Aaron's Blog

I am building my AUV to be a configurable platform on which to try out a bunch of ideas, so I am going to need some sort of hatch through which I can assemble, repair, and modify the internal components.

My first thoughts were to make a sort of beveled seat you can see here. The beveled seat in the hull mates to a complimentarily beveled plug piece. You have a rubber layer between the two pieces, and the more water pressure there is on the plug the tighter the seal.

Unfortunately after talking this design over with some friends who has a lot of experience deploying things deep in the ocean, I decided not to use the wedge hatch. Apparently at depth even slight deformations of the hull can corrupt the seat seal, and the hatch will sweat water into the pressure chamber. My maximum target depth for this is only 250 feet, but I would like the design to be able to stay submerged for very long periods – so a sweating seal is a problem.

Right now the leading contender is using the pressure rated caps Blue Robotics sells for their watertight enclosures. Here you can see a hull design where a 6-inch plug has been added towards the nose. The tank you can see is a 24 Oz CO2 tank and pressure regulator. I’m checking that I can install the tank and regulator through the hatch.

I have not found a hatch position that I like yet, but the one pictured is as close as I have come. Things get a lot easier if I switch the CO2 tank to be screwed on from outside the pressure hull, while keeping the pressure regulator contained in the hull, so that’s what I am likely to try next.

One nice part about using the Blue Robotics pressure flanges – is that they offer clear outer faces of an optical quality. So I can easily add internal cameras looking outward through the hatches. So if I can make this hatch style work, I will probably add several smaller ones for camera and sensor mount points.

Whoop! I finally have a hull design. I found a STL model for the type of shark I wanted my design to mimic. The white you can see poking up behind the red shark is my hull design.

The elliptically shaped flats on the hull are for mounting fins. This approach will let me push out the problems of fin design until later. I’m hip deep in problems as it is. I also like that it will let me repair the fins, try out multiple fin designs, and remove them for transport. I’m going to shop off the tail as well, but I am debating how to add control surfaces, so I left it intact for now.

Here you see the outer surface of the hull. I am trying a cast acrylic hull design. The wall thickness will be just over an inch. So there is much less room inside the hull then it initially seems. Since things are going to be tight I modeled up the critical parts first so I could play with different layout designs and test fit.

The ballast system is driven off of a 24oz CO2 tank, which you can see got pushed out into the tail. Electronics will be housed inside a 4inch pressure chamber. The weird grey cylinder is a bilge pump being repurposed for propulsion and steering. I am still figuring out where to put another pump, ballast tanks, ballast weights, batteries, and some other goodies.

I have a lot more work to do on this – but I am happy how it is shaping up.

One of the problems the AUV project needs to solve it making electric connection through a pressurized hull. I found this interesting tutorial for making making watertight electrical connectors from pvc.

Initially I was drawn to Rays for a bio-inspired design. However, eventually I would like to use the active ballast to have the AUV be able to glide over large distances and Rays don’t really glide through the water in the right way.

So right now I am thinking I will use the general design of a small shark. After some quick measurements though I’m going to blow out its belly to be more like a grey whale. That should still give more storage and decent flow over the frame.

I am toying about adding two smaller fins – if I am right it could help with glide stability – but man it does not look weird. I will probably ditch the two fins, but keep the long thin tail. Since I am going to be printing all the hull mold pieces in chunks on my 3d printer – I also am strongly incentivized to minimize hull surface area.

The Softrobotics Toolkit Manta-Rae project is well documented project using soft robotics techniques to make an manta-ray inspired UAV.