slightly larger than the outer diameter of the PVC pipe. This allows them to slide along the pipe.
These members were left adjustable in order to fine tune the thruster position and were sealed to be water tight with custom PVC plugs and PVC cement.
MOUNTS
The thruster mounts were custom fabricated from PVC “T” joints. A milling machine was used to create a radius in the “T” joint equal to the radius of the thruster body. The thruster body was mated to the radius in the “T” joint and secured in place using hose clamps. The “T” joint-thruster assembly was then mounted to the frame using an adjustable “T” joint as discussed above.
To prepare the cameras to fit into their mounts the heavy ballast of the cameras were removed and the ballast holders were drilled almost completely off. This drilled end was then inserted into a small length of PVC pipe. A single hole was drilled through both the PVC pipe and the end of the camera. The PVC pipe was secured to the camera using a nut, bolt and two washers. The small PVC pipe was then cemented into a “T” joint on the frame.
THRUSTERS
The thrusters were constructed from Johnson Mayfair 650GPH Bilge Pump Replacement Cartridges. The bilge pump replacement cartridge was a water proof motor with an impeller coupled to the shaft. The
custom fabricated in the machine shop. The shaft adapters extend the shaft by two inches allowing adequate clearance behind the propellers to allow for unobstructed flow. The shaft adapter was fixed to
the shaft adapter was threaded, with the propeller screwed on and secured with Locktite Threadlocker.
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Figure 6 - Shaft Adapter
The thruster arrangement was calculated and then refined during testing. The final thruster arrangement was chosen so that the forward thrusters surge the ROV without pitching, the lateral thrusters sway the ROV without yawing, and the vertical thrusters heave the ROV without pitching or rolling.
CONTROL SYSTEM
The construction of the control system is separated into several steps: Basic Stamp wiring, control box construction, HB-25 installation and depth gauge wiring.
BASIC STAMP 2
S X
WIRING
A Super Carrier board was purchased to hold the Basic Stamp 2sx, a 9V battery to power the Stamp and the joystick, and the RC and Analog to Digital circuitry. The Basic Stamp 2sx was plugged into the Super
Carrier board. The RC circuitry and Analog to Digital circuitry were soldered onto the board. Each RC circuit consists of a capacitor connected from the Stamp pin input to ground, a variable resistor wire connected to the stamp pin and a resistor that bridged the gap between Stamp and RC circuit to protect the stamp from current overload. The ground wire for the joystick was also soldered to the ground rail.
The Analog to Digital chip was mounted in an eight pin socket. This socket was soldered onto the Super
Carrier board. The eight pins were connected to three Stamp pins, the depth gauge ground wire, two five volt connections, a straight ground connection, and a ground connection through a resistor.
CONTROL BOX CONSTRUCTION
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The control box was built to house the Basic Stamp 2sx and its Super carrier board, the four HB-25 motor controllers and the LCD screen. The box has openings for the depth gauge wire, the joystick wire and the input and output power cord for the thrusters. The control box had to hold the HB-25 controllers off of the base of the box to ensure the cooling fans located on the bottom of the HB-25s could properly circulate air. The LCD screen is mounted on the top of the control box so that it can be seen by the user while the box is closed. The box is made from balsa and ply wood, with hinges and a magnetic latch to keep the box closed and mostly splash-proof.
HB-25 INSTALLATION
The HB-25 motor controllers were attached to the control box on pedestals with screws. Two of eight wires contained within the main tether were attached to each of the four HB-25 controllers. Each pair was attached to a thruster at the other end of the tether. Each HB-25 was also attached to the 12V battery in the same fashion. Care was taken to attach the positive and negative wires to the HB-25s in the correct fashion. The HB-25s were finally connected to the Basic Stamp 2sx through the Super Carrier board.
DEPTH GAUGE WIRING
The depth gauge was wired to the ground rail of the Super Carrier board. Three nine volt batteries were connected in parallel, and then connected straight to the positive depth gauge wire, and to the ground rail/ negative depth gauge wire through an on/off switch. This provides enough power to the depth gauge to produce reliable readings at all operating depths.
TETHER
Fifty three feet of eight strand, neoprene-jacketed, power cable was ordered to create the tether. A small length of cable was used to connect the motor controllers to the battery. The rest was used to relay the power from the motor controllers to the thrusters. The main challenge during the construction of the tether was waterproofing the submerged end of the tether. Without proper sealing, water would be able to enter the tether and seep up, corroding the entire length of the tether. Any exposed copper wire could also short if exposed to water.
