KARR Kristian's Aquatic Roving Robot

Kristian Charboneau KARR Technical Report 12/2015

Contents Frame

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Water Tight Containers (WTC)

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Thrust

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Electronics

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Main Electronics Compartment

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Battery Compartment

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Camera

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Tether

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Operator Controls

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Frame KARR’s current frame is made out of 1/2” PVC pipe. Originally the frame was going to be made out of aluminum, however due to changing the water tight container design the original frame (already partially built) wasn’t the correct size. There were two main factors for using PVC instead of aluminum for the new frame. The first was design. By using PVC tube instead of square or flat aluminum (what I had easy access to) the frame itself could support the WTC’s without any other brackets or hardware. The second factor was cost, as PVC is much cheaper than aluminum. The PVC is painted with Rustoleum 2-in-1 (paint and primer) grey spray-paint. The frame’s overall shape is that of a simple box. This reduced complexity simplifies the amount of PVC joints and pipe necessary, reducing cost and build time. The upper portion (or main portion) of the frame houses the two water tight containers (WTC’s) and vertical motors. The lower portion of the frame acts as the base and supports the lateral motors and camera. After realizing the original frame wouldn’t work with the new WTC’s the main challenge in redesigning the frame was how to secure the WTC’s. The advantage with this frame design is that the frame itself supports the WTC’s without the need for separate brackets or supports. To support the WTC’s the center of the frame consists of four PVC tubes running front to back. Each tube is covered by heatshrink to prevent the frame from scratching the acrylic. Two heat-shrink covered hose clamps secure each WTC to the frame. This is a simple and inexpensive way to secure the WTC’s to the frame while providing easy access.

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Figure 1: Frame

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Water Tight Containers (WTC)

Figure 2: Water Tight Containers

KARR uses two 4 inch series watertight enclosures from bluerobotics.com. These are available in kit form, allowing you to pick from a number of end-cap styles and specify a custom tube length. I chose the standard length tube with a clear end-cap and 10-hole aluminum end cap for both enclosures. The design is similar to what I originally wanted to build, however, lacking access to a lathe I decided to try a different design for the WTC. This design would have been made out of aluminum, and instead of using end-caps that fit inside the tube, with o-rings for sealing, would have used a flat end-cap bolted to a flange on each end, with a gasket for sealing. A few weeks into the build (after I had started the original frame) I came across these Blue Robotics enclosures. Since they were similar to what I had wanted in the first place and probably more reliable than what I was working on, I decided to use these instead.

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Image

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from bluerobotics.com Image from bluerobotics.com Blue Robotics also sells a solution for cable entry. Called “cable penetrators”, these are basically a hollowed out bolt with an o-ring between the end-cap and the bolt head. Marine epoxy is used to seal the wire(s). These offer a semi-removable solution, as the bolt can easily be removed from the end-cap, however the wire can’t be easily removed from the bolt.

Thrust KARR is equipped with 8 thrusters for maximum maneuverability. The 4 horizontal thrusters utilize a vectored thrust configuration. This allows the rov to move forward/backward, left/right, and rotate left/right. The front two vertical motors are wired together and the back two vertical motors are wired together. This allows the rov to move up and down and pitch forward and backward, enhancing maneuverability. The thrusters are made from inexpensive bilge pumps. While less efficient and robust, these are much cheaper than off-the-shelf brushless rov thrusters. Each pump is rated for 12V and draws 3 to 5 amps during operation in the water. To make a bilge pump based thruster the front outer shell is removed, after which the impeller can be pried off and replaced with an RC boat or submarine propeller. Because the propeller’s set screw doesn’t seem to reliably hold the propeller on during operation, each propeller is super-glued to the output shaft. Conduit hangers are used to secure the modified bilge pump to

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Figure 3:

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the frame. Two conduit hangers are bolted end-to-end, one side holds the bilge pump, while the other side clamps onto the frame. Heat-shrink is used to prevent the metal hanger from scratching the frame.

Figure 4: Motor Due to the mechanical seal of the bilge pumps the reliability over time and at depths larger than in a swimming pool or small lake doesn’t seem to be that great. In that case a more robust thruster would be ideal. However, because this is a hobby project with limited funds and no need for operating in anything deeper than a small lake this thruster system should be adequate.

