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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Another Coilgun

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Author Post
DerAlbi
Fri Oct 30 2015, 10:59PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
So its time for a final report for the eddy current sensing system: its fucked up. But not beyond recognition.
The parasitic effect of the acceleration coils was underestimated. The coils work as good as the projectile when it comes to short-circuiting the sensor-field. Therefore inside the fully constructed setup the sensors cant distinguish between the copper of the coils and the iron of the projectile.
So what i did was to look for solutions.
It turs out the sensors need an nonconductive cavity around them with 3mm distance from sensor coil to the next conductor. This guarantees that the projectile is the dominant eddy current loss.


Here you see my testsetup. I replaced the accelerations coils by aluminium foil (the foil has the same effect as the presence of the coils). I then made some donuts from FR4 and used them as nonconductive spacer beneath the aluminium foil.
This made the 5 sensors in series work again with an overall distance of 60cm! The pure cable length created a parallel resonance again so my sensor frequency is again limited to 3MHz. Never the less it gives a strong readout now. I am currently positive that i can solve the problems. The key will be to wind coils that provide inner layers with less turns so the inner layers leave enough space to not interact with the eddy current sensor.
It sucks. Its really complicated to wind such coils. But once the right bobbin is created the winding is trivial.
So the conclusion: its a really good technology. Once figured out how to get it to work its way more robust than optical sensing. However another redesign is in order. This will again swallow not only months of time, it will also consume moths of salary again -.-
...but i also can incorporate all i learned from this build again. Necessary steps i guess.
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DerAlbi
Mon Nov 09 2015, 11:28PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
So maybe is time for a new update. Its going slowly as allways.
Since i am doing a new version i want to incorporate new ideas regarding further improvement of the system. To make new coils i need to decide first for a new barrel.
Until now i had a ID=10mm, OD=12mm tube made of carbon fiber. This introduces a 1mm air gap between the coils and the projectile which is 10mm in diameter. This is too big! I want to go for 0.5mm air gap or less by compromising the mechanical strength the barrel gives to the design. (i can achieve mechanical strength otherwise)
Right now i am investigating in Teflon tubing or Teflon plates that i can hopefully insert into the coils. Teflon should work fine as a barrel material. Its low friction should prevent wear+tear so it can be thin without compromising lifetime.
The main problem is that it is fucking soft and not form-stable at all. I cant buy a Teflon tube with the right dimensions like ID=10.1mm, OD=11mm. I cant widen a bought tube since the inner diameter is not relieable and i cant reduce a normal 12mm OD to 11mm.
One either needs a specially manufactured tube (where i wouldnt be sure that any company would promise me the tolerances i wish for) or be more creative.
The second thing is that i ordered a Teflon plate with 0.5mm thickness. I hope i can cut it to the right width and mybe just put it inside a coil where its spring force is enough to press the teflon against the inner side of the Inductor.

Maybe the Teflon is just a completely wrong idea. Maybe i need something more like POM which is way more relieable in holding its shape. There are even mixtures of PTFE and POM. However those solution are quiteexpensive for a prototype. Grrrr. There is just no way to do it right and cheap.

One additional thought on the barrel: i want to make it as structurally weak as possible. This has no technical reason but legal ones. Its something about the german gun law.

Maybe its time for you to break this monolouge and give me some input where i can get usefull material to make a good barrel
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BigBad
Wed Nov 11 2015, 12:33AM
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 599
Fiberglass is probably the material of choice, except for any legal issues.

Friction ought to be a non issue; if your projectile is touching the sides you've messed up your timing; magnetically, due to Earnshaw's theorem coil guns are either unstable laterally or longitudinally, but not both, so you want it laterally stable, and longitudinally unstable and then electronically control the long axis.
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DerAlbi
Wed Nov 11 2015, 03:55AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
Hmmh only because the forward pulling force is dominant it does not mean that gravity and the magnetic asymmetry does not create lateral forces..
(the closer you get to one side to a coil the more attracted the projectile becomes to this side)
However i agree with you that friction is not a major influence to efficiency but the barrel wall is touched - the projectile certainly does not hover though the barrel.
My thought was by having low friction the durability of the barrel increases naturally. Fiberglass.. is similar to my current carbon fiber or similar to PCB-material. In my experience those materials become rough and quite weak when they are exposed to abrasive action. (since the fibers becomes exposed when the epoxy is gone) Thus ever increasing their roughness.. increasing friction.. increasing abrasive action.. increasing roughness....
Do you know what i mean? Do i have irrational fears?

Meanwhile i found a company who has Teflon tubes ID=10mm and wallthickness=0.5mm in their standard stock. I will phone them tomorrow. Maybe i can get a sample... but 10.0mm ID is not good for me actually. i need something like 10.1mm

Concnerning the durability of the barrel material i made an interesting experience: i tried to cut a teflon tube with a diamant saw blade. NO CHANCE. A diamant blade seems to work abrasive which seems extremely useless on teflon. A simple knife went though like it was nothing however.
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BigBad
Wed Nov 11 2015, 10:03PM
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 599
DerAlbi wrote ...

Hmmh only because the forward pulling force is dominant it does not mean that gravity and the magnetic asymmetry does not create lateral forces..
(the closer you get to one side to a coil the more attracted the projectile becomes to this side)

That's not quite correct; it depends on distance. If the projectile is close to the active coil, then yes, it is laterally unstable then, but if the projectile is slightly further away; it is pulled towards the centre of the coil; and in-between is a neutral point. Note that gravity is a small force in coil guns, and can be neglected.
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DerAlbi
Thu Nov 12 2015, 09:05AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
I slept about it.. i dont really get my head around it. But i think you are basically right with the forces But glass fiber still does not win my heart.

So small little progress again. (just some stupid tests with teflon)
I bought 0.5mm teflon sheets and just put it inside my existing coil. If i cut it to precice width it might actually work.

Well.. i didnt cut it precicely The spring force makes it quite tight around the circumfence, but at the gap its not so nice
Here a better angle:

Left is ok, right.. not so much. you see a little bigger gap between the teflon and the coil.
The next picture shows it even better, but it was really hard to take. The projectile just slides out at any small angle


Note that this is a 10mm Projectile in a 12mm ID Coil with just a 0.5mm thick wall. Naturally there is a gap left and its not tight. But it might be a start.
So maybe i just start winding 11mm inner diameter coils.

