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guitardude012

Sun Sept 07 2008, 07:03AM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Just wanted to say Hi and give a quick preview of a new coilgun to come (my 3rd).

The current plan is to use 4 stages, 400V rated caps(350V charge max for now) and V-switches. I am using lasers after every coil to do timing and will hopefully be able to do control (setting output velocity). The total will be just shy of 2.5kJ until I get these 4 stages working.

I have built a cart for this gun, as I imagine it containing more caps/stages if I make enough progress that I feel limited by the current design. It was necessary to make it 4' long due to the 4 stages and high velocity.

I have already achieved >2.69% efficiency with the first stage shooting at a low 160V using two paralleled SCR's and an excessive air gap around the projectile (new projectile material will be ordered once I have funds again).

The first half(3900uF) of the first stage is already quite powerful(237J shot):

Charging is very rapid via a 110VAC->220VAC transformer, then rectified(full wave).

My uC will data log and transmit to PC so optimization should be quick. I already have an LCD readout of velocity on my gun.

I spent last week solving some hard differential equations for this thing and should have some cool results soon. I am out of funds for at least 2 weeks so the other stages will have to wait until then.

guitardude012

Sun Sept 07 2008, 08:57PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Sorry I am still in the 1%s efficiencies, I was timing the projectile before it had completely left the coil and the t ail was still braking.

Arcstarter

Sun Sept 07 2008, 11:14PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Very nice cart :P. Let me guess, mallory 3900uf 400 volt capacitors?

guitardude012

Sun Sept 07 2008, 11:39PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Arcstarter wrote ...

Very nice cart :P. Let me guess, mallory 3900uf 400 volt capacitors?

Correct =) Thanks

Backyard Skunkworks

Mon Sept 08 2008, 12:38AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Sweet setup!

Can't wait for videos!

Firefox

Mon Sept 08 2008, 01:41AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Still, 1% is better than my 'it didn't move attempts.' Given, I've never tried hard so...

guitardude012

Mon Sept 08 2008, 04:50AM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Here is a scope view of the sensor voltages falling as the .0155kg projectile exited at 17m/s :

In this case I was firing at 330V so efficiency was about 1.05%. It does better in the 150-200V range and I need to play with a lot of parameters still.

I need to work on the position sensing, the rise/fall time is too slow.

Killa-X

Mon Sept 08 2008, 07:44PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

id say nice project, but excuse me for asking....How do you figure the efficiency of the gun? Dont forget to play smart though with the charger :) i used a camera one and my newly charged energizers fried the transformer. Those things are like 1500ma or so xD

guitardude012

Mon Sept 08 2008, 08:03PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Killa-X wrote ...

id say nice project, but excuse me for asking....How do you figure the efficiency of the gun? Dont forget to play smart though with the charger :) i used a camera one and my newly charged energizers fried the transformer. Those things are like 1500ma or so xD

Not sure if you are asking for formulas or how I actually measured things.

I had two lasers a set distance apart and watched them being triggered to determine velocity. The problem I mentioned earlier was that the projectile was still braking when I read the velocity in circuit, combined with a poorly tuned sensor (though this gives me a hint of what to expect once I can turn the coil off faster)

For efficiency I just used projectile kinetic energy / (initial capacitor energy(J) - remaining capacitor energy)

for Projectile Kinetic energy I used 0.5*mass*(velocity^2)

Now I have to decide how I will trigger the SCRs with a microcontroller.

I had good luck getting some nice metal for free this morning ASTM A848, I will keep you all posted when I can compare the results.

Can you explain your warnings about charging in more detail?

Thanks

Lessons this weekend: Don't try to make one resistor work for bleeding and dumping.

P.S. using my new equations it may be possible to get an idea of friction and bullet stability in the barrel. WTG Differential Equations!

guitardude012

Wed Sept 10 2008, 02:53AM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Very exited, I upgraded to two caps on the first coil and got to 1.3766% efficiency tonight without sacrificing initial capacitor voltage(330V in this case) to do so!

Launched a monstrous .0292kg slug at just over 20m/s!

Can't wait until I get the other three coils up and going. I still have a lot of work to do on this coil yet and am not even using a V-Switch or diagonal half bridge.

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