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New Coilgun Project

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Steve Conner

Fri Oct 12 2012, 08:15AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Where is the length of the projectile taken into account in the force simulations above? It looks as if you're computing the force on an infinitely short projectile element. You need to integrate it over some finite length to get a physically meaningful result.

The bigger the change in inductance when the projectile is inserted, the more efficient a coil/projectile system will be. This is why adding external iron helps. The back EMF generated by the coil can be approximated as L*di/dt + I*dL/dt, the first term represents stored inductive energy that has to be dissipated or returned to the bus, the second represents energy transferred to the projectile. There is also a IR term, energy wasted in the coil resistance, so a coil geometry that maximises Q factor is also of interest. The "Brooks coil" is an optimal air-cored coil, but maybe you should be taking the projectile into account as an iron core.

Having a projectile shorter than the coil may help by providing a force-free "drift zone" that the projectile can fly through without suffering suckback. This would allow some time for the coil current to be quenched.

Yandersen

Fri Oct 12 2012, 02:54PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Steve, you keep insisting that inductance change gives a pull to the bullet. If it is so, than what pulls the bullet after iron is saturated? In "normal" gauss, during the shot pulse, the field inside is multiples of saturation limit (up to hundreds of Teslas), and pull force increases linearly together with Amps going through the coil. However, saturated iron does not changes inductance anymore.
The more realistic model, I think, would be to consider a saturated bullet as a permanent magnet with a constant field, which interacts with a coil considered as a second magnet, both are attracted to each other. If so, than pull force, as a multiplication of magnetic strength of both magnets, will grow linearly with the grow of coil strength. And yeah, before saturation both magnets increase their strength together, that's why pull force before saturation has a square dependency on the current.
The external iron is a must, according to practical results. But it's benefit lower for powerful coils, as it works well only until saturated - when coil just starting to energize. Personally, I use a stacks of few iron washers with cut as a holders on both sides of a coils, cluing them to the barrel with epoxy and insulating them with paper on epoxy. Works very well as holders for the coils, and it is so easy to pass a wire out through the cut. :)

Saz43

Fri Oct 12 2012, 11:36PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

I think there's a lot of confusion in general as to how coilguns operate, so here's a graphic I put together that explains it.

Yandersen

Sat Oct 13 2012, 05:42AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Perfect explanation! IMHO, should be putten in first post at some topic for the beginners. Howether, it still does not give a clear answer for the question "which shape is better?" and I assume, that this initiated our discussion. I think, we better just stick to our opinions - I like short coils, Saz prefer long ones, and one day we will see, who was right, If I ever finish my 80J gauss with 8 stages. Howether, my technology is different - I use non-polar caps and recuperate energy instead of dissipate leftovers with a help of dempher diode, so not only coil shapes will contribute to the result if it will ever be achieved. Will see! :)

Yandersen

Wed Oct 17 2012, 01:44AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Saz, rereading this topic I've got few thoughts about your design. You say that you do not have an oscilloscope and you are using IR transistors to trigger the stages. So you can not be sure if triggering is precise, because you did not measured the reaction time of those devices. Ones I have, being simply pulled with a resistor, have a hundred microseconds of no-reaction and few hundreds for transition in response to IR emitter switched on. That's why I bought IR diodes once and build the precise trigger achieving the reaction time far below microsecond. Howether the microcurrents they produce in response to light change were overwhelmed by the EMI noise from the energized coil, so some of the shots the bullet was flying out slowly or even just stuck inside the coil, repeatedly energized by oscillating IR trigger. So some glitches may happen due to triggering circuit, and without oscilloscope you can not determine if it is a case. You sure you have no way to get it?
And question about 8th coil - does it still glitching? May it be voltage dropdown problem happening by the time the last coil energized?

Saz43

Thu Oct 18 2012, 05:51AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Hey Yanderson,

About the triggers, there may be a chance that the EMP from the coils is causing some spurious triggering but I doubt it. Every stage is triggered by a comparator that can be tuned via pot over a range of sensitivity. By threading a metal wire down the barrel, I tuned the sensitivity of each comparator to only trigger the coil with the IR path fully blocked. That means there has to be a significant voltage drop across the IR transistor and I don't think the EMP will be enough to cause that. Also, the IR transistor is sticking through a powdered Iron ring, so most of the magnetic field should rout around it instead of through it. I have good results for the first 7 stages so if anything is going on, it's not adverse to performance after 30+ test shots.

The 8th stage comparator failed even before a test shot, likely due to being accidentally zapped with too high a voltage somewhere. I replaced the comparator and everything works now, although I haven't fired the 8th stage yet. Currently the whole accelerator is detached from the circuit board so it can be integrated into the gun frame.

Yandersen

Sat Oct 20 2012, 05:28AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Oh, okay then. Yeah, mine was triggered by 50mV, that's why EMI was a problem even with full metal shielding.
Good luck, Saz, everyone want to see rapid fire gun of your quality, so have a pot of mana! :)

Saz43

Tue Oct 23 2012, 05:53AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Thanks!!

After 2 weeks of cutting circuit boards and soldering, most of the switching circuitry is in place. Next task is to install the +50V bus bar, glue the rest of the plastic case in, add the LEDs, then wire everything together.

Saz43

Mon Oct 29 2012, 03:19AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Hey, it's starting to look like a coilgun!

Zero

Mon Oct 29 2012, 12:20PM

Registered Member #3588 Joined: Sat Jan 08 2011, 11:19AM
Location:
Posts: 36

Looks good so far.
Cant wait.

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