The outer jacket of the tether was removed a few inches from the end, exposing the eight wires with their own waterproof jackets. The last centimeter of each wire was stripped to expose the copper wires underneath. The main cable was sealed using heat-shrink tubing and epoxy. Epoxy was placed on the eight wires, and a piece of heat shrink tubing was shrunk around the bundle to cause the epoxy to fill the gaps between each wire. This segment of heat shrink was covered with a second, larger piece of heat shrink that had a layer of glue that melted when the heat-shrink was activated. This piece of heat shrink was used to seal the first heat shrink with epoxy to the main cable’s outer waterproof jacket. Each stripped wire was attached to a thruster wire, and covered with a piece of heat shrink tubing. This
14 process waterproofed the thruster connections and the main cable, preventing short-circuits during ROV operation.
BALLAST SYSTEM
All of the heavy ballast is inside the frame of the ROV. Fifteen inches of rebar were added to both the port and starboard pipes in the keel of the ROV. The combined weight of the rebar is 1.9 kg. Lead balls, weighing 0.5 kg, were added to the front keel pipe of the ROV. The lead balls were placed in the middle of the pipe and secured in position by placing foam on both sides of the lead balls. Small pieces of balsa wood were placed over the Styrofoam and held in position using epoxy.
To add buoyancy, two PVC tubes were added above the port and starboard pipes on the top of the ROV.
The pipes are 2 inches in diameter and 4 inches in length. End caps were secured to either side of the pipe using sealing glue to ensure that they are waterproof. The air tanks were secured to the top pipes using tie wraps and Styrofoam.
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TESTING
Cameras and thrusters were tested as they arrived and were assembled. After assembling the ROV it was tested at the Department of Oceanography to pass inspection. After inspection testing was complete, further modifications to the ballast system were made. The frame members were put into positions as determined by the pool test and the frame was glued together. The final propellers were obtained and mounted on the thrusters. The final test was performed at the pool and tower tanks at the
Department of Oceanography to determine if the design requirements had been met. Finally the ROV was tested at the Tancook Island ferry dock in Chester Nova Scotia to determine its seaworthiness.
Results from these tests can be found below.
CAMERA TEST
One camera was tested to determine its capabilities.
METHOD
The camera was attached to a PVC pipe in order to hold the camera under the water in the tow tank. It was connected to a laptop and video capture software was used to record the video taken. A laminated eye chart was held underwater and video was captured from several distances. The lights were then turned off and the test was repeated. A piece of scrap metal with white writing on it was also submerged in order to simulate the surface of a ship hull.
RESULTS
The test did not meet the eye chart requirements; however, it was able to easily see the writing on the piece of scrap metal. It was determined that the quality of the video is sufficient to inspect a ship hull and that the requirements were unnecessarily high. In order to meet this requirement a much more expensive camera system would need to be purchased.
PROPELLER TEST
Different sizes and configurations of propellers were tested to determine the optimal propeller to provide maximum thrust for the thrusters.
METHOD
A shaft adapter was built to attach a propeller to a thruster. Various 2-blade, 3-blade, and 4-blade propellers made of brass and plastic were tested. The thruster was mounted at the bottom of a cross made of PVC pipe. A force gauge was attached at the top. The assembly was secured in the overflow tank beside the tow tank and was free to rotate. As power was provided to the thruster the cross would rotate and pull the force gauge. A fuse was connected to the thruster that limited the current to the thruster’s maximum of 5 A. A multimeter was used to monitor the current throughout the test to avoid
exceeding the maximum allowable current. The propeller which provided the most amount of thrust was chosen.
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RESULTS
The following table shows the results of the propeller test. The 50 mm plastic propeller was chosen for all four thrusters based on these results.
Table 1 - Propeller Results
Material # Blades Size (mm) Force (N) Current (A)
Brass 4 70 6 4.5
Brass
Brass
3
3
65
40
6.2
6
4.5
4.5
Plastic
Plastic
Plastic
Plastic
4
3
3
2
60
60
50
40
10
12.5
14
8
4.5
4.5
4.15
4.5
Plastic
Plastic
Plastic
3
2
2
40
120
110
9
9
5
2.5
4.68
4.4
In some cases the current was not able to reach the set maximum of 4.5 amps. This is because the propeller applied less torque to the thruster and the thruster reached its maximum speed (rpm). The larger propellers applied more torque on the thruster and the thruster was not able to reach its maximum speed as the current was limited with the fuses and user input. Removing the fuses and allowing more current to pass through the thruster would likely cause it to overload.
