Hi

This may sound crazy.  How about a quadcopter designed for underwater use.  Brushless motors operate perfectly well submerged and the rest can be waterproofed quite easily.  Idea being that one would use small props, basically the quad would operate in a different medium but essentially it should still stabilise as it does in air.  I thought about escs (which don't like water) getting hot but they could be exposed, covered with tectyl or some other waterproofing spray.

As a fail-safe, waterquad would be slightly buoyant so in case of power loss, it would merely float to the surface.  One would then have keep throttle on all the time to stay submerged and power up to sink as opposed to normal operation.  I know that 2.4ghz does not work underwater but the older 35mhz systems do.  For fpv, 1.3ghz could be used ??  Also I wonder which flight controller would work best, kk2 maybe for cost-effectiveness. 

Has this been attempted before and if so, any success?   Couldn't seem to find anything on this concept.  I'm itching to start building but somebody please stop me if I'm wasting my time. 

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I started on an ROV project this weekend so this is timely. I did briefly consider acoustic wireless, the thought process being '56k modem->speaker->microphone->modem'. Very quick Google returned some results for acoustic modems but seeing the scale of the challenge opted for a wired tether.

Something I did consider, showing some RF ignorance here, but using the 3DR telemetry radios, if one were to connect these back to back with coax (instead of putting antennas on)... How far away from a working setup would this be? Presume it would need some padding or impedence matching.

The nice thing about the floating antenna idea is you could get a GPS on it too, because I bet that doesn't work underwater either :)

I suppose the drawback to this is being so close to the water you are going to have a very short horizon, and maybe even occlusion from waves, before you even get into drag and paying out the cable etc

Sorry if I'm playing catchup here but is the water/air interface the issue? What's rf proper faction like if both antenna are in the water?

If you mean using coax instead of transmitting through air ? then yes this would work but wouldn't that be the same thing as a tether ? and would it fly with the coax on ? If you are going to use a coax tether then there are better utilization of it than using 3DR.

The problem with RF is not really the interface though that does present losses the problem is 

absorption of rf in water

after all microwave ovens rely on the efficient absorption to heat stuff. The micro waves are much higher frequency but there is a pay off higher absorption with higher frequencies and lower data rates with lower frequency. There doesn't seem to be a sweet spot that will do both at any reasonable distance under water. Low frequencies work well under water and indeed submarines use them to communicate with the land but generally the data rate is so slow the most common message sent is please contact us,, this would result in the sub surfacing (or deploying surface antennae) and using conventional sat technology. 

Yes you can put GPS in the floating buoy and use dead reckoning to calculate where the ROV is under water but the inaccuracies are so huge it again becomes point less. I tried getting rid of the errors by using two GPS modules and one fixed in a known spot with the errors communicated to the other for correction but my simple GPS receivers don't allow me to get at the raw data and thus without spending 100's that didn't work either..

ultra sonic s are OK for all of it bar the live video. By placing some land based TX you can calculate the position of the buoy and a similar set up underwater to get the location of the craft this can also be used to communicate simple controls. Maybe radio above water and ultra sonics under but still no video. My favorite solution is for the Quad to carry a payload consisting of a data link (possibly standard WiFi)  and fiber optic tether. I lands on the water and as it sinks the payload is left on the surface relaying the data. This would work providing you have enough weight caring capacity and battery life on the quad. Some sort of ballast tanks would be needed to keep the quad as light as possible in the air but be neutral buoyant in the water to preserve battery life. The standard quad blades work ok under water providing you stall them before they hit the water (breakage) and your motors will go slow enough as the rpm under water would be a fraction of the rpm in air...

you need to use low frequency 3-300 khz TxRx that do not exist  for sale to the public wifi variant is being made but expensive,  second option is to use communication relay floating a devise on the surface of the water for communication wireless wired communication in water last is all tethered.

http://www.dspcomm.com/l/aquacomm-underwater-wireless-modem.html?gc...

you can make low frequency RF links its not that difficult but the antennae lengths start getting ridiculous to get any reasonable distance. Again though you set up a perfect 300 hz RF link with a county length antennae and you still wont get the data through put for live video....

The link you've posted OG is for ultra sonic not RF. And yep there sis no doubt this works but as I said in my post a few up , the data rate is pretty poor unless you pay big money and even then it's not great...

Wow.. lot of things you have been discussed.. which indicates that its almost impossible to have underwater wireless live video transmission using present technology.. i think then i have to go with your favorite idea.. which is i think more than sufficient for my project.. but i will keep on searching  for wireless high speed communication.. one or the other day it should be done..  

can we use same propellers which we use in air?? doesn't it have any negative effect? i mean torque required and also the load on the motors?

