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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I set up a system to track down the performance of coilguns looking at voltage and current waveforms. The system is simply a 10Mhz storage o-scope, a 10:1 compensated probe, and a rogovsky current measurement system (4mOhms eq shunt resistance). The waveforms are stored in chopped mode
The pictures were taked with a casio exilim Digital camera and processed to obtain the actual image.
This will be used to tune the performance of my last coilgun 5 stage pistol, to determine decay time, and peak switch voltage
This with the projectile
Dryfire
the waveforms belong to my old single stage v-switch pistol, notice the small second peak belonging to the second scr shutting off the main scr
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
WaveRider wrote ...
Hi Mero. That hump in the current at the trailing end of the voltage waveform seems strange to me. Is there any explanation for that?
Cheers!
The hump on the voltage waveform is indeed strange, but probably a measure artifact, the cap themselves have a ESR, maybe it is due to the IR losses (the voltage is measured at cap ends). Another explanation can be due to a transient generated by one scr closing and one opening, or maybe the inductance of the capacitors. Did it appear on your waveform? If not it may be a compensation issue of my probes (1Khz square used for adjusting). The current probe is active and properly terminated so it don't suffer strange effects at coilgun frequencies
I am curious what happens with halfbridge coilgun. With the voltage and current probe i will be able to calculate the recovery efficiency (Vend^2-Voff^2/2*C) /(Ioff^2*L/2) where Ioff is the coil current immediately after the IGBT switch off, same for Voff and Vend is the final capbank voltage, (Vend^2-Voff^2/2*C) is the energy recovered, and (Ioff^2*L/2) is the total coil energy when the IGBT's switch off. The value should be in the 50-60% range (2-3J for a 5J coil energy), better save than sorry
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Hi Mero, I was referring to the small hump in the final part of the current curve.. (I looked at my first post and it was a bit unclear.) When I went to Evgenij's site, that cleared it all up for me..
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cool coilgun waveforms
than sorry