I also had contact with a really nice company. They have 10x0.5mm teflon tubing in their standard stock which i might get a sample of if they find a piece with plus-tolercance. They also talked to me about alternative materials.
In the automobile industry the welding robots use teflon tubing for guiding their welding wire. To make it really durable they add ceramic particles to the teflon. The only problem is that the thin wall thickness might become a problem. The teflong and the ceramic clog together during manufacturing.
Anyway: for prototyping the barrel situation is a mess. For a series production.. its doable but who cares without a prototpye -.-.
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Karmaslap
Tue Jan 05 2016, 08:38AM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
Over 20% average efficiency? Is this calculated by subtracting the energy saved with half bridge design, or just using total energy in capacitors and muzzle energy?

I think it's safe to say that I've read every single post in the electromagnetic accelerators sub forum lver the last 2 years. I just finally made an account. I am looking forwards to when you are comfortable sharing more of your theory in setitng up the coilgun and circuitry: many of your forum threads are great reads. Hopefully you will eventually share some of the higher-level concepts for the design (once prototype is done) with us as well. (Nothing too specific if it is a secret, but some of the ideas you had to wring efficiency while still getting good power).
This is my first post as well. Last I checked 4hv was last december while working on a mini coilgun with some of my college friends on a weekend. I am hoping to spend some time learning FEMM, much more circuitry (two semesters of college for circuits and still don't understand a lot of this) and make a lower-power prototype just to learn basics and get good efficiency/correspondence to models before I try to make it bigger (everyone seems to make a camera gun, then buy the biggest cap bank they can and make a huge gun, that isn't engineering).

I certainly am looking forwards to seeing more of your project!
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DerAlbi
Tue Jan 05 2016, 07:45PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
Awww such nice comment :) tell me you are female and just marry me!

The 20% Efficiency (its actually extrapolated to be 40%) is calculated by KineticEnergy/UsedElectricalEnergy. I have a lot of energy left in the caps after a shot, but still... for the next shot its only relevant to recharge the used electrical energy. Still with such high efficiency i can have way smaller caps than most others. (670J for a 100J CG which will hopefully have 23% eff due to aluminium wire. but thats music for the future.)

Considering my theory and stuff: i actually do not do any magic stuff. I very much think that i am just doing everything (hopefully) right. Yet I still learn a lot from others even after my first prototype. But a complete sharing is difficuilt and i doubt many would read it. Currently i do not update my project thread here because i do not very much technical related stuff. I am really thinking about making coilguns commercially available. I think it would sell.. even for the higher price point.. maybe not in america but.. and mybe not in highest numbers but its worth a try. Not only its a technical couriosity but it also can be made very safe and projectiles are reusable (20ct-30ct/pcs). Thus i am currently fighting for the coilguns legal definition with the goal of making it free at 18+ like crossbows and stuff. The main point there is that it does not have a barrel as the law defines it (while having comparable projectile energy to crossbows). But to make authority see the same facts as i do is quite hard. They dont understand the technology. Naturally they fear it. They tell the story as they see it with no regards to the technical truth. State and believe/religion. It never got independed.
This is not only for me but i also feel its important for the coilgun community to finally get a clear statement. Allready a 10th-grade student got kicked in the ass by the state for beeing interested in electronics (coilguns) here. I hate that. But in the long run i cant see how i could finance a law evaluation when facing too much resistance. Its just all so messed up.. i need investors

The only technical stuff i did is that i made a super ultra mega cap charger of doom from hell. Its hyper ultra super mega efficient (93..95%, mainly core losses) and thus providing 410W..450W output power without any heatsink while its possible to concetinate several modules while the PCBs are only (W/L/H) 45x90x30 mm in size and cost is <20$ if you would produce a 1000 of them.. It would fit even in a slim coilgun and provide scalable fire rate.

Now on your stuff:
last december while working on a mini coilgun with some of my college friends on a weekend. I am hoping to spend some time learning FEMM, much more circuitry (two semesters of college for circuits and still don't understand a lot of this) and make a lower-power prototype just to learn basics and get good efficiency/correspondence to models
You are absolutely on the right track! It brings a tear to the eye to finally read this here
My secret goal would be to make a FEM-Independend Coilgun model using just LT-Spice. My problem is currently that i do not have any time nor motivation (because i have a different working system). Technically all i am not sure about is how to handle the changing coupling factor for adjacent coils when the projectile passes through both. This would be perfectly suited research for my students however the university wont appreciate research for private projects

everyone seems to make a camera gun, then buy the biggest cap bank they can and make a huge gun, that isn't engineering
Yes its a fundamental problem with coilguns. They are both easy and fascinating projects to get something moving.. but as soon as you want anything better it becomes very complicated very fast. Only a few go this route. Even fewer succeed. As understandable it is i still did not learn to appreachiate the simple systems.

two semesters of college for circuits and still don't understand a lot of this
Reality flash: you never will! Well.. at least not if you rely on education alone. Higher education gives you the tools for understanding but not the actual understanding. Therefore you need to "practice in practice". And here you seem to be active enough to seperate yourself from others. Thats good! Just dont get stuck on one project for too long and even if you change your project also change your subject. I know people who practice an electronic hobby but the virtually do nothing else than the same ZVS stuff over and over again. Tesla-Coil, Induction-heater, Cap-Charger.. whatever.. its all the same and the Mosfets burn for the same reason every time . Check out microcontrollers, OpAmps and even HF-Stuff. Learn LT-Spce. Its priceless. Well.. it really is

I certainly am looking forwards to seeing more of your project!
As said.. i fear there is not much comming soon, sry I will have a meeting soon the the consultant who evaluate the technical stuff for our authorities. I hope they will provide me with a specification with which they would back me up in my quest for 18+. After that i will try to build a small system with not more than 15J E_kin. That level of patheticness will hopefully lower the resistance and the motivation to invent their own version of the technical truth. Then, if my daring plan eventually succeeds i will get back to the big thing but i dont see it before 2017 realisticly. But a lot of firmware and proof of concept for upcomming software and mechanical challanges will be happened until then.

Thats it for today.
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Karmaslap
Thu Jan 07 2016, 11:30PM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
Sorry, I'm a guy! I replied to this post on my iPad a while ago, but it crashed and didn't post, and I had typed a large reply so I was annoyed and am just now getting back to it.

You have already offered to help with small questions I have, so thanks for that, and if you hate writing technical stuff then no need? Except those that do browse would all love to read it.
Do you post anywhere else? 4hv is the best forum for technical help for projects I have found. Other forums ban topics that people here thrive on, but 4hv does not show up on Google search very easily.

You don't do any "magic" but you seem to use half-bridges, and a projectile much longer than the coils to fit more coils on the barrel? Those are both "untraditional" designs.

