Electromagnetic BB gun
rpopeye, Thu Jun 07 2018, 04:34AMSo, this is my baby. It’s not much in terms of specs, but the special thing about it is that it’s as good as an average CO2 BB gun, and as far as I know that’s a first for an electromagnetic BB gun, plus it looks cool :)
Here are the specs:
Electromagnetic BB Gun
type: coil gun
munitions: standard 4.5mm steel BBs
muzzle velocity: 70 - 130 m/s (selectable)
kinetic energy: 0.8 - 3.0 Joules
rate of fire: 20 rpm
full weight: 3.6 kg (including optics, clip and battery)
length: 70 cm
battery: 2.5 Ah Li-Ion, more than 300 shots on a single charge
magazine: auto loader with a standard 19 rounds clip
manufacturing process: 3D printed
It's a standard coil gun. A series of electrically powered coils along the barrel create a running electromagnetic field from the back of the barrel towards the front, which attracts and accelerates the steel BB. Because the switching of the coils is controlled through software, it's very easy to change the muzzle velocity and in this design there are 7 selectable muzzle velocities, from 70 m/s to 130 m/s. The coils are powered from a high voltage capacitor bank, which in turn is charged up from the battery by means of a DC-DC converter.
The BBs are automatically loaded between shots from the clip into the barrel thanks to a servo mechanism, and at the moment of producing the shot there are actually no moving mechanical parts in the device.
One great thing about it is that it’s very cheap to operate, no need to buy CO2 cartridges, just charge the 30 Wh battery for less than a cent and it’s good for more than 300 shots.
Re: Electromagnetic BB gun
rpopeye, Thu Jun 07 2018, 05:19AM
And here is a picture of the coils
48 stages, mosfet-switched, 1 mosfet per stage, open-loop (no feed back) configuration
rpopeye, Thu Jun 07 2018, 05:19AM
And here is a picture of the coils
48 stages, mosfet-switched, 1 mosfet per stage, open-loop (no feed back) configuration
Re: Electromagnetic BB gun
DerAlbi, Fri Jun 08 2018, 02:18PM
Hmmh. Complicated to comment something. It was obviously a LOT of work and that alone is worth some cheering
130m/s and shooting round balls is quite usable despite the 3J.
However to reach 3 Joule it seems a lot like there had to be an easier way than 48 stages and 3.6kg.. i do however realize that your projectile is extremely small, so its hard to push energy in it.
The 2.5Ah LiIon has how many cells in series?
Assuming just one cell (in your favor) the battery provides around 32.4kJ of energy. If that is only good for 300 shots your system efficiency seams to be only 2.7% assuming 3J/Shot. If you have 2 cells in series, the efficiency drops to only 1.35%. This seems almost unrealistically low for a multi-stage concept
Are you sure the BBs are actually magnetic (and as good as pure iron balls)? It seems very strange to me
I love the idea of having such a reload mechanism and using a standard magazine and standard ammo for it.
Nevertheless interesting to see your approach!
DerAlbi, Fri Jun 08 2018, 02:18PM
Hmmh. Complicated to comment something. It was obviously a LOT of work and that alone is worth some cheering
130m/s and shooting round balls is quite usable despite the 3J.
However to reach 3 Joule it seems a lot like there had to be an easier way than 48 stages and 3.6kg.. i do however realize that your projectile is extremely small, so its hard to push energy in it.
The 2.5Ah LiIon has how many cells in series?
Assuming just one cell (in your favor) the battery provides around 32.4kJ of energy. If that is only good for 300 shots your system efficiency seams to be only 2.7% assuming 3J/Shot. If you have 2 cells in series, the efficiency drops to only 1.35%. This seems almost unrealistically low for a multi-stage concept
Are you sure the BBs are actually magnetic (and as good as pure iron balls)? It seems very strange to me I love the idea of having such a reload mechanism and using a standard magazine and standard ammo for it.
Nevertheless interesting to see your approach!
Re: Electromagnetic BB gun
rpopeye, Fri Jun 08 2018, 02:41PM
The BBs are only 0.35 gram each, at 130m/s it's 3J max. But the main goal here was velocity rather than kinetic energy.
Because the BBs are really small (4.5mm in diameter), the coils have to be short too, so can't accelerate much with a single coil, hence 48 of them.
