Is ethernet cable really that horrible?


#1

Hi,

I'm just curious about how horrible drag resistance *really* is using a standard ethernet cable is. I've heard from some other forum threads how the OpenROV has so much available power it "jumps out of the water...is there any way to tone down the power", and I've also seen / read how people are jumping through hoops to get bandwidth and power down and back to the ROV. Like tether length had to be restricted from 100M to 50M, framerate of camera can only be X, number of cameras can only be one, etc...

From what I understand a standard ethernet cable would solve both of these issues (obviously not a ton of voltage / amperage via power over ethernet), but the limiting factor is its cross section and stiffness. Ok, fair arguments about those limitations, but my question is who has *really* done the ethernet test in water and can whole heartedly say "Yea, it sucks, my ROV was being dragged all over the place from the currents." or "Yea, it sucks because I couldn't control my ROV the cable was so stiff?" I can understand the hypothetical situation of it not being ideal, but sometimes you just have to do the physical test.

I know about the tether decision tree (http://openrov.com/forum/topics/tether-decision-tree) and the only negative about standard 4 wire ethernet is "heavy and more rigid", has it been reevaluated recently?

Thanks.


#2

Marcus,

I think you're absolutely right about it being worth it to reevaluate that. I'd say that weight is the biggest factor with using Ethernet cable (compared to drag and stiffness) but also, we've been striving to use the thinnest, simplest tether we can as a design philosophy- especially as conditions get more challenging, or require more advanced maneuverability, having the most minimal tether possible majorly effects what the ROV can do. We know that a very thin tether is what we want ultimately, so we're trying to push the technology in that direction for a better long-term result.

Hopefully we'll be able to do some demonstration videos with different tethers in the near future, but if anyone out there wants to beat us too it, I think it would be very valuable to see that.

It's good to get fresh perspectives on this stuff every now and then- thank you Marcus.

Eric


#3

i think i also will start with an ethernet tether as a test.

just attach some floating devices about every 10 meters will do the job i guess


#4

I have 150 ft. of CAT 5e cable on mine. I am not using the ethernet to twisted pair adapters. I did by them and try them, but am having trouble staying connected. So I removed them, and added RJ45 connectors to the wire (so it is a normal network connection). I have tried connecting and running the unit with 100 meg/bit and 10 meg/bit switch. The only diffence with the speed is the undating of the video from the USB camera on my laptop. I still have problems with being disconnected when running the thruster motors at much speed. My OpenROV has enought power to pull the wire around in my test tank. Since I am only using 2 pair of the wire for the network connection, I am sending 13.5 volts down the other 2 pair. I am limiting the current to 0.7 amps. One pair for + voltage and one pair for ground. I left the unit on for 58 min. running lights and the vertical thruster on and off over the weekend in my test tank and then finally lost the connection. Using the horz. thursters I loose the connection in a couple of min.


#5

John, this is great data. We're planning to do some similar testing once we get our pool set up.

Thank you!

Eric


#7

Whats testing result?