Gerenal questions about tethers


#1

Hello,

I am one of the people at http://hackerspace.gr that took interest to the OpenROV project, and have some first questions for you, if you don't mind :)

a) What is the usual (expected?) length of a tether used for ROVs?

b) Is the tether used for data, power, or (as I saw in another thread) both?

c) What is the typical cost of tether?


#2

Tethers will vary by application. Keeping this mind:

a) tether length is dependent on operating depth. For my projects, I've used ~100 ft tethers. For ethernet (CAT5 cabling) based systems, the max length is 100 meters / 328 feet before requiring active components to repeat the signal, at least per spec.

b) tethers will often carry power and data. the power requirement can be avoided if you use onboard batteries. Personally, I keep batteries on the surface; therefore, my tethers will have a power line running down as well.

c) when using an ethernet cable and "lamp cord" style power cords, the cost of a tether is pretty cheap. The expensive component is when you come to the sheathing. The expandable braided sleeving can be expensive when you buy it in great lengths. The braided rope for sleeving is another option that I've seen that seems to work well.


#3

Are there ANY examples of tetherless ROVs?


#4

Jinxcat,

I read the notes from your hackerspace meeting and I'm really excited to see that people are talking about this! Greg had a great reply which I think gets most of the info you need across, but I can also say that for OpenROV, I've been picturing a 100m tether length using very thin and low cost wires like this which are light enough to not need to worry about adding buoyancy compensation and are only a few cents per meter. I like being able to send both power and data over the tether (power can be sent at high voltage allowing lower current and thinner wire), however OpenROV can carry 8 "C" sized batteries, which at around 5000mAh each, would power the ROV for an hour or so.

It seems like you guys already explored a lot of the options for wireless, but certainly low frequencies can be used for low bandwidth communications as well as acoustic technologies. David and I are meeting with Pierros today, so perhaps some more thoughts will come together this afternoon.

I'm also very interested in using low cost acoustics for location finding- I wonder if it would be possible to build a a small, simple, acoustic beacon (maybe using a piezo element) that goes on the ROV that can be heard from a series of similarly simple underwater microphones which could trilaterate the ROVs placement in 3D. Just a thoght.

I'm excited about this discussion, and I look forward to seeing where it goes!


#5

Glad to see the pad got your attention :)

We are exploring the tetherless model for a lot of reasons... of course, it takes power consumption into a whole different level. When you are saying the batteries can power the ROV for an hour or so, we really need to get some hard facts on power consumption of components, in regards both to engines and all the electronics.

From what we discussed, VLF needs *very* lengthy antennas, and I presume it also requires more power. Hence, we moved to "higher" ground :)

What I would be very grateful for would be if there is any previous work done on tetherless designs and more importantly, if there are any resources (people?) regarding marine accoustics.


#6

Without a tether you have to spend LOTS of money on acoustic modems to get even poor quality video. For this class of ROV I would say no tether = no video.


#7

Agreed