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Electronic Control of Pressure

Electronic Control of Pressure

Electronic Control of Pressure

(OP)
Hello all,

I'm trying to electronically control pressure(0-75psi) release from a CO2 tank, and was hoping to receive some guidance as to how I can achieve that. I'm aware of the 'electro-pneumatic regulators,' but they just prove to be too expensive. Cheapest I found was in the $250 area, can't justify spending that much.

If not being able to set a desired pressure, at least meeting three pre-set pressures, say 15-30-60psi, but being able to cycle through those pressures on demand from an ECU. How exactly would one achieve that?

I have a pretty slick manifold with poppet valves controlled by solenoids under my disposal, if someone can see how that can help in this picture.

If anyone's wondering what this is for - it's essentially a new mechanism for a beverage launcher that some of us were discussing earlier. I decided to start a new thread, since that was revolving around DC motors and actuation, which by the way proved to be too inefficient and costly - luckily I was able to return most of the parts. Hence, the new direction.

RE: Electronic Control of Pressure

(OP)
Just came up with this idea. Please critique.

Use the manifold (rated 350psi) which has an intake and an exhaust port, and 4 other ports. (As said before, I have this under my disposal already)

I would attach 3 manual pressure regulators at the outlet of each port. Set those at desired pressures. Use fittings to connect all the output of the 3 regulators to a storage tank of about a reasonable size(7-10cu-in), and then a quick exhaust valve to connect the storage tank to the barrel. Here is where I don't have much experience; how would I trigger the mechanism and make sure the previous "used" pressure is vented before filling with the desired pressure?

I might be assuming a lot of things here, please go ahead and call me out.

RE: Electronic Control of Pressure

Single pressure transducer, use that signal that as input to control a solenoid. Check ebay for surplus or old stock pressure transducers, I've used Omega before but that's just an example to get you started.

RE: Electronic Control of Pressure

For your idea, if you used the 60 psi regulator and then there was 20 psi left in the tank, when you switch to the 15 psi regulator on your next launch it should bleed the tank back down to 15 psi, so I don't think it would be a problem.

RE: Electronic Control of Pressure

(OP)
1gibson,

Thanks. That is true. So, there would be a benefit to connect all outputs from the regulators.

Now, the question is would I supply the output from the regulators directly to the storage tank? Would I then set the pressure transducer to read from the storage tank to trigger the solenoid valve, which I would set between the storage tank and barrel, for launch?

A relief valve is also probably needed to be set in the line.

RE: Electronic Control of Pressure

Simple "bang-bang" pressure control.

Two solenoid valves.

Valve 1 goes between your high-pressure tank and your launch tank.

Valve 2 vents launch tank to atmosphere.

Control sequence (Starting from zero psi in launch tank).

Open valve 1 for a short time.

Check pressure.

If pressure is too low then open valve 1 for a short time.

If pressure is too high then open valve 2 for a short time.

If pressure is just right you're done.

Repeat until done.

RE: Electronic Control of Pressure

I like MintJulep's idea, but I'd restrict one of the solenoids so they don't both flow the same rate. I think that would make it easier to converge on the desired pressure, rather than overshooting above and below in and endless loop.

RE: Electronic Control of Pressure

No need to restrict as the delta-P across the two valves will be very different in this application so Valve 2 will always exhaust slower than valve 1 will charge.

For extra credit "a short time" does not need to be a constant. It can be relatively longer when you are far from the target pressure and relative shorter when you are close - for Valve 1. Opposite for Valve 2.

RE: Electronic Control of Pressure

In some fluid power applications, a solenoid is driven by a pwm signal to control the output pressure.
(automotive automatic transmission)
There's enough flow/leakage in the system that no separate bleed is needed.
Of course, that requires a solenoid that can survive being driven that way.

It would require a single solenoid to control pressure, and a single pressure feedback signal.

Jay Maechtlen
http://www.laserpubs.com/techcomm

RE: Electronic Control of Pressure

(OP)
I'm presuming the setup would look something like:

Please let me know if I'm overlooking, underestimating something.

Thanks fellas!

RE: Electronic Control of Pressure

(OP)
Of Course there's a pressure transducer in line with the storage tank. Is that the best way to read pressure from the tank? Those also are a bit pricey...sigh.

RE: Electronic Control of Pressure

Using some imagination that CO2 knows what lines it follows and electricity similarly knows then yes, that looks about right.

RE: Electronic Control of Pressure

If you can afford a bit of a delay before firing and aren't too bothered about exactly when the thing goes bang, how about this as a scheme (adapted from 1gibson's first suggestion) for giving you pretty much continuous control:

Reservoir connected to source cylinder through a bang-bang solenoid valve and an orifice

Reservoir permanently vented to atmosphere through a much smaller orifice

Reservoir pressure monitored using a transducer.

Protect the reservoir with a burst disk (needs to be able to make a much bigger hole than the orifice in the filling line).

Arduino controlling the solenoid valve and the main firing valve in response to the transducer.

Charge your reservoir only in the last few seconds before firing.

When the "Stand to!" command comes, Pulse the filling valve repeatedly (perhaps adapting the pulse length to moderate the filling rate to compensate for pressure variations in the source cylinder). Stop as soon as the transducer reading exceeds your target pressure.

Then monitor the decay in reservoir pressure as gas bleeds off through the small orifice. Trigger the gun as pressure falls through the desired value.

