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

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Author Post
DerAlbi
Thu Sep 24 2015, 08:29PM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
Increasing voltage will not necessarily increase performance. It may increase the current slightly, but that may only cause deeper saturations.. so not *much* gained. Your LiPos will have a current limit and having more in series only increases the chance of having a bad one in series and that will ultimately limit the current. Even if all LiPos are fine, its not the voltage that transfers the power, its the current...
The only way to get away with higher voltage is to also increase turn-count in your coils. This will lower the current but keep the turns*current constant.
That would be totally fine then, but it wont increase your power transfer.. it basically would just increase the LiPo lifetime due to lower peak current.

Increasing the stages would be the way to go. But you have to check, if you can do that actually (with a proportional energy increase). The faster the projectile gets, the more power (current*voltage) must be provided or in other words: the same energy, but shorter time.
As i see the current in your design as a limiting factor it might be that the additional stage wont push as much energy as the 2nd or 3rd stage.
It really sucks, that you cant do a good measurements currently (how is the oscilloscope going? ).. it would be really beneficial to see how close your LiPos are allready to dying to get some starting point for a debate thats more based on facts instead of fear and believe. So currently i just can give you my opinion based on gut feeling..

If no schematic, can you tell me the topology? Have you used a Halfbdige, or do you use the typical reverse diode across the coil?
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ben5017
Mon Sep 28 2015, 12:34PM
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Joined: Thu Oct 14 2010, 04:23PM
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Posts: 154
Good points albi. (BTW I plan on ordering a scope this week)

I am using paralled bi-directional tvs diodes for clamping

I have decided to stay on 12s and add 2 stages.

I got most of the modifications roughed out on CAD this weekend.

Removed the fuse box and the injector, replaced gear motor/encoder with a stepper motor, This allowed the drum mag to be moved back enough to fit 2 more stages with only increasing overall length an additional 3/8"



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Yandersen
Tue Sep 29 2015, 08:56AM
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Joined: Fri Sep 28 2012, 04:54PM
Location: Canada
Posts: 340
Ben, paralleling TVS is not good - due to a variance in breakdown voltage most of the power may be dissipated by the TVS with lowest BD voltage. Better to put TVS with smaller breakdown voltages in series.

I also do not agree with Albi about the batteries. The more cells you put in series the better, but the coils must be rewound as well - more turns with thinner wire. After all, your input power is U*I, and it is better to aim for higher U. The pull force is I*turns, therefore the more turns the coil has the better, but you need more U to ensure that current can rise fast enough.
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DerAlbi
Tue Sep 29 2015, 12:02PM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
Nice form of disagreement. Just repeat, what i say
The only way to get away with higher voltage is to also increase turn-count in your coils. This will lower the current but keep the turns*current constant.

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ben5017
Tue Sep 29 2015, 02:39PM
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Yan,

Thanks for the heads up on the TVS diodes. Makes perfect sense after you educated me. I will be ordering new ones this week.


I am interested to see what gains in performance i can make after rewinding better and tighter(with epoxy) coils and tuning them for the new 50v setup.

(performance numbers given on previous page were using same loose and sloppy coils i made & tuned for 25v. With Delta KE of approx 5j/STAGE)

Even an increase in KE/stage of 10% would add up, especially with additional 2 stages.

30j KE might be possible

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DerAlbi
Tue Sep 29 2015, 04:04PM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
I think you should wait until you get the oscilloscope to measure if/how your swtiching and everything works on your setup right now. I think if you really do the effort to rewind the coils you should maximize the learning effect. That works best if you do good before- and after-measurements.
(for some reason i dont believe that TVS are really effective at such currents, since they absorb the energy.. but not in such orders..., yeah.. i know.. thats not based on facts.. sry)
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ben5017
Tue Sep 29 2015, 04:31PM
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Albi,

I agree. Should have the scope here in a couple weeks. Might as well get one while my electronics are still in breadboard stage..

Remaking the coils will be the last upgrade done once it is shooting consistent with 6 stages.

I still have to modify the v3 frame and cut new pieces on router. + additional electronics and coding for 2 more stages + stepper motor control.

It will be a good bit before I focus on tuning coils.


As far as the TVS diodes, I have put around 2-3 hundred rounds through with no issues. However I am always open for ways to improve my design.
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ben5017
Thu Oct 01 2015, 12:54AM
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Started drawing up the modifications for the V3 mark 2.

First pic shows what I will be reusing once I get done removing the back half.

Last 2 pics show 2 possible configurations for the mark 2

I can either

1) add two more stages with coils using similar geometry currently used and shown measured approx 5j/stage

2) split coils in half for a total of 12 shorter stages. I have the room for the extra FETs would just be alot more work.
I could use the arduino to play with the timing and sequencing to have multiple coils on at the same time for more of a rolling mag field.









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Yandersen
Fri Oct 02 2015, 06:44PM
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Joined: Fri Sep 28 2012, 04:54PM
Location: Canada
Posts: 340
Uh, I love that last option. :) Though the shorter the coils the more precise the triggering must be and you will have to deal with higher currents which is not good considering your choosed a battery power supply. Better to maximize the length of the barrel and use the same long coils.

There is no way a battery-powered gun can be efficient, so don't bother about it and aim for light weight and rapid fire to compensate this disadvantage.

And I really don't like your reloading mechanism - it's a bulky solution. Why not using a conventional spring-pushed ammo holder with magnetic plunger to push out projectiles? It can store more ammo, feed it faster, require less space and eliminates a need for a rotation motor.

And a quick tip in case you will go with longer barrel - you may need a cap on the far end of it. The longer the power rails the higher the voltage swing could be due to their increased inductance.
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BigBad
Sat Oct 03 2015, 03:25AM
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Yes, last one looks good to me too.

Short, fat coils often tend to work well.
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DerAlbi
Sat Oct 03 2015, 10:10AM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
Depends ons what "well" means. In a coilgun its ususally equivalent to good efficiency. Since he uses Batteries, he does not care about that. I am with Vandersen here.
I optimized my coil switchting to maximize the figure of merit "Ekin*Eff^2".
A Battery design does not need that. He can purely optimize Ekin.
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BigBad
Sat Oct 03 2015, 03:13PM
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Well, long coils give poorer thrust, short, pancake style coils give higher resistance and are heavier (unless you turn several on at once in parallel).

Coils where the length is equal to the winding thickness which is equal to the aperture size seem to do the best.
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Yandersen
Mon Oct 05 2015, 01:56AM
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Joined: Fri Sep 28 2012, 04:54PM
Location: Canada
Posts: 340


Short coils (length==winding_thickness) have the highest L/R ratio, therefore have lowest possible heat dissipation per energy stored. This makes short coils ideal for recuperational designs, which is not the case.

To maximize the pull force per energy stored, one should tend to minimize the OD to squeeze all of the produced flux as close to the projectile as possible - the further a turn from projectile the less it pulls (smaller portion of it's flux is inside the barrel) and the more this turn will dissipate (longer wire).

First, Ben, you should determine the OD of your coils. Use FEMM. Make a coil with one turn and constant current (as much as your battery able to supply) and gradually increase the radius of that turn, writing down the pull force and wire resistance. From those two graphs you will be able to tell how far from the barrel the external layer of a coil can be to make a reasonable ratio of pull force per wire resistance (the wider the turn the higher it's resistance and lower the pull force).

The length of your coils should be at least equal to ID. If you can make a lot of such small coils - it is good. To choose the right wire thickness, set the constant current and gradually increase turns number: see when the projectile starts to saturate (2.2T at the tip of the projectile). Then calculate which wire thickness you need to make the required number of turns fit into the chosen coil geometry.
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DerAlbi
Mon Oct 05 2015, 07:57PM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
I am with Yan at this one. Tighter turns have more coupling, so optimizing L/R is not the right thing in a system thats not efficiency bound.

However for me it begs the question:
if the short fat (lets call it Albi ) coil is most efficient, does it means that the tranfered power is relatively large?
I mean.. what is efficiency? Is it lowest loss at low energy transer? Or is it highes energy transfer while having high loss? It both represents best efficiency.
Or in other words again: how can a coil be efficient if it does not transfer the maximum of energy?
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Yandersen
Tue Oct 06 2015, 06:42AM
Registered Member #6944
Joined: Fri Sep 28 2012, 04:54PM
Location: Canada
Posts: 340
Hm, the term "coil efficiency" may be linked to a ratio of Pull_force/Heat_dissipation, or
n=F/P
Few thoughts:
1) Up to a saturation point we have force and heat dissipation proportional to the square of the current: F~I^2. After the saturation force tend to be linearly proportional to the current. Conclusion: coil's efficiency does not depend on current before the saturation, but drops after.
2) Coil's resistance linearly depends on coil's length, therefore heat dissipation does too. But force doesn't - increasing the coil's length adds turns which are further and further away from the projectile resulting in lower pull force they produce. Conclusion: the longer the coil the less it is efficient.
3) This one is obvious - the faster the projectile goes, the more efficient coil is:
Apull=F*v,
Aheat=P*t,
therefore the faster the projectile passes the coil, the less heat will be dissipated.
Conclusion: shorter projectiles are accelerated faster, therefore lead to a higher efficiency.
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ben5017
Thu Apr 28 2016, 01:09PM
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Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 154
I decided to take what i learned from the v3 and go for a compltet redesign i am calling it the V3 MARK2
Not sure whether to start a dedicated new thread or just keep rolling with this one.