The battery is actually 3 cells, 30Wh, 108kJ. The efficiency (electric energy in capacitor bank to kinetic energy of projectile) is just about 2%, but because the DC-DC converter is not great either (at that speed of charging - 3s per recharge), the total efficiency (battery to projectile) drops to about 1% - terrible, but the goal was to get something practically usable rather than achieve high efficiency. The BBs are solid steel and totally magnetic, the problem is most likely due to their small size. If the size of the projectile is bigger I suppose the efficiency will improve too.
rpopeye, Fri Jun 08 2018, 02:41PM
The BBs are only 0.35 gram each, at 130m/s it's 3J max. But the main goal here was velocity rather than kinetic energy.
Because the BBs are really small (4.5mm in diameter), the coils have to be short too, so can't accelerate much with a single coil, hence 48 of them.
The battery is actually 3 cells, 30Wh, 108kJ. The efficiency (electric energy in capacitor bank to kinetic energy of projectile) is just about 2%, but because the DC-DC converter is not great either (at that speed of charging - 3s per recharge), the total efficiency (battery to projectile) drops to about 1% - terrible, but the goal was to get something practically usable rather than achieve high efficiency. The BBs are solid steel and totally magnetic, the problem is most likely due to their small size. If the size of the projectile is bigger I suppose the efficiency will improve too.
Re: Electromagnetic BB gun
Signification, Fri Jun 08 2018, 09:07PM
How does your timing work? i.e. things like sensing the BB position, firing the coil at the right instant, etc. It's also interesting that you have a velocity select--do you use a lower current for smaller velocity, use less active coils...? VERY impressive overall--I never thought BB's could be used in a coil gun with results like you are getting!
Signification, Fri Jun 08 2018, 09:07PM
How does your timing work? i.e. things like sensing the BB position, firing the coil at the right instant, etc. It's also interesting that you have a velocity select--do you use a lower current for smaller velocity, use less active coils...? VERY impressive overall--I never thought BB's could be used in a coil gun with results like you are getting!
Re: Electromagnetic BB gun
rpopeye, Sat Jun 09 2018, 04:30AM
The timings of the coils firing are preset, there is no sensing of the BB position. With 48 stages, adding position sensing would have made the design too complex, and I also wanted to see if it can be done without sensing, turns out it can, at least in this particular case :)
To change the velocity then is simply a matter of changing the timings of the coils firing.
rpopeye, Sat Jun 09 2018, 04:30AM
The timings of the coils firing are preset, there is no sensing of the BB position. With 48 stages, adding position sensing would have made the design too complex, and I also wanted to see if it can be done without sensing, turns out it can, at least in this particular case :)
To change the velocity then is simply a matter of changing the timings of the coils firing.
Re: Electromagnetic BB gun
DerAlbi, Sat Jun 09 2018, 07:51AM
DerAlbi, Sat Jun 09 2018, 07:51AM
but the goal was to get something practically usable rather than achieve high efficiencyThat usually goes hand in hand in terms of power density and performance/weight-ratio. I fully understand your troubles - BBs are really challenging projectiles for a coilgun. The main problem is the round shape, not its small size, btw.
Re: Electromagnetic BB gun
rpopeye, Sat Jun 09 2018, 12:11PM
rpopeye, Sat Jun 09 2018, 12:11PM
DerAlbi wrote ...
That usually goes hand in hand in terms of power density and performance/weight-ratio.
True, true. My goal was to build a BB gun using standard 4.5mm steel BBs, everything followed from that.That usually goes hand in hand in terms of power density and performance/weight-ratio.
Re: Electromagnetic BB gun
the_anomaly, Sun Jun 10 2018, 12:58AM
What material did you choose for the barrel?
the_anomaly, Sun Jun 10 2018, 12:58AM
What material did you choose for the barrel?
Re: Electromagnetic BB gun
Signification, Sun Jun 10 2018, 01:06AM
Have you ever gotten suckback of a superposition (summation of forces from each coil) that resulted in no ejection at all?
I was also wondering (in the early stages of design) if you ever used only ONE coil, and the effects you got.
