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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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V3 Coil Carbine

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Author Post
ben5017
Fri Nov 30 2012, 03:25AM Print View
Registered Member #3315
Joined: Thu Oct 14 2010, 04:23PM
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Posts: 154
After my proof of concept S.C.A.R. 1000 project, I have been lurking around the forum and have slowly been making progress on the upgraded model. I decided to lay out an experimental benchtop version to finetune the design before I complete the V3CC portable version.

Project Goals of X3CC:
-To create an easily modified experimental testing rig as a stepping stone for the final version (V3CC)
-Learn to optomize a multi-stage coil gun.

Future design Vision of V3CC:
-A 6s Lipo powered, 4+1 stage, select fire, drum fed, portable coilgun

Project Goals of VC33:
-Rate of Fire: 150 rpm
-Velocity: 50 m/s
-Efficency: 8-10%

The pictures attached show my progress to date, as well as a picture of the completed S.C.A.R 1000.

Progress:
-Injector is completed and operational.
-Optical Sensor is Completed and operational.
-Stage 1 Completed

















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PSCG
Fri Nov 30 2012, 03:41PM
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Joined: Sun Mar 27 2011, 06:07PM
Location: Greece
Posts: 136
Good to see you with a new built! Are you using the design that Saz43 follows with his own multistage coilgun?
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ben5017
Fri Nov 30 2012, 03:56PM
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PSCG:
Saz has deffinatly given me some motivation to keep making progress, and my design is somewhat similar. However there are some major diffrences.

1. It will be ran off of 6s instead of 12s
2. It will have a cap bank to increase the discharge rate as opposed to running it just off of the lipo. Hopefully it will give it some more punch.
3. It uses an "injector" to smack the projectile out of the magazine, as opposed to having the 1st stage pull the projectile out of the clip
4. It will be a 4+1 coilgun (4 stages + Injector) as opposed to 8 smaller stages
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PSCG
Sat Dec 01 2012, 09:10AM
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Posts: 136
Very interesting changes. I'd like to see how the "battery - capacitor bank" combination will work.
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Yandersen
Mon Dec 03 2012, 08:15AM
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Location: Canada
Posts: 340
Ben, you asked me to enlight my design of recuperational gauss technology, but your current battery-driven design is completely incompatible with it. See, the keypoint of mine is using non-polar caps as power source for the coils. If you are going to start from scratch, let me know. By now, my 6-stage gun shows average efficiency 20.6% (recuperation is taken into account) by accelerating 6.57gramm projectile of 9.4mm caliber to the speed 54m/s while initial energy stored in 3 caps is just 55J. If such perspective will initiate you to try my technology, I'll tell in details how you can build one.
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ben5017
Mon Dec 03 2012, 06:26PM
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Yandersen,

To clarify, I am still at the beginning stages of this build, and moreover its entire purpose is as a research and testing platform for later builds. I most definatly want to experiment with your design, with modifications of course (neither of us would get much out of simply duplicating your work).

To your point. I kind of figured that it would not be compatible with a battery driven gun. However I am still interested in seeing if it can be used to capture the leftover energy in my coil and the dissipate it elsewhere after the coil shuts off. I realize that this would not allow me to reuse the captured energy in additional stages, but i am thinking it would still be more effective way to minimize suck back, compared to an anti-parallel diode, and still have the rapid rate of fire I am looking for.

Thoughts?
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Yandersen
Mon Dec 03 2012, 06:44PM
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Posts: 340
Easy. Just put a couple of high power zener diodes in series with dempher diode. When coil disconnected from the power source, the speed of energy dissipation will be proportional to the voltage spike over the coil. Choose the highest possible voltage your commutation elements can handle.
To determine the voltage spike' magnitude check the peak current in a coil. Multiply this value by coil resistance, add 1V for dempher diode and around 1.5 values of zener diode voltage rating. Make sure zener can handle the spike current for a time it will run (this characteristics are rarely shown in the datasheet, howhether). I think, couple of 5W zener in parallel will handle the spike. To be sure, put 5 or so of them in parallel and check the voltage over them during the spike. If it is just 1-2 Volts over their rating, you don't need that much - remove some and check again. Zener diode, when burned with moderate overwhelming, become a shunt resistor with few ohms of resistance, so don't be worry much about the rest of the circuit.
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ben5017
Mon Dec 03 2012, 07:31PM
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Is this setup worth pursuing since it wont be able to use the captured energy, and therefore could not give me the same magnitude of efficiency gains you achieved? It should still give me a good boost in velocity due to almost eliminating suck back true? Can you give me a reasonable guess of % increase in velocity I can expect?