I Took the same design as the V3, Which had 4+1 Stages and 18J KE
, and designed the Mark 2 with 12 stages, and a new goal of 40J KE (which would be a nice even 50m/s)

Here is a quick sneak peak of my progress (updates and edits to follow)












Pics of the molding process i used to make the iron matrix sensor housings






After the epoxy hardened, i cut out the inner diameter and outter diameter then cut the bottoms flush with a jig attached to my router

Finished: Iron Matrix sensors




I scrapped the servo/encoder setup used in the V3 and replaced with a timing belt and stepper motor.
Shown below ate the drum mag bearing and belt tensioner setup







Next, I 3d printed the mounting system for the pistol grip






Next up was to make a coil winder:








TEST COILS FOR TUNING




SPACE FOR CIRCUITS:




I will try to post some better pictures tonight when I get home. These were just the ones i had on my phone at the moment.



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DerAlbi
Fri Apr 29 2016, 02:43AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
Wohooooooooooooo
That mechanical design! That awesomeness. That tooling!

Is that a stepper motor and revolver reload mechanism?
Any idea how heavy that will become? I guess its around 8kg in the end?
What dimensions?
A 32g projectile that slow with so much energy... how will you catch the projectiles while test shooting?

What worries me:
a) Your turn count. You counter did count negative. Now the gun will shoot backwards
b) where are the switches?
c) i dont know what a Iron Matrix sensor is
d) with increasing speed, have you considered supporting your poor battery with some capacitors? If you have an oscilloscope, i would loooove to see the voltage of the pack during a shot. (any thoughts on how much current the last stage will have?)
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Karmaslap
Fri Apr 29 2016, 04:29AM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
I'm also quite curious what the iron matrix sensors are. Are they just iron end caps with optical senslrs embedded in them?

I read this thread in high school (2012) when you started and it's one of the threads thst got me interested in making my own gun, so I am looking forward to more updates! Beautiful mechanical work
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ben5017
Fri Apr 29 2016, 12:33PM
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Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 154
CAD model used to design the V3 Mark 2







Some overall Photos, It all fit together perfectly









Albi: 8-10 Kg is probly a good ballpark range, depending on final coil sizes.
It is comfortable to hold and handel with a good solid pistol grip and front grip. Being a Bulpup design, the overal weight distribution is pretty even along the length, it is not awkward at all to handle so that makes weight a little more manageable. I may add a strap in the future, we will see once she is nearer to completion

-Drum is driven by a stepper motor and timing belt.

-overall length is 32" (will update dims when i get home)

-Slug catching setup is shown on previous page.

-Iron matrix sensor housings are simply iron powder suspended in epoxy (1/2" thick). I saw the name somewhere and liked it so i stole it :) i am Not sure if they will be worth the effort it took to make them though... (on barrys coil gun site he showed promising results using the iron matrix end caps on his low voltage coilgun albeit on a less powerfull setup., only one way to find out.....)

-Thanks for reminding me about the turn counter being backwards.... It never even crossed my mind that the rotation would be reversed when driving it off of 2 wheels like i did. I was pissed first time i used it. Idiot.
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ben5017
Tue Feb 07 2017, 05:30PM
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Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 154
Bus bars made and installed.

Lots more to do, updates to come.

MAGA





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ben5017
Wed Feb 08 2017, 02:06AM
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Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 154
Few more









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Saz43
Wed Feb 08 2017, 06:39AM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Hey nice work!
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DerAlbi
Thu Feb 09 2017, 10:33PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
It worries me is how fast your turn count decays.... i dont see much turns left for the last few coils... whats your plan about this ?
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ben5017
Fri Feb 10 2017, 04:00AM
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Joined: Thu Oct 14 2010, 04:23PM
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Posts: 154
Coils you see are arbitrary, I will determine final coil geometry through testing. plan is to optimize KE from the first stage alone, then work my way down from there.

picked up a new toy. Now i just have to figure out how to use it.

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DerAlbi
Fri Feb 10 2017, 06:32AM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
I have that one too. Usage is a little frustrating because i allways move the rotary knob while pushing it. The UI is deeply structured because those small form factor scopes just dont have enough knobs for everything. Is it already cranked up to 100MHz ? ^_^
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