A.

RE: Electronic Control of Pressure

Clarification: If you use multiple pressure regulators in parallel, either they have to be of the 'self relieving' type, or you need a solenoid dump valve to bring the outlet pressure down.

The beauty of the PWM approach is that the set of setpoints are limited only by the resolution of your pressure transducer and the controls downstream. The downside is that you need to put in some deadband, or put up with the buzzing noise.

Relatively cheap pressure transducers are available from auto parts suppliers; they're called oil pressure senders, and typically comprise a variable resistor, whose variable leg is moved by a diaphragm or bourdon tube.



Mike Halloran
Pembroke Pines, FL, USA

RE: Electronic Control of Pressure

(Jay posted while I was typing!)

A.

RE: Electronic Control of Pressure

I like that orifice idea, adds some suspense!

You can get a pressure transducer for 20-30 bucks on ebay, look harder.

RE: Electronic Control of Pressure

(OP)
You will see after this attachment that I'm more visual. This was the initial design idea, as per 1gibson: http://i59.tinypic.com/11b6gd5.png

This setup requires only one solenoid, as I have the multiport manifold already.

I'll prepare another diagram with what zeusfaber listed.

RE: Electronic Control of Pressure

Why not have a servo motor control something like a ball-valve... the angle in the servo can be related to orifice area, which in turn can be related to flow rate through the orifice. Once you have a curve for that it's easy to control a desired flow rate by turning the servo.

Servo motors are generally very cheap, I know they're usually in the $20 range. Even a stepper would work and they're not bad... $50 tops. The wafer ball valve or something like that, I'm not so sure... I see them in the $40 range on up to a few hundred dollars.

Experience: accumulated knowledge over time.

Talent: the ability to use experience.

Which is more valuable?

RE: Electronic Control of Pressure

of course, old printers have stepper motors - scrounge and strip a crappy old printer, have your choice of steppers and support goodies!

Jay Maechtlen
http://www.laserpubs.com/techcomm

RE: Electronic Control of Pressure

Try this:
From the CO2 tank to a manifold.
From the manifold to three self relieving regulators.
After each regulator a solenoid valve.
All three solenoid valves feed a second manifold.
The second manifold will feed the surge/storage tank.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Electronic Control of Pressure

There's plenty of E/P & I/P transducers on ebay for ~$30 USD.

I think you're wasting your time with anything else.

RE: Electronic Control of Pressure

(OP)
Hey all, thought I should share my progress so far.

I've gone with a CO2 tank and a regulator, two solenoid valves (one for fill, the other for launch), a pressure transducer which I found for 35 bucks. They're all controlled by an arduino, which was not much of a pain at all to setup.

The system is a little too efficient, to be honest; at only 30 psi it shot close to 50 feet. To remedy that, the barrel length will be decreased, and the storage tank volume will be decreased.

Thank you all for contributing solutions and helping me with this project. Of course, I've only got the launching system done. I still have the unloading of the beverage from the refrigerator to the launching system to still work on, and some programming, so that it can all be controlled using your phone!

Pretty excited. Will keep you all updated.

RE: Electronic Control of Pressure

Just in case, make sure to check distance for a can with condensation on it, before you bring the thing inside.

RE: Electronic Control of Pressure

(OP)
Hey all,

Curious if anyone has a cheaper/quicker solution to this:

So, the refrigerator and the entire contraption is placed over a lazy susan (it can sustain the weight). I'm trying to automate the swivel. What is the best way to achieve this?

The first thing that popped in my head: http://imgur.com/ddRoymW

Essentially the square box represents the base of the refrigerator, and the inner circle with teeth will be laser cut or CNC'd out of MDF or acrylic. Both of which I have access to. The problem with this is I really don't know how much motor torque I would need. My assumption is not that much - still don't know until I get my hands on the lazy susan, which arrives on Saturday.

Just wondering if anyone has a different solution to rotating the lazy susan 120 degrees and back. Perhaps one that wouldn't require me to use a CNC or laser machine.

Thanks guys.

RE: Electronic Control of Pressure

That's easy.

You need a nice circle of moderate thickness below the fridge, say a 3/4" plywood circle with a nice smooth edge.

You drive it with a timing belt, and a small timing pulley on your motor. You can probably get an XL belt that's long enough.

To keep the timing belt from slipping off the plywood disc, you need a slightly larger aluminum disc below it, and probably one above it just for fun.

To keep the timing belt from slipping rotationally, you just embed two timing pulley teeth in the plywood at 180 deg from each other. For limited rotation, you really only need one tooth, but I'd use two anyway.

I would not, e.g. just cut the timing belt and nail it to the disc or something like that. The belts are not hard to cut, but they are hard to terminate reliably.

If you mount the motor really close to the disk, you may need an idler on the back of the belt to get enough wrap around the motor pulley.

Mike Halloran
Pembroke Pines, FL, USA

RE: Electronic Control of Pressure

I would put the teeth 100 degrees apart so that they are both always in contact with the belt.
How about a used electric window mechanism from a junked car? You could use a link arm from the operating arm to some spot on the plywood disk.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Electronic Control of Pressure

(OP)
MikeHalloran,

Haha. I have. Though, I'm proud that my launching allows for any distance to be accomplished.

Pneumatic is the definition to robustness after this project.

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