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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Some advices for my first coilgun

Author Post
Pinkamena
Tue Nov 29 2011, 03:22PM Print View
Registered Member #4237
Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 112
Good day!

I am currently designing a coilgun that I am planning to build next summer. I have spent the last days reading up on the subject, doing simulations, and trying to get a grip of what design would be most effective. But I am a little blown back at how difficult it is to derive the perfect design, due to all the different factors involved, like coil turns, wire width, projectile weight and size, inductance, pulse width and length, etc. So I thought I should finally make a user here (I have lurked quite a bit already), and ask for some guidance. I am getting the feeling that when it comes to coilguns, experience beats calculations.

The coilgun will be powered by my 3,6kJ cap bank that I built last summer. It works like a treat! You can see two pictures of it below.

I have for now thought of using a 4 or 3-stage design, splitting the large cap bank into smaller ones. The projectile is a 200-300 gram iron rod, 2-3 cm diameter. The cap bank will be split into 3 or 4 smaller banks, one for each stage. I have literally no idea of how many turns my coils should have, how long they should be (except they need to be the same length as my projectile), and how thick of a wire I should use. I'll use a PVC tube for a barrel.

So, yeah, I'd very much like to hear your input on this. Please let me know how I can improve this design, or how you would have built it! The only thing that's set is the capacitor bank size, everything else can still be changed. Oh, and I've already worked out a good way to get the timing of the coils right, so at least that's out of the way!

I am eager to hear your suggestions.






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Saz43
Tue Nov 29 2011, 05:19PM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 292
Sounds like a good project, and I think you're off to a good start. The multistaging is a great idea, but only if you plan on taking the time to tune each stage. Many people choose to make a multisage gun but don't put any time into tuning it, so they still end up with 2% eff or worse, which defeats the purpose and extra work.

Add one stage at a time, calculate its efficiency seperate from the other stages (simple math) and tune it until it's giving good performance. Depending on how you switch your coil, the first stage should get at least 2%, and the following stages should get 3% to 10%. Therefore, it would make sense to make the first stage less energetic than the following ones to get the low efficiency "pre-acceleration" or "injection" out of the way then hit it with the real power in the higer efficiency 2nd, 3rd, and 4th stages.

Balance that with the fact that, as the projectile speeds up, well timed coils will have shorter and shoret pulses so you will be able to add less and less energy efficiently. My best reccomendation is to use simulations and math to create a spreadsheet that predicts the timing and kinetic energy added for each coil before you go and buy or build anything. I'm more than happy to provide examples of the math if you ask, but judging by the pictures you posted I think you know what to do.

Good luck and I hope you finish!
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Turkey9
Tue Nov 29 2011, 06:35PM
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Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
If you have the funds or are lucky enough with your scavenging, I would try and build a multistage coilgun using IGBT H-bridges. I would make a small kicker coil to start the projectile going (something that doesn't need to be tuned perfect but only uses enough energy to start the projectile going down the barrel) and then have the remaining 3-4 stages switched by the IGBTs.

Here's the trick: Don't split up your bank. Since the IGBTs can be turned off (unlike SCRs) you don't have to worry about the first stage completely draining your bank. This will also cut down on the need for tuning (it will still need it, but not by physically changing the coils). I would make each coil pretty large so that the voltage on the bank won't drop much per stage. You can use an optical sensor before each coil tied directly to your IGBT for triggering. When the signal goes high, turn on the IGBT. When it goes low, turn off the IGBT.

Your main concern with this design is finding an IGBT that can withstand the current. To get an idea what current to expect, I would first start by picking the length of your projectile. Then you know your coil length too (1.5x projectile length). Start calculating inductances - either using Barry's calculator or the raw equations - for each layer added to the coil. Then using Barry's RLC sim you can find the peak current for each inductance. Keep adding layers until you find that the circuit is overdamped. The peak current for that coil is what I would use to find the right IGBT.

I know that this system is pretty complicated but you should be able to get the best efficiency out of it. Plus, that's what I would do!