BALLAST TEST
To determine the amount of heavy and light ballast needed for the ROV, two ballast tests were conducted.
INITIAL BALLAST TEST
The volume of the ROV was determined experimentally in order to calculate the buoyant force needed to make the ROV positively buoyant.
METHOD
A large tank was completely filled with water. The ROV was temporarily waterproofed and placed in the tank. The tank was allowed to overflow, and the change in volume in the tank after the ROV was removed was equal to the volume of the ROV. A sonar measuring device was used to measure the
17 distance from the device to the water surface. An original reading was taken before ROV was placed in the tank, and then read again once the water had drained from the tank.
RESULTS
Calculations done after this test show that 0.36 kg of heavy ballast need to be added to make the ROV neutrally buoyant. These calculations can be found in Appendix A.
The result of this test was improved in a second ballast test. The ROV was placed in the tow tank and weight was added to it to fine tune the ballast. During this experiment it was determined that approximately 0.5 kg would need to be added into the bottom front pipe.
FINAL BALLAST TEST
During testing it was found that while the ROV was turning it would roll significantly, thereby decreasing the efficiency of its movements. This was due to the ROV having a low righting moment. In order to improve the righting moment more buoyancy and ballast was added in order for the ROV to remain upright while in motion.
METHOD
Rebar was added to the bottom pipes on the starboard and port sides of the ROV. This adds significant weight to the bottom of the ROV without increasing the drag forces. PVC pipes of 2 inch diameter filled with air were added to the port and starboard sides of the ROV. The length was determined experimentally in the tow tank. An initial length of 19 inches was used. The ROV was placed in the tow tank.
RESULTS
It was found that it floated just under the surface. No further adjustments were made. The final ballast
Table 2 - Final Ballast
Added Light Ballast 1.95 Liters of air
Added Heavy Ballast 1.9 kg rebar
0.5 kg lead balls
INSPECTION TEST - MONDAY MARCH 17 TH
The ROV was assembled and tested in order to pass inspection. Inspection goals were to prove that the
ROV was functional. Work was done to optimize the ballast system and frame arrangement during the test.
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The test took place at the Department of Oceanography in the pool tank. It is 15 m in diameter and 3.5 m deep. Viewing windows encircle the tank near the bottom.
METHOD
The ROV was deployed and driven around the tank by various group members. A large plastic tube was placed in the pool and the ROV drove into and back out of the tube.
RESULTS
The ROV was able to move forward and backward, up and down, laterally, and turn. It was able to drive to the bottom of the pool and return to the surface with no difficulty. It was able to drive to the middle of the pool and return to the wall.
The ROV successfully completed the tube mission. It navigated to the tube and drove into it. After becoming temporarily stuck in the tube, the ROV drove back out of the tube the way it drove in.
The ROV was not glued during the inspection and water leaked in. This caused the ROV to be negatively buoyant and made it difficult to keep the ROV at the surface. Additionally, it was determined that a greater righting moment was necessary to keep the ROV level in the water during operation. More work was done on the ballast system after draining the ROV to improve stability and buoyancy.
Initially the ROV pitched upward while driving forward. The position of the horizontal thrusters was adjusted to keep the ROV level. Similarly, the lateral thruster was adjusted to prevent yaw motion when moving laterally.
FINAL TEST - FRIDAY MARCH 28 TH
After the inspection test the ROV was finished. The ballast system was installed, correct propellers were attached to the thrusters and the ROV was waterproofed. Final testing occurred at the Department of
Oceanography to determine if the ROV met the design requirements set at the beginning of the year.
POOL TANK
The ROV was tested at the pool tank to determine if the shallow water requirements were met before moving to the deep water requirements.
