The Typewriter Repairmen
The Typewriter Repairmen
Adult team
(picture of ROV)
Members:
Jim Forbes
David Forbes
Steven Forbes
Janet Forbes
Team Website:
Record of changes (delete for final version)
0.1 / May 3, 2009 / Initial version, format
0.2 / May 8, 2009 / First Jim revision
0.3 / May 9, 2009 / Added strategy, etc
Table of Contents
Section Page
Mission Strategy
Design Revisions
Techniques Learned......
Preliminary Results/Testing
Summary
References
List of Figures
List of Tables
Appendices
Abstract
The Typewriter Repairmen robotics team designed and built an underwater Remotely Operated Vehicle for use in the 2009 National Underwater Robotics Challenge. The team analyzed the mission requirements, and designed a relatively small, robust, robot with all the necessary degrees of movement, a versatile vision system, a simple manipulator, and limited instrumentation. Cost, construction methods, and materials availability factors drove many of the design features. Reliability and ease of repair were incorporated into the design. Team members used commercial software tools to model the mechanical and electronics designs, and learned new building techniques.
The Typewriter Repairmen
Design Rationale
The team used a systems engineering (SE) approach to the design and development of a remotely operated vehicle (ROV). As part of the SE process, the team identified the requirements and design constraints from the given competition rules, developed alternative concepts and selected the most appropriate via a tradeoff analysis. The team designed and developed the system, tested the system and redesigned as necessary to optimize it. The SE approach ensured the management/traceability of requirements. The team developed a tradeoff matrix to determine the mission prioritization and design function emphasis.
(insert trade off matrix)
Following determination of mission priorities, the team “brainstormed” various design solutions to the problem. This design step identified alternative methods to meet the overall requirements and emphasize the mission prioritization. The following describes the selected design and its characteristics.
The Robot: A steel strap frame with heavy steel bottom rails that holds a large diameter polycarbonate tube laterally at front-top for the cameras/electronics, and two 120mm diameter, 300mm long buoyancy tubes across the sides at top. Buoyancy adjustment is made on the surface with petcock valves
Polycarbonate tube: The tube is 3mm wall thickness, 150mm diameter and 250mm long. Two 12mm thick, turned aluminum end plugs with O-rings and flanges keep water out and provide mounting for electronics and connectors.
Motors: Four bilge pump motors with 75mm diameter ducted propellers for up/down (1), forward/reverse/rotate (2) and sideways (1) movement. Each motor has a bi-directional speed controller mounted in the tube that is powered by the surface 48V battery pack.
Manipulator: One motor to squeeze and release two 50mm square vertical paddles. Located about 150mm in front of robot, flush with bottom of rail surface.
Cameras: Two Radio Shack security cameras with original lenses, mounted 100mm apart on centers for stereovision. The cameras and lights are mounted in a sectioned boat that can tilt under servo control over a 90degree range, from straight ahead to straight down.
Lights: One 12V 20 watt flood for distance driving, four 3W LED lights for close work.
Motor controllers: Each motor controller has a PWM input that commands speed and direction in a linear way.
Batteries: On the surface, four 12V 12AH AGM sealed lead-acid wired in series to make 48V. A 15amp fuse protects the wiring, and limits total power to 750watts, well below the 1700 maximum allowed.
Solar charger: A 75W solar panel will be used to charge the four batteries in parallel during the daytime event. .
Tether: Two Ethernet cables and one zip cord braided together with a 19mm foam flotation strip. Total length is 22m, floating length 18m.
Surface control box system using two Vex transmitters: One will use joystick for forward/reverse/rotate and the other for up/down and sideways. The other Vex transmitter will be used for camera manipulation, one joystick for camera tilt up/down, the other for manipulator squeeze/release, and buttons for floodlight on/off. Ability to add up to 3 extra control functions as needed.
Telemetry: Pressure gage with 3 digit DVM readout, audio recorder, and separate audio output signal, two NTSC video outputs, temperature probe with 3digit DVM readout.
LED power: There are two series strings of 2 LEDs each. A 6.8 ohm dropping resistor matches them to 12V.
Motors. Johnson Motors makes the motor in the Rule pump. The shaft seal is a rubber disc that rubs against the motor housing, providing a large surface area with low force. The motor wires are molded into a rubber boot that fits in an oblong hole in the housing. The housing pieces are thermally welded. The motor itself is a standard brushed type with a noise suppressor on the commutator.
Mission Strategy
The mission will be completed in several steps.
First trip:
- Bring basket down, drop off near glacier
- Free ROV
- Turn on lights
- Find and identify anchor, mooring chain, and volcanic vent
- Rescue science package, take it to surface
Second trip:
- Bring new science package down
- Deliver new science package
- Collect core and biological samples and put in basket
- Surface with basket
Third trip:
- Enter tunnel
- Find and measure depth mark
- Retrieve core sample, surface
Fourth trip (if time allows):
- Free anchor
- Attach mooring chain
- Measure volcanic temperature and record sound
- Surface
Design Revisions
Techniques Learned
- O ring seals. O ring seals offer many advantages including simplicity, re-useability, and high pressure resistance. The team researched O rings by reviewing manufacturers literature. Using this information, they designed the end plugs for the SCULL, designed the correct groove width and depth to achieve optimum O ring compression, and taper of the opening in the polycarbonate tube. They also researched different types of O ring materials, and selected EPDM as the most appropriate for use in water. Testing showed that proper O ring lubrication is important for ease of assembly, and that disassembly is tricky because of the large size of the tube. Disassembly screw holes were added to solve this problem.
2.
Preliminary Results/Testing
The team made a camera and light testing rig. It has two 80mm polycarbonate domes glued to two ABS Tees, which are glued on the end of a long piece of smaller ABS pipe. One tee holds the test subject camera, the other tee holds the light. Tested the Radio Shack camera and 20W MR16 flood light in a swimming pool at night. Also tested a 3 Watt LED.
Thruster testing. The team made a simple Bollard Pull testing setup for use in a bathtub or utility sink. The test thruster is clamped to the bottom of a pivoting arm, and a fish scale is used to measure the pull at the top of the arm. The test setup includes an analog ammeter to approximate the motor’s current draw. The arm has a two to one ratio, so the scale readings need to be factored.
Tests were performed with various motors, propellers, and with and without ducts/nozzles. The most efficient performance was from the Rule 1100 GPH motor driving a 75mm brass model boat propeller, with a nozzle.
Camera Boat Testing. We have been concerned about glare from the lights, affecting the cameras. A quick experiment was completed to see how much of a problem it would be, by placing the light/camera assembly into the tube, and turning on the light and camera, in a dark room. This does not show what will happen with the LEDs on, or with the tube in water, but does seem to indicate that it will not be as serious a problem. With the mount assembly about two millimeters from the tube, there is very little glare. With it flush, there is none, and with it pulled away further than two mm, glare does become noticeable. Shown is the setup with the planned spacing, and a photo of the display showing that glare is negligible.
Summary
References
Power Sonic “Sealed Lead-Acid Batteries Technical Manual”
Parker O Ring Handbook, ORD 5700
Motorola Senseon MPX4250/D Datasheet, Integrated Silicon Pressure Sensor
MK SLA Battery Datasheet
Bulgin Standard Buccaneer IP69 Connector Datasheet
Team Website:
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