In general though, I would just say to use thick wire (14 gauge and greater) and definitely make it multi-stage.
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Pinkamena
Tue Nov 29 2011, 08:37PM
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Joined: Tue Nov 29 2011, 02:49PM
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Posts: 112
First of all, thank you both for your detailed comments. Much appreciated!

Saz43 wrote ...

Sounds like a good project, and I think you're off to a good start. The multistaging is a great idea, but only if you plan on taking the time to tune each stage. Many people choose to make a multisage gun but don't put any time into tuning it, so they still end up with 2% eff or worse, which defeats the purpose and extra work.

Add one stage at a time, calculate its efficiency seperate from the other stages (simple math) and tune it until it's giving good performance. Depending on how you switch your coil, the first stage should get at least 2%, and the following stages should get 3% to 10%. Therefore, it would make sense to make the first stage less energetic than the following ones to get the low efficiency "pre-acceleration" or "injection" out of the way then hit it with the real power in the higer efficiency 2nd, 3rd, and 4th stages.

Interesting. I had thought of having more power in the first stage, and less in the next. But I now see that this approach is wrong! Calculating efficiency is a walk in the park, I study physics.

Saz43 wrote ...

Balance that with the fact that, as the projectile speeds up, well timed coils will have shorter and shoret pulses so you will be able to add less and less energy efficiently. My best reccomendation is to use simulations and math to create a spreadsheet that predicts the timing and kinetic energy added for each coil before you go and buy or build anything. I'm more than happy to provide examples of the math if you ask, but judging by the pictures you posted I think you know what to do.

Actually, I would very much like if you could give me an example of how to calculate the kinetic energy added per stage. I will definitively plan everything very well in advance, but what's giving me the biggest headache in this project, is deciding exactly how many turns each stage should have. More turns increases resistance, which increases pulse length, which decreases magnetic flux. But at the same time, more turns adds magnetic flux. I am confused.


Turkey9 wrote ...

If you have the funds or are lucky enough with your scavenging, I would try and build a multistage coilgun using IGBT H-bridges. I would make a small kicker coil to start the projectile going (something that doesn't need to be tuned perfect but only uses enough energy to start the projectile going down the barrel) and then have the remaining 3-4 stages switched by the IGBTs.

So you think I should have the kicker coil powered separately from my cap bank? I can do that. I have some big caps lying around I could use to power it.

Turkey9 wrote ...

Here's the trick: Don't split up your bank. Since the IGBTs can be turned off (unlike SCRs) you don't have to worry about the first stage completely draining your bank. This will also cut down on the need for tuning (it will still need it, but not by physically changing the coils). I would make each coil pretty large so that the voltage on the bank won't drop much per stage. You can use an optical sensor before each coil tied directly to your IGBT for triggering. When the signal goes high, turn on the IGBT. When it goes low, turn off the IGBT.

Interesting approach. I searched for IGBTs in the electronics store I use, they didn't seem too expensive. But will this not give a lot of power to the first coils, and less to the others? I thought (after reading Saz43s comment) that the last coils needed the most power. I have already designed a circuit to switch an SCR on when a beam of light is broken by the projectile, shouldn't be too difficult to alter it to work with an IGBT.

Turkey9 wrote ...

Your main concern with this design is finding an IGBT that can withstand the current. To get an idea what current to expect, I would first start by picking the length of your projectile. Then you know your coil length too (1.5x projectile length). Start calculating inductances - either using Barry's calculator or the raw equations - for each layer added to the coil. Then using Barry's RLC sim you can find the peak current for each inductance. Keep adding layers until you find that the circuit is overdamped. The peak current for that coil is what I would use to find the right IGBT.

I know that this system is pretty complicated but you should be able to get the best efficiency out of it. Plus, that's what I would do!

In general though, I would just say to use thick wire (14 gauge and greater) and definitely make it multi-stage.

