Thin Tethers

[AHarris]

I have noticed that the majority of ROVs use extremelly thin tethers, usually made with Cat5 cable or telephone cable. I have been told that for an ROV to function, the tether needs to be thick to keep resistance to a minimum otherwise you can't get enough power.
To create a ROV with a small tether, do you need to have on board batteries or is it something else?

[Rover]

Yes, ROVS with thin wire sized tethers usally have an onboard power source. Short tethers with cat 5 or telephone wire have been used to operate smaller ROVS with low power requirements, but the voltage drop over the length of the tether limits the power consumption.

Jim

[AHarris]

Thanks for the help, with my previous ROV I had this massive tether which had about eight wires running through it (and they where thick wires) which made it stupidly bulky.

[martinw]

This ROV has a very thin tether, hence the name, like a spider's web

http://www.oceaneering.com/advanced-tec ... spiderbot/

[derelicte]

that is neat, but how do you get power down a fiber optic cable?

[martinw]

It has on board lithium batteries (or did when I saw it last). The fibre has a strength member attached to it and is bailed out of the back of the ROV like a spiders web. Apparently one of the hard parts was getting the fibre and strength member to bail out of the back flat and not in a coil.

[Chas 100]

I have noticed that the majority of ROVs use extremelly thin tethers, usually made with Cat5 cable or telephone cable. I have been told that for an ROV to function, the tether needs to be thick to keep resistance to a minimum otherwise you can't get enough power.
To create a ROV with a small tether, do you need to have on board batteries or is it something else?

My tether is EnduroFlex 18 AWG 8 Conductor rated for 20 million cycles, very flexable and is only 5/16 OD. I run 24 volts at surface thru 150 foot tether and that gives me around 14.7 volts at rov figuring in 4 amps. I run the thrusters with PWM speed controls

[Asbjørn]

Thin tether doesn't necessarily mean low power. Power is a balance of current and voltage.
Say you have a cable that can handle 5A, at 12V you have 60W power. If you double the voltage you have 120W in the same thin wire. At 48V you have four times the power, 240W. You get the picture. The electronics in the ROV normaly run on 5 or 12V, so you'll need a power regulator.
You could also benefit from having a Lithium battery pack in the ROV to handle the peak power consumption, when the power drain is less than the tether can handle, it will charge the battery.

Voltage drop through the tether is less if you use A/C current, and the higher the voltage, the less power is lost. This is why the long distance electric grid uses 10,000V +.

[marcane]

This is actually wrong, DC carries 1.4 times more power at the same diameter cable. But its cheaper to make higher voltage AC than DC, so you have to calculate how much power you need and how big the resistance will be in the cable..

By the way, has anyone any link where they sell Cat5/6 tethers? I have batteries onboard and need Cat 5 tether..

Marcus

[aquajack]

I use cat 6. It works well because it has 4 pairs, 2 for drive 1 for up and doown and one for the manipulator