I'm sure that the people reviewing it will be fearful of "coilguns", or whatever that translates to in German? When I am talking to my friends, I am working on a coilgun. When I am talking to someone I think might not like me playing with a gun, I'm designing an "Electromagnetic linear accelerator" and this does a LOT to make people feel more at ease. They even hear it and get bored if they don't like engineering (some people are weird I guess) but if I said coilgun to them, they would immediately think it was dangerous. You should be very careful what you refer to it as :) If people are going to be ignorant and fearful of something, then there is nothing wrong with showing them the same thing from an angle they won't be scared of.

The only technical stuff i did is that i made a super ultra mega cap charger of doom from hell. Its hyper ultra super mega efficient (93..95%, mainly core losses) and thus providing 410W..450W output power without any heatsink while its possible to concetinate several modules while the PCBs are only (W/L/H) 45x90x30 mm in size and cost is <20$ if you would produce a 1000 of them.. It would fit even in a slim coilgun and provide scalable fire rate.

You seem quite proud of this, and rightfully so! that's a pretty small charger, and being able to place modules together at that price range is just about unbeatable (I think, at least.) Admittedly the charging system is what I know the least about when I look at schematics people post. I can analyze everything else, but not tranformers or figure out the power it can deliver. Will come once I mess with them, I guess.

You are absolutely on the right track! It brings a tear to the eye to finally read this here

aww, thanks. Also because I am more interested in the designing and construction than the finished gun itself, so I don't mind spending time with a small prototype. Also because I'm a poor college student!


I would love to combine LTSPICE with FEMM, then edit it so that you can model the force on the projectile to give it a velocity and position that change. Run through a lot of iterations, and you have your entire shot and can focus on optimizing timing in the simulation more easily. You could simulate the entire gun at once, see the magnetic field intensity and plots as well as the current vs time for each the coils, see the finished video of the projectile accelerating, coasting, and accelerating again after the simulation is done. I can see exactly what I want in my head, but it would be a greater challenge to make the program than it would be to make the gun. Maybe if there were hundreds of people actively here who would use it I would want to.

Reality flash: you never will!

I'll be more familiar with it, at least! in class we were told "here is a resistor, here is a capacitor" etc. and given everything idealized, then had to do hours of homework to know how to analyze basic circuits made of them. Diodes, the professor spent a few minutes on one day so we would know what the symbol was, and never mentioned a "gate voltage" even. There is a large difference between what my book shows and what I see when I go online to start buying the parts.

also, good advice to play with different stuff

A shame you won't have much to update, but the little tidbits are quite interesting. You should try and do some other work, like projectile design, while you wait :)
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DerAlbi
Fri Jan 08 2016, 12:32AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
I had typed a large reply so I was annoyed

now think how we feel just kidding

I would love to combine LTSPICE with FEMM,
Small missconception here: you dont combine the both programs. You simulate the force versus position in FEMM and use that data to feed a model in LTspice. there the projectile position would just be a voltage source.... basically you can model the coilgun as a circuit.. but thats complicated and quite some work. It gets even more complicated if saturation will happen and if you have multistage with magnetic coupling. Specially the latest i have currently no solution for it.

I'm sure that the people reviewing it will be fearful of "coilguns", or whatever that translates to in German? When I am talking to my friends, I am working on a coilgun. When I am talking to someone I think might not like me playing with a gun, I'm designing an "Electromagnetic linear accelerator" and this does a LOT to make people feel more at ease. They even hear it and get bored if they don't like engineering (some people are weird I guess) but if I said coilgun to them, they would immediately think it was dangerous. You should be very careful what you refer to it as :)
I dont like hiding the truth behind words. Its a gun and its dangerous. Doesnt mean i will be a mass murderer only because i have fun building it. Nor others are because they like shooting with it. I allways just explain thats basically the same as a crossbow (energy wise). And most people relax then because the dont put it in the context of war and crime compared to normal guns.
Coilgun is a Coilgun in German too maybe sometimes a "gauss-gun". But its interesting to hear that from america.. i thought you guys were all about guns and constitution and defending your freedom all day long

And yes.. big thanks to 4hv at this point. Its the highest quality forum i have experienced so far for electrical engineering. I am currently not really active in any other community.. engineers are a complicated type of human and i am amongst them as others might have experienced.. damn.. this forum lacks a red blushing smiley
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Karmaslap
Fri Jan 08 2016, 04:54AM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
Oh, you just want to model how the projectile changes the circuit in LTSPICE. I'd like a program that does both, because a FEA could model the heat from the electronics too, or the actual output of a generator constructed with the circuit.

It's also a linear accelerator, and a majority of them made by people are harmless when it comes to the actual projectile. The capacitors are not, but that shouldn't get attention else the government start banning us from taking apart electronics. A design like yours is unique, of course, and could be dangerous, but even a .22 is more dangerous. People stop thinking clearly when they hear "gun" so explaining it a Different way might give them a chance to see the real picture.

We have a lot of people who believe the exact opposite of that (sigh) but I dont want to get politicsl on the forums.

No kidding. Theres a picture I'll esit in here in a few hours when i can to describe it. Other forums you can ask questions in, but have to mislead people on what it's for, or not mention.

Like on physicsforums I wrote about shaping the field for basically a "linear motor electric chainsaw" which is not a lie, something I plan to build for fun, but didnt want to mention a coilgun also because on a circuits forum i know that is an instant account ban. Hallelujah for 4hv.



Reply from Eugen's thread we hijacked- edited on here to avoid double post.

The flight characteristics are independent of the coilgun's design so long as they both spit the projectile out lengthwise.
Nohohohooooo! Thats what i try to say for about 2 posts now Bad designed coilguns that have any suckback effect going on accelerate the projectile lateraly instable! A bad coilgun therefore will spit out the projectile with more parasitic lateral momentum than a proper designed CG. This is important.

Thats a question for my own thread, but the short answer is that its way more complicated than that^^ The eddy current loss is not as important as the fact that there is one at all. And its not a guidance.. its a detection.

putting grooves on it so it spins itself might be easier.
No way. In a conventional gun, thats no problem. The additional friction is just holding back the gas that most likely will just build up more pressure in the mean time - this might even be beneficial. In a coilgun that friction is directly contributing to bad efficiency. You can not afford it in any way. If you want to spin it, spin it before the shot.
And...... "taofledermaus" on youtube is a nice channel to watch. but he is focussed on supersonic bullet design. but one thing i learned: even a smooth-bore-shotgun can shoot quite precise.

If you talk about whats necessary then everything is the barrel Its also wouldnt work without the capacitors. that cant be the fact they nail me with.


