Homebuilt ROVs Forum

Hi everyone!

I was absent from the forum for a while due to other obligations.
Yet, this summer I managed to make a few dives with the ROV and I also managed to destroy it 2 or 3 times.
Anyway, I learned a lot through these dives about the equipment and its behavior underwater.

After our initial testing to 14-15m, (https://www.youtube.com/watch?v=KAT13i_dbgI) and after re-sealing the fuses in the battery box, I was pretty sure that all will be OK at least to 20m.
So, on our first dive at the end of July we had a blown up buoy at 17m! (See photo). The buoy was made of hydraulic PVC pipe (Ø75mm – 6Atm). Obviously the 6Atm pressure must be applied from the inside of the pipe, not the outside!

The next day I made a new buoy for 10Atm which made it through the summer and reached at 21m depth without any explosions or leaks.

After that, we made another dive aiming at 20m. At 9m the U/D motor failed. After a little search, I discovered that the DPDT relays were not responding to the orders of the switch. I concluded that they had been burned since the load (approx. 5A at sea surface) was close to their capacity (8A). So, I thought, at 9m depth, the loading is higher and the relays failed.
I decided to make a new relay control and dive again. This time I followed a different wiring with 2 SPDT relays (instead of 2 DPDT mini-relays) for each motor (http://www.robmeyerproductions.com/bows.html)[/url] that were much stronger (30A) and readily available at my home town (also quite cheap 3.5Euros each).
After some days, I had prepared the new control. It worked fine until I pot it (in liquid glass). When the liquid glass cooled, I found out that the left motor wasn’t responding!. The relays though were responding (you could hear the characteristic ‘click’ when the relay switched position). I had to go for some dives anyway, and I needed the ROV for its cameras so I took it along even without one motor.
We tried it at 21m, and at this depth the bottom camera failed (something about the wiring) and the battery made a short-circuit burning the wires, and most probably the relays. All in one dive!

I was so disappointed after all these failures that I thought to cut the tether and drop the thing to Marianna Trench.
Yet, after 6 weeks, I can see things more clearly (I still don’t want to touch the ROV) and I have the courage to share the experience.
There are some positive things after all these (failed) trials:
1) The bildge pump motors (Johnson 1600 gph) worked fine after 2 dives below 15m (one at 21 and one at 19).
2) The 10Atm PVC buoy held at 21m.
3) The motors, with PC cooling fans as props (7 blade – Ø70mm) are quite strong in pushing down the ROV with a speed of approx. 18-20m/min (I have no records about the Upward speed).

I also decided to change the frame (copper pipe) because it is too big for my needs and too heavy (6.5 kg). I’m thinking of making a new one either of Polypropylene (http://www.borealisgroup.com/pdf/litera ... _10_BB.pdf) pipes (I can’t find PVC pipes of small diameter in Greece) or Aluminium.

My major problem is the sealing of the battery. I’m thinking of using an Otterbox (if I find a small enough battery that fits to an otter box then I might use one.) Has anyone really tried it to its limits (30m)?. How can I seal the holes that I’ll open for the wires?
Any other suggestions would be most welcome!

Thanks Triton for good story and video. Battery Box: Can you obtain 100mm PVC pipe of 10mm or greater wall thickness in your country? Over here it is known as PN 18 and in the US as "schedule 80" I believe. Recently a plumber friend gave me a short length of the PN18 about 100mm dia (cannot buy short lengths from plumbing supplies). I think it is commonly used for water mains in the street. Consider getting someone with a lathe to machine an O-ring groove in each end, then make your own caps. If you can do a bit of fibreglassing then maybe consider making your own pressure-hulls. 3 layers, fibreglass on the inside, microballoons/epoxy as a core (syntactic foam) then fibreglass on the outside. Very strong and very light.
Wire entry: You may have read this one http://www.homebuiltrovs.com/howtosealingwireexits.html beware that water can run between the individual strands of wire if using stranded cable. A search of the site search.php?keywords=potting+epoxy+wires ... mit=Search

Thank you Rossrov for your suggestions.
The ideal solution about battery & relay waterproof box is something that can be re-opened easily for charging the battery and replacing a burnt relay etc.
If I understand correctly either the PVC tube (I don't know if I can find such, I'll make a search) or the fiberglass solution, will be fixed. Am I right?
I may go to a someone with a lathe to check if an aluminium box with o-ring and screw-cap can be made (and check also the cost).

I'm still thinking about it....

Hi Triton,

What type of batteries are you using? The reason I ask is if you use lithium polymer you could oil fill you box/tube they're housed in to make it do deeper.

I don't know your design in any detail but if the relays are in the same box you could also drill holes in the relay housing and let them fill with oil also.

Just a thought

Martin

Personally I would avoid threaded caps where it is critical to keep water out. Bolted or clamped is the way to go. Not sure what you meant by "fixed" regarding the PVC and fibreglass. Do you mean permanently sealed? If so, no, everything should be accessible, plus, consider having a separate external socket to charge into, and cover that with a waterproof cap. Be aware of explosion risk of some battery types.

I made a separate charge connector which can be opened without risk of the main electronics getting wet. Made from a garden tap adaptor and a threaded coupling from the hardware store. Bullet connectors (left over from HobbyKing speed controls) are potted in place, but I noticed that some of the connectors had a hole in the side, so was careful not to let epoxy run in. The mixed epoxy I had left over I tipped into the coupler to seal the other end. O-ring and grease are visible. Sealed well, but would not use this approach on the main hull
Posted some O-ring stuff you may find useful viewtopic.php?f=13&t=1204

Hi Martin and Rossrov,

thanks for the info and ideas and sorry for the delay of my answer.

Martin, I had thoughts about oil filling things (battery, motors etc) but I don't like the idea in general as it seems a little 'dirty' during maintenace and also quite dirty for the environment in case of an accident. Up to now I had potted the battery and all electronics in liquid glass. You have to throw it away if it is broken but it's 100% waterproof up to (theoretically) a lot of pressure. Anyway thanks again for the suggestion.

Rossrov, thanks for the images and the info. I should study a little on how to make such seals like yours. Might that this is the safest way to go. The goal is at least 30m (4 Atm). Have you tried it at such pressures?

I haven't decided yet what kind of batteries to use. Until now I run on a 12V bike battery (too big, too heavy). I'm thinking of using LiPo in order to get a little more voltage (18V) since my motors run up to 24V. I'm hesitant though, as LiPo tend to be a little bit sensitive to harsh treatment (and they're certainly not made for ROVs!). Many say though, that if you follow the rules with LiPo they can hold without problems for a long time. LiPo's are also very small and light (and expensive)...

Alexis

Cool. Was just showing that a separate enclosure or waterproof connector for charging (and in this case a basic means of power on/off switching) is preferable to opening the battery/electronics enclosure each time you charge. I've used the O-ring as a sealing washer, not how an O-ring is designed to function, and only down to about 4 or 5 meters, the water depth where I have been testing. Also, the main hull shown elsewhere on the forum uses cheap drainpipe (maybe like your old buoyancy!) screw fittings and only suitable for a few metres.
Personally i would keep away from oil except where a shaft exits through a seal, as in thrusters. Lead-acid batteries have vents, even if you cannot see them, that release pressure in case something goes wrong. Potting lead-acids not a good idea. Also they take too long to charge, can create explosive gas....