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Electromagnetic BB gun

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rpopeye

Thu Jun 07 2018, 04:34AM

Registered Member #62069 Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13

So, this is my baby. Itâ€™s not much in terms of specs, but the special thing about it is that itâ€™s as good as an average CO2 BB gun, and as far as I know thatâ€™s a first for an electromagnetic BB gun, plus it looks cool :)

Here are the specs:

Electromagnetic BB Gun
type: coil gun
munitions: standard 4.5mm steel BBs
muzzle velocity: 70 - 130 m/s (selectable)
kinetic energy: 0.8 - 3.0 Joules
rate of fire: 20 rpm
full weight: 3.6 kg (including optics, clip and battery)
length: 70 cm
battery: 2.5 Ah Li-Ion, more than 300 shots on a single charge
magazine: auto loader with a standard 19 rounds clip
manufacturing process: 3D printed

It's a standard coil gun. A series of electrically powered coils along the barrel create a running electromagnetic field from the back of the barrel towards the front, which attracts and accelerates the steel BB. Because the switching of the coils is controlled through software, it's very easy to change the muzzle velocity and in this design there are 7 selectable muzzle velocities, from 70 m/s to 130 m/s. The coils are powered from a high voltage capacitor bank, which in turn is charged up from the battery by means of a DC-DC converter.

The BBs are automatically loaded between shots from the clip into the barrel thanks to a servo mechanism, and at the moment of producing the shot there are actually no moving mechanical parts in the device.

One great thing about it is that itâ€™s very cheap to operate, no need to buy CO2 cartridges, just charge the 30 Wh battery for less than a cent and itâ€™s good for more than 300 shots.

rpopeye

Thu Jun 07 2018, 05:19AM

Registered Member #62069 Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13

And here is a picture of the coils
48 stages, mosfet-switched, 1 mosfet per stage, open-loop (no feed back) configuration

1528348732 62069 FT182289 Img 4587 Small

DerAlbi

Fri Jun 08 2018, 02:18PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Hmmh. Complicated to comment something. It was obviously a LOT of work and that alone is worth some cheering

130m/s and shooting round balls is quite usable despite the 3J.
However to reach 3 Joule it seems a lot like there had to be an easier way than 48 stages and 3.6kg.. i do however realize that your projectile is extremely small, so its hard to push energy in it.

The 2.5Ah LiIon has how many cells in series?
Assuming just one cell (in your favor) the battery provides around 32.4kJ of energy. If that is only good for 300 shots your system efficiency seams to be only 2.7% assuming 3J/Shot. If you have 2 cells in series, the efficiency drops to only 1.35%. This seems almost unrealistically low for a multi-stage concept

Are you sure the BBs are actually magnetic (and as good as pure iron balls)? It seems very strange to me

I love the idea of having such a reload mechanism and using a standard magazine and standard ammo for it.
Nevertheless interesting to see your approach!

rpopeye

Fri Jun 08 2018, 02:41PM

Registered Member #62069 Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13

The BBs are only 0.35 gram each, at 130m/s it's 3J max. But the main goal here was velocity rather than kinetic energy.
Because the BBs are really small (4.5mm in diameter), the coils have to be short too, so can't accelerate much with a single coil, hence 48 of them.
The battery is actually 3 cells, 30Wh, 108kJ. The efficiency (electric energy in capacitor bank to kinetic energy of projectile) is just about 2%, but because the DC-DC converter is not great either (at that speed of charging - 3s per recharge), the total efficiency (battery to projectile) drops to about 1% - terrible, but the goal was to get something practically usable rather than achieve high efficiency. The BBs are solid steel and totally magnetic, the problem is most likely due to their small size. If the size of the projectile is bigger I suppose the efficiency will improve too.

Signification

Fri Jun 08 2018, 09:07PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

How does your timing work? i.e. things like sensing the BB position, firing the coil at the right instant, etc. It's also interesting that you have a velocity select--do you use a lower current for smaller velocity, use less active coils...? VERY impressive overall--I never thought BB's could be used in a coil gun with results like you are getting!

rpopeye

Sat Jun 09 2018, 04:30AM

Registered Member #62069 Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13

The timings of the coils firing are preset, there is no sensing of the BB position. With 48 stages, adding position sensing would have made the design too complex, and I also wanted to see if it can be done without sensing, turns out it can, at least in this particular case :)
To change the velocity then is simply a matter of changing the timings of the coils firing.

DerAlbi

Sat Jun 09 2018, 07:51AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

but the goal was to get something practically usable rather than achieve high efficiency

That usually goes hand in hand in terms of power density and performance/weight-ratio. I fully understand your troubles - BBs are really challenging projectiles for a coilgun. The main problem is the round shape, not its small size, btw.

rpopeye

Sat Jun 09 2018, 12:11PM

Registered Member #62069 Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13

DerAlbi wrote ...

That usually goes hand in hand in terms of power density and performance/weight-ratio.

True, true. My goal was to build a BB gun using standard 4.5mm steel BBs, everything followed from that.

the_anomaly

Sun Jun 10 2018, 12:58AM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

What material did you choose for the barrel?

Signification

Sun Jun 10 2018, 01:06AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Have you ever gotten suckback of a superposition (summation of forces from each coil) that resulted in no ejection at all?

I was also wondering (in the early stages of design) if you ever used only ONE coil, and the effects you got.

EDIT: I was also wondering (not asking for every answer) about things like the dimensions/specs of each (identical?) coil WRT the 4.5mm BB...ID, OD, length, number of turns, current and cap specs (C,V) per coil, and wire AWG. PLEASE forgive me for the swamping questions, but I am so fascinated with your results, and I had some unique ideas of my own which I previously thought would be useless to attempt--you have changed my mind!

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