...if you run two tanks, thats four solanoides and two supply tanks plus the tubing and fittings of added weight.
Why two tanks? I think one tank should have no problem feeding a ballast tank and a trim tank. When I looked into this method I just had the air tank feed a plenum that the solenoids lead off of, so if you need to blow the ballast tank and adjust trim then you just open two solenoids corresponding to the tanks you want to blow. I dont know how much it matters but I always planned to use air lines of equal lengths for all the tanks (read: all the air lines are as long as the longest required) so when you open the solenoid the plenum moves the same volume of air the same distance and in my mind that makes things smoother.
Im hung up on piston tanks right now though, a single hull with a piston on each end gives you both ballast and trim with two motors. But then you do have to deal with O rings and piston seals and depending on your hull layout that could result in a catastrophic failure on the part of your control board as the pressure hull floods.
Silver dollar wrote:I'm no expert on this, but can't you use the pump connected to a check valve and a solenoid valve to let the pressure out through a separate pipe.
That is a possibility, any good places to buy solenoids?
Kjetil wrote:
Is the any small pumps that is reversable and will not let pressure throught when its stopped?
Nice idea. Like somebody else suggested, a check valve on the pump line will keep the air in. You probably need a second vent solenoid then.
I've been toying with the idea of charging two buoyancy tanks with compressed air at 120 psig. I would then use these to blow ballast for an "emergency blow". No idea if it will work yet.
I have ordered some solenoids, and I have found a check valve.
I'm able to pump in 0,9 liter of water in to a 1,5 liter bottle with a pressure of 2 bar. When I bleed of after the check valve the air pressure is pushing all the water out. But at 10 meter depth there will be 1 bar of water pressure, so I think that I have to reverse the pump to be able to get all the water out of the tank. I just place the solenoid so it will overide the check valve.
The system you show is commonly known as "soft ballast" because the effect changes with depth and can cause a run-away. The system is very simple, just a dump valve on the highest point of the tank and a large hole in the bottom. A pressurized air line is just fed through the hole in the bottom to anywhare in the tank. These are never used in commercial ROVs as they are inherently unstable in that they get heavier as they go down and lighter as they come up and are very sensitive to even small changes in depth.
An alternative system, that I think is where you are going is a "hard ballast system". this is widely used in manned submarines for trim and is much more stable although more complex. In this system, a sealed tank is filled with a small quantity of water, which is then pumped in and out of the tank. Because the tank has a fixed displacement, the bouyancy changes with the quantity of water remaining in the tank. The main problem with this system for smaller vehicles is how to deal with the changing pressure in the tank, and leakage through the pump as you've found. In a submarine, the ballast is usually pumped in and out of the hull, which is so large the change in pressure is insignificant and can easily be made up.
Another method is to use a pneumatic ram connected to the air supply with the rod end vented. A spring is connected to the rod. The air pressure obviously has to overcome the spring pressure and the max pressure for depth. As the air is fed in, it moves the piston and displaces the water on the rod side with air on the opposite side of the piston. A second valve vents the pressurized air to the sea allowing the spring to push the rod back to displace the air with water, increasing the effective weight. I would suggest using an old first stage diving regulator with the spring adjusted to the minimum required pressure to avoid blowing up the ram. If the ram is placed in view of the camera, the rod position will indicate the level of ballast. This type of system is widely used on commercial ROVs to pressure compensate the hydaulic systems, which has nothing to do with trim but operates on the same principle. In this case the spring is inside the ram, which keeps it all nice and neat.
Once you have pressurized air onboard you can do a whole bunch of other fun stuff with it such as moving a trim weight forward and back to affect the trim angle, drive manipulators, pan and tilt units, etc...
By the way... if you want to drop weights, avoid dropping lead, it's very unfreindly to the environment. Gravel or concrete are OK, but iron has a higher density, an old shackle works pretty well.