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Efficient multistage coilgun

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Ben Solon

Tue Sept 18 2012, 05:31AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

I havnt had much experience in coilgunning other than rigging my 8200uF cap at 340v to a mousetrap switch and an uncalculated coil to an unmatched slug... It did punch a hole in a can though.

But

I am feeling in the mood for a nice multistage coilgun with all the features. 100% closed loop sensing with a micro to time it. First off, I would like to point out that I want it to be many stages with a small slug. I haven't measured anything, but I think I want to shoot a <=10g slug at >=100m/s. yes, a sniper rifle of sorts. First thing I want to as is about efficiency. What is the maximum efficiency i could get if I do everything right? Timing to coupling. I'm hoping to ot have to use more than a kj of tank energy, and still convert enough of it to meet my specs. 10% seems doable, saz just got 8% in his 8 stage proto. I just want a heads up on where to expect difficulties to set in

I should add that I WILL make this happen, so even I end up with 20 stages and 30kj, I'll do it. It just won't look like much of a rifle anymore.

2Spoons

Tue Sept 18 2012, 06:24AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

First of all I'd be looking to eliminate most obvious loss mechanisms, such as eddy loss in the slug (by laminating).
Then I'd use an energy recycling scheme such as the attached schematic. I've drawn transistors but substitute your switch of choice. In this scheme both transistors are turned on to power the coil, then when both are turned off the diodes return any remaining energy to the storage cap. This also has the advantage of turning the coil off very fast - much faster than simply putting a clamp diode across the coil. Downside is more complicated drive electronics, and two switches per coil, but that's no biggie.

Disclaimer : this is theoretical - I haven't built a coil gun myself, though I've thought about it a lot.

Ben Solon

Tue Sept 18 2012, 04:01PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

So if I where to go and build an efficient slug, could I go and do something like rolling a steel sheet into a cylinder and then braze over the surface and grind the braze into slug shape? Or is it laminating the air around the coil that you're talking about? This will be the hardest part- a easy to "mass" produce projectile that minimizes losses and holds shape on impact.

I already plan for extreme electronics going to be going on, It needs to time a good number of stages. I also already planned on recycling the energy somehow, do thanks for the idea. I'll build on it.

And I just remembered a post about powdered iron projectiles. If they proves the best route then I can just make a jig to pour epoxy iron slugs

Saz43

Tue Sept 18 2012, 06:40PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

This sounds like a really cool project, I don't think anyone has built a high-power mutli-staged half-bridge coilgun before.

ben123324 wrote ...

I am feeling in the mood for a nice multistage coilgun with all the features. 100% closed loop sensing with a micro to time it.

If you have closed loop, there's no need for a timer. That's the point of closed loop, no feed-forward timing is required.

wrote ...

First off, I would like to point out that I want it to be many stages with a small slug. I haven't measured anything, but I think I want to shoot a <=10g slug at >=100m/s. yes, a sniper rifle of sorts.

There's an important thing to understand here. A lighter projectile actually takes more power to accelerate to a given energy over a given distance than does a heavier projectile. This is counterintuitive, but true, and is the reason why big coilguns require big projectiles to reach high kinetic energy.

wrote ...

First thing I want to as is about efficiency. What is the maximum efficiency i could get if I do everything right? Timing to coupling. I'm hoping to ot have to use more than a kj of tank energy, and still convert enough of it to meet my specs. 10% seems doable, saz just got 8% in his 8 stage proto. I just want a heads up on where to expect difficulties to set in

The maximum theoretical efficiency of a linear accelerator accelerating an object from rest is 50%. My 8-stage actually got up to 13.16% on the 7th stage (havenâ€™t tested the 8th yet) but the overall efficiency is 6.52% since the first stages are slower. I don't want to discourage you, but I think with your 50J proposed muzzle energy, you can expect a much lower overall efficiency unless you plan for your gun to be 10 feet long. The reason is power transfer and efficiency are mutually exclusive in a coil, you can't have both at the same time. The more efficiently your coil transfers power, your projectile will induce a stronger voltage in the coil and reduce current flow- and thus magnetic force. The solution is to have lots and lots of low power, high efficiency coils that slowly yet efficiently add energy- the drawback is your launcher starts to get too long to be practical.

wrote ...

