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Senior Project assistance - Automated pneumatic tennis ball

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 11, 2008 12:49 pm

Team PSU wrote:I searched for some pictures of this actually built, but all I could find were more gif and idea threads. Anyone have any pictures of the thing built? I believe we'll be able to implement the idea, possibly with an electric solenoid because we're going to have an additional power source (not just CO2).


You don't need solenoids in this case, it is the gas pressure that does all the work.

As to the blow forward valve, I've never seen it done for tennis balls but clide certainly made it work for golfballs while my own experiments were on a smaller scale ;)

The "tee" version would be much simpler to make though, and since you already have other complex mechanisms to deal with, the "add simplicity" maxim applies here. The reason I personally went for the blow-forward breech is because I wanted to use non-spherical projectiles in a hopper as opposed to a tubular magazine, but in your case for tennis balls the Tee breech is perfect.

keep_it_real wrote:1. Some CB tests have been done and they found that .8:1 works best for pneumatics.


Unlike combustions, a larger chamber on a pneumatic will always equal more power. As you get bigger and bigger though, the increase will be smaller until it reaches a point where it's simply not worth it.

Choosing the ideal ratio in this case depends on what pressure you intend to operate with. If you're using say 30 psi, I would go for around 0.5:1 using a reasonably long barrel. This will give you adequate power with good air efficiency. Think of it this way, any pressure you have in the barrel just as the projectile leaves the muzzle is air pressure that is no longer pushing the projectile and therefore is wasted.

I would strongly recommend downloading GGDT to help determine the ideal ratio.
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Unread postAuthor: jimmy101 » Tue Nov 11, 2008 1:33 pm

You might consider a "paddle-interrupter" type loader. For example, Markfh11q's design. It is much simpler and easier to build than Jack's first design. (Jack's second design is often hard to get to work well since it tends to fire more than one round with each cycle.)

The interruptor feed rate is going to be limited to how fast the paddle resets and how fast the next ball will fall. Iwould think it should be able to reload in a couple seconds at most.
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 11, 2008 5:25 pm

jimmy101 wrote:(Jack's second design is often hard to get to work well since it tends to fire more than one round with each cycle.)


Given a short enough burst of air this won't be the case, however I agree that a "paddle" can easily be incorporated and would eliminate the potential for double loading.
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Unread postAuthor: Team PSU » Wed Nov 26, 2008 1:26 pm

Just wanted to drop in and give you all an update...I've been pretty busy finishing up design work and getting home for Thanksgiving break. So here's whats up:

I've got 12" x 3" chamber with cap, and fittings down to 1" to go into the sprinkler valve. I've also got the fittings to come out of the sprinkler valve, turn 90 degrees up to the 24" x 2.5" barrel. I was lucky enough to find some SDR21 2.5" and a 2.5" coupler for free :o .

I also have the reloading system figured out, just need to get a 2.5" Tee. We are going to use this idea:
Image
A magazine that holds 6-8 balls will be mounted to the stem portion of the tee. At the top of the 2.5" magazine will be a screw on (cleanout) cap and some type of gas shock that will force feed the balls into the barrel. To keep from loading more than 1 ball into the barrel, an actuator will be installed into the side of the magazine (near the Tee) that will extend and retract when our Oopic micro-controller tells it to. Near the cleanout cap, a small relief hole will be drilled to prevent damage to the magazine.

We will implement this method first, if this proves to not work; there is another method ready to try.

The only thing left to get is order a sprinkler valve (none of my local stores carry them?), 2.5" tee and some fittings, and then a C02 bottle with Palmer Pursuit Stabilizer.
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Unread postAuthor: roboman » Wed Nov 26, 2008 10:31 pm

If range isn't a concern, then just use an unmodded sprinkler valve. I built a couple water balloon launchers that work just fine with them. Besides, it would be much easier to actuate with your controller (I'm assuming you're using a microcontroller board). Otherwise, you should go with a 2-port 2-way solenoid valve as your pilot (1/4" should suffice).
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Unread postAuthor: SEAKING9006 » Thu Nov 27, 2008 2:12 pm

