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Project Log- Coilgun testing

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DerAlbi

Sat Dec 24 2016, 06:54AM

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

Hmmmmh. Thanks for linking the video in

Somehow the penetration power is a little low for the speed he shows. I guess his projectile is something like 8mm diameter and 30mm length which makes it around 12g projecting 50J kinectic energy. (just a guess). From my experience 50J hit way harder. Specially with sharp tip. Also if his capacitors are really 660J this means an efficiency of only 8% which is ridicules low given the number of stages.
Somehow if his projectile were a bit larger than i guessed and the speed was in feet per second i would believe it more. Still doesnt explain the efficiency. Well maybe.
To me it looks like he has a capacitor for every stage meaning this is a badly* designed SCR design.
"Badly" because:
a) he seems to use the same wire over and over again which would be inherently wrong - a timing nightmare- However i am not sure if he at least tries to vary the coil outer diameter.
b) later stages need more current, so the ESR of the small caps will destroy performance where later stages usually have the best performance..
c) his exit velocity is noticeable inconsistent which is the result of his sensor evaluation (not having a control loop). Firing one stage after another purely on sensor input yields those results. Those results are also possible with fixed timing that is designed for maximum output energy but it does not look that way.

His projectile rotation is interesting. i would love to see shots with and without those rotations over a larger distance. His mechanical solution is also pretty neat.

Karmaslap

Sat Dec 24 2016, 11:29AM

Registered Member #58215 Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65

I check youtube once a week for coilgun videos. There was just one where someone tested spin stabilization, though only for close range penetration and not for long-range accuracy. Nobody seems interested in shooting their coilguns more than a few feet into pillows or books.

The projectile looks a lot wider than 8mm... More like double that. He's also got a pointy tip which is no good unless it's not ferromagnetic which has to hurt, but the mass is a lot more than 12g from the looks of it.

I paused the video this time through and you can see bundles of capacitors with what looks like an IGBT placed on each one going into the coils, one bundle per stage. That and the well-designed wooden housing is impressive to me, this guy found something that worked and took it to completion with the largest portable rifle he could make.

A) Why not use the same wire throghout the coil and design? Most people just want to buy a big spool and use that for everything. His cable management is pretty, at least.

B) Pretty sure he has those in parallel, look like little photoflash capacitors

DerAlbi

Sat Dec 24 2016, 12:48PM

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

Its definitively a SCR design. Those packages arent IGBTs and the gate drive is not suited to drive a voltage controlled input (waaaay too long cable).

The diodes arent freewheeling diodes (look how they connect to a common light colored wire). He charges all caps in parallel but discharges single groups via SCR.
The coil length is around 3cm. Take a look here:

This resistors body length is 7.5mm. 4 of them fit into one coil meaning roughly 30mm coil length. I guess he followed the common knowledge "projectile length = coil length". Now lets draw lines on the projectile and push them to the side for analysis:

So the diameter fits a little more than 3x into the projectile length. Hmmh ok. Maybe its 10mm or even 12mm. It depends if, while designing the coils, the projectile length is measured to the nose of the tip or to the base of the tip. I guess its 10mm and the overall length is 30mm. This makes the weight approx 18g.

So again if this was 90+m/s this would equate to 73J so 11%eff. Sorry but 73J will hit much much harder than what was shown in the video. However in feet per second this would equate to only 7.5J which is not right either. Strange.

A) Why not use the same wire throghout the coil and design? Most people just want to buy a big spool and use that for everything. His cable management is pretty, at least.

Yes you are right, however if you spend so much time and money into a project you are not "most people". For best bang per buck you want a (optimal and) constant coil geometry and to adjust the timing you change the wire diameter to reach different inductances.

largest portable rifle

Hehe. You think this maybe 2m long monster with all the copper in it is even close to be portable? I would think if you hold it in you hand the barrel will bend downwards and you fall over

Karmaslap

Sat Dec 24 2016, 09:30PM

Registered Member #58215 Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65

I don't see how the circuit he has wouldn't be suitable for driving IGBTs, those cables aren't that long, but I also don't know how to differentiate between an IGBT, MOSFET, or SCR from looks alone when the packaging is the same.

I wonder who is responsible for that "common knowledge". The best coil shape for a field:inductance ratio doesnt care how big the projectile is, just how big the wire is.

I was planning on adjusting the geometry to change inductance and still don't see why I wouldn't as long as the coils fit, who cares how it looks

It's gotta be light enough to carry around. That makes it portable! With some high carbon steel or aluminum helping support he could be setting it up with a tripod. Close to what I'm planning for mine: I don't care if I can run around with it and shoot, I won't be playing rambo, but I do want to set it up and actually use it at range

DerAlbi

Sun Dec 25 2016, 04:14AM

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

I don't see how the circuit he has wouldn't be suitable for driving IGBTs, those cables aren't that long, but I also don't know how to differentiate between an IGBT, MOSFET, or SCR from looks alone when the packaging is the same.

If it were IGBTs, all caps would be in parallel, they are clearly not. Regarding the IGBT gate drive, please think about the current loop that those cables pose. The whole coilgun is nothing else than a big transformer (from a parasitics point of view) and while an induced voltage hurts the IGBT (false turn on), an induced current does not necessarily turn on a SCR. You usually cannot distinguish components by packaging, however i have seen those packages (the triangular tab) for SCRs but not for IGBTs. Put all those things together (the overall circuit, careless gate drive, and the overall medium efficiency) there is just one Element that could be accountable for that: SCR.

I wonder who is responsible for that "common knowledge". The best coil shape for a field:inductance ratio doesnt care how big the projectile is, just how big the wire is.

??? This common knowledge is correct. Diameter wise there is just one optimal shape that is OuterDiameter = 3x InnerDiameter which promises you the highest L/R time constant - this is however only important to a halfbridge design.. in a SCR design the puter diameter can be used for tuning. Length wise you want to be as big as the projectile. See the difference here:

Sorry, hand drawn with my mouse using paint, true work of art ^_^ . Of course the green line is not accurate and it actually stretches along the x-axis depending if the coil is too long or too short, however the most important feature - that is the longer dead zone inside the coil - is represented.

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