I should add that I WILL make this happen, so even I end up with 20 stages and 30kj, I'll do it. It just won't look like much of a rifle anymore.

I'm going to throw out a wild guess, and say with multiple, carefrully designed half-bridge stages and 50J planned muzzle energy, you might achieve 3%, so expect to have around 1.7kJ which is pretty reasonable, just be safe with that kind of stored energy.

I'd like to weigh in on a few of the other things that have been mentioned, simply because I would love to see this project be successful.
1. I wouldn't spend time on machining special projectiles- just use nails. IMO the small gains in efficiency won't be worth all the added effort. I haven't seen research on this but I have also never seen a significant variation in results based on projectile material.
2. I also wouldn't spend time on flux-augmentation, or encasing your coils in Iron powder or something of the like. Research shows that this is only effective for low power coilguns. For energies like you intend to get, your projectile will probably be saturated and any gains will be small. Unless of course you go for the 10 foot series of low power coils.
3. Remember, complex circuitry adds nothing to performance. Often the simplest solutions are the best.

Ben Solon

Tue Sept 18 2012, 09:05PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

Saz43 wrote ...

This sounds like a really cool project, I don't think anyone has built a high-power mutli-staged half-bridge coilgun before.

ben123324 wrote ...

I am feeling in the mood for a nice multistage coilgun with all the features. 100% closed loop sensing with a micro to time it.

If you have closed loop, there's no need for a timer. That's the point of closed loop, no feed-forward timing is required.

what i meant by using a micro was having the micro efficiently trigger switches(igbt's) on receiving optogate signals that way i can use the micro to inhibit a delay if necessary in line with the closed loop system. i'll end up using one anyways for the power supply monitoring and sensing/displaying, so why not. an interrupt routine to flip a digital pin's state takes in the order of 500nS with no prescaling.

wrote ...

There's an important thing to understand here. A lighter projectile actually takes more power to accelerate to a given energy over a given distance than does a heavier projectile. This is counterintuitive, but true, and is the reason why big coilguns require big projectiles to reach high kinetic energy.

good to know, thanks. if a heavier projectile is easier to speed up anyways then why not? it'll add more kick at no extra cost.

wrote ...

i aim for a barrel somewhere in the order of 1 meter. that way i can fit many stages along the length and get the projectile going in a straight line before letting it go. i can put as many stages as needed along the way to make it doable. each stage will be matched to the projectile and projectile speed of course. as long as i get 100m/s in that barrel i am happy.

can i ask how long your 8 stage gun is?

wrote ...

wrote ...

I should add that I WILL make this happen, so even I end up with 20 stages and 30kj, I'll do it. It just won't look like much of a rifle anymore.

hmm. looks like it time to make a power pack XD 2kj isnt going to make it look like much of a gun, so maybe a rigid backpack with 1/2" plexi to shield the user from detonations?

because i'm using igbts and a relative low current per stage, it can all be one big bank. its going to take some thought, but 500V is the most i want to charge to, and that implies ~15mF of capacitance. i might find an ingenious method of fitting it in the gun, but i don't think so if i want it to keep up appearance as well.

the projectiles could be iron powder with even less effort than solid steel/iron. and there's no downside to them. filing/hack-sawing nails < pouring mixture into molds.

i didn't want to do any flux enhancement anyways, so good.

almost last, i don't want to just say screw it and be the guy who does brute force on the energy, so i have to have some complexity to it.

and last:

wrote ...

This sounds like a really cool project, I don't think anyone has built a high-power mutli-staged half-bridge coilgun before.

wrote ...

I would love to see this project be successful

thanks a lot! honestly you're the person who i value those words from the most in this project! i'm a big fan of your gun, and its the main inspiration of this one of mine. congrats on ~1MEG views too!

Saz43

Tue Sept 18 2012, 09:57PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Cool, thanks!