If you're already dealing with electronics, then I feel that my design might just be right up your alley. Moreover, it doesn't require too terribly much fabrication to get working. Simply moving the switches back or forward will fine tune the timing.
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Unread postAuthor: Ragnarok » Thu Nov 27, 2008 2:21 pm

Team PSU wrote:We are going to use this idea:
Image

If I'm painfully honest, I don't think that idea will work. What will almost certainly happen is that the Bernoulli effect will draw out all the remaining balls from the magazine (and indeed the effect was used in the Vogt pattern machine gun) - or raw pressure will blast them out through the bottom of the magazine. Neither being desirable effects, you'll need to find something else as an option.

The magazine needs to be sealed off from the barrel in operation, having it exposed to pressurized air at any time is asking for trouble.
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Unread postAuthor: Pilgrimman » Thu Nov 27, 2008 2:27 pm

@Ragnarok

I though the same thing. What they need is a flap before the mag that will flip up and block the mag off, or a blow-forward bolt design.
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Unread postAuthor: MrCrowley » Thu Nov 27, 2008 2:33 pm

Pilgrimman wrote:@Ragnarok

I though the same thing. What they need is a flap before the mag that will flip up and block the mag off, or a blow-forward bolt design.

That's probably the best option, the flap. I remember markfh11q tried it out once and with a bit of tinkering it could be pretty reliable.
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Unread postAuthor: SEAKING9006 » Thu Nov 27, 2008 2:38 pm

Why use another moving part, when the whole thing can be done with one? I say blow forward bolt (especially mine) will be the simplest way to achieve reliable full-auto fire. Especially since this guy seems to already have a grip on basic electronics, so my autocannon should be a breeze. I'd be nice to see a prototype of the bolt system working, since I don't really have the time to make one anymore.
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Unread postAuthor: Team PSU » Fri Nov 28, 2008 11:06 pm

Ok, thank you for the suggestions. But Ragnarok, did you see in my post where an actuator will extend/retract at the proper times to prevent the balls from double-loading into the barrel?
SEAKING9006, you have quite the design. It looks as if it could work, but is more involved than we're looking for at this point.

Also, I realize the magazine will be exposed to some high pressure. However, with a relief hole at the top of the magazine (the furthest point from the barrel) I feel the losses will be kept minimal because of the plunger that will will used to force-feed the balls in the downward position (towards the barrel).

We are going to attempt this method of reloading because it's very simple - and at this point simplistic is best for this portion of our project. The tennis ball cannon is only a very minor portion of the project.

If our method proves to be incorrect, a sliding sleeve around the barrel will be the next design attempted.

Thanks and hope everyone had a good Turkey day!
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Unread postAuthor: TurboSuper » Sat Nov 29, 2008 2:34 pm

Can't a pair of spinning wheels pretty much do the same thing without all the...plumbing?
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Unread postAuthor: rp181 » Sat Nov 29, 2008 4:59 pm

Some reasons why a "plumber's gun" gun is better than spinning wheels:

1. Power draw- he already mentioned that this would have to be limited.
2. Power - In general, to get anything good out of somthing as big as a tennis ball will need some beefy motors for good performance.
3. Accuracy on spinning wheels suck, ive tried some. You could make 1 wheel slower than the other for some spin, but that puts more resistance on the motors, so you wont get AS good as performance.
4. Spudguns have lower maintanence. It kind of depends on how you make it, but that how it would seem.
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Unread postAuthor: Team PSU » Sat Nov 29, 2008 9:20 pm

Well put rp181.

Additionally, how much cooler is a pneumatically powered tennis ball cannon, rather than a pair of spinning wheels. The intention is not to lob tennis balls for serves, we need a cool/wow factor. We have a set amount of money to complete a mechanical, electrical, and computer controlled component of a project that integrates these three aspects.

Thanks!
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Unread postAuthor: SEAKING9006 » Sat Nov 29, 2008 9:24 pm

Just use solenoid valved (sprinkler valves :wink: ) to operate small pneumatic cylinders to operate a breech.

I can't believe we didn't think of that BEFORE. :x
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