METHOD
Tests were run in order of risk to the ROV with the low risk tests run first. The ROV was deployed and driven in all directions similarly to during the inspection test. This determined the number of degrees of freedom of the ROV. The ROV completed a 180° turn to determine its turning radius, and power was shut off while the ROV was at the bottom of the pool to test its failsafe float to surface. The ROV was driving to the bottom of the pool and back to the surface to determine its up and down speeds and was
19 driven 5 m forward to determine its forward speed. The ROV was deployed, driven, and recovered by one person to test its ease of operation. Finally the ROV was driven into a wall at full speed to determine its durability.
RESULTS
The results from this test can be found in Table 3 below.
Table 3 - Pool Tank Results
Design Requirements
Stable under water in order for ROV to sit level in optimal conditions
Ability to crash into a wall at full speed and not sustain critical damage
Ability to reach a minimum depth of 30 ft
Ability to operate for a minimum of three continuous hours
3 or more degrees of freedom
Result
ROV sits level in still water
No damage visible after crashing at full speed
Reached depth of 33 ft
Three hour test used 15% of the battery
4 degrees of freedom; surge, yaw, heave, sway
Positively buoyant Failsafe to float to surface in event of power failure
Ability to reach a minimum forward speed of 1 knot and minimum up and down speed of 0.5 knots
Ability to turn within a 1 m radius
Ability to be controlled by one person
0.66 knots forward, 0.58 knots up and
0.24 knots down
0.3 m turning radius
Ability to be controlled by each team member
Overall the ROV performed better during this test than during the inspection test. During the final test the ballast system kept the ROV stable both while stationary and while moving. As well the ROV was positively buoyant.
The requirements that were not met were the up and down speed and forward speed. It was determined this requirement was not met because the thrusters were not able to provide the necessary power. In order to increase the power provided by the thrusters significantly more expensive thrusters would need to be purchased.
All other requirements were met or exceeded during the pool test.
TOWER TANK
The tower tank, located at the Department of Oceanography is 3.66 m in diameter and 10.46 m (33 ft) deep. This is ideal for testing the depth requirement. A spiral staircase wraps around the tank and leads
from the top of the tank to the bottom. There are windows along this staircase that look into the tank.
The tank is lined with a net in order to catch any components should they fall off during the test.
20
METHOD
The ROV was deployed and driven to the bottom of the tank using the vertical thruster. This test was completed twice to ensure repeatability. After returning to the surface power was sent to the thrusters to ensure no components had failed. Video was returned from both video cameras during and after the test to ensure that they were fully functional. The depth gauge was tested by sending the ROV to various depths. During this test the ROV was viewed from the surface and from windows at various depths.
RESULTS
Results of the tests can be found in Table 4 below.
Table 4 - Tower Tank Results
Design Requirements
Highly visible tether and ROV in order to be visible from the bottom of a standard sized pool
No leaks to key components, electrical equipment and wires at depth of 30 ft
Ability to reach a minimum depth of 30 ft
Result
ROV visible from bottom of Tower Tank (33 ft depth)
All thrusters and cameras functional after reaching a depth of 30 ft
Reached depth of 33 ft
The ROV reached the bottom of the tower tank with little difficulty. On the first test a propeller was caught in the net which required that it be brought up to the surface and untangled. This caused a fuse to the motor to blow, but no critical damage to the ROV was sustained. The horizontal propellers were removed and the test was repeated. The ROV made it easily to the bottom of the tank and returned to the top unaided. On the second test the ROV got tangled in the net two feet above the bottom of the tank (30 ft). The ROV had to be pulled up in the net to untangle it. No critical damage was sustained during this test.
The depth gauge was able to control the depth of the ROV. When the set depth was changed the ROV successfully reached the depth with little overshoot, and hovered within the specified range of 1 foot.
The depth gauge made it easy to control the depth of the ROV and to reach a set depth within its operating range.
CHESTER TESTING
The ROV was launched from the Tancook Island ferry dock in Chester Nova Scotia. The ROV was driven under several large fishing boats and video was recorded from the surface. The ROV was able to navigate under the ships and maintain constant depth using the depth gauge. The video returned from the ROV was good quality with sufficient lighting. The propellers and other equipment under the boats
were visible from the video screen. This test proved that the ROV is fully functional in an actual inspection environment.
MISCELLANEOUS TESTING
In addition to the tests listed above, several smaller tests were performed during the course of the semester to meet other design requirements.
21
GEOMETRY
The final ROV was measured and weighed to ensure that it is within the range specified by the design requirements. The final dimensions of the ROV are 18 ¾” in by 18” by 23 ¼”. This translates to 4.5 ft 2 .