My 8-stage coilgun has about 20 inches of coils, it would be about 17 inches without the flux caps. Personally I'm scared of powerpacks, I wouldn't want to strap something to my back that could detonate an Elephant if it shorted the terminals. With a meter long barrel, I think you will have plenty of room for 1.7kJ worth of capacitors. You might need a bipod at the end, but that would look kinda cool.

Ben Solon

Wed Sept 19 2012, 12:17AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

ok, thanks! the pack was sort of a joke from a 20kv backpack coilgun thread not to long ago. all in good humor. i wouldn't put any form of stored energy with a potential to explode anywhere that would inhibit the ability to chuck it and run!

the 20" barrel gives me hope yet though because i have a 500v 8200uF lytic i got new for under 10$. if i can just squeeze 5-6% total from the cap then i get 50J give or take from the caps voltage charge.(i'm not going to push 500v into it).

funny thing here: the numbers i spat out earlier are actually mathematically correct! somewhere deep in my mind i knew that 50J accelerates a 10g projectile to 100m/s. don't ask me how...

i still want that 100m/s figure, but with a heavier projectile the velocity will drop regardless of the ease of using it. any thoughts?

btw saz, the tripod idea is good! take a look at nichicons nx series for the 500v 8200uF cap. if i can get all my energy out of that, then the overall volume/weight of the energy storage could be reasonable. and i'd like to mention that one of the reasons for picking this project is the fabrication and function of the gun mech itself. i recently invested in a cnc machine, and i am eager to test it out in a big complex project like this. i'm going to be using a clip of sorts to feed the chamber, bolt action style. plus it has to look good too right!?

Saz43

Wed Sept 19 2012, 05:22PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

ben123324 wrote ...

the 20" barrel gives me hope yet though because i have a 500v 8200uF lytic i got new for under 10$. if i can just squeeze 5-6% total from the cap then i get 50J give or take from the caps voltage charge.(i'm not going to push 500v into it).

With a long barrel consisting of several closed loop, half-bridge stages I think you should be able to achieve that. If you were successful it would beat the muzzle energy record for a portable coilgun- which is currently 28J (on the internet at least).

wrote ...

i still want that 100m/s figure, but with a heavier projectile the velocity will drop regardless of the ease of using it. any thoughts?

For a given mass, there are 2 things you can do to boost velocity- increase power (which reduces efficiency) or add more stages (which increases length).

wrote ...

plus it has to look good too right!?

Absolutely! It's always such a shame to see a well designed coilgun mounted into a duct tape and plywood frame.

Yanom

Thu Sept 27 2012, 11:46PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

can anyone tell me why exactly more statges increases overall efficiency?

Saz43

Fri Sept 28 2012, 06:26AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

In short, higher projectile velocity means more efficient power transfer from coil to projectile. Later stages have a higher initial velocity since the projectile has already been accelerated by previous stages. To understand why, here's my best explanation:

First understand the physics that make a coilgun work
When current starts to flow through a coil it magnetizes the projectile. The magnetized projectile establishes a magnetic field perpendicular to the charge flow through the coil, which exerts a Lorenz Force on the coil. The coil is fixed, so the projectile begins to move.

Then understand how projectile speed effects coil current
The projectile, now a moving magnet, induces a voltage in the coil that opposes the voltage driving the coil (Faraday's and Lenz's Laws), and thus acts against the forward current. The voltage induced in the coil by the projectile is proportional to projectile speed. Now there are 2 places power delivered to the coil can go. Most of it will become heat according to P=I^2*R. The more current (I) that flows in the coil, the more power will be lost here. The rest goes to the projectile. The faster the projectile moves, the stronger voltage it induces in the coil which reduces coil current, reducing heat losses and increasing efficiency. However, lower current also means lower magnetic force and less power delivered to the projectile. So you have to choose between high projectile energy and high efficiency.

Then understand how multiple stages can get you both
The solution is to add small amounts of energy at high efficiency but over a long period of time, so energy adds up to a high final muzzle energy. This can be done with a super long coil. But a super long coil with lots of wire has high resistance and thus much waste heat, so we choose to have several shorter coils wired in parallel to keep the resistance low and fire them sequentially to keep the interaction focused on the projectile as it travels through.

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