EDIT: I was also wondering (not asking for every answer) about things like the dimensions/specs of each (identical?) coil WRT the 4.5mm BB...ID, OD, length, number of turns, current and cap specs (C,V) per coil, and wire AWG. PLEASE forgive me for the swamping questions, but I am so fascinated with your results, and I had some unique ideas of my own which I previously thought would be useless to attempt--you have changed my mind!
Signification, Sun Jun 10 2018, 01:06AM
Have you ever gotten suckback of a superposition (summation of forces from each coil) that resulted in no ejection at all?
I was also wondering (in the early stages of design) if you ever used only ONE coil, and the effects you got.
EDIT: I was also wondering (not asking for every answer) about things like the dimensions/specs of each (identical?) coil WRT the 4.5mm BB...ID, OD, length, number of turns, current and cap specs (C,V) per coil, and wire AWG. PLEASE forgive me for the swamping questions, but I am so fascinated with your results, and I had some unique ideas of my own which I previously thought would be useless to attempt--you have changed my mind!
Re: Electromagnetic BB gun
rpopeye, Sun Jun 10 2018, 04:21AM
Has anybody tried with a metal tube cut on one side along the axis to prevent eddy currents?
I've seen such designs but never got around to trying it.
The problem with the acrylic tube is that it's flexible and has to be mounted to an external rigid structure to ensure straightness.
rpopeye, Sun Jun 10 2018, 04:21AM
the_anomaly wrote ...
What material did you choose for the barrel?
It's an acrylic tube.What material did you choose for the barrel?
Has anybody tried with a metal tube cut on one side along the axis to prevent eddy currents?
I've seen such designs but never got around to trying it.
The problem with the acrylic tube is that it's flexible and has to be mounted to an external rigid structure to ensure straightness.
Re: Electromagnetic BB gun
rpopeye, Sun Jun 10 2018, 04:34AM
Because the BBs are so small the coils have to be short too, hence a lot of them to get to 130m/s
The exact parameters of the coils vary along the barrel due to the different velocity of the projectile as it gets accelerated.
There is an optimal set of parameters for each coil, however due to the express non-linearity of the whole process it's impossible to calculate with formulas, at least I haven't found any such. The other option is to use a physics simulation software or trial-and-error to adjust the parameters :)
rpopeye, Sun Jun 10 2018, 04:34AM
Signification wrote ...
Have you ever gotten suckback of a superposition (summation of forces from each coil) that resulted in no ejection at all?
I was also wondering (in the early stages of design) if you ever used only ONE coil, and the effects you got.
yes, if the pulse that fires a coil is too long it will suck back the projectile or at least will reduce it's velocity, so the pulse length for each coil has to be of optimal length. In the beginning I tried with a single coil and the maximum velocity that I got with 4.5mm steel BBs was 20m/s.Have you ever gotten suckback of a superposition (summation of forces from each coil) that resulted in no ejection at all?
I was also wondering (in the early stages of design) if you ever used only ONE coil, and the effects you got.
Because the BBs are so small the coils have to be short too, hence a lot of them to get to 130m/s
The exact parameters of the coils vary along the barrel due to the different velocity of the projectile as it gets accelerated.
There is an optimal set of parameters for each coil, however due to the express non-linearity of the whole process it's impossible to calculate with formulas, at least I haven't found any such. The other option is to use a physics simulation software or trial-and-error to adjust the parameters :)
Re: Electromagnetic BB gun
Signification, Sun Jun 10 2018, 10:19AM
YES, A thin non-conductive barrel is definitely better, but the wire needs to be close to the projectile ...perhaps you could pot an entire (optimized) barrel/coil assembly in some kind of hard non-conductive material...ending up with a solid cylinder of embedded coils whose OD is that of the coils. Coat the inside of the cylindrical mold with pam non-stick spray--this has ALWAYS worked great for me in sliding off the mold of fiberglass/wood molds for multi-shot embedded coil quarter shrinkers. Another approach--use a thin cardboard cylinder intended to stay on.
Signification, Sun Jun 10 2018, 10:19AM
YES, A thin non-conductive barrel is definitely better, but the wire needs to be close to the projectile ...perhaps you could pot an entire (optimized) barrel/coil assembly in some kind of hard non-conductive material...ending up with a solid cylinder of embedded coils whose OD is that of the coils. Coat the inside of the cylindrical mold with pam non-stick spray--this has ALWAYS worked great for me in sliding off the mold of fiberglass/wood molds for multi-shot embedded coil quarter shrinkers. Another approach--use a thin cardboard cylinder intended to stay on.