The weight of the ROV is 10 lb. Both the size and the weight of the ROV are small enough to meet design specifications.
The tether was measured to determine if it meets the design requirement of 50 ft. The final tether length is 53 ft, meeting the requirement.
CONCLUSIONS
Throughout the testing procedure, all of the design requirements were verified. Table 5 contains the
results of all tests performed.
Table 5 - Testing Results
Design Requirements
Geometry:
Small and light enough for one person to lift
(max 18 ft 3 , 40 lbs)
50 ft tether to provide power to the ROV, to control the motors and to provide feedback from the camera
Highly visible tether and ROV in order to be visible from the bottom of a standard sized pool
Quality and Durability:
Colour video with image quality sufficient to read the 10/25 line on an eye chart
Lighting sufficient to read the 15/35 line on an eye chart in dark conditions
No leaks to key components, electrical equipment and wires at depth of 30 ft
Stable under water in order for ROV to sit level in optimal conditions
External cage to protect key components
Result Achieved
Able to be lifted by each team member Met
53 ft tether controls motors and provides feedback from both cameras
ROV visible from bottom of Tower
Tank (33 ft depth)
Able to read 8/40 line in light
Able to read 8/40 line in dark
All thrusters and cameras functional after reaching a depth of 30 ft
ROV sits level in still water
PVC frame protects thrusters and
Met
Met
Not Met
Not Met
Met
Met
Met
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Built out of corrosion resistant materials suitable for operation in saltwater environment
Ability to crash into a wall at full speed and not sustain critical damage
Ability to reach a minimum depth of 30 ft
Ability to operate for a minimum of three continuous hours
Maneuverability:
3 or more degrees of freedom cameras
ROV constructed out of PVC, plastic and stainless steel which are corrosion resistant
No damage visible after crashing at full speed
Reached depth of 33 ft
Three hour test used 15% of the battery
4 degrees of freedom; surge, yaw, heave, sway
Positively buoyant
Met
Met
Met
Met
Met
Failsafe to float to surface in event of power failure
Ability to reach a minimum forward speed of 1 knot and minimum up and down speed of 0.5 knots
Ability to turn within a 1 m radius
Ability to be controlled by one person
Timing and Deliverables:
As per MECH 4010 guidelines
One submersible will be built
Prototype to be built for less than $3500
0.66 knots forward, 0.58 knots up and
0.24 knots down
Documentation:
User’s manual including setup and operating instructions, hazards, and troubleshooting guide
Build instructions with pictures and technical drawings
0.3 m turning radius
Able to be controlled by each team member
User's manual found in Appendix D
Met
Not Met
Met
Met
Met
Build instructions found in build report, technical drawings found in Appendix
C
As per MECH 4010 guidelines
One submersible was built
Final cost is $2637
Met
Met
Met
Met
Upon completion of the testing phase, it was determined that 19 of the 22 design requirements have been met or exceeded. The design requirements which were not met include the speed requirements and the camera quality requirements. It was determined that these requirements were not met due to insufficient equipment. In both cases, a significant amount of money would be necessary to upgrade the equipment. Recommendations for future prototypes are included later in this report.
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BUDGET
A summary of the budget can be found in Table 6 below. A detailed list of all the materials and
components purchased for the project as well as their cost can be found in Appendix B – Detailed
Budget. The total cost for the project is $2637 which is lower than the initial budget found in the
December Report.
The project is financed entirely by the Mechanical Engineering Department at Dalhousie University and
Shell Canada. The department donated $3000; however, not all of the money was required. These funds have been donated on the condition that the department keeps the completed model.
Table 6 - Budget Summary
Category Expected
Cost
Actual
Cost
Control System $1,269.00 $1,415.00
Thrusters $215.00 $105.00
Frame
Cameras and
Light
Ballast
Power Supply
$210.00 $136.00
$580.00 $460.00
$57.00
$620.00
$65.00
$309.00
Misc
Total
$131.00 $147.00
$3,082.00 $2,637.00
Percent of
Expected
112%
49%
65%
79%
114%
50%
112%
86%
There are some differences between the expected budget and the actual budget of the project. The depth gauge is more expensive than originally anticipated which increased the cost of the control system. There is a slight increase in the cost for ballast system as different options were tested before deciding on the final solution. As well, miscellaneous supplies such as tape, adapter cables, and cleaning supplies caused the increase in this category. The team was able to obtain propellers for free and manufactured shaft adapters in house which decreased the cost for thrusters. As well, a flashlight was not purchased as the cameras are equipped with LED lights which are sufficient for inspection. The tether that was purchased required only waterproofing at the end, not the entire length, and the shrink wrap required was provided by the department. This drastically decreased the cost of the power supply.