If the projectile were moving in a perfect vacuum, you would be correct. If moving (very) short distances, you would be correct that minimizing the lateral momentum would be the solution. The projectile is moving in air, though, so after a certain distance the two could very well be indistinguishable. Even if your projectile comes out PERFECTLY straight- it will still be subject to move like the cylinder I showed you in that article. It might be more likely to go straight, spiral, or flip than something more innacurate, but it will not travel perfectly through the air. Even the "straight" path wasn't so accurate in the pictures. The "flight characteristics" I refer to are a property only of the objects geometry and mass, nothing else. To have a stable projectile you need the good design to give the projectile a chance, but then (more and more importantly over the distance the projectile travels) the projectiles own aerodynamics take over.
^testing needed at the low speeds we are talking about, of course

Sorry, thought you referred to the system as the guidance. Why that over photogates?

No, I am saying grooves on the projectile, not the barrel. If designed to not hurt the flux linkage around the edges, moving fast through air they would spin a projectile. Shotgun slugs often have these.

I will check out that youtuber!
I am hoping they don't get you at all and decide it isnt a barrel by the laws definition. Keep updating as more happens with the legal stuff.

Also, here is 4hv's problem

high IQ, low number of members. DIY guys like to hide and not be online all day.
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DerAlbi
Fri Jan 08 2016, 12:59PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
Even if your projectile comes out PERFECTLY straight- it will still be subject to move like the cylinde
Yes yes yes.. no argument there. I still think that its a system with chaotic behavior and as such its very dependend on initial conditions (e.g. lateral momentum). Cleaning up those initial condition wont change the characterisic behavior at the end, hoewever it might delay it for quite a bit. We will see.

Why that over photogates?
Light traps are not good if you think about it in terms of mass production and if you think about a customer.
1) They can get dirty. I prefer a solution without the need for constant care.
2) My current solution is ridiculesly thin! Every light trap solution will rquire a higher coil spacing than the current 0.5mm. I won a lot of energy density there.
3) Light trap receivers are high impedance. Its simply a bad choise to have the the electronics directly between the coils. If you use Optical cable the manufacturing process becomes more work. you also have problems with alignment and everything.
-1) However i want to be fair.. the downside is that i pay for it by a more complicated coil shape. Its not a problem once the coil former is done however the prototyping will become more complicated. The complicated coil shape also has a negative impact on efficiency.. but... yeah.. as long as i can reach my specification its fine. If everything goes wrong i add another capacitor... who cares.
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Karmaslap
Sat Jan 09 2016, 05:43PM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
Yes yes yes.. no argument there. I still think that its a system with chaotic behavior and as such its very dependend on initial conditions (e.g. lateral momentum). Cleaning up those initial condition wont change the characterisic behavior at the end, hoewever it might delay it for quite a bit. We will see.

I think that's it exactly, and if a rounded cylinder flies straight for "long enough" then no reason to make a more complicated projectile.

Why that over photogates?
Light traps are not good if you think about it in terms of mass production and if you think about a customer.
1) They can get dirty. I prefer a solution without the need for constant care.
2) My current solution is ridiculesly thin! Every light trap solution will rquire a higher coil spacing than the current 0.5mm. I won a lot of energy density there.
-1) However i want to be fair.. the downside is that i pay for it by a more complicated coil shape. Its not a problem once the coil former is done however the prototyping will become more complicated. The complicated coil shape also has a negative impact on efficiency.. but... yeah.. as long as i can reach my specification its fine.

Must be a pain to have to constantly consider the customer.
I thought your detectors needed a 3mm gap from the coils on either side? Or have you fixed that since now, or did I misunderstand? .5mm is very small indeed.
Why not 3d print a coil former for prototyping? Just a few hours job if you have a nearby college with a printer, or a friend with one.

If everything goes wrong i add another capacitor... who cares

If I am understanding you are trying to perfectly optimize current to minimize heat loss while maximizing magnetic field strength, current rise and fall time, (field strength to the point where a saturation curve begins to taper off) adding another cap would ruin your efficiency

<http://coefs.uncc.edu/mnoras/files/2013/03/Transformer-and-Inductor-Design-Handbook_Chapter_2.pdf> Page 6, figure 2-4
My reading for the last day has been on saturation. I believe this material would be ideal? It has a nice Linear B-H plot until after B>=2
Compared to pure iron or electrical steel which starts tapering off to the right more quickly after ~1.6 ish T. Of course a hobbyist would have trouble obtaining some, but if price were no object, this would make for great efficiency, I think?
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DerAlbi
Sat Jan 09 2016, 06:18PM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
"~3mm" is the width of the Light trap, because the IR-LED is 3mm thick. The whole sensor was however about 4mm at this time.
Edit: I just realized that i forgot in the meantime that the Sensors will not again be only 0.5mm thick. I will change to 1mm thickness. reason? The inner wire of the coil must come out somewhere. This is basically the lower bound. So the latest stage might even have 1.5mm space.
Must be a pain to have to constantly consider the customer.
No, its the main motivation to do a high quality build and to really find the best solution. If it was only for me it would be boring because i wouldnt care about most restrictions. Basicall i allready would have stopped the project because i know its outcome since i know my models match.

Why not 3d print a coil former for prototyping?
Complicated. 3D-Printer is the next step.. However if dont think that a coil former is really good enough. I must try first. And i cant bother a friend with it... it depends on the wire which former is right... and even i can make predictions about the optimal shape i have assumed that i fit the wire in perfectly and stuff. That might not be the case. So without specification and long experience i cant bother others.
If I am understanding you are trying to perfectly optimize current to minimize heat loss while maximizing magnetic field strength, current rise and fall time, (field strength to the point where a saturation curve begins to taper off)
No i do indeed operate in deep saturation. Its a concequence of Ekin*Eff^2. A cap more does nothing to the efficiency.. it only adds weight and volume and cost. Adding a cap is merely a consequence of bad efficiency and if it does anything then it lowers the ESR of the cap bank. Not much lost there, as long as i use its energy as good as i can.
Of course a hobbyist would have trouble obtaining some, but if price were no object, this would make for great efficiency, I think?
Any coilgun with reasonable output power will operate way above saturation (and can still be efficienct if desgined correctly, even if its not most efficient at this point). No material will help beyond saturation..

To help you with saturation: just remember it that way:
Below saturation: F ~ I²
Above saturation: F ~ I.
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Karmaslap
Mon Jan 11 2016, 01:36PM
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Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
DerAlbi wrote ...

