MadSci Network: Engineering |
Hi David This looks to be an interesting project. Air, as a compressible fluid can cause some problems in an application such as this. At some depth, the air bladder you are using to control depth may be completely compressed so you will lose control. Water on the other hand is for all practical purposes incompressible and will give you very precise depth control. Maybe I don't completely understand your question, but a simpler approach might be to simply measure the pressure of the water outside the sub and control the depth by comparing the rate of change of this outside pressure. For example, if the pressure is increasing, you are submerging, if the pressure is decreasing, you are surfacing, if the pressure is not changing you are steady at some depth. As you know, pressure varies with depth. Pressure will increase 1psi for every 2.31 ft increase in depth. Submarines control their depth through the use of ballast tanks that can be fully or partially filled with water or air. When the tanks are full of water, compressed air is used to empty them and the submarine rises. What you really need to concern yourself with though is buoyancy and stability. Buoyancy is the tendency of a fluid to exert a force on a body placed in the fluid. This force is defined by Archimede's principle: "A body in a fluid, whether floating or submerged, is buoyed up by a force equal to the weight of the fluid displaced." This principle is defined mathematically as: Fb=GfVd Where Fb=buoyant force Gf=specific weight of the fluid Vd=volume of the fluid displaced Stability is another area where you may run into trouble. For a submerged body to be stable, the center of gravity must belocated beloth the center of buoyance, but for a floating body the reverse must be true. Sensors: Simple pressure sensors can be fabricated. A good reference for these circuits might be the Forrest Mimms Enginnering notebooks. There are also some model rocket altitude sensing PIC applications available on the web that you might be able to adapt. If you have the budget to buy sensors, Omega Engineering, www.omega.com, has a multitude of pressure sensors. I hope this is helpful. Greg
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