In total, the project came in $445 under budget.
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SCHEDULE
The team created a Gant chart in order to stay on schedule in the winter term. This can be found in the
December report. A summary of the milestones and their expected date and actual date can be found in
Table 7 - Schedule
Milestone
Begin Procurement
Begin Construction
Begin Testing (components)
Construct Frame
Assemble Components
Calibrate Ballast
First Assembly Test
Repair and modification from testing
Device Inspection
Expected Date Actual Date
November 19, 2007 November 20, 2007
January 7, 2008
January 7, 2008
January 14, 2008
January 28, 2008
February 22, 2008
March 5, 2008
February 4, 2008
March 10, 2008
March 5, 2008 February 26, 2008
March 14, 2008 March 17, 2008
March 18, 2008
March 19, 2008
March 18, 2008
March 18, 2008
The team was able to approximately follow the schedule set out in December. Construction began almost immediately after the build report was submitted in January. Construction of the frame was completed much earlier than anticipated. Testing of components was slightly delayed as the team had to wait for the arrival of the cameras, thrusters, and propellers. The frame was complete ahead of schedule; however, it was not cemented until just before the final assembly test. The components were assembled to the frame in time to complete the first assembly test. The first assembly test proved that the device works and the team passed inspection on time. Upon returning from this test, the device was modified slightly before further testing. The device was tested in the last week of March and first week of April to determine which design requirements were met.
PERFORMANCE
During testing several strengths and weaknesses of the ROV were identified. They are summarized below.
STRENGTHS
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The depth gauge makes controlling and maintaining a set depth easy and reliable. The operator is able to focus on inspecting while the depth remains constant. The depth gauge LCD screen outputs the desired and actual depth of the ROV. The slider on the joystick is used to input the desired depth. The slider restricts the depth input to within the normal operating range of 0 to 30 ft. This protects the ROV from increased pressure at greater depths.
The thruster circuit and thrusters are protected by fuses. If the thruster tries to draw more than 5 amps, for example if the propeller is entangled, the fuse will blow and the motor will be protected. Fuses are inexpensive and easy to replace and are available at local hardware stores.
It is not necessary to turn the ROV to inspect as the lateral thruster can be used to move along the side of the ship while the forward facing camera records a video of the ship’s side. When the ROV reaches the end the depth can be changed slightly and the ROV can move back along the ship using just the lateral thruster.
The thrusters are mounted on movable members that slide along the fixed frame. This allows for further expansion of the ROV systems to include a grappler, sensors or other equipment. The thrusters can be adjusted to change the balance of the ROV and keep it level during operation. The thrusters can be moved in order to fit large equipment onto the ROV.
It is easy for one person to operate the ROV as it is small and lightweight. Testing has proven that one person can launch the ROV, drive it using the video feedback, and pull it back out of the water. It has its own power supply and is easily transported.
The joystick is intuitive and makes steering the ROV simple. Other ROVs are controlled using multiple control devices or switches which can be confusing and require training to operate properly.
SWORDFISH can be operated with no previous experience or training.
WEAKNESSES
The primary weakness of SWORDFISH is the thrusters. They did not provide enough thrust to meet the design requirements for speed. Larger thrusters would provide more thrust and increase speed.
Increased speed, however, may make controlling the ROV more difficult.
The tether increases the drag forces on the ROV and decreases its speed. As the ROV travels to greater depths and further distances from the launch point, these drag forces increase. The foam on the ROV
26 was replaced with air tanks to increase durability; however, the foam was left on the tether to make the tether neutrally buoyant. It is unknown how long the foam will keep the tether neutrally buoyant.
The potentiometers in the joystick are inconsistent and often return unpredictable values. This causes unwanted triggering of the thrusters and the ROV can be seen driving with no one controlling the joystick. The yaw potentiometer is especially inconsistent.
The frame was designed to protect the internal components such as thrusters; however, the thrusters are still able to become entangled in objects in the water. This is due to the fact that the propellers are at the edge of the cage.
