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Some advices for my first coilgun

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LAN_403

Pinkamena

Wed Nov 30 2011, 07:29PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

Thank for your reply.
I have a question regarding the switching:

The way my switching circuit is now, an IGBT receives a signal when a beam of light is broken. When the beam is restored, the IGBT turns off. I would like to expand the signal, so I can decide exactly how long it will take until the IGBT is turned off. Unfortunately, a circuit like this is a bit out of my ballpark. I've tried many different approaches, but none works. How should I go ahead and make this work? If you have a schematic for such a "signal expander" or whatever you could call it, I'd like to have a look!

Saz43

Wed Nov 30 2011, 07:38PM

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

Pinkamena wrote ...

Thank for your reply.
I have a question regarding the switching:

The way my switching circuit is now, an IGBT receives a signal when a beam of light is broken. When the beam is restored, the IGBT turns off. I would like to expand the signal, so I can decide exactly how long it will take until the IGBT is turned off. Unfortunately, a circuit like this is a bit out of my ballpark. I've tried many different approaches, but none works. How should I go ahead and make this work? If you have a schematic for such a "signal expander" or whatever you could call it, I'd like to have a look!

Yes, the signal from the detector could trigger a simple NE555 in monostable mode which would then create a pulse with a duration of your choosing.

But the question is, why would you? Use another detector (or better, the same one) to turn it of when the projectile is in exactly the right position. That way you have closed loop control (exact time communicated), rather than open loop (pre-programmed guess).

Pinkamena

Wed Nov 30 2011, 07:48PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

I would do that, but the problem is that the projectile will be inside the coil when I want to turn it off. Kind of hard to get a beam of light to pass through a coil XD Or are you perhaps thinking of another approach?

Turkey9

Wed Nov 30 2011, 08:28PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

If the sensor is right at the start of the coil, and the projectile is the same length as the coil, then when it leaves the sensor and the IGBT turns off, it will be in exactly the center of the coil.

Pinkamena

Wed Nov 30 2011, 08:36PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

Turkey9 wrote ...

If the sensor is right at the start of the coil, and the projectile is the same length as the coil, then when it leaves the sensor and the IGBT turns off, it will be in exactly the center of the coil.

Yes, but I thought the coils needed to be a little longer (About 1.5 the length of the projectile).

Saz43

Wed Nov 30 2011, 08:42PM

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

That depends on who you ask. I'd say always make your coil the same length, Barry says 1.33, others say 1.5. DARPA's massively multistaged coil mortar had coils that were maybe only 10% of the length of the projectile (although they maye have had more than one coil on at a time, im not sure). Nothing is absolute here, but anything between 1 and 1.5 should be good for a hobby coilgun.

Also, research actually suggests you place the detector a few mm before the entrance of the coil.

Pinkamena

Sat Dec 03 2011, 03:41PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

Hi guys, just thought I'd show you what I've been doing lately.
I've made an interactive simulation in Wolfram Mathematica, showing the current as a function of time, coil thickness and air gap radius. In this simulation, time and air gap radius can be dynamically altered, to show how the current will flow in the coil. Resistance, inductance, and lenght (to always keep the projectile with a mass of 300 grams) is also calculated dynamically. The curve is "blocky" due to large increments of wire being added each time the coil thickness becomes large enough to add a new layer to the coil.

Here's an album with some screengrabs showing how current will flow in a coil using AWG18 (1.06 mm) with a resistance of 0.021 ohm/meter, and an inner coil diameter of 1.7 cm. (Notice the image that say "70ms". I forced the resistance down to 0.001 ohm to create some heavy ringing in the coil! Very fun.)

