PiSub - Need Some Thoughts

[bob_hoil]

I have recently obtained a Raspberry Pi B+, and have been debating what to do with it. After much though and some research I have decided on a possible project I am currently going to attempt, though I am still gathering resources to create it right now.

The plan is to create a sub, which will at first be primarily a camera and will allow me to test the hull of the sub and other functions. I plan on using the raspberry pi as the brains of the operation, with a Ethernet cable attaching the pi to land so I can view the video feed and send commands. I plan on powering it using a power bank battery, since I won't need overly long run times out of it.

My question is how feasible do you think this project is? I currently have a list of supplies and plans over at my website if anyone is interesting in taking a look.

http://www.bobhoil.com/pisub/

Thanks

[W. H. Heydt]

My biggest reservation would be the Ethernet cable. They are not generally designed for use in (let alone any distance under) water. The seal where it goes through the hull is likely to be a problem, as well. In addition, if the cable needs to be very long (and remember that Ethernet over UTP has a specified max cable length of 100 meters), the weight will be an issue and may adversely affect the subs stability.

I would suggest that you either find a communications method that will penetrate enough water for your intended depth and "cross range", either low frequency radio or sound are what immediately spring to mind, or you rethink what sort of cable you're using...perhaps an armored optical fiber.

I will admit that when I think "submarine" I am inclined to think of an untethered vehicle. Perhaps what you are planning to build is better characterized as a bathyscaphe or a "remotely piloted vehicle".

[BMS Doug]

My first suggestion would be to ensure that your design is watertight before you put the electronics in.


Fill the payload bay of the sub with kitchen roll (very water absorbent, should identify the slightest leak) then trial the hull + Ethernet cable with no electronic components on board and ensure that it remains dry internally.


I personally wouldn't try to add a camera dome to your sub, instead have the camera at one end of the pipe and use Perspex to give a viewing window (+ mastic to seal).

As W.H.Heydt says, a large concern would be leakage around any break in the hull, so the cable entry and the pumping mechanism / ballast tank are potential danger areas.

[DougieLawson]

My biggest reservation would be the Ethernet cable.

Would it make sense to strip the outer covering from the 8-core UTP cable, take just two of the pairs and crimp RJ45 plugs on to those (pins 1,2,3 & 6 only). That would reduce the bulk of the cable down to four thin insulated conductors and reduce the size of the holes needed in the waterproofing to a minimum.

[iinnovations]

I too echo the 'no wires' vote, but if you must, find an IP rating for immersion. Standard cable entries won't cut it. Maybe one of those big WiFi antennas? Would double as a locator beacon for you. A submersible is scary. What happens when battery dies? Bye bye.

[Ravenous]

My question is how feasible do you think this project is?

My guess is, very feasible. There have been a few posts about DIY submersibles, ROVs, AUVs etc. and I really think a simple one should be possible. (I don't remember if anyone actually came back with a practical one.) Make sure you have a good seal of course!

Once I did a little searching and found at least one DIY submersible forum (some quite impressive projects there) and plenty of enthusiasts.

There was at least one thread here about wireless/radio for an underwater vehicle too - I don't think anyone actually tried it. But, your cable version should work. You might want to look for those DIY sites to see if they know how to reliably seal a cable.

For inspiration, search for Heriot-Watt university's "Nessie" autonomous underwater vehicle. Various student groups have done a few reports as PDFs over the years. Expensive kit but great to look at!

EDIT: also have a look at :
http://www.raspberrypi.org/forums/viewt ... 37&t=19283

EDIT 2:
http://www.homebuiltrovs.com/rovforum

[BMS Doug]

Following one of the links in the thread Ravenous linked to, Here is a commercially available hobbiest ROV from which to take inspiration.

Featuring:
•Live HD video up to 1080p with wide-angle lens and tilt function
•LED lighting for low-light environments
•100-meter lightweight 2-wire tether

Design:
Very different from yours, Much more like having a Tupperware box with two 1" pipes suspended below it. These pipes contain the batteries, ensuring that the vehicle remains heavy side down.

I suspect that they have achieved neutral buoyancy, so they can raise or lower their ROV with the vertical propeller (mounted in the middle)

[davidcoton]

Would it make sense to strip the outer covering from the 8-core UTP cable, take just two of the pairs and crimp RJ45 plugs on to those (pins 1,2,3 & 6 only). That would reduce the bulk of the cable down to four thin insulated conductors and reduce the size of the holes needed in the waterproofing to a minimum.