The ROV can easily be operated by one person, but requires two people to transport it. This is due to the fact that the tether cannot be removed from the ROV and both the tether and ROV must be carried together. As well, the tether is not long enough to operate in a wide area.
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RECOMMENDATIONS
In order to improve the ROV many design improvements could be made. These were not accomplished by the team due to time and cost restraints.
Three more thrusters could be purchased and mounted in parallel with the current horizontal and vertical thrusters to increase the thrust without significantly increasing the cost of the ROV. It is expected that this would increase the speed of the ROV enough to meet our design requirements. This would only increase the cost by less than $100. This would require three more propellers to be purchased, three shaft adapters to be manufactured and more T’s to be purchased to mount the thrusters to the frame. The thrusters would need to be wired to the existing tether and the existing five amp fuses three of the motor controllers would have to be replaced with ten amp fuses. As well, six five amp fuses would have to be added underwater and would require heat shrink protection. This is to prevent one thruster from drawing the full ten amps sent down the tether. It is expected that the increased thrust provided by the additional thrusters would only allow the ROV to just meet the design requirements. This is because drag forces increase with speed.
In order to further increase the speed of the ROV, much more expensive and powerful thrusters would need to be purchased. It is expected that they would also be larger and would therefore require a new mounting system and further additions to the ballast. The control system would have to be modified to accommodate these thrusters. It is expected that this would cost at least $1000. This is an order of magnitude more expensive than the previous recommendation and would only be necessary if a significant speed increase was required.
Recommendations for the tether include extending the length and changing the ballast. The camera and depth gauge tethers are 100 feet while the tether for the thrusters is only 50 feet. Increasing the tether to 100 feet would cost approximately $115. This would require the cables to be spliced together and waterproofed using heat shrink wrap. This would significantly improve the range of the ROV. Changing the tether ballast to another material such as rope buoys would increase the life and reliability of the tether ballast system. It is expected that this modification would cost under $200.
To decrease transportation problems associated with the longer tether, an underwater connection could be used to connect the tether to the ROV. This would make transporting the ROV and tether much easier as they can be separated. A suitable underwater connector would need to be found and installed properly to prevent leaks.
Purchasing a new joy stick would decrease the variation in the signals sent to the motor controllers. It is recommended that an industrial joystick should be purchased and would cost approximately $150.
A sheath should be constructed to enclose the propellers and protect them from obstacles such as netting and seaweed. This would protect both the propellers and any sensitive underwater structures.
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CONCLUSION
SWORDFISH is an inexpensive, reliable and innovative solution for underwater inspection. The thruster arrangement allows the ROV to sink to the required depth and move laterally through the water to efficiently inspect ship hulls. Two waterproof cameras allow for easy inspection of the sides and bottom of the ship with no risk of leaks. The ROV is easy to control with the depth gauge providing feedback to the control system to keep the ROV at the depth desired by the operator. The small frame constructed out of PVC pipe makes it easy to transport and launch the ROV at the desired location.
The chosen design is simple and able to efficiently complete the task of inspecting ship hulls. All of the components except for the control system are made for underwater use. The control system is located above the surface in a dry environment. This configuration increases reliability of the ROV and decreases the risk of leakage.
Testing was done in multiple stages. Individual components such as the cameras, propellers, and ballast system were tested in the tow tank and the completed ROV was tested in the pool tank and tower tanks at the department of oceanography. The final test was a sea trial in Chester Nova Scotia. These tests proved that 19 out of 22 of the design requirements were met.
The ROV was built for $2637 which is $445 less than our expected budget and $863 less than our design requirements. The project roughly followed the schedule set out in the first semester with all deliverables completed on time.
This project was very successful overall; however, there are a few components which could be improved. The depth gauge, lateral thruster, and frame flexibility are highlights of the design. The tether and thruster power are items that could be improved upon. Given more time and money the ROV would be able to meet all of the design requirements.
APPENDIX A – BALLAST CALCULATIONS
Table 8 shows the data collected during this test.
Table 8 - Ballast Test
Tank Width
Tank Length
28.0625 inches
120 inches
Initial Distance to Water’s Surface 10.54 inches
Final Distance to Water’s Surface 10.60 inches
Weight of ROV 3.22 kg
The following data shows the calculations.
