Have you thought about using multi-mode optical fiber cables for the tether? Either with a battery on the robot or in addition to the 2 wires you are using for power.
If you use an optical fiber for the data, you might be able to send only 12v (or 24v) instead of the 120v of power through the cable which should make it much safer.
The 120v (or 240v over here in Europe) seem to be a big safety issue if you take this project for research at universities or even schools. Electrocuting half a school class doesn't sound like good publicity.
Additional benefits would be the data rate and the cable length. At shorter distances (up to 100 meter) you could go as fast as 100 Gb Ethernet - not sure why you would need that, but 1 Gb Ethernet would be useful if you have more then one HD camera attached. With 1 Gb you could go up to 1000 meter in cable length.
An additional benefit would be you have a stronger tether if you use two cables (2 wires power + optical) in case something breaks.
Do you have a source of multi-mode optical fiber cabling and couplers in mind?
The stuff I use in my one-off experimental projects will be too expensive for this community (unless of course you get them from donations/auctions or the junk pile, etc).
No, not really.
A quick look on ebay and Amazon shows you get 30 meter of multi-mode cable for about $20, it probably isn't the lightest/best cable, but I guess for $20 it is worth a try. That is with plugs at the end. Longer bulk cable is cheaper, but you need the tools to add the connectors to it (or you need to know someone who can do it for you - that's nothing you would be doing daily)
The part I'm not sure about is how to get it to electrical from there. For above the water there are lots of options converters, switches, network cards, etc. If you can't find a used one somewhere a new one will probably cost $100 to $200 :-(
The biggest problem is finding something small, low power for the robot. A small switch with one optical port would be great, it would be possible to plug in network cameras.
But then with all that stuff on the robot, 12V to 24V might not be enough.
I guess this would need more research, but I still see 120/240V through a thin cable under water as a big problem in educational environments so any alternatives to that would be great.
I just posted some information on PoE (Power over ethernet) for the tether in the Onboard Electronics section that may be of interest to you.
Multimode fiber is not very expensive to buy in places that sell fiber, but it degrades faster than copper cable, unless is protected with very good water insulation.
The problem with circuitry for fiber signal communication is that is going to raise too much the price of the device.
You guys have hit the main issues perfectly. I looked into fiber fairly extensively early on. The advantages are higher bandwidth, lower density (closer to neutrally boyant), longer range, and the possibility for it to be extreamly thin and degrade naturally if lost underwater. The overwhelming disadvantage was finding a practical devise that could convert between fiber and a common electrical protocol (such as Ethernet or USB). Such devises do exist, but are generally large and expensive. The other issue is that re-terminating damaged tether would be very hard.. in fact, nearly impossible, and the tether would be very fragile. At the end of the day, a single twisted pair of stranded copper wire is almost as low profile, is very easy to come by, and can be used to communicate standard protocols with dozens of devises that are small and cheep.
Hi, these media converters are the perfect solution - very cheap (< 100$ both) and only over one single mode fiber (simplex): Cheap Media Converter
Hyphen- the converter you link to is the best fit I've seen yet! Please let me know if you pursue it.
Yes, I plan to use it with this camera setup (native H.264 encoded stream): USB HD Camera on BeagleBone
For some years I dreamed to fly a FPV plane over IP with such a camera setup - but sadly the transmission power (which is available) is not allowed here in Germany - so I must switch over to the ROV scene :o)