The maximum rate at which air can leave the chamber must be balanced with the maximum rate that air can enter the chamber so the output can be varied over a wide range. The maximum exit rate is dictated by the size of the hole the air can flow through when the solenoid valve is wide open. I used a Clippard EV-3-12 solenoid valve I had sitting around, which from the given air consumption rate (0.6 scfm at 100 psi supply pressure), has a roughly 0.025" flow path. I wanted a slightly lower air consumption rate, so I made my own 1/64" (0.016") orifice fitting by filling a 10-32 fitting with epoxy and drilling a 1/64" hole in the set epoxy.
|Nothing to sneeze at - you'll lose the drill bit!|
The air inlet area must be even smaller than this if it will allow pressures to be controlled over a wide range. Luckily a needle valve I have can be slightly cracked open to just allow enough flow that allows a good range output with decent (5-10 second) re-pressurization times.
The solenoid valve takes 12 volts to open. An IRF510 transistor connected to an Arduino's digital output pin allows the amount of time the solenoid valve spends open and closed to be adjusted to change the equilibrium pressure in the chamber. You can't just use the Arduino PWM output to throttle the valve, though - the on/off frequency used by the PWM pins is way, way faster than the amount of time the valve takes to open and close (which is 5-10 milliseconds from the valve specs). When the output signal to the valve is greater than 50%, I programmed the valve to spend 20 milliseconds off, followed by a variable amount of time on proportional to the output signal. Vice versa for when the output is less than 50%.
I then connected the pressure chamber to the pneumatic actuator of a ball valve I had sitting around. Although flow control with a ball valve is usually a pretty dicey affair (not that I haven't seen it done), this ball valve should now be able to somewhat precisely control flow! I see another day of boredom avoided with that!
Here's the whole shebang in action: