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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Project Log- Coilgun testing

Author Post
Karmaslap
Fri Jan 15 2016, 02:30AM Print View
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
I am currently working on putting together a small coilgun. The goals of the project aren't big- I just want to see how well simulations and theory can line up with reality when building a coilgun, and create a small change-friendly prototyping platform. Progress will be slow, as I put school first and I really like to think things through and simulate them before testing. I will also likely stop along the way for other little projects. The purpose of this thread is to serve as a log for me for what I've accomplished, as well as to ask questions and provide some incentive for me to keep moving.

Goals:
-Create small coilgun with available materials
-Have an "acceptable" correspondence to the theoretical values with regards to efficiency and projectile energy.
-Learn for future coilgun projects
-Stay under 50$ budget

Materials:
-16 camera photoflash capacitors totalling ~2250 uF. Some of them do not have their capacitances labeled and I guessed based on their size and shape, but this should be a respectable amount of energy at 330V (~120J)
-8mm diameter x whatever length I want steel projectiles. No idea on the carbon content of them, but they were pretty easy to cut with a belt saw. I initially cut them to 40mm length for a 5x length to diameter ratio- what should be the upper limit for projectile stability in the event I decide to try to spin them (which I will eventually try with a high-rpm hard drive disk motor).
-Assorted resistor values, and some small diodes. Most electrical components which can be found at a University.
- Arduino for timing if I decide to go with a controller (hopefully I can do this with logic gates)
- a few power supplies from cameras to charge the capacitors- 4 if they all work
-Assorted wires, connectors, switches, gates, misc items that might prove useful

Needed Materials:
-IGBTs- per Deralbi trumpeting the benefits these have in a half-bridge configuration over SCRs, I'll be ordering some offline when it comes time to use them over a normal mechanical switch.
-Photogates/phototransistors for multistage- My college didn't have any in any of our components bins, but I can order offline.
-Barrel for the 8mm projectiles- I can 3d print things about 1/2mm thick but don't want a 3d printed barrel if I can find something better. The printer will help with other things such as coil forms, though.
-Components for a better power supply/control circuitry for the halfbridge etc.
- Magnet wire for the coils! If I had some, I would be winding up a prototype now instead of posting on the forum. I need to do some basic optimization work for the projectile size and capacitors I have to determine the size of the inductor I want, and what wire I can use for it to get a low resistance. I am thinking 18AWG wire just from what I've seen, it shouldn't be burning up.
-Various miscellaneous materials (some copper rails for the capacitors, epoxy for coils, whatever else I need when I run into it).

Initial Plans:
-Run brief simulations and determine necessary materials to order
-Begin single stage gun, and log data.
-Expand to 2 and then 3 stages (3 being the max I plan for this project)
-Test various coil timing, projectile, loading mechanisms, power supplies, and control architecture to learn what works for a later larger scale project

I am lucky enough to be on a college campus with access to their eelctronics lab, machine shop, 3d printer, and underclassmen willing to press the button to fire the coilgun with exposed wiring.

Some Pictures
Projectiles and the bar (from a printer) they were cut from:



Badly taken shot of the lab I get to work in: Pretty much whatever tools I need, scopes, power supplies, whatever. I'd like to get started on the project before I graduate and have to buy all this stuff for myself.


Capacitors and charger circuits, soldered into random formats from the small gun I built last year with some friends, and some caps I found that a graduated senior left after trying to make a taser...


As this thread is mainly for me to record my own progress, I will be editing updates onto the first post so that it isn't bumped to the top and so that I don't run into any trouble for double posting.

Edit 1/16-
Found two coils that my friends and I made when we originally got all the camera capacitors. Hooked one up to a single 80uF cap charged to 360V and found a small projectile to fit into the pen we used as a barrel, and it shot pretty well (4-5% efficiency) a 5 gram projectile or so. Was fun lauching it across the lab.

Naturally, I hooked up ALL the capacitors. It takes a long time to charge with one photoflash circuit. Fired, but didn't work- the projectile appears to be getting stuck where the coil is wrapped around the barrel. It seems the coil is crushing the plastic pen barrel enough to stop the projectile. Did a quick simulation and seems like the pulse is twice as long now (or more as I don't know how to get LTSPICE to show/simulate the current rising through the inductor, it just magically starts at the peak when I set up an RLC circuit)

I didn't expect it to work all that well, and I'm using REALLY thin wire. I'm rethinking that I wanted to get 18AWG wire- of course I'll look at the resistance increase, but I'll check that vs the number of turns that can fit in a coil and how the inductance increase of more turns +more resistance changes the waveform more closely.

If anyone has an opinion on the diameter of wire I should use, I'd be interested.

Most importantly, I'd like to order IGBTs for the gun (I would like to get enough of the size I pick for a single half-bridge, or to fire two stages with a single LGBT. I don't mind putting them in parallel either, which it seems others have, but I'm not sure that paralleling them is very helpful? With two, if each has a small internal resistance, then in parallel their equivalent resistance is 1/2 of what a single one is, meaning that there is the same current traveling through two in parallel as there would be through one alone, but I've seen people here say that it helps?

I am looking at a ~450A peak which should, of course, be very brief (maybe 2 ms above 100A). I'm not entirely sure what IGBT to pick for this as mouser and digikey list their specs for continuous current, and I have no experience with them (or ordering any electrical parts offline, which I plan to remedy). I could order some and try them, but that could get expensive fast and there are others with experience to ask.



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Karmaslap
Sat Jan 16 2016, 11:59PM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
[QUESTION]
How does the "Continuous Collector Current" correspond to the peak current I can put through? Is that what I should be looking at? Can I get a 100A IGBT, or two in parallel? ($6.00 cost) or do I need a much higher rating? (400A- 20$ or so) being safe, 550A+ peak is only 5$ more than the 400A ones, but if it isn't necessary and the lower one is perfectly safe I'll go cheaper of course.

Looking at datasheets I am guessing that "I_CM" is the max peak collector voltage as it has a time next to it. In this case I could use
<http://www.mouser.com/ProductDetail/IXYS/IXGX120N60A3/?qs=sGAEpiMZZMv4z0HnGdrLji290Jq8RNevx%252bS3kPRRMvw%3d>
This IGBT and would have enough to buy 4 of them along with whatever wire I need. Does this look acceptable?
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Blackcurrant
Mon Jan 18 2016, 02:19AM
Registered Member #2989
Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 71
if you have 450A at 2ms then that igbt may be able to handle it as the data sheet for it says 600A at 1ms

A lot of the manufacturers have application notes and how to use them (Ti ST microchip ixys etc)
look for something to explain all the device data sheet parameters
a few other things to think about wound be

The drive circuits required for the igbt
mounting the igbts (some have isolated mounting tabs that make things easier ie isoplus247)
IGBT protection (like snubber circuits)

When buying parts see if you can buy direct from manufacturer or look around as sometimes they are far cheaper on other sites, also the college may get a discount from somewhere. You also maybe able to get free samples from manufactures.

Build a simple and reliable discharge circuit.
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Karmaslap
Mon Jan 18 2016, 03:11AM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
Thanks for the reply and advice. I won't order the IGBTs until I'm done modeling what wire size/coil turns and geometry to use.

I was under the impression that I could make a drive circuit fairly easily for them, though I have yet to look into how. I think making a driver circuit for a MOSFET is covered in one of my electronics labs this semester.

Thanks for the tip, I will look for those to help me read datasheets. Most of it is sensibly marked, at least.

I didnt think IGBTs needed such protection? I think by snubber you mean a diode antiparallel to the capacitor for when the current is shut off. I plan on hooking the IGBTs up in a half-bridge and getting rid of the need to do such a thing.

My discharge circuit right now is just the coil itself for safety discharges, I should probably up the level of safety for the whole project.

Update 1/17/16
Tried doing some LTSPICE simulations regarding what wire guage to use for the coils. Tested 10 AWG, 18AWG, and 24AWG with various coil geometries (inside diameter fixed for the projectile). 18AWG looks the closest to what I want so far: with a 40mm long 25mm diameter coil with 9mm wide opening (1/2mm barrel width if I could do that somehow, maybe rolled and epoxy hardened paper) and my cap bank, I can get to a 4ms wide pulse. The problem would be that it is very underdamped, but adding in a resistor to help that lowers the peak current by a large amount. Am hoping the IGBTs will nicely switch to cut off the current and maybe recharge the capacitors. If anyone has an opinion on wire diameter go ahead and share, otherwise I think I will do more simulations to see what the "optimal" diameter is, and then use it for now until I can order more wire and actually test.

All I have tried for is a low pulse time with high peak current, I haven't looked at the number of turns and how this will affect fhe magnetic field strength yet but best to get a feel for how one changes before worrying about the field strength.
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DerAlbi
Mon Jan 18 2016, 09:44AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 504
and it shot pretty well (4-5% efficiency) a 5 gram projectile or so
Beeing the ass, i say: unlikely On a SCR-based (or just mechanical switch?) design this is possible but achieving it on the first try? Lucky.. or systematic error.
1) Check your capacitors, if their capacitance value is actually as stated. These things can have -20%, and this is most likely (i feel like they produce +/-2%, and design the caps to -82% of their rated capacity, just to shit on the customers head )
2) Double check your speed measurement.
3) Make sure you measure way beyond the coil. (at least one projectile length). You should currently suffer from suckback. if you measure too close to the coil, then you might get false readings.
4) Depending on your measurement use 2 sensors to verify the speed. You may simply shoot the projectile through an other (unconnected) coil. Just hook up your oscilloscope to the open ends... the remanent magnetized projectile will induce a voltage in the shape of x*exp[abs(x)^3]. The time between maximum and minimum of that waveform divided by your projectile length will be the speed i guess. If not... its interesting too!
I didnt think IGBTs needed such protection? I think by snubber you mean a diode antiparallel to the capacitor for when the current is shut off. I plan on hooking the IGBTs up in a half-bridge and getting rid of the need to do such a thing.
IGBTs need to be protected against the stray inductance and ESL of your cap bank. Please look at "active clamping" circuits. Snubbers wont do it. Without protection your IGBTs are dead after the first test.
I can get to a 4ms wide pulse. The problem would be that it is very underdamped
Wow. What time constant does your coil have? (L/R)

Am hoping the IGBTs will nicely switch to cut off the current and maybe recharge the capacitors. If anyone has an opinion on wire diameter go ahead and share
Wire diameter can only be too thin for a certain speed, but not really too thick. <- This is only true for an IGBT-Design. But its way too dependend on coil and projectile geometry to make an educated guess. Just remember: the thicker, the higher the current. Respect your switches.