Re: Electromagnetic BB gun
rpopeye, Sun Jun 10 2018, 11:17AM
thanks for the advice about molding, you are right that the closer the coil ID to the projectile the better, so I was thinking about embedding the coils in some sort of hard resin, no need for the acrylic tube then, plus the added benefit that the coil wire is firmly fixed and can't move (in a air coil no matter how tightly wound, the wire would still move a little when running the current through it, which produces an annoying clicking sound when firing the coils), but haven't gotten around to actually doing it yet
rpopeye, Sun Jun 10 2018, 11:17AM
thanks for the advice about molding, you are right that the closer the coil ID to the projectile the better, so I was thinking about embedding the coils in some sort of hard resin, no need for the acrylic tube then, plus the added benefit that the coil wire is firmly fixed and can't move (in a air coil no matter how tightly wound, the wire would still move a little when running the current through it, which produces an annoying clicking sound when firing the coils), but haven't gotten around to actually doing it yet
Re: Electromagnetic BB gun
ShieldExperiments, Sun Jun 10 2018, 04:21PM
As for your idea about a slit down the center metal barrel, I remember reading about a man doing that a long time ago, I think it was Sam Barros - the Powerlabs guy. Hold on...
Found it!
He used a brass tube with a milled out slot as you describe.
That said, he hasn't given any updates on this, or any of his projects for a few years, so I have no idea how any of it turned out.
ShieldExperiments, Sun Jun 10 2018, 04:21PM
As for your idea about a slit down the center metal barrel, I remember reading about a man doing that a long time ago, I think it was Sam Barros - the Powerlabs guy. Hold on...
Found it!
He used a brass tube with a milled out slot as you describe.That said, he hasn't given any updates on this, or any of his projects for a few years, so I have no idea how any of it turned out.
Re: Electromagnetic BB gun
DerAlbi, Mon Jun 11 2018, 08:39AM
..a guy who measures his coilguns in kJ stored energy... always a sign that you should immediately leave the site, go search for a more credible source.
DerAlbi, Mon Jun 11 2018, 08:39AM
..a guy who measures his coilguns in kJ stored energy... always a sign that you should immediately leave the site, go search for a more credible source.
Re: Electromagnetic BB gun
_Eugen_, Mon Jun 11 2018, 11:14AM
Rpopeye, great work!
As far as I know this is the fastest amateur coilgun yet. Two points:
1) Why don't you use halfbridge (or other configurations) to reduce a number of power switches (MOSFETs)? 48 stages may have been realized by 6Ñ…8 matrix i.e. 14 switches. Even taking into account "high-side" driving needed for half of them, the profit would be obvious.
2) Accuracy on your video is quite well. May be it is the best achievable for the ball-shaped accelerator. I conducted some assessments here. It can even be said that the main problem with BBs in coilguns is the accuracy, not low efficiency.
Also the community is very interested in more detailed information about your accelerator (especially geomterty of the coils and wire characteristics).
_Eugen_, Mon Jun 11 2018, 11:14AM
Rpopeye, great work!
As far as I know this is the fastest amateur coilgun yet. Two points:
1) Why don't you use halfbridge (or other configurations) to reduce a number of power switches (MOSFETs)? 48 stages may have been realized by 6Ñ…8 matrix i.e. 14 switches. Even taking into account "high-side" driving needed for half of them, the profit would be obvious.
2) Accuracy on your video is quite well. May be it is the best achievable for the ball-shaped accelerator. I conducted some assessments here. It can even be said that the main problem with BBs in coilguns is the accuracy, not low efficiency.
Also the community is very interested in more detailed information about your accelerator (especially geomterty of the coils and wire characteristics).
Re: Electromagnetic BB gun
rpopeye, Mon Jun 11 2018, 05:12PM
rpopeye, Mon Jun 11 2018, 05:12PM
DerAlbi wrote ...
..a guy who measures his coilguns in kJ stored energy... always a sign that you should immediately leave the site, go search for a more credible source.