"~3mm" is the width of the Light trap, because the IR-LED is 3mm thick. The whole sensor was however about 4mm at this time. So the latest stage might even have 1.5mm space.
Must be a pain to have to constantly consider the customer.
No, its the main motivation to do a high quality build and to really find the best solution.

Why not 3d print a coil former for prototyping?
Complicated...... So without specification and long experience i cant bother others.

Of course a hobbyist would have trouble obtaining some, but if price were no object, this would make for great efficiency, I think?
Any coilgun with reasonable output power will operate way above saturation (and can still be efficienct if desgined correctly, even if its not most efficient at this point). No material will help beyond saturation..

To help you with saturation: just remember it that way:
Below saturation: F ~ I²
Above saturation: F ~ I.


1.5mm is not a whole lot to lose even over all the coils. Shame there is no way to wrap the coils to eliminate this.

I thought maybe you might have easy access to a printer. Most people ai know with one are more thwn happy to have any excuse to use it, especially for something productive. I can just walk for 5 minutes and choose a printer to use, so I can make a few prototypes to get it right quickly. Thought you might have the same access at the school.

F ~ I^2 below saturation because F~ B_1 *B_2, the field strength of the coil and the induced field in the projectile. Once saturation occurs, B_2 (projectile) becomes constant (or close enough to ignore) so F~I because only B_1 is increasing. However, typical Iron/steel saturates at 1.6-2 T with 2 being pure iron which is not the best projectile, and this stuff not only magnetizes to above 2T more easily, it has a higher cap at saturation. So vs steel you'd be getting 1.3x the force on the projectile or so. assuming they are the same weight that is a lot more acceleration, which means higher speed and the current spends less time in the coil.

I am back at school now, so I can see what parts I have. I think my friends and I ruined most of them, though, from playing around.
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DerAlbi
Mon Jan 11 2016, 05:10PM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
You will never get me to optimize the projectile Lets do Math to end this.

A aproximation to staturation is:
f(x, Bsat, Sharpness) := Bsat * tanh[ (x / Bsat)^Sharpness ]^(1/Sharpness).
Where x is the "should-be-B-Strength", Bsat is the saturation point and Sharpness is a parameter how hard it saturates (the higher the sharper)
Now the B-behavior for the projectile might be f(x, 2T, 1.3).
So Force ~ x*f(x, 2T, 1.3).

Now lets see how Bsat=1.7 to Bsat=2.0 changes things! Its a theoretical improvement of 2.0/1.7=1.17, so 17%
Now it depends on your coilgun. I might go up to 4T.... so lets see..
f(4T, 2T, 1.3) / f(4T, 1.7T, 1.3) = 1.167. So 16.7%.

A better measurement might be just to integrate from 0T to 4T for both variants to include all possible benefits during magnetization.
integrate(x*f(x, 2T, 1.3), 0T, 4T) / integrate(x*f(4T, 1.7T, 1.3), 0T, 4T) yields only 13.8% improvement.

So vs steel you'd be getting 1.3x the force on the projectile or so
Far off.

The material of the standardized projectiles i use is "1.0718" or "1.4305". If anyone finds data on that, i am happy to see it
To sumarize, let me put it this way: its easier to optimize it for a bad material or a normal material with sufficient headroom. Then just push less power if the projectile behaves really good and be happy about the higher efficiency. There is no point in optimizing it in a way that puts boundarys for the user. In the end.. if i need to add some coils more to make the output power relieable thats just the way it is. As said.. its also releases the stress to other components if the projectile is good. Flexibility is more important to me at this point. I would also be ok with the fact that different projectiles have different properties. Thats the way it is with guns.
I am however a bit humble realizing that i dont know what kind of steel i use. It simply uses "ISO 2338" 40x10. But thats no material., its just a toelrance. The shop where i bought it says just "steel" wohoo!
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Karmaslap
Mon Jan 11 2016, 09:22PM
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Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
I do not understand what you mean by "sharpness" with regards to saturation.
I do know that if you use 2.1 and 1.6 for the saturation values, which I used instead of 2.0 and 1.7, you get 1.30 or a 30% increase... And that's a lot. I guess your customers wouldn't want expensive projectiles? With a properly tuned setup such as yours, the gun will work with various materials even if not perfectly optimized with faster rise times/etc. for a better projectile.. So if you try shooting other things and recording the difference, you don't lose much. You're buying your projectiles from the hardware store? Not online?! Hahaha

If I can't convince you to try to optimize a better projectile, at least find some other ones and mess around once your prototype is done in the future!
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DerAlbi
Mon Jan 11 2016, 10:40PM
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Location: Dresden, Germany
Posts: 517
Yes yes, 30% is a lot if true. But honestly i do not find any material definitions that include saturation behavior. Without that its still just guessing if the increase can be 30% or not. It could also be true that the usual materials are totally fine and close to each other concerning Bsat.
I bought my projectiles online but there were no detailed information.

I wonder if the gun could just analyse the µr of the projectile and make conclusions about the prjectile. I have to see when it comes to that.
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Karmaslap
Mon Jan 11 2016, 11:49PM
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Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
DerAlbi wrote ...

Yes yes, 30% is a lot if true. But honestly i do not find any material definitions that include saturation behavior. Without that its still just guessing if the increase can be 30% or not. It could also be true that the usual materials are totally fine and close to each other concerning Bsat.
I bought my projectiles online but there were no detailed information.

I wonder if the gun could just analyse the µr of the projectile and make conclusions about the prjectile. I have to see when it comes to that.

Transformer cores are the only research available. The only equation I know of is the one for magnetic force, so I'm basing it off of that until I can do some tests.

µr and saturation are totally different!!! µr Is useful to analyze any external material you are using, but the B-H curve is what you use for saturation. Some materials could be in the 10,000-20,000 range of µr (sorry, no units hehe) while others are only ~100 and the B-H curve looks almost identical!

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DerAlbi
Mon Feb 01 2016, 09:28PM
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Location: Dresden, Germany
Posts: 517
Maybe its time for another update. I am still active.

First, the electronics:
1) I designed a flyback capacitor charger as mentioned above. PCB is on the way.. electrical components will need some time. I am very interested in testing if it works out. Its basically all about thermal management. The combined power loss of the whole charger module is around 20..25W for 430W Output power. I realistically say that the maximum duty cycle of operation is maybe 50% so the whole thing only dissipates, lets say, 15W worst case. 6W alone are pure core losses of the flyback, some Watts are the winding resistance and the rest (maybe 7W) is on the electrical components. But this power is spread out quite evenly over many components. The switching mosfets will have maybe around 1.5W hopefully dissipated by the PCB, and most of the residual losses are in passive deices that may actually run hot without damage. It was an interesting project to design. With the goal of high power density and low cost it becomes necessary to get to the limits of the components. And you realize that you are often bound by fear... running a mosfet at 160°C just doesnt feel right, but it is, if its specified for 175°C operation.