I am looking at a ~450A peak which should, of course, be very brief (maybe 2 ms above 100A). I'm not entirely sure what IGBT to pick for this as mouser and digikey list their specs for continuous current, and I have no experience with them
Your head is stuck to the SCR-design. Half-bridges work different... you will need much less current. I think you are safe with anything above 150A with maybe 300-400A. 1m-Pulse current. What is way more important than current capability is security for your IGBTs: i have a dedicated microcontroller sensing the switching of the IGBTs. The dedicated µC does have the power to completely shut down everything. So if my main controller crashes or i am halted in a debug breakpoint, nothing explodes. I see this as mandatory. No way around it.

With two, if each has a small internal resistance, then in parallel their equivalent resistance is 1/2 of what a single one is, meaning that there is the same current traveling through two in parallel as there would be through one alone, but I've seen people here say that it helps?
Do not start paralleling IGBTs. Been there.... . They do also not have an "internal resistance" like Mosfets. They are infact like a bad PNP-Transistor from emitter to collector. They have a saturation voltage.

4x IXGX120N60A3
Acceptable.. yes. They will do the job just fine. I think for a 320V design they might be a little overkill. Maybe you find 400V IGBTs for the same price with more current. On the other hand: if you are fine with a little uglyness then: i have IXGX320N60A3 (the bigger bother) just laying around here. They do not have any legs anymore (the are cut directly where the legs become thin). For the price of those 4 IGBTs i could just give you 10. I have no need for them.. i need 650V rated stuff at least. The missing legs are really only a mechanical problem. Thick legs of a diode soldered onto them and they will fit into any PCB again.
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Karmaslap
Mon Jan 18 2016, 08:09PM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
Your reply is very informative as always

i say: unlikely On a SCR-based (or just mechanical switch?) design this is possible but achieving it on the first try? Lucky.. or systematic error.

just a mechanical switch, and with a (fairly) well-wrapped coil of some small diameter wire, with a projectile only slightly shorter than the length of the coil and one that fits the barrel to a fifth of a millimeter ish to take advantage of all the flux. Not to mention, using only one little capacitor. Not the whole bank.

Depending on your measurement use 2 sensors to verify the speed. You may simply shoot the projectile through an other (unconnected) coil. Just hook up your oscilloscope to the open ends... the remanent magnetized projectile will induce a voltage in the shape of x*exp[abs(x)^3]. The time between maximum and minimum of that waveform divided by your projectile length will be the speed i guess. If not... its interesting too!

I measured by hitting a wall some distance (15 feet or so) away and calculating the projectile drop time to solve for speed. Not the most accurate way but one of my friends was in the lab and it was the quickest way to find speed.

A sense coil! What you are using for yours. I will give it a shot, if it works well I could skip photogates completely.
IGBTs need to be protected against the stray inductance and ESL of your cap bank. Please look at "active clamping" circuits. Snubbers wont do it. Without protection your IGBTs are dead after the first test.
Wow. What time constant does your coil have? (L/R)

Will do. Don't want to break anything expensive or have a danger I could eliminate.

It was a coil I was simulating, not a real one I tested. Have to order wire before testing. I'll edit in those vslues when I go back to the lab and see what it was.

Wire diameter can only be too thin for a certain speed, but not really too thick. <- This is only true for an IGBT-Design. But its way too dependend on coil and projectile geometry to make an educated guess. Just remember: the thicker, the higher the current. Respect your switches.

Thanks for the advice
Your head is stuck to the SCR-design. Half-bridges work different... you will need much less current. I think you are safe with anything above 150A with maybe 300-400A. 1m-Pulse current. What is way more important than current capability is security for your IGBTs: i have a dedicated microcontroller sensing the switching of the IGBTs. The dedicated µC does have the power to completely shut down everything. So if my main controller crashes or i am halted in a debug breakpoint, nothing explodes. I see this as mandatory. No way around it.

More current through the coil and a faster pulse time =more force on the projectile as it passes through, half bridge design or not?

I'll keep the microcontroller thought as I design but I am still figuring what parts to get. Was hoping to not use any microcontrollers, yet. If I need to, I have an arduino on hand.

Acceptable.. yes. They will do the job just fine. I think for a 320V design they might be a little overkill. Maybe you find 400V IGBTs for the same price with more current. On the other hand: if you are fine with a little uglyness then: i have IXGX320N60A3 (the bigger bother) just laying around here. They do not have any legs anymore (the are cut directly where the legs become thin). For the price of those 4 IGBTs i could just give you 10. I have no need for them.. i need 650V rated stuff at least. The missing legs are really only a mechanical problem. Thick legs of a diode soldered onto them and they will fit into any PCB again.

