I need to control an air valve variable from 0psi to 150psi (not just on/off). I don't know the actual volume at this point but it is likely less than 5cfm. Is there such thing as a variable air solenoid? Or do I have to use something like a valve connected to a servo? It needs to respond about as fast as you could move a ball valve by hand if you were to move it quickly. Also this is a hobby project so cost needs to be kept low.
You're looking for a pressure "regulator" to do this. It's the same idea as your valve-connected-to-a-servo scenario, but regulators have a spring loaded ball inside them to control the pressure whereas a valve just opens an orifice.
Any home improvement store should have them for under $10.
No, I should have expanded a bit more. The variable air flow is to be controlled by an arduino. So to simplify, I would read a potentiometer and set the air flow to correspond. turn the pot all the way to the left and it is at 0 flow. Turn it all the way to the right and it is at full flow. Anywhere in between as well. Set the pot at half way and the valve would be opened half way.
A little bit of google found this: http://www.grainger.com/Grainger/SPEEDAIRE-Regulator-4ZM08?Pid=search Looks like you could rig up some mechanism with a stepper motor to turn the pressure adjustment knob. I'd assume many others exist.
Yes I've googled quite a bit. I have found absolutely nothing that already exists as a variable air solenoid. If such a thing exists it may not be called that however.
I am confused. Are you trying to regulate air flow or air pressure? - Scotty
flow - sorry if I confused anyone else.
I would add a servo to something like this http://www.amazon.com/In-Line-Air-Flow-Regulator-Gage/dp/B00173AZ40 as it will likely be a lot cheaper than finding a ready built controllable pressure/airflow regulator. It won't be supper accurate, but if you add a pressure/flow sensor after the valve you can implement a closed loop controller on the Arduino and probably get fairly good accuracy.
A pressure regulator, like the one I linked above will only control pressure, not air-flow. Think of it like a voltage regulator. It will make sure you always have a constant (voltage) pressure on one side, but that will only equate to a constant (current) air-flow if there is a constant resistance involved. True volumetric airflow valves do exist, but they, like current limiting power supplies, are a lot more complicated and more expensive.
Alternatively, if you could live with a few discrete airflow options you could build the pneumatic analog of a resistor ladder adc (google that) with a bunch of different sized tubes (resistors) and a handful of solenoid valves. I probably wouldn't really do it this way, but it sure would be cool if someone did.
Ok thanks. Low accuracy is ok. I figured I may have to use a servo and a valve based on the fact that I never found a ready made device on google.
freakdaddy:
Ok thanks. Low accuracy is ok. I figured I may have to use a servo and a valve based on the fact that I never found a ready made device on google.
That is probably due to the fact that measuring air flow is not as easy as you think. How are you going to measure the air flow to know if the flow is too high or too low? Perhaps if you gave the details of the intended application there might be easier solutions.
I don't need to measure the flow. I just need to control it. It's for a arduino controlled rocket motor! (actually a compressed air motor) When the kids throttle up, the valve needs to open and increase air flow proportionally to the throttle position. If the throttle is at 75% it really doesn't matter exactly what the air flow is as long as it seems reasonably close to the astronaut. The control is a wireless joystick. Right now I'm setting up to use a valve and a servo.
Right now I'm setting up to use a valve and a servo.
Probably the easiest way to go.
Electronic proportional flow valves do exist but they're expensive. Some work like you're thinking - basically a stepper operated valve. And others use a PWM signal to control a solenoid valve:
http://www.omega.com/ppt/pptsc.asp?ref=FSV10&Nav=prel03
Would pulsing the air cause any problems for your rocket? Because I was wondering about driving a simple air solenoid with a PWM signal. Higher the duty cycle the more net flow you'll get.
EDIT: Thinking about it, a cheap but relatively fast switching air solenoid may not pulse all that much when driven by a PWM signal. Have a look at this paper:
http://advantech.gr/med07/papers/T06-001-456.pdf
Recently had an opportunity to work on a vehicle (Infiniti) that had an Idle Air Control valve with a stepper motor control. Perhaps some automotive devices would work. Not all automotive valves are just open/closed.
- Scotty