I am a bit confused, isn't 30Wh equal to 108kJ, that's the stored energy inside the battery => 2.5Ah * 11.1V = 28Wh..a guy who measures his coilguns in kJ stored energy... always a sign that you should immediately leave the site, go search for a more credible source.
Re: Electromagnetic BB gun
rpopeye, Mon Jun 11 2018, 06:35PM
To answer your questions:
1) Because PMOS transistors are much worse than NMOS, a half bridge would have much worse performance than a single NMOS. We could of course use an NMOS for the high side too, but that would complicate the circuit considerably. The real problem however is that the mosfets operate close to their safe-area border, so one transistor can only switch a single pulse and would need some off time after that to dissipate the heat deposited in its crystal by the high current. It might be possible to design a switching pattern where every mosfet turns off for a few pulses per every pulse it fires, but imagine the complexity of the design then.
2) I can't tell the exact parameters of the coils, that's the secret sauce, isn't it :)
rpopeye, Mon Jun 11 2018, 06:35PM
_Eugen_ wrote ...
1) Why don't you use halfbridge (or other configurations) to reduce a number of power switches (MOSFETs)? 48 stages may have been realized by 6Ñ…8 matrix i.e. 14 switches. Even taking into account "high-side" driving needed for half of them, the profit would be obvious.
2) Accuracy on your video is quite well. May be it is the best achievable for the ball-shaped accelerator. I conducted some assessments here. It can even be said that the main problem with BBs in coilguns is the accuracy, not low efficiency.
Also the community is very interested in more detailed information about your accelerator (especially geomterty of the coils and wire characteristics).
Interesting assessment about the accuracy of coil guns, I think you are spot on there. I was hoping that the magnetic field and the air going between the projectile and barrel would stabilize the BB in the center of the barrel, not so.1) Why don't you use halfbridge (or other configurations) to reduce a number of power switches (MOSFETs)? 48 stages may have been realized by 6Ñ…8 matrix i.e. 14 switches. Even taking into account "high-side" driving needed for half of them, the profit would be obvious.
2) Accuracy on your video is quite well. May be it is the best achievable for the ball-shaped accelerator. I conducted some assessments here. It can even be said that the main problem with BBs in coilguns is the accuracy, not low efficiency.
Also the community is very interested in more detailed information about your accelerator (especially geomterty of the coils and wire characteristics).
To answer your questions:
1) Because PMOS transistors are much worse than NMOS, a half bridge would have much worse performance than a single NMOS. We could of course use an NMOS for the high side too, but that would complicate the circuit considerably. The real problem however is that the mosfets operate close to their safe-area border, so one transistor can only switch a single pulse and would need some off time after that to dissipate the heat deposited in its crystal by the high current. It might be possible to design a switching pattern where every mosfet turns off for a few pulses per every pulse it fires, but imagine the complexity of the design then.
2) I can't tell the exact parameters of the coils, that's the secret sauce, isn't it :)
Re: Electromagnetic BB gun
_Eugen_, Mon Jun 11 2018, 09:19PM
You are right but there are already realized portable projects with half-bridge configurations - not simple of course ( here and here , only in Russian but pictures give enough insight).
_Eugen_, Mon Jun 11 2018, 09:19PM
It might be possible to design a switching pattern where every mosfet turns off for a few pulses per every pulse it fires, but imagine the complexity of the design then.
You are right but there are already realized portable projects with half-bridge configurations - not simple of course ( here and here , only in Russian but pictures give enough insight).
that's the secret sauce, isn't it :)Oh I see :). Ok could you only say if the coils are the same on all stages? I am asking because of my own calculative investigation (not connected with any particular coilgun project).
Re: Electromagnetic BB gun
ShieldExperiments, Tue Jun 12 2018, 01:59AM
@DerAlbi: Yes, I know about the issues here, but his other work had always made up for it in after the fact analysis, even with all the flaws, but I agree that his site is hardly the best. It was just a similar design which I had seen.
Additionally I seem to recall a 9 stage coilgun pistol on one of the old russian forums using this barrel design. If I recall correctly it was halfbridge, with something like 20% total eff. I don't remember which forum it was on, but it was beautifully implemented.
