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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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63J Coilgun Completed!

Author Post
OwlFowl
Sun May 11 2008, 04:02AM Print View
Registered Member #1441
Joined: Sun Apr 13 2008, 03:40PM
Location:
Posts: 10
63J CG

Thanks to help from this forum, I've managed to finish my cg project ahead of time!
I've completed this on 5/8/08 and have been testing with it ever since.

Here's a short vid:
63J Coilgun

Here are the specs:

barrel: 7od, 6id
voltage: 400v (I'm using 2 camera charge circuits so they only charge to 300V according to the voltmeter)
capacitance: 1400uf (3x470uf caps)
coil: 12layer 40mm length 18awg (i followed a person's advice on using 18awg)
projectile: 22mm 6mm in d
switching method: SCR + diode

*edit*
velocity: 22m/s
efficiency: 1.9%


Velocity and efficiency are still unknown because I am having trouble finding the velocity using ballistic equations. I've fired it 2.5cm level from the ground straight across the room (about 3.2m), the projectile never touched the ground and yet it's final height is sometimes higher than the initial height. I've done this multiple times and I got ranges +/- 5cm!

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uzzors2k
Sun May 11 2008, 07:36AM
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Asker, Norway
Posts: 1308
Nice coil gun!

There are three common ways of measuring speed. One is to make a PIC speed-trap, but that's the most elaborate.

If you have a microphone, a PC with audio editing software and two sheets of paper there's an easier way. Basically set the two sheets up at a known distance, put the microphone just as far from each sheet and shoot through the paper while recording. When you look at the audio signal later there will be two spikes when each sheet was penetrated. The time between these spikes will be the time the projectile used to cover the known distance. V = S / T This is most accurate with higher energy projectiles, because some kinetic energy is lost to penetrate each sheet. Not much, but with low energy coil guns enough to seriously slow the projectile down.

The final and simplest method is fairly accurate and only requires a stop-watch. You point the coil-gun straight up and shoot, while measuring the time between firing and when the projectile hits the ground. With some basic high school physics one finds that the initial speed is given by: V = 9.81 * (T / 2)

I used all three methods and there were no big gaps between the measured speeds.
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flannelhead
Sun May 11 2008, 08:35AM
Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
That's a nicely finished one!

I believe it's kinda pain to wait for the camera circuits to charge 112J... If you want to develop it further, make a more powerful boost converter.
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OwlFowl
Sun May 11 2008, 05:01PM
Registered Member #1441
Joined: Sun Apr 13 2008, 03:40PM
Location:
Posts: 10
Yeah I should have done a boost converter since it takes me 1 minute to charge it to 300v

Uzzors:
If you have a microphone, a PC with audio editing software and two sheets of paper there's an easier way. Basically set the two sheets up at a known distance, put the microphone just as far from each sheet and shoot through the paper while recording. When you look at the audio signal later there will be two spikes when each sheet was penetrated. The time between these spikes will be the time the projectile used to cover the known distance. V = S / T


I've used your method and I got some pretty accurate results!
Results
Using the audio editing program I assumed the first spike is caused by the sound when firing the projectile because it makes this "thump" sound. Thus I took the last two spikes.
*The papers are 10cm apart, and the projectile is approx 4grams

trial 1: v=29.0m/s, eff=1.5%
trial 2: v=37.4m/s, eff=2.5%
trial 3: v=31.5m/s, eff=1.8%

I'm not sure whether trial 1+3 or trial 2 are outliers, since the spikes are really unclear. So I'll just assume efficiency as 1.5-1.8%, which is very, very poor

Just a thought: since my caps really charged up to 300v, is it really a 112J source? This would really greatly change the efficiency.
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rp181
Sun May 11 2008, 05:50PM
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
thats 63 joules..... i think you forgot the divide by 2 part.
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Quantum Singularity
Sun May 11 2008, 06:46PM
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
Sounds like he used the V rating of the caps, not the actual charge. Always use the actual voltage you have charged them too, of course there will be less energy when at 300V as compared to 400V (roughly 50% less energy since the V is squared in the equation).
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Barry
Sun May 11 2008, 07:36PM
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 298
Good job! That coilgun looks great. Excellent performance figures, too!

When using acoustic measurements, be sure to account for the speed of sound in air: 340.29 m/s at sea level. This will tend to improve your velocity figures, compared to a simple "distance divided by time" calculation.

Here's the equation for velocity, assuming the PC's microphone is next to the first sound spike:

v = (distance)/(time) = d / (t - d/s)

where d is the distance (meters), t is the flight time (seconds), and s is the speed of sound (340.29 m/s at sea level)

Placing your paper sounding boards only 10cm apart is not very much for 30 m/s. It will be easier to identify sound spikes with more separation. I like to shoot across the room (3 meters) into a sheet of cardboard.

Cheers, Barry
PS - Finished ahead of time? What planet are you from?!

Edit - speed of sound is 340.29 m/s (not 182.88)
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uzzors2k
Mon May 12 2008, 07:01AM
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Asker, Norway
Posts: 1308
If your coil gun makes noise when it fires you can loose the first sheet. Instead of a second sheet of paper I used a tin can which makes much more noise. Since there are only two noise spikes measuring time is easier. Here's what one of my audio signals looked like.



To confirm your microphone measured speeds you should try the vertical shot method too.

How much does your projectile weigh?
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Backyard Skunkworks
Mon May 12 2008, 09:04PM
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
About 5 grams, from his kinetic energy calculations.
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OZZY
Tue May 13 2008, 11:36AM
Registered Member #511
Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55
According to Google the speed of sound at sea level is 340.29 m/s

Nice coilgun by the way. It looks to have good performance in the video. How are you setting the start position? I hope you can make some accurate speed measurements.

OZZY
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OwlFowl
Tue May 13 2008, 08:47PM
Registered Member #1441
Joined: Sun Apr 13 2008, 03:40PM
Location:
Posts: 10
Thanks for all your comments and input!

Barry:
Excellent performance figures, too!


Not anymore! I simply used a box as my sound detection and placed my cg 1m away. For these two trials I used 1 new and 1 used battery. Here are my results.

It seems like the measurements are more precise when I moved the sound target 1m away instead of the mere 10cm. From that, I've gotten:
the speed of sound is already factored in the t values, and
the mass of the projectile is 0.005kg
joules stored in capacitors is 63J NOT 112J

trial 1: t=0.04495233
v=22m/s
e=1.9%
trial 2: t=0.04712933
v=21m/s
e=1.8%

My goal in this project was to obtain a velocity more than 20m/s or an efficiency of 2%+
I would say it was pretty successful considering I've only used 1 new battery in the experiment. Time to get new C batteries

OZZY:
How are you setting the start position?


I used a bolt and nail to hold the projectile in place, and I measured the velocity of the projectile for each mm the nail is protruding from the bolt. I found that with 11mm-12mm of the nail sticking out from the bolt I can fire with maximum velocity. Considering the length of the nail it would mean that the projectile is just resting outside of the coil.

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