Electronics KARR uses an electronic control system for movement and data gathering. The two main parts of this system are the Propeller Chip micro-controller and a laptop running a python program. The Propeller Chip is responsible for managing the motor controllers and lights while the python program controls the entire system. The electronics are split up among two water tight containers. One container is referred to as the main electronics compartment, while the other is called the battery compartment. Each container has a “sled” inside that secures the various components. The main electronics use a sled made from wood, while the battery compartment uses a polycarbonate sled. The base of each sled consists of two PVC 7

Figure 5: half-rings super-glued to the bottom of each sled. These provide a base that conforms to the inside of the WTC and sits well on the ROV when the WTC has been removed for servicing. The next few sections detail how each portion of the system works.

Main Electronics Compartment The main electronics compartment contains the Propeller micro-controller, orientation sensor, lights, voltage regulators, and 4 motor controllers. The Propeller Chip receives commands from the host computer and generates servo signals for the motor controllers. The Propeller Chip is a 32-bit, 8-core micro-controller programed in an OOP like language called SPIN. This micro-controller was chosen because of my familiarity with the platform and its versatility. There is no dedicated hardware for things like PWM or serial interfaces, rather this is emulated with software. This allows for a very streamlined system to built, without any extraneous functionality taking up resources. The multicore-core design also allows for future expansion and more processing power for complicated tasks. The motor controller model used is the HB-25, from Parallax, Inc. These are overkill for this project, considering that each controller can handle up to 25 amps and the motors only draw 5 at most. However they were purchased with the intent of being used for later projects that may require more power. Right now they work well as an off-the-shelf solution for rapid prototyping. The only downside to these controllers is their size. Three 3-Watt leds are used for illumination, supplementing the camera’s built-in lights. A variable output step-down voltage regulator is used to power the leds. This allows for manual dimming using the regulator’s built in potentiometer that governs the voltage output. The LEDs are glued onto a piece of aluminum. This aluminum serves two purposes: secure the LEDs to the tray, and act as a heat-sink. Without a proper heat-sink the LEDs can overheat, causing damage to themselves

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Figure 6:

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or other components in the hull.

Battery Compartment The battery compartment houses the battery and related power circuitry. The battery used is a 3S 50C LiPo battery. This was chosen for it’s relatively low weight to energy-density ratio and current output characteristics. The battery’s balance cable is connected to a voltage monitor. This monitor produces a loud alarm when the battery voltage gets below a certain threshold, which helps prevent over-use of the battery. An automatic-reset automotive fuse is used to protect the system, especially the battery, in case of a short circuit. One of the challenges of building a water-tight system is how to conveniently turn that system on and off. To accomplish this KARR uses a magnetic switch system. An Adafruit push-button power switch breakout is used to monitor a magnetic switch. When a magnet is placed near the switch it triggers the breakout which energizes the coil on a 60 amp automotive relay. This relay then provides power to the rest of the system. This setup allows KARR to be turned on and off without opening up the water-tight container.

Camera KARR’s camera system consists of an off-the-shelf waterproof camera purchased from eBay. These cameras have a built in LED system and power/video tether. The available tether lengths vary, however this one has a 65 foot tether. The end of the tether has a DC barrel jack for connecting to a 12V power source and a yellow RCA cable for video. The rated resolution is 600 TV Lines. The image quality is ok, and there is a certain amount of noise, though it doesn’t make the video feed unusable. Overall it is a decent system, the only downside being the field of view is a bit too narrow.

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Figure 7: Camera

Tether

11 The tether is one of the most important parts of the ROV, as it

carries the video signal and control signals between the ROV and operator controls. Since KARR uses onboard power, there are only two cables in KARR’s tether: a 100ft CAT 5 cable and a 65ft video/power cable for the camera. The CAT 5 cable is for the USB over CAT extender. This connects the Propeller Chip and 9 DOF Orientation sensor to the operator’s computer. The camera cable is built into the camera and carries video and power for the camera and its built-in lights. The two cable are zip-tied together to help prevent the cables from becoming tangled. The usable tether length is limited to 65 feet by the camera cable.