Thanks, I'll be more comfortable ordering now.

Wow! How much would shipping be from germany to idaho, though? I haven't analyzed yet but it seems that doubling the voltage and halving capacitance (series combination) had a nice effect on the waveform.

I will not be able to work on this for a few days while I stay ahead on school but will try creating a sensing coil and finalizing thought-work for what coil wire to get so I can order everything by next weekend. I hope to print a test coil form and get some epoxy to make coils next weekend, if I can avoid getting stuck on simulating what would be best and get it done. I already ran into problems with my coil crushing the barrel and causing the projectile to get stuck when I hook up the whole capacitor bank to the small wire coil I was using for fun, so I know I need to prevent the coil from moving.
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DerAlbi
Tue Jan 19 2016, 02:39AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 504
Not to mention, using only one little capacitor. Not the whole bank.
That could imply that you are below saturation. maybe thats the key. I still dont trust such high eff

A sense coil! What you are using for yours. I will give it a shot, if it works well I could skip photogates completely.
I use a whole different concept.. has nothing to do with induced voltages. What i proposed is nothing that works in a compact design. You need to make sure that the voltage is only induced by the still magnetized projectile and not by the field of the coil. So you need some distance to the acceleration coils. You cant really sense the projectile with that. Its just a short test, to verify your speed measurement. I used a coil that was left. Photogates are the way to go in the long run.
Your current measurement doesnt sound so bad actually.. but it really depends on the starting angle... too many unknows for a precice result. Specially considering the kick-back. it will likely lift the muzzle up.

More current through the coil and a faster pulse time =more force on the projectile as it passes through, half bridge design or not?
In an SCR-Design (whats what you currently have even if you dont usw SCR) a thicker wire makes a shorter pulse with higher current. The overall energy in the coil stays the same (ignoring capacitor ESR). The timing can be good or bad.. the pulse form should correspondent to the projectile speed.
In a Halfbridge: you do not allways want a fast current rise, as you put current through the coil while its relative pullforce is low compared to its maximum pull force (based on projectile position, basically the same reason as for the SCR-design). But you can simulate a higher inductance by using PWM on the haldbridge. This is only feasable for the first some stages. As soon as the current becomes higher, you cant handle the switching loss anymore.
This is most relevant for the first stage. Any other stage can easier compensated by a later turn on. The efficiency will rise, but it will not have the optimal energy transfer (because you only use a small portion of the pull force curve).

I'll keep the microcontroller thought as I design but I am still figuring what parts to get. Was hoping to not use any microcontrollers, yet. If I need to, I have an arduino on hand.
SCR-Design do not demand µC... so its fine for the beginning. As soon as you go to Half-bridge, then there is no way around it if you want to use the full flexibility. And as soon as you use one µC you need to use two for redundancy. So 2 Arduinos will do. Sry, buts thats the minimum..

Wow! How much would shipping be from germany to idaho, though?
I think its about 3.70€... so maybe 4$? 10 suckers are about 100gramms. I would have allready included that in my offer.. no worries. The parts are basically worthless to me... but if i give them away for free, i know you will just destroy them carelessly You will destroy them anyway (thats why i offer 10 instead of 4) but you know what i mean. I am also greedy as hell. No, i actually worked 2 weeks to get 50 of them But its so long ago.. it basically feels like the dept has been paid. Anyway.. thats what you are going for...


As said... no legs. But cleaned and working.

I haven't analyzed yet but it seems that doubling the voltage and halving capacitance (series combination) had a nice effect on the waveform.
Well.. you end up with double the energy. And double ESR (more damping, "dampening?") Its not only the electronics that change its also the acceleration behavior. It accumulates.
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Karmaslap
Tue Jan 19 2016, 04:17AM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
I am undoubtedly below saturation but probably close. I trust the basic math to be close enough to accurate and I'm not worried and don't care if it's off a bit. I have the barrel being held by 2 sets of helping hands to stabilize it from kicking, and I used a level to make sure it had a flat shooting angle. It's close enough until I get above 10 m/s with the larger projectile, then I have to set up a backstop instead of firing across the lab full of expensive equipment.
Darn. I was thinking that sounded like your sense coils.

My mechanical switch stays on until the current drops to 0 essentially, because I can't switch it on and off so fast, so I see why you call it an SCR design. I hadn't even heard of a PWM controller until you just brought it up. Pulse width modulation, I have yet to see how it will help as you describe over trying to get the shortest possible pulse and careful initial placement (better, I plan on "kicking" the projectile into the coil to trigger the first shot in the final 3-stage design). I will find out later. It seems there are other advantages to the half-bridge than the ability to switch the current on and off quickly and use the inductor kick-back to recharge the caps. I suppose an arduino could even measure the waveform if I got fancy enough with it. Another arduino is cheap compared to destroying a bunch of electronics! Thought I might want to get a Raspberry Pi instead so I have one of each.