Hmm... Yes, a little more complicated than what I had in mind, but I can make it work. Multistage is a no-brainer! But should I not use thinner wire for the first stages, and thicker for the last ones, in order to increase current per stage, thus increasing magnetic flux? I was under the impression that the magnetic flux should increase, and pulse lenght decrese, per stage.
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Saz43
Tue Nov 29 2011, 10:02PM
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 292
To clarify, you want a small amount of energy in the first stage because it will be the least efficient. After that, you want to have considerably more energy in the 2nd stage, but from there you will use less and less energy in subsequent stages because as the projectile goes faster, there will be less and less time for your coil to act on it as it flies through faster and faster.

I'll do an example of the math and post it when I get home.
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Turkey9
Wed Nov 30 2011, 12:51AM
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Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Pinkamena wrote ...

But should I not use thinner wire for the first stages, and thicker for the last ones, in order to increase current per stage, thus increasing magnetic flux? I was under the impression that the magnetic flux should increase, and pulse lenght decrese, per stage.



Are you thinking thinner wire with same number of turns will decrease the magnetic flux because of it's higher resistance than thicker wire?

When working with coilguns, the resistance from the wire doesn't have a noticeable effect on the peak current when compared to it's inductance. What the resistance of the coil does do (along with the ESR of the capacitors) is damp the waveform. What you'll want is to vary the number of turns with the same thickness of wire. More turns, higher inductance, less current, longer pulse.

The kicker coil from a different supply will take care of the efficiency concerns Saz was talking about. It will be terribly inefficient but that won't matter as it will be only a small percentage of the total energy. To take care of the issue of most of the energy being used in the first stage, you need to make your coils very large. The high inductance should help make sure that only the proper amount of energy is used in each coil. This however is where there is some uncertainty: how many turns? This is where you might have to guess or experiment. If the coil is too large, the current won't build up very high (and the magnetic field) by the time the projectile has reached the center and the coil is switched off. If the coil is too small, most of the energy of your bank will be used in the first stage, or worse, all of it.

For the IGBT, I doubt the ones you found in your store would work. They will need to handle at least 1000A @ your bank voltage.

Now that I think about it, the IGBT approach will work better the faster the projectile is traveling initially. Ideally, what you want is for the voltage on your coil to look like the very first part of the normal RLC waveform where the voltage is decaying the slowest. The faster your projectile is moving, the shorter time the IGBT is on and the less voltage decay occurs. Anyway... you can play around with a system like that and figure out the best configuration. There aren't vary many good studies of this type of coilgun on this site.
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Saz43
Wed Nov 30 2011, 02:40AM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 292
As promised. The question on how to design a coil comes up all the time, so I figured I'd make something that explains it all at once. Maybe if I have more time in the future I'll make it in a nicer format.

Note that there are several assumptions and guesses required here, this is not an exact process, but it will result in a much better coil than you would get without it.


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Turkey9
Wed Nov 30 2011, 07:13AM
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Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Nice Saz. I have a couple questions...

You say that efficiency is hurt by high peak current... Why is that? Is it because with a high peak current the system is under damped and much of the energy will be wasted in the protection diode? Or is it that with a high peak current and a critically damped system the pulse length would be too long? Maybe change that to a ratio of the pulse length to the peak current...
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Pinkamena
Wed Nov 30 2011, 11:31AM
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Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 112
Turkey9 wrote ...

Are you thinking thinner wire with same number of turns will decrease the magnetic flux because of it's higher resistance than thicker wire?

When working with coilguns, the resistance from the wire doesn't have a noticeable effect on the peak current when compared to it's inductance. What the resistance of the coil does do (along with the ESR of the capacitors) is damp the waveform. What you'll want is to vary the number of turns with the same thickness of wire. More turns, higher inductance, less current, longer pulse.

Thanks for clearing that up.

Turkey9 wrote ...

For the IGBT, I doubt the ones you found in your store would work. They will need to handle at least 1000A @ your bank voltage.

I tried out Barrys RLC simulator, and I found that the coils never really got more than 350A. Also, can I not put some IGBTs in parallel to make them able to handle more current? I might have missed the point of an IGBT H-bridge, but it seems logical to me to do that.

