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Topic: Cable for submarine robot (Read 5563 times) previous topic - next topic

cr0sh


If it works I'll tell you,but I think it won't be before september-october.


A few things - cable related-wise:

1) Have you thought about how heavy 100 meters of cable will be - will your ROV have enough power to overcome the mass to move it?
2) Can you provide enough power down such a cable (taking into account resistance losses, etc) to move the entire mass of cable and ROV?
3) What about your cable management system? How are you going to pay out and pull back in that amount of cable (and keep it organized and untangled)?
4) Is the cable jacket certified to withstand the water pressure at that depth? What will you do to keep water out?
5) Most ethernet cable is made with solid-core wires - these don't stand up to repeated flexing. Multi-stranded wire ethernet cable does exist - but it ain't cheap. The problem is, you want something even more flexible than that ($$$$); such specialist cable does exist, but it makes the stranded stuff look inexpensive.

As far as keeping water out of the ROV itself:

I would look into using BLDC motors and an on-board battery pack; run the motors in the water - spray them with silicone lube before immersion, and do a complete tear-down and cleaning after the run. The battery pack should be sized to allow the ROV to move the mass of the tether/cable. Add floats to the tether to make it more bouyant; I can't tell you how to manage 100 meters of tether/cable, though. Some kind of winch and spool with slip-ring connections would be needed, most likely. On the interior of the ROV, you'll want to displace the air with oil (mineral oil would work well - it's non-conductive, so you can immerse your electronics in it, and it won't spoil).

That's about the limit of my knowledge of these things; most homebrew ROVs don't even get close to 100 meters in depth...
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

MichaelMeissner

I forgot to mention there was a kickstarter project for an open source underwater ROV recently.  You might check them out to see what the competition is doing, and possibly get some ideas.  So far, while they also plan to do 100' depth, they've only tested it to 20'. http://www.kickstarter.com/projects/openrov/openrov-the-open-source-underwater-robot?ref=card.

zoomkat

An option might be to keep all power on board and use a wireless router on the rov and a laptop on the surface with a wireless adapter. To connect the two use very thin coax/shielded type wire as a long antenna wire between the two.
Google forum search: Use Google Advanced Search and use Http://forum.arduino.cc/index in the "site or domain:" box.

giacomomarelli

I just saw the video of the OpenROV that MichaelMeissner linked (thank you!), it's a very interesting project. They obviously use battery to supply power to the ROV, I think I'll be better using rechargeable batteries too. Just need to be shure they supply enough current for the motor I'm using and all the other electronics.
For the communication they use a Ethernet to Single Twisted Pair Adapter. Wouldn't that be better? It looks more simple.
I connect the Arduino and the web cam to a router and the go trough the single twisted pair adapter.

To cr0sh: Thanks for the advices. We already went trough those problems and we still need to choose the motor for the Rov. (We where thinking to get this: http://it.rs-online.com/web/p/motori-cc/2483671/, it has 3500rpm max without load, 10Ncm torque. It can't go faster than 3000rpm because of the oil-seil we have.)
An ethernet cable isn't actually the best thing for a ROV, but if I use the adapter I'd just have 2 wires...



Far-seeker


I just saw the video of the OpenROV that MichaelMeissner linked (thank you!), it's a very interesting project. They obviously use battery to supply power to the ROV, I think I'll be better using rechargeable batteries too. Just need to be shure they supply enough current for the motor I'm using and all the other electronics.


Having on-board power is a good idea, IMHO.  It would allow the sub to react to a lose of communication (i.e. if the cable was damaged or snapped), like trying to resurface by emptying ballast tanks or turning on flashing emergency lights and emmiting a low frequency sound that could be tracked with a hydrophone.  Then you'd have a greater chance to get your sub back if something went wrong. 

zoomkat

Quote
For the communication they use a Ethernet to Single Twisted Pair Adapter.


I have my arduino ethernet shield connected to my router using cat3 four conductor telephone wire in a 10BaseT setup. I suggest you start by making test housings of your ROV and lowering them to the max depth you intend to encounter. Probably a good learning experience.
Google forum search: Use Google Advanced Search and use Http://forum.arduino.cc/index in the "site or domain:" box.

radman

100m is a long way down. It is going to be very dark, you are going to need power for lights. As was mentioned cable weight and management will be a problem. The pressure at 100m will 10 atmospheres above the surface pressure. Are you going to pressurise the interior of the ROV or are you hoping to keep a watertight seal against 10 atmospheres? How will you connect the cable to the ROV preventing water entering either the ROV or the cable sheaths?

