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Coilgun

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Unread postAuthor: more_eggs » Mon Feb 07, 2011 6:45 pm

Sup ya'll.

I havn't been around in a while due to mother nature going to town on Australia and all that. But at last I'm back into rhythm and its time to build something.
I know its not really what you see around here, but I'v decided to make a coil gun :D

I'm sure a lot of you already know what it is, but for those who don't, a coil gun works of electromagnetic fields. The general gist, is that using some for of transformer (often disposable camera flash charging circuits) a capacitor bank is charged to a high voltage (300V+). Once charged, all this stored energy is dumped into a coil designed specifically for the type of projectile being used. This creates a big electromagnetic force for a couple of milliseconds, enough to accelerate the projectile into the coil, and by the time it reaches the center of the coil, the magnetic field should cease to exist. The projectile then continues out of the barrel.

Now, for all those who are about to tell me that I can't achieve a massive fps which can go through 7 feet of solid concrete and cause a minor earthquake in the southern hemisphere, I know this.

I do however hope to achieve somewhere around 150-200 fps, using small caliber projectiles. Likely 3mm ball bearings. HOPEFULLY these small projectiles won't prove to be a b***h due to the whole "bigger ferromagnetic things are more effected than smaller ones" idea.

Ok, so onto how I plan to achieve this.
Charging Circuit:
Well first up I suppose I will go the generic way and use disposable camera circuits. Not just one though, multiple crammed together to greatly reduce the charging time, hopefully to around a few seconds.
Only problem with these is that they are very inefficient, something like 16% efficiency which might not cut it. So later in the year when I get access to printing circuit boards at uni I might make a DC-DCHV boost transformer. Much more efficient and faster.

Capacitor Bank:
Found myself a nice large 1000uF 450V capacitor on fleabay. This saves me using multiple smaller caps. I will probably charge these to about 400v depending on the capabilities of the charger circuit.

Loading:
I plan on this gun being semi auto, which means it will have two coils. One main coil for projectile acceleration, and a second much less powerful coil as an injector coil. The beauty of this project is that nothing needs to be air tight! Sweet relief.
Basically the injector coil is what will be activated when you pull the trigger, it will pull a rod through it which will push the round forward to the main coil. As the round reaches the main coil, the pushing rod will then close a contact switch which will fire the main coil. The pushing rod will be spring loaded so it will be pulled back to its originally position. This should all happen very very fast. Iv attempted to explain this process in a pic below.

Triggering:
This will be handled by a relay switch due to the large currents and voltages that travel through the circuit.

The rest:
The whole gun will be housed in a sexy plexiglass handgun enclosure, with all your usual flashing led's and such that come with an electrical project like this.
Voltage of the capacitors will be read on a digital display.

Thats about all I can think of. One hell of a rant, but its been a while :P
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Unread postAuthor: Technician1002 » Mon Feb 07, 2011 8:32 pm

You will need to study a few subjects to get this to work at all.

1 Inductance. This will limit your coil current risetime.

2 Resonance. The LC tank of capacitor and coil will have a resonant frequency and produce a damped wave. Where is the projectile duration the decay? You may need a way to rapidly squelch coil current.

3 Eddy Currents. Your projectile if conductive may have considerable repulsion due to induced current. Now does this compare to attraction? This may become more of an induction heater than a linear motor.

Good luck. That type of system is generally inefficient requiring huge input energy for low output kinetic energy.

What is your AC RF background?
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Unread postAuthor: more_eggs » Mon Feb 07, 2011 9:07 pm

Believe it or not, it actually isnt that hard to get them working well.
Here is a good example:

http://www.youtube.com/watch?v=fEb9x-PlV2E

As for the coil current rise time, I think I will just go for trial and error to see at what distance placing the projectile gives the best results. i.e the projectile experiences the whole wrath of the field :D
If thats what you mean...

As for eddy currents, they shouldn't be an issue if the projectile doesn't hold magnetism once the magnetic field has ceased. Correct?

My AC RF background?
Dont exactly know what your asking for, but Im currently studying infomechatronics engineering. Did a subject last semester based around Linear Circuits and Systems which was all about filters bla bla bla...
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Unread postAuthor: more_eggs » Mon Feb 07, 2011 9:11 pm

Oh and think you could elaborate on your second point a little?

