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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Simple mechanical coilgun model

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TheMerovingian
Fri Feb 10 2006, 10:04PM Print
TheMerovingian Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Extract from the old board:
yeah, now i understand.

I need to know if this calculus that i do in my simulator is somewhat correct:

The behauviour of the LCR circuit was described before but i added the variable inductance and a backEMF generated from the moving projectile.

Now i have I on time and compute B of the lone solenoid (solenoid shell with
finite lenght) at the center of the projectile. Then i calculate the magnetization with my model (But hyperbolic tangent gives almost the same results). I calculate the dipole multiplying by the projectile volume.

knowing the dipole moving in a mag field i obtain the magnetic potential energy and taking the derivative along the moving dyrection of the projectile i get force.

I have problems in calculating inductance so for now i keep it constant

It seems to give quick'n'dirty results but the behauviour is essentually correct.

I cannot attach images to show you the GUI.


i have done some simulations for my halfbridge coilgun
(three stages, each calculated separately with the speed of the previous one)

obtaining
a speed of 46,4m/s (6g)
a muzzle energy 6,45 (with a total energy consumption of 78J)
total efficiency 8.2%

the turn off time is when the projectile releases the photogate (-6mm from coil). The suck back is present and can be eliminated controlling the pulsetime (i have a strand of 555 controlling the pulses)

The results does'n look weird for such a simple model. maybe using your reluctance model and adding eddy losses would boost the accuracy
1139609067 14 FT0 Cg Simu
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Psyko
Fri Feb 10 2006, 11:21PM
Psyko Registered Member #81 Joined: Thu Feb 09 2006, 11:57AM
Location: Paris, France
Posts: 43
what is this software, Mero ? Seems to be really useful. Is there a place where we can download it ?
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TheMerovingian
Fri Feb 10 2006, 11:38PM
TheMerovingian Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
The software is still incomplete. I will release a beta when i am sure it is reliable
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WaveRider
Mon Feb 13 2006, 12:56PM
WaveRider Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Looks reasonable, Mero. Send me your simulation parameters and I'll run a camparison with my simple model...

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TheMerovingian
Mon Feb 13 2006, 05:13PM
TheMerovingian Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Run a simulation with these parameters:
coil turns 320
coil lenght 30 mm
coil internal diameter 8 mm
coil external diameter 18.4 mm
no ferromagnetic shield around coil

coil inductance :388uH
coil resistance :657mOhm
Capacitance: 1000uF
initial voltage : 300V
timestep: 0.1us

topology: free running scr + diode (no quench resistor apart the coil itself)
switch voltage drop 1.4V diode voltage drop 800mV

projectile diameter: 5.5mm
projectile lenght: 30mm
projectile weight: 6 grams
saturation magnetization: 2.1 tesla
initial permeability : mu_r 5000
initial position : -30 mm (measured from center of coil to center of projectile)
initial velocity : 0 m/s

Results:
18,78 m/s
1,05 J kinetic energy

(peak velocity 21,58 m/s, suck back present)

peak force 89,3N

Greetings for your promotion cheesey

Cheers tongue



1139851592 14 FT346 Cg Simu2
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TheMerovingian
Wed Feb 15 2006, 12:30AM
TheMerovingian Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I added static friction to the model calculating it for my barrel (measuring the angle at which che projectile falls off) but it doesn't really make any difference in results. Maybe can bu used for rifled barrels since determining the coefficient is very easy.

Waverider I'm awaiting for your simulation to compare the results
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WaveRider
Wed Feb 15 2006, 08:09AM
WaveRider Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Hi Mero... Been burdened with lots of stuff to do lately. I'll try to do it today!!

