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

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Author Post
Wed Dec 14 2016, 01:53PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
Its about 1kg of plastic equal to 50€ worth of filament.
Yes i am using copper for the prototype because aluminium is extremely expensive because it would be custom made with minimum 10kg per wire diameter which is quite a lot of wire considering its aluminium. For the prototype its not an issue. It will be uncomfortable heavy but it will work.
Square aluminium is also only available with >=0.5mm "diameter". My first coils would need 0.25mm so this prototype now also explores how i can adjust to thicker wire through PWM switching in the first few stages and what it does to the projectile and efficiency.

The prototype in the beginning exploded at stage 16 due to mechanical failure of a diode in the half bridge. Look at the beginning of the thread for photos of exploded parts, It was the end of the big prototype. Since then i did never shoot again.

Building the Coil-Winder now adds a bit delay to the project but being able to wind the coils is important and it also has the benefit that i am developing software for the microcontroller architecture that i am going to use in the end.

Meanwhile i have implemented the stepper motor drive and acceleration profiles and i am confident that this coil winding machine can do up to 3 turns per second. Its quite fast compared to hand winding however its quite slow considering its a machine.
The problem is the stepper motors loosing torque as they become faster. The fact that i added a 5:1 gear is therefore a problem, however i need it to ensure i have sufficient torque. As consequence i would need a way bigger stepper motor for direct drive of the winder, which means more cost and also much more difficulties driving the motor since there is no payable of-the-shelf solution anymore.
Anyway. first experience with a coil winder. prototype. good enough.
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Mon Jan 09 2017, 07:27PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
First very primitive test inductor.

Its just a single layer of 0.2mm wire to see how precise the steppers and the wire tension algorithm handle the job.
Its denser than hand wound for sure. Not by much however.
Wound with 2 turns per second. 90% of the turn is completed until the horizontal correction is done. i will have to learn about the different behaviors. i guess with such fine wire this is ok, with thicker wire maybe not. But its in no way a limitation of the machine, its just the way i programmed it to do the job. i can also go gradually side-wards of course.
The start and the end of the inductor is not messed up. i programmed it that way to start and finish with a few turns of larger wire distance.
Now i need a desktop program to make more complex movements.
The white between some turns is not bad wire distance.. its resin that comes through.. light does the rest. (i wanted to do a finished coil just for fun.. therefore i glued it in place directly - taunting few minutes. )
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Sat Feb 18 2017, 09:20AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
So its time for another update again. There was not much progress made to be honest because i am catching up on tooling here. In a sense this much is progress but in terms of the actual gun coming to life.. hmmh. As i posted above i am still building the coil winding machine. The PC-Control program is basically done and ready to use and what is still missing is a good coil former and actual knowledge how to wind a coil. Layer-changes are a bitch.
For the coil former i actually bought a CNC milling machine off Ebay (CNC3020T with 800W Spindle) which was a lot of work to get operational and of course its quite time consuming to get into all the software associated with it - tool path generation and so on.
However with this now i am quite capable of doing a hole lot of stuff in combination with the 3D-Printer. I honestly feel amazing right now. I feel the difference in quality i am able to produce if i can make the proper tools for the job. 3D-Printing the coil former was not an option btw, because the layered (surface-)structure of the print disturbs the coil turns. The only solution is to sand the surface which in turn will produce dimensional errors. So milling is indeed the right way to do it.
Here is a side wall of a milled coil former:

I wont go deeper in to its features because this part is not finished yet an the next iteration of the design is milling right now... i mill in POM plastic which is good for not adhering to epoxy.. the surface is really smooth but the material is a bitch to work with. Its too ductile and therefore hard to cut cleanly. The tool marks you see are purely optical.

As for the whole coil winder side project i want to share my doubts with you. I catch my self loosing my focus to such side project which now results in the whole CNC stuff and so on. I get side tracked a lot which does not help the project at all. Its sometime really depressing how much work is ahead of me when i realize how much of the work i dont even realize. Its always like a factor of pi more.
My problem with the whole project is that i really want to achieve something new and like Barry said - "next gen". Its impossible to invent stuff in this field because everything thats viable has been done.. its only possible to improve the current state of the art by just doing everything the absolutely right way or at least as good as i can do.
This results for example in a more complex coil shape which in turn takes time to perfect. I try to keep telling myself that all for a greater good, so when my prototype is finished i have something to show - and not only that.. i also extent my skill set so if the future ever brings me to a point where i will beg for money to make the gun bigger people can trust me. Yet i realize that conveying this trust is impossible and cheap marketing bullshit would be way more effective.
I also do not know of any alternative to how to make my desired coil shape except having everything custom made.

Anyways. I am not far from making my first coil fully automatically and in a repeatable way. I will share the results with you. Time will tell if the machine could improve the L/R-time constant of my coils which is currently 1.7ms. Actually i think it wont change much.
On the one side my coil geometry becomes worse because the inner layers have less turns to accommodate an air cavity inside the coil (this is required by my projectile sensor system) which reduces power density. On the other hand i really optimized the inner shape of the coil to reduce the air gap and to reduce overall wire length by a few percent.
Additionally the overall design (i hope) will experience an energy density increase of >10% because the coils are only separated by 1.0mm thick PCBs instead of 4mm wide spacers which hosted those unreliable light traps.

