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Nøkken 3 - ROV (Underwater Exploration Vehicle)
Nøkken 3 - ROV (Underwater Exploration Vehicle)

Nøkken is a supernatural being that lives in small inland waters and is a shapeshifter. It tries to trick people into drowning, often by changing shape into a horse, which when mounted would charge into a lake and take you to the bottom with it. I've never liked water, and a certain illustration by Theodor Kittelsen came to mind when I saw the LEDs glowing.

Nøkken

30th July 2012

This project builds upon my experiences with Nøkken mark I and II, which were able to provide some footage but either leaked in shallow water or failed outright. In the first iteration of Nøkken the battery was inboard and a thin tether was used, motors and LED power was sent through homemade feedthroughs. This worked quite well, but there were leaks in the feedthroughs, the lid used to access the battery, and in a crack which developed after only being submerged two meters. This showed me that I could not rely on my homemade wire feedthrough, nor on the body of the vessel itself. I decided to move the battery on land, and pot the entire setup in wax. This would allow me to completely seal the unit once, and without a compressible substance inside of the ROV there would be no place for water to leak into. This would have worked alright, had the wax actually provided a good seal, rather than shrink and crack. With the small air pockets left by the wax, water was able to leak into the feedthroughs once again, wreaking havoc on the electronics. Not only that, but the new umbilical cord required 14AWG wire to send 12V at up to 10A down to the ROV. The cord was so heavy the motors were unable to move the ROV, and instead it had to be dragged about. Taking wisdom from this, I had a few new ideas for the next iteration of Nøkken:

* Use prefabricated cable/connector for ALL wire feedthroughs.
* Use a single, thin, umbilical cord
* Use a redundant waterproof camera, along with a navigation camera
* Pot with something other than wax

The first thing I did was fix the tether problem. The cat5 cable I used to begin with worked flawlessly and was easy to procure. Since I only use four of the eight wires in the cable for video and communications, it was an obvious choice to use the other half of the cable to send power down to the ROV. The problem is that the loop resistance is 24 Ohms with the 30 meter cat5e cable I had, with doubled up pairs. The ROV operates at 12V and requires 10A when all motors are running and the LEDs are at maximum power. The solution is to use a high voltage across the cable, which in my case was 130V DC. This was done using a run-of-the-mill 12V-to-120V 150W inverter. At the ROV a switch mode LED power supply intended for mains operation was used to step down the voltage again. This was cheaper, better and faster than trying to design something myself, and it works perfectly. The video signal did not like having a noisy power line as a neighbor in the umbilical cord, a problem I fixed by full-wave rectifying the output of the inverter. Since the cheap inverter used a squarewave approximation of a sine-wave, the rectified and filtered result was near perfect DC. Like with the previous ROVs, the video signal is simply sent raw through a twisted pair in the Ethernet cable, and converted by an EasyCap USB device. The software I use for this is VirtualDub. Communications are provided by a FTDI USB UART with RS-485 converter, also sent down a twisted pair in the Ethernet cable.



A GoPro Hero HD2 was chosen as the redundant camera. The reason for choosing it was the standard issue waterproof housing which is rated down to 60 meters, small form factor, and heavy marketing campaign. (Seriously, I see these cameras in the strangest places.) This camera will be set to record to an internal memory card, and will record everything regardless of whether the ROV itself is controllable. The low-light performance isn't quite what I had hoped for, so a redundant lighting system may have been a good idea as well. It turns out the stock housing distorts the image underwater, so for the GoPro to actually be useful I need to buy a new diving housing.


The ROV housing itself was constructed using a sheet of plexiglas, which was welded first using acetone to hold the pieces in place, then glued later with epoxy. The potting substance used this time is laminating epoxy, commonly used to repair fiberglass constructs in marine applications. It's very expensive, but resilient and easy to use thanks to it's low viscosity. I made the mistake of not minimizing the potting volume before I starting pouring the epoxy. I had purchased 1,8 liters of resin thinking it would be more than enough, but I quickly saw that was not the case. With the insane cost of the stuff I was not willing to buy another batch, so I quickly chopped up some blocks of wood which I threw into the ROV shell. I'm not sure if the resin is simply exothermic while hardening, or if it reacts with wood, but either way this resulted in a foamy mass forming around the wood blocks. I don't think this will be a problem, but it doesn't look good. Had I known this before hand I would have used sand as fill mass, since it is chemically inert and inherently waterproof. "Standard Buccaneer" connectors from Bulgin were used for all connections on the ROV. This includes the motors which are permanently fastened. These connectors are waterproof down to 100 meters for 12 hours if assembled correctly. The LEDs were mounted inside of the ROV to save on connectors and the added complexity of waterproofing them.

 

As I mentioned at the end of the mark I and II article, I had problems with Microsoft's .NET update. For the sake of reliability I simplified the PC software and ROV firmware so only one-way communication is possible. The ROV will now only receive motor control and LED brightness packets. Two prefabricated L298 motor controller boards are used to control the motors. Any similar H-brigde motor controller unit can be controlled by the ATmega8. The schematic for the control board, firmware for ATmega8 and corresponding windows software can be downloaded here.

First Dive!

I took the ROV out to the coast again this year to try it out. I dare say it was a great success, even without HD video from the GoPro.



Did this project whet your appetite for ROVs? If so check out these links, and you'll be building one in no time.

Links:

Homebuilt ROVs
Jason Rollette's ROV
Submarineboat.com
ROV submersible (PVC) on Instructables
Underwater-ROV on Instructables
Swedish DIY ROV
Remotely Operated Vehicle T-bot
Engineering Projects - Remote Operated Vehicle (ROV)
German ROV (pretty proffesional)
The Sea Lion
JAWSS
Tiny German model ROVs, maybe for an aquarium
XKCD's ROV





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This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.

Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.