Designing an ROV – the big questions

In January 2016, we decided to build a remote operated submersible (ROV). But what had we just bitten off? Was it more than we could chew? So many questions needed answering. I’d been rhuminating on this for 25 years, so I had a lot of answers already, but Andy and Quentin were just getting used to the idea of building an ROV and hadn’t had time to work through all the implications. I bombarded them with ideas, each with pros and cons and walked them through the picture in my head. There are many different ways to build an ROV, the choices we make will shape things like operating range, equipment requirements, hull design, etc.

In this post, I’m going to talk about some of those decision points and give you a chance to think about your own design. In the next post I’ll talk about the decisions we made for our project.

Tethered or Wireless?
There are almost insurmountable restrictions on the effective range and bandwidth of both radio and audio communications under water. If you go down the wireless path you’re going to need some serious electronics skill and be prepared to operate within 10 to 50 meters of the surface or a submersible ‘garage’. There are no off-the-shelf solutions here, you’re in a lonely zone inhabited almost exclusively by research engineers. The problem is seawater absorbs radio signals almost as well as a faraday cage. Sound waves travel through water quickly and cover long distances, but because seawater doesn’t have a uniform density and there tends to be stuff in it, signals are distored and reflected, making the effective bandwidth barely high enough for a scratchy phone call. Visible light absorbs really fast as well, with blue light travelling only a few hundred meters in ideal conditions.

On the other hand, if you have a tether (a physical link between the surface and the ROV), you’ve got the added problem of managing the drag and bulk of the tether. But your power and communication options are vastly improved. A common solution people go for is using twisted pair ethernet cable. It doesn’t give you much power, but it will give you 100m of fast communications link. Greater distances are difficult without going to fibre optic – the rolls royce of long-distance communication.

Onboard power or surface supplied?
Only an option with a tether. Surface supply gives potentially indefinate running time and reduced weight and space in the ROV, but requires a bigger heavier tether and there are some considerations around sending power over long distances without too much loss.

If you’re going to keep batteries on board, this increases the amount of space you need, makes your ROV a lot heavier and limits your running time accordingly. But if your tether was ever cut, you could build in a fail-safe return-home mechanism, which isn’t possible with all of your power coming down the wire.

Photos? Live video feed?
A photo or video signal takes a lot of bandwidth to send back to the surface. This is very difficult with wireless over short range and, at the time of writing, impossible over long range (1000m+) even for the military. Sonic communications has a bandwidth far too low to be useful for this. Basically if you want images or video, you’re going to need a tether, there’s just no getting around it.

Your comms link could be simple a simple bundle of wires with motor control wires going direct to the surface but with more than a couple of motors it starts getting impractical, because the tether gets proportionally more bulky. For that matter, copper becomes impractical at 1000m as well – there’s too much line loss for power transmission unless you’re dealing with very high voltages.

A composite control signal gives you a more versatile and lighter tether, but it requires a bit more electronic skill and a but more eqiupment at both ends.

How will you control the ROV?
If you’re savvy with electronics and microcontrollers there are loads of inexpensive options for onboard computer control. Theres no simple solution that does evening – the more you want it to do, the more complex it gets. In my opinion, you can’t build a serious ROV without an onboard computer, it just makes things a lot easier in many ways and it allows you to expand your feature set without much effort, but you need some basic knowledge to work with them, so the learning curve is steeper to begin with.

Propulsion
This is where I have dozens of designs in my head, some more bizarre than others. Will you use electric motors or try building futuristic magnetohydrodynamic engines? If you do, email me because the idea is fascinating!

Any rotational shaft drive needs to be waterproofed somewhere – either a pressure seal around the shaft or fully sealed motors running in water. Or you could pressurise and maintain an ambient-pressure hull and poke your motor shafts out through a moon-pool. Or you could build fins like a fish. Or maybe you don’t need them at all and only want to go up and down on a rope. How many degrees of freedom to you want? How fast do you want to go? Will you need to fight any current? Is there any risk of tangling or jamming your motors? It’s not hard to see why commercial ROV motors have a lot of zeros in their price tags, but there are economical alternatives for the hobbyist if you’re prepared to build them yourself.

Bouyancy
How will your ROV manage it’s vertical position? Will you maintain neutral bouyancy with a fixed-displacement or will you employ a dynamic displacement device to control it. Will you have dive-planes, or will you simply ‘drive’ it to the seafloor. You could drop weights to control your bouyance.If you build a pressure vessel, you’ll need to make sure it can cope with the depths you’re going to and won’t drown your expensive computer equipment on the first dive. Or will you dispense with floating entirely and build a bottom-crawler that drives into the sea from the beach?

If you don’t already know Archimedes principle of bouyancy, or how pressure changes with depth, you really do need to get familiar with these concepts before you start:

https://en.wikipedia.org/wiki/Archimedes%27_principle
http://oceanservice.noaa.gov/facts/pressure.html

Launch and retrieval
Don’t forget that you need to be able to launch and retrieve this ROV. It’s easy to build something big and heavy to reduce the effect of tether drag and currents, but then it gets unweildy. If you’re building something to go deep but you don’t have a boat, that can also be a bit of sticking point, so be practical about it – the price tag gets exponentially higher with every meter deeper for many reasons.

Can you actually do this?
Really? You’re even asking this question? Of course you can. Or even if you can’t you’re going to have a lot of fun trying and you’ll learn a lot along the way.

Do I really think we’ll get this project to 1000m? I have no idea. Logic dictates that we have the skills to be able to do it, but there’s a ton of logistical hurdles to overcome first. But that just makes it more worthwhile because if everyone was doing this it wouldn’t be quite so special.

These are the questions that will shape your project in the biggest ways. I hope I’ve given you food for thought.

In the next post I’ll talk about what we decided on and why. Did we choose correctly? Whos knows. Will we have to change anything? Probably, but so what. The point is that we’re doing it.