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Coilgun Project Dwell Time

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mrtran1224

Thu Mar 08 2007, 10:12PM

Registered Member #572 Joined: Thu Mar 08 2007, 09:43PM
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Posts: 1

Hello, everyone first post for me.

Recently I got interested in coil guns and my knowledge of electronics is not too high. But there are some issues about coil guns that I am not too certain about.

First of all I was wanting to build my first single stage coil gun with a 450V capacitor at 3900 microfarads. I am planning to use a 4 inch long projectile.

Well I am assuming that I could reach a maximum of 3% efficiency with a 10 gram projectile. From my calculations the pojectile would travel 48 meters per second. This would means with a short power surge the projectile would have to pick up all 11.82 joules.

If the maximum efficiency came from a coil around the same length as the projectile (4 inches) than the projectile would be most efficient if the current is switched off when it was half way (2 inches) into the coil. Am I right? So the projectile would have to accelerate up to 48 meters per second within two inches.

I calculated the projectile would accelerate 22677 meters per second using [(V^2)=2(a)(x)]

I used the acceleration to calculate the time I would reach maximum speed [(x)=(.5)(a)(t^2)]
and I got a time of 2.11 milliseconds

.

I also looked at Barry's Coil gun's website and I used the RLC simulation. According to Barry the current should die away when the projectile is midway through the coil.

And if I use a IGBT I should switch off the coils at 2.11 milliseconds?

Are all my calculations and assumptions correct? I am assuming constant acceleration so I can use newtonian equations. Because a time of 2.11 milliseconds is really short. I don't know If I going about designing my coil gun correctly.

Simon

Fri Mar 09 2007, 12:49AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

I sympathise with wanting to work everything out theoretically but coilguns aren't too great for that. These basic relations aren't going to be very accurate. They'll give you ballpark quantities. (Some board members are doing very sophisticated stuff with FEA modelling but that's very experimental.)

Flinging a piece of metal with a coilgun is surprisingly simple to do. I'd recommend starting off basic. Just wind a big coil - Barry's site might give you rough ideas - and dump the energy from the cap straight into it.

That's the fun of coilgunning. You can start off extremely simple and by improving the switching, tinkering with the coils, etc, you can work the efficiency up.

ConKbot of Doom

Sat Mar 10 2007, 04:40PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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Posts: 329

Single coil isnt the best way to go for high effiency anyway.. I believe that while back there was a 6 stage coil gun that had a very high efficiency. and it was very nicely done up my some college students.

I agree with Simon, not just with this, but with any project, dont dive in all the way, start at one place and work from that figuring out one bit at a time.

Simon

Tue Mar 13 2007, 01:40AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

ConKbot of Doom wrote ...

Single coil isnt the best way to go for high effiency anyway..

That's true, but multiple coils is vastly more complicated. A good multicoil gun is made up from good single coils, anyway.

wrote ...

I agree with Simon, not just with this, but with any project, dont dive in all the way, start at one place and work from that figuring out one bit at a time.

That's right. It's especially true of CGs, which are very simple in principle but can be very, very complicated in the fine details.

kalenedrael

Tue Mar 13 2007, 07:08PM

Registered Member #5 Joined: Thu Feb 02 2006, 04:18AM
Location:
Posts: 14

The acceleration you calculated is perfectly reasonable. It translates to about 2400G, and with a projectile mass of 10g, that's around 240N of force, not too much. Any sort of gun will need a fairly extreme barrel acceleration to achieve a useful velocity. Of course assuming constant acceleration in a coilgun with a rapidly changing magnetic field is accurate, but for these purposes it should be fine.

I recommend simply tinkering around with coil size and the position of the projectile with respect to the coil in order to optimize projectile speed, to start with. The calculated figure of 2.11ms is probably not very accurate and at worst can harm the efficiency of your system. Besides, the circuitry you would need to shut off the IGBT at the required time is probably beyond a first project.

As an example, I helped some friends build a coilgun for their physics project. We initially had grand plans for a multistage system that could propel slugs with ridiculous speed. Later we figured this was way too much work and just used a bigger capacitor bank for more impressive results. I'm sure you'll find that even the most basic coilgun (like ours ended up being) is fun to play with and improve upon later.

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