This post doesn’t really have anything to do w/ electronics, but rather the mechanical design of the air cannon itself. Revision 1 was pretty basic, just a single, small pressure tube (like a pressure tank on a compressor, just made out of PVC instead) and a tiny gas valve rated to 150PSI that ran on 120VAC. The gun was fired simply by turning a 120V switch on and then off again. This design was quickly retired for Revision 2 to get better range, be safer, and be able to shoot a variety of objects, particularly golf balls.
Revision two has two large pressure tanks which are easily capable of storing enough air to quickly shoot objects out of long barrels. Supposedly the longer the barrel, the faster the object in the barrel will come out of the barrel, provided there is enough air to continue accelerating the object. I’m pretty sure at some point you reach diminishing returns, or you have an exceptionally large pressure tank. Most of my barrels are about 3-4 feet long. The cannon facilitates multiple barrels by all using the same threaded connector to mount to the air valve. The valve used in this design is a RainBird sprinkler valve that is activated via a solenoid. The valve is rated to up to 150PSI. The maximum I’ve run the cannon at is about 110-120PSI. Much more attention was paid to selecting parts that are all pressure rated. Pressure rated parts have a pressure rating printed on the side, or have NSF-PW stamped into the fittings. Non-pressure rated PVC is a cellulose construction, which has many air pockets and such in the PVC to make it lighter and cheaper to produce. The problem is these air pockets make for excellent weak spots in the pipe and since ruptured PVC acts like a frag grenade, you don’t want weak spots in the PVC. Also with pressure rated PVC, the smaller the pipe diameter, the greater its maximum rated pressure is. This is great and all except for selecting a pipe for a pressure tank. I settled on a 3″ pipe since that is still rated to 200PSI and is readily available. 4″ pipe is hard to come by and it’s rather expensive, not to mention it has a lower maximum pressure rating.
Also with this pressure rating, it’s not necessarily an air-pressure rating. Yes a PSI of whatever is the same as a PSI of air, but I think you’re not supposed to use PVC for compressed air because of the failure mode of PVC. When PVC fails, it doesn’t just get a hole in it or a little tear; it fractures and can fracture into lots of small, sharp pieces of plastic. If water or some other nearly incompressible medium was in the pipe and it failed, there’s suddenly no or very little pressure to fling these little bits of eye-gouging spears into everyone in the near vicinity. This is not the case with something compressible such as air as when the air is allowed to escape from the PVC through a fracture, the air expands rapidly and shoots these particles all over the place and probably into people, much like a pipe bomb or frag grenade. In fact, this very “explosion” is what makes the air cannon work in the first place, but in a controlled manner.
The picture at the top of the post shows Revision 2 being fired. And the video below the picture shows the air cannon controller working and the cannon firing at ~110PSI and then ~40PSI so that the golfball can actually be seen on camera.
Revision 2 worked quite well until I tripped over the air hose supplying air to the cannon and that cracked off a small bit of PVC that connects the external air connector to the pressure tanks. Luckily the cannon wasn’t pressurized, and the crack was discovered as the gun was filling. The piece that cracked off wouldn’t have exploded as it was cracked off nearly all the way through the pipe, but it may have shot off the pipe if enough pressure was built up. Unfortunately since the cannon is painted (to protect it from UV from the sun), it is nearly impossible to repair it without possibly compromising the structural integrity of the pipe when paint is removed (so that PVC glue can melt another piece of PVC onto the existing pipe).
With the demise of Revision 2, Revision 3 needed to be created before the following summer. There isn’t a lot to be improved upon Revision 2, but at a minimum, there needed to be a way for the PVC portion of the cannon to survive after tripping over the air hose. Revision 3 also has a much larger, more direct path for the air to exit out of the air valve, which should allow for quicker object acceleration out of the barrel. The air tank capacity is roughly the same as Revision 2, but the pressure tanks were taken from two large ones to four smaller tanks. This was merely an done to make the cannon look meaner, but it could actually offer the possibility of getting more air to the valve quicker than the previous design. It should start to become obvious that the only bottleneck for slowing down or limiting the air going into the barrel being the RainBird sprinkler valve.
Revision 3 before being painted:
Apparently the RainBird valve opens slowly when using solenoid activation, and supposedly it can be improved. However, it still needs to be electronically activated for it to remain safe in the sense that it’s hard to accidentally fire the cannon. I think I’ll be leaving the valve alone for the time being until I can make the cannon’s controller measure the velocity of the object leaving the barrel. That way I can see if a modified valve offers any kind of velocity improvement over the unmodified valve (more velocity = more distance). Revision 3 currently awaits a final coat of paint and a wooden platform to mount it on. Revision 3 will probably also have a plywood cover to be placed on it when the cannon is pressurized to make sure no one is hurt if a PVC failure does occur.