Designing an ROV – we have a plan!

I’ve known for a long time that I didn’t want to build just another ‘hobby’ submersible using plastic pipes and tape. Whatever I built needed to be worth the effort. So I decided that I wanted to go deep. Very deep. This would necessitate doing a proper job and would be something to differentiate this project from the myriad of other DIY submersibles being built.

If you haven’t already read my post on what sort of questions shape the design of an ROV, you can read it here.

The plan is to build in two stages. Stage 1 has the simple design goal of getting to 100m. Stage 2 needs to get to 1000m. In reality there is nothing simple about it. A bunch of DIY ROVs out there will happily operate at 100m, but 1000m is well beyond most and for good reason. So, that makes it worth the effort.

Quentin, Andy and I got together and I brain dumped some of the more sensible design ideas I’ve had. Propulsion has always been a sticking point because water-sealing rotating shafts under immense pressure is hard and water and electricity get very exciting when mixed. However with the recent surge in aerial drone technology and the increased accessibility of the Chinese market, the answer dropped right into or laps – three phase brushless electric motors. Provided we could use small motors designed for the drone market, everything stays cheap and easily obtainable. Scale up and it goes south because the the market turns industrial and you pay through the nose. Why three-phase motors instead of normal DC motors? There are no electrical contacts and therefore no moving parts to seal, you simply drop the while armiture into epoxy to keep the water out and you’re done… well, sort of.

Next, we considered telemetry – how to control the ROV. At 100m there are more options, but the whole point was to go deep so we wanted a system that could work at 1000m as well. Fibre optic was chosen because it has the range and bandwidth to provide a clear, high deginition video stream from any depth we ventured to and the fibre itself it light and easy to handle.

We opted for on-board batteries, although this wasn’t a clear cut decision. What swayed it in the end was the tether – a communications-only tether is light and cheap compared to a composite tether capable of delivering high voltage DC current over great distances, along with the extra electrical gear needed to convert the power at each end. Also, having on-board batteries opens the way for semi-autonomy on the ROVs part. For example, if the tether is severed, the ROV could initiate its own emergency surfacing procedure, which it couldn’t do with surface supplied power.

All these decisions made, we started looking at pressure vessels. We have to keep the water away from our electronics control gear and we have to provide enough bouyancy to make it neutrally bouyant. This means keeping the displacement (volume) and the weight of the ROV constant, which means maintaining a consistent pressure inside the hull – and it’s just easier if this is atmospheric pressure at sea level. We decided to build a simple pressure vessel and not muck around with ambient pressure and have to work with on-board compressed gas bottles – maybe in a later project. For our purposes, we settled on an 18kg LPG gas cylinder – they’re easily obtained, tested to high pressures and can be readily modified. In our case, we found one that had been decomissioned because of a faulty valve, which we threw away anyway, so we knew it was still sound.

Now for some maths – our cylinder is factory tested to 3Mpa, or 30 bar. Which means that under normal use it’s gauranteed to safely hold that pressure when filled. This means that the actual burst pressure is considerably higher. For our purposes we’re going to reverse the pressure – the the high pressure on the outside. The burst pressure is not simply reversed, but it’s close enough for our purposes provided we don’t get too carried away. So, 30 bar is equivalent to 300m of seawater. Which means that a unmodified empty LPG tank would not implode at 300m under water.

We’re going to cut a big hole in one end and weld a few other bits onto it, so we don’t know quite what effect that will have on the strength of the tank, but it should be plenty good enough to get to our stage 1 goal.

The other important piece of maths if the bouyancy calculation. The water capacity (WC) stamped on the tank is around 20 litres, which is the internal volume of the tank. We’re interested in the external volume, which will be a bit bigger, maybe 23 litres. To make our ROV neutrally bouyant, it needs to weigh exactly the same as the volume of seawater that would have been there if an ROV wasn’t occupying it’s space – 23kg. The empty LPG tank weights about 8kg, so that means that our motors, batteries, electronics, and ballast weights need to add up to about 15kg. We can adjust it a bit later, but if we’re not in this ballpark with wiggle room we’re going to have a bit of work to do.

Based on our design, I’ve done my best finger in the air guess at the specifications for this ROV: