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Yandersen

Mon Dec 03 2012, 10:45PM

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

Without knowing any of the technical parameters? Nothing. :)

You mean cap in parallel with the battery, in general? Just try with and without a cap and see the difference. Better to have a battery that can give the desired current so to not waste mass on cap. But if battery is weak, then you can assume your coilgun is driven from the cap - in this case you need a really big one, with low ESR and, preferably, low ESL too. Strong battery doesn't require cap helper, while weak battery need a big cap which can store multiple of energy consumed by coils. If your battery alone can push few hundreds of amps into your coils, then I don't think that cap will do much difference. But if your battery can't give the crank current you need, then your gun operate from cap and it would be better to add a charger. No rapid fire in this case, of course.

ben5017

Tue Dec 04 2012, 03:07AM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

My Lipos will be more than enough to drive the coil. I am talking about using a parallel non polar cap bank so that i can capture the energy that would be wasted, while still driving it from the battery

Yandersen

Tue Dec 04 2012, 03:56AM

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

Well, maybe you will recuperate few joules of energy. Will it be enough to drive another coil? I think, forget about the caps in your design. When you discharge cap onto coil, pulse time is predetermined. Your system uses optical triggering, so here is another incompatibility. Mine has no any sensors - each subsequent stage shots immidiately after previous one is done discharging back to it's cap. Pulse time differs due to inductance which is smaller for each subsequent stage. So my gauss has no feedback. And recuperation is relatively small, actually - less than half of energy used returns back to cap. See the table (7th and 8th stages are not done):

Oh, and take into account that cap is repolarized when first coil discharges onto it.

Steve Conner

Tue Dec 04 2012, 10:21AM

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

The best way I know of recovering energy is using the diagonal half-bridge. It is expensive in components as it needs two IGBTs, two diodes and one high-side gate drive.

Using non-polar capacitors is all very well, but they will be left charged with the wrong polarity for the next shot.

ben5017

Tue Dec 04 2012, 04:37PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Steve,

I am more interested in eliminating suck back as completely as possible to increase velocity, and not as concerned with reusing the energy to gain efficiency. That being said would an h-bridge be as effective of removing leftover coil energy as absorbing it with a non polar cap bank would be?

Steve Conner

Tue Dec 04 2012, 05:10PM

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

Probably about the same. In theory the non-polar capbank will give a half sine wave of current, the H-bridge gives a triangular pulse.

ben5017

Tue Dec 04 2012, 06:15PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Ok, sounds to me that non-polar caps would be a much simpler and cheaper solution to limit suck back, if energy recuperation was not a factor.

Would the H-Bridge make it possible to recoup energy into a capacitor bank in parallel with my batteries? Or would the caps be "full" with no capacitance left to take in the recouped energy since it would always be connected to my battery and therefore always â€œtopped offâ€.

Ash Small

Tue Dec 04 2012, 07:32PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

How about disconnecting the battery before firing, then, after firing, re-connect, but with reversed polarity, so that the battery 'tops up' the capacitor, then disconnect and fire again (still with reverse polarity,), then re-connect the battery as it was originally connected, and repeat.....

I know it sounds complicated, but it would only take a handful of 'fets and a chip to switch them.

Steve Conner

Tue Dec 04 2012, 08:48PM

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

Yes, but the same "handful of fets and chip" could be used to implement the diagonal half-bridge.

The fets might need to be bigger since they carry the firing current, but regular electrolytics are so much cheaper per joule than non-polar capacitors, you probably still win.

The half-bridge circuit dumps the energy back into the DC bus. If you have a capacitor in parallel with your battery, you can view the recovered energy as recharging the capacitor a little bit, but mostly recharging the battery.

Yandersen

Wed Dec 05 2012, 12:07AM

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

Steve, when non-polar caps used, two coils operate from one cap: first coil inverses cap's polarity once, second coil reverses it back. So there are no waste at all - whatever left from two stages is left in cap so charging it next time is easier. All you need is to disconnect the charger before firing so it will not be damaged when cap's polarity temporarily inversed by even-numbered coils.

Ben, burning leftover energy on zeners is the fastest possible way of energy dump. And the higher the voltage, the faster energy will dissipate. It is even faster than dump it onto cap. This way suckback will be eliminated - guaranteed. And all you need is a couple of paralleled high power zeners in series with your dempher diode.

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