2) I also designed a "motherboard" for the flyback module hosting some important functionality. The thing is that the flyback modules are made to be stacked or concatenated and work together as a multi-phase design. So its useless to include voltage control on every single PCB... its done once and its done on a motherboard!

3) As i am currently concerned with the power management i am also looking around for a final solution there.
First the battery: as i want to be able to go for really high power like 1.5kW burst (3 shots/sec) i need to think about a good solution. A good solution is lightweight and easy. So what would i need? I honestly dont know. I think delivering 150-200 shots per battery charge seems reasonable. If you dont reuse the projectile thats around 50-75$ worth of steel (~4.2kg). Given my efficiency i can predict that with around 30Wh of energy should deliver the said amount of shots. But the 30Wh come in many shapes and sizes: LiFePO4 - i love them. But they are heavy and the cell voltage is quite low (3.2V). If you want a long lifetime you need at least some cell-ballancing. The low voltage means high current. While the batteries are happy to deliver that, i am not happy to work with such high currents for obvious reasons. So either you go for really high cell count like 10S or you decide for a different solution.
..And thats what i am going to do: faced with modern LiPo-Technology i could again decide for different paths: going for “high voltage+low current” or go for “less voltage+high current”. Here the 30Wh are a problem. Really high voltage like >30V only comes in really big packs that are basically just dead weight for me. I could combine two reasonable small 4S-Packs to get a usefull voltage in a small form factor, but honestly.. it would be a pain. You couldnt change packs easily and having 2 of them laying around is in it self just nonsense. Its an obvious bad solution. Soo high battery voltage sadly issnt an option for me. What else? I found LiHV-Packs that claim to have 4.35V max. voltage per cell and a plateau of 3.8 instead of the regular 3.6-3.7V. Luckily enough the do come in a 6S-1.3Ah pack rated for 65C continuous (4mOhm ESR per cell) which would be wonderful for me. They should deliver ~160 shots while they consume only 225g of my weight budget.

4) Having decided for a battery pack i want to deliver an all-in-one solution since not everybody wants to be bothered with an external battery charger. Also i feel that cell balancing issnt only useful for charging alone but also while discharging. Additionally i want to provide a really broad input voltage range like 12-36V so everybody can connect anything to it and it will somehow work and charge. So i am currently all over the design of a battery charger. The LiHV batteries arent happy with high charge currents, so they are only rated for 2C (=2.6A). So i need to provide a ~70W charger with flexible and powerful balancing.
I decided to build a SEPIC converter for constant current charging and synchronous flyback balancing. Being microcontroller driven the thing should hopefully be quite a flexible solution capable of measuring battery ESR of the individual cells and stuff. When its done it will be a good power distribution controller hosting everything connected with the internal power management.

So thats it. Its all just prototyping and proof-of-concept stuff. I concentrate to get anything in a useful form factor already, but it are still just building blocks for a later final design.

There is also legal stuff going on.
I talked to lawyers. Luckily the interesting nature of the project makes it a "more than usual for free"-deal for me. So with their advice i contacted finally the right people in my city that they may give me an official license to build a coilgun.
Sadly its still not going so well To get a license to make a weapon you need either be a 3 year educated gunsmith or a state-approved weapons dealer. Either of those qualifications are needed to fill out the paper work. Soooo... bad for me However the guy i talked to was also interested in the topic and we basically both agree that such an license is not necessary since i am clearly qualified to build a linear motor as i am an electrical engineer So we looked at the paperwork just imagining i had the right qualifications and we pretty soon got stuck again. The paper asked for which kind of weapon i am allowed to build or trade. Guess what: “coilguns”, “linear motors” or “linear accelerators” arent listed. Soo its obvious that the paperwork issnt practical for me and there is currently work going on to find an other solution for me.
The guy said the basic problem is that the German BKA initiated the legal problem by declaring the coilgun as a "weapon by law" and not as a "toy" as other 130J-shooting-machines are classified legaly. And yes, air pressure driven guns shooting crossbow-bolts at >400fps are "toys" in Germany. Amazing!
Being able to actually show a coil and a projectile he quickly agreed that a coilgun does not have a barrel as stated by the BKA, however the BKA has higher authority so while its a calming statement, its useless. Their classification without understanding the technology just sucks and puts barriers in my way that dont need to be there. I mean.. if they dont understand, they should just ask.
Its just far from nice. My silent hope is currently that the obvious incompatibility to the paperwork will force the BKA to reconsider the issue maybe even asking for a more detailed explanation so the can judge reasonably.
As explanation what the BKAs problem is: they declared the coils already being the barrel, even if you hadnt anything in it (like a tube or a rail like system). That sucks. Things that dont even touch the projectile cant “guide” it. A train doesnt follow the tunnel, it always follow its rails.

Soo whoever reads this... thanks, but why?
And btw: this post i spell-checked. i really tend to use double-L where its not supposed to... sry
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Finn Hammer
Tue Feb 02 2016, 12:15PM
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Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 716
DerAlbi wrote ...

You will never get me to optimize the projectile Lets do Math to end this.

(snipped a lot of math that´s over my head)

I am however a bit humble realizing that i dont know what kind of steel i use. It simply uses "ISO 2338" 40x10. But thats no material., its just a toelrance. The shop where i bought it says just "steel" wohoo!



With regard to the magnetic saturation of steel. Steel being -> Iron alloyed with Carbon, up to about 2%
Before I went into Tesla Coiling, I did tube amplifiers and fieldcoil loudspeakers.
In the old days, around just pre WW2, fieldcoil speakers were the way to build a radioset, because the field coil in the speaker doubled as the choke in the high voltage supply pi filter, believe it or not.
Soft iron is an old term describing steel with very little carbon content.
The carbon content is a major factor, when it comes to decreasing the the saturation flux density, and since I was into building horn speakers, I wanted the highest possible flux density attainable, without having to buy the expensive Permendure, which contains up to 50% Cobalt, and which is getting Unobtanium status these days.
Anyway, I became obsessed with the idea, that what they called "soft iron" in the olden days, might not be available today off the shelf.
So I built a test rig, where I could measure these things, by submerging a voice coil with known wire length into a circular magnetic field, and then measure the force of the push from the coil, relative to the current passed trough it.