Saz43 wrote ...

As promised. The question on how to design a coil comes up all the time, so I figured I'd make something that explains it all at once. Maybe if I have more time in the future I'll make it in a nicer format.

Note that there are several assumptions and guesses required here, this is not an exact process, but it will result in a much better coil than you would get without it.

Thank you very much, this will undoubtfully be valuable to me. I have a few questions though.
1. What turkey just asked.
2. What is a v-switch, I see you put it under efficiency pros. Also, what do you mean with Flux augmentation?
3. You say that the difference in t(rlc) and t should be less than 0.01 ms. Is this correct, or did you mean to write second instead of millisecond? I feel that would make more sense.
4. You say that a small/light projectile is bad for efficiency. How can I decide how big the projectile should be? Is it simply "Bigger is better"?
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Saz43
Wed Nov 30 2011, 07:17PM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 292
Turkey9 wrote ...

Nice Saz. I have a couple questions...

You say that efficiency is hurt by high peak current... Why is that? Is it because with a high peak current the system is under damped and much of the energy will be wasted in the protection diode? Or is it that with a high peak current and a critically damped system the pulse length would be too long? Maybe change that to a ratio of the pulse length to the peak current...


High peak current or just high current in general hurts efficiency because it tends to saturate the projectile. After the magnetic field is strong enough to saturate the projectile, the projectile will respond poorly to any further increase in field strength, you're familiar with the phenomenon but for anyone who isn't, link. You can get around this to some extent with a projectile made of material with a higher saturation point, but most coilguns use steel nails or screws, which already have a pretty high H_sat.

Pulse length mismatch and poor commutation (current wasted in diode) are also on that list but mostly are separate issues.


Pinkamena wrote ...

Thank you very much, this will undoubtfully be valuable to me. I have a few questions though.
1. What turkey just asked.
2. What is a v-switch, I see you put it under efficiency pros. Also, what do you mean with Flux augmentation?
3. You say that the difference in t(rlc) and t should be less than 0.01 ms. Is this correct, or did you mean to write second instead of millisecond? I feel that would make more sense.
4. You say that a small/light projectile is bad for efficiency. How can I decide how big the projectile should be? Is it simply "Bigger is better"?


1. See above
2. V-switch: . Flux augmentation:
3. I did mean 0.01 ms, coilgun current pulses are measured in ms, so the agreement should be within a fraction of a ms if you calculations are going to be relatively precise.
4. Heavier is better for efficiency, but bad for speed performance since heavier projectiles go more slowly. The best way is to do some research here, find other coilguns like yours that were successful, and look at the size of the projectile they used. The math I showed you will also help you predict how a given projectile will work. If it's way to light or heavy, you'll have a tough time getting t and t_RLC to converge for a realistic coil, bank & eff value.

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Pinkamena
Wed Nov 30 2011, 07:29PM
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Thank for your reply.
I have a question regarding the switching:

The way my switching circuit is now, an IGBT receives a signal when a beam of light is broken. When the beam is restored, the IGBT turns off. I would like to expand the signal, so I can decide exactly how long it will take until the IGBT is turned off. Unfortunately, a circuit like this is a bit out of my ballpark. I've tried many different approaches, but none works. How should I go ahead and make this work? If you have a schematic for such a "signal expander" or whatever you could call it, I'd like to have a look!
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Saz43
Wed Nov 30 2011, 07:38PM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 292
Pinkamena wrote ...

Thank for your reply.
I have a question regarding the switching:

The way my switching circuit is now, an IGBT receives a signal when a beam of light is broken. When the beam is restored, the IGBT turns off. I would like to expand the signal, so I can decide exactly how long it will take until the IGBT is turned off. Unfortunately, a circuit like this is a bit out of my ballpark. I've tried many different approaches, but none works. How should I go ahead and make this work? If you have a schematic for such a "signal expander" or whatever you could call it, I'd like to have a look!


Yes, the signal from the detector could trigger a simple NE555 in monostable mode which would then create a pulse with a duration of your choosing.

