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Chrono Spikes

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Chrono Spikes

Unread postAuthor: rp181 » Sun Nov 23, 2008 8:45 pm

For my coilgun, i have chrono'd it several times, and every time, i get 3 "spikes". Why is there three? I have been using the second 2 to measure, but whats causing the first spike?

I am using an IR/photo transistor setup, 5.3" apart. The projectile is 1" long. Could the magnetic field be causing this?.

An image of it is attached.
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The Waveform from mic input, audacity was used, image.
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Unread postAuthor: TurboSuper » Sun Nov 23, 2008 8:51 pm

Is the chrono and the coilgun connected to the same power supply?
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Unread postAuthor: rp181 » Sun Nov 23, 2008 8:59 pm

If it was, the LED would explode :).

The IR led is a just a AA battery, and the capacitor bank for the coilgun is 330v, 955J. I don't get this spike when i just drop something through, only when coilgun is fired. I don't know if its because of the higher speed or the magnetic fields.
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Unread postAuthor: Ragnarok » Sun Nov 23, 2008 9:01 pm

Assuming this doesn't appear on pneumatic/combustion chronoing, this peak will be a result of the magnetic field. A coilgun can kick out multiple tesla, and the rate of magnetic flux density rise will cause inductive currents in nearby conductors.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: rp181 » Sun Nov 23, 2008 9:18 pm

I have not tried to chrono anything else aside from the coilgun and falling objects. I am kind of doubting induction as a cause because there is no loops and the wires are twisted together, which should cancel any inductive currents out.

Is it safe to assume the last 2 depressions represents the actual entering and leaving the IR detector's range?
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Unread postAuthor: TurboSuper » Sun Nov 23, 2008 9:25 pm

Yeah, the sine-wave-ness of the first wave makes it seem more like some kind of glitch rather than anything else, the last two seem like something genuinely blocking the IR beam.
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Unread postAuthor: jimmy101 » Mon Nov 24, 2008 12:33 pm

Yep, inductive coupling between the HV and the audio circuit. Not really surprising, you are dumping a heck of a lot of power into the coil, even if it is an incredibaly crappy radio transmitter it'll still act like a transmitter and the sound card will pick that up.

Sometimes what looks like a problem is actually an incrediably useful thing. Use it to your advantage!

The chony data is very useful for analyzing your coil gun. The voltage versus time across the coil is even more useful than the chrony data.

You can let the HV circuit "talk" to the chrony circuit and get both the voltage verus time and the speed data. As you know, you have to be careful. If the full energy of the coil gun gets anywhere near the sound card "something bad will happen".

You can safely extract a signal from both the cap disharging and the speed trap gates. You might want to take a look at my coil gun page. The gun sucks but the data system is, IMHO, the best (and simplest) I've ever seen for a coil gun. In the graph below the first peak is the voltage versus time trace of the cap disharging through the coil. From that you can get the RLC of the gun, which is probably the most important characteristic. The second two peaks are the chrony data.
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Unread postAuthor: rp181 » Mon Nov 24, 2008 1:12 pm

Ok thanks, but just how far will this "signal" travel? The laptop was around 4 feet away.

So the steeper the 1st peak, the faster the current rise, right? And how come my peaks are reverse of yours?
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Unread postAuthor: jimmy101 » Mon Nov 24, 2008 4:22 pm

rp181 wrote:Ok thanks, but just how far will this "signal" travel? The laptop was around 4 feet away.

Off-hand, as the inverse square of the distance. Double the distance get 1/4 the signal strength. Given the energy levels you are using I wouldn't be surprised if you couldn't detect (by radio) the coil gun firing from a long way away. If you had a spark gap in the circuit (for example, you were using a trigatron as the main power switch) it may well be detectable from hundreds of miles away.

rp181 wrote:So the steeper the 1st peak, the faster the current rise, right?

Hard to say. Depends on the characteristics of the system as a radio transmitter. I would'nt count on its behavior being predicatable or interpretable. Go with a hardwired link between the HV cap and the sound card. See my web page for one way to do it. Basically just put the cap's voltage through a very large high impedance voltage divider in parallel with your coil. Something like a 10Meg ohm and a 10K ohm resistor divider. That'll step the voltage down 1000:1 and drop the current blead into the sub-milliamp range. In my setup I monitor after the SCR trigger so there is a very fast voltage rise when the SCR triggers. The failing edge of the voltage curve is the cap+coil disharge curve. From that you can measure the RLC time constant of your setup as it is actually firing a projectile.

rp181 wrote:And how come my peaks are reverse of yours?