I will PM you regarding the IGBTs- I'm sure I'll blow things up while I learn. I always have safety glasses on :)

Hooking capacitors up in series doubles the max voltage, halves the capacitance if they are identical, but you end up with the same amount of energy. And double ESR but it doesn't seem like the ESR matters a whole lot compared to the resistance of the circuit (or it does, and I should be factoring it in...)
Doubling the voltage and halving capacitance in simulations is giving me a waveform 2/3 as wide with a +1/3 current peak. I like that. I think I will go with 650V instead of 330.


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DerAlbi
Tue Jan 19 2016, 04:50AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 504
ooking capacitors up in series doubles the max voltage, halves the capacitance if they are identical, but you end up with the same amount of energy.
Pls check that again. Double voltage quadruples energy. Halving Capacitance halves the energy. ..so still factor 2 remains. Or put it this way: why should the energy density of one capacitor halve just because you use 2 of them?
Voltage only matters in an SCR-Design for pulse shaping. In a halfbridge you dont care much.. the higher the voltage the less current. Basically the power remains constant. But in a Halfbridge you cant go above 500..550V with 600V IGBTs.

Thought I might want to get a Raspberry Pi instead so I have one of each.
I would be against that. You dont want to run linux in a real time sensitive application. I figure programming the ARM standalone (without operating system) is way above your head. Use 2 Arduinos. They will do fine. But no, you do not want program complexity - the more you have the more goes wrong. You only want the flexibility.

Doubling the voltage and halving capacitance in simulations is giving me a waveform 2/3 as wide with a +1/3 current peak.
You achieve the same with double capacity (double energy), and a lower inductance (higher peak, shorter current rise. Think about it how everything goes together. (This is not a suggestion, just a side node.. there are many ways to skin a.. coilgun.)

I hadn't even heard of a PWM controller until you just brought it up.
Just to clarify: its not about a PWM-controller, its about the PWM-Signal <- that will be generated by an Arduino. Its not a pure PWM.. A halfbridge has more than 2 switching states. You need High/Low-Side load sharing and stuff. Buts thats for later when it comes to it... thats why i urge you to use a controller. And a second one if stuff goes wrong


The output of the Main µC is connected over 470 Ohm. The Guard-µC senses the lines and if a fault is detected (wrong switching order, too long on-time) the guard just switches its sensing inputs to outputs and actively pulls all Gate-signals to GND overruling the 470 Ohm. This is the default startup behavior. In order to enable the Gate-outputs the Main-µC needs to communicate to the guard that stuff will happen.
The guard is programed once with a known-to-be-working program. Then software development of the Main-µC is without danger.
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Karmaslap
Tue Jan 19 2016, 06:17AM
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DerAlbi wrote ...

Voltage only matters in an SCR-Design for pulse shaping. In a halfbridge you dont care much.. the higher the voltage the less current. Basically the power remains constant. But in a Halfbridge you cant go above 500..550V with 600V IGBT.

Use 2 Arduinos.

You achieve the same with double capacity (double energy), and a lower inductance (higher peak, shorter current rise. Think about it how everything goes together. (This is not a suggestion, just a side node.. there are many ways to skin a.. coilgun.)


Actually did the math, redid, checked, don't know why I thought differently. Turned to google. I used this site for theory and read all through it. Let's play: find the math error <http://coilgun.info/theorycapacitors/capacitors2.htm> Says something for doing the work yourself instead of using someone elses, I suppose. He uses 2C and C/2 instead of C and one or the other.

How would higher voltage be less current in a half-bridge? Why would it act differently? That makes no sense to me. You want the highest possible current because that gives you the highest magnetic field. To get the highest possible current, you need more voltage.

Ok. I'm sure I could use the Raspberry pi but it's just a matter of which is easier to use. I'll cross that bridge when I get to it, though.

I'm trying to think of how it all fits together. I'm simulating different shapes of coils- long and skinny, short and fat, in between. Trying to optimize peak current, minimize inductance and resistance, and maximize the energy imparted as the projectile travels. Combining FEMM sims and LTSPICE sims.

I will ask about using microcontrollers and the PWM and such later when it is relevant for me to be thinking about it. The safety program ensures it all won't break, then you have your constraints for the main controller. Of course that is the smart way to design.

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DerAlbi
Tue Jan 19 2016, 07:51AM
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How would higher voltage be less current in a half-bridge?
Assume you push a certain number of Joules in a Projectile. Lets say... E=10J. Those 10J is Force*AccelerationLength. To pass the AccelerationLength given a mean speed v gives you a travel time (t). Basically i just describe: you do need a certain time to push the 10J. This time is constant as long as you push 10J and have equal initial velocity. Lets say the time is t=1ms.
The 10J within 1ms equates to a Power. (Energy/Time = Power, J/s = W) So your coilgun has a mechanical power of E/t=10kW. Given your efficiency is.. lets say.. 10% inevitably you need an electrical power of 100kW. Those 100kW can be achieved by 100V, 1kA... or 500V and 200A. since Power = Voltage*Current.
So the higher the voltage the less current.

You want the highest possible current because that gives you the highest magnetic field.
Well if you put it that way there is no need for a halfebridge design. If you have absolutely no efficiency optimization in mind, then go ahead Realistically, above a certain current your efficiency will drop so far.. its almost below 0. A certain coil shape with a certain projectile can only push a certain amount of energy with a certain efficiency. These are physical limits... You are building a motor here. The higher the power, the bigger the engine! Since your engine will on someday have a fixed size, you will have someday a fixed power. This power will fed by P=U*I.