Cheers..
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TheMerovingian
Wed Feb 15 2006, 01:58PM
TheMerovingian Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I have compiled your program on windows but have some problems in translating the data since your simulator asks me for gap lenght and i am forced to write the difference in diameter between the projectile and the coil inner radios (my simulator does'nt threat shields cry ) . I recalculate Bsat with your advices in the readme. There is a little suckback but less than predicted in my simulator. Also the current predicted in your simulator is bigger (faster discharge, this explains why less suckback), i calculated the inductance using wheeler formula and the variations on ind0 with a reverse formula on the force.

Anywais your simulator says 17.5 m/s that's pretty close to my prediction but i don't really know if i calculated this correctly since there is the shield.
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WaveRider
Thu Feb 16 2006, 09:31AM
WaveRider Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Hi Mero.

Bravo! Glad to hear that you got my code to work on your computer..

I ran a simulation with these values (based on what you gave me)
# Coil Length (m) = 3.000000e-02
# Projectile length (m) = 3.000000e-02
# Effective gap length (m) = 2.500000e-03
# Effective core radius (m) = 4.000000e-03
# Firing capacitance (F) = 1.000000e-03
# Number of turns in coil = 3.2e+02
# Low-field core relative permeability = 5.000000e+03
# Saturation flux density of core (Wb/m^2) = 1.000000e+00
# Saturation magnetisation of core (A/m) = 7.957747e+05
# Coil loss resistance (Ohms) = 6.570000e-01
# Mass of armature (kg) = 6.000000e-03
# Simple RLC (SCR-like) drive.
# Initial capacitor voltage = 3.000000e+02
# Initial coil current (A) = 0.000000e+00
# Initial armature speed (m/sec) = 0.000000e+00
# Initial armature position (m) = 0.000000e+00
# Time step (sec) = 1.000000e-06
# Print out every 10 time steps.
# Tolerance for the non-linear solution (Newton-Raphson) = 1.000000e-08


I used a slightly larger time step, but convergence was still very good. Here are the results:

# Maximum force on projectile (N) = 1.124648e+02
# Minimum force on projectile (N) = -4.859833e-01
# Average force on projectile (N) = 2.552838e+01
# Maximum projectile speed (m/s) = 1.758019e+01
# Projectile exit speed (m/s) = 1.751668e+01
# Minimum projectile speed (m/s) = 5.775584e-07
# Maximum current magnitude (A)= 3.026064e+02
# Maximum capacitor voltage (V)= 2.999992e+02
# Minimum capacitor voltage (V)= -7.553270e+00
# Maximum core flux density magnitude (T)= 3.504001e+00
# Time at which pulse is switched off (sec) = 3.349520e-317
# CPU seconds used in computation. = 0.110000


As you can see, my results came out very similar to yours. A couple of comments are in order:


  • 1. With open coils, you can play games with the gap length to simulate the effects of the open coil. This is a bit of a fudge, though. This model specifically counts on the confinement of the field by the shield.

    2. The model breaks down for thick, multilayer windings (like yours). Errors arise because the radially directed magnetic field becomes significant along the length of the core. EDIT: Furthermore: many of the flux lines do not couple into the armature /EDIT. The model assumes current is concentrated in a shell surrounding the core. The closer we come to this ideal, the better it works.

    3. The specific solution will, of course, depend strongly on the magnetisation model. If you used a different saturation model, the results will certainly be different.

    4. It's bloody difficult to get exactly repeatable experimental results, since even slight misplacements of the armature can vary results considerably!


Great work, Mero.. Sorry about the delay...

Cheers.
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TheMerovingian
Thu Feb 16 2006, 10:48AM
TheMerovingian Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Bravo! Glad to hear that you got my code to work on your computer..
I simply compiled your sources imported in the Opensource Watcom c++/fortran compiler and gave it a go.




It would be cool to do a graphical used interface for your program. Since they are complementary (mine no shiedls, your with shields) it would be cool to see the waveforms in real time. Translating it to basic would be problematic for me dead . Maybe doing some c++ dlls and calling it from basic would work.


Great work, Mero.. Sorry about the delay...

Cheers.
No problem Bill, I know that you are busy cheesey .


Cheers..
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