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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Since pc sound cards can sample up to 200Khz i decided to connect the output of the rogowsky coil integrator dyrectly to sound card and log it with a virtual oscope program. After that the data was transferred to excel and a graph plotted. The current was rescaled according to peak detector value shown on display.
The graphs show two shots: a dryfire and a projectile fire. The waveform was measured on the scr side so don't surprise is the current falls very rapidly (on the coil it doesn't happen and falls exponentially).
As you can see the (purple) in-coil projectile waveform is lower in value and last a bit longer (slower discharge due to additional inductance).
I don't understand the small overshoot before the current fall, may it be due to the scr, stray inductance, or diode reverse recovery capacitance (but it is an ultrafast one). Anyway it is cool to log the waveforms. Rogowsky coils are fantastic.
Edit: Sorry about the attachment. They don't work properly. I will post a link tomorrow pointing at the excel file
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
I did exactly the same thing a couple of years ago, it's a great substitute if you dont have access to a real 'scope. Especially for coil gun stuff, as you dont really need much bandwidth for that. I am surprised about the figure of 200kHz though, why would a souldcard not stop at the end of the audible range, i.e. 20kHz?
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
You are right. The badthwitch stops at 20Khz but the sampling rate can be up to 200Khz , meaning one sample each 5 microseconds, that's perfect for coilguns. The only defect of this procedure is the "baseline lose" effect due the the rogosky passive integration. So you need excel to correct it with an exponential fit.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Some better quality soundcards sample at up to 192kHz even though, like joe doh pointed out, there's no real need for sample rates higher than 45-50kHz to cover the audible frequency range of 20Hz to 20kHz.
Even if the passive integrator didn't cause droop, soundcards are still AC coupled and won't respond accurately to very low frequencies (much below 20Hz)
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Coilguns can be considered operating in the 500Hz range. Also i use a gaussian baseline correction. See this graph. Notice the dip on the waveform foot. This is the projectile suchback. In line with simulations
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logging the coilgun waveform with pc sound card