To get the highest possible current, you need more voltage.
Yes+No=Yo? In a coil the current is: dI = U*dt / L.
- So yes. You can maximize U.
- You could however also maximize dt. Ok. dt is fixed, due to the limitations above. (1ms as representative number)
- You can also minimize L.

a) A lower L is done by less turns.
Since L ~ n^2 you can take away half the turns, you get 1/4 the Inductance.
1/4 the inductance will yield 4x the initial current given a constant U
4x the initial current yields double the force with half the turns. (4x current * 0.5x turns = 2x Force).

b)
Lets do the same with doubling U.
Double U gives you the double peak current.
The number of turns stay the same.
2x Current * 1x Turns = 2x Force.

Both version [a) and b)] have incrased your force. You want however the Force to be constant, because in the end you have constant power.

To counteract the higher force in a) you will turn down the voltage (so the current does not rise as high)
To counteract the higher force in b) you will increase the number of turns, to get slower current rise in the given time

Remeber: this are only the basic thoughts for a Halfbridge. A SCR-Design is inherently different - inherently crappy .
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Karmaslap
Thu Jan 21 2016, 11:02PM
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Thanks for the information, it is helping me simulate and brainstorm a lot.
I guess I pick wire and IGBTs I can afford at this stage, and then design according to them. Solve for the voltage I am at and number of turns to give me my peak current that corresponds to the IGBTs, and then use that. I'll have to expand my knowledge as I go as IGBT half-bridges are a lot more advanced than the simple switches I thought they were.
There is a point where you can't just hakve the number of turns and get more force- this because you have to keep your current at a level the IGBTs can handle. In a design going purely for Kinetic Energy, then, It would be desirable to have a higher voltage to get the same current, but with more turns. The downside of course is a longer rise time (and longer shut off time after the switch is flipped, I think?) There must be an "optimal" point for the given materials where it is better to just ad anither coil, a point decided by your design constraints.
Putting together a materials list for actual ordering of parts, and then I can design accordingly to the parts I have. That will help me learn faster than theorizing. I desoldered the sketchy setup my capacitors had, and want to mount them to something more securely. I am thinking of taking a copper pipe or other cheap length of copper and cutting and straightening it into small bars to drill holes in and solder the caps too. Anyone have a better/safer idea?

Edit 1/22/16
Got permission from the professors to work on the project. It took a senior deciding to work on a 10kV can crusher for his senior project for them to okay me for 350V. Also got permission to use all lab materials including the oscilloscopes- regarding measurements with a scope during a discharge, does anyone have any advice on how to safely set up the measurement? The scopes we use have earth ground as a reference, and I'm afraid of shorting the circuit hooking it up to the wrong place. Thinking I may hook up the ground to the negative lead of my charging circuit, and the tip to the location I want to measure (immediately before the inductor). I should be able to capture the waveform of the discharge like this, but if anyone has advice... I would love to not break a scope. Our scopes on the 10x setting (probe only goes to 10x) are rated for 600V and 10Mhz I believe.

Edit 3/29/16
Project isn't dead, just got put on hold. I got mono and all the associated problems and have been focusing on school. Next up I am ordering diodes and switches.
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Karmaslap
Tue Dec 20 2016, 09:37PM
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Went to an electronics surplus store and while they didn't have a lot of things I was hoping to find, I did pick up this for fun:


Thinking I'll connect a few disposable camera charging circuits to it and make a single-stage gun just to have fun with. It doesn't really fit with my actual longer term coilgun project but it'll be fun and show off a basic gun. I can also use it as a testbed for spin stabilization and a few other things.

With regards to my actual project: Did more simulation work. I have a few ideas for a more non-conventional coilgun. I believe that ready access to large capacitors and (as DerAlbi has discussed in his thread) SCRs are the two largest reasons for hobbyist coilguns bwing so inefficient.

I'm simulating one design that makes use of batteries and another that uses a large number of much smaller capacitors. I think with the battery one I can reach speeds in excess of 200 m/s with 30% efficiency for a 10mmx30mm iron projectile cylinder (10mm diameter). Those estimates are from paper math and FEMM simulation. I'll have to do a lot of testing before I can finalize the design for a prototype but mostly the cost of the whole system is a few hundred dollars out of my budget. I'm graduating in a few months, though, so hopefully I'll have more money then for the project.

