A 3D printed, raspberry-brained ROV project

[gwenaskell]

Hi!

A few weeks have passed since my last post, and many things have changed this then! Not for the best at first, as I've had some disappointments with the original hull design that led me to revamp most of it. To the extent that I had to update this thread's name, because fiberglass & epoxy won't be used anymore. But it is going to save me from a lot of additional uncertainties and difficulties in the future.

Here's the full story: I tested the assembly with the parts that were already printed in a basin to test the joint between the upper and lower hulls. It happened to be a disaster. No way to prevent water from flooding in at an observable rate even with screws fully tightened. FYI, screws were placed roughly 4-5 cm away from each other.
In the meantime, I made a new plexiglas forming attempt with a positive mold and a vaccum cleaner, as I saw somewhere on the web. I pressed a square of plexiglas maintained within a wooden frame against the mold, on which I drilled small holes that were connected to the VC. But a 4mm-thick layer of plexiglas is not something easy to bend, and it did not even came close to fit on the surface of the mold. Furthermore, as predictable, I faced a problem of punching on the center of the layer, that was pressed against the top of the mold. The constraints being concentrated on that small area, had a terrible result on the transparency of the glass there.

Anyway - back to basics! As anticipated earlier, I'm falling back to using a good old PVC tube, that will be much easier to work with, especially regarding the watertight joint. For the front glass, now that i have a perfectly round shape, I will simply use an acrylic dome like this one found on Amazon:

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This is a cheap CCTV protection, but thick enough to adapt to a ROV. I think it would also be good to design some sort of bumper that would protect it from scratches.

I'm still using a 3D printed hull, though. I designed a new one that will surround the WTC. But since it won't hold the pressure (it will be flooded with water), fiberglass won't be necessary anymore.

Here's the new look:

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I was recently searching for devices that would help connecting wires underwater, especially the ethernet cable and the control and power wires of the gripper (TBD) to the container. What would you guys recommend?

[fryslan76]

Still a nice and slick design, in several messages are people talking about hull penetration styles but a lot of people use bare copper in epoxy (yeah you will end up using it afterall).

[gwenaskell]

Hi,

Yeah, I'm thinking about using some copper/nickel water pipes connectors. One filled with epoxy, holding the outside cables, screwed to another one through the pvc pipe.

Meanwhile, 3D printing is going on...

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I divided the hull in several sections to reduce risks and consequences of printing failures. As pieces are made of ASA, which dissolves in acetone like ABS, I can join them afterwards by applying dissolved plastic between each section.
I'm thinking, though, that I would be better off assembling them with screws, so that I'm allowed to change sections that would break afterwards. I'll think about it this week-end.

[asesorplaza1]

Good morning
As you have already been told on occasion, getting a printed piece to be waterproof is quite difficult, the more pieces you have, the more you complicate the waterproofing of the joint between the pieces, you will have to do an excellent job of finishing so that your project does not become a kitchen strainer.
Greetings from Spain.

[Oddmar]

DIY underwater connectors...back up to the beginning for through-hull instead of inline.
https://www.youtube.com/watch?v=EKNNDPGr1Aw&t=665s

Another tutorial about potting through-hull connectors.
https://www.youtube.com/watch?v=hHHGozByLF8

[gwenaskell]

Hi,

Well, thanks! The second video is almost exactly what I thought about, great! I bought these pipes connectors a few weeks ago:

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My idea was to trap the cables in the left one with silicone/epoxy, glue the right one to a hole in the WTC and screw them together. So that I can disconnect the wires from the hull anytime if I want to. I'll use four connections like this one for all the cables (ethernet, motors+steppers, lights, a few sensors and the gripper afterwards)



Back to the hull:

I took a break and did not posted for a while, but I managed to build the bottom of the hull entirely. So nice to see the thing taking shape!

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I said that I would not require epoxy on this new model. That was before noticing how easy it was to crack the parts that came out of the printer! My mistake was to draw very thin edges (1.2 mm) on the sides, which makes them fragile between each printed layer. Once a crack occurs somewhere, it can easily propagate...

So, I took the resin and fiberglass back from the cave and started laminating the whole thing! Here you can see the result. I used two layers of fiber.

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I know, it's a bit rough - but at least it's strong enough to withstand shocks now!

The white color comes from a primary paint for plastics and resin I applied before laminating. But I tested without paint on a wasted part, and the adherence did not seem bad either.


that's what it looks like with the pvc pipe and the bottom frames:

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[fryslan76]

Nicely done, fortunaly you won't see the epoxy from the outside.

[gwenaskell]

Hola! Back with a few updates. Things take time but the stuff is still going on!

I progressed on the inner electronics assembly. I used a frame of metal rods, on which are threaded 3D-printed supports to hold the electronics. I'll ultimately add nuts to hold them.

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I also tested a few candidates to seal the back of the WTC. I think I could use a flat axial seal with one or two o-rings, as shown on the schema.

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This way I might not even require screws cause the pressure is gonna squeeze the flat seal against the WTC. I just hope the pressure is not gonna bend the plexiglas, or actually break it if I drill the 17mm holes needed to put the four cable glands in place. (I'm afraid it could weaken the plexiglas).
chment]

The candidates for the axial seal (from left to right, foam, silicone and rubber).

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Rubber was unsuccessful but the two others performed quite well on a quick-and-dirty test bench.


In the meantime I've started to look after a good ethernet tether. (Yeah, never to late to think about a core part of the rov, right?)

From my investigations I concluded that I might be able to get by with a non-rugged PVC ethernet cable, i.e. special waterproof cables may not be required for short dives. However, it's important to find a stranded wire rather than a solid one, but that's hard to find in lengths of 75-100m.

Otherwise, have you guys managed to work with other types of tethers that could handle video stream? I've seen the blueRobotics fathom tether which uses only 2 wires but it is quite expensive and requires a specific interface.