Thoughts on buoyancy tanks/pumps

[parktrot]

I am in the process of designing my very first ROV. In typical fashion I am probably bitting off more than I can chew. I am planing on diving at a spring fed lake in northern Wisconsin with a max depth of 110 feet. I am particularly interested in mounting a rather robust manipulator. I wish to have a substantial ROV and am prepared to mount the required thrusters to deal with the mass. I am currently experimenting with various ways to control the buoyancy. At this point my plan involves having two 6 or 8 inch diameter pvc pipes with 350 GPH bilges mounted in the "tank" to pump the water out and bilges mounted outside to pump water in. I have run a few bathtub tests on a 4" diameter "tank" and have been able to have successful "dives" as well as "surfaces" so long as I have a one way check valve installed between the pump and the tank or in the case of the the surface system between the pump and the lake. The problem is when the two systems are combined...when the surface system is engaged (ie the pump inside the tank is turned on) it creates enough negative pressure to pull the check valve on the "dive" system open thus pulling water in as fast as it pumps it out. My latest thought is to add a electro magnetic solenoid or servo valve between the dive pump and the tank so that I can completely close off the intake when not in use. Any thoughts you have will be very much appreciated.

[parktrot]

UPDATE:

I have just ordered a solenoid actuated sprinkler valve. I am hoping that I can use this as a valve on the intake (or dive) pump so that I when I evacuate the tank I am not pulling any water in....Does anyone see any problems with this theory?

[SoakedinVancouver]

UPDATE:

I have just ordered a solenoid actuated sprinkler valve. I am hoping that I can use this as a valve on the intake (or dive) pump so that I when I evacuate the tank I am not pulling any water in....Does anyone see any problems with this theory?

I think you will find that sprinkler valves require back pressure to be able to operate (smaller electrical current requirement since part of the work is done by the existing pressure) . This may or may not be a problem depending on your design?

Do keep us posted on your experiments with these!

[parktrot]

Well...You beat me to the post. You are exactly right. I can not say that I am too surprised. I had heard of this problem but had to see for myself. As you mentioned sprinkler valves require additional pressure to open. I can now state that a 350 GPH bilge pump does not provide enough pressure to open said valve. I even tried to modify the spring inside the valve to have less resistance but nothing worked. So back to the drawing board. If I can not get this figured out soon I am going to have to abandon my plan of having a buoyancy system and switch to the more simple design of the Seafox. Does anyone have any other thoughts of how to remedy this problem???? Granted it has only been about 24 hours but my only thought at this point is to try to attach a stepper motor or a large waterproof servo to the top of a conventional valve (aka a hose bib) and have to turn to open/close the valve. I have tried to find a manufactured option but the only one I can find is from a manufacturing plant in China that has a minimum quantity of 1000. Anyone need 998 waterproof valves??

[SoakedinVancouver]

I suggest you try the Subcommittee website, they are a club running shallow, fresh water RC submarines (some of them really amazing reproductions of the real things!) and they do have ballast systems, as well as people selling pre-made outfits. The website is simply http://www.subcommittee.com

They also (used to?) publish a great hobby magazine for their members, full of practical and historical information. Now that I write this, I regret not having kept my membership up to date!

[parktrot]

Thanks I checked out the above mentioned webpage. It didnt seem to be exactly what I was looking for but it did give me some ideas. I spoke with an engineer at Jabsco this week and selected a different type of pump (not a bilge). I ordered it on Amazon and as soon as it gets here I hope to try it out in my test rig. I will post how it goes.

[parktrot]

I ordered the wrong pump so I guess my tests are going to have to wait until next weekend. Grrr

[sthone]

I hate when that happens.

[parktrot]

Quick update:

Finally got the right pump AND more importantly the time to test it! I built a air tight housing to hold the pump and ran a line through the the housing from the "lake" (as its still frozen in Wisconsin right now it was actually a bathtub) into a test ballast tank. I am happy to report that unlike the bilge pumps this pump was able to pump water out of the tank without letting any go back into the tank! It would seem that at this point I have the buoyancy situation under control (famous last words)... Now on to ROV design! I am really getting into using Solidworks and am considering building the ROV out of 6061 T6 aluminum instead of PVC... Any thoughts?

[Seadragon]

oil based trim system.pdf

My crappy drawing feel free to make it better visually if you are a better graphic designer than me

(252.47 KiB) Downloaded 1804 times

Quick update:

Finally got the right pump AND more importantly the time to test it! I built a air tight housing to hold the pump and ran a line through the the housing from the "lake" (as its still frozen in Wisconsin right now it was actually a bathtub) into a test ballast tank. I am happy to report that unlike the bilge pumps this pump was able to pump water out of the tank without letting any go back into the tank! It would seem that at this point I have the buoyancy situation under control (famous last words)... Now on to ROV design! I am really getting into using Solidworks and am considering building the ROV out of 6061 T6 aluminum instead of PVC... Any thoughts?

aluminum has a higher specific gravity than PVC, so I see some issues of bouyancy. The PVC would give a higher displacement than the aluminum. Maybe a combination with aluminum in key structural areas but not a lot would work.

As for ballasting tank a good you could use an expansion tank like the use on reverse osmosis systems and oil instead of air to change the displacement the RO tank has a membrane that expands and contracts based on water on one side and air on the other the air pressurises the water and forces it out. with the oil instead of air it forces the water out of the tank since oil is non compressable you have no differential pressure issues and you have control over the overall ROV displacement for depth and attitude control(if you use 2 or 4 tanks you can use control valves to give yourself 3 axis attitude control with a junction box to hold the pumps and 3 way vales with an expansion tank attached to the junction box)

oil based trim system.pdf

My crappy drawing feel free to make it better visually if you are a better graphic designer than me

(252.47 KiB) Downloaded 1804 times

I know you all would like to pitch the whole ROV up or down or angled while keeping thrusters off to avoid dustout near the bottom..

These are ideas to help everyone go deeper.