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Microphone chronograph

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Microphone chronograph

Unread postAuthor: saefroch » Tue Oct 19, 2010 11:38 am

I am thinking of calculating muzzle velocity using the method that I believe Technician1002 used with his QDV cannons, firing a magnetic projectile past wires and using a sound-recording program like Audacity to record the changes in current as the magnet passes each part of the coil.

Is there any chance that I fry something in my computer by firing a small neodymium magnet down a 1/4" stainless steel barrel with the wires against the barrel (insulated, of course)?
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Unread postAuthor: Technician1002 » Tue Oct 19, 2010 1:01 pm

Drop a magnet down a copper or other conductive tube.
[youtube]http://www.youtube.com/watch?v=pcVG6c_OvYU[/youtube]
You may decide placing the pickup on a non conductive barrel would work much better.
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Unread postAuthor: saefroch » Tue Oct 19, 2010 4:49 pm

Ummmmmmm I must be missing something. I don't see the relevance of that video, or how it answers my question.
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Unread postAuthor: Ragnarok » Tue Oct 19, 2010 6:28 pm

saefroch wrote:I don't see the relevance of that video

Passing a magnet down a conductive tube creates eddy currents in the tube - these produce a opposing magnetic field that resists the motion of the magnet.

Basically, firing a magnet from a conductive barrel will have a dramatically lower muzzle velocity than firing another otherwise identical projectile. This applies regardless of whether the metal in question is magnetic.

In short, magnet based measurements cannot be used with conductive barrels.

That said, I wouldn't recommend it even otherwise - you could quite easily fry computer parts if you didn't know exactly what you were doing.
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Unread postAuthor: Technician1002 » Tue Oct 19, 2010 6:36 pm

The conductive stainless barrel will both slow the magnet and short the moving magnetic field due to the current induced in the barrel by the moving magnet.

As far as damage to a computer is concerned, unless the coil has too many turns, the voltage will not be too high and damage the computer. I used 4 turns per pickup and used a DJ mixer to adjust the volume. Use the Line level input, not the Mic input.

In low speed magnet drop tests, I used more turns to test the concept. Voltage is directly proportional to;

Flux (magnet strength) X Speed X Number of turns.

The actual formula to calculate it is Number of flux lines crossed per second = voltage. More turns = more flux lines crossed, stronger magnet = more flux lines, faster = more lines crossed per second.

This initial test generated voltage peaks of less than 100 mV.
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Reference;
http://www.tpub.com/neets/book2/1b.htm
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Unread postAuthor: saefroch » Tue Oct 19, 2010 7:23 pm

Ragnarok wrote:Passing a magnet down a conductive tube creates eddy currents in the tube - these produce a opposing magnetic field that resists the motion of the magnet.

Haven't studied E&M yet, that's in a few months. Thanks a bunch for the explanation.

Ragnarok wrote:you could quite easily fry computer parts if you didn't know exactly what you were doing.

This is now looking like a less viable option. I'll just go with audio recording of firing and impact. Thanks for the explanations.
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Unread postAuthor: jimmy101 » Fri Oct 22, 2010 5:02 pm

A light based chrony is pretty easy, and uses the PCs sound card. Easy enough to directly mount the detectors onto the gun, which makes alignment a lot easier. You probably already have all the parts needed for the holder. Just need a couple phototransitors (radioshack), some wire and a PC.
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Audio is OK but it works best if you have a large open area to do the recording. That reduces the number of echoes and makes the analysis much easier.
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Unread postAuthor: Technician1002 » Fri Oct 22, 2010 5:40 pm

Ragnarok wrote:In short, magnet based measurements cannot be used with conductive barrels.


There is actually a way to do this in a conductive barrel. A pair of small holes are made in the barrel and a magnet with a coil on the magnet is bolted into the holes. A non magnetized projectile is fired past the pick ups and in passing the magnet, the induced current is picked up as a change in flux on the magnet. This is fed to the sound card.

This type of pickup is commonly used in crank angle sensors in cars as the passing of gear teeth past the end of the magnetized sensor induces current in the pick up coil in the sensor assembly. This will both sense Iron and non ferris conductors such as copper or lead as the induced current will provide a flux change.

Some use a pick up coil. Others use a hall effect sensor.

Gas engine crank angle sensor
Image

Edit I found a Youtube video that shows all three of the sensor types. The drawing of the magnetic sensor is excellent showing the magnet and coil. A passing metal object alters the flux which is picked up by the coil.
[youtube]http://www.youtube.com/watch?v=RuIislTGOwA[/youtube]
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Unread postAuthor: JDP12 » Fri Oct 22, 2010 6:12 pm

Thanks for sharing the Jim's Homemade Chrono link... I lost it awhile back and have been wanting to make one.
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Unread postAuthor: MrCrowley » Fri Oct 22, 2010 6:42 pm

With help from both Jimmy101 and Technician1002, I tried both methods, light and audio. I made a thread about it here.
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