EDIT! Well, someone already beat me to the punch. The one I'm thinking of is the second one that Eugen linked above
ShieldExperiments, Tue Jun 12 2018, 01:59AM
@DerAlbi: Yes, I know about the issues here, but his other work had always made up for it in after the fact analysis, even with all the flaws, but I agree that his site is hardly the best. It was just a similar design which I had seen.
Additionally I seem to recall a 9 stage coilgun pistol on one of the old russian forums using this barrel design. If I recall correctly it was halfbridge, with something like 20% total eff. I don't remember which forum it was on, but it was beautifully implemented.
EDIT! Well, someone already beat me to the punch. The one I'm thinking of is the second one that Eugen linked above
Re: Electromagnetic BB gun
DerAlbi, Tue Jun 12 2018, 07:29AM
Concerning the Power-Labs guy: afaik also his conclusion were bullshit. The problem with a limited knowledge is that your conclusions are based on whats inside your horizon.. I know that i went over the site once and left in pure anger - sources like this hinder everyone in achieving great things. The most offensive is the Donate-Button however..
Anyway: a slotted barrel works, but the slot also removes a lot of strength. Not sure that it will be a good solution to mechanical problems.
I also know the work of that russian.
The first link is actually new to me (i cant search russian sites - limited language skills). The 100/s with 13J @ 15%eff sound reasonable however. His 300J cap bank sounds like a heavy overkill.
Before i started, the second link was the state-of-the-art build for what i remember.. but its "only" 14% efficiency for the little energy he pushes (~6J?) with his 1kg device.
Concerning the BB-gun... i know too little about the build, but i feel like that the half-bridge approach like in the Russian links may not be practical with such a short projectile. With only a few 10µs actionable time a half bridge will be challenging. His switching scheme would demand a switching frequency well above the capabilities of even very good IGBTs.
DerAlbi, Tue Jun 12 2018, 07:29AM
Concerning the Power-Labs guy: afaik also his conclusion were bullshit. The problem with a limited knowledge is that your conclusions are based on whats inside your horizon.. I know that i went over the site once and left in pure anger - sources like this hinder everyone in achieving great things. The most offensive is the Donate-Button however..
Anyway: a slotted barrel works, but the slot also removes a lot of strength. Not sure that it will be a good solution to mechanical problems.
I also know the work of that russian.
The first link is actually new to me (i cant search russian sites - limited language skills). The 100/s with 13J @ 15%eff sound reasonable however. His 300J cap bank sounds like a heavy overkill.
Before i started, the second link was the state-of-the-art build for what i remember.. but its "only" 14% efficiency for the little energy he pushes (~6J?) with his 1kg device.
Concerning the BB-gun... i know too little about the build, but i feel like that the half-bridge approach like in the Russian links may not be practical with such a short projectile. With only a few 10µs actionable time a half bridge will be challenging. His switching scheme would demand a switching frequency well above the capabilities of even very good IGBTs.
Re: Electromagnetic BB gun
_Eugen_, Tue Jun 12 2018, 08:38PM
Let us calulcate something (cannot persist myself :). Assume coil length 4,5 mm (equal to the length of BB, longer coil would be ineffective). At speed 100 m/s progectile will pass it in 4,5E-3/100 = 45 µs.
It is not very high frequency even for most powerful "slow" IGBTs (althow there are special families which are faster but less powerful). Of course very well built driver must be ensured.
Another question arises.
Rpopeye says that MOSFETS are used it means the voltage doesn't exceed 100 V (higher voltages require IGBTs to constrain the cost of the scheme in a reasonable limit). So the circuit must supply (2/48) = 42 mJ in 45 µs (suggesting the energy increment is equal in all stages). This means about 1 kW "pure" power or about 1 kW/100V/1% = 1000 Amps current (according to 1 % efficiency which is reasonable for such "uncomfortable" projectile).
A huge value. There is inconsistency somewhere.
_Eugen_, Tue Jun 12 2018, 08:38PM
Let us calulcate something (cannot persist myself :). Assume coil length 4,5 mm (equal to the length of BB, longer coil would be ineffective). At speed 100 m/s progectile will pass it in 4,5E-3/100 = 45 µs.
It is not very high frequency even for most powerful "slow" IGBTs (althow there are special families which are faster but less powerful). Of course very well built driver must be ensured.
Another question arises.