Volume removed from the tank:
ππππ’ππ = (πΏππππ‘β)(ππππ‘β)(π₯π·πππ‘β)
ππππ’ππ = (120 πππβππ )(28.0625πππβππ )(0.06πππβππ )
ππππ’ππ = 218.4 ππ 3
ππππ’ππ = 3.58 πΏππ‘πππ
Required Ballast:
Density of water = 1kg/Liter
Buoyant Force = (3.58 Liters)(1 kg/Liter)
= 3.58 kg
Required Ballast = Buoyant Force - mass of ROV
= 3.58 – 3.22
= 0.36 kg
Table 9 - Ballast Test
Difference in Water Depth 0.06 inches
Volume of ROV 3.58 liters
Required Ballast 0.36 kg
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APPENDIX B – DETAILED BUDGET
30
APPENDIX C – TECHNICAL DRAWINGS
31
32
APPENDIX D – INSTRUCTION MANUAL
INSTRUCTIONS:
Failure to follow proper startup procedure can lead to equipment damage and personal injury.
SETUP
Read all instructions thoroughly before starting setup procedure
1.
Insert camera power plug into 12V battery. Plug camera cords into display screen. Ensure cameras are functioning properly.
2.
Plug 9V battery into microcontroller board.
3.
Turn depth gauge switch to ‘ON’
4.
Place SWORDFISH into water, keeping tether clear of propellers
5.
Attach red lead wire from control box to positive 12V battery terminal (red cap)
6.
Attach green lead wire from control box to the negative terminal (black cap)
CAUTION - THE MOTOR POWER LEADS MUST BE ATTACHED TO THE CORRECT POLARITY.
FAILURE TO ATTACH THESE LEADS CORRECTLY WILL SEVERELY DAMAGE THE ELECTRONICS. TO
TEST FOR CORRECT POLARITY, REMOVE ALL FUSES FROM THE MICROCONTROLLERS AND
BRIEFLY CONNECT POWER. IF THE GREEN LEDS LIGHT UP ON THE MOTOR CONTROLLERS,
REPLACE FUSES AND CONTINUE. IF THE GREEN LEDS DO NOT LIGHT UP, IMMEDIATELY SWITCH
POLARITY.
OPERATION
If setup has been properly completed, SWORDFISH will be ready to operate. Place the thin end of the joystick towards the operator. The switches on the top of the control stick should also face the operator, and the joystick should be comfortable to grasp.
Depth is set by using the slider on the base of the joystick. Moving the slider all the way clockwise around the base will set the depth to zero feet. Moving the slider all the way counterclockwise will set the ROV depth to ~30 feet.
The set depth and actual depth are reported on the LCD screen. SWORDFISH will automatically try to reach and maintain the depth set by the user.
To move the ROV in the horizontal directions (sway, surge, and yaw) move the joystick in the corresponding directions.
RECOVERY
33
When the ROV needs to be recovered from the water, drive the ROV as close to the person recovering
SWORDFISH as possible. Disconnect the battery from the control box before removing the ROV from the water. Manually remove the ROV from the water by pulling up the tether. Turn the depth gauge switch to ‘OFF’. Disconnect the 9V battery from the microcontroller board.
SOFTWARE INSTALLATION
The microcontroller requires a nine pin serial cable to update software. All software must be written in
PBASIC code, available from www.parallax.com
. The 9V power supply must be connected to the microcontroller when updating software. In the expected life of SWORFISH, the software should not require updating.
TROUBLESHOOTING:
Table 10 - Troubleshooting
Problem
Single thruster not responding
Solution
Disconnect power
Check fuse, replace if broken
Connect power and ensure light on motor controllers are on
Ensure microcontroller is powered on
Check connection to microcontroller
All thrusters are not responding
Ensure 9v battery connected to microcontroller is not low
Cameras are not responding Ensure that camera cables are connected properly to the view screen
Check fuse located at tip of cigarette lighter power plug
Ensure ROV is not deeper than 30 ft in the water column Depth gauge or joystick report values beyond 0-30 ft on LCD screen
If the ROV value is erroneous, ensure that depth gauge switch is in the
‘on’ position, and that the ROV is in the water
Thrusters trigger without input
If the SET value is erroneous, ensure that the depth slider is at least one millimeter away from either end of the slider slot on the joystick
Ensure that the joystick is centered on all three axis
