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Registered Member #62069
Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13
the_anomaly wrote ...
What material did you choose for the barrel?
It's an acrylic tube. Has anybody tried with a metal tube cut on one side along the axis to prevent eddy currents? I've seen such designs but never got around to trying it. The problem with the acrylic tube is that it's flexible and has to be mounted to an external rigid structure to ensure straightness.
Registered Member #62069
Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13
Signification wrote ...
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.
yes, if the pulse that fires a coil is too long it will suck back the projectile or at least will reduce it's velocity, so the pulse length for each coil has to be of optimal length. In the beginning I tried with a single coil and the maximum velocity that I got with 4.5mm steel BBs was 20m/s. Because the BBs are so small the coils have to be short too, hence a lot of them to get to 130m/s The exact parameters of the coils vary along the barrel due to the different velocity of the projectile as it gets accelerated. There is an optimal set of parameters for each coil, however due to the express non-linearity of the whole process it's impossible to calculate with formulas, at least I haven't found any such. The other option is to use a physics simulation software or trial-and-error to adjust the parameters :)
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
YES, A thin non-conductive barrel is definitely better, but the wire needs to be close to the projectile ...perhaps you could pot an entire (optimized) barrel/coil assembly in some kind of hard non-conductive material...ending up with a solid cylinder of embedded coils whose OD is that of the coils. Coat the inside of the cylindrical mold with pam non-stick spray--this has ALWAYS worked great for me in sliding off the mold of fiberglass/wood molds for multi-shot embedded coil quarter shrinkers. Another approach--use a thin cardboard cylinder intended to stay on.
Registered Member #62069
Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13
thanks for the advice about molding, you are right that the closer the coil ID to the projectile the better, so I was thinking about embedding the coils in some sort of hard resin, no need for the acrylic tube then, plus the added benefit that the coil wire is firmly fixed and can't move (in a air coil no matter how tightly wound, the wire would still move a little when running the current through it, which produces an annoying clicking sound when firing the coils), but haven't gotten around to actually doing it yet
As for your idea about a slit down the center metal barrel, I remember reading about a man doing that a long time ago, I think it was Sam Barros - the Powerlabs guy. Hold on... Found it! He used a brass tube with a milled out slot as you describe. That said, he hasn't given any updates on this, or any of his projects for a few years, so I have no idea how any of it turned out.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
..a guy who measures his coilguns in kJ stored energy... always a sign that you should immediately leave the site, go search for a more credible source.
Registered Member #57984
Joined: Thu Nov 19 2015, 09:44AM
Location:
Posts: 58
Rpopeye, great work! As far as I know this is the fastest amateur coilgun yet. Two points: 1) Why don't you use halfbridge (or other configurations) to reduce a number of power switches (MOSFETs)? 48 stages may have been realized by 6Ñ…8 matrix i.e. 14 switches. Even taking into account "high-side" driving needed for half of them, the profit would be obvious. 2) Accuracy on your video is quite well. May be it is the best achievable for the ball-shaped accelerator. I conducted some assessments here. It can even be said that the main problem with BBs in coilguns is the accuracy, not low efficiency.
Also the community is very interested in more detailed information about your accelerator (especially geomterty of the coils and wire characteristics).
Registered Member #62069
Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13
DerAlbi wrote ...
..a guy who measures his coilguns in kJ stored energy... always a sign that you should immediately leave the site, go search for a more credible source.
I am a bit confused, isn't 30Wh equal to 108kJ, that's the stored energy inside the battery => 2.5Ah * 11.1V = 28Wh
Registered Member #62069
Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13
_Eugen_ wrote ...
1) Why don't you use halfbridge (or other configurations) to reduce a number of power switches (MOSFETs)? 48 stages may have been realized by 6Ñ…8 matrix i.e. 14 switches. Even taking into account "high-side" driving needed for half of them, the profit would be obvious. 2) Accuracy on your video is quite well. May be it is the best achievable for the ball-shaped accelerator. I conducted some assessments here. It can even be said that the main problem with BBs in coilguns is the accuracy, not low efficiency. Also the community is very interested in more detailed information about your accelerator (especially geomterty of the coils and wire characteristics).
Interesting assessment about the accuracy of coil guns, I think you are spot on there. I was hoping that the magnetic field and the air going between the projectile and barrel would stabilize the BB in the center of the barrel, not so. To answer your questions: 1) Because PMOS transistors are much worse than NMOS, a half bridge would have much worse performance than a single NMOS. We could of course use an NMOS for the high side too, but that would complicate the circuit considerably. The real problem however is that the mosfets operate close to their safe-area border, so one transistor can only switch a single pulse and would need some off time after that to dissipate the heat deposited in its crystal by the high current. It might be possible to design a switching pattern where every mosfet turns off for a few pulses per every pulse it fires, but imagine the complexity of the design then. 2) I can't tell the exact parameters of the coils, that's the secret sauce, isn't it :)
Registered Member #57984
Joined: Thu Nov 19 2015, 09:44AM
Location:
Posts: 58
It might be possible to design a switching pattern where every mosfet turns off for a few pulses per every pulse it fires, but imagine the complexity of the design then.
You are right but there are already realized portable projects with half-bridge configurations - not simple of course ( here and here , only in Russian but pictures give enough insight).
that's the secret sauce, isn't it :)
Oh I see :). Ok could you only say if the coils are the same on all stages? I am asking because of my own calculative investigation (not connected with any particular coilgun project).
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Electromagnetic BB gun
