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It's never finished...

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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It's never finished...

Unread postAuthor: Tep » Sun May 17, 2015 7:59 am

Soooooo....

I took apart the "finished" gun that I posted a few months ago and rebuilt it to feed directly from the regulator paintball style.

I also rebuilt the solenoid to QEV attachment: now when the solenoid driver strikes the pin of the presta valve that pilots the QEV the air that is released pushes the driver back re-cocking it and closing the valve super quickly.

I built a circuit that allows the solenoid to be driven from a capacitor bank that are charged by two 9V batteries (seen onboard in the picture in a project box. Adding or removing capacitors allows me to adjust the dwell time of the solenoid I currently have it opening the valve for 30 milliseconds. This also allows the trigger to be pulled and held while only actuating the solenoid one time (like a semi auto marker) the capacitors recharge when the trigger is released.

The regulator is set to 400psi.

The tank takes 650 from my fridge compressor.

I get about 50 shots before I start to loose pressure and about 80 or so before it is empty / not firing properly.

The QEV is modded with a spring and a 0.32mm hole in the piston. For high pressure this is the best setup I have had.

I can get about 2-3 shots a second but it is limited by a number of factors including length of stroke of the solenoid, venting of exhaust gas around the solenoid driver and refill speed of the chamber by the reg (I could use an expansion chamber but I'm trying to keep it compact.

The barrel on there is a shorty muzzle loaded 8mm aluminium number that shoots 8mm steel ball bearings It's ported and is quiet enough to shoot inside without pissing off anyone.

I also have a collection of other barrels including a blow through bolt type and a "Barrage barrel" type that I made with a neodymium magnetic detent and the hopper in a vertical bottom fed position (0 double feeds and surprising fire rates / power). None of them look like much because they are all sleeved in polly like the one on there.


The red dot is held on to the top of the QEV with epoxy :roll:


Photo on 17-05-2015 at 8.11 pm.jpg


Photo on 17-05-2015 at 8.12 pm.jpg


Just a little bit of exposed wiring... :roll:

I might upload some schematics of all this MS paint style so it makes more sense to everyone.

It's basically a hammer valve piloted QEV with an electric hammer and gas re-cocking.









Spudding's not dead it's just resting.




Edit:

Schematic

overview.jpg
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Re: It's never finished...

Unread postAuthor: mark.f » Sun May 17, 2015 11:07 am

Looks nifty. :wink:

Although wouldn't using all of your capacitors and varying the voltage accomplish the same thing as switching them out? Not very important, just a thought.
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Re: It's never finished...

Unread postAuthor: Tep » Sun May 17, 2015 8:16 pm

Sort of... but higher voltage = a harder push from the solenoid (force is proportional to the square of the voltage, minus losses). Changing the number of capacitors (or the amount of capacitance) is what affects the duration of the capacitor discharge time.

Using the calculation

t=R.C (where t = time in seconds R = resistance in ohms and C is farads)

And measuring the two constants we know.

R= 9.9ohms (on my multimeter - let's call it 10ohms for maths)
C= 3000micro farads (3x1000micro farad caps) we get a time constant of

10x0.0003 = 0.003 seconds or 30ms

if I were to use 4 x 1000 microfarad caps I would get roughly 40ms

The "t" value represents a time taken for a capacitor to dump roughly 60 percent of its charge and by 5 t's the capacitor is basically totally empty.

The charges after the initial time interval aren't really that important to me as the solenoid is blown back open by the exhaust gas from the QEV at that point.

There are some graphs in this link explaining capacitor discharge behaviour (for anyone who want's to know more / would like to build their own version of this circuit).
http://www.electronics-tutorials.ws/rc/rc_2.html



The TLDR version:
Yes it would but opening the valve requires a lot of force on the solenoid for a very short period. If I lower the voltage I get less initial force because solenoid force is equal to the voltage squared. The rate of discharge is determined by total capacitance vs resistance
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Re: It's never finished...

Unread postAuthor: Doodmens » Thu May 28, 2015 4:48 pm

If I understand it correctly... by opening the solenoid, a bolt fits through and presses the presta valve, and the air escaped returns the bolt, right? But then what kind of solenoid do you have? I have one that only opens up a tiny lil hole and not the whole thing (don't really know how to explain, sorry). And also, the QEV you use, is it the regular $5 ebay QEV or something really fancy? Because it holds high pressure.

PS does the solenoid also close when the gun recocks? Or is there some sort of timing system? (I am not able to comprehend everything you've drawn or written)

PPS Really cool gun! You can be proud of that!
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Re: It's never finished...

Unread postAuthor: grumpyoldman » Fri May 29, 2015 6:34 am

Really good job! What's the brand name of the regulator you have your CO2 bottle? Any problems with it? I'd like to try one out. Thanks!
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