You should try to experiment in a swimming pool first. In the first 10m of your dive the pressure will double halving the volume of any compressible spaces.

Riva

If you get a length of CAT-5 cable seal the ends and chuck it in a bath/pool is there enough air between the conductors and the sheath to make it buoyant? Or could you put neutrally buoyant floats every few meters on the cable to reduce the weight problem. You will still suffer inertial problems and be susceptible to water currents though.

query

#23
Aug 15, 2012, 01:47 pm Last Edit: Aug 15, 2012, 01:53 pm by query Reason: 1
Hi.
i have just started a similar project. Since posting last night, I did a bit more research and found a sight where a guy has built an ROV and has plans for another using ethernet cable (cat5) Check out "Building BOB" and thier project "Building Bart". Also check out Roborealm. Software to receive video and control an arduino or other serial interface.
My plan is to use 100m of cat5 and run the ROV at about 30m. the weight of the cable shouldn't be a problem as when it is in water in will "weigh" a lot less. I think a bouy on the surface with the cable feeding through it and running directly down to the ROV will be the way to go. then a few fishing net bouys to float the rest of the cable. I saw a picture of a winding drum for the cable that had a shaft coming out the center with 5 or 6 copper disks that had the wires from the pc stretched over the copper disks so you could wind the cable with out twisting one end. Ill see if I can find a pic. Found it. Attached below. also check out "homebuiltrovs"
Good luck.

MichaelMeissner

I was curious about the weight, and 500' (152m) of bulk cat5e/6 cable weighs about 14-16 pounds (6.3-7.5kg), including the box and spool it comes in.  Fiber of course weighs a lot less, but you do have to factor in the weight of digital to fiber conversion.

query

In physics, buoyancy (play /?b??.?nsi/) is an upward force exerted by a liquid, gas or other fluid, that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the column than at the top. This difference in pressure results in a net force that tends to accelerate an object upwards.

eg. 7mm dia cable x 100m is pi x 3.5 squared which is about 3.8 kg
So the cable would "weigh" 3.8 kg less in water.

could be wrong.
Also a little negitive weight in the cable help to keep the rov down...
There is a drag issue though, having to tow 100m of cable. Especially if there is any current...

Far-seeker


100m is a long way down. It is going to be very dark, you are going to need power for lights. As was mentioned cable weight and management will be a problem. The pressure at 100m will 10 atmospheres above the surface pressure. Are you going to pressurise the interior of the ROV or are you hoping to keep a watertight seal against 10 atmospheres? How will you connect the cable to the ROV preventing water entering either the ROV or the cable sheaths?

You should try to experiment in a swimming pool first. In the first 10m of your dive the pressure will double halving the volume of any compressible spaces.


Very good idea about starting at shallower depths. 

However, just because giacomomerilli wants a 100 m cable doesn't mean that is the sub's intended maximum depth.  100 m just means that the farthest potential straight-line distance between the sub and it's top-side controller in any direction (Max. Depth^2 + Max. Range^2 = Cable Length^2), not just straight down.  For example, 100 m of cable would allow a range of a little less than 99.5 m at a 10 m depth.

radman

Far-seeker, sorry  I did assume he wanted to go deep. If he stays shallow but at the end of the tether then the weight of the cable becomes as issue as the ROV will have to pull the cable.
I think the cable will describe one half of a catenary as in the cables of a suspension bridge?

Ordinary Cat5 cable is probably bouyant on the surface but it will lose bouancy at depth as it gets compressed. Sounds a bit complicated to work out. He could just swim/dive with a spring balance attached to some cable to find out the forces involved.

radman

I took a look at homebuiltrovs it is great to see video from working ROVs. It would have been good to see video of the ROVs operating. Note the target depth for some designs was 3m.

giacomomarelli

I just want the cable as long as possible, and I think 100 meters is plenty. That doesn't mean I want to go at 100 meters deep. I'm going to use it (after all the tests) in a lake and it means that 100 meters is the distance between the pc and the robot. I still don't know what the maximum depth will be.

I'm not working on it at the moment, I'll post more info as soon as I get back to work.

Thanks for all the advices and help!!!

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