No. Much easier to seal the single multicore cable, the outer sheath will be waterproof if it is not damaged. The good news is the cable is low voltage and will not be hazardous even if it gets damaged. Water getting to batteries is a different story (Very Bad News (TM)).

The IP rating for immersion is IP68, you can get a suitable gland but ensure it fits properly and is done up tight. Seal the gland in the hull with a good silicone sealant. You could have a single cable from the sub to above water on shore, or you could use IP68 rated RJ45 connectors (RS and probably other suppliers -- Google). Check that all seals are watertight beyond the maximum operating depth before adding electronics or batteries, since joints that are OK under shallow water may fail at the increased pressure of even moderate depth.

[BMS Doug]

The IP rating for immersion is IP68, you can get a suitable gland but ensure it fits properly and is done up tight. Seal the gland in the hull with a good silicone sealant. You could have a single cable from the sub to above water on shore, or you could use IP68 rated RJ45 connectors (RS and probably other suppliers -- Google). Check that all seals are watertight beyond the maximum operating depth before adding electronics or batteries, since joints that are OK under shallow water may fail at the increased pressure of even moderate depth.

Don't forget to add an external strain relief to the cable or the effort of pulling the cable along may be enough to dislodge your waterproofing. As the cable is also your emergency recovery method a solid strain relief is vital.

[davidcoton]

Don't forget to add an external strain relief to the cable or the effort of pulling the cable along may be enough to dislodge your waterproofing. As the cable is also your emergency recovery method a solid strain relief is vital.

Agreed. I would not use the cable alone for recovery. Tape a nylon line to the cable, attach firmly to the hull, and leave a loop of slack in the cable (long enough for any angle between line and sub).

[BMS Doug]

I would not use the cable alone for recovery. Tape a nylon line to the cable, attach firmly to the hull, and leave a loop of slack in the cable (long enough for any angle between line and sub).

That is a better plan.






I would go for the suggested IP68 connectors rather than the cable gland, snag the outer sheath on something sharp and your ROV will get wet inside.

[Ravenous]

I'd also suggest a water sensor of some sort mounted down in the lowest part of the hull - so you get an alert if a slow leak starts. (There are various simple water level detectors out there, which are supposed to trigger when wet.)

If a fast leak starts then you've got no chance, it's hair drier time!

[W. H. Heydt]

Water getting to batteries is a different story (Very Bad News (TM)).

Pretty much the worst version of that is salt water getting into lead acid batteries. That results is evolving Chlorine gas...and that will pretty much eliminate any risk of biological activity in that space.

[Ravenous]

I currently have a list of supplies and plans over at my website if anyone is interesting in taking a look.

http://www.bobhoil.com/pisub/

By the way I had a look, and I don't think the little peristaltic pump is suitable for depth. My reason is: subs probably need a very high pressure pump (or some other high pressure air/gas source) to push the water back out of the trim tanks again, to start resurfacing. Unless you have thrusters/aquaplanes to resurface instead, or some other method. This is another case for looking at the DIY subs people have built to see how they control buoyancy (I gather this could get complicated due to the compression of the hull, etc.)

(I suppose you could just let air out of a tank to sink straight to the bottom, then use a little CO2 cylinder to force water back out again to resurface...? That would be a straight down-then-up trip each time.)

[bob_hoil]

Hey everyone, thanks for all the great replies and input! I'm at work typing this from my phone so I will go into more details when I get home.

First I plan on making the hull and testing it with the cable attached before placing electronics inside. I have a couple questions also for the cable.

Will a cable rated for direct bury be good enough for low depths not over let's say 30'?

I was also looking into using co2 for forcing the water out too. Was hoping for a solution that would allow Mr to freely adjust depth. I will do some research on this area.

For the hull I was going to try the end caps that screw into the pvc. That would allow for easier matienence if it is water proof enough under any pressure.

I will post more later, I'm doing some research once I get home.

[aTao]

Use STP cable. UTP might not work quite right when it has a screen (the water) also you can continuously measure the resistance of the cable screen to the water, this will give you quick indication of a leak. You could place a bare wire in the bilges and connect that to the screen as well.

