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Simple mechanical coilgun model

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LAN_403

WaveRider

Thu Feb 16 2006, 12:50PM

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

Just to let you know that I plugged your geometry into my FEM based code and it's chugging away as I write. In a few hours, I'll have some results.. At this point in the simulation (with the FEM), the current-rise is much slower than the simple model, indicating significantly higher inductance as a result of the thick winding layer..

Here is a pic of the field lines early in the sim..

The armature is the thin rectangle at the bottom left. The coil is the fat block in the middle and the top rectangle is an external shield (whose permeability is set to 1.0...that of free space...it really isn't there).
With "fat" open coils and thin projectiles, many of the field lines don't seem to interact with the projectile..

TheMerovingian

Thu Feb 16 2006, 01:13PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

Heres the calculations i have done . Used the polinomial magnetization model
]1140095622_14_FT363_coilgun_force_expressio.pdf[/ file]

The inductance i calculated was derived from the wheeler formula, while the variations on the inductance are calculated in a semi-empirical way.

I had a slower current rise as you can se from the graph, that's why there is more suckback

rise rate = 5,55E05

What fem program is that?

WaveRider

Thu Feb 16 2006, 02:23PM

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

What fem program is that?

It's Waverider's Magic Graphical Coilgun FEM simulator.. I like to call it "BigShot1.0"

TheMerovingian

Thu Feb 16 2006, 02:45PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

any chances to port it on windows :D ?

WaveRider

Thu Feb 16 2006, 02:59PM

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

If you are volunteering.. I guess the chances would be good!

TheMerovingian

Thu Feb 16 2006, 03:14PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I feel an irresistible need to install Linux on my pc

Seriously this program needs a big and complex graphical interface that are way too much for my weak programming skills.

You should put an announcement to search volunteers for windows porting.

Cheers..

WaveRider

Fri Feb 17 2006, 09:55AM

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

Mero, went through your force calculations.. I guess you are aware that B depends not only on the current, but also the presence of the magnetic material in the coil. Your expression for B does not include the effects of the core. This follows through in your energy discussion too. The energy of the magnetic field depends not only on the core dipole moment, but also on the field that surrounds the core... That's why the problem is not so easy...it's all coupled together!

Anyway, I had to shut my computer off last night, so the simulation did not run to completion, but it made it most of the way there! Here's a gif movie of the projectile

It's kinda interesting at the beginning..You can clearly see the effect of the armature saturating early (watch the field lines on the left being drawn in to the center early on in the simulation).

Here's the circuit dynamics:

There is a little suckback at the end and the armature exit velocity of 19.2m/s (max velocity of 20.7m/s). The timing of the current peak indicates an empty-coil inductance of about 410uH...not to far from your estimation of 388uH... Peak force was calculated to be 72N.

TheMerovingian

Fri Feb 17 2006, 10:44AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I know that it's not easy but i needed a simple/fast model helping me design/understand coilguns. I know that your FEM simulator is the closest to the reality and obtaining less than 20% error on final velocity makes me very happy. Also your model does consider eddy currents and residue magnetization so the results are very precise.

The projectile saturates early? This is because the current rises rapidly since the inductance is close to the coil inductance alone so the field establishes rapidly.

Your simulator is beautiful Bill, when you will release a commercial version?

WaveRider

Fri Feb 17 2006, 06:06PM

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

Your simulator is beautiful Bill, when you will release a commercial version?

Thanks! Commercial version.... I don't think there would be enough buyers out there to warrant the hard work.. I may release the research code under GPL after I finish my paper and fix a few nagging bugs..if there is enough interest..

TheMerovingian

Fri Feb 17 2006, 10:47PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I'll be happy to use your software for simulations. May need to find a linux emulator or install a linux distribution to run it.

Just for curiosity i forced your inductance value in my simulator (this feature is because it is impossible to wind a perfectly packed coil, so after building the coil i measure inductance and resistance with an RCL multimeter).
The new simulations show that:
Ipeak = 247 A (difference +3% respect to Bigshot)
Peak force = 89N (difference +23% respect to Bigshot)
Peak speed = 21,78 (difference +5% respect to Bigshot)
Exit speed = 18,78 (difference -2% respect to Bigshot)

Simulation parameters except inductance are the same.

] 1140216421_14_FT363_merossimulationforcespeed.pdf[ /file]
] 1140216421_14_FT363_merossimulationcorrentvoltage. pdf[/file]

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