But the question is, why would you? Use another detector (or better, the same one) to turn it of when the projectile is in exactly the right position. That way you have closed loop control (exact time communicated), rather than open loop (pre-programmed guess).

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Pinkamena
Wed Nov 30 2011, 07:48PM
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I would do that, but the problem is that the projectile will be inside the coil when I want to turn it off. Kind of hard to get a beam of light to pass through a coil XD Or are you perhaps thinking of another approach?
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Turkey9
Wed Nov 30 2011, 08:28PM
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Location: Boulder, Co
Posts: 661
If the sensor is right at the start of the coil, and the projectile is the same length as the coil, then when it leaves the sensor and the IGBT turns off, it will be in exactly the center of the coil.

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Pinkamena
Wed Nov 30 2011, 08:36PM
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Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 112
Turkey9 wrote ...

If the sensor is right at the start of the coil, and the projectile is the same length as the coil, then when it leaves the sensor and the IGBT turns off, it will be in exactly the center of the coil.



Yes, but I thought the coils needed to be a little longer (About 1.5 the length of the projectile).
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Saz43
Wed Nov 30 2011, 08:42PM
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Posts: 292
That depends on who you ask. I'd say always make your coil the same length, Barry says 1.33, others say 1.5. DARPA's massively multistaged coil mortar had coils that were maybe only 10% of the length of the projectile (although they maye have had more than one coil on at a time, im not sure). Nothing is absolute here, but anything between 1 and 1.5 should be good for a hobby coilgun.

Also, research actually suggests you place the detector a few mm before the entrance of the coil.

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Pinkamena
Sat Dec 03 2011, 03:41PM
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Posts: 112
Hi guys, just thought I'd show you what I've been doing lately.
I've made an interactive simulation in Wolfram Mathematica, showing the current as a function of time, coil thickness and air gap radius. In this simulation, time and air gap radius can be dynamically altered, to show how the current will flow in the coil. Resistance, inductance, and lenght (to always keep the projectile with a mass of 300 grams) is also calculated dynamically. The curve is "blocky" due to large increments of wire being added each time the coil thickness becomes large enough to add a new layer to the coil.

Here's an album with some screengrabs showing how current will flow in a coil using AWG18 (1.06 mm) with a resistance of 0.021 ohm/meter, and an inner coil diameter of 1.7 cm. (Notice the image that say "70ms". I forced the resistance down to 0.001 ohm to create some heavy ringing in the coil! Very fun.)





EDIT: I almost forgot, here's the code in case you have Mathematica and want to try it out yourself.
r0 := x
Slider[Dynamic[ri], {0.0056, maxri}]

diam := 0.00206 (*diameter wire*)
rw := 0.012 (*ohm/meter*)
convert := 39.37
Slider[Dynamic[time], {0, 0.1}]

d := 7870 (*mass per m3*)
v0 := 290 (*cap voltage*)
c := 92.8*10^-3 (*cap capacitance*)
w := 0.300 (*weight projectile*)
magconst := 1.25663706*10^-6 (*magnetic constant*)

L := 0.5 w/{d*{ri^2}*Pi} (*lenght*)
maxL := 0.2
minri := Sqrt[{0.5 w}/{d*Pi*maxL}]
maxri := 0.013
maxr0 := 0.07
minr0 := ri + diam
turnsPrLayer := L/diam
wireLength := turnsPrLayer*2 Pi*{Sum[{ri + i*diam}, {i, 0, maxLayers}]}
turns := maxLayers*turnsPrLayer
beta := L/2 ri
alpha := r0/ri
maxLayers := Floor[{r0 - ri}/diam]

(*Inductance formula*)
res := wireLength*rw (*Resistance ledning*)
induc := {{31.6*turns^2*ri^2}/{6 ri + 9 L + 10 {r0 - ri}}}*10^-6

(*Currentformula*)
betaI := Sqrt[{1/{induc*c}} - {res^2/{4*{induc^2}}}]
alphaI := res/{2*induc}
curr := {v0/{betaI*induc}}*Exp[-alphaI*time]*Sin[betaI*time]