EDIT: I almost forgot, here's the code in case you have Mathematica and want to try it out yourself.

r0 := x
Slider[Dynamic[ri], {0.0056, maxri}]

diam := 0.00206 (*diameter wire*)
rw := 0.012 (*ohm/meter*)
convert := 39.37
Slider[Dynamic[time], {0, 0.1}]

d := 7870 (*mass per m3*)
v0 := 290 (*cap voltage*)
c := 92.8*10^-3 (*cap capacitance*)
w := 0.300 (*weight projectile*)
magconst := 1.25663706*10^-6 (*magnetic constant*)

L := 0.5 w/{d*{ri^2}*Pi} (*lenght*)
maxL := 0.2
minri := Sqrt[{0.5 w}/{d*Pi*maxL}]
maxri := 0.013
maxr0 := 0.07
minr0 := ri + diam
turnsPrLayer := L/diam
wireLength := turnsPrLayer*2 Pi*{Sum[{ri + i*diam}, {i, 0, maxLayers}]}
turns := maxLayers*turnsPrLayer
beta := L/2 ri
alpha := r0/ri
maxLayers := Floor[{r0 - ri}/diam]

(*Inductance formula*)
res := wireLength*rw (*Resistance ledning*)
induc := {{31.6*turns^2*ri^2}/{6 ri + 9 L + 10 {r0 - ri}}}*10^-6

(*Currentformula*)
betaI := Sqrt[{1/{induc*c}} - {res^2/{4*{induc^2}}}]
alphaI := res/{2*induc}
curr := {v0/{betaI*induc}}*Exp[-alphaI*time]*Sin[betaI*time]

(*F (a, b) formula*)
F := beta {ArcSinh[alpha/beta] - ArcSinh[1/beta]}

(*H0 equation*)
H0 := turns*curr*F/2*beta*ri*{alpha - 1}

(*Plot3D[curr, \
{x,0.012,0.1},{y,minri,0.01},PlotPoints->25,AxesLabel->{thickness,\
airgap},ViewPoint->Front, PlotRange->{-100,100}]*)
 "time in seconds:" Dynamic[time]
"air gap radius:" Dynamic[ri]
"Length of coil:" Dynamic[L]
"turns per layer:" Dynamic[turnsPrLayer]

Dynamic[Plot[curr, {x, minr0, maxr0}, PlotRange -> {-100, 1000}, 
  AxesLabel -> {coil thickness, current}, Filling -> Bottom]]
Dynamic[Plot[turns, {x, minr0, maxr0}, PlotRange -> {0, 5000}, 
  AxesLabel -> {coil thickness, wireturns}]]
Dynamic[Plot[maxLayers, {x, minr0, maxr0}, PlotRange -> {0, 30}, 
  AxesLabel -> {coil thickness, layers}]]
Dynamic[Plot[wireLength, {x, minr0, maxr0}, PlotRange -> {0, 1000}, 
  AxesLabel -> {coil thickness, wirelength}]]
Dynamic[Plot[res, {x, minr0, maxr0}, PlotRange -> {-2, 10}, 
  AxesLabel -> {coil thickness, resistance}, Filling -> Bottom]]
Dynamic[Plot[induc, {x, minr0, maxr0}, PlotRange -> {0, 0.04}, 
  AxesLabel -> {coil thickness, inductance (H)}]]

Turkey9

Sat Dec 03 2011, 07:41PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Nice. I don't know if mathematica can do this, but you should do a 3D plot with time on one of the axis. The peak current for each coil will be in different places as the turns vary so a 3D plot would be really interesting to see.

Pinkamena

Sun Dec 04 2011, 03:09PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

That can be done, but it is VERY time consuming to plot it. I made a 3D plot with coil x=outer radius, y=coil inner radius, and z=current, and it took over 8 seconds to calculate it every time I changed the time variable. It is much easier to look at the 2D plot with a fixed inner coil radius.

EDIT:
With an inner coil radius of 1 cm, and an outer coil radius of around 1.5 cm (still AWG18), I am getting a peak magnetic field strength of over 6000 kA/m (This is at 1 ms, @1800A. The magnetic field quickly drops 0, takes about 150ms). Does this seem very unlikely? I could also mention that at these coil specifications, the wire length is around 28 meters.

Jack A

Mon Jan 02 2012, 02:24AM

Registered Member #2975 Joined: Wed Jul 07 2010, 12:19AM
Location:
Posts: 28

Hi all! Just wondering, all of you have been talking about how the first stage is the least efficient, why is this? (time in coil perhaps?)

Cheers!!!

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