![]() ![]() In my experience this is the most difficult to control. In this method, gravity stays always the same. When the balloon is filled with compressed air, it will expand and displace more water, and thus increase buoyancy force. Submarine 3.0 was equipped with an air compressor and a balloon. They rotate the opposite direction from each other to prevent yaw. The two red Lego constructs in front are the propellers for controlling depth. Maybe the only problem I see is that the weighing has to be very accurate, otherwise the sub is always sliding up or down. You see the submarine immediately start moving up or down, and when you stop pressing the buttons, it will quickly slow to to a halt due to water resistance. You get fast response to radio control buttons. In my experience this is the easiest to control. Gravity and buoyancy stay always the same while the propellers exert force. In this method the submarine is weighed to be neutrally buoyant. Submarine 2.0 had propellers pushing water up or down. I’ve tried three methods for controlling the depth of a submarine. Red Lego bars under the lid help it withstand water pressure. The problem was finally fixed when I added red Lego beams under the lid to support it from bending. That was enough to prevent the submarine from rising from the bottom, as the propellers didn’t have enough thrust force. It resulted in a loss of buoyancy, which I measured to be 66 grams. That is like a frigging small human standing on it. I calculated that in 1.5 meters of depth, a normal swimming pool depth, the water pressure will press the lid with 35 kg force. I had first hand experience of it with my Submarine 2.0 that had a soft plastic lid. That’s why it is surprising how large it is. When you as a human dive in a swimming pool, you don’t feel the pressure. Or if the hull compresses under water pressure, it’s volume of displacement will decrease, which will again make it sink faster. If the submarine leaks, it’s mass will increase, which will make it sink faster. That assumes the submarine stays unaffected during the dive. If your submarine is 5 grams negatively buoyant, meaning it has slightly more mass than is being displaced, it will sink at the same downward force from surface to bottom. ![]() Note that the buoyant force does not depend on depth. In this example, the mass of the submarine, including the hull, motors, propellers, tungsten pellets and everything else, should be 2400 grams. To keep the submarine in neutral buoyancy, not moving up or down, the gravity should be equal to the buoyant force. 2400 ml of water weighs about 2400 grams. If the submarine volume is 2400 ml, then the displacement is also 2400 ml. It equals the weight of the liquid being displaced. In mathematics, this would be a double integral: force / acceleration -> velocity -> position.īuoyancy is an upward force caused by the surrounding liquid. The depth is the result of these forces over time, following Newton’s second law. The forces in a submarine are buoyancy and gravity. ![]() ![]() You need to drive it there, the same way a car needs to be driven to position X by pressing the gas pedal and the brake pedal. What physical principle makes the submarine go to depth X? Then I realized, it doesn’t go to depth X automatically. I struggled with this question with my first submarine. So, a lot of new things, that’s the way I like it. Extra weight had been steel plates or lead pellets in previous subs, but now I wanted to try tungsten pellets to save space for other stuff. The previous subs had a separate Li-Po battery for radio control, which I wanted to get rid off and use only one battery onboard. I would make the end caps myself, which would enable me to use very thin material, good for magnetic couplings to transfer a lot of torque. For the hull I would try a see-though acrylic plastic cylinder, which would be narrower than the previous hulls and therefore faster underwater. The whole idea that you could measure depth with a pressure sensor felt very intriguing. I could buy a pressure sensor and use that as an input for the controller. I had some previous experience using PID with a Reaction Wheel Inverted Pendulum, so I knew the basics and had some parts already available, e.g. I thought PID control might resolve the problem. Those submarines were always either at the bottom or at the surface. This project began from a simple question: how to keep a submarine depth at a constant level? Unsteady depth had been a nuisance with the three submarines I made earlier, especially subs 1 and 3. ![]()
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