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Modeling of coilgun dynamics

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WaveRider

Fri Jun 23 2006, 10:07AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Just finalised my latest paper on modeling coilgun dynamics.

It describes my finite element method where losses and material saturation are taken into account. As well as reluctance coilguns, it will also model Thompson (induction) coilguns quite easily...

If there is enough interest, I can make the finite element software that was used to generate the results for this paper available on my mass accelerators site. This will also probably give me an excuse for improving the code as well as including more geometry examples (disc launchers, etc..)

Simon

Fri Jun 23 2006, 11:21PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Great! I'll be giving it a read.

Are there any results you can summarise for the practical builders here?

WaveRider

Sun Jun 25 2006, 01:35PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

wrote ...
Are there any results you can summarise for the practical builders here?

Hi Simon..
I'm putting together some case studies which illustrate some of the results that might interest practical builders. This takes a bit of time, but I hope to have a draft out later this summer...

Do you have any particular questions that you'd like me to address?

Yohan

Wed Jun 28 2006, 02:51PM

Registered Member #194 Joined: Fri Feb 17 2006, 07:52PM
Location:
Posts: 19

Hey Wave, long time...

This might not be a question for the practical builder but, have you given any attention to a constant voltage source feeding an R,L circuit instead of just the R,L,C?

mike0t4ever

Thu Jun 29 2006, 04:47AM

Registered Member #321 Joined: Wed Mar 15 2006, 03:33AM
Location:
Posts: 14

just let C get very large (approximately infinity) and you get a const V source

in a small capacitance gun the capacitor is fully discharged (assuming ideal capacitor) and the inductance of the coil maintains a decaying current (R/L=time constant)

with a const voltage source (or an infinite capacitor) the current rises asymptotically to V/R (also with a R/L time constant)
with a projectile this is lower since the V= source voltage - velocity induced voltage

where velocity induced voltage depends on the speed of the projectile, its position, and the current
also if the velocity induced voltage is higher than the source voltage than the coil is outputing energy by slowwing the projectile

WaveRider

Thu Jun 29 2006, 08:29AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Indeed, I just make the capacitor very large (3e5-1e6 uF). The current rises asymptotically, as you indicated, to approx Vc / R. During the firing, current will drop to V/(R+Rd).. (Rd is the dynamic resistance that arises as a result of projectile motion.) If you think in terms of "velocity induced voltage", projectile induced EMF opposes the applied voltage when entering the coil and adds to when exiting....the reverse of the situation you describe...

mike0t4ever

Sat Jul 01 2006, 03:14AM

Registered Member #321 Joined: Wed Mar 15 2006, 03:33AM
Location:
Posts: 14

yeah that's right L' is +ive when entering and -ive when exiting (slipped my mind)

WaveRider

Mon Jul 03 2006, 11:42AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

In the old forum, I posted some movies of the magnetic flux behaviour during a firing cycle. I have repeated some of the runs to look at the effects of varying sheath material (or the absence of a field concentrating sheath). More specific details on the firing currents, velocities will be given on my website...
However, these are some of the early results.

Open coil, steel armature (Bsat=2.0T) no armature loss (sigma = 0)

This is a simple lossless armature accelerated by a an open coil. Flux lines penetrate the armature immediately at current switch-on (since the armature is not conductive). The upper rectangle represents the coil windings and the lower (moving) one is the armature.

Open coil, steel armature (Bsat=2.0T), conductive (sigma=4.0e4 S/cm)

Notice that the flux lines take time to penetrate into the projectile. This lowers the efficiency and the final velocity a bit. There is also a noticeable "flux-drag" effect at the end of the firing cycle.

Coil enclosed in lossless ferrite (Bsat=0.5T), lossless armature

1151924575 29 FT11903 Ferritesheathnoloss

By enclosing the coil in a ferrite sheath, acceleration performance can be enhanced. The upper "inverted U" polygon is the sheath. The coil windings are located in the rectangle nestled in the inverted "U" of the sheath.

Coil enclosed in lossless ferrite (Bsat=0.5T), lossy steel armature (sigma=4.0e4S/cm)

1151926110 29 FT11903 Ferritesheathwloss

Like the previous example...field concentration by means of the ferrite sheath. Efficiency is reduced somewhat because of the armature losses. There is a delay in force at beginning of firing cycle because of time-delay in field penetration into the armature. There is also a drag effect at the end of the firing cycle as a result of residual field and armature motion.

Thin lossless steel sheath with lossless steel armature (Bsat = 2.0T)

Here we model the effect of a thin (2.5mm thick) steel sheath. Note the escaping field lines (because of saturation of the sheath). This is what might be expected for a coil enclosed in a sheath of packed steel rods (electrically insulated from one another).

Lossy steel sheath and armature (Bsat=2.0T, sigma = 4.0e4S/cm)

This final example coil is enclosed in a lossy 2.5mm thick steel jacket (much like a steel pipe). The armature is also lossy... Notice the finite time it takes for the flux to penetrate the shield and armature. A "flux drag" effect can be seen toward the end of the firing cycle.

EDIT 20060704
Added more detailed results to my Mass Accelerators web page. I focused on the two aspects:

1. enclosing the coil in a flux-enhancing shield,
2. and an initial attempt to quantify the effects of armature eddy currents on coilgun performance.

WaveRider

Wed Jul 12 2006, 10:56AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

An example of a "short" coilgun stage...

Many launchers in the literature seem to have stages that are wider than thay are long... Thought I would model one here!

WaveRider

Mon Aug 28 2006, 12:30PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Hi all.. Hope everyone had a great summer break!

I didn't have much time for projects this summer, but I did manage to run some simulations to study the effects of ferromagnetic shields and losses on reluctance coilgun performance... I put together a brief report on the subject found here. Or, just go to my mass accelerators page and look for the document titled â€œNotes on the effects of metallic coil sheaths and armature losses in reluctance coilgunsâ€. Hopefully it offers some useful insight for practical designs...

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