What if I were to charge nonpolar caps directly off of the lipos and use it to fire the coils?, could it then recoup and use the energy if I gave the batteries some sort of protection so they couldn’t see the reverse voltage, kind of like a hybrid between a cap bank and direct lipo driven coils gun
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Yandersen
Mon Dec 03 2012, 07:57PM
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Posts: 340
Well, eliminating suckback may give you up to 10-20% of efficiency in total, while recuperation will not increase bullet's energy but doubles efficiency by reusing energy leftovers. If you would wire your coils in a proper shape (length is equal to the inner diameter, 3x outer diameter to inner), use the shortest possible projectile and add external iron (at least washers at the ends of a coils), then you can be totally sure about those numbers. But by now, just put zeners in series with your dempher diode and see the difference.
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ben5017
Mon Dec 03 2012, 08:12PM
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what do you think about my proposed "hybrid" setup
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Yandersen
Mon Dec 03 2012, 10:45PM
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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.
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ben5017
Tue Dec 04 2012, 03:07AM
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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
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Yandersen
Tue Dec 04 2012, 03:56AM
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Joined: Fri Sep 28 2012, 04:54PM
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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.
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Steve Conner
Tue Dec 04 2012, 10:21AM
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.
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ben5017
Tue Dec 04 2012, 04:37PM
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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?
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Steve Conner
Tue Dec 04 2012, 05:10PM
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.
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ben5017
Tue Dec 04 2012, 06:15PM
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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 “topped off”.
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Ash Small
Tue Dec 04 2012, 07:32PM
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Joined: Sun Nov 14 2010, 05:05PM
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Posts: 4118
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.
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Steve Conner
Tue Dec 04 2012, 08:48PM
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.
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Yandersen
Wed Dec 05 2012, 12:07AM
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Joined: Fri Sep 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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Saz43
Wed Dec 05 2012, 04:10PM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Steve Conner wrote ...

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.


Keep in mind that the half bridge will dump close to peak coil current back into the battery (100's of amps), which is probably not very good since LiPos typically have a 1C charge rate. Although it will only be for a millisecond or so, so maybe it's not a big deal. Adding a capacitor in parallel to the battery makes little difference in terms of energy recovery.
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Steve Conner
Wed Dec 05 2012, 05:29PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
How is this any worse for the battery than the equivalent discharge current? Surely for small increments of charge, the physics work the same both ways.
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ben5017
Wed Dec 05 2012, 06:38PM
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While I am still waiting for my order of igbt's and caps to come in, I got to work building the clip and locking mechanisms. It is made from a heavily modified Nerf clip and some 1/2" plastic pipe tees. The clip snaps into place with almost no play what so ever. This system will eventually be optimized and redesigned for the portable V3CC project to come, even though this initial attempt works well. As soon as my igbt's arrive I can test the injector and clip as a complete system.
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ben5017
Wed Dec 05 2012, 06:43PM
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Joined: Thu Oct 14 2010, 04:23PM
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Clip pics didnt upload for some reason last post, so here they are





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Saz43
Sat Dec 08 2012, 03:16AM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Steve Conner wrote ...

How is this any worse for the battery than the equivalent discharge current? Surely for small increments of charge, the physics work the same both ways.


Does it though? Every LiPo I've ever seen specifies a separate charge and drain rate. For example, my LiPos specify a 50C discharge rate, 100C burst rate, and a 1C charge rate, which comes with a warning not to charge over 1C or else damage can occur.

See #6 under charging instructions:

Ben, I admire your use of existing hardware. Keep up the good work.
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Hanzie
Sat Dec 08 2012, 04:06PM
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Joined: Sat Sep 08 2012, 12:22PM
Location: The Netherlands
Posts: 32
Saz is right, LiPos don't like high charge rates AT ALL. Though some may have a 65C continuous discharge, a charge rate of anywhere over 5 should be very much avoided. They will go boomzies with fire. Lots of fire. ^^
Also, you should always use a special LiPo-charger when charging your batteries as they need to charge differently from nickel or lead based cells.
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Ash Small
Sat Dec 08 2012, 04:22PM
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Joined: Sun Nov 14 2010, 05:05PM
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Posts: 4118
.....And with a firing rate of 150 rounds per minute (5 rounds in 2 seconds) the LiPo's are unlikely to be happy with that amount of current being dumped back into them.

Worst case will be fire or explosion, best case will be very short battery life, I assume.

It may well be cheaper in the long run to use non-polarised caps, and an appropriate switching circuit for the batteries, etc. if you wish to re-claim the wasted energy.

EDIT: Assuming 10% efficiency, if you use zeners to 'dump' the wasted energy, you'll need 10 times as many LiPo's for the same number of shots as you'd need if you use non-polar caps, etc. to re-claim the wasted energy anyway.
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Steve Conner
Sat Dec 08 2012, 04:42PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I'm pretty sure some brushless ESCs for RC cars incorporate regen braking. If your theory was right, that wouldn't be possible without destroying the batteries.
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Yandersen
Sat Dec 08 2012, 07:40PM
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Joined: Fri Sep 28 2012, 04:54PM
Location: Canada
Posts: 340
Hey Ben, yesterday night I've got enlighted by ingenious idea for your battery-driven coilgun! I've invented the circuit which will recuperate energy, shorten the discharge AND charge time for your coils, will consume less of the battery energy and will not charge it at all! See the schematic:

How it works.
As in your current design, coil is driven by IGBT transistor. Howhether, when it is switches off, current will deplete much faster due to discharge onto C2 cap. When coil is completely discharged, C2 will contain all recuperated energy. The less is C2 capacitance, the higher the voltage it will have. Then... Once projectile left the coil, you trigger SCR1 to return energy back to source - C2 will discharge through the coil in opposite direction, charging C1 to the voltage level which is higher than the battery voltage.

Wait a sec, the schematic has a bug - after current depletes, C1 will discharge back onto C2 through D2... Okay, give me some time, I think I know how to deal with this problem...
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Hanzie
Sat Dec 08 2012, 07:58PM
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Joined: Sat Sep 08 2012, 12:22PM
Location: The Netherlands
Posts: 32
Steve, I've got some RC cars too. (I got way too many hobbies... xD) It's true they use the engine to brake, but as far as I know it doesn't charge the batteries with it. It's usually just very little reversed throttle. Hope that explains stuff a bit. ^^
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