Photos will be forthcoming, but here's a description of the planned fueling system (edit - photos now added):
- The fuel comes from a propane cylinder, with a propane regulator set to minimum pressure (it says 1 bar, but I haven't checked).
- The propane enters a 3-way 12V solenoid valve, built for pneumatic automation projects. It's a non-piloted valve, which requires more power but works better at low pressures.
- When the solenoid is activated, the propane port is opened, and connected to the metering port. This port is plugged, which creates a small dead volume inside the valve. This is my metering chamber.
- When the solenoid is deactivated, the propane port is closed, and the (now propane-filled) metering chamber gets connected to the outlet port. The result is a small, precisely metered puff of propane. I can send a new pulse to the solenoid for another puff.
- A tiny Arduino clone (Pro Micro form SparkFun) controls power to the valve, in order to create the desired number and length of pulses.
Each puff from the valve gives 3 ml of gas, which I found out by capturing 10 puffs in an inverted medicine measuring cup underwater. An open and close time of 100 ms each is sufficient for metering accuracy, giving me 5 puffs per second. 50 ms was too short, and 500ms didn't improve metering accuracy visibly over 100ms.
I need 60 ml or 20 puffs for my chamber, which will take 4 seconds to meter. I could reduce the number of puffs by increasing the regulator pressure or meter dead volume, but for now I think the tweaking accuracy afforded by lots of puffs will be the most useful. Also, having the regulator set at the end stop means I don't have to worry about adjusting it carefully.
The fueling tube is 4mm (ID 2.5mm) pneumatic tubing, which I will stick through the inside of the open ball valve and into the chamber, in order to inject the gas in the center of the chamber. I'll also plug the end of the fueling tube, and add transverse holes that will spray the gas turbulently in all directions. When fueling is complete, I'll retract the tubing, close the ball valve, and let 'er rip.
There will be more parts to the injector before I consider it done. The most important part is adding a 12V mattress inflator, also controlled by the Arduino, so that I can replace the air through the same port before injecting new fuel. I plan on using a larger hose for this, and placing the gas hose loosely inside the air hose, so they they both can be connected at once and the rest done by the push of a button.
Planned functionality and tests:
- I haven't actually injected and fired the cannon itself yet. I'll have to do that to see if my math is on point.
I'll have to see if the concentric tube idea works, and how fast the inflator can replace the air in the chamber. I normally do this by filling the chamber with smoke, and seeing how fast it clears. The inflator is at the airfield, so I won't get it until the weekend.
- The circuit is breadboarded right now, and wires and tubing are dangling about on my desk. It's also a bit over-engineered, with optocouplers for the motor transistors. Good freewheeling diodes and design shoud keep any transients well away from the micro, and I'll need the optos for another project (A mains-powered, Arduino-controlled stroboscope).
- The final circuit will be mounted in a box and/or on a board. The slender propane cylinder also needs a way to stay upright, or liquid will mess up the regulation. I could possibly strap the other parts to the cylinder, but I want the option of using other size cylinders in the future. I may mount everything in a small briefcase or something.
- The circuit is powered by 4S 18650 lithium cells (16V fully charged, 12V empty). I'll get (or 3D print) a better battery holder, and probably have the Arduino keep an eye on the cell voltages. Charging will be done by a separate charger that monitors individual cells.
- I think the mattress inflator will draw too much current, especially at 16V, so I may need to put more cells in, or control it through PWM if it delivers enough air at lower RPM. A soft start is probably a good idea in any case.
- I may want to use the batteries for cannon ignition as well. Right now it has a 9V battery assisted by a capacitor. I don't want to be tethered during firing though. If the system works consistently every time, I'll probably just charge a cap from the batteries, enough for one ignition attempt.
- Priming the fuel line by injecting enough propane to expel any air (2 puffs shoud suffice)
- Emptying the fuel line between the propane bottle and valve, after closing the bottle (testing showed that this took 30 puffs - these will have decreasing pressure, which won't give accurate fueling, so this gas will just be vented)
- Monitor circuit functionality: solenoid, fan motor, battery voltage, program halt with error indications of something's wrong.
- Maybe in the future, settings for different chamber volumes.
Edit: now with photos!