Edit: i have HUGE problems with my router:

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Thu Mar 30 2017, 07:12PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
I give up.
Not with the project but with coil winding. I manage to get good results for the first few layers, but my complicated coil shape is a bitch. Its not worth the trouble and the hunt for the perfect coil is wasting my time - i think my wire is not reliable enough (tolerance in diamter..). I have decided to continue to work on the electronic side of things and when the PCBs are ready i will wind coils again and i will live with the ulgy results.

More details: the coils we want as coilgunners are called "orthocyclic. It is well known that the first layer of such coils is the most problematic one and if there is the slightest error there it will amplify in later layers. Also the layer change on the side of the coil has a certain behavior that is sometimes hard to deal with. In addition my coil former is not perfectly round - it has rather sharp corners.. those corners make things worse in terms of orthocyclic winding. Turn crossing that usually moves a bit jumps there instead leaving a gap that will disturb later layers.
This is a systematic problem so i will never get pleasing results.
Now i have to think about what "pleasing results" are actually are: i have wound a lot of coils for my bigger prototype that started the thread. OF course there were good ones and bad ones, but in terms of electrical properties the ugly kids never performed any worse. So there is a lot of aesthetics involved thats more important to a human as to the actual electronics. As is it often the case.
Also.. is it worth it? When i think about the future of the project i see square aluminum wire as the way to go. This wire will have totally different properties. It wont have the grooves to auto-guide it self and normal helical winding should be possible. I think the worst that can happen is that a layer change twists the wire for 90°.. and i actually dont know if that would be so bad. So getting everything right now is not indicative for the future of the accelerator i think i should care less. The fact that i do not use square aluminium wire right now is purely cost related. I cant get it for cheap and i cant get it in small quantities.
However i learned a lot.
- I made a coil winding machine ( i wouldnt build it again that way which is progress enough.
- used stepper motors for the first time[/list]
- I used STM32 for the first time in a custom project which gave me the chance to get a good hardware abstraction layer in software (this is also the microcontroller of choice in my final design)
- I also used QT as GUI for the PC
- I also finally got my milling machine/router (i never know when to use those terms) running and i changed my CAD from SolidWorks to the free Fusion360 which also increased the broadness of my abilities[/list
All in all it was a huge gain not only for my work shop but also for me, but i have to move on.
I do know now how my coils will look like, which width they will be and at least i know that it can be done.

So back to electronics. I cant really go into details because i wasnt in touch with the whole circuit and PCB design stuff however i know for certain a lot of my electronics is driven by the desire for quality yet still designing on edge. The quality part of this sometimes causes decision that arent cheap or easy to implement. I think this has to change at least for the electronics for the acceleration stages.
It may very well become another fail, however its worth exploring if caring about every parasitic effect is actually worth it or if i can get away cheap. i just have a lot ideas in my head.
Every coil has a SCR in series.. this SCR has to be turned on. But how to do that in a asym half bridge with all the voltage swings... the clean way is a gate drive transformer for every SCR. But this sucks. Lets see what i come up with

Again: sorry for all who wanted to see a complete coil wound... i can still make a video if you like.. its not like its not working but the tolerances mess things up pretty quickly.
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Sat Apr 01 2017, 07:20PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
Ok, last post left you guys hanging... not nice. I got the SCR-Fireing visualized and i am.. idk.. i think it might work.
Have a look

The usual way to fire the SCR in that situation is to use a small transformer. In this way the control electronics can be completely independent of the violent things that goes on in the half bidge. In reality the ground of the control circuit is connected to the ground of the half bridge, however this ground connection is far from reliable. The problem with this configuration is the transformer involved. its just expensive.. and in a multi stage design you pay with the number of stages in this part of the circuit.

My alternative solution uses a PMOS to pull the SCR gate high. When the low-side IGBT is turned on the gate is basically ground referenced, thus the gate should fire. The issue here is again the bad ground connection. There is a lot of inductance in the wires and a lot of ringing involved when the SCRs turn off. So the "ground" of the SCR is by far in any way clean. This rises two concerns, one of which is actually really interesting:
First the "dont care"-issue: due to ringing or some interference the SCR could fire too late or refuse to fire for some time. i dont regard this as a big problem because its assured that the SCR will eventually fire. And the time scale is way too small to care... i mean even 5µs delay means 200kHz eigenfreiquency of the ground loop and i thing this should be way higher - even in a bigger gun.
The second issue is interference and the implied parasitic triggering of the gate. If an SCR turns on by accident its certain death to the IGBTs. I hope the capacitor and pull down resistor at the gate is enough to manage the problem.
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Mon Apr 03 2017, 01:12PM
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: Alfreton, UK
Posts: 407
I would go for the conventional circuit. There are so many things that could go wrong with mismatched ground references, apart from the severe consequences if anything goes wrong. It could save you a lot of head-scratching later on when things would otherwise be exploding unexpectedly.
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Mon Apr 03 2017, 07:54PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
Thats not the reassurement i need!
Still thanks for that - this forces me to further explain myself - because i thought about the situation a lot during schematic level design..
If I use some 60V PFET in the new design I actually have immunity up to -48V on the low side of the coil because only when the mosfet avalanches there can be a high enough current flow to accidentally trigger the gate.
Positive ground swings arent so bad either because the diode protects the circuit from current flow. All that can happen is some parasitic current flow which is easily compensated by the 22nF (its a capactitve voltage divider xx pF vs 22nF). The more i get into the circuit the more i like it.