I understand that the LC circuit will create some form of sinusoid. According to my simulator it will be quite underdamped. How will this have an effect on the projectile, and how does its position make a difference during the decay?
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Unread postAuthor: saefroch » Mon Feb 07, 2011 9:14 pm

The eddy currents will be active while the magnetic field from the coils is present, but will no longer be significantly present after the current in the coils has dropped to almost 0. I personally do not think that the eddy currents will be as much of an issue as Tech says, after all these devices have been built in the past. However I do think he is quite correct in saying that this will be more of a science project than a really impressive launcher.

-It is however of note that this is coming from a mere high school student currently studying magnetism.
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Unread postAuthor: more_eggs » Mon Feb 07, 2011 9:22 pm

Oh I completely understand that it isnt going to be a powerhouse or anything like that.
For me it is like a science project sort of, just something new I want to start messing around with. Also doing a whole unit (subject) on electromagnetism and Faradays law and such this coming semester. One of the assessments is to make a working model to show Faradays law, a sweet coil gun would get me some nice marks I think :)
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Unread postAuthor: ramses » Mon Feb 07, 2011 9:51 pm

I recommend an SCR to switch the main capaitor; the relay will probably weld shut. Multistage coilguns can help to avoid projectile saturation.
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Unread postAuthor: more_eggs » Mon Feb 07, 2011 10:00 pm

I considered an SCR but a high rated relay should be fine. I'v seen them used in a few applications like this and have good life, no welding.
Multi stage is in the future when I can afford more parts such as optical sensors. Will also require more knowledge on my part, which is also a future thing :P
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Unread postAuthor: jackssmirkingrevenge » Mon Feb 07, 2011 11:47 pm

I havn't been around in a while due to mother nature going to town on Australia and all that. But at last I'm back into rhythm and its time to build something.
I know its not really what you see around here, but I'v decided to make a coil gun


Typically Aussie, feet wet and decides now's the right time to start playing with electricity :D

Interesting design, and I like the loading/firing coil combination, good luck!
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Unread postAuthor: Technician1002 » Tue Feb 08, 2011 2:31 am

Hmm, I'm going to have to dig into my high power SCR stock. I have a few decent size ones on hand. I have a hockey puck of unknown rating.. I'll have to look that up, and a few 300 amp rated stud mount at about 600 volt PRV.
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Unread postAuthor: more_eggs » Tue Feb 08, 2011 3:09 am

Hmm, I'm going to have to dig into my high power SCR stock. I have a few decent size ones on hand. I have a hockey puck of unknown rating.. I'll have to look that up, and a few 300 amp rated stud mount at about 600 volt PRV.


I take it your gonna make one too?
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Unread postAuthor: Technician1002 » Tue Feb 08, 2011 5:50 am

Probably not. Maybe a coin shrinker instead.
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Unread postAuthor: more_eggs » Tue Feb 08, 2011 6:29 am

You'll need some serious volts or some serious capacitance
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Unread postAuthor: Ragnarok » Tue Feb 08, 2011 10:46 am

For the record, a spherical projectile is about as inefficient as possible for a coilgun projectile. Your projectile should be as close to a perfect fit cylinder for your solenoid as you can manage.
In my own designs, I have compromised with a number of aerodynamic features (a rounded nose, and a hollow boat-tail), but that's because I want it to actually shoot straight.

Still, even with those compromises, I'm just about managing to model efficiencies of 45J kinetic from ~100J electrical*, which isn't that shabby for a reluctance coilgun (although I expect the hard copy probably won't be quite as good).
On the other hand, you'll probably be struggling to muster even a single percentage point of efficiency with those tiny spheres.

*Which should be fun with a 2-3 kW mains charger. Full auto coilguns FTW.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 08, 2011 10:52 am

Ragnarok wrote:For the record, a spherical projectile is about as inefficient as possible for a coilgun projectile. Your projectile should be as close to a perfect fit cylinder for your solenoid as you can manage.


Sounds like a good argument for sawn-off nails :D
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