(*F (a, b) formula*)
F := beta {ArcSinh[alpha/beta] - ArcSinh[1/beta]}

(*H0 equation*)
H0 := turns*curr*F/2*beta*ri*{alpha - 1}

(*Plot3D[curr, \
{x,0.012,0.1},{y,minri,0.01},PlotPoints->25,AxesLabel->{thickness,\
airgap},ViewPoint->Front, PlotRange->{-100,100}]*)
 "time in seconds:" Dynamic[time]
"air gap radius:" Dynamic[ri]
"Length of coil:" Dynamic[L]
"turns per layer:" Dynamic[turnsPrLayer]

Dynamic[Plot[curr, {x, minr0, maxr0}, PlotRange -> {-100, 1000}, 
  AxesLabel -> {coil thickness, current}, Filling -> Bottom]]
Dynamic[Plot[turns, {x, minr0, maxr0}, PlotRange -> {0, 5000}, 
  AxesLabel -> {coil thickness, wireturns}]]
Dynamic[Plot[maxLayers, {x, minr0, maxr0}, PlotRange -> {0, 30}, 
  AxesLabel -> {coil thickness, layers}]]
Dynamic[Plot[wireLength, {x, minr0, maxr0}, PlotRange -> {0, 1000}, 
  AxesLabel -> {coil thickness, wirelength}]]
Dynamic[Plot[res, {x, minr0, maxr0}, PlotRange -> {-2, 10}, 
  AxesLabel -> {coil thickness, resistance}, Filling -> Bottom]]
Dynamic[Plot[induc, {x, minr0, maxr0}, PlotRange -> {0, 0.04}, 
  AxesLabel -> {coil thickness, inductance (H)}]]
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Turkey9
Sat Dec 03 2011, 07:41PM
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Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Nice. I don't know if mathematica can do this, but you should do a 3D plot with time on one of the axis. The peak current for each coil will be in different places as the turns vary so a 3D plot would be really interesting to see.
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Pinkamena
Sun Dec 04 2011, 03:09PM
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Joined: Tue Nov 29 2011, 02:49PM
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Posts: 112
That can be done, but it is VERY time consuming to plot it. I made a 3D plot with coil x=outer radius, y=coil inner radius, and z=current, and it took over 8 seconds to calculate it every time I changed the time variable. It is much easier to look at the 2D plot with a fixed inner coil radius.

EDIT:
With an inner coil radius of 1 cm, and an outer coil radius of around 1.5 cm (still AWG18), I am getting a peak magnetic field strength of over 6000 kA/m (This is at 1 ms, @1800A. The magnetic field quickly drops 0, takes about 150ms). Does this seem very unlikely? I could also mention that at these coil specifications, the wire length is around 28 meters.
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Jack A
Mon Jan 02 2012, 02:24AM
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Posts: 28
Hi all! Just wondering, all of you have been talking about how the first stage is the least efficient, why is this? (time in coil perhaps?)

Cheers!!!
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Jack A
Thu Jan 05 2012, 06:58AM
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bump bump
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Turkey9
Thu Jan 05 2012, 08:28PM
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Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
In my thinking, it is partly due to overcoming static friction which is always greater than kinetic friction. Other than that, the statement is mostly based on experimental results from many different people.

Also, double posting, especially to get attention, is against the rules.
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Saz43
Fri Feb 24 2012, 12:27AM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 292
Turkey9 wrote ...

In my thinking, it is partly due to overcoming static friction which is always greater than kinetic friction. Other than that, the statement is mostly based on experimental results from many different people.

Also, double posting, especially to get attention, is against the rules.


I'm bumping this thread for good reason, I've got a theoretical answer to this question.

