Electronic – Signal and power cable over long distances

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I am working on a hobby project where I am trying to build a ROV (Remotely Operated Vehicle) for underwater use. The ROV will have some electronic parts, that are powered with 12v and 5v, that I have not selected yet. (Brushless DC motors, servos, LED lights, video camera, microcontroller,…)

For mobility I am concidering to use one or more 12v batteries (car bat. or lead bat.) on land and a laptop with an microcontroller for controll.

So I want to use a cable that has 4 cores, two for power and two for data.

I would like to have at least 100 meters of cable, but a range from 200 to 300 meters or even longer would be much better. Is this possible ?

What cable should I choose (low cost is priority) ?
Whats about the voltage loss over distance ?
Is it better (and safe) to invert the 12v DC from the battery(s) to AC and transform back into DC at the ROV ?
And what would be the best way to design the data transmission ?

The data that comes from my laptop to the ROV will be mainly control signals that a microcontroller at the ROV will interpret. The data that is coming from the ROV back to the laptop, is video and sensor data.

If the data (mainly the video data) transfer would be a problem, I was thinking of only using a cable for power but using rf communication for sensor data, control and live video. I don't want to mess with underwater low frequency transmission (since I think this is way to complicated) so I was thinking of using an antenna that floats on the water surface. (Connected to the ROV over a fixed length cable, that would limit me in operation depth)

I am happy to hear all of your ideas and advises.

Thanks.

Best Answer

Real ROVs often run anything up to 1,000V or so at the head end of the cable (Nothing like that when it gets to the vehicle under load of course), sending significant power down to the vehicle is problematic.

The other gotcha is that you really want the cable to be neutral buoyancy overall at the vehicle, and that is a lot harder then it sounds if you have lots of heavy copper in there.

If I was doing it I would put the batteries (Lead acid is good) on the vehicle, not at the surface (No need for Lithium, that is weight efficient, but you need ballast anyway to make the thing submerge), and would look seriously at fibre for the data link (Small switch with an SFP at each end, 300M is easy, and a 300M drum of single pair glass is not exactly expensive in the scheme of underwater electronics, a very quick search implies ~£150 which is not bad for a 300M gigabit speed link).

Now unless you have played with underwater doings a LOT you will discover very quickly that keeping water out is HARD, forget 300M, just doing it at snorkel depths will give you fits, I forsee lots of PUR resins and weird issues with osmosis in your future.