I'm still going to design and simulate things in the meantime, so I'll get started and ask here with regards to my first issue:

I need to know the position of the projectile +-.5 mm. Anyone have ideas for this? I was thinking a laser rangefinder placed behind the projectile but the systems I have found affordable are not so accurate. I have also considered capacative sensors on the inside of the barrel because they can be made very thin, or just small loops of wire placed inside the barrel/between coils and some interpolation.
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DerAlbi
Wed Dec 21 2016, 01:20AM
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I think with the battery one I can reach speeds in excess of 200 m/s with 30% efficiency for a 10mmx30mm iron projectile cylinder (10mm diameter)
You are talking about an 18g projectile at 200m/s == 375J projectile energy. are you sure you have not forgotten any decimal point in the 200m/s..
also be aware of that the current involved in the coils increased with the square of the velocity (for a battery design) and with the third power in a capacitor design (if you pous constant energy per stage).
I need to know the position of the projectile +-.5 mm. Anyone have ideas for this? I was thinking a laser rangefinder placed behind the projectile but the systems I have found affordable are not so accurate. I have also considered capacitive sensors on the inside of the barrel because they can be made very thin, or just small loops of wire placed inside the barrel/between coils and some interpolation.
I tried building a range finder circuit what would have an appropiate sampling rate. It did fail mainly due to missing optics. Also bending the barrel and reflections inside the barrel are hard to deal with. Capacitive sensors can be made and i would be interested in their design. However i would think that evaluating capacitive sensors is quite complex and you cannot reuse the same circuit for multiple capacitors (at least its complicated). I use the phase shift of high frequency resonant inductive sensors since this can be cascaded easily but the circuitry is a bitch. I would say the most feasible option is the light trap.
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Karmaslap
Wed Dec 21 2016, 02:34AM
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the 30% efficiency is my guess from the work... i do not think it will be super accurate without recuperating energy.

not sure what you mean in your second paragraph reminder about coil current and velocity

A laser-based sensor may not work for me because of what I want to do with the shaping and spinning of the projectile. I would not need to chain capacitors but I could potentially by analyzing an AC signal running through the chain. i think inductive sensors might be best because I can use extremely thin wires for the sensors.


I'll be relying heavily on PWM and rapid changes from a microcontroller already, but with a few MHz clock speed I think things will execute well enough.

I won't be buying hardly anything until I am done designing. I need to purchase a lot of my own equipment for when I cannot use the school's too. Deralbi do you have your own scope?

edit: wow. dont do math and drive.
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DerAlbi
Wed Dec 21 2016, 06:41AM
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The 30%eff is reachable, this wasnt an issue for me. I just thought about my 100m/s estimates are already reaching 800A at 300V while you speak of 200m/s meaning 2.4kA and your voltage is even less, since you talk about a battery driven design so you current will be even higher. I dont see how you will manage this kind of current in a halfbridge. I know you projectile is smaller than mine.. however the ballpark figures still apply since this is close to an order of magnitude too crazy to be reasonable.
I am not sure but if your models or math predict that you can reach 200m/s with a 18g projectile there is something fundamentally wrong.
Can you elaborate how you get those figures?
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Karmaslap
Wed Dec 21 2016, 08:29AM
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A higher number of smaller stages carefully controlled so that the maximum force possible (center of coil in most designs) is always as close to the projectile as possible

and then a lot of stages. Big capacitors are terrible for efficiency because you end up dumping most of the energy before the projectile is in the optimum position, and then a big C messes with the time constant so a lot of energy isnt available for the small window that is optimal for maximizing the force on the projectile. then your projectile hits halfway into the coil and coasts until your system fires the next coil. this means that
A. half of every coilgun barrel is wasted as far as energy going into yhe projectile is concerned
B. most energy is dumped at a poor time

I'm still not sure at all what you mean with 800A and such... I do not plan on exceeding 400A but again, more stages. You are making a portable gun as a primary requirement, I only wish that it be able to be carried and set up by one person. without caps it gets lighter.

and of course: Simulating my design in FEMM and supplementing with paper and having things actually work are very, very different. When i get back home (in over a week) from christmas I can send you what i actually did
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hen918
Wed Dec 21 2016, 07:49PM
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I think what DerAlbi means is this:
The lower your current, the wider your pulse width, the wider your pulse width, the less energy you can put in the right place, timewise and obviously spacewise.
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Karmaslap
Wed Dec 21 2016, 08:42PM
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Ah.. total power delivery and pulse shaping, though in a coilgun current is all that matters. I get what you mean now, thanks hen


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Karmaslap
Sat Dec 24 2016, 12:38AM
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Found this pretty cool video. I hope he shows off more of his design. He can hit 90 m/s with this one which seems to be running off of relays or something
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DerAlbi
Sat Dec 24 2016, 06:54AM
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Hmmmmh. Thanks for linking the video in
Somehow the penetration power is a little low for the speed he shows. I guess his projectile is something like 8mm diameter and 30mm length which makes it around 12g projecting 50J kinectic energy. (just a guess). From my experience 50J hit way harder. Specially with sharp tip. Also if his capacitors are really 660J this means an efficiency of only 8% which is ridicules low given the number of stages.
Somehow if his projectile were a bit larger than i guessed and the speed was in feet per second i would believe it more. Still doesnt explain the efficiency. Well maybe.
To me it looks like he has a capacitor for every stage meaning this is a badly* designed SCR design.
"Badly" because:
a) he seems to use the same wire over and over again which would be inherently wrong - a timing nightmare- However i am not sure if he at least tries to vary the coil outer diameter.
b) later stages need more current, so the ESR of the small caps will destroy performance where later stages usually have the best performance..
c) his exit velocity is noticeable inconsistent which is the result of his sensor evaluation (not having a control loop). Firing one stage after another purely on sensor input yields those results. Those results are also possible with fixed timing that is designed for maximum output energy but it does not look that way.