First I want to discount the friction theory. Barrel friction should be negligible compared to the forces exerted by the coils. I've got experimental evidence for this. My three stage coilgun gets 3.5, 8.3, and 10% efficiency on respective stages. If i gently budge the projectile into the breech (static initial conditions), the coilgun accelerates it to 27m/s. If I use my finger to knock it pretty hard (at least a few m/s) into the breech, still 27m/s, no change. This was done multiple times with just one stage and with all 3 stages- always the injection velocity made no consistent difference on exit velocity. So from that I conclude that the difference in performance is not due to static friction. Furthermore, I suggest that injection velocity does not directly improve efficiency.

Here's why (I theorize) the later stages are more efficient: not because of initial velocity, but because of initial projectile magnetic field. Consider the projectiles point of view. The first stage imposes the greatest change, from zero flux through the projectile to a very strong magnetizing field- this rapidly changing magnetizing field induces eddy currents in the projectile, which along with hysteresis are responsible for the additional losses. As the projectile exits the first coil and enters the second, it's still magnetized because current in the first coil is still ramping down as current ramps up in the second coil. As the projectile moves faster and faster, time between stages reduces, and ramp-up and ramp-down overlap more and more. The less the magnetizing field changes between stages, the less eddy current and hysteresis losses will be experienced in the projectile. Eventually the projectile sees a constant magnetizing field, and no eddy currents or hysteresis occurs at all.

What do you think? This is based on my beginners knowledge of magnetics and may have some holes, but I'm convinced static vs sliding friction is not the explanation. Please critique!
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Turkey9
Fri Feb 24 2012, 05:25AM
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Location: Boulder, Co
Posts: 661
Do the currents in the coils really overlap? I would think that current would have to be at zero by the time the projectile reached the center of the coil to prevent suck back. Magnetic field is after all proportional to the current. It doesn't have a phase change like between voltage and current. So by this thought, there would never be any overlap in the current ramp down and ramp up between the coils.

Not saying that the magnetizing of the previous coil doesn't occur. I'd be interested to see a trace of coil current as the stages fire that can relate them all on the same timeline. Same with magnetic field. I might do this using hall sensors on my current multistage design.

An interesting experiment: Have a coil that has no purpose except to magnetize the projectile. Have it so that while on, the projectile is in the initial position for the accelerating coil and doesn't move at all. Then turn in off just before the first coil fires. Compare the velocity with the a shot without the magnetizing coil on and see if there is a difference.
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Saz43
Fri Feb 24 2012, 07:29AM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 292
Turkey9 wrote ...

Do the currents in the coils really overlap? I would think that current would have to be at zero by the time the projectile reached the center of the coil to prevent suck back. Magnetic field is after all proportional to the current. It doesn't have a phase change like between voltage and current. So by this thought, there would never be any overlap in the current ramp down and ramp up between the coils.


I'm thinking of the case where the coil is the same length as the projectile. So if you open the switch when the center of the projectile is at the center of the coil, the nose of the projectile is already about to exit the first coil and enter and trigger the next coil. If the current in the first coil takes long enough to quench and if the projectile is moving fast enough and if the coils are close together, there will be some overlap. I guess you could also test this theory by spacing the coils far apart so as to ensure that the projectile isn't magnetized in between.
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Turkey9
Fri Feb 24 2012, 08:38AM
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Location: Boulder, Co
Posts: 661
I really want to understand what is causing the efficiency increase in a multi-coil design. There are many different ideas that seem to make sense to me, but they are still just theories. I think I'm going to try and make a projectile that has an accelerometer attached to it to get a good idea of the forces over time while accelerating in a coil. The projectile will push a light thin plastic rod with the accelerometer board attached to it. This of course will only work on at most 2 stages, but would be interesting to see. Once I get my project done, I'm going to experiment with different injection velocities as well. The hobby community needs a good source of data to answer common questions that no one has a solid answer to.
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Saz43
Fri Feb 24 2012, 06:57PM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 292
Turkey9 wrote ...

