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Coaxial sentry gun

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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Coaxial sentry gun

Unread postAuthor: oilfieldtrash21 » Sat Feb 22, 2014 10:25 pm

What's up guys! This is my first time posting but I've been a long time lurker. I've built a few projects from the information in this forum and it's turned in to a lot of fun and now pretty much an obsession. I have been tinkering with an arduino board for making a sentry gun. It tracks motion through the use of a webcam and also has the ability to output a signal to fire a trigger. Most people are using them to fire existing paintball or airsoft guns but for me to do the same would be too easy and where is the fun in that? I'm basically wanting to build a coaxial for the compactness plus it would be my first. The smaller and lighter the cannon the less load that would be put on the servos and the quicker they will be able to react. I plan on using a 1/2" port 3 way valve actuated by a 12V DC solenoid that will be triggered by the arduino for multiple firing rates. After looking at all the different designs of piston valves being built, I had an idea for one that would make the whole cannon even more compact but I wasn't sure how well it would work. The plan is to build the spool as shown in the drawing and use a return spring to help "stabilize" the piston. The air supply would enter the rear through the 3 way valve. The first piston "A", would be machined down as much as possible to allow maximum air flow through it. The second piston "B" would have two ports machined in it and reed valves would be attached to cover the ports and only allow air to flow in one direction. Once the chamber is filled, the solenoid is fired and shifts the 3 way to exhaust the area behind the piston and well you get the rest. As far as the feeding system, I'm still working on that part. It will be pretty similar to a motorized system on a paintball gun. I'm just curious if anyone has attempted this before or might have some insight on the advantages/disadvantages of this design. Keep in mind that none of this is to scale, only drawn this way to show the concept. Thoughts, ideas or criticisms?
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Re: Coaxial sentry gun

Unread postAuthor: jackssmirkingrevenge » Mon Feb 24, 2014 1:03 am

Welcome to the forum :)

I don't quite get the need for the complicated piston... what level of performance are you expecting from this sentry gun, is going to be "safe"?

A full auto BBMG seems to be a better idea for a sentry gun, a nice spray of BBs gives you a much better chance to hit the target, and depending on the pressure of your gas supply you can make it pretty devastating:

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Re: Coaxial sentry gun

Unread postAuthor: oilfieldtrash21 » Mon Feb 24, 2014 2:02 pm

Thanks!!

I came up with that design so the whole cannon would be more "inline", I don't see how it's any more complicated than some of the other designs here. I was just curious if anyone had every tried a "reed" type valve for this or if it was even feasable in this application. Keep in mind this isn't a final drawing, just a rough draft to explain my thought process. The reed valve works well in a 2 stroke engine, though the same could apply here.

I'm not really looking to spray BBs, I'm after more of a selectable rate of fire determinded by the rate at which the solenoid is pulsed by the software. This project is a lesson in design and programming for my kids and I. Our goal is to build the sentry to "defend" an area of our yard by firing airsoft rounds (at a safe pressure) at them as they attack. The sentry's tripod will have a target attached, that when hit would disable the sentry. This will give them hours of fun plus allow us to learn to tweak the software to improve motion tracking and anticipating movement.
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Re: Coaxial sentry gun

Unread postAuthor: jackssmirkingrevenge » Tue Feb 25, 2014 10:10 am

oilfieldtrash21 wrote:I came up with that design so the whole cannon would be more "inline", I don't see how it's any more complicated than some of the other designs here. I was just curious if anyone had every tried a "reed" type valve for this or if it was even feasable in this application. Keep in mind this isn't a final drawing, just a rough draft to explain my thought process. The reed valve works well in a 2 stroke engine, though the same could apply here.


A reed valve should work, though in practice unless you put o-rings on your piston a one-way valve is not needed as enough air can flow around it to fill the chamber. Even with o-rings, you can make them in such a way that they leak air in one direction and not the other, just like a biciycle pump head.

I'm not really looking to spray BBs, I'm after more of a selectable rate of fire determinded by the rate at which the solenoid is pulsed by the software. This project is a lesson in design and programming for my kids and I. Our goal is to build the sentry to "defend" an area of our yard by firing airsoft rounds (at a safe pressure) at them as they attack. The sentry's tripod will have a target attached, that when hit would disable the sentry. This will give them hours of fun plus allow us to learn to tweak the software to improve motion tracking and anticipating movement.


In this context, why not use the solenoid as a valve directly?

Something like this will give you excellent performance for airsoft even with pressures in the 30-50 psi range, and you can pulse it to shoot say 1 or 2 BBs per second easily to give you a satisfying "dakka dakka dakka" sort of firing:

http://www.ebay.com/itm/1-4-NPT-Electri ... 43b075ab59

A simple paintball gun style hopper can give you a large ammunition reservoir as well:

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Re: Coaxial sentry gun

Unread postAuthor: oilfieldtrash21 » Wed Feb 26, 2014 11:36 am

That's a simplistic solution to the sentry gun idea, seems to me that will work very well for that application. I appreciate the advice. Using the paintball style hopper I could make a scaled down replica of this: http://en.wikipedia.org/wiki/File:Phala ... 6W-003.jpg Thanks for the help!!







On another note, I understand what you are saying about not needing the one-way valve if the piston wasn't completely "air tight". The reason I wanted to test out the idea of that design was, if it was successful and feasable, I could use a much higher pressure on a larger cannon and use the volume of the chamber to increase velocity instead of relying on just the pressure and volume in the air supply line like in your suggestion. Seems like to me the openings in the piston that would be covered by the reeds would allow for a faster fill rate to the chamber between shots than blowby around the piston. Either way, it was just an idea.
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