His projectile rotation is interesting. i would love to see shots with and without those rotations over a larger distance. His mechanical solution is also pretty neat.
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Karmaslap
Sat Dec 24 2016, 11:29AM
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Location: Boise, Idaho
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I check youtube once a week for coilgun videos. There was just one where someone tested spin stabilization, though only for close range penetration and not for long-range accuracy. Nobody seems interested in shooting their coilguns more than a few feet into pillows or books.

The projectile looks a lot wider than 8mm... More like double that. He's also got a pointy tip which is no good unless it's not ferromagnetic which has to hurt, but the mass is a lot more than 12g from the looks of it.

I paused the video this time through and you can see bundles of capacitors with what looks like an IGBT placed on each one going into the coils, one bundle per stage. That and the well-designed wooden housing is impressive to me, this guy found something that worked and took it to completion with the largest portable rifle he could make.

A) Why not use the same wire throghout the coil and design? Most people just want to buy a big spool and use that for everything. His cable management is pretty, at least.

B) Pretty sure he has those in parallel, look like little photoflash capacitors
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DerAlbi
Sat Dec 24 2016, 12:48PM
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Its definitively a SCR design. Those packages arent IGBTs and the gate drive is not suited to drive a voltage controlled input (waaaay too long cable).


The diodes arent freewheeling diodes (look how they connect to a common light colored wire). He charges all caps in parallel but discharges single groups via SCR.
The coil length is around 3cm. Take a look here:

This resistors body length is 7.5mm. 4 of them fit into one coil meaning roughly 30mm coil length. I guess he followed the common knowledge "projectile length = coil length". Now lets draw lines on the projectile and push them to the side for analysis:

So the diameter fits a little more than 3x into the projectile length. Hmmh ok. Maybe its 10mm or even 12mm. It depends if, while designing the coils, the projectile length is measured to the nose of the tip or to the base of the tip. I guess its 10mm and the overall length is 30mm. This makes the weight approx 18g.

So again if this was 90+m/s this would equate to 73J so 11%eff. Sorry but 73J will hit much much harder than what was shown in the video. However in feet per second this would equate to only 7.5J which is not right either. Strange.

A) Why not use the same wire throghout the coil and design? Most people just want to buy a big spool and use that for everything. His cable management is pretty, at least.
Yes you are right, however if you spend so much time and money into a project you are not "most people". For best bang per buck you want a (optimal and) constant coil geometry and to adjust the timing you change the wire diameter to reach different inductances.

largest portable rifle
Hehe. You think this maybe 2m long monster with all the copper in it is even close to be portable? I would think if you hold it in you hand the barrel will bend downwards and you fall over
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Karmaslap
Sat Dec 24 2016, 09:30PM
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I don't see how the circuit he has wouldn't be suitable for driving IGBTs, those cables aren't that long, but I also don't know how to differentiate between an IGBT, MOSFET, or SCR from looks alone when the packaging is the same.

I wonder who is responsible for that "common knowledge". The best coil shape for a field:inductance ratio doesnt care how big the projectile is, just how big the wire is.

I was planning on adjusting the geometry to change inductance and still don't see why I wouldn't as long as the coils fit, who cares how it looks

It's gotta be light enough to carry around. That makes it portable! With some high carbon steel or aluminum helping support he could be setting it up with a tripod. Close to what I'm planning for mine: I don't care if I can run around with it and shoot, I won't be playing rambo, but I do want to set it up and actually use it at range
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DerAlbi
Sun Dec 25 2016, 04:14AM
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I don't see how the circuit he has wouldn't be suitable for driving IGBTs, those cables aren't that long, but I also don't know how to differentiate between an IGBT, MOSFET, or SCR from looks alone when the packaging is the same.
If it were IGBTs, all caps would be in parallel, they are clearly not. Regarding the IGBT gate drive, please think about the current loop that those cables pose. The whole coilgun is nothing else than a big transformer (from a parasitics point of view) and while an induced voltage hurts the IGBT (false turn on), an induced current does not necessarily turn on a SCR. You usually cannot distinguish components by packaging, however i have seen those packages (the triangular tab) for SCRs but not for IGBTs. Put all those things together (the overall circuit, careless gate drive, and the overall medium efficiency) there is just one Element that could be accountable for that: SCR.

I wonder who is responsible for that "common knowledge". The best coil shape for a field:inductance ratio doesnt care how big the projectile is, just how big the wire is.
??? This common knowledge is correct. Diameter wise there is just one optimal shape that is OuterDiameter = 3x InnerDiameter which promises you the highest L/R time constant - this is however only important to a halfbridge design.. in a SCR design the puter diameter can be used for tuning. Length wise you want to be as big as the projectile. See the difference here:


Sorry, hand drawn with my mouse using paint, true work of art ^_^ . Of course the green line is not accurate and it actually stretches along the x-axis depending if the coil is too long or too short, however the most important feature - that is the longer dead zone inside the coil - is represented.
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