I really want to understand what is causing the efficiency increase in a multi-coil design. There are many different ideas that seem to make sense to me, but they are still just theories. I think I'm going to try and make a projectile that has an accelerometer attached to it to get a good idea of the forces over time while accelerating in a coil. The projectile will push a light thin plastic rod with the accelerometer board attached to it. This of course will only work on at most 2 stages, but would be interesting to see. Once I get my project done, I'm going to experiment with different injection velocities as well. The hobby community needs a good source of data to answer common questions that no one has a solid answer to.


Great! I'm surprised that this hasn't been done yet. Please keep us updated on your progress and findings because this is something I'm really interested in. You might also consider testing magnetic vs. spring injection at the same velocity to determine if it's a kinetic or EM effect

On another note, this experiment casts some doubt on my theory of overlapping current. He gets an efficiency boost from 4% to 14% from the first to second coil, and the current has plenty of time to fall to zero in between. Does magnetization take some time to wear off? I need to do more reading to understand this fully. I'll let you know if I find anything interesting and relevant to your research.

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Forty
Mon Feb 27 2012, 07:27PM
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Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
I don't think an accelerometer would be useful unless it could measure 100's of g's (example calc: 0 to 20m/s in a 2cm length -> ~1000g)

Couldn't any residual magnetization be found from the BH curves of a material?

If the projectile material does follow the BH curves and has a residual magnetization, then successive coils wound in the same direction as the first would cause the projectile to be attracted to the coils more strongly i imagine. If that is the case, then the first coil is acting on an initially unmagnetized projectile and so would have less attraction. This, along with the law of inertia and to a lesser extent the static>kinetic friction would decrease the efficiency of the first coil.
I suppose you could test the residual magnetization idea by placing a permanent magnet outside of the coil such that a non magnetized projectile would be attracted to it, but a projectile magnetized in the direction of the coils B field would be repelled.
Another method would be to reverse the current direction on a 2 stage gun's 2nd stage. If the projectile is magnetized when it reaches the second coil, it will be slowed down compared to when the coil current was in the other direction.

Residual current in the coil would be a bad thing after the projectile passes through the center because it would experience a force in the opposite direction of its motion (suckback)
Another interesting thought: If the residual magnetization of the project does occur (to an appreciable extent) then that would seem to suggest that the negative current spike of an underdamped coil would cause an additional repulsive force if it occurred as the projectile left the coil.

any thoughts? I'm a bit hesitant to believe anything i said, despite it initially making sense.
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Saz43
Wed Feb 29 2012, 04:20PM
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Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 292
Forty, the residual magnetization thing is kind of what I had in mind when I was thinking that the first coil does most of the magnetization of the projectile. I think there is some validity to that and it would be interesting to experiment with magnetic and non magnetic injection techniques.

However after some more reading I've found that, like most questions about coilguns, the efficiency question has already been answered by James Paul and he points to a different cause.

He states that the cause of poor efficiency in linear motors is a low motion induced voltage. He explains the principles and derives some basic equations for the performance of a linear motor here. See page 4 for the results. I’ll sum them up here:

With a typical clamped inductive load (coil with no projectile), 100% of the supply energy becomes thermal energy, dissipated in the coil’s resistance. When you introduce the projectile, some of the supply energy is converted into kinetic energy of the projectile. From the supply’s point of view, this energy loss appears as an induced voltage of opposite polarity created by the moving magnetized projectile as it passes through the coil. Also, the induced voltage impedes current flow through the coil, which reduces the thermal energy wasted in the coil’s resistance.
This answers the question as to why the later stages are more efficient. Induced voltage is proportional to velocity- so as the projectile moves faster, it induces greater voltage in the coils, causing less energy to heat the coil and more energy to transfer to the projectile.

Here we see the effect illustrated clearly, thanks again to James Paul (in the first graph). You can see what James calls a “knee” in the slope of the leading edge of the current pulses for the second and third coil. These knees indicate the point where the projectile enters the coils, and the induced voltage causes a reduction in coil current, and thus a reduction in coil heat dissipation. No such knee is present in the first coil’s current plot because the projectile is initially at rest, with nearly 100% of the energy going to heat since there is no induced voltage until the projectile gets moving.
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