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My school coilgun poject

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LAN_403

Skynet

Sun Mar 18 2018, 11:13PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Update:
I've just started to wire up the second stage and my LEM current sesors arrived so I am trying to mess with them ;) When it will be done I'll try to do some tests on realy low voltage.
WP 20180319 00 02 24 Pro

DerAlbi

Mon Mar 19 2018, 04:24AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Ohh!! Current sensors! Yes yes yes, i hope you have a digital oscilloscope, so you can share current waveforms. I just made analog isolators with 1MHz bandwidth for up to +/-5V input and 2kV isolation voltage including x1/x10/x100 optional gain.. but they will take some time

The cool thing about your 2 coil setup is that your 2nd coil seems to be movable. (or is it glued in place?)
It would be a great experiment to change the distance between the coils, measure their coupling factor and see how the individual efficiency of the 2nd coil changes. Thats something i cant do with my setup and it really bothers me. To be honest i am currently at a low point with my own build. Not due to disappointment, but due to lack of understanding why it performs much better than expected. Somehow there is some magic energy transport between stages, but i cant figure out what it is or where it comes from. As you can see by my word choice "magic" i cant even define the mechanism at play

I hope current measurement sheds some light.

Btw, for consistent measurements, i would suggest you add a 3rd (maybe even a 4th) light trap so you can determine the projectile speed reliably after acceleration has finished..
For now 7.71km/h sounds like its alive ^_^

Skynet

Tue Mar 20 2018, 02:37AM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Made some progress again :)
Lot of wiring done but not at the end yet :D Got 2 stages to work at 24V (just testing firing algorithm) without any protection at all. Eff for 1st coil 3.92% and second 11.76% but it's not much precise (need one more optogate just for 1st stage).

Next steps are current sensors and some dumb charger to get more voltage. Also ordered another 4 IGBT modules (Toshiba MG200J2YS50) it's some cheap used crap but it will do the job for now.

1521513454 11734 FT158837 Wp 20180320 03 00 17 Pro

1521513454 11734 FT158837 Wp 20180320 03 00 03 Pro

DerAlbi

Tue Mar 20 2018, 04:34AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

May i ask how you define your efficiency? There are several ways:
1) Overall shot:
1.1) Ekin / Initial_Ecap
which is equivalent to the SCR design (implies weight- and volume efficiency)
1.2) Ekin / Used_Ecap
which takes into account the advantages of a half bridge design, however it says nothing about if you have a viable build or not
2) Individual Coil
[EKin_Coil(n) - Ekin_Coil(n-1)] / [ Final_ECap(n) - Final_ECap(n-1) ] = "Gained Ekin from coil n" / "Used Ecap from coil n"
which is more a design verification and tells more about if an individual coil is driven correctly

There is merit to all of those definitions, so i ask.
11% at this velocity sounds pretty amazing (no matter which definition)

Skynet

Tue Mar 20 2018, 01:30PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

I calculate efficiency separately for individual coils. So "Gained Ekin from coil n" / "Used Ecap from coil n" but for now I just count that it eats all energy from cap (1-2V on caps after shot is negligible for now) so it's not much accurate :D
Working on voltage feedback from caps to processor that will give me posiibility to count with recuperation.

DerAlbi

Wed Mar 28 2018, 11:39PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

More updates!!

Skynet

Fri Mar 30 2018, 01:36PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Not much to write about started to wire up more stages, and still playing with current sesnors and algorithm for current regulation. More progress would be done after easter break ;)

WP 20180328 21 09 34 Pro

Skynet

Wed Apr 04 2018, 08:47PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Just playing with more voltage :D Remaked firing algorithm so now i dont know speeds but hopefully it will be fixed soon to be able to get some results ;)
On this shot end speed shoud be 12,5m/s and final voltage on caps was around 20V.

DerAlbi

Wed Apr 04 2018, 09:41PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

The kickback is a bit scary

Fixing your acceleration path to your table might help. (it also consumes energy to move the coils)
Looks like roughly 2.5J.. whats in the box to stop the projectile?

Skynet

Wed Apr 25 2018, 02:28PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Time for updateeee :P
So I almost wired up everything for 4 stage test for weekend.
Also did some measuring and for now we are at 30,41m/s with caps charged to 200V ;)

Video from test there:

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