Rpopeye says that MOSFETS are used it means the voltage doesn't exceed 100 V (higher voltages require IGBTs to constrain the cost of the scheme in a reasonable limit). So the circuit must supply (2/48) = 42 mJ in 45 µs (suggesting the energy increment is equal in all stages). This means about 1 kW "pure" power or about 1 kW/100V/1% = 1000 Amps current (according to 1 % efficiency which is reasonable for such "uncomfortable" projectile).
A huge value. There is inconsistency somewhere.
Re: Electromagnetic BB gun
DerAlbi, Wed Jun 13 2018, 08:11AM
..the russian original applied a constant current drive to the coils. I did the calculations and also got 45µs. However my further conclusion were different: to make a constant current actually reasonable constant you need multiple switching cycles within the 45µs time window. This might leave you with a period of 4.5µs or 9µs depending on how many cycles you want to have. And this is actually unreasonable for IGBTs since this is >100kHz.
In the context of your calculations it actually makes not much sense to speak of a frequency since you implied a single pulse without repetition.
I dont think you have much inconsistencies. I am not sure where the 2 comes from in 2/48 (you assumed 2 Jules?)
Even if just 2 Joules, a coil needs _more_ energy in it than the energy it transfers mechanically. So the actual value might be much larger. But the voltage might be higher and the current lower. You can easily get 600V Mosfets with less than 30mOhm on-resistance (or higher - i assume the Mosfets to be a major problem for the efficiency)... Its not far from doable, but far from practical [cost].
Hmmh. Its a bit of a mystery how he did it without selling his home to be honest. But that is actually fine with me ^_^
..and he is actually demonstrating outdoors... maybe not without reason
Not sure if this is actually a push forward in technology or just the same old design build to a size that no one ever thought necessary [but still cool enough; 100m/s is a dream i still have]
DerAlbi, Wed Jun 13 2018, 08:11AM
..the russian original applied a constant current drive to the coils. I did the calculations and also got 45µs. However my further conclusion were different: to make a constant current actually reasonable constant you need multiple switching cycles within the 45µs time window. This might leave you with a period of 4.5µs or 9µs depending on how many cycles you want to have. And this is actually unreasonable for IGBTs since this is >100kHz.
In the context of your calculations it actually makes not much sense to speak of a frequency since you implied a single pulse without repetition.
I dont think you have much inconsistencies. I am not sure where the 2 comes from in 2/48 (you assumed 2 Jules?)
Even if just 2 Joules, a coil needs _more_ energy in it than the energy it transfers mechanically. So the actual value might be much larger. But the voltage might be higher and the current lower. You can easily get 600V Mosfets with less than 30mOhm on-resistance (or higher - i assume the Mosfets to be a major problem for the efficiency)... Its not far from doable, but far from practical [cost].
Hmmh. Its a bit of a mystery how he did it without selling his home to be honest. But that is actually fine with me ^_^
..and he is actually demonstrating outdoors... maybe not without reason
Not sure if this is actually a push forward in technology or just the same old design build to a size that no one ever thought necessary [but still cool enough; 100m/s is a dream i still have]
Re: Electromagnetic BB gun
rpopeye, Wed Jun 13 2018, 07:46PM
sorry I didn't reply the last few days, I was a bit busy
to answer some of the questions:
- the coils have different parameters along the barrel, in general due to the smaller and smaller time the projectile spends inside a coil as it is being accelerated, coils further down the barrel have a smaller number of windings (shorter pulse requires less inductance to reach maximum current)
- the mosfets are indeed 100V (optimum in terms of price/performance ratio - $1.5 per mosfet) and the maximum current is about 350A
- efficiency from capacitor bank to kinetic energy is actually 2%, it's 1% when including the DC-DC converter
- the BOM is about $200
- I am not saying that a matrix design wouldn't work, I am saying that it wouldn't be able to pass as high currents as the simple one-NMOSFET-per-stage design
rpopeye, Wed Jun 13 2018, 07:46PM
sorry I didn't reply the last few days, I was a bit busy
to answer some of the questions:
- the coils have different parameters along the barrel, in general due to the smaller and smaller time the projectile spends inside a coil as it is being accelerated, coils further down the barrel have a smaller number of windings (shorter pulse requires less inductance to reach maximum current)
- the mosfets are indeed 100V (optimum in terms of price/performance ratio - $1.5 per mosfet) and the maximum current is about 350A
- efficiency from capacitor bank to kinetic energy is actually 2%, it's 1% when including the DC-DC converter
- the BOM is about $200
- I am not saying that a matrix design wouldn't work, I am saying that it wouldn't be able to pass as high currents as the simple one-NMOSFET-per-stage design
Re: Electromagnetic BB gun
rpopeye, Wed Jun 13 2018, 07:55PM
I am highly skeptical about portable designs that state efficiencies on the order of 15% and high velocities, only unquestionable video demonstrations can convince me that such beasts exist. I mean DARPA got it only up to 25% with a 2m long mortar that used inductive drive and insane voltages and currents, and a Ghz radar to sense the position of the projectile.