It came about nicely, that a very common type of steel, construction steel, with 0.15% carbon content, would saturate at 2.05Tesla, woila!

You will have to take my word from this aging Toolmaker, that this is true, since I have long lost all documentation, I`m afraid.
0.15%Carbon, and you are good to go!

Cheers, Finn Hammer
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hen918
Tue Feb 02 2016, 08:44PM
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Location: Alfreton, UK
Posts: 407
DerAlbi:
LiPo all the way... My favourite battery topology. With the current craze for large drones and high powered RC equipment the power density, as well as the energy density available, has gone up and up; the price lower and lower. They just need careful handling, although not as much as some like to think. A good enclosure and careful monitoring of the charging (Obviously this can be done automatically) are what are required.
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DerAlbi
Wed Feb 03 2016, 12:32AM
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Location: Dresden, Germany
Posts: 517
Thx Finn for your information I heard that high carbon steel does spark more when grinded, but since i have no reference, i cant really test it.

And thx hen for your backup. My luck with LiPos wasnt great so far. I allways had issues with bad cell matching leading to deep discharge during some project work. Also charging issnt fun because the cheap chargers take ages for the balancing, specially once one cell is damaged.
But thats basically my fault because i never used any battery control during the battery use. Having that should simplify things hopefully.
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DerAlbi
Sun Apr 17 2016, 11:38PM
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Location: Dresden, Germany
Posts: 517
Time for updates again..
I had a lot of fun exploding mosfets and destroying 4 layer PCBs (Actually it was very frustrating and expensive and a lot of work and i am still silently crying at night)

There was some proof of concept going on.
1) User interface
I tried capacitive sensing as user interface to a capacitor charger. Meaning: I can place capacitive sensors in regions that are typically touched when the coilgun is handled or even when someone aims. This can then enable the capacitor charger and make the coilgun ready to shoot. A cap bank consumes quite some power through leakage which is a practical problem in a power density optimized portable design with a as small as possible battery pack. I am now confident that i can hold the capacitor voltage on a stand-by voltage level and only let it consume power (charge the capacitor fully) when the coilgun about to be used. By placing capacitive sensors at the grip and trigger i can find out when the coilgun is about to be shot.
I also experimented with vibration sensors. I let the power management have a switch with 3 positions: Force-Off, Force-On (which are self explanatory) and a StBy-position. In this StBy-mode i sense a vibration sensor and when the gun is handled (the Test-PCB in this case) the coilgun switches on automatically and stays on for some time. This makes the coilgun usable quickly in any situation (like in a typical American self defense scenario). The vibration sensor is bad enough that a careful position change does not trigger it but when you hasty grab it (or tap on it) it will turn on..

2) Battery management
I implemented a battery charger (based on a SEPIC-topology) with active balancing (based on bidirectional flyback topology).
The SEPIC topology accepts 8V-30V and turns it into a constant current which can be controlled by a microcontroller. Its good for 50W and is around 90% efficient so it can be cooled by PCB copper.
The balancing is done by 6 small flybacks which either can supply or draw current to individual cells. I prefer this over resistive discharging of overcharged batteries which wastes power and does not perform good (fast). The downside is that its much more expensive. The current version seems to be way too powerful.. the cells are balanced in no time which is kind of a waste since a permanent balancing will never transfer much power - just small power over long time. I am however not sure how much balancing capability i am supposed to provide when a battery pack is bad... i figure 1/4 of the charging current is fine.
Here is the PCB with the charger and the user input stuff. The power management draws 40µA when off... thats ok.






3) Capacitor charger...
I developed quite a nice flyback topology which is quite efficient. Its utilizing a multiphase design which can use as many transformers as i concatenate. In this case i tried 2 transformers on 2 PCBs. One PCB is good for supplying 350W average charging power and consumes only 108g of the weight budget. Quite light!




It was supposed to deliver 420W per PCB but when the core material of the transformer gets hot (100°C), its unsafe. The worst issue i had here is that i had no under voltage lockout. So as soon as my power supply current limit kicked in, the logic remained in an undefined state (due to insufficient voltage) where it almost every time destroyed the mosfets (Switching transistors do not like to be in diode configuration and conducting 10A...) This was a serious mistake from my side... i was so fixated on the topology that i forgot the small things around a circuit.
My graveyard after one day...


The charger is controlled by a CPLD... the internal schematic development also cost some lives.. but now it finally runs, its really nice. The whole PCB does not need any cooling due to its exceptional high 93.5% efficiency (with main loss being the transformer cores). This is reached by active energy recycling which reuses up to 20W of power which would otherwise be dissipated. Additionally the mosfets are controlled by valley-switching which reduces turn-on losses.
This is how the drain-voltages of the 2 mosfet switches look like:


One drawback is that valley-switching does not work when the capacitors are completely discharged. So i need a precharge mechanism to give the caps just a few volts prior to charging.. (i solved it with a diode from VCC to C+ so the caps had at least ~20V). Of course the charger is designed to charge a series connection of capacitors (2x 300V series) and it has excellent voltage sharing eliminating the need for a balancing circuit for the cap bank.

So i am currently working on a single PCB design to get a small prototype (maybe 5-6 coils) working for the authorities.
So far i have achieved quite a lot. Topology wise i have not many open issues...
Solved:
  • Cap charger
  • Battery charger
  • User interface and sensors
  • Halfbridge design
  • Projectile sensing

Unsolved:
  • Law conform projectile guidance
  • Reload mechanics
  • Case

So these are mainly mechanical issues now. I guess i need to buy a 3D printer soon to begin experimenting.
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DerAlbi
Fri Apr 22 2016, 03:32PM
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Location: Dresden, Germany
Posts: 517
3D-Printer arrived.
I am working on the mechanical problems now.. sooo this is my version of a projectile guidance that should comply with german law:





Its NOT tube shaped and should therefore be comparable with crossbow rails, because in the end.. this is a rail system.
Of course the german authorities are still thinking the actuall coil is the barrel, but without mechanical contact i dont see how actual guidance is given by the coils. Guidance is one criteria to make something a barrel. Additionally if coils become a part of a licenced weapon, the inductor industry in germany will break down since its not allowed to sell gun-parts.

The rails are made out of round 1mm FR4. For later some wear resistant material can be used. There is close to no mechanical force acting on the system.. so i still look forward to ceramic particle reinforced teflon.