[W. H. Heydt]

Will a cable rated for direct bury be good enough for low depths not over let's say 30'?

No. 30' of water will exert 15psi trying to push the water into *everything*. Also...assume that your sub will at some point, be likely to wind up on the bottom of whatever body of water you are using it in, so plan a safety margin around the max depth. (Note that doing that is impractical for most budgets if that body of water is an ocean. But some bodies of water are surprisingly deep. There is 350 feet of water under the middle of the Golden Gate Bridge, and Lake Tahoe--surface elevation over 6000 feet--is about 1600 feet deep.)

[bob_hoil]

Will a cable rated for direct bury be good enough for low depths not over let's say 30'?

No. 30' of water will exert 15psi trying to push the water into *everything*. Also...assume that your sub will at some point, be likely to wind up on the bottom of whatever body of water you are using it in, so plan a safety margin around the max depth. (Note that doing that is impractical for most budgets if that body of water is an ocean. But some bodies of water are surprisingly deep. There is 350 feet of water under the middle of the Golden Gate Bridge, and Lake Tahoe--surface elevation over 6000 feet--is about 1600 feet deep.)

Okay thanks for that tip I will look into getting some correct cable then.

I have been thinking if I have space within the interior I may add some cross bracing to help it keep its shape better under pressure.

[cyrano]

I've been testing an Otterbox in a pond nearby for months now. When I first dropped it into the pond, the DIY gasket for the UTP started leaking within days. The second one hasn't failed since june last year. There's just a temp/humidity unit in the Otterbox, and power is supplied over the utp cable. No RPi just yet.

Someone in Australia did depth tests and found the Otterboxes to be reliable up to -100 m. My aspirations don't go that deep

Ethernet worked fine for simutaneous motor control and video feedback up to 100 m cable. And unshielded did better under water, despite physical compression. I suppose cable capacity with the shielded cable was too much for that length. Haven't tested longer cables. I'm still on the lookout for heavy duty armored UTP but these are rather expensive and heavy...

I see no way to make a reliable wireless connection under water. Wifi works upto maybe 1 m depth and even then it's not reliable.

I aim to go with neutral buoyancy and a small screw driven cylindrical expansion tank. I haven't even started a design for that, however.

[bob_hoil]

Thanks for sharing your experiences! Will definitely help me with making decisions on what to use exactly for my sub. I am currently working on whipping up a test sub hull right now. I already have a few parts but have to make another run to the hardware store and pick up a few more essentials before it is ready for any sort of testing.

One question I have is attaching the pieces of pipe together such as the end caps. What would be a good glue or type of rubber cement for this job? The usual kind used with pvc that I can get locally?

[Mandrewpi]

Don't forget water clarity you might not be able to see anything in murky water

[W. H. Heydt]

I see no way to make a reliable wireless connection under water. Wifi works upto maybe 1 m depth and even then it's not reliable.

I'm not surprised. That's why I earlier suggested using low frequency as an option. The USNavy uses extremely low frequency (ELF) for long range communications with submarines. Not much bandwidth, though.

[BMS Doug]

One question I have is attaching the pieces of pipe together such as the end caps. What would be a good glue or type of rubber cement for this job? The usual kind used with pvc that I can get locally?

The standard glue for these pipes welds the plastic of the end cap to the pipe, once they are together it will be very difficult to separate them (which is what makes such a good watertight seal). you could have this glue holding one end shut but if you use it on both ends you will have to cut the pipe to re-open it.

A silicone bathroom sealant may not be able to cope with the required pressure.

[Ravenous]

Let's face it waterproof camera housings exist for reasonable depths...

I thought your piece of pipe, but with a thick piece of acrylic or glass stuck over the end, then sealed with bathrom sealant, should work for a few metres at least... the camera faces the acrylic window.

It would take ages to apply and take off though. Not very practical but for a quick and dirty test...

[bob_hoil]

Okay so I am going to begin assembling a test hull, and will be placing it underwater for a length of time for testing.

I am working on creating something so I will be able to slide out the components also such as the raspberry pi. This will either be a cage with the components mounted to it with the diameter of the inside of the sub so it slides out, or a sort of arc that will slide in and out of the bottom of the sub. I will have to disconnect components that are not removable, but will allow for me to work on the sub more easily.

I will post pictures here and up on my site once I get some assembly done. These tests will be going on without an electronic components inside of course.