List and Trim

Got a question for everyone.

How are you controlling or what have you thought about using to control the list and trim for your ROV?

I have seen a lot of talk about Buoyancy but nothing about attitude control. If you don’t know what I am talking about for List and Trim … just think of your ROV and where the Center of Gravity is and how the object is going to rotate. The Center of Gravity is more than likely not going to be dead center and things are going to get real interesting when your front end is extremely light and the rear end has the batteries etc. To keep a stable platform you are going to be burning a lot of thruster power just to stay level and that really depends on where your thrusters are.

I plan on using hard tanks that I can vent and blow water out of while using a triple axis acceleration sensor to set the level in the tanks when the unit first submerges. I am trying to make sure that my weight is distributed correctly but once it gets to depth, pressure compression can change things. My total goal is to acheive zero bubble for list, trim and buoyancy … then I can use minimal thrust for movement in any axis.



That’s a really interesting idea. I like it. Would the tanks fill with water or would the be pre-pressurized with air and blow that out to adjust buoyancy?

On our BlueROV, for static stability we rely on the center of buoyancy being a few inches directly above the center of gravity, so that the ROV hangs like a pendulum. It also has 6-DOF thrusters so that attitude can be controlled directly. The ROV is symmetric left-to-right and the watertight enclosure can be adjusted forward/backward to change center of buoyancy.


There would be two types of tanks in reality. There would be ballast tanks that when the air is vented out of them that the unit would be slightly negatively bouyant. This would allow you to sink etc. and pretty much drive around. This tank system would have vents on the top and open grate system on the bottom. If for some reason your unit started to flood or if you loose power, the system could automatically blow the tanks and it would force the unit to the surface. I hate loosing crap on the bottom!

The second type of tanks would be internal list / trim tanks. These tanks would be partially full of air or water to adjust the trim. Kinda of like an automatic system that allows you to adjust trim on the fly. If your weight distribution is preset on the surface … there is little you can do about it under the water. So the idea was to have fluid tanks to allow you to shift water (or whatever) around different points on the unit to level it out. Also assume you have some sort of grapple arm and you want to bring “X” amount of weight up with you … the front end or whereve the arm / weight is will drastically change the attitude of the craft and shift how you have to use your thrusters … and that is if you have them fore / aft etc. If you have a ballast system and the tanks are all along the unit with the major volume tanks being on the ends etc. you could shift weight around and again move your CG point to compensate for the new mass.

The valve system for the internal trim tanks can be cheap because you are not worried about pressure … the tanks would be “soft” tanks inside of your pressure proof containers with external high pressure line coupling it. The only pressure valves you need to deal with are for the “hard” tanks that can handle a pressure rating above your lowest depth. This way you can ensure you have adequeate volume / pressure in your blow cylinders to get you back to the surface.

Now the cost is going to be something that will knock your standard DIY types out that are looking for low budget systems like the OpenRov thing … but as the price of your toys goes up, I like to consider it as an insurance policy that my package is coming home.