3D-Printer: Zortrax M200.
I have problems with the print bed.. it came bent, so that i cant level it. Obviously the small, flat parts worked. There are test holes inside the print to test for printer issuses. Indeed, the printer need to widen holes by 0.2mm to make them as specified.
The small indents where the rails are suposed to sit, should actually close more than this, so i glued the FR4 in place (and did it badly -.-). The printer minimum wall thickness prevents sharp edges. Later this will be fabricated using PCBs.. it should be fine there (so the rails just clip in and stay there)
The actual model looks like this:


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Karmaslap
Fri Apr 22 2016, 04:15PM
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Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
Good updates! You're going ahead now instead of just meddling with law. With your previous findings and experience I'm definitely looking forwards to the control schemes and a working prototype.

Frustrating about the printer, did you call them up to say it was defective?

I've taken microelectronics since I last read through your thread, it's a lot easier to follow now
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DerAlbi
Fri Apr 22 2016, 04:58PM
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Location: Dresden, Germany
Posts: 517
Thy for the feedback.... but how is YOUR project? "femalified"?
looking forwards to the control schemes
I am currently indeed working on an cover-everything-in-detail block scheme. Unfortunately i dont see it finished soon, as it evolves with the mechanical problems solved. For example the reload mechanism: how to transport the projectiles to the starting position?
Optoins are:
  • Servo (270° revolver magazine?)
  • Stepper motor (more complex, but better control) feels like overkill
  • anything else? i dont like the idea of spring driven systems here, since the projectiles are awfull heavy and big.
  • anything gravity driven does not work since the acceneraltoin path is already on top of the gun

Frustrating about the printer, did you call them up to say it was defective?
I just have it since yesterday and learning to use the thing. I noticed the fault immediately (because it wouldnt auto-calibrate) but only now i could actually give the problem a name. Unfortunately its friday. No quick solution will be found here

I've taken microelectronics since I last read through your thread, it's a lot easier to follow now
Strange.
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Karmaslap
Sat Apr 23 2016, 01:03AM
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Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
Hasn't come along much in the last month at all, still a list of parts. I have built a can crusher in the meantime, helping a senior with his project and I'll post it up in the next week or two. I also got a job this summer which will give me enough extra to continue work on the project and even upscale a lot.

why not a server motor that grabs the projectile from a clip and elevates it above the others enough that they won't be affected by the first coil? Saves you the length of the projectile if you don't have to push it forwards or anything? I'll draw what I mean in paint if you like.

I'm sure you have plenty to do while you wait for it, though! It's good you got a printer. Plenty you could use it for on your project. Going to try printing a coil form when it is working?

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DerAlbi
Mon Apr 25 2016, 01:32AM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
Ok.. after 2 days playing around a realized some problems.
What did i do?
  • Testing out 3D printing materials; i have never 3D-printed before, so there is a lot to learn. And since there are a lot of parameters to play with and even an easy print takes quite some time (~1h) its hard to get a quick overview.
  • I played around modeling the gun, to get a feeling what mechanical challenges arise.


I realized that a small coilgun, even with quite high efficiency, will be an ugly child that not even the mother likes.



The problem is that is over proportional much electronics is inside a small gun. There are elements that every gun needs, like a battery, battery charger, cap charger, halfbridges, timing control, projectile sensing and so on.. these elements take up space no matter how powerless the gun is. But once they are there they grow less than linear with the power demand.
This implies that a small gun will have a bad power density. So the bigger you build the gun, the more credible the design becomes. Also a larger gun provides additional space inside the butt stock while a hand gun has no such space luxury.
A small handgun [20J output power, 6 stages (copper), 100J cap bank, 20cm length] becomes really ugly pretty fast.

I have never touched a gun nor seen one in action in my life... sry you americans, keep laughing, we also laugh at you
One issue i have is width... i have no idea what is appropriate or what looks still ok or what is 'just right'. The acceleration path needs at least 40mm width, the cap bank can either be 52mm wide and relatively thin or it could fit inside the 40mm of the acceleration path and be pretty high bringing the high of the gun up to 15cm (with 20cm length? wtf) UGLY.
So a width of 55mm will be necessary to fit everything in. And seeing this on a ruler, its pretty bulky. not sure how this ever can be made attractive.

To be honest.. esthetic design is actually not important right now. but i want to do it as practice. I need it. Obviously.

Its also a good training to solve the mechanical challenge of reloading. I am currently thinking about a revolver magazine. Its reloaded with an entry on the backside of the gun, then rotates and put the projectile in front of the first acceleration coil. The problem is that this does not leave the projectile in a good starting position wasting valuable energy through bad initial efficiency. But an additional motion is really complex.
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Karmaslap
Mon Apr 25 2016, 10:40AM
Registered Member #58215
Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 62
You are still wanting to first build a small prototype to show off how harmless these devices are?

A gun with the dimensions you describe (3.6x as long as it is wide, 1.3x as long as it is tall) would feel very odd if you wanted it to seem like a typical firearm, but you can always choose to go with a more "science fiction" type look that gives you more internal volume. I believe the most popular coilgun on youtube (Saz from forums I think) shows this off well- it's just a cool looking box, but it LOOKS like a gun and has plenty of volume because it doesn't have any narrow areas.

The most important thing is comfort. A 20 cm long gun is just big enough that it would be uncomfortable to hold with one hand, but too short to feel like an smg and use with two hands. You should slightly upscale for the sake of comfort. Making the whole gun longer also means you don't have to make it so tall.

From little sketches and looking at gun concept art on google images, it seems that very small one-hand weapons can easily achieve a 1:1 length/height ratio and be aesthetically pleasing. This diminishes quickly with length, with the exception of a long high capacity clip (a skinny area, not much volume) adding length. For two handed weapons, a greater than 2:1 L:H is pretty necessary to ensure it doesn't look weird. You can get away with more height near the stock of the weapon than the barrel. Two reasons for this: looks, and front hand position cannot be too low unless the whole weapon has a "heavy" feel to it, which is not suitable for a coilgun that isn't shooting very large projectiles. It feels "off" and I've heard others comment on it before.


I will edit in some images of what I mean tomorrow. It's just about maximizing volume and keeping the "feel" correct.

Width: near the back of the gun, 2.3 inch width is about the max of what you want, but it's more visual. The gun can taper slightly towards the middle get a bit more volume as long as the top of the gun is more narrow. The 2" size is a good one to go with for the whole top, but you can get more width by increasing towards the middle and then tapering back down. Most handguns are only 1" wide, and rifles stocks are about the same, but that increases to ~3" in the middle as well.

You should definitely see if you can get on to fire a rifle or see one in action. I would bet that your police department might let someone watch. When I was 10 my dad asked the local SWAT team if we could join them for training and it was one of the best experiences I've had, and they were friendly about it the whole time (which we kelpt short of course)
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