rpopeye, Wed Jun 13 2018, 07:55PM
I am highly skeptical about portable designs that state efficiencies on the order of 15% and high velocities, only unquestionable video demonstrations can convince me that such beasts exist. I mean DARPA got it only up to 25% with a 2m long mortar that used inductive drive and insane voltages and currents, and a Ghz radar to sense the position of the projectile.
Re: Electromagnetic BB gun
_Eugen_, Wed Jun 13 2018, 09:00PM
2 DerAlbi
2 Rpopeye:
Thanks, little information is better than nothing. Now the values become more realistic. 2 % total efficiency means you may have 4 or 5 % efficiency on the last stages which convert 1000A to 200 A peak current - quite tolerable for 350 A MOSFETs you spoke about. Of course 100V initially stored in caps are lowered to maybe 70V till that moment and must be compensated by more current as DerAlbi points, but reserve still takes place.
_Eugen_, Wed Jun 13 2018, 09:00PM
2 DerAlbi
..to make a constant current actually reasonable constant you need multiple switching cycles within the 45µs time window. This might leave you with a period of 4.5µs or 9µs depending on how many cycles you want to have..Yes but if the coils are similar, you may have multiple pulses (ensuring approx. stable current) in the beginning of acceleration, and only one (nearly triangle) pulse in the last coil which nevertheless reaches the same current in its peak. Such a configuration lowers the efficiency and I argued with Dantist (an authour of the coilgun I linked above) about that matter, but he insists his scheme to be optimal (maybe because construction profits are more critical).
2 Rpopeye:
Thanks, little information is better than nothing. Now the values become more realistic. 2 % total efficiency means you may have 4 or 5 % efficiency on the last stages which convert 1000A to 200 A peak current - quite tolerable for 350 A MOSFETs you spoke about. Of course 100V initially stored in caps are lowered to maybe 70V till that moment and must be compensated by more current as DerAlbi points, but reserve still takes place.
and a Ghz radar to sense the position of the projectileinteresting solution :)
Re: Electromagnetic BB gun
DerAlbi, Wed Jun 13 2018, 09:13PM
A 25%eff induction launcher does not speak against my 26%eff reluctance coilgun or the Russian 15%eff design.
Of course my 40m/s arent really high velocities but the concept should be good up to 100m/s until i reach semiconductor limits. Of course thats nothing against induction coil guns, but they are a completely different class of devices.
DerAlbi, Wed Jun 13 2018, 09:13PM
I am highly skeptical about portable designs that state efficiencies on the order of 15% and high velocities, only unquestionable video demonstrations can convince me that such beasts exist. I mean DARPA got it only up to 25% with a 2m long mortar that used inductive drive and insane voltages and currents, and a Ghz radar to sense the position of the projectile.Dont mix induction coilguns with reluctance coilguns. Different concepts, different problems, different limitations.
A 25%eff induction launcher does not speak against my 26%eff reluctance coilgun or the Russian 15%eff design.
Of course my 40m/s arent really high velocities but the concept should be good up to 100m/s until i reach semiconductor limits. Of course thats nothing against induction coil guns, but they are a completely different class of devices.
Re: Electromagnetic BB gun
rpopeye, Thu Jun 14 2018, 06:08PM
Here is a link to the DARPA paper about the inductive mortar launcher
rpopeye, Thu Jun 14 2018, 06:08PM
Here is a link to the DARPA paper about the inductive mortar launcher
Print this page