Operator Controls The operator controls consist of a laptop, Logitech F310 gamepad, and portable tv. The laptop (running Linux or OS X, though Windows may work with a little modification) runs a python program, referred to as the “Operator Console”, responsible for taking the gamepad input, processing it, and sending movement data to the micro-controller. It uses PyGame to read the gamepad and PySerial to send data to the micro-controller. The main part of the program, ROV.py contains most of the logic. It does however rely on several external modules: error_handler.py, HID.py, Packet.py, and profiler.py. Error_handler manages non-critical error messages generated by the software, HID hides the complexity of getting input from the gamepad, Packet contains methods for generating and using communication packets for the micro-controller, and profiler contains a simple system for recording execution time for various parts of the program. The software is designed to be access from the command-line, however, due to its use of PyGame requires that a graphical session be running. Unfortunately this complicates is use with SSH. # Research This a collection of resources I have used in the development of KARR. Organized by catagory.

• Python • http://effbot.org/tkinterbook/tkinter-hello-tkinter.htm • http://stackoverflow.com/questions/14554582/installing-python-pygame-on-mac • http://scottlobdell.me/2014/02/capture-input-from-an-xbox-controller-in-python-on-mac-os-x/ • http://stackoverflow.com/questions/929103/convert-a-number-range-to-another-range-maintaining-ratio • http://www.ridgesolutions.ie/index.php/2013/02/22/raspberry-pi-restart-shutdown-your-pi-from-python-code/ • https://wiki.python.org/moin/UdpCommunication • http://stackoverflow.com/questions/15396628/sending-strings-between-to-python-scripts-using-subprocess-pipes • Propeller Chip

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• http://learn.pimoroni.com/tutorial/propeller-hat/propeller-ide-getting-started • https://www.parallax.com/sites/default/files/downloads/P8X32A-Web-PropellerManual-v1.2.pdf • Tutorials • http://docs.bluerobotics.com/watertight-enclosures/ • http://docs.bluerobotics.com/tutorials/cable-penetrator/ • http://docs.bluerobotics.com/vent/ • http://docs.bluerobotics.com/tutorials/cable-penetrator/#installation • http://docs.bluerobotics.com/tutorials/vacuum-test-plug/ • Other ROVs • http://www.homebuiltrovs.com • http://www.homebuiltrovs.com/rovforum/ • Various tech reports from NURC (h2orobots.org) and MATE (http://www.marinetech.org/rov_competition/), particularly these teams: Typewriter Repairmen, ASU Nasa Space Grant Robotics, and Jesuit Robotics. • Research relating to the tech report • http://pandoc.org/getting-started.html • http://tex.stackexchange.com/questions/139139/adding-headers-and-footers-using-pandoc • https://blog.ghost.org/markdown/ • http://kevin.deldycke.com/2012/01/how-to-generate-pdf-markdown/ • https://puppetlabs.com/blog/automated-ebook-generation-convert-markdown-epub-mobi-pandoc-kindlegen • https://github.com/PharkMillups/beautiful-docs • Misc • https://www.raspberrypi.org/forums/viewtopic.php?p=436687 • http://betterexplained.com/articles/aha-moments-when-learning-git/ • http://pyqt.sourceforge.net/Docs/PyQt5/ 13

• http://zetcode.com/gui/tkinter/ • https://pitchersduel.wordpress.com/2005/10/22/pygame-gui-comparison-2/ • http://www.tuxradar.com/content/code-project-build-ncurses-ui-python • http://stackoverflow.com/questions/27805077/display-io-stream-from-raspberry-pi-camera-as-video-in-pygame • http://pyui.sourceforge.net/ • http://electronics.stackexchange.com/questions/19669/algorithm-for-mixing-2-axis-analog-input-to-control-a-differentialmotor-drive • https://www.youtube.com/watch?v=gwLPoWLgPbk

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Quantity Item 1/2” PVC Pipe

Price/unit

Total Price

Source Notes Home Depot

Quantity Item 1 PropStick USB 1 60A SPST Relay 1 Resettable Fuse 9 DOF Fusion 1 Orientation Sensor 1 6” BreadBoard Magnetic Switch Push-button Power 1 Switch Breakout USB over CAT 5 1 Extenders

Price/unit $49.99

$16.99

$5.95

Total Price

Source

Notes

$49.99 Parallax, Inc Misc Misc Adafruit Industries $16.99 Radioshack Adafruit $5.95 Industries Amazon.com

From Previous Project

Mechanical Total: Electrical Total: Project Total:

$0.00 $72.93 $72.93