The phase of the peaks depends on how they are coupled (communicate) with the soundcard. So don't worry too much if the peaks are positive or negative. You can explain why they are one way or the other but bottom line it doesn't matter. You really just care about the shape of the curve and when it occurs, the phase is irrelevant.

Your signal looks to be inductive coupled so you are getting both positive and negative voltage swings. I think it'll be hard to interpret exactly what you are recording. Best to purpose-design a link between your coil setup and the soundcard.

BTW, what's the velocity you get from this recording? There are no time markings so I can't figure it out for myself.
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Unread postAuthor: psycix » Mon Nov 24, 2008 4:30 pm

Maybe you are measuring light brightness and thus when the projectile passes the brightness drops.
If we consider the sine wave at the start as noise, then you see two blocks of where the projectile blocked off the sensors.
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Unread postAuthor: rp181 » Mon Nov 24, 2008 6:00 pm

I dont think i want to risk connecting it to the laptop directly, never done this kind of thing before, or any voltage divider for that matter.

I've been using a silicon steel projectile Which measured at 15.125 m/s, with an efficiency of the coilgun at .359%. The output joules was 3.43J. Very bad, but i just found a steel projectile which works MUCH better, but i gotta wait for the laptop to chrono it.

EDIT: My switch is just 2 copper plates contacting, alot less sparking than you would think. The plates are 2" X 4" X .24".


Instead of posting a new thread, i started some stuff with Supercavitation. I dont have a very good camcorder right now, and this is taken at 30fps with digital camera. Is this really super cavitation? It was fired with a coilgun, so no air was intentionally pushed under the water.

http://www.youtube.com/watch?v=s1XdxtHuA9w
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Unread postAuthor: Fnord » Mon Nov 24, 2008 6:30 pm

How far away from the gun is the chrono setup? About 6 inches?
(not the laptop, the phototransistors themselves)

As to cavitation: You have some air in the barrel, right? I think there may be a little being pushed out, then dragged along in the vortex behind the projectile. Also, the air behind the projectile in the barrel will be pulled forward, and may have enough momentum to exit the barrel slightly.
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Unread postAuthor: rp181 » Mon Nov 24, 2008 6:35 pm

The Coilgun has the chrono built in (http://rp181.110mb.com/index.php?p=1_9_The-Coils). I was using the far right coil. The phototransistors are under the black rings immdeiatley right of the 2 left coils.


Cavitation: Ile try to completley submerge the coilgun, and see if i get any results.

EDIT: I tried completley underwater, and nothing :( . What kind of speed do i need for a flat .5in projectile to super cavitate?
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Unread postAuthor: psycix » Tue Nov 25, 2008 11:33 am

Wikipedia wrote:A supercavitating object uses this phenomenon in a much larger (and sustained) manner (hence the name supercavitation). A supercavitating object's main features are a specially shaped nose, typically flat with sharp edges, and a streamlined, hydrodynamic and aerodynamic shape. When the object is traveling through water at speeds of above roughly one hundred miles per hour / 44.7 m/s, the nose deflects the water outward so fast that it flies free of the surface.

http://en.wikipedia.org/wiki/Supercavitation


Different shaped objects would give different minimal velocities, but as you can see, you need roughly 3 times the speed you're currently at to get it supercavitating.
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Unread postAuthor: jimmy101 » Tue Nov 25, 2008 11:59 am

If you are using a homebrew mechanical switch to fire this thing you are almost certainly generating a spark. Some of the first radio transmiters were just spark gaps, the RF will travel for miles and can be picked up by TVs, radios, tape recorders etc. The sound card will all most certainly pick up your switch closing. Heck, even if you unplug the chronometer gates it'll probably still pick up the RF from the switch closing.

It sounds though like you are on the right track. You might want to check your ammo. People think all steels are equally attracted to a magnet. This is not true. High grade stainless steels are only weakly attracted to a magnet (or coil) and are terrible coil gun ammo. Might as well try launching a hunk of wood. The crappier the steel the better (usually) it is attracted to a magnet and the better it'll perform in a coil gun. IIRC, the best ammo is a hunk of iron instead of steel.

The efficiency of coil guns always sucks. 0.3% isn't all that bad.
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