 

There are some new quadcopters that claim to go underwater, I think you can find them in Kickstarter.  One solution to communication is a floating antenna, this will keep you in contact.  Managing the cable will be one of the challenges, good luck, I think this is something that could be really great and FAA could not regulate.

The only question I have is why a quadcopter? what issues with typical submersibles would this solve.  A quadcopter in air is easier to fly, can turn in a very short radius, hover, and other practical aspects that other aerial vehicles cannot do.  

Make sure you consider the difference in physics between a quadcopter supporting its weight in air and a marine vehicle with near-neutral buoyancy.

Remember that a quadcopter tilts to generate a forward force and accelerate. When you're maintaining altitude, this force is basically your (thrust * sin(tilt angle)). That's a lot for a quadcopter that has to hold it's weight up in the air. For example, a 2 kg quadcopter tilted at 10 degrees will generate 0.35 kgf forward.

Underwater vehicles, on the other hand, are usually closer to neutrally buoyant so the "thrust" term is much closer to zero. For example, if you are close to neutral and need just 0.1 kgf to maintain depth, then a 10 degree tilt would only provide 0.02 kgf. That won't do much.

That basically means that you'll have to tilt the vehicle much further to get the same lateral force. If you're neutrally buoyant, you'd have to turn a full 90 degrees to change direction without also ascending or descending.

Also, if you're positively buoyant, then all the tilting dynamics work out in reverse, i.e. you would pitch nose up to go forwards instead of backwards.

Just some things to think about.

-Rusty

i think these might be the advantages

1. Its a hybrid which will do both the workings of quadcopter  and a submersibles.

2. we can reach distant place more faster through air than in water.. once we have reached the distance we can dive into the water.

3. i have thought of giving it a robotic arm. which could do some once its submerged..

what i am doing is just a prototype.. a small step into world of new kind of vehicle..

as we all know 70% earth is covered with water.. and i think this kind of vehicles will be required in the future..

there might be few other applications also.. that we just have to find out..

yes.. i have seen this part.. i am using same method as you said.. it will be neutrally buoyant and.. and propellers will turn 90 degrees.. now i have to work on its buoyancy factor.. i have to build it by myself i think.. because the ballast tanks which are readily available in market which could get into a depth 25 to 30 m are out of my budget.. The depth was given to me by my project guide because he thinks idea is  more important than its limits also we have less budget and for my for my project its more than enough if it goes a max of 30 depth..  

An underwater quadcopter is certainly technically possible, but beyond the bouyancy and waterproofing issues it is important to understand some other items as well.

Yes brushless quadcopters can work underwater - for a while.

In salt water (ocean) once or twice would be a fair guess due to the corrosion playing absolute havoc with the teeny tiny ball bearings / let alone the aluminum and steel bits that touch each other and form great electro galvanic batteries (dedicated to corrosion).

In fresh water you can probably make quadcopter motors last a lot longer if you clean and re-lubricate their bearings after every submersion.

Then there is the matter of the difference in density of water and air, the requirements for a motor propeller combination to be effective both in the thin stuff (air) and water are hugely different.

Much more torque and much less RPM in water and really an entirely different set of requirements for each.

Various inventors (and even the military) at various times have designed and tested assorted flying submarines and for the reasons mentioned here and by others there have been no significant successes. 

It can probably be done, but my guess is that to produce a practical implementation is - unlikely.

Best Regards,

Gary

As long as the water is pretty clear of minerals or salt it is surprising how long those brush less last. I must have used one for at least 20 hours under water doing tests etc and it still runs great, all I do is rinse the thing off and leave it in a breezy area in the garage to dry out. I have even used a hair dryer to dry it but of course don't use any heat or you maybe driving any grease left in the bearing out. Also WD40 isn't the best as it'll wash out the grease. A quick re-grease after drying would probably help to.

Vectored thrust is the way forward if your insistent on using the same props and motors, as mentioned above the same prop and motor set up will work providing the motors will give enough torque at very slow speeds. Maybe dream up a cunning magnetic coupling thing and use different props. The up and down is handles OK by the quad props but maybe an extra one for forward / reverse and a rotation prop or two thrust and use them to turn as well (skid steer)  it would involve increasing the weight by two motors and props (or coupling system) but may work out better in the end...

surface float idea works, tether control is a problem. One idea i tried was a multiplier fishing reel with a motor for retraction but this may be too heavy if you are going to carry this on a quad copter .. The other idea I thought of but didn't try was a normal little Dc motor and a magnetic coupling to a spool , that way it would put a constant pressure on the tether retraction and slip if it pulled too hard (ie snagged).  

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