Also the conventional circuit does have issues too. The problem is there in the first few stage where inductance is high - the current does not rise fast enough to reach the latching current during the very short time the SCR is fired by the 4.7uF pulse capacitor. The pulses last only some micro seconds there, so its also not the most reliable circuit - however its failure behavior is kind of better.

If you find problems in my logic please argue with me - this is important ^_^
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Thu Apr 13 2017, 08:51PM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
I was bored.
I opened up a coil - just to see how high the fill factor is. Here is the picture.

1) obvious: the conductor fill factor is quite high. I think its as high as it can get - meaning ~90% (i think the limit of orthocyclic winding is 91%).
2) a defect propagates though the layers. You can see a diagonal line at the left side. There was one turn just a liiittle out of place and it never got back to normal. Keep in mind that i wound the coil manually and hand-corrected every turn.
3) the second layer is on top of the first layer. Its the region where the wire goes from one gap to the next one.

The most devastating implication is the high fill-factor. previously i thought that i have quite a bad fill factor and thought that when switching to aluminium using a rectangular wire can make up for at least some of higher resistance. But it seems rectangular wire has no benefits when it comes to fill factor.. i guess its just easier to wind (which is also important).
I have to revisit my old thoughts on how efficiency develops with a worse conductor.. because aluminum will hit a lot harder than i guessed.
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Fri May 05 2017, 03:00AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
All PCBs are finished. 10 different PCBs, 500eur order.
I am currently redesigning the case. I have imported the outlines of the PCBs and the most critical connectors to the CAD-Tool (Fusion360) to see if everything lines up as planed.
The whole thing is full of stuff. Totally crowded, every volume used.. bigest clearance inside the case is like 5mm.

The accelerations path with its sensor-PCBs is not shown. Also the plastic components are hidden except the trigger section (transparent) to give a reference what you are looking at.
I also included some component information. (If anyone gives a f.)
I just realize that the USB-connector that is mentioned is not shown. Its on the other side; it protrudes behind the HB-Power supply, the connector is soldered on the bottom side. Sry.
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Mon May 15 2017, 06:36AM
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 517
Did i mention that i redesign the case? What a shitload of work. Also i decided to go with the flow and try to make something presentable - meaning adding funky outlines beyond of whats functionally important. My biggest issue here is that i am not really an artist. More an autist Hehe, sry.
This is also the week where i expect some PCBs to show up. As far as i am informed the first batch will be all the 70um PCBs which are the Cap-Bank and the Halfbridge. I hope the other PCBs will follow soon. Man... guys. You dont believe how pumped i am. This is the climax of 2 years of concept work. Yes, it has been 2 years since the last projectile was accelerated. Since then i worked hard, refined, tested, dismissed and accepted solutions... everything thats inside the gun has been evaluated - it should work. However nothing was ever assembled - it always was just concept work with a lack of real world context. Now everything comes together and i hope for the best fearing the worst. I know that some concepts never worked in my tests, but i learned from the failures and i was confident that this iteration will work. However confidence seems to be a volatile thing as the PCBs become reality. Hoooly fuck. I suffer from PTSD. Just this time it means Pre-traumatic-stress-syndrome.
Anyway. The fun part:

So i want go go for a 2 color scheme. Black is just neutral enough and does not become dirty or filthy from handling the plastics. I plan to use a green or orange mid-line which gives some kick to the design. I also opted for showing my balls - meaning the caps arent just encapsulated in a box, but they show through for all the sexiness like a really tight thong .
I am currently not sure if i will add this barrel stub thingy in the front. i am also working on giving the top side some structure, so the shape is more interesting to look at. However this is quite a challenge since modeling such stuff takes hours at a time and the motivation is low if you just dont know how the idea turns out. i currently dismissed 2 designs already.

I am pretty happy how the cap-section/sextion turned out... here is a 1:4 model i printed over night. Notice that the printer fucked up (because the model wasnt really made for printing this small) and it gave the thing some horizontal structure on its upper surface. I think this will be my next option for improving the looks.

And yes, a small show off. Small albi being proud, sry.

So not sure about the size. I think for the specs (20J, 40m/s) it ended up pretty bulky. On the other hand i will be able to empty my 6-proj-magazine in the time most youtube videos realize they should edit/cut/remove the cap charging process from the video while still being smaller and not much weaker. This form factor is not the best advertisement imho. But if you think about scaling up.... and when you realize that its just about adding stages and adding a few caps and keep in mind that a bigger gun will inherently have more space for electronics, i can imagine that this can be attractive.
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