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DerAlbi

Sat Oct 03 2015, 10:10AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Depends ons what "well" means. In a coilgun its ususally equivalent to good efficiency. Since he uses Batteries, he does not care about that. I am with Vandersen here.
I optimized my coil switchting to maximize the figure of merit "Ekin*Eff^2".
A Battery design does not need that. He can purely optimize Ekin.

BigBad

Sat Oct 03 2015, 03:13PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Well, long coils give poorer thrust, short, pancake style coils give higher resistance and are heavier (unless you turn several on at once in parallel).

Coils where the length is equal to the winding thickness which is equal to the aperture size seem to do the best.

Yandersen

Mon Oct 05 2015, 01:56AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Short coils (length==winding_thickness) have the highest L/R ratio, therefore have lowest possible heat dissipation per energy stored. This makes short coils ideal for recuperational designs, which is not the case.

To maximize the pull force per energy stored, one should tend to minimize the OD to squeeze all of the produced flux as close to the projectile as possible - the further a turn from projectile the less it pulls (smaller portion of it's flux is inside the barrel) and the more this turn will dissipate (longer wire).

First, Ben, you should determine the OD of your coils. Use FEMM. Make a coil with one turn and constant current (as much as your battery able to supply) and gradually increase the radius of that turn, writing down the pull force and wire resistance. From those two graphs you will be able to tell how far from the barrel the external layer of a coil can be to make a reasonable ratio of pull force per wire resistance (the wider the turn the higher it's resistance and lower the pull force).

The length of your coils should be at least equal to ID. If you can make a lot of such small coils - it is good. To choose the right wire thickness, set the constant current and gradually increase turns number: see when the projectile starts to saturate (2.2T at the tip of the projectile). Then calculate which wire thickness you need to make the required number of turns fit into the chosen coil geometry.

DerAlbi

Mon Oct 05 2015, 07:57PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

I am with Yan at this one. Tighter turns have more coupling, so optimizing L/R is not the right thing in a system thats not efficiency bound.

However for me it begs the question:
if the short fat (lets call it Albi

) coil is most efficient, does it means that the tranfered power is relatively large?
I mean.. what is efficiency? Is it lowest loss at low energy transer? Or is it highes energy transfer while having high loss? It both represents best efficiency.
Or in other words again: how can a coil be efficient if it does not transfer the maximum of energy?

Yandersen

Tue Oct 06 2015, 06:42AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Hm, the term "coil efficiency" may be linked to a ratio of Pull_force/Heat_dissipation, or
n=F/P
Few thoughts:
1) Up to a saturation point we have force and heat dissipation proportional to the square of the current: F~I^2. After the saturation force tend to be linearly proportional to the current. Conclusion: coil's efficiency does not depend on current before the saturation, but drops after.
2) Coil's resistance linearly depends on coil's length, therefore heat dissipation does too. But force doesn't - increasing the coil's length adds turns which are further and further away from the projectile resulting in lower pull force they produce. Conclusion: the longer the coil the less it is efficient.
3) This one is obvious - the faster the projectile goes, the more efficient coil is:
Apull=F*v,
Aheat=P*t,
therefore the faster the projectile passes the coil, the less heat will be dissipated.
Conclusion: shorter projectiles are accelerated faster, therefore lead to a higher efficiency.

ben5017

Thu Apr 28 2016, 01:09PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

I decided to take what i learned from the v3 and go for a compltet redesign i am calling it the V3 MARK2
Not sure whether to start a dedicated new thread or just keep rolling with this one.

I Took the same design as the V3, Which had 4+1 Stages and 18J KE
, and designed the Mark 2 with 12 stages, and a new goal of 40J KE (which would be a nice even 50m/s)

Here is a quick sneak peak of my progress (updates and edits to follow)

Pics of the molding process i used to make the iron matrix sensor housings

After the epoxy hardened, i cut out the inner diameter and outter diameter then cut the bottoms flush with a jig attached to my router

Finished: Iron Matrix sensors

I scrapped the servo/encoder setup used in the V3 and replaced with a timing belt and stepper motor.
Shown below ate the drum mag bearing and belt tensioner setup

Next, I 3d printed the mounting system for the pistol grip

1461848657 3315 FT147269 Grip Attachment

1461848657 3315 FT147269 Grip Attachment 2

Next up was to make a coil winder:

TEST COILS FOR TUNING

SPACE FOR CIRCUITS:

1461848918 3315 FT147269 Electronics Space

I will try to post some better pictures tonight when I get home. These were just the ones i had on my phone at the moment.

DerAlbi

Fri Apr 29 2016, 02:43AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

!amazed Wohooooooooooooo

That mechanical design!

That awesomeness.

That tooling!

Is that a stepper motor and revolver reload mechanism?
Any idea how heavy that will become? I guess its around 8kg in the end?
What dimensions?
A 32g projectile that slow with so much energy... how will you catch the projectiles while test shooting?

What worries me:
a) Your turn count. You counter did count negative. Now the gun will shoot backwards

b) where are the switches?
c) i dont know what a Iron Matrix sensor is

d) with increasing speed, have you considered supporting your poor battery with some capacitors? If you have an oscilloscope, i would loooove to see the voltage of the pack during a shot. (any thoughts on how much current the last stage will have?)

Karmaslap

Fri Apr 29 2016, 04:29AM

Registered Member #58215 Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65

I'm also quite curious what the iron matrix sensors are. Are they just iron end caps with optical senslrs embedded in them?

I read this thread in high school (2012) when you started and it's one of the threads thst got me interested in making my own gun, so I am looking forward to more updates! Beautiful mechanical work

ben5017

Fri Apr 29 2016, 12:33PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

CAD model used to design the V3 Mark 2

1461932093 3315 FT147269 V3 Mark 2 Iso 1

1461932093 3315 FT147269 V3 Mark 2 Iso 2

Some overall Photos, It all fit together perfectly

Albi: 8-10 Kg is probly a good ballpark range, depending on final coil sizes.
It is comfortable to hold and handel with a good solid pistol grip and front grip. Being a Bulpup design, the overal weight distribution is pretty even along the length, it is not awkward at all to handle so that makes weight a little more manageable. I may add a strap in the future, we will see once she is nearer to completion

-Drum is driven by a stepper motor and timing belt.

-overall length is 32" (will update dims when i get home)

-Slug catching setup is shown on previous page.

-Iron matrix sensor housings are simply iron powder suspended in epoxy (1/2" thick). I saw the name somewhere and liked it so i stole it :) i am Not sure if they will be worth the effort it took to make them though... (on barrys coil gun site he showed promising results using the iron matrix end caps on his low voltage coilgun albeit on a less powerfull setup., only one way to find out.....)

-Thanks for reminding me about the turn counter being backwards.... It never even crossed my mind that the rotation would be reversed when driving it off of 2 wheels like i did. I was pissed first time i used it. Idiot.

ben5017

Tue Feb 07 2017, 05:30PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Bus bars made and installed.

Lots more to do, updates to come.

MAGA

1486488603 3315 FT147269 Thumbnail Img 008

1486488603 3315 FT147269 Thumbnail Img 0081

1486488603 3315 FT147269 Thumbnail Img 0082

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